Changes in Soil Organic Matter, Nitrogen and Phosphorus Contents during Decomposition of Pear Branches

Yaxuan ZHONG, Rukeyanmu Matistic, Aikebaier·Yilahong*, Turnisa Matiturum, Setivaldi Abdushik

1. College of Resources and Environment, Xinjiang Agricultural University, Urumqi 830052, China; 2. Agricultural Economy and Agricultural Technology Service Station of Kan Township, Qapqal Xibe Autonomous County, Yili 835100

Abstract [Objectives] To investigate the changes in soil organic matter, nitrogen and phosphorus content in the decomposition process of Korla fragrant pear branches by indoor mixed culture. [Methods] The branches of Korla fragrant pear in the orchard were collected and returned to the field for a period of 150 d for indoor mixed culture. [Results] Different ages of Korla fragrant pear branches have different effects on soil nutrient content during the simulated return to field decomposition process. Compared with the control in the same period, the treatment of returning to field reached a significant level (P<0.05). Compared with the control, the average values of organic matter, total nitrogen and available phosphorus content in treatment 1 and treatment 2 increased by 2.16 times and 1.93 times, 61% and 59%, 5.88 times and 6.88 times, respectively; compared with the control, the average increase performance of the alkaline hydrolysis nitrogen content of the two treatments was basically the same, and the treatment 2 was the best; compared with the control, the average total phosphorus content of treatment 1 and treatment 2 increased but not significantly. [Conclusions] The contents of soil organic matter, nitrogen and phosphorus were all increased during the decomposition of pear branches, and the overall improvement effect of 10-year-old trees was better than that of 5-year-old trees. Returning the pruned branches to the field can provide a reliable theoretical basis for solving the problem of organic fertilizer shortage in orchards, and also can ensure technical support for improving soil fertility and improving the rhizosphere micro-environment of pear trees.

Key words Korla fragrant pear, Decomposition of branches, Soil nutrient content

1 Introduction

Korla fragrant pear (PyrussinkiangensisYu) belongs to Xinjiang pear species and is one of the most common characteristic fruits in Xinjiang[1]. The survey results in 2020 show that the planting area of fragrant pears in Korla City was 30 900 ha, with a total yield of 402 700 t, an increase of 175 000 t from 227 700 t in 2019, an increase of 76.86%[2]. However, for a long time, due to the hot-sales of Korla fragrant pear, it is common for fruit farmers to blindly pursue the yield of fragrant pears and use a large amount of chemical fertilizers. Improper long-term fertilization will not only cause many problems such as the decline of orchard soil fertility, soil compaction and salinization, but also reduce the utilization rate of nutrients absorbed by fruit trees. Quantitative changes lead to qualitative changes, and the decline in soil fertility directly affects the quality and taste of fragrant pears. The traditional clear tillage method will accelerate the consumption of soil organic matter and destroy the soil structure[3]. In recent years, there have been extensive studies on how returning organic materials to fields can increase soil organic matter and improve soil fertility[4]. Soil management models in modern orchards are divided into two types: soil management and soil improvement. Among them, growing grass in orchards is a measure commonly implemented in developed countries producing fruit trees, and it is also an important orchard soil management model in China[5-7]. As an important way to utilize fresh grass, returning the raw grass to the field plays a positive role in improving the soil and balancing the nutrient cycle of the farmland ecosystem[8].

