Dryland Soil Carbon Dynamics Under Durum-Forage Sequences.

Cropping sequence may influence dryland soil C sequestration and fractions. We evaluated the effect of four years of durum-forage sequences on dryland soil C fractions at the 0- to 120-cm depth in eastern Montana. Cropping sequences were conventionally-tilled durum-barley hay (D-B), durum-foxtail millet hay (D-M), durum-(Austrian winter pea + barley) hay (D-P/B), durum-fallow (D-F), and no-tilled continuous alfalfa (CA). Carbon fractions were soil total C (STC), particulate total C (PTC), microbial biomass C (MBC), and potential C mineralization (PCM). Total C input from above- and belowground crop residues and rhizodeposition were greater in D-F, D-M, and D-P/B than in D-B and CA. The STC was greater in CA than in D-B at 0- to 15-cm but was greater in D-B and D-M than in CA at 15- to 60-cm. Similarly, PTC was greater in CA than in other treatments at 0- to 15-cm but varied with treatments and soil depths at 15- to 120-cm. The PCM was greater in CA than in other treatments at 0- to 120-cm. The MBC varied with treatments and soil depths but was greater in CA than in D-B at 0- 120-cm. Although total C input was lower, no-tilled continuous alfalfa increased C sequestration at the surface soil and microbial biomass and activities at the surface and subsurface soils than other cropping sequences, probably due to increased root C input and reduced soil disturbance. In the conventional tillage system, durum followed by (pea + barley) hay increased microbial biomass and activities compared with durum followed by barley, probably due to reduced crop residue C input.