Winter Cover Cropping Influence on Nitrogen in Soil

Winter cover crops may affect the short- and long-term N availability in soil depending on the quantity, quality, and degradation rate of biomass returned to the soil. We examined the effects of several cover crops on soil inorganic and organic N levels in a winter cover crop-silage corn (Zea mays L.) double-cropping system that was initiated in 1987. High biomass N concentrations (BMN) in the above- and belowground biomass of the leguminous cover crops corresponded to high levels of inorganic N and water-soluble N, but low levels of water-soluble C and carbohydrate compared with the nonleguminous cover crops. The BMN above which there was net N mineralization 4 wk after residue incorporation was 17.9 g N kg -1 . The organic N from the aboveground biomass degraded rapidly. The first-order rate constants for the degradation of organic N and C in the cover crops were significantly correlated. This, coupled with a significant correlation between the soil organic N (SON) levels and cumulative biomass C added, indicated the importance of biomass C inputs in organic N retention in the soil. The cover crops had variable short- and long-term effects on soil N availability. Whereas rye (Secale cereale L. cv. Tetra Petkus) and annual ryegrass (Lolium multiflorum Lam. cv. Billion) were ineffective in increasing soil inorganic N levels, they were more effective than hairy vetch (Vicia villosa Roth subsp. villosa), Austrian winter pea (Lathryrus hirsutus L.), and canola (Brassica napus L. cv. Santana) in increasing SON accumulation because of a higher biomass potential and a larger input of biomass C.