Dryland soil carbon dynamics under alfalfa and durum-forage cropping sequences

Forages grown in rotation with or without cereals to sustain dryland soil water content and crop production may influence C dynamics. We evaluated the effect of alfalfa (Medicago sativa L.) and durum (Triticum turgidum L.)-annual forage cropping sequences on above- (stems+leaves) and belowground (roots) biomass C inputs and dryland soil organic C (SOC), particulate total C (PTC), microbial biomass C (MBC), and potential C mineralization (PCM) at the 0–120cm depth. Cropping sequences were continuous alfalfa (CA), durum-barley (Hordeum vulgare L.) hay (D-B), durum-foxtail millet (Setaria italica L.) hay (D-M), durum-Austrian winter pea (Pisum sativum L.)/barley mixture hay (D-P/B), and durum-fallow (D-F). The experiment was conducted in a Williams loam (fine-loamy, mixed, superactive, Typic Argiustoll) from 2002 to 2005 in eastern Montana, USA. Except in 2003, aboveground biomass yield and C content were lower in CA than in other treatments from 2002 to 2005. Similarly, belowground biomass yield and C content were lower in D-F than in other treatments from 2003 to 2005. In 2005, soil surface residue amount and C content were greater in D-F than in other treatments. The SOC at 0–15cm was greater in CA than in D-B and D-M. The PTC at 0–15cm was greater in CA than in other treatments, but varied with treatments at other depths. The PCM at 0–120cm was greater in CA than in other treatments. The MBC at 30–120cm was greater in CA and D-P/B than in D-B. The proportion of SOC in PTC, PCM, and MBC at 0–120cm was greater in CA or D-P/B than in D-B and D-F. Although aboveground biomass C input was lower, greater belowground biomass C and a relatively undisturbed soil condition probably increased C storage at the surface layer and microbial biomass and activity at the surface and subsurface layers under alfalfa than under annual durum-forage sequences. Greater aboveground biomass C and fresh residue accumulation during durum phase, however, increased C storage in the surface residue under durum-fallow than under other cropping sequences. Perennial forages, such as alfalfa, can increase dryland soil C sequestration and biological soil quality by increasing microbial biomass and activity compared with annual cropping systems due to greater belowground biomass C input and continuous root growth.