Pedosphere 27(1): 76--85, 2017
ISSN 1002-0160/CN 32-1315/P
©2017 Soil Science Society of China
Published by Elsevier B.V. and Science Press
Soil respiration, microbial biomass and nutrient availability in soil after addition of residues with adjusted N and P concentrations
T. T. NGUYEN and P. MARSCHNER
School of Agriculture, Food and Wine, The University of Adelaide, Adelaide 5005 (Australia)
ABSTRACT
      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.
Key Words:  C/N ratio, C/P ratio, microbial activity, nutrient immobilisation, nutrient mineralisation
Citation: Nguyen, T. T. and Marschner, P. 2017. Soil respiration, microbial biomass and nutrient availability in soil after addition of residues with adjusted N and P concentrations. Pedosphere. 27(1): 76-85.
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