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

• Published in: Global change biology 2018-04, Vol.24 (4), p.1417-1427
• Main Authors: Averill, Colin, Waring, Bonnie
• Format: Article
• Language: English
• Subjects: Acidic soils; Acidity; Availability; Biological activity; Biomass; Carbon; carbon storage; Computer simulation; Cycles; Decay; Decomposition; Dynamics; Ecosystem models; Ecosystems; Environment models; Environmental changes; Frameworks; Interactions; Leaf litter; Mathematical models; Microbial activity; microbial ecology; Microorganisms; Mineral nutrients; Mineralogy; Nitrogen; Nutrient dynamics; nutrient limitation; Organic matter; Organic soils; Osmotic stress; Predictions; Soil; Soil chemistry; Soil dynamics; Soil environment; Soil mineralogy; Soil pH; Soil stresses; Soils; Storage
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.13980

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 the 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 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 that 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. Understanding nitrogen limitation of decomposition has direct consequences for understanding and forecasting soil and ecosystem carbon storage potential. Observational evidence suggests that nitrogen can limit microbial decomposition, but many nitrogen fertilization experiments directly contradict this conclusion. Here, we resolve this apparent contradiction by synthesizing covarying effects of nitrogen on microbial element limitation and soil acidity, and use a mathematical model to demonstrate how the net outcome for soil carbon storage depends on interactions between microbial phys