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13C isotopic signature and C concentration of soil density fractions illustrate reduced C allocation to subalpine grassland soil under high atmospheric N deposition
Citation:
Volk, M., S. Bassin, M. Lehmann, Mark G Johnson, AND Christian P Andersen. 13C isotopic signature and C concentration of soil density fractions illustrate reduced C allocation to subalpine grassland soil under high atmospheric N deposition. SOIL BIOLOGY AND BIOCHEMISTRY. Elsevier Science Ltd, New York, NY, 125:178-184, (2018). https://doi.org/10.1016/j.soilbio.2018.07.014
Impact/Purpose:
Globally, terrestrial soils annually release roughly 10X more CO2 to the atmosphere than fossil fuel combustion. CO2 release from soils comes primarily from microbial decomposition and from root respiration. Factors affecting fluxes of CO2 into and out of soils are poorly understood, particularly in high elevation ecosystems. Grassland soils may respond to changing climate patterns and increased atmospheric N deposition by increasing plant productivity and carbon storage, or they may become stronger sources of atmospheric CO2 released through increased decomposition processes. Scientists at the Western Ecology Division worked collaboratively with scientists at Agroscope Zurich and the University of Basal to better understand how climate drivers influence C dynamics in a subalpine grassland ecosystem in Switzerland. In a seven-year field study they followed 13/12C isotope ratios to track C fluxes to describe the C sink/source properties under increased levels of N-deposition. Isotopic ratios coupled with soil density fractionation allowed them to follow which organic matter pools (i.e., younger vs older) were accumulating or being lost through decomposition processes, and to follow how seasonal climate patterns affected the responses in the different N treatments. Previous analyses at the site showed that C stocks increased over the seven-year study, but that N-deposition did not lead to consistent increases in C, despite a strong effect on plant growth. Surprisingly, the N effect on C concentration and δ13C in the biologically most active soil fractions was most pronounced in the intermediate N treatment, not in the highest N treatment. Measurements of stable isotopes of CO2 demonstrated a significant influence of climate and canopy development with maximum shifts in utilization of specific forms of C, indicating that metabolic shifts occurred early and late in the growing season. Overall, the seasonal dynamics were modulated by the N deposition treatments. Collectively, the results show the complex interaction of soil carbon with climate, seasonal temperature shifts, and N fertilization. Although the isotopes were useful in identifying important patterns, the complexity of interactions occurring indicates that future predictions will be difficult without a better understanding of the underlying processes occurring in these fragile subalpine ecosystems.
Description:
We followed soil C fluxes in a subalpine grassland system exposed to experimentally increased atmospheric N deposition for 7 years. Earlier we found that, different from the plant productivity response, the bulk soil C stock increase was highest at the medium, not the high N input as hypothesized. This implies that a smaller N-deposition rate has a greater potential to favor the biological greenhouse gas-sink. To help elucidate the mechanisms controlling those changes in SOC in response to N deposition, we produced four soil density fractions and analyzed soil organic C concentration [SOC], as well as δ13C signatures (δ13CSOC) of SOC components. Soil respired CO2 (δ13CCO2) was analyzed to better distinguish seasonal short term dynamics from N-deposition effects and to identify the predominant substrate of soil respiration. Both at the start of the experiment and after 7 years we found a strong, negative correlation between [SOC] and δ13CSOC of the soil density fractions in the control treatment, consistent with an advanced stage of microbial processing of SOC in fractions of higher density. During the experiment the [SOC] increased in the two lighter density fractions, but decreased in the two heavier fractions, suggesting a possible priming effect that accelerated decomposition of formerly recalcitrant (heavy) organic matter pools. The seasonal pattern of soil δ13CCO2 was affected by weather and canopy development, and δ13CCO2 values for the different N treatment levels indicated that soil respiration originated primarily from the lightest density fractions. Surprisingly, [SOC] increases were significantly higher under medium N deposition in the <1.8 fraction and in bulk soil, compared to the high N treatment. Analogously, the depletion of δ13CSOC was significantly higher in the medium compared to the high N treatment in the three lighter fractions. Thus, medium N deposition favored the highest C sequestration potential, compared to the low N control and the high N treatment. Clearly, our results show that it is inappropriate to use plant productivity N response as an indicator for shifts in SOC content in grassland ecosystems. Here, isotopic techniques illustrated why atmospheric N deposition of 14 kg N ha−1 yr−1 is below, and 54 kg N ha−1 yr−1 is above a threshold that tips the balance between new, assimilative gains and respiratory losses towards a net loss of [SOC] for certain soil fractions in the subalpine grassland.
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DOI: 13C isotopic signature and C concentration of soil density fractions illustrate reduced C allocation to subalpine grassland soil under high atmospheric N deposition