CN110922276A

CN110922276A - Preparation of modified sludge hydrothermal carbon material and application of modified sludge hydrothermal carbon material in non-point source pollution emission reduction

Info

Publication number: CN110922276A
Application number: CN201911116409.9A
Authority: CN
Inventors: 冯彦房; 薛利红; 褚清南; 杨林章; 花昀; 何世颖; 李德天
Original assignee: Jiangsu Academy of Agricultural Sciences
Current assignee: Jiangsu Academy of Agricultural Sciences
Priority date: 2019-11-15
Filing date: 2019-11-15
Publication date: 2020-03-27
Anticipated expiration: 2039-11-15
Also published as: CN110922276B

Abstract

The invention relates to a preparation method of a modified sludge hydrothermal carbon material and application thereof in non-point source pollution emission reduction, wherein wet sludge subjected to anaerobic digestion treatment and a solution containing 0.8-1.2M magnesium citrate and 0.8-1.2% of H₂SO₄Wherein the ratio of wet sludge to reaction medium solution is 1: 1-4 w/v; placing the mixture of the wet sludge and the reaction medium solution in a high-pressure reaction kettle, and carrying out hydrothermal carbonization for 1-2h under the conditions of 300 ℃ at 250 ℃ and 4-10MPa to prepare a modified sludge hydrothermal carbon material; collecting the modified sludge hydrothermal carbon material, and then drying the modified sludge hydrothermal carbon material for later use; the modified sludge hydrothermal carbon material can be applied to the paddy field soil, can inhibit ammonia volatilization, reduce nitrogen loss in paddy field surface water, improve the utilization efficiency of the rice on nitrogen and increase the rice yieldThe amount has positive significance for sustainable rice production.

Description

Preparation of modified sludge hydrothermal carbon material and application of modified sludge hydrothermal carbon material in non-point source pollution emission reduction

Technical Field

The invention relates to a preparation method of a modified sludge hydrothermal carbon material and application of the modified sludge hydrothermal carbon material in non-point source pollution emission reduction.

Background

Nitrogen is a major nutrient element for plant growth. The amount of nitrogen fertilizer used is expected to increase from 105 megatons in 2010 to 180 megatons in 2050. In rice fields, nitrogen fertilizer often exceeds plant demand, and most of the nitrogen fertilizer is due to ammonia (NH)₃) Is lost by volatilization. N caused by nitrification and denitrification of soil microorganisms₂O and N₂Drained and lost in runoff during storms and in-season drainage. Loss of nitrogen can reduce available nitrogen in plants and cause environmental problems. Worldwide, the annual NH is estimated₃The volatility loss amounts to 32Tg, results in atmospheric pollution by the formation of particulate matter (e.g., PM 2.5. mu.m. diameter. ltoreq.2.5), eutrophication of water and soil acidification after atmospheric nitrogen deposition. N runoff causes pollution of rivers and lakes. Therefore, effective strategies are urgently needed to be made to reduce the loss of the nitrogen in the rice field and improve the utilization efficiency of the nitrogen in the rice.

Biochar is a carbonaceous residue (hereinafter referred to as hydrocar) obtained by cracking an organic material by hydrothermal carbonization (HTC) under oxygen-limited conditions (hereinafter referred to as pyrolytic carbon) or in the presence of water. In the paddy field, NH₄ ⁺Is the predominant form of inorganic nitrogen that is bioavailable. In flooded paddy soil, NH₃Volatilization is promoted and the surface water causes runoff of available nitrogen.

Sludge is a by-product of wastewater treatment plants. Many countries do not recommend direct use of sludge for agriculture due to concerns about pathogens, micropollutants, organic harmful substances and heavy metals. If the sludge can be transformed to be nontoxic and can obviously promote agricultural production and soil modification, the method has good application value in industry and can realize regeneration and cyclic utilization of the sludge.

Disclosure of Invention

In order to overcome the defects, the invention provides a preparation method of a modified sludge hydrothermal carbon material and application of the modified sludge hydrothermal carbon material in non-point source pollution emission reduction, which can inhibit ammonia volatilization, and improve soil nitrogen fixation capacity and rice nitrogen utilization rate.

In order to solve the technical problems, the technical scheme adopted by the invention is as follows: a preparation method of a modified sludge hydrothermal carbon material comprises the following steps:

the first technical proposal is that wet sludge after anaerobic digestion treatment and sludge containing 0.8-1.2M magnesium citrate and 0.8-1.2% H₂SO₄Wherein the ratio of wet sludge to reaction medium solution is 1: 1-4 w/v; placing the mixture of the wet sludge and the reaction medium solution in a high-pressure reaction kettle, and carrying out hydrothermal carbonization for 1-2h under the conditions of 300 ℃ at 250 ℃ and 4-10MPa to prepare a modified sludge hydrothermal carbon material; collecting the modified sludge hydrothermal carbon material, and then drying the modified sludge hydrothermal carbon material for later use;

or, the second technical proposal is that wet sludge after anaerobic digestion treatment is dried to obtain dried sludge, and the dried sludge contains 0.8 to 1.2M magnesium citrate and 0.8 to 1.2 percent of H₂SO₄The reaction medium solution is mixed, wherein the ratio of the dried sludge to the reaction medium solution is 1: 6-12 w/v; placing the mixture of the dried sludge and the reaction medium solution in a high-pressure reaction kettle, and carrying out hydrothermal carbonization for 1-2h under the conditions of 250-300 ℃ and 4-10MPa to prepare a modified sludge hydrothermal carbon material; collecting the modified sludge hydrothermal carbon material, and drying the modified sludge hydrothermal carbon material for later use.

The magnesium citrate has carboxyl, so that the abundance of the carboxyl on the surface of the hydrothermal carbon is increased, the adsorption and the fixation of ammonium radicals are facilitated, and the loss of the ammonium radicals converted into ammonia gas is reduced; the slow-release fertilizer effect is achieved, namely, the ammonium roots are held to prevent loss when the concentration of the ammonium roots is high, and the ammonium roots are released to provide nutrients for rice when the concentration of the ammonium roots in the rice field is low.

