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Core Ideas Soil C and N dynamics were still affected by past fertilization treatments (8 yr). Soil C and N dynamics were still affected by past tillage systems (3 yr). Ammonia‐oxidizing bacteria were affected by ammonium concentration and pH. Ammonia‐oxidizing bacteria and archaea were associated with N mineralization. Poultry litter (PL) is widely used as an alternative to inorganic fertilizers (IF) in conventional (CT) and no‐tillage systems (NT). The objective of this work was to evaluate the legacy effect of a 16‐yr factorial study that combined fertilization (PL or IF) and tillage (CT or NT) treatments on soil C and N pools and microbial communities. Soil samples were collected from the 0 to 5 and 5 to 20 cm depths after 8 and 3 yr of suspending fertilization and tillage, respectively. Samples were analyzed for total organic C (TOC), total organic N (TON), N mineralization rate (k0), and abundance of ammonia‐oxidizing bacteria (AOB) and archaea (AOA). The TOC and TON concentrations (range: 5.8–20.8 and 0.5–1.8 g kg−1, respectively) were affected by tillage system and fertilizer source. Legacies of fertilization and tillage affected k0 (range: 0.20–0.87 mg kg−1 d−1) which was correlated with TOC and TON (r2 ∼ 0.9). The abundance of AOA (5.3–7.6 log10 copies g−1) and AOB (7.1–8.9 copies log10 g−1) was greater in CT than NT in the 0 to 5 cm layer, whereas the opposite was observed in the 5 to 20 cm layer. Both AOA and AOB were associated with k0 (r2 = 0.5 and 0.8, respectively), suggesting the dependence of ammonia oxidizers on mineralization of the organic N pool. Our results indicate that 8 and 3 yr after fertilization and tillage treatments were suspended, soil C and N and microbial dynamics were still being influenced by previous management practices.

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Poultry litter (PL) is widely applied on grazing lands in Georgia. However, it is not clear how its long-term use affects soil microorganisms and their function. We examined changes in activity and community structure of ammonia-oxidizing archaea (AOA) and bacteria (AOB) in a grazing land with a history of PL application and compared it to treatment with urea ammonium nitrate (UAN). Soil samples (0-15 cm) were collected in 2009 (after 15 yr of PL application) and in 2013 (after 2 yr of no application). The abundance and community composition of ammonia oxidizers (AO) were determined with molecular techniques that targeted Nitrification potential (NP) was used for measuring their activity. Abundance of AO was significantly higher in PL (7.41 and 7.10 log copies g soil for AOB and AOA, respectively) than in UAN plots (6.82 and 6.50 log copies g soil for AOB and AOA, respectively) in 2009. This is consistent with NP, which was higher in PL (0.78 mg NO -N kg h) than in UAN (0.50 mg NO-N kg h) plots in 2009. The abundance of AO and NP decreased in 2013. There was no treatment effect on the composition of AO. Correlation analysis suggested that AOB were functionally more important than AOA, indicating the need to target AOB for efficient management of N in PL-receiving soils. Overall, the difference in nitrification between PL and UAN was mainly caused by the change in AO abundance rather than composition, and AO were not negatively affected by an increase in PL-derived trace metal concentrations.