Keywords

1 Introduction

As a powerful measure to regulate soil nutrient cycle, reduce nitrogen fertilizer loss, and maintain and improve soil organic matter level, crop straw returning has been paid more and more attention by people. A large amount of straw is produced under the traditional agricultural planting system. Straw is a high-quality renewable organic resource [1], but the phenomenon of straw burning in production is still common [2], which not only wastes resources, but also harms the environment [3, 4]. Straw returning has many advantages such as increasing soil organic matter content, improving soil physical and chemical properties, increasing soil aggregates, promoting soil moisture and fertilizer conservation, and ensuring high corn yield [5,6,7]. Farmland management measures.

Predecessors have done a lot of work on the impact of straw returning to soil on carbon and nitrogen. These works mainly focused on the initial and final state of straw returning to the field, and focused on the impact of the initial and final state of straw returning to the field on crop yields. There are few studies on the dynamic changes of soil carbon and nitrogen caused by the release of straw nutrients and the application of chemical fertilizers. It is generally believed that the C/N ratio of corn stalks is higher than that of rice stalks, and it is more difficult to decompose, and the nutrients in it cannot be released quickly for the current crops to use [8]. Due to the differences in the chemical composition of different plant straws, they will have different effects on soil pH after they decompose in the soil [9]. Returning straw to the field can significantly increase the soil microbial biomass, leading to an increase in soil available nitrogen (small straw C/N) or a decrease (large straw C/N). Chen Xingli et al. [10] conducted indoor cultivation experiments on corn stalks and showed that the application of chemical fertilizers or organic fertilizers can significantly increase the nitrogen content of the stalks and reduce the C/N ratio, which is conducive to the decomposition and release of the straw's own nutrients. Ding Xueli et al. [11] further research showed that within a certain range of nitrogen fertilizer application, the soil microbial biomass continued to increase with the increase of inorganic nitrogen application. This is mainly because straw usually contains a high C/N ratio, and a certain amount of active inorganic nitrogen needs to be applied from the outside to make up for the insufficient supply of active nitrogen in the soil during the straw decomposition process, and to meet the nitrogen absorption by the straw degrading microorganisms., To maintain a high number and activity of microorganisms, thereby increasing the rate of mineralization and decomposition of straw. Chen Shanghong et al. [12] showed that continuous straw returning to the field can increase the soil organic carbon content under the condition of two-cropping and upland rotation in a year, and the straw returning to the field can increase the soil organic carbon content more than no-tillage.

Returning straw to the field can increase soil nitrogen supply. The research of Hong Chunlai [13] showed that the total nitrogen in the paddy soil increased from 0.205% to 0.229% after returning the straw to the field, and the alkali hydrolyzed nitrogen increased by 25.5mg·kg-1. The results of the fertilization experiment in the micro-area of straw returning to the field in northern Liaoning showed that the returning of straw to the field significantly increased the soil available nitrogen, the treatment with the largest amount of returning to the field increased the available nitrogen by 11.18%, and the total N content was also significantly increased [14]. The results of Wang Shuping [15] showed that the addition of corn stalks and root stubble can significantly increase the nitrogen content of the soil. Compared with single application of chemical fertilizers, they increase by 0.096 g/kg and 0.094 g/kg, respectively. Li Xiaoyong et al. [16] analyzed the test results of returning straw to the field to improve soil fertility and showed that the total nitrogen content of the soil increased by 28.5% ~ 40.1%, and the content of alkali hydrolyzed nitrogen increased by 13.2% ~ 30.8%. Long-term application of chemical fertilizers, especially nitrogen fertilizers, can increase the total N and available N content in the soil. However, the applied inorganic nitrogen fertilizer seldom accumulates in the soil organic matter. Only when organic carbon (such as returning straw to the field) is increased at the same time, can the organic nitrogen content be increased, and its mineralization can be improved, which is conducive to biological absorption of nitrogen [17]. The combined application of straws and chemical fertilizers can promote the conversion between the mineralization and fixation of organic nitrogen in the soil, so that the distribution of various humus nitrogen in the soil and the C/N ratio are continuously coordinated and updated, and the nitrogen cycle in the soil-plant system is accelerated Role [18].