Magnesium ions in the magnesium citrate are loaded on the surface of the hydrothermal carbon, so that effective retention of anions such as nitrate and phosphate can be realized. The carbon material has poor anion adsorption effect, and the adsorption of anions can be enhanced by adding magnesium ions. Meanwhile, the magnesium ions are nontoxic and harmless and are beneficial elements for the growth of crops.

The addition of sulfuric acid can play a role in enhancing the carbonization of the sludge, and the hydrothermal reaction temperature is relatively low and is only more than 200 ℃, so that the complete carbonization is not facilitated; the addition of sulfuric acid can enhance the process. In addition, some heavy metal elements contained in the sludge can be washed and dissolved out by sulfuric acid solution, so that the sludge carbon is harmless and can be recycled.

The drying sludge is ground to decompose the sludge into small particles, contact between the sludge particles and a reaction medium is increased, the hydrothermal carbonization reaction efficiency is improved, the modification of the sludge hydrothermal carbon is more uniform, and the modified sludge hydrothermal carbon has better performances of inhibiting ammonia volatilization, and improving the nitrogen fixation capacity of soil and the utilization rate of rice nitrogen.

Before the sludge drying agent is used, the content of copper, zinc, chromium, mercury, lead, cadmium, arsenic, nickel, mineral oil and polycyclic aromatic hydrocarbon in the dried sludge or wet sludge is detected, and the content of the copper, the zinc, the chromium, the mercury, the lead, the cadmium, the arsenic, the nickel, the mineral oil and the polycyclic aromatic hydrocarbon is required to meet the pollutant limit value standard of an A-grade sludge product in the agricultural sludge pollutant control standard GB 4284-. If the standard is not met, the use purpose needs to be changed.

Preferably, the moisture content of the wet sludge is 50% -90%; the preparation process of the wet sludge comprises the following steps: activated sludge produced by municipal sewage plants is subjected to anaerobic digestion to obtain wet sludge. The wet sludge is directly mixed with the reaction medium solution without grinding.

Preferably, the water content of the dried sludge is 1-10%; in the second technical scheme, the prepared dry sludge with the water content of 1-10 percent can be directly used (the preparation method is the same as the second technical scheme). The water content of the dried sludge cannot be too high, which can affect the reaction efficiency of hydrothermal carbonization, the abundance of carboxyl groups on the surface of finally formed hydrothermal carbon and the concentration of magnesium ions, and can also reduce the separation and removal of harmful substances in the sludge.

Further, the dried sludge is ground, sieved by a sieve with 10-30 meshes, and then mixed with the mixture containing 1M of magnesium citrate and 1% of H₂SO₄The reaction medium solution of (1) is mixed, wherein the ratio of the dried sludge to the reaction medium solutionIs 1:10 w/v.

The modified sludge hydrothermal carbon material is collected in a centrifugal collection mode; the modified sludge hydrothermal carbon material is dried for later use, which means that: drying the centrifugally collected modified sludge hydrothermal carbon material at the temperature of 60-70 ℃ until the weight is constant.

The invention also provides a modified sludge hydrothermal carbon material which is prepared from sludge subjected to anaerobic digestion treatment, magnesium citrate and H₂SO₄The mixture of (a) is prepared by hydrothermal carbonization reaction. The modified sludge hydrothermal carbon material can be prepared by the preparation method.

The invention also provides an application method of the modified sludge hydrothermal carbon material, which comprises the following steps:

the modified sludge hydrothermal carbon material is fully mixed with the paddy field soil in an application amount of 0.1-1.5%, and after at least three times of continuous fertilization, the modified sludge hydrothermal carbon material always inhibits the volatilization of soil ammonia, increases the absorption of soil to ammonium nitrogen and increases the utilization rate of rice to nitrogen. Specifically, the mass ratio of the modified sludge hydrothermal carbon material to the soil is 0.1-1.5%, preferably 0.8-1.5%, and more preferably 1%.

The invention also provides the application of the modified sludge hydrothermal carbon material in inhibiting ammonia volatilization in the field, increasing nitrogen fixation capacity of soil, improving nitrogen content and yield of rice grains and increasing continuous planting time of rice, wherein the modified sludge hydrothermal carbon material is fully mixed with the soil in the field by 0.1-1.5% of application amount during application; preferably, the rice is planted in soil.

The invention also aims to provide the application of the modified sludge hydrothermal carbon material as a fertilizer for inhibiting ammonia volatilization in the field, increasing the nitrogen fixation capacity of soil, improving the nitrogen content and yield of rice grains and prolonging the continuous planting time of rice.

Compared with the prior art, the invention has the beneficial effects that: hydrothermal carbonization (HTC) is a promising sludge treatment technology. In the invention, three sludge hydrothermal charcoals generated by mixing sludge hydrothermal charcoal and water (SSHW), sludge hydrothermal charcoal and 1M magnesium citrate (SSHM) solution, and sludge hydrothermal charcoal, 1M magnesium citrate and 1% sulfuric acid (SSHMS) solution are applied to a rice soil column test. The effect of the different modified sludge hydrothermal chars on ammonia volatilization, soil nitrogen (N) retention and rice growth was evaluated. The results show that SSHMS reduced the cumulative ammonia volatility measured after three applications of fertilizer N compared to the control. SSHM and SSHMs reduced ammonia volatilization by 20.3% and 41.2%, respectively. In addition, after the fertilizer is supplemented for the first time, the three sludge hydrothermal carbons can increase the absorption of soil ammonium nitrogen; however, after the second supplemental fertilization, addition of only SSHMS significantly increased soil ammonium nitrogen uptake. Among the three sludge hydrothermal carbons, SSHMS has the strongest influence on the retention of soil ammonium nitrogen and on the inhibition of the loss of ammonium nitrogen in field runoff. This is due to the lower surface pH and pore diameter, larger adsorption pore volume and higher carboxyl functional group content of SSHMS, thereby promoting ammonia adsorption. In addition, the increased soil nitrogen retention increases the nitrogen content and yield of rice kernels. The invention provides a new method for recycling sludge into valuable fertilizer, which can inhibit ammonia gas volatilization and nitrogen loss in field water and improve the utilization efficiency of rice on nitrogen if applied to the soil of a rice field, and has positive significance for sustainable rice production.

