Permafrost response to temperature rise in carbon and nutrient cycling: Effects from habitat-specific conditions and factors of warming

Permafrost is warming at a rate of two times faster than the rest of the Earth's surface. However, there is still a lack of a quantitative basis for predicting the functional stability of permafrost ecosystems in carbon (C) and nutrient cycling. We compiled the data of 708 observations from 89 air-warming experiments in the Northern Hemisphere and characterized the general effects of temperature increase on permafrost C exchange and balance, biomass production, microbial biomass, soil nutrients, and vegetation N dynamics via a meta-analysis. Also, an investigation was made on how responses may change with habitat-specific (e.g., plant functional groups and soil moisture status) conditions and warming variables (e.g., warming phases, levels, and timing). Warming downregulated net ecosystem C exchange generally via stimulating ecosystem respiration (15.6%) more than photosynthesis (6.2%). Vegetation usually responded to warming by investing more the C to the belowground, as belowground biomass increased much more (30.1%) than aboveground biomass (2.9%). Warming had a minor effect on microbial biomass. Warming increased soil ammonium and nitrate concentrations. What's more, a synthesis of 70 observations from 11 herbs and 9 shrubs revealed a 2.5% decline of N in green leaves. Compared to herbs, shrubs demonstrated a stronger response in respiration and had green leaf N declined by a greater extent. Not only in dry conditions did green leaf N decline with warming but also in wet conditions. Warming in non-growing seasons would negatively affect soil water, C uptake, and biomass production during growing seasons. Permafrost C loss and vegetation N decline may increase with warming levels and timing. Overall, our findings suggest that besides a positive C cycling-climate feedback, there will be negative feedback between permafrost nutrient cycling and climate warming.