Nitrogen mineralization: Challenges of a changing paradigm

Until recently, the common view of the terrestrial nitrogen cycle had been driven by two core assumptions-plants use only inorganic N and they compete poorly against soil microbes for N. Thus, plants were thought to use N that microbes "left over," allowing the N cycle to be divided cleanly into two pieces-the microbial decomposition side and the plant uptake and use side. These were linked by the process of net mineralization. Over the last decade, research has changed these views. N cycling is now seen as being driven by the depolymerization of N-containing polymers by microbial (including mycorrhizal) extracellular enzymes. This releases organic N-containing monomers that may be used by either plants or microbes. However, a complete new conceptual model of the soil N cycle needs to incorporate recent research on plant-microbe competition and microsite processes to explain the dynamics of N across the wide range of N availability found in terrestrial ecosystems. We discuss the evolution of thinking about the soil N cycle, propose a new integrated conceptual model that explains how N cycling changes as ecosystem N availability changes, and discuss methodological issues raised by the changing paradigm of terrestrial N cycling.