Active fractions of organic matter in soils with different texture

Relationships between soil organic C (SOC), soil microbial biomass C (SMBC), mineralizable C and N during a 21 d incubation, and basal soil respiration (BSR) were evaluated on eight soil types from Texas that varied in soil texture (7–45% clay) and organic matter. The portion of SOC as SMBC increased with increasing clay content, whereas the relationships of mineralizable C and N and BSR to SOC were not affected by soil texture. The ratio of BSR-to-SOC averaged 1.4 ± 0.4 mg mineralizable C g−1 SOC d−1. The amount of mineralizable C and N and BSR per unit of SMBC, however, decreased with increasing clay content, indicating that the soil microbial biomass (SMB) was more active in coarse-textured soils than in fine-textured soils. The average specific respiratory activity was 29 mg mineralizable C g−1 SMBC d−1 with 10% clay and 11 mg mineralizable C g−1 SMBC d−1 with 40% clay. The C-to-N ratio of the mineralizable fraction was 10 ± 3 and not affected by soil texture. The established relationships between active soil organic matter (SOM) fractions and soil texture could be used in models predicting SOM turnover.