Effects of Method for Returning Straw to Field on Soil Properties, Straw Decomposition and Nutrient Release

Xiang LI, Qiao ZENG, Shanchao CAO, Yueli MA, Yangli ZHANG

1. College of Chemical Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China; 2. Shaanxi Agricultural Product Processing Technology Research Institute, Xi’an 710021, China; 3. Pucheng Experimental Station of Weinan Agricultural Science Research Institute, Pucheng 715204, China; 4. Fufeng County Agricultural Product Quality and Safety Center, Baoji 722200, China

Abstract [Objectives] To alleviate the influence of meteorological conditions on soil environment (temperature and water content) and maintain high and stable grain yield. [Methods] Taking Sunzhen Experimental Station of Weinan Academy of Agricultural Sciences as the experimental base, the effects of returning double-crop wheat and corn straw to field (Twm), returning single-crop corn straw to field (Tm), returning single-crop wheat straw to field (Tw) on soil temperature, water content, straw decomposition rate and nutrient release, soil organic matter and bulk density were studied systematically. [Results] Twm treatment could effectively alleviate the effects of meteorological conditions on soil temperature and water content. The decomposition rate of straw treated with Twm was 4.7% higher than that of Tm treatment, 3.8% higher than that of Tw treatment, 10.5% higher than that of Tm treatment, and the decomposition rate of straw showed a trend of "first fast, then slow and then fast". The release of nitrogen from straw was basically similar to that of straw decay, and the release of potassium and phosphorus increased at first and then remained basically unchanged. The release rate of potassium was the highest, followed by phosphorus and nitrogen. The content of soil organic matter in Twm treatment increased by 11.67% annually, an annual average of 0.998 g/kg. The soil bulk density of Twm treatment decreased by 0.058 g/cm3 annually, an annual average of 4.29%. The fundamental reason is that Twm treatment provides conditions (temperature, water content, nutrition) for microbial growth, reproduction, enzyme production and biochemical reaction, and increases the exchange capacity of soil and external water, heat, gas and fertilizer. [Conclusions] It is expected is to help people change their understanding of returning straw to field from "quick harvest" to "fertilizer transformation".

Key words Method for returning straw to field, Straw decomposition, Soil organic matter content, Soil bulk density

1 Introduction

In 2020, Shaanxi has a wheat planting area of 964 200 ha, with a yield of 4.132 5 million t, and a corn planting area of 1.179 4 million ha, with a yield of 6.201 6 million t[1]. According to the ratio of wheat grass to grain (1.23) and the ratio of corn straw to grain (1.52)[2], the amount of straw in Shaanxi was 14.509 4 million t in autumn and summer in 2020. If it is fully returned to the field, the amount of straw returned is 6 768.7 kg/ha, slightly larger than the national recommended amount of 4 500 kg/ha[3]. Straw contains 47% carbon, 0.72% nitrogen, 2.8% phosphorus, 1.34% potassium, medium and trace elements on average. After returning straw to field, the carbon in straw effectively replenishes the consumed organic matter in the soil[4]. The soil water storage capacity was improved[5], and the nitrogen, phosphorus and potassium in the straw were gradually released with the decomposition of the straw[6], reducing the application amount of chemical fertilizer and increasing the production efficiency of agriculture. The amount of medium and trace elements in straw are coenzymes of plant enzymes, which can be supplemented to the soil in time after returning to the field, which can improve the stress resistance of plants[7]. This paper is supported by the Key Industrial Chain Project of Shaanxi Province and the Key Scientific and Technological Research and Development Project of Xi’an City—"Technical Development and Demonstration of Comprehensive Utilization of Straw Resources in Farmland Soil". The effects of meteorological conditions on soil temperature and water content under different methods were systematically studied in Sunzhen Experimental Station of Weinan Academy of Agricultural Sciences since 2016. This paper reveals the internal relationship between annual decomposition rate of straw, release of nutrients (carbon, nitrogen, phosphorus, potassium), soil organic matter content, soil bulk density and the number of microorganisms and enzyme activity. The buffering effect of soil temperature and water content on climatic conditions in Twm treatment was greater than that in Tm and Tw treatments. Straw decomposition rate, nitrogen, phosphorus and potassium release, organic matter content increased and soil bulk density decreased. Under the wheat-corn rotation system, the treatment mode of double-crop returning straw to field is worth popularizing. The purpose of this study is to improve people’s re-understanding of returning straw to field, realize the transformation from "quick harvest" to "fertilizer transformation", and promote the scientific and refined development of returning straw to field. The study is rarely reported.