In this experiment, corn and rice straw were used as materials to study the effects of two treatments with and without fertilizer on the physical and chemical properties of the soil after returning the straw to the field under flooded culture conditions. The soil pH, electrical conductivity, organic matter and nitrogen content were measured for 16 days. The dynamic changes of soil carbon and nitrogen after returning straw to the field and fertilizing were discussed. In order to obtain the dynamic relationship between chemical fertilizers and the decomposition process of straw, the aim is to provide a scientific basis for the rational return of straw to the field and the full utilization of straw resources.

2 Materials and Methods

2.1 Soil Material

The paddy soil used in the experiment was taken from the Agricultural Meteorological Experiment Station of Nanjing University of Information Technology in November 2021 (32°03′N, 118°51′E). The local multi-year average temperature is 15.6℃, and the multi-year average precipitation is 1100 mm per year. The test soil was a stagnant paddy soil (Grey genus), the soil texture was loamy clay, and the clay content of the topsoil was 26.1%. The soil of the tillage layer was collected after the late rice harvest in 2011, and the sampling depth was 0–20 cm. The collected soil was air-dried, and the stones, rice roots, etc. were removed and stored for later use. The basic physical and chemical properties of the soil are as follows: soil bulk density 1.57, pH (water-soil ratio 2.5:1) 7.90, conductivity 0.35µs/cm, organic matter content 8.54g/kg, total nitrogen 739.66mg/kg, ammonium nitrogen 13.42mg/kg.

2.2 Straw Material

The straws used in the experiment were rice straws and corn straws collected after the crops were harvested in November 2021, which were air-dried and ground for later use.

2.3 Experiment Method

There are six treatments in this experiment: -FS (no fertilization and no straw), -F + M (no fertilization and corn stalk), -F + R (no fertilization and rice straw), + FS (no stalk fertilization), + F + M (fertilization plus corn stalks), + F + R (fertilization plus rice stalks). Fertilization treatment is 0.2gN/kg soil, calculated as (NH4)2SO4; 0.15gP2O5/kg soil, calculated as Ca(H2PO4)2·H2O; 0.15g KCl/kg soil; straw treatment is 5%, that is, every 100g soil add 5g straw. The rice and corn stalks are air-dried and ground and mixed with soil, chemical fertilizer and sufficient water. Weigh 200g of the mixed soil in a disposable plastic cup and cultivate in a greenhouse at 40℃ for 16 days to measure soil pH, conductivity, organic matter, total nitrogen, ammonium nitrogen and nitrate nitrogen and other indicators.

Among them, the pH and conductivity are measured by electrode method (water-soil ratio is 2.5:1). The soil organic matter is determined by the potassium dichromate external heating method, the sample is digested with concentrated H2SO4 and titrated with FeSO4; the total nitrogen in the soil is determined by the Kelvin method, the sample is added to the catalyst, the sample is digested with concentrated H2SO4, and the nitrogen analyzer is used After distillation, the distillate was titrated with H2SO4 (0.005M). Use the standard curve method (see Fig. 1) to calculate the results.

Fig. 1.
figure 1

Standard song for total nitrogen determination

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2.4 Experiment Method

The experimental data using Excel 2007 basis calculated and analyzed using single factor analysis of variance (one-way ANOVA) differentially average significance test, p < 0.05 as significant difference, p < 0.01 for the difference was significant, the LSD method Multiple comparisons.

3 Results and Discussion

3.1 Effects of Straw Returning and Fertilization on Soil Structure

Returning straw to the field under crushed conditions can increase the total porosity of the soil and reduce the bulk density of the soil, and there will be no disadvantages such as the loss of soil nutrients due to the looseness of the soil and the uneven size of the soil voids. In this experiment, both corn stalks and rice stalks increased the porosity of the paddy soil. After the straw is crushed and added to the soil, the bulk density of the soil can be reduced. This may be because the crushed straw provides a better decomposition environment for soil microorganisms, which accelerates the decomposition rate of the straw, and its secretion products are beneficial to the soil aggregates. Formed, thereby improving the stability of the soil structure and improving the structural condition of the soil.

