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 v aried in soil texture (7-45% clay) and organic matter. The portion of SOC as SMBC increased with increasing clay content, whereas the relati onships of mineralizable C and N and BSR to SOC were not affected by s oil texture. The ratio of BSR-to-SOC averaged 1.4 +/- 0.4 mg mineraliz able C g(-1) SOC d(-1). The amount of mineralizable C and N and BSR pe r unit of SMBC, however, decreased with increasing clay content, indic ating that the soil microbial biomass (SMB) was more active in coarse- textured soils than in fine-textured soils. The average specific respi ratory activity was 29 mg mineralizable C g(-1) SMBC d(-1) with 10% cl ay 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 organ ic matter (SOM) fractions and soil texture could be used in models pre dicting SOM turnover. Published by Elsevier Science Ltd