Drought and plant phenology influence apparent temperature sensitivity of soil respiration more than seasonal changes in enzyme activities

Marion Schrumpf1, Enrico Weber1, Markus Reichstein1
1 Max-Planck-Institute for Biogeochemistry

O 2.8 in Environmental controls on fluxes and processes in ecosystems

17.07.2014, 11:40-12:00, H18

Soil respiration is a major carbon flux in terrestrial ecosystems which can potentially have a positive feedback to climate change. However, the temperature sensitivity of soil respiration and the influence of abiotic and biotic factors potentially controlling it are still debated. We established a field experiment where peppermint plants were grown at an agricultural field site for two years at (1) constant optimal soil moisture, (2) drying-rewetting cycles in summer, and (3) ambient soil moisture. Trenching and weeding was used exclude autotrophic respiration in additional bare soil treatments, but due to the missing root water uptake, drying treatments were not effective. Soil respiration was measured with manual and automated chambers from June 2011 to October 2012 at all treatments. Soil samples were taken at all treatments for four drying-wetting cycles at the end of the drying period and two weeks after rewetting. Soil carbon and nitrogen, microbial biomass and the kinetics and temperature sensitivity of the activities of extracellular enzymes (beta glucosidase, beta xylosidase, N-acetylglucosamidase) phosphatase were determined.

While drought reduced soil respiration rates during drying-rewetting cycles relative to optimal soil moisture, temperature was the most important control on soil respiration. Fluctuating soil moisture conditions in the ambient moisture and drying-rewetting treatments reduced the apparent seasonal temperature sensitivity of soil respiration relative to the optimum soil moisture treatment (Q10 of 4.1 for optimum soil moisture and 2.9 and 2.8 for the drying-rewetting and the ambient treatment, respectively). Separating the dataset of the plots with drying rewetting cycles into phases with optimum soil moisture and dry periods shows that contrary to our expectation, Q10 was greater for dry (4.2) than wet (3.4) treatments. Bare soil treatments only had an average Q10 of 2.6. While substrate depletion might have contributed to the smaller Q10 at the bare soil treatments, this result suggests that seasonal changes in root respiration were responsible for the greater apparent Q10 at the vegetated sites. We plan to use daily temperature variations in soil respiration with data from the automated chambers, to determine respective seasonal changes in Q10, but analyses are still in progress. Lab incubations of peppermint plants in carbon-free substrate showed that root respiration had on average a Q10 around 1.8 with a slightly higher for drought-stressed plants. Activities of extracellular enzymes showed no clear seasonal pattern and neither the Q10 of Vmax (averages between 1.7 and 2.5), nor of Km were affected by the time of the year. Assuming that results of enzyme assays represent  field conditions, this suggests that differences in quality and quantity of enzyme activities were not responsible for the observed greater apparent temperature sensitivities of soil respiration measured in the field.

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last modified 2014-06-19