There are two specific methods of soil improvement: one is to use animal-source organic fertilizers that need to be decomposed before application; the other is to use grass instead of fertilizer, also called returning straw to the field. Returning straw to the field is an effective approach to using straws and has been strongly recommended by the government and scientists[9]. Generally, straw returning to the field is mainly used in crops. Crop straw is rich in nitrogen, phosphorus, potassium, organic matter and some trace elements. These nutrients can be reused by plants after the straw rots[10-11]. Under the condition of returning all the straw to the field, the potassium, most of the phosphorus and part of the nitrogen in the straw can be supplemented to the soil[12]. Previous studies have shown that tree litter is also an important factor influencing soil physical and chemical properties. The humus formed by the decomposition of litter contributes to the long-term carbon sequestration of the soil and releases the nutrients bound by organic matter, which is an important mechanism for soil self-fertilization, and also an important reason for a large amount of CO2to return to the atmosphere[13]. As the basic carrier of nutrients, litter is an important component of the forest ecosystem and an important link in the material cycle[14]. Studies have shown that more than 90% of the N and P absorbed by forest plants come from the nutrients returned to the soil by the decomposition of litter[15-16]. The application of organic fertilizer in orchards can improve the soil, increase soil organic matter and alleviate the contradiction between supply and demand, and its beneficial effects on the growth of fruit trees have gradually been valued by orchard farmers[17]. Through branch composting, the orchard solid waste can be turned into a new type of organic fertilizer with comprehensive nutrients, so as to realize the resource utilization of pruned branches, which is a new method of soil improvement. Compared with litter returning, there are few related researches on discarded branches returning to field. In practice, fruit tree pruning and shaping are carried out every year in the orchard to ensure the reasonable distribution of branches and adequate nutrition. The branches after pruning fruit trees in spring contain a lot of organic material, but most of them are discarded in the orchards. These pruned branches are not easy to rot, and will pollute the environment when burned. Is there an easier way than composting branches? Can directly "returning branches to the field" increase soil nutrients?

In view of these problems, we simulated the effects of decomposition of pear orchard branches on soil organic matter, nitrogen and phosphorus contents under the condition of indoor mixed culture, and observed the changes in soil nutrient content after cultivation, in order to provide a reliable theoretical basis for pruning branches and returning them to the field to solve the problem of organic fertilizer shortage in orchards, provide a theoretical basis and technical support for improving soil fertility and improving the root micro-environment of orchards, and provide a certain theoretical basis for the large-scale promotion of this technology in the future.

2 Materials and methods

2.1 Overview of the study areaThe experimental site is located in Awati Xiangli Town (41°37′ E, 86°24′ N), Awati Township, Korla City, Bayinguoleng Mongolian Autonomous Prefecture, Xinjiang Uygur Autonomous Region. The altitude is 912 m, and the experimental site has a temperate continental climate with an annual average temperature of 10.7-11.2 ℃. The soil types of the orchard are mainly irrigation silting soil and fluvo-aquic soil.

2.2 Preparation of test soil samples and branch samplesWe collected the original soil from the local pear orchard in Korla for indoor mixed culture. The test soil samples were collected from Awati Xiangli Town, Awati Township, Korla City, Bayinguoleng Mongolian Autonomous Prefecture, Xinjiang Uygur Autonomous Region. The initial soil organic matter content (w, the same below) was 18.64 g/kg, the total nitrogen content was 0.07 g/kg, the alkaline hydrolysis nitrogen content was 6.99 mg/kg, the total phosphorus content was 0.79 g/kg, and the available phosphorus content was 7.93 mg/kg, the pH was 7.84, the bulk density was 1.33 g/cm3, and the soil moisture content was 20.94%. Branches were collected by randomly selecting robust 5-year-old and 10-year-old pear sample trees in 3 Korla fragrant pear orchards. According to different levels and orientations, plant samples of each individual plant were collected from the upper, middle and lower parts of the trees and 4 different directions from southeast to northwest by using high-branch shears or manual tree climbing. The healthy and disease-free branches were selected, the branches were cut with a length of <1 cm, and the samples in different directions were mixed evenly in proportion to obtain the mixed samples. The element composition of branches of different ages is shown in Table 1.

Table 1 Element composition of branches of different ages

Branches and soil samples collected in the field should be stored in an incubator with built-in ice in time. The soil samples in the 3 orchards were randomly selected according to the "S" curve at 5 locations, and the soil was drilled at 40-60 cm. After being brought back to the laboratory, soil samples were mixed and the roots and debris were picked out. Before each time of indicator measurement, samples should go through nine steps of "air-drying, sorting, removing impurities, grinding, sieving, mixing, bottling, preservation, and registration". Branch samples brought back to the laboratory should be washed with distilled water, wiped, weighed, dried in a 70 ℃ incubator, and crushed through a 60-mesh sieve for later use. In addition to the blank control, 120 g of soil and 6 g of organic materials crushed to 0.25 mm were weighed in each "small pot" and then went through shading treatment.

