BIOMASS PRODUCTION AND NITROGEN ACCUMULATION IN PEA, OAT, AND VETCH GREEN MANURE MIXTURES

Interest in the use of green manures has revived because of their role in improving soil quality and their beneficial N and non-N rotation e ffects. This study evaluated biomass production, N content, radiation interception (RI), and radiation use efficiency (RUE) of pea (Pisum sa tivum L.), oat (Avena sativa L.), and hairy vetch (Vicia villosa Roth) mixtures. Treatments were a three-way factorial of pea genotype ('Cen tury' vs. 'Tipu'), pea planting density (90 vs. 224 kg ha(-1)), and cr opping mixture (sole-cropped pea vs. pea planted with a mixture of oat and hairy vetch). A mixture of oat and vetch without pea was also pla nted. Treatments were planted in early June on a Caribou gravelly loam (coarse-loamy, mixed, frigid Typic Haplorthods) in Presque Isle, ME, in 1993 and 1994. Biomass production and radiation interception were m easured by repeated sampling. Mixture biomass was affected by a year x pea density interaction: respective yields for mixtures containing lo w-density and high-density pea were 770 and 880 g m(-2) in 1993 vs. 82 0 and 730 g m(-2) in 1994. Mixture N content paralleled biomass produc tion and averaged 20.9 g m(-2) across all treatments. While pea sole c rops did not consistently produce biomass or N equal to three-species mixtures, the two-species mixture of oat and vetch did, yielding 820 g m(-2) of biomass and 21.7 g m(-2) of N, averaged over the 2 yr. Multi ple regression showed that 61% of the variability in mixture biomass p roduction was accounted for by a combination of early-season pea RI an d midseason total mixture RUE. Economic analyses showed that rotations including these green manures may be economically competitive with a conventional rotation of barley (Hordeum vulgare L.) undersown with cl over (Trifolium spp.) in a potato (Solanum tuberosum L.) production sy stem.