Distribution of organic 14C and 15N in particle-size fractions of soils incubated with 14C, 15N-labelled glucose/NH 4, and legume and wheat straw residues

Abstract

Comparisons have been made of the recoveries and distributions of total and isotope-labelled microbial biomass C and organic C and N in particle-size fractions of a Vertisol and an Alfisol decomposing 14C, 15N-labelled glucose/NH 4; and of the Vertisol decomposing labelled legume and wheat residues. Recoveries after fractionation of soil weight, and of labelled organic C and N and biomass C, were high (> 96%) in most treatments. In both the strongly-aggregated Vertisol and the weakly-aggregated Alfisol, each amended with 14C-glucose/ 15NNH 4 and incubated for 3 days, the bimodal patterns of distribution of residual organic 14C and of biomass 14C were very similar to each other and to that of native biomass C. Major proportions of residual organic 14C and biomass 14C were located with the fine silt-size (20-2 μm) fraction, which, in the Vertisol under the conditions of soil dispersion, was dominated by clay microaggregates. A minor peak of residual organic 14C and biomass 14C was located in the fine sand-size (250-50 μm) fraction from both soils. Residual organic 15N was similarly bimodally-distributed. Nevertheless, the clay-size (2-0.1 μm) fraction of both soils accounted for higher proportions of organic 15N than of organic 14C, and in the case of the Alfisol, higher proportions of organic 14C than of biomass 14C. The relatively large differences in the cation exchange capacities of the clay- and fine silt-size components of the almost fully-dispersed Alfisol, in contrast to the similar cation exchange capacities of these size fractions in the partially-dispersed Vertisol, are considered to be contributory factors to the differences between the soils in residual 14C and 15N distributions. The more evident partial separation of biomass 14C, organic 14C and organic 15N in the fine particle size fractions of the Alfisol than in those of the Vertisol is thought to have arisen from the greater opportunities in the former soil for metabolic turnover and preferential adsorption of exocellular nitrogenous organics to its clay-size than its silt-size constituents. However, the cation exchange capacity of fractions was not the sole factor determining residue distribution throughout the total size-range of the fractions. Expressed as concentrations on a fraction CEC basis, organic 14C, biomass 14C and organic 15N were most concentrated in the coarse sand-size (> 250 μm) fraction of both soils (probably from the association of the labelled microbial residues with partly degraded plant fragments). In the cases of the Vertisol amended with 14C, 15N-labelled legume or wheat materials and incubated for 49 days, the distribution patterns of labelled organic residues were very similar to those respectively from the glucose/NH 4-amended Vertisol. The main exception was that the sand-size fractions from the plant residue-amended Vertisols were relatively enriched with total organic 14C, due very probably to the persistence of some undecomposed plant constituents. Both the similarities and differences in the distributions of labelled organic 14C and 15N and biomass 14C in soil particle-size fractions have been related to the influence of substrate and soil properties and of incubation time on the extent of decomposition of primary substrates and of C and N turnover. Broadly, the results suggest that there is a widespread and somewhat uniform early response of the soil microflora to a soluble substrate such as glucose and NH 4, i.e. the distribution of the new growth (and of immobilized 14C and 15N) within the soil matrix was strongly determined by the location of the existing microflora. For plant residue substrates (after an incubation period sufficient to allow turnover of the relatively unprotected components of biomass 14C), the distributions within particle-size fractions of labelled biomass and immobilized 15N were also broadly similar to that of native biomass C. In such circumstances, the distributions of biomass C and 14C in the fractions were probably determined by factors of substrate supply and protection from predation within the unfractionated soil matrix; factors which appeared to be unrelated to fraction CEC.