Grass-legume bicultures as winter annual cover crops

Grass-legume bicultures as winter annual cover crops may combine the N scavenging ability of grasses and the biological N-2 fixation capacity of Legumes to improve N management in crop production systems of the southeastern USA. A 3-yr field experiment was conducted on a Norfolk loamy sand (fine-loamy, kaolinitic, thermic: Typic Kandiudults). The focus of this research was to examine the differences among legume monocultures and grass-legume bicultures with regard to early spring dry matter (DM) and N accumulation, and related effects on soil inorganic N levels and subsequent corn (Zea mays L.) yield. Austrian winter pea [Pisum sativum E. subsp. sativum var. arvense (L.) Poir.], crimson clover (Trifolium incarnatum L.), common vetch (Vicia sativa L.), and hairy vetch (Vicia villosa Roth) were grown in monoculture and in bicultures with rye (Secale cereale L.), oat (Avena sativa L.), and wheat (Triticum aestivum L.). Aboveground plant material was harvested in early March, late March, and mid-April. Biomass was separated into component species and analyzed for total N and C concentrations. Averaged over 3 yr, legume component DM accumulation in monoculture and biculture ranged from 0.87 to 2.53 Mg ha(-1), with a ranking of Austrian winter pea > hairy vetch > common vetch > crimson clover. For the same period, the grass component DM accumulation ranged from 1.31 to 2.28 Mg ha(-1), in the order rye = oat > wheat. Three year mean N accumulation values for the legume component followed the same relative ranking and ranged from 24 to 93 kg N ha(-1). Grass factor N content ranged from 18 to 39 kg N ha(-1) in the order rye > oat > wheat. For all bicultures, the average C:N ratio over the 3-yr experiment was <30, suggesting that net N mineralization would occur from the decomposing cover crop residues. Profile soil inorganic N (0 to 90 cm) was greater in legume monoculture than in grass-legume biculture: treatments, indicating the ability of grasses to capture sail N. Corn yield was affected by the treatments in 1 of 3 yr, with greater yields following a Legume monoculture than a grass-legume biculture. Collectively, these results suggest that grass-legume bicultures as winter annual cover crops have the potential to utilize residual soil NO2 and thereby minimize leaching while adding fixed N to cropping systems in the southeastern USA.