2 Materials and methods

2.1 MaterialsStraw decomposition agent, self-made by Agriculture and Forestry Technology Extension Engineering Center of Shaanxi University of Science and Technology; soil conditioning agent, made by Agriculture and Forestry Technology Extension Center of Shaanxi University of Science and Technology; urea, ammonium dihydrogen phosphate, potassium sulfate, purchased from agricultural material market of Pucheng County.

2.2 Methods

2.2.1Base selection. The Pucheng Sunzhen Experimental Station of Weinan Academy of Agricultural Sciences is located in the east of Pucheng County (109°20′17″-109°54′48″ E, 34°44′50″-35°10′30″ N). It has a warm continental climate, which is characterized by four distinct seasons (warm spring, hot summer, cool autumn, cold winter), sufficient sunshine and less rainfall. The annual average temperature is 13.7 ℃. The temperature in the coldest month (January) is -1.4 ℃, and the extreme minimum temperature is -16.7 ℃; the highest temperature is 26.7 ℃ in July, and the extreme maximum temperature is 41.8 ℃. The average number of annual sunshine hours is 2 228.9, the average annual rainfall is 519.9 mm, the extreme maximum rainfall is 876.1 mm, and the minimum rainfall is 271.8 mm.

2.2.2Experimental design. returning straw to field experiment was conducted in Sunzhen Experimental Station of Weinan Academy of Agricultural Sciences for 4 consecutive years from 2016 to 2019. Four fields (each 21 m2) with basically the same fertility conditions were selected as experimental fields. An isolation zone of 0.5 m was set between the fields, and three groups of replicates were set for each experiment. With non-returning straw to field as control (CK), returning double-crop wheat and corn straw to field (Twm), returning single-crop corn straw to field (Tm), returning single-crop wheat straw to field (Tw) were used as treatments. From the end of May to the beginning of June every year, the "YANMAR" harvester was used to harvest wheat, which required that the stubble height should not exceed 10 cm, the straw length should be 3-5 cm, and it should be spread evenly. Before ploughing deeply, 150 kg/ha urea, 250 kg/ha superphosphate, 200 kg/ha potassium sulfate and 30 kg/ha self-made straw ripening agent were scattered evenly on the straw surface. Then, the machine was used for deep ploughing, and after leveling the ground, "Shandan 650" was sown as required. After the corn matured in early October, the corn grain harvester was used to harvest. It was required that the stubble should not exceed 10 cm and the straw crushing length should not exceed 10 cm. Before rotary tillage, urea, calcium superphosphate, potassium sulfate and self-made straw ripening agent were mixed according to the proportion of 225 kg/ha, 250 kg/ha, 230 kg/ha and 30 kg/ha, respectively, and spread evenly on the straw surface. The rotary tiller was used to rotate the root for 1 or 2 times, and the wheat "Weimai 8" was sown after soil preparation. Other tasks (fertilization, irrigation, management) were the same.

2.2.3Determination items and methods. (i) Determination of soil temperature and water content. At different growth stages of wheat and corn after sowing in each experimental area, according to the "S" sampling method, the temperature and water content at underground 5, 10, 15 and 20 cm were measured by TZX-5X-G multi-parameter soil moisture meter at 8:00, 12:00 and 20:00 every day, and the average temperature and water content of soil at different stages were calculated.

(ii) Decomposition rate of straw and release of nutrients (nitrogen, phosphorus, potassium). A 30 cm×50 cm nylon mesh bag was sewn, and 15 g of wheat and corn stalks with length of 3-5 cm were mixed with soil and put into the bag. Before the experiment, the bag was immersed in the suspension of soil and water (1∶1.5) for 10 min. Before sowing corn and wheat, the bag was buried in the soil layer according to the shape of "S". In different growth stages of wheat and corn, the "S" sampling method was used to sample in each experimental plot, and the content of dry weight, nitrogen, phosphorus and potassium of straw was determined. The decomposition rate of straw and the release rate of nitrogen, phosphorus and potassium were calculated according to the formula:

