Soil Respiration, Microbial Biomass and Nutrient Availability in Soil After Addition of Residues with Adjusted N and P Concentrations

Microbial activity and nutrient release are known to be influenced by organic matter properties, but it is difficult to separate the effect of C/N ratio from that of C/P ratio because in most plant residues both ratios are either high or low. An incubation experiment was conducted to investigate the effects of reducing the C/N and C/P ratios of slowly decomposable plant residues (young eucalyptus leaves, mature wheat straw, and sawdust) to those of rapidly decomposable residues (young kikuyu shoots) on soil respiration, microbial biomass, and N and P availability. The C/N and C/P ratios of the former were adjusted to 15 and 89, respectively, by adding N as (NH4)2SO4, P as KH2PO4 or both and residues were added at 10 g C kg−1 to a silt loam. Soil respiration was measured over 21 d; microbial biomass C (MBC) and available N and P were measured on days 0, 7, and 21. Compared to the unamended soil, addition of kikuyu increased cumulative respiration 20-fold, MBC concentration 4 to 8-fold, and available P concentration up to 4-fold, whereas the increase in available N concentration was small and transient. Cumulative respiration and MBC concentration were low in the sawdust-amended soil and were not influenced by reducing the C/N and C/P ratios. Cumulative respiration with original wheat and eucalyptus was 30%-40% of that with kikuyu. Reducing the C/N ratio alone or both C/N and C/P ratios increased cumulative respiration and MBC concentration 2-fold compared to the original wheat and eucalyptus, whereas reducing the C/P ratio had little effect. Throughout the experiment, the available N concentration after addition of residues with reduced C/N ratio increased in the following order of eucalyptus < wheat < sawdust. By independently lowering the C/N and C/P ratios, microbial activity was more limited by C and N than P. However, lowering the C/N ratio of very slowly decomposable sawdust had no effect on soil respiration and MBC concentration, suggesting that other properties such as concentration of poorly decomposable C compounds limited decomposition.