Contrasting effects of substrate and fertilizer nitrogen on the early stages of litter decomposition

Commonly observed positive correlations between litter nitrogen (N) concentrations and decomposition rates suggest that N frequently limits decomposition in its early stages. However, numerous studies have found little, if any, effect of N fertilization on decomposition. I directly compared internal substrate N and externally supplied inorganic N effects on decomposition in sites varying in soil N availability. I decomposed eight substrates (with initial %N from 0-2.5) in control and N-fertilized plots at eight grassland and forest sites in central Minnesota. N fertilization increased decomposition at only two of eight sites, even though decomposition was positively related to litter N at all sites and to soil N availability across sites. The effect of externally supplied N on decomposition was independent of litter N concentration, but was greater at sites with low N availability. The inconsistent effects of substrate and externally supplied N may have arisen because decomposers use organic N preferentially as an N source; because inorganic N availability across sites or with fertilization induced changes in microbial community attributes (for example, lower C:N or greater efficiency) that reduced the response of decomposition to increased inorganic N supply; or because the positive correlation between litter N or site N availability with decomposition was spurious, caused by tight correlations between litter or site N and some other factor that truly limited decomposition. These inconsistent effects of substrate N and external N supply on decomposition suggest that the oft-observed relationship between litter N and decomposition may not indicate N limitation of decomposition.