TILLAGE-INDUCED SEASONAL-CHANGES IN SOIL PHYSICAL-PROPERTIES AFFECTING SOIL CO2 EVOLUTION UNDER INTENSIVE CROPPING

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. Reduced tillage increases SOM in the long term, but there is limited information on the in situ seasonal changes in soil physical and biological properties that affect SOM dynamics. Our objectives were to: (i) determine the effect of tillage (conventionally disked (CT) and no tillage (NT) in a sorghum (Sorghum bicolor (L.) Moench.)- wheat (Triticum aestivum L.)/ soybean (Glycine max (L.) Merr.) 2-year rotation sequence and a wheat/soybean double-cropping sequence on the seasonal dynamics and soil depth distribution of gravimetric soil water content (SWC), soil temperature, bulk density (BD) and water-filled pore space (WFPS); and (ii) relate soil CO2 evolution to changes in these physical properties. Treatments had been in place for 9 years at the beginning of sampling. 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 properties were measured 57 times during a 2-year period. Soil water content with NT was greater than with CT at the 0-50 mm depth during the fallow period of sorghum-wheat/soybean. In contrast to the surface, SWC at the 50-125 mm depth with CT was greater than or equal to that with NT in all crop sequences. Tillage reduced soil BD, especially at the 50-125 mm depth in all crop sequences, but exhibited large seasonal dynamics at the 0-50 mm depth. Greater temporal variation in SWC and BD occurred due to tillage effects. Soil temperature at 50 mm depth at sunrise averaged 1.2-degrees-C greater with NT than with CT during May, September and November, perhaps due to reduced heat loss with residue cover. The mean rate of soil CO2 evolution with CT was 1.55, 1.95 and 2.45 g CO2-C m-2 d-1 in sorghum-wheat/soybean, sorghum-wheat/soybean and wheat/soybean, respectively. The corresponding soil CO2 evolution with NT was 9% greater, 12% greater, and not different compared with CT, respectively. Large seasonal differences in soil CO2 evolution occurred with respect to tillage regime. Soil CO2 evolution was highly related to soil temperature, moisture and temperature-moisture interactions. Regression models of soil CO2 evolution on soil temperature, moisture and day of the season explained from 65 to 98% of the temporal variation, depending upon crop sequence, tillage regime and season. Day of the season was related to non-linear residue decomposition and a polynomial function that expressed crop root respiration and microbial respiration due to rhizodeposition. There were significant tillage and crop sequence interactions with soil temperature and moisture, 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 upon crop sequence, suggesting that the dynamics of C sequestration/mineralization had changed during the 10-year period.