Effects of long-term biogas slurry application on soil properties and crop yield in the North China Plain

Abstract

Zhoukou is located in the southeast of Henan Province, China, which is the core area of grain production. Lime concretion black soil is one of the main low-yield soil types in this region, which has the characteristics of heavy clay, compact soil layer, poor aeration and low organic matter content, so it is urgent to carry out the task of improving the soil fertility. Biogas slurry contains a variety of nutrients, which can improve soil physical and chemical properties, increase soil enzyme activity, improve crop biological properties and promote crop growth. However, the application of continuous large concentrations will have a negative impact on the growth of crops. Reasonable application of biogas slurry is particularly important in order to ensure the yield and quality of agricultural products, and does not pollute the soil and groundwater environment. In this study, we systematically analyzed the effects of different proportions of combined application of biogas slurry and chemical fertilizer on the physicochemical properties and enzyme activities of different soil layers in the of lime concretion black soil of the Huang-Huai-Hai Plain, as well as the effects on the growth and development of winter wheat, and determined the appropriate amount of biogas slurry suitable for Zhoukou area, so as to provide a scientific basis for the improvement of the soil of the of lime concretion black soil, and provide a reference for the application of biogas slurry in Ukraine, the improvement of soil texture and the realization of sustainable agricultural development. Soil bulk density comprehensively reflects the tightness and porosity of soil, and has a significant impact on water, fertilizer, gas, heat conditions in the soil and agricultural production. In this study, there were significant differences between different soil layers, and between each treatment and CK (сontrol). After adding biogas slurry, the soil bulk density was significantly reduced, especially BS50 (50 % nitrogen from biogas slurry and 50 % nitrogen from fertilizer during winter wheat planting) soil bulk density was the smallest, and it was within the suitable bulk density range for winter wheat, which was conducive to the growth and development of winter wheat and the increase in yield. In 0-10 cm soil layer, percentage of particle aggregates with size >2 mm was the highest for BS50 and for CF (only chemical fertilizers) was the lowest, which are 54.48 and 39.27 %, respectively. Percentage of 0.25-2 mm particle size aggregates was the highest for CF with a value of 53.61, which is significant different from others. Soil samples from BS50 had the lowest percentage of aggregates 0.053-0.25 mm with a value of 2.03, which is not significant different from BS100 (100% biogas slurry), but is significant different from other treatments. For aggregates of <0.25 mm particle size, CF had the smallest percentage - 1.94, but there is no significant difference between treatments. Long-term application of chemical fertilizer significantly decreased the number of large aggregates and increased the number of small aggregates, and application of biogas slurry was beneficial to increase the composition of large aggregates. The stability of soil aggregates varies in different soil layers and under different treatment conditions. Biogas slurry treatment is beneficial to increase the MWD (mean weight diameter) of 0-10 cm, 10-20 cm, 20-40 cm force-stable aggregates and water-stable aggregates, with BS50 being the largest. The size order of MWD of force-stable aggregates and MWD of water-stable aggregates in the vertical direction is 0-10 cm>10-20 cm>20-40 cm, and both decrease with the increase of soil depth. The combined application of chemical fertilizers and biogas slurry made the MWD of force-stable and water-stable aggregates significantly higher than those of other treatments, increasing the stability and agglomeration of the soil. In the 0-10 cm soil layer, BS50 has the smallest percentage of aggregate destruction (PAD) value of 36.52, which is 45.0 % and 46.74 % lower than CK and CF respectively. The PAD of CF increased by 3.27 % compared to CK. In the 10-20 cm soil layer, the PAD value of BS50 is 35.70, which is 46.26 % and 47.71 % lower than CK and CF respectively. In the 20-40 cm soil layer, the PAD value of BS50 combined with biogas slurry and chemical fertilizers was the lowest (33.33), which was significantly different from each treatment. The PAD of CF was increased compared with CK. The single application of chemical fertilizer CF will make the stability of soil aggregates worse and destroy the aggregation of aggregates; while the combined application of organic fertilizer and chemical fertilizer will reduce the degree of damage to aggregates and facilitate the aggregation of aggregates. Reasonable