Drawings

FIG. 1 FTIR spectra for SSHW, SSHM and SSHMS. Au: absorbance units; SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 2C 1s XPS spectra of SSHW, SSHM and SSHMS. SSHW (fig. 2A): sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM (fig. 2B): in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS (fig. 2C): 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 3 Basic Fertilization (BF), first fertilization (SF1) and second fertilization (SF2) on day 7, sludge hydrothermal charcoal on soil NH₄ ⁺-N (FIG. 3A), NO₃ ^--effect of N (fig. 3B) concentration and pH (fig. 3C). According to DuncanThe multi-range test shows significant differences in the list indicated by different letters when P ≦ 0.05(n ═ 3). SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 4 Basic Fertilization (BF), first supplemental fertilization (SF1) and second supplemental fertilization (SF2) 7 days after fertilization, sludge hydrothermal charcoal application on average NH in field water samples₄ ⁺-N (FIG. 4A), NO₃ ^--N (FIG. 4B) and pH (FIG. 4C). According to Duncan's multiple range test, the list denoted by different letters shows significant differences when P.ltoreq.0.05 (n.3). SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 5 shows ammonia (NH) accumulated in the paddy soil within one week after the application of the sludge hydrothermal charcoal pair (A) to the Basic Fertilizer (BF), the first supplemental fertilizer (SF1) and the second supplemental fertilizer (SF2), respectively₃) Effect of volatilization loss (cumulative ammonia volatilization 7 days after fertilization); (B) NH per unit yield₃And (4) volatilization loss. According to Duncan's multiple range test, the list denoted by different letters shows significant differences when P.ltoreq.0.05 (n.3). SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 6 shows the effect of sludge hydrothermal carbon treatment on nitrogen absorption by rice plants. According to the Duncan test, at P ≦ 0.05(n ═ 3), the list indicated with different letters shows significant differences. SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 7 shows elements C, O, P, Ca, Mg and S of different modified sludge hydrothermal carbon materials. (A) SSHW; (B) SSHM; (C) sshms. sshw: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 8 SEM images of different modified sludge hydrothermal carbon materials. (A) SSHW; (B) SSHM; (C) sshms. sshw: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

FIG. 9 application of sludge hydrothermal charcoal to NH within one week after Basic Fertilization (BF), first supplemental fertilizer (SF1) and second supplemental fertilizer (SF2)₃Loss of volatilization (characteristic of daily change in ammonia volatilization). According to Duncan's multiple range test, the list denoted by different letters shows significant differences when P.ltoreq.0.05 (n.3). SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC. Wherein, black dots: comparison; red diamond grid: SSHW; blue square: SSHM; green triangle: SSHMS.

Detailed Description

The following describes embodiments of the present invention with reference to examples.

A preparation method of a modified sludge hydrothermal carbon material comprises the following steps:

wet sludge treated by anaerobic digestion is mixed with water containing 0.8-1.2M magnesium citrate and 0.8-1.2% H₂SO₄Wherein the ratio of wet sludge to reaction medium solution is 1: 1-4 w/v; subjecting the mixture of wet sludge and reaction medium solution to high pressureHydrothermally carbonizing for 1-2h in a reaction kettle at the temperature of 250-300 ℃ and under the pressure of 4-10MPa to prepare a modified sludge hydrothermal carbon material; collecting the modified sludge hydrothermal carbon material, and then drying the modified sludge hydrothermal carbon material for later use;

or drying wet sludge treated by anaerobic digestion to obtain dried sludge, wherein the dried sludge contains 0.8-1.2M magnesium citrate and 0.8-1.2% H₂SO₄The reaction medium solution is mixed, wherein the ratio of the dried sludge to the reaction medium solution is 1: 6-12 w/v; placing the mixture of the dried sludge and the reaction medium solution in a high-pressure reaction kettle, and carrying out hydrothermal carbonization for 1-2h under the conditions of 250-300 ℃ and 4-10MPa to prepare a modified sludge hydrothermal carbon material; collecting the modified sludge hydrothermal carbon material, and drying the modified sludge hydrothermal carbon material for later use.

The moisture content of the wet sludge is 50-90%; the preparation process of the wet sludge comprises the following steps: performing anaerobic digestion treatment on activated sludge generated by a municipal sewage plant to obtain wet sludge; the water content of the dried sludge is 1-10%; the dried sludge can be replaced by dried sludge with the water content of 1-10% which is prepared in advance.

Before use, the content of copper, zinc, chromium, mercury, lead, cadmium, arsenic, nickel, mineral oil and polycyclic aromatic hydrocarbon in the dried sludge or wet sludge is detected, and the content of the copper, the zinc, the chromium, the mercury, the lead, the cadmium, the arsenic, the nickel, the mineral oil and the polycyclic aromatic hydrocarbon are in accordance with the pollutant limit value standard of the A-grade sludge product in the agricultural sludge pollutant control standard GB 4284-.

Specifically, the dried sludge is ground, sieved by a sieve of 10-30 meshes, and then mixed with the mixture containing 1M of magnesium citrate and 1% of H₂SO₄The reaction medium solution is mixed, wherein the ratio of the dried sludge to the reaction medium solution is 1:10 w/v.

The application method of the modified sludge hydrothermal carbon material comprises the following steps: the modified sludge hydrothermal carbon material is fully mixed with the paddy field soil in an application amount of 0.1-1.5%, and after at least three times of continuous fertilization, the modified sludge hydrothermal carbon material always inhibits the volatilization of soil ammonia, increases the absorption of soil to ammonium nitrogen and increases the utilization rate of rice to nitrogen.

