Despite decades of research on decomposition responses to elevated rates of atmospheric N deposition, the coupling between concurrently changing carbon (C) and nitrogen (N) cycles remains a key uncertainty in understanding feedbacks between the terrestrial C cycle and climate change. For example, existing coupled models do not consider the full suite of linked C-N processes, particularly belowground, that could drive future C-climate feedbacks. Most mechanistic studies of N effects on belowground C cycling have focused on litter decomposition, demonstrating that in many instances N accelerates the decomposition of labile litter pools, but slows that of more slowly cycling C. However, compared with litter, mechanistic studies of the effects of N on the dynamics of soil organic matter (SOM) are rare. We review what is known about the patterns and underlying mechanisms of N effects on litter decomposition, present a conceptual model of the pathways by which N might alter soil C dynamics, and evaluate evidence to support this model.
In our conceptual model, N interacts with climate and soil texture to influence both biochemical stabilization and physicochemical stabilization, and thus microbial decomposition of SOM. Microbial activity is the primary bottleneck to decomposition on annual timescales and is influenced by litter quantity and chemistry, microbial necromass, and microbial community composition and physiology (via enzymatic potentials). On the other hand, for physicochemically protected OM, microbial access is the primary bottleneck to decomposition, creating variation on timescales of years to millennia. Microbial access is influenced by physical protection of SOM in aggregates, and chemical protection of SOM on mineral surfaces as mediated by exchangeable cations and through direct adsorption. Nitrogen interacts with climate and soil texture to influence plant litter inputs in the short term, and soil aggregate formation in the long term. Additionally, N inputs influence SOM accessibility by altering cation-bridging that enhances chemical protection of SOM onto minerals and microaggregate formation, via soil acidification and leaching of base cations and increased solubility of hydrolyzing cations. As seen for litter, N addition generally increases the decomposition of more labile SOM, but decreases the decomposition of more slowly cycling SOM. However, the relative importance of N effects on biochemical and physicochemical stabilization in contributing to these patterns is uncertain, having been little evaluated.