Microbial activity is not always limited by nitrogen in Arctic tundra soils

Abstract

Both primary productivity and decomposition appear to be limited by low soil nitrogen (N) availability throughout much of the Arctic tundra active growing season, making these ecosystems among the most N-limited in the world. Climate warming may potentially stimulate microbial activities such as N mineralization, which could have cascading long-term effects on Arctic tundra ecosystems. Despite previous evidence of N limitation to microbial decomposition in Arctic tundra, N may not limit microbial activity throughout the entire active season. Labile carbon (C) may be co-limiting for portions of the active season when there is relatively high inorganic N and/or low labile C availability. To assess seasonal variation in the controls on microbial activity, we conducted a series of laboratory incubations with soils collected at the beginning and peak of the active season in two years to examine intra-seasonal and annual variability of soil microbial N limitation in an Arctic moist acidic tundra (MAT) soil. The soil incubations were set-up in a factorial design with treatments of added N or DI water as a control and incubation temperatures of 5 °C and 15 °C. We measured chloroform-labile C and N (as a proxy for microbial biomass), extractable nutrients, C-mineralization and potential enzyme activities. In contrast to previous studies, we found that these metrics of microbial activity were not consistently stimulated by N additions; rather, added N was primarily immobilized by microbes resulting in decreased chloroform-labile C:N ratios. Stimulation of C mineralization by N addition was short-lived and variable between our two sampling dates within a single active season. Additionally, there were differences in temperature sensitivities of C mineralization and contrasting effects of N amendment on enzyme activities between the two study years. These findings suggest that, at times, other factors co-limit microbial activities in MAT soils. The current dogma of universal N-limitation to microbial activity may need to be refined in light of these results, to more accurately predict the fate of the large amounts of C currently sequestered in Arctic tundra soils.