Soil Microbial Biomass and Organic Component Alterations in a No-Tillage Chronosequence

A no-tillage (NT) chronosequence that had been continuously cropped to maize (Zea mays L.) for 0, 1, 2, 4, 7, 9, 15, or 20 yr on a Westmoreland silt loam (fine-loamy, mixed, mesic Ultic Hapludalf) was examined for differences in microbial biomass, and soil organic C, N, P, and S. In the plowzone of the NT sites, biomass-C, total C (TC), soluble organic C (SOC), total Kjeldahl N (TKN), organic P (OP), and organic S (OS) levels were generally greater in the soil surface (0 to 7.5 cm) layers than in the 7.5- to 15-cm layers. In contrast, biomass-C under conventional tillage (CT or 0-yr NT site) in the soil surface layer was ∼50% of that in the 7.5- to 15-cm layer, whereas levels of the organic components were nearly identical. Biomass-C and organic component levels in the soil surface layers under NT were from 27 to 83% greater than those under CT. Opposite tillage method effects on these properties were usually found for deeper soil layers. Soil organic components, but not biomass-C, were significantly (p ≤ 0.05) related to years under NT in the soil surface layer. Only biomass-C was significantly (p ≤ 0.10) related to years under NT in deeper soil layers. When just “typical (i.e., nonmanured, moderate N-rate)” sites were included in the regression models, only biomass-C and SOC reservoir contents (total to 45 cm) varied significantly (p ≤ 0.10) with years under NT. Soil biomass-C reached a maximum (786 kg·ha furrow-slice⁻¹) in the soil surface layer after only 1 yr under NT, approaching a level nearly equivalent to that under an improved pasture, then equilibrated in about 10 yr to a level approximately 30% greater than that under CT. These observations suggest that continual NT induces a predictable dynamic in soil biomass-C, but not soil organic components, that is generally insensitive to a range of management differences. As a consequence, management practices designed to improve nutrient use-efficiency, especially by controlling microbial mineralization/immobilization activity, should not only take into consideration tillage method but years under the tillage method as well. Contribution from the USDA-ARS, North Atlantic Area.