[Impacts of freeze-thaw process on soil microbial nutrient limitation in slope farmlands of the Chinese Mollisol region].

Understanding the impacts of freeze-thaw action on soil microbial nutrient limitation can provide important support for sustainable utilization of black soil resources. We analyzed the impacts of freeze-thaw action on soil microbial nutrient limitation on a slope farmland located in a typical thick Mollisol region of Keshan County, Heilongjiang Province. We examined the responses of soil microbial nutrient limitation to soil erosion rates through measuring soil nutrient, soil microbial biomass, and soil enzyme activities before and after freeze-thaw under natural conditions, and estimated the soil erosion rates by ¹³⁷Cs tracing technology. The results showed that: 1) soil erosion rates of slope farmland ranged from 479.31 to 7802.33 t·km^-2·a^-1, with an average value of 2751.02 t·km^-2·a^-1. 2) Under freeze-thaw process, soil water-soluble organic nitrogen and microbial biomass carbon (MBC) of slope farmland significantly decreased by 27.9% and 37.3%, respectively. However, the freeze-thaw process did not affect soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), water-soluble organic carbon, soil available phosphorus, microbial biomass nitrogen and phosphorus. 3) Under freeze-thaw action, the activities of β-1,4-glucosidase, L-leucine aminopeptidase and β-1,4-N-acetyl-glucosaminidase significantly decreased by 43.2%, 11.0%, and 25.5%, respectively. The results of the enzyme quantification vector model indicated that soil microorganisms were limited by carbon and phosphorus availability. The freeze-thaw action weakened the relative carbon limitation of soil microorganisms and strengthened the phosphorus limitation. 4) The structural equation model analysis indicated that freeze-thaw action had a direct positive effect on relative phosphorus limitation and a negative effect on relative carbon limitation in soil microorganisms. Soil erosion rates had a direct negative effect on relative carbon limitation of soil microorganisms. 5) Soil erosion rates had significantly negative influences on SOC and TN before and after freeze-thaw, TP after freeze-thaw, MBC after freeze-thaw, and vector length before freeze-thaw. Overall, freeze-thaw action reduced the activities of soil carbon and nitrogen acquisition enzymes and further changed the resource limitation of soil microorganisms. Our results could improve the understanding of the mechanisms regarding freeze-thaw action impact on the limitation of soil microbial resource in the Chinese Mollisol region and provide scientific support for nutrient management of slope farmland.