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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.