Reduction of Nitrogen Fertilizer Requirements and Nitrous Oxide Emissions Using Legume Cover Crops in a No-Tillage Sorghum Production System

Abstract

Nitrous oxide (N2O) emission from denitrification in agricultural soils often increases with nitrogen (N) fertilizer and soil nitrate (NO3−) concentrations. Our hypothesis is that legume cover crops can improve efficiency of N fertilizer and can decrease N2O emissions compared to non–cover crop systems. The objectives of this study were to (a) evaluate the performance of summer leguminous cover crops in terms of N uptake and carbon (C) accumulation following winter wheat and (b) to quantify the effects of summer leguminous cover crops and N fertilizer rates on N2O emissions and grain yield of the subsequent grain sorghum crop. Field experiments were conducted in the context of a wheat-sorghum rotation for two seasons in Kansas. Treatments consisted of double-cropped leguminous cover crops following winter wheat harvest with no fertilizer applied to the following grain sorghum or no cover crop after wheat harvest and N fertilizer rates applied to the grain sorghum. The cover crops were cowpea (Vigna unguiculata L. Walp.), pigeon pea (Cajanus cajan L. Millsp.), and sunn hemp (Crotalaria juncea L.). The three N treatments (were 0, 90, and 180 kg·N·ha−1). Fallow systems with 90 and 180 kg·N·ha−1 produced significantly greater N2O emissions compared with cropping systems that received no N fertilizer. Emissions of N2O were similar for various cover crops and fallow systems with 0 kg·N·ha−1. Among cover crops, pigeon pea and cowpea had greater C accumulation and N uptake than sunn hemp. Grain yield of sorghum following different cover crops was similar and significantly higher than fallow systems with 0 kg·N·ha−1. Although fallow systems with 90 and 180 kg·N·ha−1 produced maximum sorghum grain yields, N2O emissions per unit of grain yield decreased as the amount of N fertilizer was reduced. We conclude that including leguminous cover crops can decrease N fertilizer requirements for a subsequent sorghum crop, potentially reducing N2O emissions per unit grain yield and providing options for adaptation to and mitigation of climate change.