Variation in the C:N ratio of substrate mineralized during forest humus decomposition

Abstract

Laboratory incubations of sieved (<2mm) forest humus were used to study the response of C and N mineralization to perturbation. Considerable variation in the ratio of mineralized C to mineralized N was observed. This ratio widened with increasing temperature. At constant temperature, addition of P stimulated CO 2-C evolution and reduced NH 4 +-N production, also widening the C:N ratio of substrate mineralized. Addition of weak base stimulated mineralization of N more than C, reducing the C:N ratio of substrate mineralized. Addition of weak acid, mineral-N, or excessive amounts of water inhibited CO 2-C evolution while stimulating production of NH 4 +-N, resulting in a “negative correlation” between the two, and reducing the C:N ratio of substrate mineralized still further. Results were interpreted in terms of effects on microbial biomass. A relatively benign treatment (P addition) may promote microbial growth and respiration, reducing net N availability. A moderate perturbation (addition of weak base) favors new organisms growing partly at the expense of microbial necromass. These organisms will mineralize some necromass-N, increase net N mineralization, and reduce the C:N ratio of substrate mineralized. Under severe conditions (addition of acid) the C:N ratio of substrate mineralized approaches that of the microbial biomass itself, suggesting that the biomass is the primary substrate mineralized. Microbial mortality is likely to be a significant factor affecting the supply of N in field situations, and should be included in any general model of soil N mineralization processes.