1 materials and methods

1.1 production of three modified sludge hydrothermal carbon materials

Sludge samples were collected from a sewage treatment plant in Nanjing, China. And collecting the activated sludge dry mud cake. The preparation process of the activated sludge dry mud cake comprises the following steps: performing anaerobic secondary digestion treatment on activated sludge generated by a sewage treatment plant to obtain wet sludge with the water content of 70%, and then drying to obtain dried sludge with the water content of 8%. The secondary digestion is to divide the digestion tank into two parts, the activated sludge is firstly digested in the primary digestion tank (provided with a heating and stirring device and a gas collecting hood for collecting the methane), and the discharged sludge is sent to the secondary digestion tank after 7-12 days of vigorous digestion reaction. The second-stage digestion tank is not provided with a heating and stirring device, sludge is continuously digested by the waste heat of the sludge from the first-stage digestion tank, the digestion temperature is 20-26 ℃, the gas production amount accounts for about 20% of the total gas production amount, and the sludge can be collected or not collected.

Grinding the dried sludge, sieving with 30 mesh sieve, mixing with 1M magnesium citrate and 1% H₂SO₄The reaction medium solution is mixed, wherein the ratio of the dried sludge to the reaction medium solution is 1:10 w/v (g/ml).

HTC (hydrothermal carbonization) of sludge is carried out in a hydrothermal reactor under high pressure (8MPa, high pressure is helpful for heavy metals in sludge to be separated and organic matters to be decomposed), and the solid-to-liquid ratio is 1:10(w/v, unit is g/mL). The solid is sludge particles which are ground and then pass through a 30-mesh screen, and the liquid is any one of 1-3: 1, deionized water; 2 reaction medium solution containing 1M magnesium citrate solution; 3 contains 1M magnesium citrate solution, 1% H₂SO₄The reaction medium solution of (1). The reactor was sealed and heated at 260 ℃ for 1h, then allowed to cool to room temperature overnight. The modified sludge hydrothermal carbon material resulting from HTC was collected by centrifugation and dried at 70 ℃ until constant weight. Three modifications were produced using different reaction mediaHydrothermal carbon material: SSHW, using deionized water as reaction medium; SSHM, 1M magnesium citrate is used as a reaction medium; and SSHMS to contain 1M magnesium citrate and 1% H₂SO₄The reaction medium solution of (2) is a reaction medium. 1% of H₂SO₄Finger H₂SO₄The concentration of the entire reaction medium solution (1 g of sulfuric acid molecule in 100mL of the reaction medium solution).

1.2 arrangement of the soil column experiment

A column of PVC soil 30 cm in diameter and 50 cm in height was filled with the paddy field soil. The paddy soil is collected from Jiangsu Yixing in China. The soil column test is carried out at the experimental point of agricultural academy of sciences of Jiangsu province. Each column was filled with 35kg of rice soil. The soil was air-dried and ground, passed through a2 mm sieve and mixed thoroughly with different sludge hydrothermally carbons at an application rate of 1% (w/w, mass%) i.e. the modified sludge hydrothermally carbons were applied at a rate of 1% of the soil. The soil had the following characteristics: pH 6.38 (soil: water 1:2.5, g/mL), organic matter 2.28%, total nitrogen 1.56g kg^-10.96 kg of total phosphorus^-1Total potassium 4.12g kg^-1. Individual rice plants were then sown in each soil column. Controls without nitrogen fertilizer application or modified sludge hydrothermal charcoal were included in the experiment. All treatments were repeated three times.

In the whole rice growth period, 240kg of N ha urea is applied to all three hydrothermal carbon treatments^-1. Nitrogen fertilizer is applied in three times: a Base Fertilizer (BF), a first nitrogen supplementing fertilizer (SF1 after tillering) and a second nitrogen supplementing fertilizer (SF2 after heading), wherein the mass ratio of the nitrogen fertilizers applied for three times is 4: 4: 2. applying base fertilizer in the amount of 96kg of N ha one day before transplanting^-1、90kg P₂O₅ha^-1And 120kgK₂O ha^-1In the form of urea, calcium superphosphate and KCl. Rice plants were transplanted at 29 months 6 and 2018 and harvested at 9 months 11 and 2018. Submerging all potted plants to a water level of 3-5 cm; the season middle drainage is carried out from 8 months and 7 days to 18 days.

1.3 hydrothermal carbon characteristics

The pH value of the hydrothermal carbon is determined by the soil-water ratio of 1:2.5 (w/v) measurement (which means a soil to water ratio of 1:2.5, g/mL, and soil to which modified sludge hydrothermal charcoal has been added). The total C, N, H, O and S content of the hydrothermal charcoal was determined by means of an elemental analyzer (EL III; elemental analysis systems, Ltd., Germany). The surface morphology of the samples was observed by scanning electron microscopy (SEM, Quanta200, FEI, Netherlands) at 2000X in combination with SEM-EDS. Elemental mapping (C, O, P, Ca, Mg and S) was performed by SEM. The surface functional groups were characterized by fourier transform infrared spectroscopy (FTIR) on an Agilent Cary 660FTIR analyzer (ca). The C1s spectra of the surface functional groups were characterized using X-ray photoelectron spectroscopy (XPS) technology using X-ray photoelectron spectroscopy of the AXIS UltraDLD model (Kratos, uk) and AlKa radiation (1468.6eV) excitation. The X-ray source voltage was set to 15KV and the current to 10 mA. The broad spectrum scanning energy is set to be 80eV, and the step length is 0.5 eV; the fine spectral scan energy was set at 20eV and the step size was 0.1 eV. Specific surface area for adsorption and desorption (SSA), Pore Diameter (PD) and Pore Volume (PV) were measured using NOVA 1200 analyzer and parameters were calculated by Brunauer-Emmett-teller (bet) method. In addition, thermogravimetric analysis and differential scanning calorimetry (TG-DSC) analysis were performed simultaneously using SDT Q600 (usa) to evaluate ash content of the hydrothermal char.

