Combining theory and experiment to understand effects of inorganic nitrogen on litter decomposition.

It has been long recognised that mineral elements, and nitrogen in particular, play an important role in determining the rate at which organic matter is decomposed. The magnitude and even the sign of the effects are, however, not universal and the underlying mechanisms are not well understood. In this paper, an explanation for the observed decreases in decomposition/CO2 evolution rates when inorganic nitrogen increases is proposed by combining a theoretical approach with the results of a 6-year litter decomposition-forest nitrogen fertilisation experiment. Our results show that the major causes of observed changes in decomposition rate after nitrogen fertilisation are increases in decomposer efficiency, more rapid formation of recalcitrant material, and, although less pronounced, decreased growth rate of decomposers. This gives a more precise description of how inorganic nitrogen modifies decomposition rates than the previously loosely used "decrease in microbial activity". The long-term consequences for soil carbon storage differ widely depending on which factor is changed; stores are much more sensitive to changes in decomposer efficiency and/or rate of formation of recalcitrant material than to changes in decomposer growth rate.

The fate of 15N-labelled nitrate additions to a northern hardwood forest in eastern Maine, USA.

We followed the movements of 15N-labelled nitrate additions into biomass and soil pools of experimental plots (15×15 m each) in a mid-successional beech-maple-birch-spruce forest in order to identify sinks for nitrate inputs to a forest ecosystem. Replicate plots (n=3) were spray-irrigated with either 28 or 56 kg N ha-1 year-1 using 15N-labelled nitric acid solutions (δ15N = 344‰ ) during four successive growing seasons (April-October). The 15N contents of foliage, bolewood, forests floor and mineral soil (0-5 cm) increased during the course of treatments. Mass balance calculations showed that one-fourth to one-third of the nitrate applied to forest plots was assimilated into and retained by above ground plant tissues and surface soil horizons at both rates of nitrate application. Plant and microbial assimilation were of approximately equal importance in retaining nitrate additions to this forest. Nitrate use among tree species varied, however, with red spruce showing lower rates of nitrate assimilation into foliage and bolewood than American beech and other deciduous species.