Land-use type affects gross nitrogen transformation and this information is particularly lacking under varied low temperature conditions. In this study, the effects of land-use type (forest vs. grassland) and temperature (10 vs. 15°C) on gross N transformation rates under aerobic conditions were investigated using the 15 N isotope pool dilution technique in the laboratory. Soils were collected from forest and grassland sites in China and Canada. The results showed that gross N mineralization and immobilization rates were significantly higher in forest soils than in grassland soils, while the reverse was true for gross nitrification rates. The higher TC and lower SOCw concentrations in the Chinese soils relative to the Canadian soils were related to the greater gross N mineralization rates and lower gross N immobilization rates in Chinese soils. The greater gross N mineralization rates and lower gross N immobilization rates resulted in much higher inorganic N accumulation and that may increase the risk of NO 3 − leaching in the Chinese soils. Increasing temperature significantly increased gross nitrification rates in grassland soils and gross N immobilization rates in forest soils, suggesting that grassland soils maybe more vulnerable to N loss through NO 3 − leaching or denitrification (when conditions for denitrification exist) and that conversion of grassland to forest soils may exert less negative effects on the environment by promoting the retention of N and decreasing the production of NO 3 − and subsequently the risk of NO 3 − leaching under increasing temperature by global warming.

Purpose Land use type is an important factor influencing greenhouse gas emissions from soils, but the mechanisms involved in affecting potential greenhouse gas (GHG) emissions in different land use systems are poorly understood. Since the northern regions of Canada and China are characterized by cool growing seasons, GHG emissions under low temperatures are important for our understanding of how soil temperature affects soil C and N turnover processes and associated greenhouse gas emissions in cool temperate regions. Therefore, we investigated the effects of temperature on the emission of N 2 O, CO 2 , and CH 4 from typical forest and grassland soils from China and Canada. Materials and methods The soils were incubated in the laboratory at 10°C and 15°C under aerobic conditions for 15 days. Results and discussion The results showed that land use type had a large impact on greenhouse gas emissions. The N 2 O emissions were significantly higher in grassland than in forest soils, while CO 2 emissions were higher in forest than in grassland soils. Grassland soils were weak sources of CH 4 emission, while forest soils were weak sinks of atmospheric CH 4 . The global warming potential of forest soils was significantly greater than that of grassland soils. Soil pH, C/N ratio, and soluble organic carbon concentrations and clay content were dominant factors influencing the emissions of N 2 O and CO 2 , respectively. Increasing temperature from 10°C to 15°C had no effects on CH 4 flux, but significantly increased N 2 O emissions for all studied soils. The same pronounced effect was also found for CO 2 emission from forest soils. Conclusions Indications from this study are that the effects of land use type on the source-sink relationship and rates of GHG emissions should be taken into consideration when planning management strategies for mitigation of greenhouse gas emissions in the studied region, and temperature changes must be taken into account when scaling up point-or plotbased N 2 O and CO 2 flux data to the landscape level due to large spatial and temporal variations of temperature that exist in the field. The reader is cautioned about the limitation with incubation studies on a limited number of samples/locations, and care need to be exercised to extrapolate the result to field conditions.