ARS | Publication request: Carbon and Nitrogen Fractions in Dryland Soil Aggregates Affected by Long-term Tillage and Cropping Sequence

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 6, 2009
Publication Date: N/A

Interpretive Summary: In dryland soils in the northern Great Plains, conventional farming systems, such as spring tillage with spring wheat-fallow, have resulted in a decline in soil aggregation and organic matter by 30 to 50% of their original levels in the last 50 to 100 yr. While tillage reduced soil organic C and N concentrations, fallowing further decreased their levels by reducing the amount of crop residue returned to the soil and increasing the mineralization of soil organic matter during fallow. Therefore, improved management practices are needed to enhance soil aggregation and C and N fractions to reduce soil erosion and increase soil quality and productivity. We evaluated the 21-yr effect of tillage and cropping sequence combinations on C and N fractions in aggregates of a Dooley sandy loam at 0- to 20-cm depth in eastern Montana. Tillage and cropping sequences were no-tilled continuous spring wheat (NTCW), spring-tilled continuous spring wheat (STCW), fall- and spring-tilled continuous spring wheat (FSTCW), fall- and spring-tilled spring wheat-barley (1984-1999) followed by spring wheat-pea (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 4.75-2.00 mm aggregate-size class at 0- to 5-cm but was greater in STW-F than in STCW in 2.00-0.25 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 at 0- to 5-cm but MBN varied with treatments, aggregate-size classes, and soil depths. The NH4-N concentration was greater in FSTCW than in STW-F in 4.75-0.25 mm size class but PNM and NO3-N were greater in FSTW-B/P than in FSTCW in <2.00 mm size class. The SOC, STN, POC, PON, PCM, and PNM were greater in 4.75-2.00 mm than in <0.25 mm at 0- to 5-cm but MBC was greater in <0.25 mm than in 4.75-0.25 mm size class at both depths. Reduced tillage with annual cropping can improve soil aggregation, C and N sequestration, and microbial activities in 4.75-0.25 mm size class compared with the conventional system, such as STW-F. Because of greater proportion of aggregates and intermediate levels of organic C and N concentration, C and N can be sequestered mostly in small macroaggregates (2.00-0.25 mm size class) by using these management practices. Inclusion of pea in the crop rotation can increase N mineralization and availability in <2.00 mm size class.

Technical Abstract: 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, frigid, Typic Argiboroll) at 0- to 20-cm depth in eastern Montana. Tillage and cropping sequences were no-tilled continuous spring wheat (Triticum aestivum L.) (NTCW), 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 4.75-2.00 mm aggregate-size class at 0- to 5-cm but was greater in STW-F than in STCW in 2.00-0.25 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 at 0- to 5-cm but MBN varied with treatments, aggregate-size classes, and soil depths. The NH4-N concentration was greater in FSTCW than in STW-F in 4.75-0.25 mm size class but PNM and NO3-N were greater in FSTW-B/P than in FSTCW in <2.00 mm size class. The SOC, STN, POC, PON, PCM, and PNM were greater in 4.75-2.00 mm than in <0.25 mm at 0- to 5-cm but MBC was greater in <0.25 mm than in 4.75-0.25 mm size class at both depths. Reduced tillage with annual cropping can improve soil aggregation, C and N sequestration, and microbial activities in 4.75-0.25 mm size class compared with the conventional system, such as STW-F, and inclusion of pea in the crop rotation can increase N mineralization and availability in <2.00 mm size class.