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SOIL BIOLOGY & BIOCHEMISTRY

Carbon and Nitrogen Fractions in Dryland Soil Aggregates Affected by Long-term Tillage and Cropping Sequence

USDA-ARS, Northern Plains Agricultural Research Lab., Sidney, MT 59270

^* Corresponding author (upendra.sainju{at}ars.usda.gov).

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), NH₄–N, and NO₃–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 <2.00-mm size class at 0 to 5 cm but MBN varied with treatments, aggregate-size classes, and soil depths. Compared with other treatments, NH₄–N concentration was lower in STW-F in the 4.75- to 0.25-mm size class but PNM and NO₃–N were greater in FSTW-B/P in the <2.00-mm size class. Long-term reduced tillage with continuous spring wheat increased soil aggregation, C and N sequestration, and microbial biomass and activities in all aggregates but increased tillage intensity with spring wheat-barley/pea rotation increased N mineralization and availability in small aggregates compared with the conventional STW-F.

Abbreviations: FSTW-B/P, fall- and spring-tilled spring wheat-barley (1984–1999) followed by spring wheat-pea (2000–2004) • FSTCW, fall- and spring-tilled continuous spring wheat • MBC, microbial biomass C • MBN, microbial biomass N • NTCW, no-tilled continuous spring wheat • PCM, potential C mineralization • PNM, potential N mineralization • POC, particulate organic C • PON, particulate organic N • SOC, soil organic C • STW-F, spring-tilled spring wheat-fallow • STCW, spring-tilled continuous spring wheat • STN, soil total N