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Soil moisture changes, arising from seasonal variation or from global climate changes, could influence soil nitrogen (N) transformation rates and N availability in unfertilized subtropical forests. A N dilution study was carried out to investigate the effects of soil moisture change (30-90 % water-holding capacity (WHC)) on potential gross N transformation rates and NO and NO emissions in two contrasting (broad-leaved vs. coniferous) subtropical forest soils. Gross N mineralization rates were more sensitive to soil moisture change than gross NH immobilization rates for both forest soils. Gross nitrification rates gradually increased with increasing soil moisture in both forest soils. Thus, enhanced N availability at higher soil moisture values was attributed to increasing gross N mineralization and nitrification rates over the immobilization rate. The natural N enrichment in humid subtropical forest soils may partially be due to fast N mineralization and nitrification under relatively higher soil moisture. In broad-leaved forest soil, the high NO and NO emissions occurred at 30 % WHC, while the reverse was true in coniferous forest soil. Therefore, we propose that there are different mechanisms regulating NO and NO emissions between broad-leaved and coniferous forest soils. In coniferous forest soil, nitrification may be the primary process responsible for NO and NO emissions, while in broad-leaved forest soil, NO and NO emissions may originate from the denitrification process. [ABSTRACT FROM AUTHOR]

Abstract: The effects of 4years of simulated nitrogen (N) and sulfur (S) depositions on gross N transformations in a boreal forest soil in the Athabasca oil sands region (AOSR) in Alberta, Canada, were investigated using the 15N pool dilution method. Gross NH4 + transformation rates in the organic layer tended to decline (P <0.10, marginal statistical significance, same below) in the order of control (CK, i.e., no N or S addition), +N (30kgNha−1 yr−1), +S (30kgSha−1 yr−1), and +NS treatments, with an opposite trend in the mineral soil. Gross NH4 + immobilization rates were generally higher than gross N mineralization rates across the treatments, suggesting that the studied soil still had potential for microbial immobilization of NH4 +, even after 4years of elevated levels of simulated N and S depositions. For both soil layers, N addition tended to increase (P <0.10) the gross nitrification and NO3 − immobilization rates. In contrast, S addition reduced (P <0.001) and increased (P <0.001) gross nitrification as well as tended (P <0.10) to reduce and increase gross NO3 − immobilization rates in the organic and mineral soils, respectively. Gross nitrification and gross NO3 − immobilization rates were tightly coupled in both soil layers. The combination of rapid NH4 + cycling, negligible net nitrification rates and the small NO3 − pool size after 4years of elevated N and S depositions observed here suggest that the risk of NO3 − leaching would be low in the studied boreal forest soil, consistent with N leaching measurements in other concurrent studies at the site that are reported elsewhere. [Copyright &y& Elsevier]

Abstract: Soil pH can be affected by land use change and acid deposition and is one of the primary regulators of nutrient cycling in the soil. In this study, two soils from adjacent forest and grassland sites in central Alberta were subjected to different pH treatments to evaluate the short-term effects of pH on soil gross N transformations using the 15N tracing technique and calculated by the numerical model FLUAZ. For the forest soil, gross immobilization increased faster than gross N mineralization rates with increasing soil pH, leading to a declining pattern in net N mineralization rates; however, none of those rates changed with pH in the grassland soil. In contrast, the increase in pH significantly stimulated gross and net nitrification rates while soil acidification decreased gross and net nitrification rates for both the forest and grassland soils. The ratio of gross nitrification to gross immobilization rates (N/IA) was significantly increased by KOH addition but declined to nearly zero by HCl addition for each soil. The low and high KCl addition treatments partially or completely inhibited gross nitrification rates, respectively, but gross mineralization was less sensitive to salt additions than the nitrification process. We conclude that based on the short-term laboratory incubation experiments both pH and salt (osmotic effect) affected gross N transformations and pH had contrasting effects on gross and net nitrogen mineralization but not on nitrification in the adjacent forest and grassland soils. [Copyright &y& Elsevier]