2.3 Experimental designBecause this experiment was an indoor mixed culture, in order to avoid the interference caused by different nutrient content absorbed by different fragrant pear trees in the potted plant, after picking out the soil samples of the 40-60 cm soil layer of the orchard, the roots and debris were picked out and no other crops were planted at the same time. Except for the control, the amount of organic material added to each pot was 6 g. The experiment was carried out from May 20, 2021 to October 17, 2021. During the experiment, the field water holding capacity should be guaranteed. In view of the fact that the orchard soil is yellow-aquic soil, the soil moisture should be controlled to 75% of the saturated water content, so as to achieve the ideal state of returning Korla fragrant pear branches to the field. In this experiment, three treatments were set up: treatment 0: blank control (CK), treatment 1 (the crushed 5-years old branches were applied), and treatment 2 (the crushed 10-years old branches were applied). The dynamic changes in soil nutrients within 150 d under indoor mixed culture were observed.

In this experiment, we analyzed the dynamic changes in soil nutrient content within 150 d of Korla fragrant pear branches decomposing, to obtain the degree of improvement of soil fertility in different periods of a certain treatment.

2.4 Sample determinationSoil organic matter was measured by potassium dichromate external heating method, total nitrogen was determined by Kjeldahl method, alkali-hydrolyzed nitrogen was measured by alkali-solution diffusion method, and total phosphorus and available phosphorus were measured by molybdenum-antimony anti-colorimetric method[18], all were repeated three times.

2.5 Data processingThe data processing system SPSS 25.0 was used to perform one-way analysis of variance to analyze the significance of the differences in soil chemical properties among different treatments in the same period. Duncan’s method was used for multiple comparisons (α=0.05), and finally Excel 2021 was used to plot the chart. The data were expressed as (mean±standard deviation).

3 Results and analysis

3.1 Changes in soil organic matter content during the decomposition process of different treatmentsAs shown in Fig.1, during the whole culture process, except for the control, the change trends of the two treatments were first increased and then decreased. Compared with the control group at the same period, both treatment 1 and treatment 2 reached a significant level (P<0.05). Except for no significant difference on the 30thd between the two treatments, there were significant differences in the other nine periods (P<0.05). Specifically, the periods with the highest soil organic matter content in treatment 1 and treatment 2 were the 60thand the 45thd, respectively, reaching 62.43 and 67.90 g/kg, which were separately 2.71 and 2.93 times higher than the control in the same period. Compared with the control, the mean values of soil organic matter in treatment 1 and treatment 2 both increased by 2.16 and 1.93 times, respectively. The period with the largest difference between treatment 1 and treatment 2 and the control was separately the 60thand the 45thd after simulated returning to the field.

Note: Different lowercase letters indicate significant differences over the same period (P<0.05), the same in following figures.Fig.1 Changes in organic matter content during branch decomposition

3.2 Changes in soil total nitrogen content during branch decomposition of different treatmentsAs shown in Fig.2, there was a significant difference between treatment 1 and treatment 2 at 15thand 150thd (P<0.05). There was no significant difference between treatment 1 and the control group at the 45th, 75th, 90thand 105thd (P>0.05). Except that there was no significant difference between treatment 2 and the control group at the same period on the 45thd and 75thd, there were significant differences in the other 8 periods (P<0.05), which separately increased by 0.05, 0.06, 0.03, 0.02, 0.03, 0.06, 0.04 and -0.02 g/kg compared with the control. The period with the highest soil total nitrogen content in treatment 1 was the 150thd after the simulated returning to the field, and the 135thd in treatment 2. Compared with the control soil total nitrogen, treatment 1 and treatment 2 both increased by 61% and 59%, respectively. Compared with the control, the mean value of soil total nitrogen in treatment 1 and treatment 2 increased by 61% and 59%, respectively. The period with the largest difference from the control was on the 30thd after the simulated returning branches to the field. The total nitrogen content of treatment 1 and treatment 2 reached 0.11 and 0.09 g/kg, which were 2.92 and 2.24 times that of the control in the same period. Compared with the control in the same period, the total nitrogen of the two treatments increased significantly.