Decomposition rate of straw=(Straw dry weight before experiment-Straw dry weight at a certain stage)/straw dry weight before experiment×100%

(1)

Release rate of nitrogen, phosphorus and potassium=(Straw dry weight before experiment×Content of nitrogen, phosphorus and potassium in straw)-(Straw dry weight at a certain stage×Content of nitrogen, phosphorus and potassium in straw)/Straw dry weight before experiment×content of nitrogen, phosphorus and potassium in straw×100%

(2)

(iii) Determination of soil organic matter content and bulk density. The content of soil organic matter was determined according to GB9834-88 and the bulk density was determined according to NY/T1121.4-2006.

(iv) Determination of soil microorganism and enzyme activity. According to the "S" sampling method, the sampling point was determined in each experimental area, and 500 g of 0-10 cm topsoil was taken at each sampling point. The soil was fully mixed by dichotomy, packed in a self-sealing fresh-keeping bag and stored in a refrigerator at 4 ℃. Plant, root and animal residues were removed from the soil before determination, and then sifted through a 20-mesh sieve and sealed in a self-sealing bag. Potato-sucrose agar medium, beef extract peptone and Gaoshi No.1 medium were used to culture and investigate the colony number of bacteria, fungi and actinomycetes under different treatments. The soil enzyme activity was determined by the method of Guan Songyin[8]. The catalase activity was expressed by the volume of 0.02 mol/L potassium permanganate consumed per unit of soil weight, and the cellulase activity was determined by 3, 5-dinitrosalicylic acid colorimetric method.

3 Results and analysis

3.1 Effects of method for returning straw to field on soil temperature and water contentTaking 2018 as an example, the soil temperature and water content of wheat and corn were measured at different growth stages. The experimental results are shown in Table 1.

It can be seen from Table 1 that no matter what kind of treatment, the soil temperature and water content changed with the change of atmospheric temperature and rainfall. The reason is that there are three main sources of soil energy, namely, solar radiation, biological heat and subterranean heat. In general, only 5% to 15% of the radiant energy can be stored in the form of thermal energy in soil and vegetation, which has a great impact on soil temperature.

Under the same other conditions, the soil temperature of wheat during overwintering, jointing and flowering stage showed the trend of "Twm>Tm>Tw>CK", indicating that Twm, Tm and Tw treatments were beneficial to the safe overwintering of wheat and provided conditions for wheat jointing and flowering. However, during wheat harvest, corn jointing, flowering and harvest, the soil temperature showed a trend of "Twm

It can also be seen from Table 1 that under the same other conditions, the soil water content was in the order of Twm>Tm>Tw>CK. The main results are as follows: (i) The operation of deep ploughing and rotary ploughing when the straw is returned to the field breaks the ploughing layer of the soil, increases the water permeability and air permeability of the soil, which is beneficial to the storage of water. (ii) With the decay of straw, the "-OH" in cellulose and hemicellulose molecules is exposed, which is easy for water molecules to form hydrogen bonds and lock the water in rainfall and irrigation in the soil. Therefore, returning straw to field is the main factor to alleviate the influence of meteorological conditions on soil temperature and water content.

Table 1 Effects of different methods for returning straw to field on soil temperature and water content

3.2 Effects of method for returning straw to field on decomposition rate of straw and the release of nutrients (N, P, K)

Taking 2018 as an example, the decomposition rate of straw and the changes of nutrients (N, P, K) were measured in different growth stages of wheat and corn. The experimental results are shown in Table 2.

As can be seen from Table 2, the decay rate of straw treated with Twm was 4.7% higher than that of Tm and 3.8% higher than that of Tw, and the decay rate of wheat straw was 10.5% higher than that of corn straw. Straw decomposition showed a trend of "fast first, then slow and then fast", which was the same as the view of many scholars[9-13]. The reason is that when the straw is returned to the field, the straw and the microbial agent carrying nutrients are buried in the ground at the same time. The microorganisms germinate under suitable water and temperature conditions, and use their own nutrients to grow and reproduce. The produced enzymes decompose the starch, cellulose and hemicellulose in the straw into small molecules of sugars, amino acids and other nutrients, which improve the nutrition for the growth of microorganisms and accelerate the decomposition of straw. With the decomposition of straw and the accumulation of decomposition products, the products have an inhibitory effect on enzymes, so the decomposition rate of straw is low in the middle stage of returning straw to field. When wheat and corn bloom, the plant height begins to set, the root system basically grows into contact with the underground straw, and the root tip secretes some substances to accelerate the decomposition of the straw.

