Are decomposition and N release from organic mulches determined mainly by their chemical composition?

The influence of chemical composition on the decomposition and N release rates from samples of 11 organic mulches enclosed in nylon mesh bags was assessed under field conditions at the University of California, Riverside. Time was adjusted by temperature and the cumulative temperature-adjusted days (tad) were used to model the pattern of the decay and N release. The chemical composition of the mulches significantly affected their decay. In descending order of significance, the concentration of the polar extractable carbon fraction (CP) and the acid insoluble fraction (CAI) were significantly correlated with decomposition during the year of study. Correlation was positive with CP and N and negative with CAI (mostly lignin). The CP was selected as the best predictor for mulch decomposition during the early and intermediate phases of this process (36 and 195 tad), but CAI was selected as the best variable for predicting the fraction of the initial mulch mass remaining at the end of the study (397 tad). N was immobilized, as indicated by temporary increases in N masses in mulches above initial conditions, in shredded redwood, pine trimmings and in two of three compost mulches. Immobilization was most pronounced during the first 36 tad of the study, with a maximum rate that varied from 6 to 11.5% above the initial N concentrations. At the end of the study N releases ranged from 97% of initial N (grass clippings) to only 8% (one of the composts.) The CP was selected as the best predictor for N remaining in the four sampling dates (397 tad) and explained from 52 to 68% of the variation in N release as a percentage of initial N content.