fertilizer application, especially the BS50 treatment of biogas slurry and chemical fertilizers, has positive significance in maintaining the stability of soil aggregates and improving soil quality. pH is an important chemical property of soil, which can affect the solubility of nutrient elements and ultimately affect the supply of nutrients in the soil. In this study, CF treatment can reduce pH, and application of biogas slurry can increase pH. Biogas slurry itself is alkaline and contains high cation content. The increase of base ions can effectively inhibit the soil acidification trend caused by long-term application of chemical fertilizer to a certain extent, and the acid-base environment of soil solution can be improved. The application of biogas slurry in agricultural planting can significantly increase the nitrogen content of the soil, thereby satisfying the absorption of nutrients by crops. In 0-10 cm, 10-20 cm, and 20-40 cm, the total nitrogen content is highest by the BS50 treatment. The total nitrogen content is 0-10 cm>10-20>20-40 cm, the total nitrogen content decreased with the increase of soil depth. Biogas slurry contains a large number of microorganisms, which can promote the propagation of soil microorganisms and enhance the activity of related enzymes. Application of biogas slurry has a certain impact on the soil available phosphorus content. Among all soil layers, the available phosphorus content of BS50 was the highest, which was 25.59 mg /kg, 22.50 mg/kg and 20.48 mg/kg, respectively, and there was a significant difference between BS50 and other treatments. In terms of soil depth, soil available phosphorus content decreased with the increase of soil depth, and the available phosphorus content was 0-10 cm>10-20 cm>20-40 cm, indicating that soil available phosphorus mainly concentrated in the surface layer. The content of soil available phosphorus was different in different soil layers and different treatment conditions, which indicated that the application of biogas slurry could improve the supply level of soil available phosphorus, but more is not always better, increasing the proportion of biogas suspension to BS75, B100 leads to a decrease in phosphorus content. In agricultural production, biogas slurry should be applied reasonably according to soil conditions and crop needs to increase soil available phosphorus content and promote crop growth. Potassium has obvious effects on improving crop yield and agricultural product quality. Available potassium is an effective potassium that can be directly used by plants and plays an irreplaceable role in ensuring the normal growth and development of plants. Compared with CK, the content of available potassium in soil of each treatment increases to varying degrees. In different soil layers, the available potassium content of BS50 treatment combined with biogas slurry and chemical fertilizer was the highest, which was in 0-10 cm - 173.71 mg/kg, 10-20 cm - 156.63mg/kg and 20-40 cm - 144.06 mg/kg, respectively, and significantly increased compared with other treatments. Biogas slurry increased the content of available potassium in soil, which was beneficial to the absorption of potassium by winter wheat. Reasonable application of fertilizer and biogas slurry has positive significance to increase soil available potassium content, which is conducive to crop growth. Application of biogas slurry had an effect on soil organic matter content. In 0-10 cm, 10-20 cm and 20-40 cm soil layers, the soil organic matter content was BS100>BS75>BS50>BS25>CF>CK, and the soil organic matter content increased with the increase of biogas slurry concentration. There were significant differences between all treatments and CK (p<0.05), and the soil organic matter content of BS100 was the highest (24.24 g/kg, 20.16 g/kg and 18.32 g/kg, respectively). Compared with CF, organic matter increased by 32.82 %, 24.44 % and 17.84 %, respectively, with the increase of soil layer, the organic matter content of different treatments decreased, and the organic matter content was 0-10 cm >10-20 cm > 20-40 cm. We studied the effects of different proportions of biogas slurry and fertilizer on soil enzyme activity. The results showed that: the application of biogas slurry increased the activity of sucrase in soil, and with the increase of biogas slurry application, the activity of sucrase first increased until B75 and then decreased (BS100). The activity of sucrase with BS75 was the highest in 0-10 cm, 10-20 cm and 20-40 cm, which was 15.28 mg/g, 30.73 mg/g and 31.28 mg/g, respectively. Sucrase provides energy for organisms in soil, increases the intensity of organic carbon accumulation, decomposition and transformation, and increases soil fertility level. Application of biogas slurry and chemical fertilizer can increase soil urease activity. The urease activity of combined application of biogas slurry and chemical fertilizer (BS25, BS50 and