TILLAGE AND CROP EFFECTS ON SEASONAL DYNAMICS OF SOIL CO2 EVOLUTION, WATER-CONTENT, TEMPERATURE, AND BULK-DENSITY

Crop management practices impact soil productivity by altering the soil environment, which in turn affects microbial growth and decomposition processes that transform plant-produced C to soil organic matter (SOM) or CO2. Long-term reduced tillage increases SOM, but little is known about the seasonal dynamics of soil CO2 evolution as affected by tillage and crop. The objectives were as follows: (1) To determine the effect of tillage (conventionally disked (CT) and no tillage (NT)) in monoculture sorghum (Sorghum bicolor (L.) Moench,), wheat (Triticum aestivum L.), and soybean (Glycine max (L.) Merr.) on the seasonal dynamics and soil depth distribution of, gravimetric soil water content (SWC), soil temperature, bulk density (ED), and water-filled pore space (WFPS). (2) To relate soil CO2 evolution to changes in these parameters. The soil was a Weswood silty clay loam (fine, mixed, thermic Fluventic Ustochrept) located in southcentral Texas. Soil CO2 evolution, using the static chamber method with alkali absorption, and physical parameters were measured 57 times during 2 years. Soil water content was greater under NT than under CT during the fallow period in all crops, but not always during the growing season. Soil ED was reduced shortly after tillage, but increased to levels observed under NT during wet and cold intervals in the fallow period and during the growing season in all crops. Soil temperature at sunrise was 0.7-1.6 degrees C greater under NT than under CT during spring and autumn months in all crops. Mean soil CO2 evolution was greater during the growing season than during fallow in all crops. Mean soil CO2 evolution during the fallow period was not different between tillage regimes in sorghum (1.25 g CO2-C m(-2) day(-1)), but was greater under NT compared with CT in wheat (1.88 vs. 1.65 g CO2-C m(-2) day(-1)) and in soybean (1.04 vs. 0.86 g CO2-C m(-2) day(-1)). During the growing season, soil CO2 evolution was greater under NT compared with CT in sorghum (2.04 vs. 1.84 g CO2-C m(-2) day(-1)) and in soybean (2.08 vs. 1.58 g CO2-C m(-2) day(-1)), but was lower under NT compared with CT in wheat (2.09 vs. 2.52 g CO2-C m(-2) day(-1)). Mean soil CO2 evolution during both periods was related to soil organic C (SOC) under CT, but not under NT. The combined effects of soil temperature, SWC (or WFPS), temperature-water interactions, days after harvest during the fallow period, and days after planting during the growing season explained 73 to 92% of the seasonal variation in soil CO2 evolution depending upon tillage regime and crop. In situ soil CO2 evolution responded to soil temperature and SWC differently depending upon tillage and crop management, suggesting that C budgets of agroecosystems derived from climatic data alone could be misleading. Conversion from CT to NT increased C sequestration in soil, but soil under NT released the same or more C as CO2 depending on crop during Year 9 and Year 10 after initiation, suggesting that the dynamics of C sequestration/mineralization had changed during the 10 year period.