Maize and cover crop sequence in the Pampas: Effect of fertilization and water stress on the fate of nitrogen

Abstract

Cover crops are well known for their positive effects on erosion processes, soil organic matter, soil physical properties, weed populations and nitrate (NO₃) leaching. In this work, we evaluated the fate of nitrogen (N) from fertilizer in maize (Zea mays L.) and then in ryegrass (Lolium multiflorum) as cover crop, in the conditions of the Argentine Pampas. To this end, a field experiment was carried out at the School of Agriculture, University of Buenos Aires, Argentina (34°36′ S, 58°29′ W). The design of the experiment was factorial with three replications. We applied to maize two levels of N (0 and 140 kg N ha⁻¹ [125 lb N ac⁻¹] ammonium nitrate target with ¹⁵N) and two levels of water (50% and 100% of crop evapotranspiration). ¹⁵N was determined in both the soil and plants. Maize plants and the soil organic fraction were the main sinks of fertilizer N, depending on the water treatment. The N from fertilizer remaining as NO₃ in the soil (0 to 1.5 m [0 to 4.92 ft] depth) at maize harvest was 8% in plots subjected to water stress compared to 3% in the nonstressed. Nitrogen losses due to volatilization were minor. Total N (soil and fertilizer) accumulated in ryegrass tissues plus NO₃ remaining in the soil were higher in cover crop plots than in bare soil (130 versus 51 kg N ha⁻¹ [116 versus 45.5 lb N ac⁻¹]). The N in the soil organic matter originating from fertilizer significantly decreased between maize harvest and cover crop harvest. This soil organic N that originated from fertilizer mineralized at high rate (around 47% in six months), suggesting it was in more labile. This mineralized N can be subjected to potential losses during following months.