Soil Nitrogen Mineralization in Mixtures of Eastern Gamagrass with Alfalfa and Red Clover

Abstract

The amount and rate of soil N mineralization often influences the productivity and persistence of a grass–legume mixture. This research investigated soil N availability in monoculture and binary mixtures of alfalfa (Medicago sativa L.) or red clover (Trifolium pratense L.) with eastern gamagrass [Tripsacum dactyloides (L.) L.] on sandy and clay loam soils near Manhattan, KS. 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 than with gamagrass in monoculture at the end of 1996. At the midseason of 1997, soil inorganic N was three‐ to ninefold higher at the clay loam site, but at the sandy site, only alfalfa monoculture was three‐ to fivefold higher than the other treatments in both years. Soils under alfalfa at both sites 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 (r2 > 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.