Decomposition of plant material in Australian soils. I. The effect of quantity added on decomposition and on residual microbial biomass

Abstract

14C and 15N-labelled legume (Medicago littoralis) material was added in amounts ranging from 23.0 to 92.1 kg nitrogen ha-1 to a calcareous, sandy-loam topsoil and allowed to decompose under field conditions for up to four years. Characteristic curves for decomposition were obtained; residual organic 14C accounted for about 65% and 18% of input after four weeks and four years respectively. About one-half of the 15N in the added plant material remained in organic residues after four years. However, the amounts of residual organic 14C and 15N were only approximately proportional to the amounts originally added. The proportions of plant material 14C and 15N mineralized calculated from recovery data differed significantly with the amount of substrate added; the greater the amount of plant material added, the more extensive its decomposition. Carbonate in soil became labelled during plant residue decomposition. Levels of carbonate 14C were low, however, and were directly related to the total residual 14C levels, whether in soils sampled four weeks or four years after substrate addition. Carbonate 14C accounted for only 1.1% of total residual 14C. Biomass 14C and 15N in soil decreased significantly with time within a 2-4 year decomposition period, the percentage decrease being greater than for 14C and 15N in the corresponding non-biomass organic pool. Further, the proportions of substrate 14C and 15N retained as biomass 14C and 15N in soils decreased significantly with increasing amounts of added substrate. From a consideration of the turnover of biomass C and N during plant residue decomposition in soils, we concluded that stabilization of extracellular substrates and metabolites alone does not account for the greater percentage retention of organic residues when lower amounts of substrate are added, but that factors leading to the accumulation of higher proportions of substrate carbon and nitrogen in microbial biomass are important.