1.4 analysis of Water and surface soil of the field

Surface water samples were collected with a syringe, filtered through filter paper, and immediately frozen in a 100mL plastic bottle at-20 ℃ until further analysis. Sampling surface soil for three times: on day 7 after each application (BF, SF1, SF2), the soil pH and soil NH were then analyzed₄ ⁺-N and NO₃ ^-Concentration of-N (soil pH, soil NH on day 7 after fertilization)₄ ⁺-N concentration value, soil NO₃ ^-N concentration values, fig. 3). The pH was measured by the same method as above (by an earth-water ratio of 1:2.5 (w/v)).

NH in field surface runoff was measured using a continuous flow analyzer for the first 7 days after each application (BF, SF1, SF2)₄ ⁺-N and NO₃N concentration and pH value (i.e. determination of NH in surface water 7 days after fertilization)₄ ⁺-N、NO₃N concentration, pH value) as shown in figure 4 (average of measurements on day 7).

1.5 Ammonia (NH)₃) Analysis of volatilization

NH₃Volatilization was performed simultaneously with sampling of the field water (considered field runoff). NH per day₃The volatile flux was estimated using a continuous flow lock method (calculating single-day NH 7 consecutive days after BF, SF1, SF2, respectively₃Amount of volatilization, fig. 9). Briefly, 80mL of a mixture of 2% boric acid, methyl red, bromocresol, and ethanol indicator was used as NH₃Absorbent for trapping volatile NH in a plexiglas chamber with an internal diameter of 15cm and a height of 20cm₃. Each measurement was taken for 4 hours (from 8:00am to 10:00am and from 14:00pm to 16:00pm) until there was no color difference between the control and the treated soil column. With 0.01M H₂SO₄Titration of NH content₃The solution of (1). During the monitoring period after BF, SF1 and SF2 (i.e. the first 7 days after urea administration), respectively, volatile NH will accumulate₃(indicating the accumulation of volatile NH 7 days after BF, SF1 and SF2₃) Calculated as NH₃Sum of volatile amount. By using the above-mentioned accumulated NH₃The volatility loss was divided by the rice yield of the corresponding treatment to calculate the yield per NH₃And (6) volatilizing.

1.6 measurement of Rice growth index, Nitrogen content of grain and yield

Rice plants were harvested manually from each pot at rice maturity to determine growth index, grain nitrogen content and yield. Determining the yield of grains and the agronomic characters related to the yield, including the number of rice ears, the number of grains per ear and the thousand kernel weight. The nitrogen content of the seeds is measured by using a Kjeldahl method.

1.7 statistical analysis

All statistical analyses were performed using SPSS 18.0 (SPSS of chicago, illinois, usa). Results were evaluated using one-way analysis of variance (ANOVA) at a probability level of P < 0.05. The Duncan multi-range test (P <0.05) was used only when the ANOVA F test indicated significant treatment effects at the significance level.

2 results

2.1 hydrothermal carbon basic Properties

Table 1 lists the physicochemical properties of SSHW, SSHM and SSHMS, in which the major elements, including C, H, N and S of hydrocarbons are listed. SSHMS has a significantly lower pH, and higher S and C content than SSHM and SSHW. For example, the C and S content of SSHMS is 15.6% and 6.64 times higher than SSHM, respectively. The contents of N and H are comparable. According to element mapping, the abundance of Mg and P in SSHMS is higher than SSHW and SSHM. The abundances of C, O, P, Ca, Mg and S as determined by SEM are shown in fig. 7 and these findings are confirmed. The residual amounts of ash, SSHW, SSHM and SSHMs at 800 ℃ were measured by TG-DSC analysis to be 59.9%, 65.0% and 66.6% of the crude material, respectively. TG-DSC analysis reflects higher ash content in SSHM and SSHMs.

Table 1 physicochemical properties of hydrothermal charcoal (n ═ 3).

PD: porous diameter (pore diameter); PV: a porous volume; SSA: a surface area; SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC. Data are mean (n-3). Different letters represent a significant difference (P)<0.05)。

SEM images showed differences in surface structure of SSHW, SSHM and SSHMs (fig. 8). SSHM and SSHMs have a more pronounced crystal structure than SSHW, which can be attributed to the accumulation of magnesium oxide and sulfate.

FTIR spectra of the hydrothermal charcoal revealed the presence of functional groups (fig. 1). 2923.5cm^-1And 2852.7cm^-1The peak at wavenumber is the tensile vibration peak of aliphatic C-H in the plane. SSHMS has a richer range of aliphatic C-H groups than SSHW and SSHM. Furthermore, the-COOH peak in SSHMS was at 1700cm^-1Peak intensity sum of (A) and (B) 1600cm^-1The peak intensity of aromatic C ═ C at (a) is significantly higher than SSHM or SSHW. Acidic functional group affecting NH₄+ is adsorbed and related to oxygen content. In addition, 785-855cm of heat energy was observed in all sludge hydrothermal carbon materials^-1The peak at (B) represents an out-of-plane elongation peak of the aromatic C-H group. SSHW at 3420cm compared to SSHM or SSHMS^-1The peak intensity of the phenolic hydroxyl group (phenol, -OH) at (C) and (C) at 1028cm^-1The peak intensity of aliphatic C-O/C-O-C is significantly reduced.

XPS of SSHW, SSHM and SSHMS gave a spectrum of C1s as shown in FIG. 2. The C1s spectra of SSHW and SSHM are very similar, with about equal numbers of different C-containing functional groups. However, in contrast to SSHW or SSHM, H is used₂SO₄The processing results in a significant difference in the spectrum of the SSHMS. For SSHMS, the most important peaks were observed around 288.7eV, representing the C-N and C-O functionalities, representing 72.03% of the surface C-containing functionalities. In contrast, the C-N and C-O functional groups account for only 32.16% and 36.25% of SSHW and SSHM, respectively. In addition, the acidic functional group O ═ C — O accounts for 13.48% of the carbon-containing groups on the surface of SSHMS, 93.4% and 53.9% higher than SSHW and SSHM, respectively. XPS shows that the result of the more abundant carboxyl on the surface of SSHMS is consistent with FTIR, and the result has an inhibiting effect on ammonia volatilization of paddy soil.