3.2 Changes of Soil pH After Returning Straw to the Field and Fertilizing

The changes of soil pH in the experiment are shown in Table 1. It can be seen from the table that the pH of the soil in the control group was 7.90 before the flooding treatment, and the soil was weakly alkaline. The soil pH decreased after adding straw and fertilizer, and the order of magnitude was CK >  + F-S >  + F + M > -F + M > -F + R >  + F + R. In the process of culture, CK, + F + M, -F + M showed a trend of first decreasing (0-8d) and then increasing (8-16d). + F-S shows a trend of decreasing. -F + R and + F + R fluctuate slightly, first increase (0-4d), then decrease (4-8d) and then increase (8-16d).

Analysis shows that returning straw to the field can adjust soil pH, reduce the damage of soil alkalinity caused by fertilization, and maintain the soil in a suitable pH environment to maintain the availability of soil nitrogen. Different types of straw returned to the field had different effects, but the difference was not significant. The decrease in soil pH during the experiment was caused by the nitrification of ammonium and the release of H+ [19]. The soil pH decreased slowly after adding straws. On the one hand, the soil redox potential was reduced due to the flooding culture, and the high-valent iron in the soil was greatly reduced and consumed a large amount of H+ [20]. On the other hand, it was due to the degradation of organic nitrogen. Chemicals cause the pH of the soil to rise.

Table 1. Changes of pH in soils with different treatments with time
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3.3 Changes Effect of Returning Straw to Field on Soil Electrical Conductivity

The soil solution has conductivity, and the strength of the conductivity can be expressed by the conductivity. Soil electrical conductivity is an indicator for determining soil water-soluble salt, and soil water-soluble salt is an important attribute of soil, and it is a factor to determine whether salt ions in soil restrict crop growth.

In this experiment, the change of soil conductivity with incubation time in different treatments is shown in Fig. 2. The order of conductivity is + F + R >  + F + M > -F + R >  + FS > -F + M > CK. It can be seen that returning straw to the field can increase the electrical conductivity of the soil, and the application of fertilizers has a significant impact on the electrical conductivity of the soil. With the continuation of the incubation time, CK and + FS did not change significantly, the soil conductivity of -F + M showed an upward trend, and the conductivity of + F + M and + F + R remained stable at 0-4d, and showed an upward trend at 4-8d, 8–16 days showed a downward trend.

Fig. 2.
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Change of conductivity of soil solution with different treatments with incubation time

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3.4 Changes of Soil Organic Matter After Returning Straw to the Field and Fertilizing

Soil organic matter is an important indicator to measure the level of soil fertility. The fertility of southern paddy soil is closely related to the organic matter content. A large number of studies have shown that returning straw to the field can increase soil organic matter and fertilize the soil [21,22,23].

The results of this experiment are shown in Fig. 3. Returning straw to the field can significantly increase the soil organic matter content. The organic matter content of the soil treated with straw increased from 0.9% before the experiment to more than 3%. The organic carbon content of paddy soil in the field treatment increased by more than 3 times on average. The order of soil organic matter on day 0 is: -F + R > -F + M >  + F + M >  + F + R >  + F-S > CK. With the increase of incubation time, the soil organic matter content in the -F + M, + F + R and -F + R 3 treatments decreased first and then stabilized and increased slightly at the end of the incubation.

The soil ignition loss rate is an important index to measure the high temperature volatile components of the soil. In existing research, it is often used to characterize the soil organic matter content. In this experiment, the effect of adding straw on soil ash and volatile matter was considered, and the ignition loss rate was determined at the same time. On day 0, the order of soil loss on ignition is: -F + R > -F + M >  + F + R >  + F + M >  + F-S = CK. With the continuation of the cultivation time, the burning loss rate of -F + M and -F + R soil showed the same changing trend as the corresponding total nitrogen. It shows that in the early stage of cultivation, the organic matter content of the soil decreases, which may be due to the accelerated decomposition of nitrogen-containing organic matter in the soil under flooding conditions, and a large amount of ammonia gas is released. Bernd et al. [24] showed that fresh organic materials contain a large amount of water-soluble substances, but there are big differences between different species. These substances are mainly amino acids, amino sugars, monosaccharides, polysaccharides and proteins, etc., all of which are extremely bioavailable low molecular weight compounds. Due to the limitation of the experimental environment, only harvested rice and corn stalks were used as organic materials in this experiment, but there are some differences between the two. This experiment shows that the organic matter content is very high at the initial stage of decomposing, and the decrease of organic matter after 8 days is caused by the mass reproduction and consumption of microorganisms, which may not decompose quickly due to the low temperature. But it can also be seen that there is a downward trend. Changes of soil organic matter content are shown in Fig. 3a and loss on ignition rate with culture time tare shown in Fig. 3b after different treatments. We can see that the organic matter content decreases faster when fertilizer is added to the soil. This is because the addition of fertilizer provides the necessary mineral nutrients for the growth of microorganisms, promotes the metabolic activities of microorganisms, and increases the decomposition rate. Therefore, straw and fertilizer Combined use can promote the increase of soil microbial metabolic activity and provide a favorable soil environment for crop growth.

