THE EFFECT OF ADDED NITROGEN ON THE RATE OF DECOMPOSITION OF ORGANIC MATTER

TLDR

Adding nitrogen to decomposing organic matter typically has little or a negative impact on microbial activity, especially for recalcitrant, high‑C/N substrates, while it can stimulate degradation of low‑C/N material; this effect is attributed to altered decomposer competition, enzyme repression, browning precursor formation, and microbial luxury uptake. Nitrogen addition increases the formation of water‑soluble, brown, recalcitrant compounds and reduces humus production.

Abstract

Summary (1) N added to decomposing organic matter often has no effect or a negative effect on microbial activity, at least in the long term. More than 60 papers are cited in support of this statement. (2) The negative effect of N is mainly found with recalcitrant organic matter with a high C/N ratio (straw, wood, etc.), whereas a positive effect of N is common for easily degradable organic material with low C/N ratio. (3) The negative effect of N could be explained by: (i) N disturbs the outcome of competition between potent and less potent decomposers; (ii) through ‘ammonia metabolite repression’, N blocks production of certain enzymes, at least in basidiomycetes, and enhances breakdown of the most available cellulose, whereby recalcitrant lignocellulose accumulates; (iii) amino compounds condense with polyphenols and other decomposition products, forming ‘browning precursors’ which are toxic or inhibitory. (4) The effect of adding N may depend on the microflora present. (5) There are indications that some microorganisms have a ‘luxury uptake’ of N when it is present in sufficient amounts, thereby delaying N mineralization. (6) The addition of N seems to increase the formation of water‐soluble , brown, recalcitrant compounds, but to decrease the amount of humus formed.