Effects of irrigation and nitrogen management on water and nitrogen use efficiency of irrigated corn

Abstract

The South Platte river basin of northeastern Colorado is an area of high agricultural production. Approximately 30% of the irrigated land is in corn production. Corn requires high levels of nitrogen (N) for maximum production. Recent studies have shown that incidence and levels of nitrate (NO3) in the ground water are increasing and also that many growers over-irrigate and overapply N fertilizer as insurance against poor yields. Nitrate is mobile in the soil environment and moves with water throughout the soil profile. Over-irrigation and/or over-fertilization may be exacerbating the NO3-ground water problem. Many soils in the area have a high clay content that are not thought to be conducive to NO3 leaching. Are growers benefiting from irrigation and N fertilizer applications beyond those recommended by university researchers, or are the over-applications costly to the grower and/or the environment? This study was undertaken to answer these questions. What effects, if any, do over-irrigation and over-fertilization have on com grain yield, N loss from the cropping system from denitrification and NO3 leaching, and do the recommended irrigation and N fertilizer applications lead to N losses and/or NO3 leaching on these clayey soils. To evaluate these scenarios, a two-part study was conducted. In the first part, com was grown under a factorial combination of three irrigation and four labeled N (15N) regimes in microplots. The high cost of 15N prohibits field scale research. Enrichment N fertilizer with 15N, a stable isotope of N, is a widely used and valuable technique in agricultural research for tracing movement of N fertilizers through the cropping system. In the second part, a field scale study was done with the same levels of irrigation and N fertilizer (unlabeled) as the 15N microplot study to evaluate the field scale irrigation and N fertilizer dynamics of the cropping system on these clayey soils. It was found that the recommended N fertilizer application produces maximum grain yields without contributing to NO3 leaching; that no benefit is derived from overirrigation or over-fertilization; and that over-irrigation significantly reduces the water use efficiency of the crop while over-fertilization significantly increases NO3 leaching, even on these clayey soils.