Sensitivity of Labile Soil Organic Carbon to Tillage in Wheat‐Based Cropping Systems

To investigate the sensitivity of labile, or active, soil organic C (SOC), such as soil microbial biomass C (SMBC), mineralizable C, particulate organic matter C (POM C), dissolved organic C (DOC), and hydrolyzable C, to changes in management, we sampled soils in a 20‐yr experiment with tillage (no‐till [NT] and conventional tillage [CT]), cropping sequence, and N fertilization treatments in south‐central Texas. Sensitivity is defined as how rapidly soil properties respond to changes in management. No‐till significantly increased the size of SOC and all labile SOC pools compared with CT, especially at 0 to 5 cm. Intensified cropping also increased SOC and these labile pools, which generally decreased with depth. Labile pools were highly correlated with each other and SOC, but their slopes were significantly different, being lowest for DOC and highest for hydrolyzable C. In our study, SMBC was 5 to 8%, mineralized C was 2%, POM C was 14 to 31%, hydrolyzable C was 53 to 71%, and DOC was 1 to 2% of SOC. Model II orthogonal regression and simple linear regression both provided similar results, indicating that both methods were appropriate for evaluation of sensitivity to changes in management; however, using our proposed equation for sensitivity to tillage, no labile SOC pool was more sensitive than SOC. Further studies are needed to examine the effectiveness of this model.