Soil nitrogen mineralization in mixtures of eastern gamagrass with alfalfa

The amount and rate of soil N mineralization often influences the productiv ity and persistence of a grass-legume mixture. This research investigated s oil N availability in monoculture and binary mixtures of alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.) with eastern gamagrass [Tr ipsacum dactyloides (L.) L.] on sandy and clay loam soils near Manhattan, K S. Soil inorganic N and in situ net N mineralization were monitored monthly during the growing seasons of 1996 and 1997, Soil inorganic N was two- to threefold higher with alfalfa, red clover, and gamagrass-alfalfa mixture th an with gamagrass in monoculture at the end of 1996. At the midseason of 19 97, soil inorganic N was three- to ninefold higher at the clay loam site, b ut at the sandy site, only alfalfa monoculture was three- to fivefold highe r than the other treatments in both years. Soils under alfalfa at both site s in 1997 had the highest cumulative net N mineralized (35-100 kg N ha(-1) yr(-1)), followed by the gamagrass-legume mixtures (15-62 kg N ha(-1) yr(-1 )) and then the gamagrass monoculture treatment (2-15 kg N ha(-1) yr(-1) A high correlation (r(2) > 0.9, P < 0.05) was found between C/N ratio of the aboveground biomass and the total net N mineralized in the 2nd yr for both sites, suggesting that litter quality is an important driving variable on N mineralization, Our results emphasize the importance of forage legumes in maintaining soil quality and productivity and quality of forage mixtures.