FOR 562: Dynamics of Soil Processes under Extreme Meteorological Boundary Conditions
Geosciences
Final Report Abstract
The response of soil processes to changing precipitation and temperature regime is a major question in the light of future climatic changes. The general aims of the Research Unit 562 were to investigate the response of soil processes and element fluxes to soil frost and thawing, soil drying and rewetting and to fluctuations of the groundwater level (fen soil). Furthermore, the aim was to resolve the physical, biological and chemical mechanisms that cause the effects. In a Norway Spruce forest (Podsol) and a fen (Histosol) we experimentally induced soil frost by snow removal and soil drought by exclusion of throughfall. Fluctuations of groundwater table in the fen site was achieved by drainage and irrigation. In addition, a series of laboratory experiments were conducted under controlled conditions. The focus was on the emission of trace gases, the element transport in solution and related processes (C- and N-mineralization, nitrification, denitrification, root growth, DOC-release, methanogenesis, redoxreactions of S and Fe, formation of flow paths and hydrophobicity of the soil). The experiments on freezing and thawing of a forest soil did to not support the hypothesis of large additional C and N losses after freeze/thaw cycles. On the contrary, we observed lower soil respiration rates in the year following the frost event as compared to the unfrozen control, despite increased fine root turnover, pointing to a larger C sink in the soil as a consequence of soil frost. However, freezing/thawing induced increased emissions of N2O and presumably also NO in the field experiment, however absolute fluxes were still small. The reason for the increased emissions of N2O was the reduced consumption of N2O in the upper soil profile in the phase of soil frost rather than an increased production. Additional nitrate losses from the soil following freezing/thawing under field conditions were low, however, the nitrate availability in the upper soil layers increased after soil frost following a surprisingly long lag phase. The field and laboratory experiments related to soil drying and rewetting contradicted the hypothesis of elevated C and N losses following wetting of a dry forest soil. On the contrary, decreased C mineralization during and after soil drought, and increased fine root production, strengthened the C sink of the forest floor. Hydrophobicity of soil organic matter and preferential flow of infiltrating rainwater amplified these effects. The substantial amount of retarded organic matter was not mineralized following improved moisture conditions. The soil microbial community and the activity of specific microbial groups were at least temporarily altered in the forest floor, though the total microbial biomass was not affected by soil drought. Gross ammonification and nitrification remained relatively high at low water availability. Nitrate leaching from the forest floor was not altered despite reduced water flux. Drought turned the forest soil into a transient sink for atmospheric N2O and reduced annual N2O emissions. Elevated consumption of N2O by denitrification exceeded the production of N2O at low water contents. Soil NO emissions followed an optimum curve with a maximum at moderate water content. The experiments with fluctuating water table in the fen soil showed that temporary drought in the fen did not increase CO2 losses most likely due to the dense, decomposed structure of the peat at the site. Methane fluxes were reduced during drought, by provision of alternative electron acceptors for respiration due to oxidative processes at low water table levels. While under permanently flooded conditions CO2- fluxes decreased, CH4 fluxes increased after consumption of alternative electron acceptors. Nitrous oxide fluxes peaked after rewetting, however, pronounced drought turned the site into a transient N2O sink. The microbial community at the site was unexpectedly stable during all manipulations. Altered process rates were caused by changes in activity rather than by changes in the cell numbers or in the microbial community structure. At the catchment scale, the wetlands are the most important drivers for runoff generation and chemistry. Overall, the results document that the effects of climatic changes on soils are process specific and characterized by different time scales with respect to response and recovery. In terrestrial forest soils, higher frequency of freezing/thawing and drying/wetting as predicted under climate change, will likely enlarge the sink function of soils for C but not for N. In case of degraded fen sites, decreasing water tables might have less effect on CO2 emissions than previously thought, but trigger a long lasting decrease of methanogenesis. Research Unit 562 focussed on soil processes and only minor emphasis was given to the plants. Hence, a deeper understanding of ecosystem scale effects of hydrological fluctuations requires future research on the response of plants and on related plant-soil feedbacks.