Fig. 3.
figure 3

Changes of soil organic matter content (Fig. 3a) and loss on ignition rate with culture time (Fig. 3b) after different treatments

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3.5 Changes of Soil Total Nitrogen Content After Straw Returning to the Field and Fertilization

As shown in Fig. 4, returning straw to the field can increase the total nitrogen content of the soil, and the change of total nitrogen in the soil is different after the treatment of different types of straw returning to the field. Returning straw to the field and applying chemical fertilizers can significantly increase the total nitrogen content of the soil, and the content is in the order of -F + M >  + F + M >  + F + R > -F + R >  + F-S > CK. Adding straw and fertilizer at the initial stage of cultivation will increase the initial total nitrogen content of the soil. The total nitrogen content of corn straw is higher than that of rice straw. During the greenhouse cultivation, the total nitrogen content of the soil showed a downward trend in all treatments. For the two treatments adding corn stalks, fertilizer had little effect on increasing the total nitrogen content of the soil after the corn stalks were returned to the field. The total nitrogen content of the soil with only corn stalks has an increasing trend in the initial stage, the total nitrogen content is the largest on the 4th day, and then there is a decreasing trend; after adding fertilizers, the total nitrogen content of the soil shows a decreasing trend during the whole process, at the end of the cultivation It is always lower than that with corn stalks only. For the two treatments of adding rice straw, the addition of fertilizer increased the total nitrogen content of the paddy soil.

Fig. 4.
figure 4

Changes of soil total nitrogen content with cultivation time after different treatments

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3.6 Changes of Soil Inorganic Nitrogen (Ammonium Nitrogen and Nitrate Nitrogen) After Straw Returning and Fertilization Treatments

As shown in Fig. 5, the addition of straw and fertilizer in four of the six treatments (except fertilization plus rice straw and CK) increased soil ammonium nitrogen content. The addition of fertilizers in the initial stage significantly increased the content of ammonium nitrogen. From the 4th day to the end of the cultivation period, the content of ammonium nitrogen in each treated soil tended to be stable. During the 0–16 days of culture, the group added with rice straw alone did not significantly contribute to the increase of ammonium nitrogen. The combined application of fertilization and straw increased the content of ammonium nitrogen in the soil.

Fig. 5.
figure 5

Changes of soil ammonium nitrogen content in different treatments

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As shown in Fig. 6, soil nitrate nitrogen in each group showed a decreasing trend during the whole culture process. For this experiment, adding straw treatment can effectively increase the nitrate nitrogen content of the soil. The effect of adding corn stalks alone is greater than that of adding rice stalks alone. The addition of fertilizer increased the nitrate nitrogen content in the soil treated with rice straw. For rice straw, the nitrate nitrogen content in soil decreased after adding fertilizer.

Fig. 6.
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Changes of soil nitrate nitrogen in different treatments

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The forms of nitrogen in the soil are divided into two categories: inorganic and organic. Inorganic nitrogen is mainly ammonium nitrogen and nitrate nitrogen, which are less in soil, and nitrogen in soil mainly exists in organic form. Under normal circumstances, the inorganic nitrogen content in the soil does not exceed 5% of the total nitrogen, and this part of the nitrogen can be absorbed by crops. The intervention of a large amount of organic carbon in crop straw will significantly change the intensity and time of the soil nitrogen mineralization/immobilization process, thereby affecting the dynamic changes of inorganic nitrogen in the soil. The C/N of straw returned to the field affected this process. The contents of NH4 + -N and NO3-N in the soil with high C/N rice straw were lower than those in the soil with low C/N corn straw. Studies have shown that when a large amount of mineral nitrogen exists in the soil, the application of crop residues can improve the retention of ammonium nitrogen and nitrate nitrogen, and reduce the amount of inorganic nitrogen (especially nitrate) in the soil concentration. While using inorganic nitrogen, microorganisms also make use of nitrogen from straw, and finally decompose.

