Soil warming, carbon-nitrogen interactions, and forest carbon budgets

Abstract

Soil warming has the potential to alter both soil and plant processes that affect carbon storage in forest ecosystems. We have quantified these effects in a large, long-term (7-y) soil-warming study in a deciduous forest in New England. Soil warming has resulted in carbon losses from the soil and stimulated carbon gains in the woody tissue of trees. The warming-enhanced decay of soil organic matter also released enough additional inorganic nitrogen into the soil solution to support the observed increases in plant carbon storage. Although soil warming has resulted in a cumulative net loss of carbon from a New England forest relative to a control area over the 7-y study, the annual net losses generally decreased over time as plant carbon storage increased. In the seventh year, warming-induced soil carbon losses were almost totally compensated for by plant carbon gains in response to warming. We attribute the plant gains primarily to warming-induced increases in nitrogen availability. This study underscores the importance of incorporating carbon-nitrogen interactions in atmosphere-ocean-land earth system models to accurately simulate land feedbacks to the climate system.

Fig. 1.

Total annual soil CO₂ efflux partitioned into soil organic matter loss, root respiration, and fine-root decomposition for the heated and control areas (in Mg·C·ha⁻¹).

Fig. 2.

(A) Annual vegetation carbon storage in the heated and control areas (in Mg·C ha⁻¹). (B) Annual vegetation in carbon storage delta (heated minus control) (in Mg·C·ha⁻¹).

Fig. 3.

The average annual effect of soil warming on the net carbon balance of the forest stand (ecosystem carbon flux) expressed as the difference between the warming-induced carbon loss from the soil (soil organic matter decay) and the gain in the above- and belowground perennial tissues of the canopy trees (vegetation carbon storage) (in Mg·C·ha⁻¹). These values are relative to the control area. Note that the ecosystem carbon flux value for year 7 is near 0 Mg·C·ha⁻¹.

Fig. 4.

The cumulative effect of soil warming on the carbon balance of the ecosystem after 7 y of warming in Mg·ha⁻¹. Increases in growing season length may also contribute to vegetation carbon storage (not shown in this figure). These values are rounded to the nearest tenth of a Megagram (Mg).

Fig. 5.

Net nitrogen mineralization in the control and heated areas. Bars represent mean net nitrogen mineralization rates of subplots (n = 10) ±1 SE in kg·N·ha⁻¹·yr⁻¹.