Diversifying cropping sequence reduces nitrogen leaching risks

Abstract

• We monitored soil mineral N during various cropping sequences. • We simulated a 20-year series of N leaching for the cropping sequences. • We found an impact of the preceding cop on N leaching after the following crop. • Catch crop and oilseed rape volunteers reduced N leaching risk. • Diversified cropping sequences can reduce N leaching by up to 40 %. Overuse of chemical fertiliser in cereal cropping systems has resulted in severe degradation of air and water quality. Diversifying cropping sequence with legumes provides a natural source of nitrogen (N), but also increases N leaching risks after their growing period. Here, we hypothesize that legumes and other break crops, i.e., crops grown to diversify the cropping sequence, reduce N leaching at the rotation scale due to their contribution to increasing nutrient use efficiency and crop N recovery of the following cereal crops. In two 4-year experiments conducted in northern France, we monitored agronomic performance and the changes in the soil mineral N content at field scale in six preceding crop-current crop combinations including winter wheat ( Triticum aestivum ), pea ( Pisum sativum L.) and oilseed rape ( Brassica napus L.). We quantified N leaching after each crop as a function of the preceding crop with a water-fluxes model based on soil mineral N content, climate data and soil characteristics. We then simulated N leaching at the rotation scale, for 20 years of climate conditions and various cropping management systems. We show that growing pea or oilseed rape reduced soil mineral N content at harvest of the following cereals (up to mean values of −28 and −19 kg N ha −1 respectively), and N leaching risks during winter of the following cereals compared to the wheat-wheat cropping sequence. Although N leaching was higher during the winter after pea was cultivated, the cumulative losses over four experimental years of the pea cropping sequences were not significantly higher than the no-break cropping sequences. Over the 20 climate years, sequences including pea, oilseed rape, volunteers or catch crops reduced simulated N leaching by up to 40 % compared to wheat monoculture. Our study confirms that N leaching not only depends on the current crop but is also affected by the preceding crop. A large potential reduction in nitrogen leaching could be achieved in many intensive cereal-growing regions with very limited cropping sequence diversity.