Microbial regulation of soil nitrogen dynamics along a stand density gradient of Mongolian pine plantations

Abstract

Establishing plantations represents an effective nature-based solution for enhancing terrestrial carbon sinks and mitigating climate change; however, plantation productivity and carbon sequestration capacity are generally constrained by soil nitrogen (N) availability. Stand density regulation is an important silvicultural practice for alleviating N limitation and maximizing carbon sequestration potential, yet the mechanisms through which stand density influences soil N availability and transformation rates remain insufficiently elucidated. Here, we aimed to explore these mechanisms by examining key N-cycling microbes and associated substrate availability. To address this aim, we selected 25 plots of 40-year-old Pinus sylvestris var. mongolica stands across a stand density gradient (350–1,500 ind·ha^–1). In these stands, we measured needle litter characteristics, soil physicochemical properties, soil microbial and ammonia oxidizer communities, and net N mineralization (N_min) and nitrification rates (N_nitri). We found that soil N availability decreased with increasing stand density, evidenced by the significantly negative relationships between stand density and soil nitrate concentrations and N_min and N_nitri. The decline in N_min was related to reduced β-N-acetylglucosaminidase activities with increasing stand density. Similarly, the decrease in N_nitri was associated with the negative effect of stand density on amoA gene abundance of ammonia-oxidizing bacteria. Collectively, these results suggest that stand density regulates soil N availability and transformation rates by mediating relevant microbial drivers and underscore the importance of stand density management in alleviating N limitation and enhancing plantation ecological functions.

Keywords

Ammonia oxidizer / Forest management / Nitrification / Nitrogen-cycling microbes / Nitrogen mineralization / Plant–soil interaction

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