Recently, global and some regional observations of soil carbon stocks and turnover times have implied that warming may not deplete soil carbon as much as predicted by ecosystem models. The proposed explanation is that microbial respiration of carbon in 'old' mineral pools is accelerated less by warming than ecosystem models currently assume. Data on the sensitivity of soil respiration to temperature are currently conflicting. An alternative or additional explanation is that warming increases the rate of physico-chemical processes which transfer organic carbon to 'protected', more stable, soil carbon pools. These processes include adsorption reactions, some of which are known to have positive activation energies. Theoretically, physico-chemical reactions may be expected to respond more to warming than enzyme-mediated microbial reactions. A simple analytical model and a complex multi-pool soil carbon model are presented, which separate transfers between pools due to physico-chemical reactions from those associated with microbial respiration. In the short-term, warming depletes soil carbon. But in the long-term, carbon losses by accelerated microbial respiration are offset by increases in carbon input to the soil (net production) and any acceleration of soil physico-chemical 'stabilization' reactions. In the models, if net production rates are increased in response to notional warming by a factor of 1.3, and microbial respiration (in all pools) by 1.5, then soil carbon at equilibrium remains unchanged if physico-chemical reactions are accelerated by a factor of about 2.2 (50% more than microbial reactions). Equilibrium soil carbon increases if physico-chemical reactions are over 50% more sensitive to warming than soil respiration. (C) 2001 Annals of Botany Company.
