Effect of soil warming and rainfall patterns on soil N cycling in Northern Europe

With climate change northern Europe is expected to experience extreme increase in air temperatures, particularly during the winter months, influencing soil temperatures in these regions. Climate change is also projected to influence the rainfall amount, and its inter- and intra-annual variability. These changes may affect soil moisture regimes, soil water drainage, soil nitrogen (N) availability and N leaching to aquatic environment and N₂O emissions to atmosphere. Thus it is important to study the effects of increased soil temperature and varying rainfall patterns on soil N cycling in arable land from temperate climates, which is a major source of N pollution. An open-field lysimeter study was carried out during 2008-2009 in Denmark on loamy sand soil (Typic Hapludult) with three factors: number of rainy days, rainfall amount and soil warming. Number of rainy days included the mean monthly rainy days for 1961-1990 as ‘normal' and half the number of rainy days of former as ‘reduced' treatments. Rainfall amount included mean monthly rainfall for 1961-1990 as ‘present' and the projected change in mean monthly rainfall for 2071-2100 as ‘future' treatments. Soil warming included increase in soil temperature by 5°C at 0.1m depth as ‘heated' and non-heated as ‘control' treatments. Automated mobile rain-out shelter and irrigation system, and insulated buried heating cables were used to impose the treatments. Soil warming, compared with unheated control, advanced winter wheat crop development, and increased the above-ground biomass and N uptake only during vegetative stage, but shortened the total crop growing period by 12 days without reducing the total above-ground biomass. Rainfall amount and rainy days treatments increased the drainage, 46% and 10%, respectively, but did not have additive effect on the drainage. In contrast, soil warming increased crop evapotranspiration (18%) and reduced drainage (41%). The projected future rainfall amount increased NO₃-N leaching (289%) compared with present rainfall amount. The study showed significant interaction between soil warming and rainfall amount (P <0.001) with heated plots reducing NO₃-N leaching both under present and future rainfall amount offsetting the adverse effect of increased future rainfall on NO₃-N leaching. Soil warming, compared to control, consistently increased the soil NO₃-N availability during the crop growing season and left higher levels of NO₃-N in the plough layer (19kgNha⁻¹) even after harvest of crop posing a potential risk of increased leaching in the following autumn/winter seasons. The results suggest that while the projected future rainfall patterns increase drainage and N leaching, warmer winters, on the contrary, seems to offset this effect through increased water and N removal by the advanced crop growth and development during winter.