Turnover of organic matter in differently textured soils - II. Microbial activity as influenced by soil water regimes

To evaluate the effect of soil texture and soil water content on decomposition of organic carbon (OC), turnover of partially stabilized C-14-labelled ryegrass residues was studied at four matric potentials in twelve differently textured soils of similar origin and cropping history. Six soils were from a naturally occurring clay gradient and had 11, 16, 21, 31, 37 and 45% clay (termed NA1 to NA6). Three clay-amended soils (CL2, CL4, CL6) and three silt-amended soils (SI2, SI4, SI6) were prepared by adding clay or silt sized organomineral complexes extracted from the NA2 soil to a portion of the NA1 soil. After C-14-labelled ryegrass had decomposed for eight months under field-like conditions, soil cores were sampled, adjusted to four matric potentials (-30, -100, -500 and -1500 hPa) and incubated at 20 degrees C for 15 weeks. The content of native soil organic carbon (SOC) in the NA soils was not related to texture. The SOC content increased with clay and silt in the CL and SI soils because of OC contained in the applied size separates. The relative CO2-evolution from CL and SI soils was lower than from the texturally corresponding NA soils, indicating a slower turnover of C supplied with the clay separate than of bulk OC. Differences in the decomposability of native SOC and residues of C-14-ryegrass were better explained by soil moisture parameters than by soil textural composition. Within each set of soils, evolution of CO2 from native SOC was highly correlated with the volumetric water content. The same was true for (CO2)-C-14-evolution, but correlations were significantly improved when (CO2)-C-14 was related to water retained in soil pores with diameters > 0.2 mu m. This indicated that the water available for the turnover of residues from ryegrass and of native SOC was retained in different fractions of the pore volume. Our study suggested that water was the main factor in controlling turnover of SOC. Texture effects were indirect and expressed through soil structure which in turn defined the soil pore system and thus the ability of the soils to retain water of different availability to the decomposer organisms. (C) 1999 Elsevier Science B.V. All rights reserved.