Soil indicators of C and N transformations under pure and mixed grass-clover swards

The changes occurring in the soil organic status under grassland swards after 3 years were examined as a function of the initial soil nitrogen supplying capacity (SNSC: 135 or 225 kg N ha(-1) year(-1)) and of the soil cover: bare soil, pure white clover, pure rye-grass or a mixture of both species. The amounts of C and N of the coarse particle size OM, the microbial biomass-C, the C and N in water extracts (microbial metabolites) and the C and N mineralization in 28-day incubation tests were used as indicators of the transformation of the organic C and N inputs to the soil under these contrasting managements. The part of the Variation of these indicators explained by the initial soil N supplying capacity decreased from the metabolite-C to the coarse plant residues-C to the microbial biomass-C (MB-C) soil compartments. The part of the variation explained by plant cover increased in the same order. For each variable, the effect of soil fertility was maintained under pure grass and was reduced or suppressed under pure legume. The soil organic status under the mixed stand was similar to that of the pure clover and intermediate between that of the pure clover and the pure grass stands. The amount of N mineralization was similar with similar N amounts in the plant residues, whereas amounts of C in the plant residues and MB-C were in the order; pure clover<mixed stand<pure grass. Higher soil respiration and microbial specific respiration in incubation tests were seen in the pure grass stand. Soil organic status was examined as a function of the amounts of C and N and the C:N ratio of the organic inputs and from their impact on the microbial activity in situ. The ratio of C mineralization to N mineralization in laboratory incubations is proposed as an indicator of the dry matter and N harvests, of the degree of accumulation of the non-harvested C and N in the plant residues, and of the risk of nitrate leaching. The optimum value of this indicator, about 14, was obtained under the mixed award. (C) 1998 Elsevier Science B.V. All rights reserved.