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

Abstract 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.