Table 2 also showed that the release law of nitrogen in straw was basically the same as that of straw decomposition, and it also showed the characteristics of "fast in the early stage, slow in the middle stage and fast in the later stage", while the phosphorus and potassium in the straw showed the trend of "sudden release in the early stage and basically stable in the later stage". The release of potassium in straw was faster than that of phosphorus and nitrogen. The reason may be that potassium exists in the vacuoles of plant cells, which is easy to break, and phosphorus exists in the cell wall of plants. If the straw is to be degraded, it must break the cell wall. Nitrogen mainly exists in protein and is difficult to degrade. The release rate of nitrogen, phosphorus and potassium in wheat straw is higher than that in corn straw, which may be related to temperature and rainfall. According to Table 1, the environmental temperature and rainfall during the growth of corn were higher than those of wheat, and the soil temperature and water content were high, so the decomposition rate of wheat straw was higher than that of corn straw.

3.3 Effects of method for returning straw to field on soil organic matter content and bulk densitySoil samples were taken 1-3 days before wheat harvest and corn harvest for 4 consecutive years in each experimental plot from 2016 to determine the soil organic matter and bulk density. The results are shown in Fig.1-2.

It can be seen from Fig.1 that the content of soil organic matter treated by CK was basically unchanged, with an average of 8.55 g/kg. Soil organic matter is actually a form of soil carbon. Although small molecular organic matter such as urea and some organic matter such as plant residues were added to the soil every year, most of these substances were degraded by microorganisms and released into the atmosphere in the form of CO2during plant growth, and rarely fixed in the soil, so the content of soil organic matter was basically unchanged when straw was not returned to the field. From 2016 to 2019, the content of soil organic matter treated by Twm increased by 3.99 g/kg, with an increase rate of 46.67% (an average annual increase of 0.998 g/kg, 11.67%); in the past 4 years, the content of soil organic matter treated by Tm increased by 2.57 g/kg, with an increase rate of 30.06% (an

average annual increase of 0.64 g/kg, 7.52%); in the past 4 years, the content of soil organic matter treated by Tw increased by 1.84 g/kg, with an increase rate of 21.25% (an average annual increase of 0.46 g/kg, 5.31%). Returning straw to field can increase the content of soil organic matter, which is the manifestation of soil fertility, so returning straw to field can improve soil fertility. The content of soil organic matter (average increase of 0.998 g/kg) treated by Twm was 0.358 g/kg higher than that treated by Tm (0.64 g/kg), and the content of soil organic matter (0.46 g/kg) treated by Twm was 0.538 g/kg higher than that treated by Tw (0.46 g/kg). Twm treatment could significantly increase the content of soil organic matter and soil fertility. The reason may be that corn yield, ratio of straw to grain were higher than wheat yield and ratio of straw to grain.

It can be seen from Fig.2 that the average soil bulk density of CK treatment is 1.34 g/cm3. From 2016 to 2019, the soil bulk density in Twm treatment decreased by 0.23 g/cm3, with a decrease rate of 17.16% (an average annual decrease of 0.058 g/cm3, 4.29%). In the past 4 years, the soil bulk density treated by Tw decreased by 0.15 g/cm3, with a decline rate of 11.19% (an average annual decrease of 0.038 g/cm3, 2.8%). In the past 4 years, the soil bulk density treated by Tw decreased by 0.17 g/cm3, with a decline rate of 12.69% (an average annual decrease of 0.043 g/cm3, 3.17%). Soil bulk density is a measure of soil looseness and the exchange capacity of air, heat, water and fertilizer between soil and environment. The higher the content of organic matter in soil is, the lower the soil bulk density is. returning straw to field can improve the exchange capacity between soil and the outside world.

