Conservation tillage induced changes in organic carbon, total nitrogen and available phosphorus in a semi-arid alkaline subtropical soil

A multi-year experiment was conducted to compare the effects of conservation tillage (no-till and ridge-till) with conventional plow tillage on organic C, N, and resin-extractable P in an alkaline semi-arid subtropical soil (Hidalgo sandy clay loam, a fine-loamy, mixed, hyperthermic Typic Calciustoll) at Weslaco, TX (26°9'N 97°57'W). Tillage comparisons were established on irrigated plots in 1992 as a randomized block design with four replications. Soil samples were collected for analyses 1 month before cotton planting of the eighth year of annual cotton (planted in March) followed by corn (planted in August). No-till resulted in significantly (p<0.01) greater soil organic C in the top 4 cm of soil, where the organic C concentration was 58% greater than in the top 4 cm of the plow-till treatment. In the 4-8 cm depth, organic C was 15% greater than the plow-till control. The differences were relatively modest, but consistent with organic C gains observed in hot climates where conservation tillage has been adopted. Higher concentrations of total soil N occurred in the same treatments, however a significant (p<0.01) reduction in N was detected below 12 cm in the ridge-till treatment. The relatively low amount of readily oxidizable C (ROC) in all tillage treatments suggests that much of the soil organic C gained is humic in nature which would be expected to improve C sequestration in this soil. Against the background of improved soil organic C and N, bicarbonate extractable P was greater in the top 8 cm of soil. Some of the improvement, however, appeared to come from a redistribution or “mining” of P at lower soil depths. The results indicate that stratification and redistribution of nutrients were consistent with known effects of tillage modification and that slow improvements in soil fertility are being realized.