Soil Microbial and Biochemical Changes Associated with Reduced Tillage

Abstract The physical, chemical, and biological soil environment for reduced or no‐till farming differs greatly from that for conventional tillage. Evaluation of the soil microbial and biochemical environment greatly aids predictions of N availability to crop plants and in optimizing management practices for reduced and no‐till soils. Surface soils from long‐term no‐till and conventional tillage plots at seven U.S. locations were characterized for microbial and biochemical components. The counts of aerobic micro‐organisms, facultative anaerobes, and denitrifiers in the surface (0–7.5 cm) of no‐till soils were 1.14 to 1.58, 1.57, and 7.31 times higher, respectively, than in the surface of plowed soil. Phosphatase and dehydrogenase enzyme activities and contents of water and organic C and N in the surface of no‐till soil were also significantly higher than those for conventional tillage. However, at the 7.5‐ to 15‐cm and 15‐ to 30‐cm depths these trends were reversed and microbial populations, enzyme activities, and water and organic C and N contents were the same or higher for conventional tillage than for no‐till. The trends in microbial populations with both tillage treatments were closely paralleled by soil enzyme activities and were also regulated by soil pH and levels of organic C and N. The surface 0‐ to 7.5‐cm of no‐till soil contained more potentially mineralizable N—20 to 101 kg/ha—than did that of plowed soils. This increased labile N reserve is apparently related to the higher microbial biomass present under no‐till soils. Maximum aerobic microbial activity with conventional tillage extends to a greater depth than with no‐till. Microbial populations under no‐till decrease rapidly below the 7.5‐cm depth. At the 7.5‐ to 15‐cm depth counts of aerobic microor‐ganisms and nitrifiers were 1.32 to 1.82 times higher on the conventionally tilled soils. However, counts for facultative anaerobes and denitrifiers were 1.23 to 1.77 times higher for no‐till soil. Also, the proportion of the total aerobic population represented by facultative anaerobes and denitrifiers for no‐till was twice that for conventional tillage. Consequently the potential rate of mineralization and nitrification is higher with conventional tillage while that for denitrification is higher with no‐till. Microbial population counts and the relative abundance of various microbial types suggests that the bochemical environment of no‐till soils is less oxidative than that under conventional tillage. Changes in tillage and fertilizer management practices required for no‐till soils should reflect the increased potential for immobilization of surface applied N and the lower levels of plant available NO 3 ‐ as compared with those under conventional tillage.