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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 delta C-13 signatures (delta C-13(soc)) of SOC components. Soil respired CO2 (delta C-13(co2)) 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 delta C-13(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 effect that accelerated decomposition of formerly recalcitrant (heavy) organic matter pools. The seasonal pattern of soil delta C-13(co2) was affected by weather and canopy development, and delta C-13(co2) 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 delta C-13(soc) 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.