SYMP 17-4 - Nitrogen limitation of decomposition and decay: How can it occur?

Thursday, August 10, 2017: 9:40 AM
Portland Blrm 251, Oregon Convention Center
Colin Averill, Department of Biology, Boston University, Boston, MA and Bonnie Waring, Department of Biology, Utah State University, Logan, UT

Nitrogen (N) limitation of terrestrial productivity is commonly observed across biomes, and soil N availability is assumed to control terrestrial productivity in many theories of ecosystem ecology. 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 litter with low N content organic matter decomposes more slowly. Furthermore, ectomycorrhizal fungi are thought to selectively mine organic N from soils, exacerbating N-limitation of decomposition and increasing soil carbon (C) storage. 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. Additionally, N-rich substrates may increase carbon use efficiency (CUE) of microbial growth, thereby increasing retention of C in the microbial biomass and ultimately, enhancing soil C storage by increasing the mineral sorption rate. These examples highlight the divergence in ideas regarding the role of N in regulating the processes of decomposition and soil C storage.


Here we present a mathematical synthesis of these concepts using a simple microbial model, resolving contrasting interpretations of the role of N in decomposition and soil C storage. We believe the discrepancy between patterns observed across gradients of litter stoichiometry vs. N addition experiments reflects differences in the nature of the N applied. Experimental N addition can drive acidification and osmotic stress, which can slow decomposition independent of any positive effect of relieving nutritional limitation of decomposition. Second, N-limitation of decomposition will increase C stored in particulate organic matter, but may decrease transfer rates of C to mineral surfaces due to a negative effect on microbial CUE. Hence, N availability may always limit the rate of decomposition, but the ultimate effect on soil C storage will depend on the relative contributions of particulate vs. mineral associated organic matter in a given soil profile.