Processes of litter and soil organic matter transformation during extreme drying/wetting as assessed by compound-specific isotope approaches
Final Report Abstract
Under temperate climate, frequency of extreme weather events such as intensive freezing or frequent thawing periods during winter and long drying periods followed by heavy rainfall in the summer might increase with effects on soil organic matter (SOM) quality and dynamics. We investigated the influence of extreme weather events (frost / drought) on SOM quality and quantity in a Haplic Podzol under a 140 years old Norway spruce forest in the Fichtelgebirge mountains (Waldstein experimental site, Bavaria, German) within two laboratory and two field studies. Lignin contents were not significantly affected by repeated freeze/thaw cycles. However, intensive frost slightly enhanced lignin mobilization in the O layer and the translocation into the B horizon. While soil frost did not influence lignin concentrations, the decomposition rate of vanillyl monomers (Ac/Ad) V decreased at the end of the frost period, these results confirm reduced mineralisation under frost. In contrast, lignin phenols were not systematically affected by the drying/rewetting-experiment and the moisture regime. The sum of PLFA (soil microbial biomass) was not affected by the frost respectively drying event, suggesting that most soil microorganisms were well adapted or recovered more quickly than the accumulation of microbial residues such as microbial sugars directly after the experiment. However, PLFA patterns indicate that fungi are more susceptible to soil frost than bacteria. The ratio of fungi to bacteria was generally not altered through drying, however, at least in the L horizon, warmer and drier weather led to a dominance of fungi while a cooler and moister regime favoured bacteria. Increasing water stress was indicated by a higher PLFA (cy17:0+cy19:0)/(16:1ω7c+18:1ω7c) ratio suggesting that the microbes suffered from water stress in the organic layer and uppermost mineral soil. While soil microbial biomass was not affected by the moisture regime, the structure of soil microbial community changed. Gram+ bacteria and actinomycetes were reduced whereas Gram-bacteria, fungi and protozoa were stimulated by the reduced moisture regime. In the subsequent summer after the freezing experience, soil microbial biomass was significantly higher at the snow-removal plots (SM) compared to the control despite lower CO2 respiration and increasing water stress indicator. These results suggest that soil microbial respiration and therefore the activity was not closely related to soil microbial biomass but more strongly controlled by substrate availability and quality. Both freezing/thawing and drying/re-wetting reduced the amount of microbial sugars due to reduced mineralisation. However, also the hydrolysable plant sugars decreased in all soil horizons. We postulated that the only possible explanation for the disappearance of plant and microbial sugars upon soil freezing or drying are chemical alterations of sugar molecules leading to SOM stabilization, also known as SOM aging. To verify these interpretations, a combination of stable isotopes and biomarker analyses seems a promising tool. This was achieved during the second project phase by establishing tracer experiments in the drying and wetting fields (13C and 15N) and by analysing the stable isotope signatures not only in the bulk SOM but also in biomarkers, which characterise chemically defined SOM pools, e.g. microbially and plant-derived sugars and phospholipid fatty acids (PLFA). The combination of biomarker and isotope (13C and 15N) analysis allows not only to deduce the real SOM turnover after drying / wetting but also to identify the involved microbial community (by 13C-PLFA) and the fate of chemically defined compounds during SOM decomposition and turnover under the experimental manipulations under field conditions. We could prove that the proposed stable isotope labelling approach is a useful innovative tool to label trees in situ with 13C and 15N without large expenditure. The experimental approach for the investigation of litter decomposition seems to be feasible, which could be illustrated by the 13C and 15N incorporation into spruce litter (needles) and the tracer detection in all investigated soil organic matter pools after 54 days such as bulk soil organic matter, heterotrophically respired CO2 , soil microbial biomass, PLFA, sugars and amino acids. Up to now, we conclude that drought significantly reduced litter mineralisation and thus slow down (or reduced) litter decomposition by 50% compared to a natural control and by 100% compared to an irrigated plot. In addition, a reduction of SOM turnover in the O layer by factor of 3 (compared to natural control) or 4 (compared to an irrigated plot) could be observed. C turnover of living microbial biomass was reduced by about 100%. While saprophytc fungi were hardly affected by soil drought, reduced activity of most soil microbial groups such as Gram+ bacteria, VA mycorrhiza and protozoa could be shown by the stable isotope approach.
Publications
- (2008). Repeated freeze/thaw cycles change soil organic matter quality in a temperate forest soil. Journal of Plant Nutrition and Soil Science 171, 707- 718
Schmitt, A., Glaser, B., Borken, W., and Matzner, E.
- (2010). Organic matter quality of a forest soil subjected to repeated drying and different re-wetting intensities. European Journal of Soil Science 61, 243-254
Schmitt, A., Glaser, B., Borken, W., and Matzner, E.
- (2011). Organic matter dynamics in a temperate forest soil following enhanced drying. Soil Biology and Biochemistry 43, 478-489
Schmitt, A., Glaser, B.
