Wheat and Soybean Rotations: Does It Improve Soil Health as Well as Enhance Crop Production?.

Understanding the impact of crop rotation on soil chemical, biochemical, and microbiological properties may strengthen our effort in maintaining soil health for sustainable agricultural production. Long-term effects of three different crop rotations on soil organic C, microbial biomass C, and activities of C- and N-transforming enzymes were investigated. The tested C-transforming enzymes included α- and β-glucosidase, α- and β-galacosidase, cellulase and invertase, and N-transforming enzymes included urease, β–glucosaminidase, nitrate reductase, L-glutaminase, L-asparaginase. Thirty-six soils were sampled in three different depths up to 30 cm from monocrop soybean, doublecrop soybean-wheat, and a modified double-cropped system. The modified doublecrop involved 3-crop per 2-year including early-season soybean/wheat/full-season soybean rotations. Results showed that cropping systems affected the activities of C- and N-transforming enzymes in similar way, suggesting that C- and N-cycling are closely coupled. Enzyme activities were generally lower in the modified doublecrop system and higher in the continuous soybean or wheat-soybean doublecrop rotations. All enzyme activities were significantly correlated with soil organic C content (r > 0.55***), suggesting that enzyme activities are sensitive and may be used to detect early changes in the soil ecosystem. With the exception of urease, L-glutaminase, and L-asparaginase, enzyme activities decreased with increasing soil depth. Crop rotation, however, did not affect microbial biomass C contents significantly. According to published data on economical evaluation of the same field experiment, higher enzyme activities and greater nutrient cycling capacity in the tested soils did not translate into greater yields and economical value. Further evaluation is warranted to guide our effort in developing soil and crop management practices that maximize both environmental and economical benefits.