Soil warming and carbon-cycle feedbacks to the climate system.

In a decade-long soil warming experiment in a mid-latitude hardwood forest, we documented changes in soil carbon and nitrogen cycling in order to investigate the consequences of these changes for the climate system. Here we show that whereas soil warming accelerates soil organic matter decay and carbon dioxide fluxes to the atmosphere, this response is small and short-lived for a mid-latitude forest, because of the limited size of the labile soil carbon pool. We also show that warming increases the availability of mineral nitrogen to plants. Because plant growth in many mid-latitude forests is nitrogen-limited, warming has the potential to indirectly stimulate enough carbon storage in plants to at least compensate for the carbon losses from soils. Our results challenge assumptions made in some climate models that lead to projections of large long-term releases of soil carbon in response to warming of forest ecosystems.

Assessing the role of fine roots in carbon and nutrient cycling.

Fine roots remain one of the most difficult and important areas to study in terrestrial ecosystems. Recent investigations have focused on carbon and nitrogen balances to assess their dynamics in natural systems. The results of these new, system-level, budgeting approaches together with those of the more conventional biomass measurement methods may provide valuable insight into fine root carbon and nutrient cycling dynamics. These findings, in turn, may facilitate a more holistic understanding of ecosystem structure and function, which is critical for the assessment and prediction of disturbances to terrestrial systems.

Nitrogen cycling and nitrogen saturation in temperate forest ecosystems.

The last decade has seen a dramatic shift in the focus of nitrogen cycling research in forest ecosystems. Concerns over nitrogen deficiencies and effects of removal in harvest have given way to concerns over excess nitrogen availability and the potential for forest decline and surface water pollution. Driving this paradigm shift is the increase in atmospheric deposition of nitrogen to forests due to industrial and agricultural activity. At the core of the new paradigm is the concept of 'nitrogen saturation' of forest ecosystems. The purpose of this review is to synthesize recent advances in research relating to nitrogen deposition effects on temperate zone forest ecosystems, and the further effects of nitrogen saturation on environmental quality.