Effects of different straw mulching amounts on soil organic carbon in apple orchards of the Loess Plateau

【Objective】Apple, a globally popular and economically valuable fruit crop, is cultivated across many regions. Apple orchard soils store significant amounts of soil organic carbon (SOC), which is crucial for maintaining soil fertility and plays an important role in the global carbon cycle. Growing concerns about soil health and climate change have turned the enhancement of SOC in apple orchards into a prominent research focus. Straw mulching, an environmentally friendly soil management practice, is widely recognized for its potential to improve apple orchard soils and raise SOC content. However, how different straw mulch amounts affect SOC remains unclear, and the key question of what level of straw mulch can achieve the most significant SOC enhancement remains unanswered. Given this, the present study focuses on apple orchards in the Loess Plateau region to investigate how different straw mulch amounts affect SOC and its fractions, and to identify the optimal straw mulch amount for SOC enhancement. The findings will provide a basis for effective soil management practices, improving orchard soil fertility and productivity while contributing to climate change mitigation.【Methods】This study was conducted in an experimental plot established in a“Red Fuji”apple orchard in Baota District, Yan’an City, in July 2021. The apple trees in this orchard were 15 years old, with a planting densi- ty of 4 m×3 m and a tree height of approximately 385 cm. The diameter at breast height of the trees was about 16.4 cm. Five plots with different straw mulch amounts (0, 100, 200, 300, and 400 g · m- 2 ) were set up in the orchard. These plots were subjected to wheat straw mulching treatments each year after apple harvest for the following three years. In October 2024, soil samples were collected from the 0-20 cm depth in each plot to determine SOC, particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and other soil physicochemical properties. To evaluate the effects of different straw mulch amounts on SOC fractions and other physicochemical properties, one-way analysis of variance was used to determine the significant differences in SOC, POC, MAOC, and other soil physicochemical properties among the different straw mulch amounts. Subsequently, Pearson correlation analysis was employed to explore the correlations between soil properties and SOC, POC, and MAOC. Finally, stepwise regression analysis was used to identify the key factors driving the dynamics of SOC, POC, and MAOC under straw mulch conditions. The application of this series of analytical methods aimed to gain a deep understanding of the mechanisms by which straw mulch affected soil carbon cycling in apple orchards.【Results】The results showed that straw mulching significantly increased the stocks of SOC, POC, and MAOC (P＜0.05). As the amount of straw mulch increased, the stock levels of these carbon fractions rose gradually, peaking at a mulch amount of 300 g ·m- 2 , with values of 22.1 g ·m- 2 for SOC, 8.1 g · m- 2 for POC, and 14.0 g · m- 2 for MAOC. Beyond this amount, further increases in straw mulch did not significantly affect these stock levels. Additionally, straw mulching also significantly improved the physicochemical and biological properties of the soil (P＜0.05). The enhancement of these properties was most pronounced at a straw mulch amount of 300 g·m-2 . At this level, the soil water content increased to 17.5%, the dissolved organic carbon content reached 164.4 mg · kg- 1 , the total nitrogen content was 0.86 g·kg-1 , the ammonium nitrogen content was 3.6 mg·kg-1 , the nitrate nitrogen content was 7.2 mg · kg- 1 , the available phosphorus content was 6.1 mg · kg- 1 , the microbial biomass carbon reached 189.8 mg·kg-1 , the β-1,4-glucosidase activity was 142.6 μmol g-1 h-1 , and the cellulose hydrolase activity was 72.34 μmol g-1 h-1 . Meanwhile, the soil bulk density decreased to 1.13 g cm- ³, the soil temperature decreased to 19.6 ℃, and the soil pH value decreased to 8.26. Correlation analysis showed that SOC, POC, and MAOC were significantly and negatively correlated with soil bulk density, temperature, and pH, while they were significantly and positively correlated with soil moisture, dissolved organic carbon, total nitrogen, ammonium nitrogen, nitrate nitrogen, available phosphorus, microbial biomass carbon, β- 1,4-glucosidase, and cellulose hydrolase activity (P＜0.05). Further analysis indicated that straw mulching mainly increased SOC stocks by increasing POC stocks, with a contribution rate ranging from 55% to 58%. Stepwise regression analysis showed that β-1,4-glucosidase was the key factor affecting SOC and MAOC stocks, while available phosphorus was the key factor affecting POC stocks.【Conclusion】 Overall, straw mulching significantly increases SOC, POC, and MAOC stocks, and the enhancement effect increases with the amount of mulch. Once the mulch amount exceeds 300 g ·m-2 , additional mulch does not significantly raise SOC, POC, and MAOC. Thus, 300 g·m-2 is the optimal mulch amount for increasing SOC in apple orchards on the Loess Plateau. This measure maximizes the carbon sequestration benefits of straw mulch while it effectively avoids resource waste, providing a scientific basis for the sustainable development of apple orchards.