Plant - soil carbon responses to warming and nitrogen

Soils contain over three times as much carbon as the atmosphere in soil organic matter, and have the potential to slow down or accelerate climate change through altered rates of plant growth and soil organic matter decomposition. Cold, northern ecosystems in particular, store vast amounts of carbon in the soil, but these stocks are vulnerable to increased carbon losses due to warming temperatures and changes in the availability of limiting nutrients such as nitrogen. In addition to the direct effects of warming and increasing nitrogen availability on organic matter decay by microbes, plants also play a major role by changing the way in which they use their photosynthates. By allocating more or less carbon belowground to roots, symbionts, or exudation, plants can alter soil carbon input rates and pathways, and thereby change the way soil organic matter responds to warming and nitrogen enrichment. Our research will examine how warming and nitrogen availability impact on carbon dynamics of plants and soil microbes in order to improve our understanding of plant-soil carbon cycling under future global change scenarios. In order to do this we will carry out experiments at a subarctic grassland in Iceland exposed to different levels of warming and nitrogen availability, tracking carbon flows from plant photosynthesis into the soil and back to the atmosphere and input this data into mathematical models to help better predict ecosystem carbon cycling feedbacks to global warming.

Soils contain over three times as much carbon (C) as the atmosphere in soil organic matter, and have the potential to slow down or accelerate climate change through altered rates of C sequestration. The vast soil C stocks of cold, northern ecosystems are particularly vulnerable to increased C losses due to climate warming. In addition to the direct effects of warming on soil organic matter decomposition, also indirect effects through plants and their C uptake and allocation can importantly alter soil C and N cycling. By allocating more or less C to roots and symbionts, plants can alter soil carbon input rates and pathways, and thereby change the way soil organic matter responds to warming and N enrichment. Our research project aimed at exploring how warming and N availability, individually and in combination, impact on the plant-soil C dynamics in subarctic grasslands. We studied grassland plots in Iceland, which were exposed to gradients of natural geothermal soil warming, and to some of which N fertilizer was being added. We found that warming led to a strong initial (i.e. within 5 years) loss of soil organic matter and microbial biomass, followed by a long-term downregulation of the responses. After 8-10 years of warming, ecosystem respiration exceeded maximum gross primary productivity during the most productive season, which caused net CO2 emissions from warmed grassland. An isotopic pulse-labelling experiment showed that warming increased and accelerated the belowground allocation of recent plant-assimilated C. The microbial groups, whose biomass was more strongly reduced under warming, increased the uptake of recent plant-derived C more than those, which were less affected by warming. Lab incubations of soil with a labile C source indicated that warming increased a negative priming effect of soil organic matter turnover and that, surprisingly, lower N availability was not associated with increased priming. The N addition treatments at the field site suggest that under warming plants but not microbes were increasingly N-limited, and that warming-induced declines in plant N availability increased belowground C allocation. N addition also caused a faster decomposition of structural litter compounds, but did not affect stabilization of litter-derived C in soil organic matter. Overall, the project findings suggest that warming leads to significant losses of C and N from subarctic grassland soil and that microbes in warmed soil are not N-limited, but C-limited and depend more strongly on the supply of recent C from plants. We conclude that belowground C allocation and C-N interactions play a key role in driving soil C dynamics of subarctic grasslands in a warmer world.