3.7 Changes of Soil C/N After Different Treatments

It can be seen from the analysis in Fig. 7 that in the early stage of cultivation, adding straw can effectively increase the C/N of the soil. The results showed that the C/N ratio of the soil showed an overall upward trend during the whole cultivation process. The changing trends of soil carbon-nitrogen ratio were different after different treatments. Under the same fertilization treatment conditions, the addition of rice straw increased the soil C/N ratio more than that of corn straw. Under the same straw treatment, the C/N ratio of each group with fertilizers was almost lower than that of groups without fertilizers. At the end of cultivation, the order of soil carbon and nitrogen ratio was: + F-S < -F-S < -F + M <  + F + M <  + F + R < -F + R.

Fig. 7.
figure 7

Changes of C/N with culture time in different treatment

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Soil carbon-nitrogen ratio is the ratio of soil carbon to nitrogen. An appropriate carbon-nitrogen ratio is helpful for microbial fermentation and decomposition. Generally speaking, the stalks of grass crops such as rice stalks, corn stalks and weeds have high carbon-nitrogen ratios. When a microorganism is decomposing organic matter, it needs to assimilate 1 part of nitrogen when it assimilates 5 parts of carbon to form its own cell body, because the carbon-nitrogen ratio of the microorganism itself is about 5:1. Therefore, it is difficult or slow for microorganisms to decompose and mineralize organic matter with a large carbon-nitrogen ratio. The optimum carbon-nitrogen ratio of microorganisms to organic matter is 25:1. If the carbon-nitrogen ratio is too large, the decomposition of microorganisms will be slow, and the available nitrogen in the soil will be consumed. Therefore, when using straw with a large carbon-nitrogen ratio (such as corn straw, rice straw, etc.) to return to the field, fertilizers containing more nitrogen should be supplemented to adjust the carbon-nitrogen ratio.

3.8 The Effect of Each Factor on the Experiment

As shown in Table 2: the influence of each factor and the interaction between each factor on each index of this experiment is significant. Among them, cultivation time had a significant effect on organic matter and loss on ignition rate, and had a very significant effect on other indicators; fertilization had no significant effect on total nitrogen content, which might be the main effect of straw addition. Because the cultivation time is short, the straw is not completely decomposed.

Table 2. The significance test of the influence of cultivation time, fertilization and straw treatment on the main basic physical and chemical properties of soil
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4 Conclusion

In conclusion, straw returning to the field can increase the organic carbon content of paddy soil, improve soil fertility, help soil environment remediation, and protect soil ecology. This paper draws the following conclusions, in order to provide a scientific basis for rationally returning straw to the field and making full use of straw resources in production practice.

(1) Returning straw to the field can significantly increase the content of soil organic matter and improve soil fertility. For paddy soil, returning rice straw to the field without fertilization is better. The combined application of chemical fertilizers can effectively reduce the carbon-nitrogen ratio, promote the decomposing of the straw in the soil, facilitate the decomposition and release of the straw's own nutrients, and avoid the competition of decomposing bacteria for nitrogen. The corn straw combined fertilization treatment has the most significant effect.

(2) When using straw with a large carbon-nitrogen ratio to return to the field (such as corn straw, rice straw, etc.), an appropriate amount of fertilizer containing more nitrogen should be supplemented to adjust the soil carbon-nitrogen ratio. At the same time, attention should be paid to the sequence of fertilization and straw returning to avoid excessive nitrogen loss and secondary pollution.

(3) In order to make full use of straw returning to the field to protect the soil ecological environment, when using rice straw with a large carbon-nitrogen ratio, fertilizers containing more nitrogen should be supplemented to adjust the carbon-nitrogen ratio and make the carbon-nitrogen ratio reach an appropriate value.