Dryland Cropping Systems Influence Microbial Communities in a Sandy Soil in a Semiarid Region.

This study evaluated sensitive soil quality parameters such as microbial biomass C and N (MBC, MBN) and enzyme activities (EAs) involved in C, P and S cycling as affected by dryland cropping systems under conventional (CT) and no-tillage (NT) practices since year 1 to 5.  The soil is an Olton sandy loam soil (Fine, mixed, superactive, thermic Aridic Paleustolls) with an average of 16.4% clay, 67.6% sand and 0.65 g kg^-1 of OM at 0-10 cm.  The cropping systems varied from low to high cropping intensity (CI) including cotton (Gossypium hirsutum) monocolture (Ct-Ct), grain sorghum (Sorghum bicolor)-cotton (Sr-Ct), cotton-winter rye (Secale cereale)-grain sorghum (Ct-Rye-Sr), and forage sorghum (haygrazer; another variety of Sorghum bicolor L.)-winter rye (Sr_f-Rye).  During the study, tillage treatments did not affect soil MBC, MBN and all EAs evaluated (i.e., β-glucosidase, β-glucosaminidase, α-galactosidase, alkaline phosphatase, phosphodiesterase, and arylsulfatase).  After 3 yrs, rotations with a winter cover crop history (Ct-Rye-Sr and Sr_f-Rye) enhanced soil MBN (1.7 times higher) and EAs (up to 2 times) compared to Sr-Ct.  After 5 yrs, Sr-Ct and Ct-Rye-Sr showed similar soil MBC, MBN, EAs, total C and organic C.  Our results revealed that it took 5 years to detect higher soil total C, MBC, and EAs under Sr-Ct compared to Ct-Ct.  This study demonstrated that differences in soil microbial communities and enzymatic activities are more affected by crop rotation than tillage management history for this sandy soil under dryland production. The significant improvements found in several soil properties (i.e., MBC, MBN, EAs of nutrient cycling, and soil organic C) as affected by a history of dryland cotton rotations with winter cover crops and/or higher biomass return crops (i.e., grain or forage sorghum) can represent changes in soil organic matter content and quality, nutrient cycling, and C sequestration in the semiarid Texas High Plains region.  It is very significant that these soil changes occurred despite summer crop failure (2003 and 2006) and lack of winter cover crops (2006) due to lack of precipitation in certain years.