Increasing the soil temperature to study global warming effects on the soil nitrogen cycle in agroecosystems

According to the GCMs air temperature will increase 3-5 degrees C above ambient in Central Europe. As one consequence the element turnover in terrestrial ecosystems should change; in particular, the large soil carbon and nitrogen pools are crucial because of their potential to further 'pollute' the globe with liquid and gaseous compounds. According to the goals of the International Geosphere-Biosphere Programme (IGBP) an ecosystem manipulation was carried out to increase the temperature of the top soil in an agroecosystem 3 degrees C above ambient and to study the effects on the nitrogen cycle. The experimental design, called HOTWORM, could be proved during a 3-month winter period to keep a constant temperature difference of 3 degrees C above ambient including freezing-thawing cycles. No changes in the soluble nitrogen pools could be measured at the end of the investigation, probably because of the shea heating period and the high spatial variability. However, N2O-N emissions were much higher in the unheated plot (0.233 kg ha(-1)) compared with the heated one (0.058 kg ha(-1)). This effect could be caused by more frequent freezing-thawing cycles in the unheated plots, which showed the highest emission rates. The importance of the reduced water content in the heated plot could not be evaluated in this investigation. Based on the experiences of this study a schedule for intensified field experiments was developed.