No abstract

Despite concerns about climate change-effects on ecosystems functioning, only little is known on how plant assimilate partitioning changes with temperature. Particularly large temperature-effects might occur in cold ecosystems where critical processes are at their temperature limit. In this study, we tested temperature effects on carbon (C) assimilate partitioning in a field experiment at the alpine treeline. We warmed and cooled soils of microcosms planted with Pinus mugo or Leucanthemopsis alpina, achieving daily mean soil temperatures (3-10 cm depth) around 5.8, 12.7 and 19.2°C in cooled, control and warmed soils. We pulse-labelled these systems with 14CO2 for one photoperiod and traced 14C over the successive four days. Plant net 14C uptake increased steadily with soil temperature. However, 14C amounts in fungal hyphae, soil microbial biomass, soil organic matter, and soil respiration showed a non-linear response to temperature. This non-linear pattern was particularly pronounced in P. mugo, with five times higher 14C activities in cooled compared to control soils, but no difference between warmed and control soil. Autoradiographic analysis of the spatial distribution of 14C in soils indicated that temperature effects on the vertical label distribution within soils depended on plant species. Our results show that plant growth, in particular root metabolism, is limited by low soil temperature. As a consequence, positive temperature effects on net C uptake may not be paralleled by similar changes in rhizodeposition. This has important implications for predictions of soil C storage, because rhizodeposits and plant biomass strongly vary in their residence time. AbstractDespite concerns about climate change-effects on ecosystems functioning, only little is known on how plant assimilate partitioning changes with temperature. Particularly large temperature-effects might occur in cold ecosystems where critical processes are at their temperature limit. In this study, we tested temperature effects on carbon (C) assimilate partitioning in a field experiment at the alpine treeline. We warmed and cooled soils of microcosms planted with Pinus mugo or Leucanthemopsis alpina, achieving daily mean soil temperatures (3-10 cm depth) around 5.8, 12.7 and 19.2°C in cooled, control and warmed soils. We pulse-labelled these systems with 14 CO 2 for one photoperiod and traced 14 C over the successive four days. Plant net 14 C uptake increased steadily with soil temperature. However, 14 C amounts in fungal hyphae, soil microbial biomass, soil organic matter, and soil respiration showed a non-linear response to temperature. This nonlinear pattern was particularly pronounced in P. mugo, with five times higher 14 C activities in cooled compared to control soils, but no difference between warmed and control soil.Autoradiographic analysis of the spatial distribution of 14 C in soils indicated that temperature effects on the vertical label distribution within soils depended on plant species. Our results show that plant growth, ...

Carbon cycling responses of ecosystems to global warming will likely be stronger in cold ecosystems where many processes are temperature‐limited. Predicting these effects is difficult because air and soil temperatures will not change in concert, and will affect above and belowground processes differently. We disentangled above and belowground temperature effects on plant C allocation and deposition of plant C in soils by independently manipulating air and soil temperatures in microcosms planted with either Leucanthemopsis alpina or Pinus mugo seedlings. Daily average temperatures of 4 or 9°C were applied to shoots and independently to roots, and plants pulse‐labelled with 14 CO 2. We traced soil CO 2 and 14 CO 2 evolution for 4 days, after which microcosms were destructively harvested and 14C quantified in plant and soil fractions. In microcosms with L. alpina, net 14C uptake was higher at 9°C than at 4°C soil temperature, and this difference was independent of air temperature. In warmer soils, more C was allocated to roots at greater soil depth, with no effect of air temperature. In P. mugo microcosms, assimilate partitioning to roots increased with air temperature, but only when soils were at 9°C. Higher soil temperatures also increased the mean soil depth at which 14C was allocated. Our findings highlight the dependence of C uptake, use, and partitioning on both air and soil temperature, with the latter being relatively more important. The strong temperature‐sensitivity of C assimilate use in the roots and rhizosphere supports the hypothesis that cold limitation on C uptake is primarily mediated by reduced sink strength in the roots. We conclude that variations in soil rather than air temperature are going to drive plant responses to warming in cold environments, with potentially large changes in C cycling due to enhanced transfer of plant‐derived C to soils.