Soil moisture effects on gross nitrification differ between adjacent grassland and forested soils in central Alberta, Canada

Background and aims Changes in soil moisture availability seasonally and as a result of climatic variability would influence soil nitrogen (N) cycling in different land use systems. This study aimed to understand mechanisms of soil moisture availability on gross N transformation rates. Methods A laboratory incubation experiment was conducted to evaluate the effects of soil moisture content (65 vs. 100% water holding capacity, WHC) on gross N transformation rates using the N-15 tracing technique (calculated by the numerical model FLUAZ) in adjacent grassland and forest soils in central Alberta, Canada. Results Gross N mineralization and gross NH4+ immobilization rates were not influenced by soil moisture content for both soils. Gross nitrification rates were greater at 100 than at 65% WHC only in the forest soil. Denitrification rates during the 9 days of incubation were 2.47 and 4.91 mg N kg(-1) soil d(-1) in the grassland and forest soils, respectively, at 100% WHC, but were not different from zero at 65% WHC. In the forest soil, both the ratio of gross nitrification to gross NH4+ immobilization rates (N/IA) and cumulative N2O emission were lower in the 65 than in the 100% WHC treatment, while in the grassland soil, the N/IA ratio was similar between the two soil moisture content treatments but cumulative N2O emission was lower at 65% WHC. Conclusions The effect of soil moisture content on gross nitrification rates differ between forest and grassland soils and decreasing soil moisture content from 100 to 65% WHC reduced N2O emissions in both soils.