Divergent Microbial Metabolic Limitations Across Soil Depths After Two Decades of High Nitrogen Inputs in a Primary Tropical Forest

Soil microorganisms play an important role in soil biogeochemical cycles and ecosystem stability. Elevated atmospheric nitrogen (N) deposition has greatly accelerated terrestrial N cycling processes and altered elemental stoichiometry of substrates, leading to changes in soil microbial metabolic limitation. However, it remains unclear how soil microbial metabolic limitation responds to long-term N additions in highly weathered tropical forests. Here, based on a two-decade N addition experiment in an N-rich primary tropical forest, we explored how chronic N additions affected soil microbial metabolic limitation across soil profiles. In contrast to the traditional view, our results demonstrated that long-term N addition had a depth-selective impact on phosphorus (P) limitation, which was enhanced at the surface soils but not at deeper soils. Soil microorganisms can acclimate to P limitation through downregulating microbial community abundance, where the relative abundance of actinomycetes could indicate P limitation status. We further found that chronic N addition alleviated microbial carbon (C) limitation through increasing soil dissolved organic C (DOC) contents at surface soil layers but intensified microbial C limitation at deeper soils. DOC contents could be the predictors of C limitation at surface soils. These findings suggest that long-term N deposition may drive varied biogeochemical consequences across distinct soil horizons, and it is necessary to consider depth-dependent microbial metabolic limitations while developing earth ecosystem models.