Microbial biomass formed from 14C, 15N-labelled plant material decomposing in soils in the field

Abstract

The decomposition of 14C, 14N-labelled medic ( Medicago littoralis) material and the net formation and decay of isotope-labelled biomass have been measured in four South Australian soils in the field over 4 yr. The field sites were in similar climatic zones but two sites received about twice as much rainfall as the others. The soils were calcareous and of similar pH, but differed in texture and organic matter content. The decomposition of the organic- 14C and organic- 15N residues were, for a given site, similar. Initially, the concentrations of labelled residues decreased rapidly, then very slowly. Decomposition rates in a heavy clay soil were significantly less than in the other soils during the first 16 weeks after incorporation of plant material, but thereafter, rates of decomposition in all soils were similar, despite differences in soil texture and climate. More than 50% of the medic- 14C had disappeared from all soils after 4 weeks of decomposition and only 15–20% of the medic- 14C remained as organic residues after 4 yr. Of the medic- 15N 60–65% remained as organic residues after 32 weeks decomposition; the percentage decreased to 45–50% after 4 yr. The amounts of 14C, 14N-labelled biomass, formed from decomposing plant material, were maximal 4–8 weeks after incorporation of plant material into the soils. In samples taken at 8 weeks from the sandy Roseworthy soil, biomass- 14C and - 15N accounted for 14 and 22% respectively of the total organic- 14C and - 15N residues present. Thereafter in this soil, the concentrations of biomass- 14C and - 15N decreased, rapidly at first then more slowly. Nevertheless, throughout most of the decomposition the rates of decrease in the concentrations of biomass- 14C and - 15N exceeded those of the non-biomass, labelled organic residues. The proportions of 14C, 15N-labelled materials accounted for in the labelled biomass varied between soils. Soils of higher clay content generally retained higher proportions of residual organic- 14C and - 14N in the biomass, even though the net rates of decomposition of total labelled residues did not differ significantly between soils during most of the decomposition.