Tillage effects on phosphorus mineralization and microbial activity

Soil respiration (CO2 evolution and O-2 uptake), alkaline phosphatase activity, resin-extractable phosphate, and dissolved organic C dynamics were compared during a 36-day incubation of soil collected from within crop rows of conservation tillage (CT) (CT = no-till and ridge-till) and moldboard plow/disc tillage (PT) plots in a high-temperature, semiarid climate after 9 years. Significantly (P <0.05) greater total soil organic C, N, and extractable P were found in CT soils. Organic C was 14.2,13.4, and 10.3 g kg(-1) in the no-till, ridge-till, and PT treatments, respectively. Organic N was 1.41 1.26, and 0.95 g kg(-1), whereas resin-extractable P was 11.4, 9.3, and 6.4 mg kg(-1) for the same treatments. CO2 production in CT treatments was 916% and 758% greater, respectively, for the no-till and ridge-till treatments than for the PT treatment. Highest O-2 uptake occurred in the CT (NT and RT) treatments, which were 435% and 359% higher, respectively, than uptake in PT. Respiratory quotients (RQs) were similar among the CT treatments, but they were significantly (P <0.05) higher than those in PT. This suggests that there may be a difference in substrate composition caused by tillage, and consequent differences in microbial respiratory activity indicate that biological controls of phosphorus mineralization are becoming better established, although absolute changes in soil organic C and N concentrations were modest. Dissolved organic C ranged from a high of > 50 mu g C g(-1) soil in CT to < 20 mu g C g(-1) soil in PT. Significant correlations between soluble P and biological activity indices (respiration, RQ, and phosphatase activity) indicate that even modest improvements in soil organic matter can have measurable impacts on P mineralization. These results suggest there may be differences in soil substrate quality that result from conservation tillage and which lead to differences in microbial activities, as well as to improvements in P cycling and soil quality, from even modest gains in soil C in high-temperature subtropical soils.