Effect of soil warming and N availability on the fate of recent carbon in subarctic grassland

Abstract

<p>Climate warming has been suggested to impact high latitude grasslands severely, 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 soil 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.  We hypothesized that warming would increase belowground C allocation, while enhanced N availability would decrease it, and that their interactive effects would be additive.</p><p>We studied a subarctic grassland located at a natural geothermal soil warming gradient close to Hveragerði, Iceland, which was established by an earthquake in 2008. We chose 14 plots along the gradient with soil warming temperatures ranging from 0 to 10°C above ambient, and fertilized a subset of plots with 50kg ha<sup>-1</sup> y<sup>-1</sup> of NH<sub>4</sub>NO<sub>3</sub> twice a year prior to the study. We performed <sup>13</sup>CO<sub>2</sub> canopy pulse labeling for an hour and tracked the <sup>13</sup>C pulse through the plant-microbe-soil system and into soil respiration for ten days after labeling.</p><p>Our preliminary results show that at higher temperatures microbial activity increased, causing higher C turnover and a higher respiration of recently assimilated C from the soil. Warming significantly decreased microbial biomass, however, the recent C allocated from roots to microbes increased. This indicates a higher microbial C-limitation and a tighter root-microbe coupling under warming. Nitrogen addition increased the allocation of recent C to roots, microbial biomass, and soil respiration. The effects of N addition and warming were additive with no interaction. Our results indicate that the microbes in warmed soil may not be N limited, but could be C limited and depend more on the supply of recent C from plants. We conclude that in a future climate with warmer soils, more C may be allocated belowground, however, its residence time may decrease.</p>