Response of Extracellular Enzyme Stoichiometric Properties and Microbial Metabolic Limitations to the Ecosystem Transition Mode Employed in Red Jujube Economic Forests on the Loess Plateau

Soil carbon (C), nitrogen (N), and phosphorus (P) cycling and microbial metabolism limitations are key factors affecting nutrient cycling and vegetation development. Extracellular enzyme activity (EEA) plays a key role in carbon and nutrient cycling in ecosystems, and their activities can serve as indicators of microbial nutrient requirements. At present, there is insufficient research on the nutrient limitations of microorganisms during ecosystem transition in abandoned jujube forests on the Loess Plateau. Four modes were selected: jujube forest replanted with Pinus tabulaeformis (CP), with Platycladus orientalis (PO), with medicinal materials (MM), and with alfalfa (AL). An abandoned jujube forest (CK) was used as a control. Soil physical and chemical properties, microbial biomass carbon, nitrogen, and phosphorus, as well as changes in the activities of β-1,4-glucosidase (BG), leucine aminopeptidase (LAP), N-acetylglucosamine (NAG), and alkaline phosphatase (AP), were studied. Analysis of changes in soil microbial nutrient limitations was performed. Compared with those in the CK treatment, the activities of soil C, N, and P extracellular enzymes significantly increased (<i>p</i> < 0.05) in the forest transition treatments, and the C:N_EEA, C:P_EEA, and N:P_EEA ratios of extracellular enzymes tended to decrease. Within the treatments, the activities of soil C, N, and P extracellular enzymes decreased as the soil layer deepened, whereas the enzyme stoichiometric ratio increased as the soil layer deepened, with significant differences observed between the soil layers. The vector model was used to quantify nutrient limitations in microbial metabolism and revealed that microbial metabolism in surface soil was limited mainly by C and P and that in the 10-20 cm and 20-40 cm layers, soil microbial metabolism was limited mainly by C and N. Correlation analysis revealed that SOC, pH, MBC, and MBN were the main factors affecting soil extracellular enzyme activity. Mantel's test revealed that (NAG + LAP), AP, C:N_EEA, and C:P_EEA were important factors affecting vector length and angle. RAD analysis revealed that microbial properties had a greater impact on enzyme stoichiometry and microbial metabolic limitations than physicochemical indicators did. This study highlights the importance of vegetation in determining microbial metabolic processes and enhances our understanding of how ecological changes in jujube forests affect soil nutrient cycling and microbial metabolic constraints on the Loess Plateau. Forest transformation modes have important impacts on soil extracellular enzyme activity and microbial nutrient limitation.