Impact of forest conversion to agriculture on carbon and nitrogen mineralization in subarctic Alaska

Land-use change is likely to be a major component of global change at high latitudes, potentially causing significant alterations in soil C and N cycling. We addressed the biogeochemical impacts of land-use change in fully replicated black spruce forests and agricultural fields of different ages (following deforestation) and under different management regimes in interior Alaska. Change from forests to cultivated fields increased summer temperatures in surface soil layers by 4–5 °C, and lengthened the season of biological activity by two to three weeks. Decomposition of a common substrate (oat stubble) was enhanced by 25% in fields compared to forests after litter bags were buried for one year. In-situ net N mineralization rates in site-specific soil were similar in forests and fields during summer, but during winter, forests were the only sites where net N immobilization occurred. Field age and management had a significant impact on C and N mineralization. Rates of annual decomposition, soil respiration and summer net N mineralization tended to be lower in young than in old fields and higher in fallow than in planted young fields. To identify the major environmental factors controlling C and N mineralization, soil temperature, moisture and N availability were studied. Decomposition and net N mineralization seemed to be mainly driven by availability of inorganic N. Soil temperature played a role only when comparing forests and fields, but not in field-to-field differences. Results from soil respiration measurements in fields confirmed low sensitivity of heterotrophic respiration, and thus decomposition to temperature. In addition, both soil respiration and net N mineralization were limited by low soil water contents. Our study showed that (1) C and N mineralization are enhanced by forest clearing in subarctic soils, and (2) N availability is more important than soil temperature in controling C and N mineralization following forest clearing. Projecting the biogeochemical impacts of land-use change at high latitudes requires an improved understanding of its interactions with other factors of global change, such as changing climate and N deposition.