C-13 isotopic signature and C concentration of soil density fractions illustrate reduced C allocation to subalpine grassland soil under high atmospheric N deposition

We followed soil C fl uxes in a subalpine grassland system exposed to experimentally increased atmospheric N deposition for 7 years. Earlier we found that, di ff erent 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 me- chanisms controlling those changes in SOC in response to N deposition, we produced four soil density fractions 13 13 and analyzed soil organic C concentration [SOC], as well as δ C signatures ( δ C SOC ) of SOC components. Soil respired CO 2 ( δ 13 C CO2 ) was analyzed to better distinguish seasonal short term dynamics from N-deposition e ff ects 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 δ 13 C SOC 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 e ff ect that accelerated 13 decomposition of formerly recalcitrant (heavy) organic matter pools. The seasonal pattern of soil δ C CO2 was 13 a ff ected by weather and canopy development, and δ C CO2 values for the di ff erent N treatment levels indicated that soil respiration originated primarily from the lightest density fractions. Surprisingly, [SOC] increases were signi fi cantly higher under medium N deposition in the < 1.8 fraction and 13 in bulk soil, compared to the high N treatment. Analogously, the depletion of δ C SOC was signi fi cantly 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.