Under-sown ryegrass cover crops mitigate increased nitrogen leaching risks in a warming climate: Evidence from a 34-year field study in south-west Sweden

The effects of repeatedly cultivating under-sown cover crops (CC) on nitrogen (N) leaching were investigated in a long-term field experiment in SW Sweden (1989–2023). Treatments with and without CC and with and without N fertilization were compared (CC_90N, 90N, CC_0N, and 0N). The impact of temperature, precipitation and the large-scale North Atlantic Oscillation index (NAOi) was also explored. N leaching was measured in separately tile-drained field plots in which also the yield of the main crop, biomass and N content of the cover crop, and soil nitrate were determined. The mean annual N concentration in drainage and the annual N leaching were significantly smaller in CC_90N than in 90N in 18 out of 34 years. A trend analysis showed that N concentrations in drainage increased significantly after 2010 in both treatments without CCs but not in those with CCs. The reduction in N leaching by growing CCs averaged 48 % (CC_90N) over the period and did not decrease over time. NAOi correlated with temperature and precipitation and showed a positive trend after 2010. NAOi was positively correlated with N leaching in 90N but not in CC_90N. Furthermore, NAOi was positively correlated with N content in CC biomass. Our results suggest that on-going climate change in Scandinavia and periods of high NAOi result in higher N mineralization and higher N leaching. The study also shows that under these circumstances, growing an under-sown ryegrass cover crop is an effective measure for environmental protection since its N uptake compensates for higher N mineralization. • An under-sown cover crop reduced N leaching on average by 48 % between 1989 and 2023. • N concentration in drainage increased after 2010 in treatments without cover crops. • Effect of the cover crops in reducing N leaching did not decrease with time. • Periods with excessive N were compensated for by higher cover crop N uptake. • Grain yield did not differ between treatments with and without a cover crop.