Nitrogen dynamics and water relations in some eastern gamagrass-legume mixtures

This study was conducted to evaluate the effects of legume inclusion on some soil nitrogen parameters (inorganic soil N, net N mineralization), biological N fixation, and soil-plant-water relations (soil water content, plant water potential, leaf conductance) in mixtures of alfalfa (Medicago sativa) or red clover (Trifolium pratense) with eastern gamagrass (Tripsacum dactyloides). Biomass and N production of mixtures of eastern gamagrass with cicer miikvetch (Astragalus cicer), birdsfoot trefoil (Lolus corniculatus), and Illinois bundleflower (Desmanthus illinoensis) were also investigated. This study was conducted at two locations in Kansas (sandy and clay loam texture) for two consecutive years. Dry matter yield of eastern gamagrass was affected by the legumes in the second year, suggesting higher competition that year. Eastern gamagrass/legume mixtures had higher yields than the grass alone, and mixtures with cicer milkvetch and birdsfoot trefoil had the highest DM production. Total crude protein production by the mixtures was higher than the grass alone. Soil inorganic N was higher in the legume stands and mixtures than with the grass alone in both years, but it was lower at the site with sandy soil in the second year, probably related to the low precipitation of that year. Soils under alfalfa had the highest cumulative N mineralized at both sites, followed by the grass/legume mixtures, and by the grass alone treatment with the lowest value. Alfalfa and red clover monocultures had up to 63% of N fixed symbiotically, but legumes in mixtures had higher proportions of N derived from this process (up to 95%). The gamagrass/alfalfa mixture had the highest amount of N fixed (108 kg N ha{dollar}sp{lcub}-1{rcub}{dollar}), while red clover alone had the lowest (34 kg N ha{dollar}sp{lcub}-1{rcub}{dollar}). Proportions of N transferred to the grass ranged from 31 to 58%, with no significant differences between alfalfa and red clover. Total N transferred varied from 12 to 35 kg N ha{dollar}sp{lcub}-1{rcub}{dollar}, being higher at the clay loam site in the second year. Patterns of soil water content throughout the soil profile at the clay loam texture site followed the order: eastern gamagrass monoculture {dollar}>{dollar} mixtures {dollar}>{dollar} legume monocultures. However, at the other site this order was not followed, but gamagrass in monoculture still had the highest soil water content at the 40 to 120 cm depth. Although no consistent pattern was obtained, lower leafwater potential was measured on some dates in plants of gamagrass in mixture at the clay loam location, suggesting that higher interspecific competition occurred during those days. At both locations, legumes alone or in mixtures had similar leaf water potentials. No differences in leaf conductance were observed between legumes in mixture and monoculture, or gamagrass mixed or alone. Growing eastern gamagrass with legumes might be an excellent system for maximizing N cycling with no major effects on plant water status.