Tillage and cropping sequence may influence C and N sequestration, microbial activities, and N mineralization in dryland soil aggregates. We evaluated the 21-yr effect of tillage and cropping sequence combinations on C and N fractions in aggregates of a Dooley sandy loam (fine-loamy, mixed, superactive, frigid Typic Argiustolls) at the 0- to 20-cm depth in eastern Montana. Tillage and cropping sequences were no-tilled continuous spring wheat (NTCW) ( Triticum aestivum L.), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley ( Hordeum vulgare L.) (1984-1999) followed by spring wheat-pea ( Pisum sativum L.)(2000-2004)(FSTW-B/P), and spring-tilled spring wheat-fallow (STW-F). Carbon and N fractions were soil organic C (SOC), total N (STN), particulate organic C and N (POC and PON), microbial biomass C and N (MBC and MBN), potential C and N mineralization (PCM and PNM), NH 4-N, and NO 3-N. Aggregate proportion was greater in NTCW than in FSTCW in the 4.75- to 2.00-mm aggregate-size class at 0 to 5 cm but was greater in STW-F than in STCW in the 2.00- to 0.25-mm size class at 5 to 20 cm. After 21 yr, STW-F reduced SOC, STN, POC, and PON concentrations in aggregates by 34 to 42% at 0- to 5-cm and by 20 to 32% at 5- to 20-cm compared with NTCW and STCW. The PCM and MBC were greater in NTCW and STCW than in STW-F in the