BS75) was higher than that of CF. With the increase of biogas slurry replacement ratio, the urease activity increased first BS50 and then decreased, indicating that the benefit of biogas slurry on soil was dose-dependent, and appropriate biogas slurry was beneficial to the improvement of soil urease activity, but excessive fertilization was counterproductive. In 0-10 cm, 10-20 cm, 20-40 cm, soil urease activity was the highest in BS50 treatment (235.33 ug/g, 337.79 ug/g and 351.45 ug/g, respectively), and was significantly different from that in other treatments. Combined application of biogas slurry and chemical fertilizer can enrich soil nitrogen pool, improve soil nitrogen transformation, increase the nitrogen source absorbed by winter wheat. Both biogas slurry and chemical fertilizer increased soil acid phosphatase activity. With the increase of biogas slurry concentration, soil acid phosphatase increased with BS50 and then decreased on BS75-100. An appropriate amount of biogas slurry could catalyze the conversion of soil phosphorus into phosphorus easily absorbed by organisms, and improve its utilization rate. The activity of acid phosphatase with BS50 was the highest in different soil layers. The order of catalase activities in 0-10 cm, 10-20 cm, and 20-40 cm soil was as follows: BS100 >BS75> BS50>BS25>CF >CK, and there was a significant difference between each treatment and CK (P<0.05), and there was no significant difference between CF and BS25 (P > 0.05), BS100 catalase activity was the highest, with 93.19 mmol/g, 95.11 mmol/g and 97.12 mmol/g, which were 12.35 %, 6.21 % and 5.86 % higher than CF, respectively. The activity of soil catalase reflects the process of soil organic oxidation, with the increase of the proportion of biogas slurry, the catalase activity increases. The biogas slurry has a significant promoting effect on soil catalase activity, which can accelerate the removal of hydrogen peroxide in organisms and soil to prevent damage, and the soil catalase activity of BS100 treatment is the largest, and the soil detoxification effect is the strongest. Long-term application of chemical fertilizers has a significant inhibitory effect on soil catalase activity, which can easily lead to the accumulation of root exudates, thereby aggravating the toxic effect of hydrogen peroxide on crops. Biogas slurry application can significantly increase soil catalase activity and accelerate the decomposition of toxic substances in soil. Under the influence of soil fertility and productivity, the vertical distribution of soil enzymes has obvious regularity. Biogas slurry can improve the soil enzyme activity of winter wheat in 0-40 cm soil layer, especially in 20-40 cm soil layer, which is conducive to delaying the aging process of winter wheat root in the late growth period, promoting the transformation of soil nutrients, improving the ability of soil to supply fertilizer to winter wheat, and meeting the demand for nutrients during the growth and development of winter wheat, which is beneficial to the improvement of soil fertility and crop yield. Finally, we measured the effects of different proportions of biogas slurry and chemical fertilizer on the growth and development of winter wheat. Both biogas slurry and chemical fertilizer application increased the yield of winter wheat, and there was a significant difference compared with CK (p<0.05). With the increase of biogas slurry proportion, the yield of winter wheat first increased and then decreased, and the yield of winter wheat with BS50 treatment was the highest, compared with CF, BS25, BS75 and BS100, the increases were 30.79 %,13.61 %, 4.31 % and 34.06 %, respectively, and there were significant differences between them (p<0.05). BS50 is the optimal combination of biogas slurry and chemical fertilizer, which can well match the nutrient demand of winter wheat at different growth and development stages, ensure the continuous supply of nutrients, promote winter wheat nutrient growth and nutrient accumulation, and achieve the best level of winter wheat yield. The yield and yield composition of winter wheat are closely related. Chemical fertilizer and biogas slurry treatment can significantly increase the effective number of ear, grain number per ear and 1000-grain weight of winter wheat (P<0.05), so as to increase production. Yield was significantly correlated with the effective number of ear, grain number per ear and 1000-grain weight, and the correlation coefficients were 0.964, 0.974 and 0.870, respectively. The correlation between yield and grain number per ear is the highest. The high yield was achieved by increasing grain number per ear. Different fertilization treatments had effects on the morphological indexes of winter wheat. Fertilization increased the plant height of winter wheat, and thus increased the ventilation and light transmittance of the bottom of the plant, which was conducive to the growth and development