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

Crop management practices impact soil productivity by altering the soi l environment, which in turn affects microbial growth and decompositio n processes that transform plant-produced C to soil organic matter (SO M) or CO2. Reduced tillage increases SOM in the long term, but there i s limited information on the in situ seasonal changes in soil physical and biological properties that affect SOM dynamics. Our objectives we re to: (i) determine the effect of tillage (conventionally disked (CT) and no tillage (NT) in a sorghum (Sorghum bicolor (L.) Moench.)- whea t (Triticum aestivum L.)/ soybean (Glycine max (L.) Merr.) 2-year rota tion sequence and a wheat/soybean double-cropping sequence on the seas onal dynamics and soil depth distribution of gravimetric soil water co ntent (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 b eginning of sampling. The soil was a Weswood silty clay loam (fine, mi xed, 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 perio d. 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 t o 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 variat ion 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 wi th CT during May, September and November, perhaps due to reduced heat loss with residue cover. The mean rate of soil CO2 evolution with CT w as 1.55, 1.95 and 2.45 g CO2-C m-2 d-1 in sorghum-wheat/soybean, sorgh um-wheat/soybean and wheat/soybean, respectively. The corresponding so il CO2 evolution with NT was 9% greater, 12% greater, and not differen t compared with CT, respectively. Large seasonal differences in soil C O2 evolution occurred with respect to tillage regime. Soil CO2 evoluti on was highly related to soil temperature, moisture and temperature-mo isture interactions. Regression models of soil CO2 evolution on soil t emperature, 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 decomp osition and a polynomial function that expressed crop root respiration and microbial respiration due to rhizodeposition. There were signific ant tillage and crop sequence interactions with soil temperature and m oisture, suggesting that C budgets of agroecosystems derived from clim atic data alone could be misleading. Conversion from CT to NT increase d 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 perio d.