Linkage between microbial functional genes and net N mineralisation in forest soils along an elevational gradient

Soil net nitrogen (N) mineralisation, the difference between organic nitrogen mineralisation and mineral nitrogen immobilisation, changes with elevation, thereby determining plant productivity and soil N cycling along elevation gradients. However, it has yet to be established how different microbial functional genes influence the rate of soil net N mineralisation along such gradients. To address this deficiency in our current knowledge, we performed metagenomic sequencing to identify soil microbial functional genes encoding enzymes involved in N cycling at five forest sites along an elevational gradient spanning a range from 1503 to 3182 m above sea level. Our results indicate that the rate of net N mineralisation follows a unimodal pattern with increasing elevation, with a peak (0.18 mg kg⁻¹ d⁻¹) being detected at the mid-high elevation site. Further, we detected a significant correlation between the abundance of genes encoding enzymes involved in denitrification and ammonia assimilation pathways and net N mineralisation rate (p < 0.05). Moreover, we established that microbial species in the phyla Cyanobacteria, Acidobacteria and Planctomycetes, harbouring keystone functional genes, play a predominant role in determining the rate of net N mineralisation. Our findings also revealed soil substrate content (ammonium nitrogen and nitrite nitrogen, soil organic carbon, and C:N ratio) and soil environment (soil temperature and soil moisture) to be the major drivers of net N mineralisation in soil via their regulatory effects on the composition of microbial communities and functional genes. Our characterisation of the microbial metagenomic basis of net N mineralisation in forest soils accordingly highlights the importance of the combined contributions of soil microbial functional genes, soil substrate, and environmental factors in determining the cycling of N in forest soils at different elevations. Highlights: Net N mineralisation rate followed a unimodal pattern along an elevational gradient. Denitrification and ammonia assimilation genes were correlated with net N mineralisation rate. Cyanobacteria play a dominant role in determining net N mineralisation rate. Soil substrate and environment determine microbial functional gene distribution.