Stimulation of organic N mineralization by N‒acquiring enzyme activity alleviates soil microbial N limitation following afforestation in subtropical karst areas

Stoichiometry of soil extracellular enzymes such as the ratios of carbon (C)‒, nitrogen (N)‒, and phosphorus (P)‒acquiring enzymes as well as their vector length and angle are used to reveal the biogeochemical equilibrium between microbial nutrient requirements and soil nutrient availability. However, the dynamics of soil extracellular enzymes activities and microbial N limitation following afforestation remain poorly understood in karst rocky desertification areas.

Soil samples were collected from Dodonaea viscosa plantations after 0, 10, 20, and 40 years of afforestation following the abandonment of croplands in a karst rocky desertification area, and a natural restored shrubland soil was served as the control. The activities of C, N, and P extracellular enzymes were measured and the stoichiometric and vector ratios of extracellular enzymes were calculated to quantify microbial nutrient limitation.

The stoichiometric ratio of soil C: N:P acquisition enzymes was 0.63:1.48:1.0, with a vector angle of 34.7 in croplands, indicating high microbial N limitation. Compared to the croplands, D. viscosa afforestation significantly increased soil C‒, N‒, and P‒acquiring enzyme activities and gradually increased C: N:P stoichiometric ratio and vector angle to 0.75:1.39:1–0.76:1.17:1 and 34.9–38.2, respectively, indicating that afforestation alleviated microbial N limitation. Furthermore, gross N mineralization and gross ammonium immobilization rates increased by 140‒278% and 340‒801% following afforestation due to the increase in soil organic C and total N contents and the > 2 mm soil aggregates. The vector angle positively correlated with β‒N‒acetylglucosaminidase activity, gross N mineralization, and gross ammonium immobilization. Notably, the vector angle of extracellular enzymes following 40‒year afforestation was still 6.63% lower than that of the shrubland soil.

Our results suggested that afforestation could substantially increase gross N mineralization through stimulating β‒N‒acetylglucosaminidase activity and gross ammonium immobilization, thereby reducing microbial N limitation. However, this limitation persists even following long‒term afforestation in karst rocky desertification areas.