For over a century, it was assumed that plants could only use inorganic sources of nitrogen and so N mineralization was seen as the focal process in the N cycle. Microbes metabolize organic substrates and release inorganic N as a waste product that plants could take up. Beginning in the 1980's however, that tidy picture began to unravel, leading to a newer model in which depolymerization--the extracellular fragmentation of nitrogenous polymers such as protein--became increasingly seen as teh rate-limiting step in the N cycle. This coincided with increased focus on the idea that plants might take up organic N directly, bypassing mineralization, and on the role of extracellular enzymes in driving soil processes. However, these parallel developments have challenges of their own that have become more apparent over the last decade. First, we still don't know, for any species of plant, how much of its in situ N-uptake is as organic N-compounds rather than as inorganic N. Second, while extracellular enzymes are clearly central to litter decay, there remain issues with interpreting the current standard potential activity measurements which use small freely mobile artificial substrates and with enzymes in mineral soil, where mineral activities (e.g. as oxidizers) and the nature of organic matter cloud the role of exoenzymes.
Results/Conclusions
In this overview talk, I will discuss developments in our understanding of the nature of internal cycling of N in litter and soils--how have they changed since the new "organic-centric" view of the N cycle crystalized a decade ago. What is current evidence of the role of organic N uptake by plants? What are some of the new tools to evaluate these questions? High on that list is microdialysis, which uses small, root-sized flow-through dialysis membranes to capture small molecules that are free to diffuse down a concentration gradient in the soil, and so to sample the soil in much the way a root does. Additionally, new approaches to modeling the soil system place increased emphasis on the specific mechanisms of decomposition, and hence on the balance between polymer breakdown and microbial activity. Ultimately, by focusing on the biogeochemical question of whether mineralization or depolymerization is the driver of soil N cycling, we are better able to integrate the multiple processes that weave together soil microbial communities, the physical/chemical structure of soil, and the critical processes of N cycling.