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The purpose of this study was to examine the effects of long‐term tillage and residue management on C and N pools in a sandy loam soil. Corn (Zea mays L.) was grown continuously since 1991 in a factorial experiment with three tillage treatments, directly seeded no‐till (NT), shallowly harrowed reduced tillage (RT), and moldboard plowed conventional tillage (CT), and two levels of residue input, roots plus corn stover (high input, HI) and roots only (low input, LI). Soils were collected at two depths (0–5 and 5–20 cm) in the fall of 2007. The soil organic C (SOC) and total N pools were greater in the NT and RT treatments than in the CT treatment. Residue management had a marginal (P < 0.1) effect on SOC and total N pools. The change in the SOC pool (0–20‐cm depth) after 16 yr showed a net loss of −6.3 Mg C ha−1 from the CT treatment and net gains of 5.1 and 10.8 Mg C ha−1 in the NT and RT treatments, respectively. Microbial biomass and the potentially mineralizable C and N concentrations in the 0‐ to 5‐cm depth were greater in the soil from the RT and NT treatments than from the CT treatment, with higher concentrations in the HI than LI residue treatment for both soil depths. Microbial biomass and potentially mineralizable C and N were also correlated with the SOC pool, which supports the use of these labile fractions as indicators of management‐induced changes in the SOC pool.

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