Fig.2 Changes in soil total nitrogen content during branch decomposition

3.3 Changes in soil alkaline hydrolysis nitrogen content during the decomposition of different treatmentsAs shown in Fig.3, there was a significant difference (P<0.05) between treatment 1 and the control at the same period on the 30thd, which was 2.53 mg/kg lower than that of the control. There were significant differences (P<0.05) between treatment 2 and the control group at the 30th, 90th, 135thand 150thd, with an increase of -0.19, -2.14, 2.51 and 4.65 mg/kg compared with the control, respectively. Compared with the control, the average improvement performance of soil alkaline hydrolysis nitrogen in treatment 1 and treatment 2 was basically the same, increasing by 1% and 4%, respectively. The period with the largest difference from the control was the 150thd after simulated branch returning to the field, which was also the period when the soil alkaline hydrolysis nitrogen content was the highest. Treatment 1 and treatment 2 separately reached 7.39 and 9.38 mg/kg, which was 0.56 and 0.98 times that of the control in the same period.

Fig.3 Changes in soil alkaline hydrolysis nitrogen content during branch decomposition

3.4 Changes in total phosphorus content in soil during the decomposition of different treatmentsAs shown in Fig.4, the total phosphorus content during the entire indoor mixed culture period was almost steady. Compared with the control, the average value of total phosphorus in treatment 1 did not increase, and the highest soil total phosphorus content reached 0.72 g/kg on the 90thd after simulated returning to the field, an increase of 6%, which was not significantly different from that of the control in the same period (P>0.05). Except for the 105thd, treatment 1 had no significant difference (P>0.05) in the soil total phosphorus compared with the control in other periods. Compared with the control, the increase and decrease of soil total phosphorus in treatment 2 were -8%, 7%, 4%, 1%, 12%, 1%, -3%, 0%, 6% and 3%, respectively. Except for the 75thd, compared with the control in other periods, there was no significant difference in soil total phosphorus in treatment 2 compared with the control in other periods (P>0.05), and the average increase of soil total phosphorus was 2%. In summary, the period with the greatest difference between treatment 2 and the control was the 75thd after the simulated returning to the field, and the total phosphorus content reached 0.71 g/kg, which was 0.12 times higher than that of the control in the same period and there was a significant difference (P<0.05).

3.5 Changes in soil available phosphorus content during different treatmentsAs shown in Fig.5, treatment 1 reached its maximum value of 11.54 mg/kg on 45thd with an increase of 36%. But there was no significant difference (P>0.05) compared with the control group in the same period. Compared with the other 9 periods, there were significant differences (P<0.05) in soil available phosphorus between treatment 2 and the control, which increased by -0.03, 3.35, 6.04, 2.43, 6.76, 3.35, 2.77, -0.57 and 2.70 mg/kg, respectively. Except the 105thd and 135thd not significantly different from the same period of control, there were significant differences with treatment 2 in the other 8 periods (P<0.05), which were 1.43, 0.56, 0.49, 1.89, 1.78, 46.96, and 5.48 times that of the control group in the same period, respectively. The highest soil available phosphorus content in treatment 1 reached 21.68 mg/kg on the 15thd after simulated returning to the field, an increase of 1.43 times compared with the control. Compared with the control, the average value of soil available phosphorus in treatment 1 and treatment 2 increased, which were 5.88 and 6.88 times of the control, respectively. The period with the greatest difference from the control was the 90thd after branch simulated returning to the field. The available phosphorus contents of treatment 1 and treatment 2 were 6.91 and 7.24 mg/kg, which were 44.78 and 46.96 times higher than that of the control in the same period.