Fig.2 Effects of method for returning straw to field on soil bulk density

4 Discussion

Soil is the stronghold of microorganisms and soil microorganisms are responsible for the transformation of matter and energy in soil, and participate in biochemical reactions such as soil oxidation, nitrification, ammoniation, nitrogen fixation and vulcanization, to promote the decomposition and transformation of organic matter. In different treatments of returning straw to field, there will be different straw decomposition rate, nutrient (N, P, K) release, soil organic matter content and bulk density, which may be related to the number of microorganisms and enzyme activity in soil. Soil samples were taken 1-3 d before summer harvest from 2016 to 2018 to determine the number of microorganisms and the activity of sucrase and catalase. The experimental results are shown in Fig.3-4.

Fig.3 Effects of method for returning straw to field on the number of soil microorganisms

As can be seen from Fig.3, the quantity of bacteria and actinomycetes in soil treated by Twm, Tm and Tw increased year by year with the passage of time, and the quantity of fungi in Twm treatment was more prominent, while the quantity of bacteria, actinomycetes and fungi in CK treatment was basically unchanged. As can be seen from Fig.4, the activity of sucrase, urease and catalase increased year by year with the passage of time, while it remained basically unchanged in CK. The reason is that with the passage of time, part of the organic carbon in the straw was fixed in the soil, which increased the content of organic matter in the soil and provided conditions for the growth of microorganisms. On the other hand, with the time of returning straw to field, the soil bulk density decreased and the soil porosity increased, which was more conducive to the growth, reproduction and development of microbial bacteria and actinomycetes, and the quantity of fungi was basically unchanged due to the inhibition among fungi.

Fig.4 Effects of method for returning straw to field on soil enzyme activity

The reason why returning straw to field can increase the number of microorganisms in soil and improve enzyme activity is related to the fact that returning straw to field can buffer the effects of climate change on soil temperature and water content. Generally, the more suitable soil conditions (temperature, moisture, air permeability, nutrients,etc.) are, the more favorable it is for the growth and reproduction of microorganisms, and the more favorable it is for the enzymatic hydrolysis reaction. Under the action of enzymes produced by microorganisms, straw in soil degrades proteins, cellulose, semi-fiber and lignin into small molecular amino acids, sugars, phenolic acids and other substances, which stimulate crop growth on the one hand, and accelerate microbial reproduction on the other hand. Bacteria are the largest class of soil microorganisms with the largest number of species. Each gram of soil can contain millions to hundreds of millions of microorganisms, and the content is even higher in fertile soil. Soil organic carbon, total nitrogen and water content have great influence on the total number of bacteria in soil. Bacteria and other soil microorganisms can participate in the formation of humus and the complete mineralization of organic matter. Actinomycetes in soil mostly live in aerobic heterotrophic state, and their main activities are to decompose cellulose, lignin and pectin in soil, which can improve soil nutrient status and facilitate crops to absorb and utilize soil nutrients directly. Fungi in soil mostly live in aerobic heterotrophic state, participate in the decomposition of animal and plant residues, and become an indispensable driving force of nitrogen and carbon cycle in soil, especially in the early stage of plant organism decomposition. Fungi are more active than bacteria and actinomycetes.

The soil treated by returning straw to field for many years has higher content of organic matter, lower soil bulk density and strong capacity to exchange water, air, heat and fertilizer with the external world, which improves the distribution of microflora in soil. The number of bacteria and actinomycetes in the soil treated by returning straw to field for many years increased year by year, but the number of fungi remained basically unchanged. The activity of soil sucrase, urease and catalase increased with the increase of the total number of microorganisms, which affected the decomposition of straw and the release of nutrients. These changes are caused by climate change in the final analysis.

5 Conclusion

Soil temperature and water content were mainly affected by climatic conditions, but returning straw to field, especially returning double-crop wheat and corn straw to field (Twm), alleviated the impact of climate change on soil temperature and water content to a great extent, accelerated the transformation rate of straw carbon to soil carbon, provided nutrients, energy and environmental conditions for microbial growth, reproduction, enzyme production and enzymatic reaction, and accelerated the decomposition and transformation of straw organic carbon.

By returning straw to field, especially returning double-crop wheat and corn straw to field (Twm) for many years, the content of soil organic matter increased at a rate of 0.998 g/kg annually, with an average increase rate of 11.67%. Soil bulk density decreased by 0.058 g/cm3annually, with an average decline rate of 4.29%. Returning straw to field can significantly improve soil fertility and exchange capacity of matter and energy between soil and the outside world, and lay a solid foundation for high and stable grain yield.