COS 145-2 - No effect of nutrient addition on soil carbon stocks despite changes in plant community composition and ecosystem carbon fluxes in an Alaskan tundra

Background/Question/Methods

The arctic is warming at twice the rate of the rest of the globe. Given that arctic ecosystems store nearly 1,000 petagrams of carbon (C), this warming may have important consequences for global carbon cycling. As tundra soils warm, nitrogen (N) mineralization rates increase, and it and phosphorus (P) become more available to plants. This increase in nutrients may stimulate plant productivity and increase the amount of C taken up by via photosynthesis, but it may also increase decomposition and ecosystem C losses. Thus, understanding how nutrient availability affects ecosystem C fluxes is crucial in predicting if the arctic will remain a C sink or if it will become a C source as temperatures warm. Here, we quantified ecosystem C fluxes, soil C, and plant community composition across an experimental gradient of N and P addition at Toolik Field Station, Alaska, USA. We hypothesized that increased nutrient availability would lead to a more shrub-dominated plant community with increased C uptake and soil C. We measured net ecosystem exchange (NEE; NEE = ER – GPP) biweekly from June through August and used the point frame method to quantify plant community in mid-July. Soils were collected in late July.

Results/Conclusions

We found no changes in soil C stocks across the nutrient addition gradient (F_1,34 = 0.19, p = 0.67). However, we did find that NEE decreased slightly (i.e. more C uptake) as N and P addition increased (R² = 0.07, p = 0.01). This change in NEE was correlated with changes in shrub density (r = -0.3, p = 0.006), which increased with N and P addition (R² = 0.3, p = 0.01). Species richness declined with nutrient addition (R² = 0.28, p = 0.01).

Our results indicate that nutrient-induced shrubification may lead to increased C uptake, which could increase the size of the tundra C sink. However, the treatments with low NEE and high shrub density also had the lowest species richness, which can lead to reductions in ecosystem function and stability, and possibly less C uptake. Furthermore, we found increases in C uptake despite no changes in soil C, which suggests that increased C uptake is coupled with more C losses from increased respiration, decomposition, or leaching. In summary, our data indicate that further increases in nutrient availability in tundra soils could lead to accelerated tundra C cycling despite little to no change in total soil C stocks.