4 Discussion

The results of study showed that the content of soil organic matter increased significantly during the simulated decomposition of branch returning to the field. Compared with the control, the mean value of soil organic matter in treatment 1 and treatment 2 increased by 2.16 and 1.93 times, respectively. The period with the largest difference from the control was the 45thd after the simulated returning to the field in treatment 2, which was consistent with the previous research results. Findings of Wang Xueminetal.[19]showed that the direct straw returning, decomposing and straw biochar returning to the field combined with nitrogen fertilizer reduction can significantly increase the organic carbon content. However, the soil organic matter content showed a trend of first increasing and then decreasing during the decomposition of simulated branch returning to the field, because the organic carbon in the branches first decomposes in the soil to form organic matter, and organic matter is decomposed by microorganisms over time, which is consistent with the research results of Pan Jingetal.[20]that deep burial of straw increased the number of microorganisms and catalase, thereby promoting the oxidation of organic matter. The results showed that the content of total nitrogen in the soil increased during the decomposition of simulated branch returning to the field. During the indoor mixed culture period, compared with the control, the average value of soil total nitrogen in treatment 1 and treatment 2 separately increased by 61% and 59%. Both treatment 1 and treatment 2 reached the maximum difference with the control on the 30thd after simulated returning to the field, which is slightly different from the results of previous studies. The test results of Gao Jinhuetal.[21]showed that with the increase of the amount of corn stalks returned to the field, the total nitrogen content of the plow layer soil would increase, but the increase was small (up to 0.3%). The increase in this study compared with corn stalks is due to two reasons: one is that the nitrogen content in fruit tree branches is higher than that in corn stalks; the other is that the two act on different soil layers. Corn stalk returning to the field generally acts on the soil of the plow layer at 0-20 cm, while the Korla fragrant pear branches, the object of this study, act at a distance of 40 cm from the surface. Therefore, it can better absorb the nutrients in the returned organic matter. The research results showed that the total phosphorus content of treatment 1 and treatment 2 in the 40-60 cm soil layer did not increase significantly compared with the control in the same period. Compared with the control, the average value of total phosphorus in the treatment 2 increased by 2%, and the period of the highest soil total phosphorus content was the 75thd after the simulated returning to the field, which is consistent with previous findings of other scholars. Wang Xueminetal.[19]found that returning straw directly to the field can increase the total phosphorus content to varying degrees. Our results showed that compared with the control soil, the average values of alkaline hydrolysis nitrogen in treatment 1 and treatment 2 were all increased but not significantly, increasing by 1% and 4%, respectively. The period with the greatest difference from the control was the 150thd after the simulated return to the field in treatment 2, which is consistent with findings of Chang Dannaetal.[22]who found that after returning the grass seeds to the field to decompose, on the one hand, the input of exogenous organic matter was increased, and on the other hand, the leaching loss of nutrients was reduced, so that the available nutrients in the soil increased to a certain extent compared with before returning to the field and the control. Because organic materials are mainly decomposed by microorganisms in the soil, and the important condition for microbial proliferation is that the C/N needs to reach 25：1-30：1, and studies have shown that crops with a high carbon-nitrogen ratio are more difficult to decompose. In this study, the carbon-nitrogen ratio was too high due to the lack of nitrogen fertilization, which would result in an insignificant increase in the content of some alkaline hydrolysis nitrogen. Studies have shown that plant decomposition is positively correlated with temperature and moisture within a certain range[8]. The reason why the content of alkaline hydrolysis nitrogen first decreased and then increased during the decomposition may be that the efficiency of microbial activity changed from low to high with the increase of the external environment temperature. Studies have shown that the content of available nutrients in soil is rapidly increased in the early stage of decomposition, because it is released in large quantities in the form of ions and input into the soil in the early stage of decomposition[23], which is similar to our findings in this study. Compared with the control, the average value of available phosphorus in both treatment 1 and treatment 2 increased, which were 5.88 and 6.88 times of the control, respectively. The period with the largest difference from the control was the 90thd after the simulated return in treatment 2. The reason for the decreasing trend of available phosphorus content during the decomposition may be that as the branches further decompose, the easily released nutrients have been released, so the available phosphorus content is not increased in the later stage[8].

5 Conclusions

The contents of soil organic matter, nitrogen and phosphorus are increased during the pear branch decomposition, and the overall improvement effect was that the 10-year-old tree was better than the 5-year-old tree. The periods with the largest differences in total nitrogen, organic matter and available phosphorus content compared with the control were the 30th, 45thand 90thd after simulated returning to the field, respectively. The period with the highest content of total phosphorus and alkaline hydrolysis nitrogen was the 75thand the 150thd after simulated returning to the field, respectively.