Individual and interactive effects of warming and nitrogen supply on CO₂ fluxes and carbon allocation in subarctic grassland

Abstract

Climate warming has been suggested to impact high latitude grasslands severely, potentially causing considerable carbon (C) losses from soil. Warming can also stimulate nitrogen (N) turnover, but it is largely unclear whether and how altered N availability impacts belowground C dynamics. Even less is known about the individual and interactive effects of warming and N availability on the fate of recently photosynthesized C in soil. On a 10‐year geothermal warming gradient in Iceland, we studied the effects of soil warming and N addition on CO2 fluxes and the fate of recently photosynthesized C through CO2 flux measurements and a 13CO2 pulse‐labeling experiment. Under warming, ecosystem respiration exceeded maximum gross primary productivity, causing increased net CO2 emissions. N addition treatments revealed that, surprisingly, the plants in the warmed soil were N limited, which constrained primary productivity and decreased recently assimilated C in shoots and roots. In soil, microbes were increasingly C limited under warming and increased microbial uptake of recent C. Soil respiration was increased by warming and was fueled by increased belowground inputs and turnover of recently photosynthesized C. Our findings suggest that a decade of warming seemed to have induced a N limitation in plants and a C limitation by soil microbes. This caused a decrease in net ecosystem CO2 uptake and accelerated the respiratory release of photosynthesized C, which decreased the C sequestration potential of the grassland. Our study highlights the importance of belowground C allocation and C‐N interactions in the C dynamics of subarctic ecosystems in a warmer world.

Decadal soil warming decreased vascular plant above and belowground production in a subarctic grassland by inducing nitrogen limitation

“…However, Callebaut (2022) showed that leaf‐level photosynthetic capacity in one of the dominant vascular plant species was reduced in warmed plots, due primarily to reduced foliar N concentrations. Nitrogen has indeed been identified to limit grassland productivity in Iceland (Leblans et al ., 2014, 2017a,b; Meeran et al ., 2023), and this limitation has been aggravated in many other northern areas with low atmospheric N deposition (Liu et al ., 2020). The strong decline in soil N–P ratio at our study site indeed suggests that N limitation in these grasslands aggravated in response to decadal warming (Fig.…”