Effects of different land use types on soil enzyme activities and their stoichiometric ratios in the black soil region of Northeast China

Soil enzyme activity is a key biochemical indicator of nutrient cycling in soil ecosystems, reflecting the activity of soil microorganisms and nutrient transformation processes. The aim of this study was to investigate the effects of different land use (farmland and forest land) on soil enzyme activities and their stoichiometric characteristics in the black soil region of Northeast China, and to analyze the relationship between soil enzyme activities and microbial nutrient limitation in response to soil physicochemical properties and environmental factors. The study area is located in the central part of the Northeast Plain, where soils are dominated by black soils, and is the core area for agricultural and forestry production. Through this study, it is expected to provide a theoretical basis for optimizing regional soil management patterns and improving soil nutrient cycling efficiency. In this study, agricultural and forest soil samples from the central Northeast Plain were selected and four extracellular enzyme activities related to carbon, nitrogen and phosphorus transformations were determined: β-glucosidase (BG), β-N-acetylamino glucosidase (NAG), leucine aminopeptidase (LAP) and acid phosphatase (AP). In addition, the stoichiometric ratios and microbial nutrient limitation of soil enzymes were analyzed, and the effects of environmental factors such as soil pH, ammonium nitrogen (NH₄⁺), and annual precipitation (MAP) on soil enzyme activities and their stoichiometric characteristics were assessed in combination with redundancy analysis (RDA) and correlation analysis. It was found that BG, NAG, LAP and AP enzyme activities were significantly higher in the forest land than in the farmland, by 74.5%, 108.7%, 80.1% and 31.9% (P<0.05), respectively. The results of redundancy analysis showed that pH was an important factor influencing regional soil enzyme activities (R²=0.74), and MAP and NH₄⁺ also played important roles in regulating soil enzyme activities, with their R² values of 0.31 and 0.30, respectively. The stoichiometric ratio analyses of soil enzymes showed that the carbon, nitrogen, and phosphorus-related enzyme stoichiometric ratios (EEA_{C: N: P}) was 1: 1.34: 1.58, and the ratio of nitrogen-phosphorus-related enzyme stoichiometry (EEA_{N: P}) was 0.84. Among them, both farmland and forest soils showed significant phosphorus limitation, and the phosphorus nutrient limitation of farmland soils was more prominent than that of forest soils, which indicated that phosphorus plays a key role in soil productivity enhancement and ecological cycling. The study showed that soil enzyme activity and microbial nutrient limitation were significantly affected by land use practices. The microbial phosphorus limitation of woodland soil was lower due to its rich organic matter and stable microecological environment. On the contrary, farmland soils were more phosphorus-limited due to long-term irrational cultivation and fertilization, which affected soil enzyme activity and nutrient cycling efficiency. pH, MAP and NH₄⁺ were the main environmental factors affecting soil enzyme activity. Based on the results of this study, ecologically adaptive soil management strategies, such as rational application of phosphorus fertilizer, optimal water management and reduction of anthropogenic disturbances, are recommended for the future to improve the efficiency of soil nutrient cycling and to promote the sustainable development of the regional ecosystem.