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Nitrogen Fixation and Mineralization Potential of Winter Annual Legume Cover Crops for Reduced-tillage Organic Corn Production in North Carolina.

Parr, Mary. 2010. North Carolina State, M.Sc.

Abstract

Sixteen winter annual cover crop varieties were grown in North Carolina to determine biomass nitrogen (N) production and N fixation potential by termination dates compatible with a roller-crimper implement, and mineralization potential of residues following roll-kill. Cover crops were tested in a completely randomized block, with termination date as a strip plot, cover crop treatments included hairy vetch (Vicia villosa) cv. AU Merit, AU Early Cover, Winter Hardy Early Cover, and Purple Prosperity, common vetch (Vicia sativa) variety unstated, crimson clover (Trifolium incarnatum) cv. AU Sunrise, AU Robin, Dixie and Tibbee, Austrian winter pea (Pisum sativum) cv. Wistler and variety unstated, berseem clover (Trifolium alexandrinum) cv. Bigbee, subterranean clover (Trifolium subterraneum) cv. Denmark, and narrow leaf lupin (Lupinus angustifolius) cv. TifBlue78, and balansa clover (Trifolium michelianum) cv. Frontier as well as bicultures of rye (Secale cereale) and AU Merit, AU Early Cover, and Austrian winter pea. Termination occurred at in mid April, early May and mid May in 2009 and late April and mid to late May in 2010. Total biomass, N concentration, C:N ratios and 15 N natural abundance was determined for biomass at all roll times. Soil N and N flux was determined under rolled AU Early Cover and AU Sunrise with KCl extraction and ion exchange resins at two week intervals for a total of 12 weeks. Hairy vetch and crimson clover had greatest overall biomass of monocultures. Mixtures had greatest biomass in 2010. Crimson clovers were easiest to roll kill in late April, Hairy vetches and Austrian winter peas didn't roll kill until early - mid May, berseem clover and common vetch roll killed in late may. All cover crops derived between 70 and 100% of their atmosphere from the air, with the exceptions of lupin and subterranean clover. B-values for vetches and winter peas were significantly affected by nodulating rhizobium strains, with inoculant b-values giving the highest δ15 N values and piedmont b-values being the lowest. Corn response to cover crop mulches was significantly affected by roll time, with corn planting in mulches where kill was attempted too early suffering from competition and poor stands. Crimson, balansa and subterranean clover mulches resulted in poor corn yields despite relatively high levels of biomass N. In 2009, soil extract N and N flux from PRS probes was greater under hairy vetch biomass than under either 0N control or crimson clover, with peak soil N between 4 and 6 weeks after roll kill. Soil N under crimson clover mulches was lower in crimson clover than 0N, suggesting immobilization. In 2010, soil N and N flux was equal to 0N for all cover crop mulch treatments at Caswell. In piedmont, there was considerable variability, with hairy vetch having the greatest overall soil N of the cover crop treatments but this was equal to or lower than the 0N for most of the season. Corn yields were greatest in hairy vetch and 150N treatments in 2009 and in the150N and hv rye treatment in 2010. Yields in crimson clover were routinely less than or equal to that of 0N. N flux as measured by PRS probes showed decreases during times of low rainfall, whereas soil extract N increased during these periods, indicating that while mineral N was in the soil, dry conditions affected nutrient movement and therefore affected uptake by the ion resin PRS probes.