High SSA and PV can increase nutrient retention in soil as these features help to increase mass transfer flux and adsorbate loading. With respect to the adsorption capacity of the modified sludge hydrothermal carbon, SSHM and SSHMs both had significantly greater SSA and smaller PD, and furthermore SSHMs had significantly greater PV (table 1). With respect to desorption, no significant difference in PV was found, with SSHM and SSHMs having greater SSA (2.01 and 1.97 times higher than SSHW, respectively); compared to SSHM, SSHW and SSHMs have significantly higher PD, 2.92 and 2.93 times higher, respectively. This finding suggests that magnesium citrate is reacted with H during HTC₂SO₄The use together can prevent the reduction of the surface PD.

2.2 sludge hydrothermal charcoal on soil pH, NH₄ ⁺-N and NO₃ ^-Influence of-N concentration

After BF, SF1 and SF2 treatments, soil pH and NH were adjusted by hydrothermal charcoal₄ ⁺-N and NO₃ ^-The effect of-N concentration is shown in FIG. 3. After BF treatment, soil NH₄ ⁺There was NO significant difference in concentration, but SSHMs significantly increased soil NO compared to SSHM, control and SSHW₃ ^-(iii) concentration (fig. 3A and 3B). After SF1 treatment, application of all three types of hydrothermal charcoal significantly increased soil NH compared to controls₄ ⁺The concentration of (c); whereas in comparison to the control and SSHW,addition of SSHM and SSHMS significantly reduces soil NO₃ ^-The concentration of (c); further, SSHMS is more effective than SSHM and NO is more potent than SSHM₃ ^-The concentration was reduced by a factor of 2.45. Addition of SSHMS after SF2 treatment significantly increased soil NH compared to controls, SSHW and SSHM, respectively₄ ⁺Concentrations 1.94 times, 1.68 times and 1.48 times; application of SSHW and SSHMS significantly reduced soil NO compared to control and SSHM₃ ^-The concentration of (c). After BF and SF1 treatment, addition of SSHMs significantly reduced soil pH compared to controls, SSHW and SSHM (fig. 3C).

2.3 pH and NH of sludge hydrothermal charcoal to surface Water₄ ⁺-N and NO₃ ^-Influence of-N concentration

Applying hydrothermal charcoal to NH at different fertilization stages₄ ⁺-N and NO₃ ^-The effect of-N concentration and field water pH is shown in FIG. 4. In the base fertilizer period, the application of SSHMS can obviously reduce NH₄ ⁺Concentration; the three types of hydrothermal carbon all have obvious reduction of NO₃ ^-SSHMS significantly reduces NO compared to SSHW and SSHM₃ ^-Is 1.32 and 1.36 times, respectively. NH detected after hydrothermal charcoal application during SF1 phase₄ ⁺No significant difference in N concentration; addition of SSHW significantly increased NO compared to control₃ ^-1.64 times, addition of SSHMS significantly reduced NO₃ ^-1.38 times. In SF2 phase, application of SSHMS resulted in NH in the field water compared to the control, SSHW and SSHM₄The concentration is greatly reduced; the addition of SSHM and SSHMS does not affect the NO of the field water₃ ^-Concentration, but the use of SSHW significantly increased NO in the field water compared to the control group₃ ^-The concentration was 1.58 times. In the whole experiment process, the pH value of the field water is not influenced by the application of the hydrothermal carbon.

2.4 application of sludge hydrothermal charcoal to NH₃Influence of volatilization

Accumulating NH in the paddy soil by applying hydrothermal charcoal within 7 days after BF, SF1 and SF2 are applied₃Volatilization and yield scale NH₃Influence of volatilization lossAs shown in fig. 5 and 9. FIG. 9 shows three modified sludge biochar material treatments, NH after fertilization₃The daily change of volatilization. As shown in FIG. 5A, administration of SSHW significantly increased NH after BF₃1.56 times, and no significant difference was detected between the control, SSHM and SSHMs. After SF1, SSHMS alone significantly reduced accumulated NH compared to control group₃And (6) volatilizing. After SF2 treatment, SSHM and SSHMS, respectively, will accumulate NH compared to the control group₃The volatilization was reduced by a factor of 1.66 and 2.05. SSHW did not cause NH compared to control₃Any significant change in the amount of accumulated volatiles. Yield scale NH₃The volatile matter is the accumulated NH of the rice in the whole growing period based on the yield₃And (4) quantifying the volatilization amount. yield-Scale NH of control and SSHW treatment as shown in FIG. 5B₃The volatilization loss is equivalent. In contrast, both SSHM and SSHMS significantly reduced the production-scale NH by a factor of 1.25 and 1.70, respectively₃And (6) volatilizing. SSHMS treatment with minimum production scale NH₃And (4) volatilization loss.

2.5 Effect of sludge hydrothermal charcoal on Rice growth, Nitrogen absorption and seed yield

Table 2 lists the effect of hydrothermal charcoal application on grain yield (dry weight, DW) and yield-related agronomic traits; the effect on nitrogen uptake by plants is shown in figure 6. Administration of SSHW, SSHM and SSHMs significantly increased kernel DW by 1.24-fold, 1.20-fold and 1.31-fold, respectively, compared to controls (table 2). Thus, the application of the three types of modified sludge hydrothermal carbons significantly improved the ear number and harvest index compared to the control. In contrast, the plant height was significantly reduced. The thousand kernel weight of SSHM and SSHMs treatments was significantly increased compared to the control group. The setting rate of all treatments was comparable. The N content in rice kernels can be significantly increased by applying modified sludge hydrothermal charcoal, which is higher than SSHW and SSHM, respectively, in SSHMS treatment (fig. 6). In addition, no significant difference was found in the N content of the straw after addition of SSHW, SSHM or SSHMS.

