5 years ago

Nitrogen limitation of decomposition and decay: how can it occur?

Colin Averill, Bonnie Waring
The availability of nitrogen (N) is a critical control on the cycling and storage of soil carbon (C). Yet there are conflicting conceptual models to explain how N availability influences decomposition of organic matter by soil microbial communities. Several lines of evidence suggest that N availability limits decomposition: the earliest stages of leaf litter decay are associated with a net import of N from the soil environment, and both observations and models show that high-N organic matter decomposes more rapidly. In direct contrast to these findings, experimental additions of inorganic N to soils broadly show a suppression of microbial activity, which is inconsistent with N limitation of decomposition. Resolving this apparent contradiction is critical to representing nutrient dynamics in predictive ecosystem models under a multitude of global change factors that alter soil N availability. Here, we propose a new conceptual framework, the Carbon, Acidity and Mineral Protection (CAMP) hypothesis, to understand the effects of N availability on soil C cycling and storage, and explore the predictions of this framework with a mathematical model. Our model simulations demonstrate that N addition can have opposing effects on separate soil C pools (particulate and mineral-protected carbon), because they are differentially affected by microbial biomass growth. Moreover, changes in N availability are frequently linked to shifts in soil pH or osmotic stress, which can independently affect microbial biomass dynamics and mask N stimulation of microbial activity. Thus, the net effect of N addition on soil C is dependent upon interactions among microbial physiology, soil mineralogy, and soil acidity. We believe our synthesis provides a broadly applicable conceptual framework to understand and predict the effect of changes in soil N availability on ecosystem C cycling under global change. This article is protected by copyright. All rights reserved.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1111/gcb.13980

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