Microbial assimilation of 14C of ground and unground plant materials decomposing in a loamy sand and a clay soil

The influence of grinding plant materials on the microbial decomposition and the distribution of plant-derived carbon in soil was measured. Ground and unground, 14C-labelled subclover leaves (Trifolium subterraneum) were added to a loamy sand and a clay soil and incubated for 42 d at 25 degrees C. More 14C and N were mineralized and less microbial biomass 14C accumulated in soils amended with unground than with ground subclover leaves. Differences in the amounts of 14CO2 and biomass 14C were established during the initial 7 days of decomposition. At this time, biomass 14C in the two soils accounted for 18-19% of the input 14C of unground leaves, and 22% of that of ground leaves. After 7 days of decomposition of subclover 14C, aliquots of the soils were gently dispersed and particles of diameters > 50 micrometer and < 50 micrometer were separated by wet sieving. Biomass 14C in the fraction of particle sizes > 50 micrometer accounted for 5-6% of input 14C in the clay soil and 8% of input 14C in the loamy sand; the proportions were little affected by grinding of the clover leaf amendment. In contrast, the amounts of biomass 14C in the fraction of particle sizes < 50 micrometer were larger with ground than with unground leaves added to soils. Thus, the increased amounts of biomass 14C in soils amended with ground leaves were mainly associated with clay plus silt size particles and microaggregates. After 7 d of decomposition, non-biomass 14C in the two soil fractions accounted for about 40% of input 14C, irrespective of soil type and particle size of the plant residue amendment. The early (< 7 days) decomposition of the subclover leaves was compared with that of ground and unground, 14C-labelled wheat (Triticum aestivum) leaves and with that of [14C]glucose. After 7 d decomposition, there was no significant effect of grinding of the wheat leaf material on the release of 14CO2, but as with subclover leaves, less biomass 14C was formed in the soils amended with unground wheat material. The initial (1-2 days) release of 14CO2, from glucose exceeded that from wheat and subclover leaves. However, within 4 days in the clay soil, the rate of evolution of 14CO2 was least from glucose. Thus, the glucose-derived residual 14C, which was mainly present in microbial biomass, was to a higher degree retained in soil than residual 14C from the decomposing plant materials. It is suggested that grinding of plant materials favours, on their addition to soil, a more intimate contact between the plant constituents and the soil matrix, thereby enhancing opportunities for the colonization by decomposer organisms that are more protected against predation. The greater association of protected cells with the soil matrix results in a higher retention of substrate-derived C in the microbial biomass and a less extensive turnover of substrate C.