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

It has been long recognised that mineral elements, and nitrogen in particul ar, 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 th is paper, an explanation for the observed decreases in decomposition/CO2 ev olution rates when inorganic nitrogen increases is proposed by combining a theoretical approach with the results of a 6-year litter decomposition-fore st nitrogen fertilisation experiment. Our results show that the major cause s of observed changes in decomposition rate after nitrogen fertilisation ar e increases in decomposer efficiency, more rapid formation of recalcitrant material, and, although less pronounced, decreased growth rate of decompose rs. This gives a more precise description of how inorganic nitrogen modifie s decomposition rates than the previously loosely used "decrease in microbi al activity". The long-term consequences for soil carbon storage differ wid ely 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.