of winter wheat and the photosynthetic efficiency of leaves, so as to increase the yield of winter wheat. In the fertilization treatment, the plant height of BS treatment was higher than that of chemical fertilizer. The plant height treated with BS25, BS50, BS75 and BS100 was 77.47 cm, 79.52 cm, 78.08 cm and 78.02 cm, respectively. With the increase of biogas slurry proportion, the plant height first increased and then decreased, and plant height with the BS50 was the highest of 79.52 cm, but still belonged to the medium plant height. This can not only prevent lodging, but also enable the winter wheat nutrient body to obtain the appropriate spatial distribution and size, meet the nutrition required for wheat growth and growth, and is more conducive to the high yield of winter wheat. The plant height is determined by several internode lengths, which extend at different stages of development, so they have different effects on yield. The first basal internode length is very important to plant lodging resistance wheat, the base of the first internode increases with fertilizer use, but the increase rate is different, compared with CK, CF, BS25, BS50, BS75, BS100 respectively increased by 1.81 %, 3.55 %, 0.68 %, 5.96 % and 6.19 %, BS100 has the greatest amplitude, increasing the risk of lodging. BS50 had the smallest increase, which was not only conducive to increasing plant height, but also had better lodging resistance. The plant height component index of wheat (І) is the proportional relationship between the length between the upper and lower nodes of the stem, reflecting the information of the spatial arrangement of photosynthetic area and the operation and distribution of compounds. Fertilization can effectively improve the plant height component index of winter wheat. If the value of I is high, the lower node is shorter, the center of gravity is lower, the lower ear node is longer, the photosynthetic area above the blade node is increased, and the resistance to falling and photosynthesis are enhanced. The increase of plant height component index with BS50 made the ear under good light condition, which was conducive to increasing the yield of winter wheat. Plant height component index was significantly correlated with the effective number of ears, grain number per ear, 1000-grain weight, yield and plant height. The correlation coefficients between sub-panicle internode and 1000-grain weight, I1 and plant height were above 0.9, accounting for a higher proportion. In the correlation analysis between yield and plant height component index, the correlation coefficient between yield and subear node was the highest (0.895), while the correlation coefficient between plant height and I1 and subear node was 0.971 and 0.954, respectively. Therefore, subear internode plays a key role in the formation of winter wheat yield. In practice, winter wheat with relatively high plant height and I1 should be selected within a moderate height range. The increase of crop yield is based on the accumulation and distribution of dry matter. The combined application of biogas slurry and chemical fertilizer had a significant effect on straw biomass and aboveground biomass (p<0.05), and the highest values of straw biomass and aboveground biomass were BS75 and BS50, with the increase of biogas slurry proportion, even the biogas slurry completely replacing chemical fertilizer, both showed a downward trend. Harvest index is the ratio of grain yield to aboveground total dry matter weight. It reflects the distribution ratio of crop assimilation products in grains and vegetative organs, and is an important index to evaluate crop yield level and cultivation effectiveness. Both fertilizer and combined biogas slurry increased the harvest index of winter wheat (P<0.05), and the harvest index of BS50 was the highest (0.48). Increasing biogas slurry within a certain range could improve both economic yield and harvest index. BS50 treatment can improve the growth and development of various organs in the later period of growth, coordinate the balanced distribution of dry matter, transport more photosynthetic products from source organs to reservoir organs, increase the flow and achieve the purpose of increasing production. The factors affecting grain yield, biological yield and substance distribution were closely related to harvest index. Above ground biomass was significantly positively correlated with effective number of ear and grain number per ear, with correlation coefficients of 0.935 and 0.944, respectively, and the harvest index was also significantly positively correlated with effective number of ear and grain number per ear, with correlation coefficients greater than 0.9 (0.911 and 0.901, respectively). Therefore, it is possible to effectively combine high biological yield with high harvest index, and it is feasible to increase the yield by increasing effective number of ears and grain number per ear.