Discussion of 3

3.1 sludge hydrothermal charcoal can inhibit NH in paddy soil₃Loss of volatilization

Application of SSHM and SSHMS to paddy field can suppress NH₃Volatilization, the greatest and most sustained effect was observed for SSHMS (fig. 5, fig. 9). This difference is attributed to the properties of the hydrothermal char and its effect on soil properties.

SSHMS processing is due to the use of H₂SO₄The solution acts as a reaction medium for HTC, resulting in short-term acidic perturbation of the paddy soil during fertilization (fig. 3C). Adjusting the pH of the modified sludge hydrothermal charcoal to near neutral 7 (Table 1) may result in precipitation of aluminum and iron as oxides in the soil, and the negatively charged organic functional groups may become the primary adsorption sites for soil ammonium, thereby preventing NH₃And (6) volatilizing.

Controlling NH₃The most important physical properties of the hydrothermal carbon for retention are its SSA, PV and PD values (table 1). Use of H in contrast to use of deionized water (SSHW)₂SO₄Larger adsorbed PV and SSA can be produced as reaction media for HTC (SSHM and SSHMs) (table 1). These results can be attributed to the acceptor H₂SO₄The rupture of macropores in the hydrothermal carbon is affected. Meanwhile, the rupture of macropores is accompanied by the formation of micropores, which is more advantageous for SSA and adsorption. Larger SSA results are consistent with increased pore size. SSHM and SSHMs can provide more adsorption sites for soil ions. Furthermore, SSHMs increases PV compared to SSHM or SSHW. Thus, SSHMS results in lower moisture retention compared to SSHM/SSHW, thereby increasing NH₄ ⁺-N. In addition, due to NH₃Is a basic gas, so that acidic surface groups (e.g. carboxyl groups) on the water carbon can react NH₃Protonation of gas to NH₄ ⁺Ions, and promotes adsorption to water carbon (e.g., SSHMS). Both FTIR and XPS spectral analysis showed more carboxyl groups in SSHMS than SSHW/SSHM. In contrast, this FTIR analysis showed more-OH groups on the surface of SSHMS and SSHM compared to SSHW. In addition, the lower the surface pH of SSHMs relative to other modified sludge hydrothermal carbon materials (SSHM and SSHW), the lower its surface pH is for NH₄ ⁺The stronger the adsorption capacity of (a). Furthermore, SSHMS has higher SSA and PV and lower PD than the respective attributes of SSHW and SSHM. The abundance of surface carboxyl groups gives SSHMS greater protection from NH₃Ability to volatilize.

3.2 improvement of sludge hydrothermal charcoal promotes the absorption of nitrogen in the paddy soil and improves the absorption of nitrogen by plants and the yield increase

The use of all sludge hydrothermal charcoal in this study increased soil NH after SF1 treatment₄ ⁺-the concentration of N. However, after SF2, addition of SSHMS alone significantly increases soil NH₄ ⁺-reservation of N (fig. 3). These results indicate that SSHMS is on NH₄ ⁺The effect of-N retention is longer. Obviously, the macroporosity and SSA of certain modified sludge hydrothermal carbon materials can promote NH in paddy soil₄ ⁺-retention of N. For example, specific PV, PD and surface functional groups in SSHMS may promote NH₄ ⁺Thereby increasing its retention in the soil. Consistent with these results, SSHMS treatment significantly inhibited NH in field surface runoff after BF and SF1 treatment₄ ⁺Loss of-N (FIG. 4).

First converting urea applied to the soil into NH₄ ⁺-N. Thereafter, the NH is generally separated by nitration₄ ⁺Conversion of-N to NO₃ ^--N. After the base fertilization period, all three types of hydrothermal charcoal increased soil NO₃ ^-Concentration of-N and prevention of NO₃ ^--runoff of N. This is due to the surface functional groups of the hydrothermal carbon increasing the adsorption capacity and increasing the anion exchange sites, which contribute to the adsorption of NO₃ ^-N and reduced nitrification rate. The increase in soil aggregation by application of hydrothermal charcoal is the immobilization of NO₃ ^--N. Lower soil NO in SSHMS treatment after SF1 and SF2₃ ^-N is associated with inhibition of nitrification, e.g. NH₄ ⁺Increase in-N retention (FIG. 3), which further reduces NO production by surface water₃ ^--loss of N. Mg on the surface of the hydrothermal carbon can be increased by using magnesium citrate as a reaction medium²⁺While promoting the formation of a crystalline structure (fig. 7-8), thereby providing more anion adsorption sites. These results improve NO in soil₃ ^-Adsorption of N and prevention of loss of surface nitrogen due to Mg loading in the water coal²⁺Providing more adsorption sites for anions. In the present invention, in SSHMSSurface neutral pH and rich carboxyl groups of (A) increase NH₄ ⁺Further inhibiting nitrogen runoff.

In addition, NH is significantly increased due to SSHMS₄ ⁺-retention of N, and NH₄ ⁺N is the predominant form of inorganic nitrogen available for rice growth, so the addition of SSHMS promotes grain N content in rice (fig. 6). SSHMS treatment increased grain yield and related growth traits in rice (table 2). The nitrogen absorption effect of rice plants is not improved before. Porosity and desorption capacity are increased by thermally converting biomass to hydrothermal char as compared to pyrolytic char. The high SSA and PD resulting from the high desorption capacity of the hydrothermal char is more likely to release nutrients, which can then be absorbed by the plant. In the present invention, both SSHM and SSHMs significantly increase sorptive SSA. However, desorption PD of SSHM is significantly lower compared to SSHMS, resulting in NH₄ ⁺The release efficiency of (2) is low. Thus, the highest kernel N content and yield were detected in SSHMS treatment.

Table 2. rice grain yield and agronomic traits related to yield. Data are mean and standard deviation (n-3). The different letters in each column in the same column represent significant differences between treatments (P < 0.05).

