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 NO3- as compared with those under conventional tillage.