DW: dry weight; SSHW: sludge hydrothermal carbon using water as a reaction medium in hydrothermal carbonization (HTC); SSHM: in the HTC, 1M magnesium citrate is used as sludge hydrothermal carbon of a reaction medium; SSHMS: 1M magnesium citrate and 1% H were used₂SO₄Sludge hydrothermal char as a reaction medium in HTC.

4 conclusion

The results show that magnesium citrate and H are used₂SO₄NH reduction by modified sludge hydrothermal charcoal with solution as HTC reaction medium₃Volatilize the N runoff in the field surface runoff and simultaneously increase the N retention in the rice field soil and the utilization of the rice to the N. Observed suppressed NH₃Volatilization can be attributed to lower soil pH. ByNH increase at larger SSA, PV and lower PD₄ ⁺The adsorption capacity of (c); SSHMs is more carboxyl-rich on the surface than other hydrothermal carbons (SSHM or SSHW). The increased retention of nitrogen in soil may be attributed to NH₄ ⁺Increased adsorption and inhibition of nitrification. Rice nitrogen uptake and yield production is increased because of the strong desorption capacity and nitrogen loss is reduced by the application of novel modified hydrochar (SSHMS) through gasification and runoff. In the future, soil microbial studies will need to be performed to better understand NH absorbed by sugars in crops₃And NH₄ ⁺-availability of N. The invention provides a new field for converting sludge into efficient rice field improvement with environmental benefits. The foregoing illustrates and describes the principles, general features, and advantages of the present invention. It should be understood by those skilled in the art that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by using equivalent alternatives or equivalent variations fall within the scope of the present invention.

Claims

1. A preparation method of a modified sludge hydrothermal carbon material is characterized by comprising the following steps:

wet sludge treated by anaerobic digestion is mixed with water containing 0.8-1.2M magnesium citrate and 0.8-1.2% H₂SO₄Wherein the ratio of wet sludge to reaction medium solution is 1: 1-4 w/v; placing the mixture of the wet sludge and the reaction medium solution in a high-pressure reaction kettle, and carrying out hydrothermal carbonization for 1-2h under the conditions of 300 ℃ at 250 ℃ and 4-10MPa to prepare a modified sludge hydrothermal carbon material; collecting the modified sludge hydrothermal carbon material, and then drying the modified sludge hydrothermal carbon material for later use;

or drying wet sludge treated by anaerobic digestion to obtain dried sludge, wherein the dried sludge contains 0.8-1.2M magnesium citrate and 0.8-1.2% H₂SO₄The reaction medium solution is mixed, wherein the ratio of the dried sludge to the reaction medium solution is 1: 6-12 w/v; placing the mixture of the dried sludge and the reaction medium solution in a high-pressure reaction kettle, and carrying out hydrothermal carbonization for 1-2h under the conditions of 250-300 ℃ and 4-10MPa to prepare a modified sludge hydrothermal carbon material; harvesting machineThe modified sludge hydrothermal carbon material is collected and then dried for later use.

2. The method for preparing the modified sludge hydrothermal carbon material according to claim 1, wherein the moisture content of the wet sludge is 50-90%; the preparation process of the wet sludge comprises the following steps: performing anaerobic digestion treatment on activated sludge generated by a municipal sewage plant to obtain wet sludge; the water content of the dried sludge is 1-10%; the dried sludge can be replaced by dried sludge with the water content of 1-10% which is prepared in advance.

3. The method for preparing the modified sludge hydrothermal carbon material as claimed in claim 2, wherein the content of copper, zinc, chromium, mercury, lead, cadmium, arsenic, nickel, mineral oil and polycyclic aromatic hydrocarbon in the dried sludge or the wet sludge is detected to meet the pollutant limit value standard of the class A sludge product in the agricultural sludge pollutant control standard GB 4284-.

4. The method for preparing the modified sludge hydrothermal carbon material according to claim 2, wherein the dried sludge is ground, sieved by a sieve of 10-30 meshes, and then mixed with the modified sludge hydrothermal carbon material containing 1M magnesium citrate and 1% H₂SO₄The reaction medium solution is mixed, wherein the ratio of the dried sludge to the reaction medium solution is 1:10 w/v.

5. The preparation method of the modified sludge hydrothermal carbon material as claimed in claim 2, wherein the modified sludge hydrothermal carbon material is collected by centrifugation; the modified sludge hydrothermal carbon material is dried for later use, which means that: drying the centrifugally collected modified sludge hydrothermal carbon material at the temperature of 60-70 ℃ until the weight is constant.

6. A modified sludge hydrothermal carbon material is characterized by comprising sludge subjected to anaerobic digestion treatment, magnesium citrate and H₂SO₄The mixture of (a) is prepared by hydrothermal carbonization reaction.

7. The modified sludge hydrothermal carbon material as claimed in claim 6, wherein the modified sludge hydrothermal carbon material is prepared by the preparation method as claimed in any one of claims 1 to 5.

8. The method for applying the modified sludge hydrothermal carbon material as claimed in claim 6 or 7, characterized by comprising the steps of:

the modified sludge hydrothermal carbon material is fully mixed with the paddy field soil in an application amount of 0.1-1.5%, and after at least three times of continuous fertilization, the modified sludge hydrothermal carbon material always inhibits the volatilization of soil ammonia, increases the absorption of soil to ammonium nitrogen and increases the utilization rate of rice to nitrogen.

9. The application of the modified sludge hydrothermal carbon material in inhibiting ammonia volatilization in fields, increasing nitrogen fixation capacity of soil, improving nitrogen content and yield of rice grains and prolonging the continuous planting time of rice as claimed in claim 6 or 7, wherein the modified sludge hydrothermal carbon material is fully mixed with the soil of the rice field by 0.1-1.5% of application amount.

10. The use of the modified sludge hydrothermal carbon material of claim 6 or 7 as a fertilizer for inhibiting ammonia volatilization in the field, increasing nitrogen fixation capacity of soil, increasing nitrogen content and yield of rice grains, and increasing the continuous planting time of rice.