Carbon Credits for Belowground C Sequestration by Alders? A Review of Empirical Evidence and Underlying Processes

Increasing belowground carbon sequestration through forest management has significant potential to mitigate rising atmospheric CO2 but is rarely used in C-credit systems. Difficulty in, and expense of, quantifying changes in soil C, along with large but somewhat ambiguous results in trials, has hindered its use in C-credit systems. Growing N2-fixing trees, however, has been widely suggested to consistently increase soil C. We review evidence from empirical and ecosystem process studies to evaluate whether consistent belowground C sequestration is present in the most widely studied N2-fixing-tree genus Alnus (alder). We searched globally for studies that included measures of change in mineral-soil C, including chronosequence studies and field trials. We found studies that also compared alder effects against non-N2-fixing species, which link to additionality calculations (relative C sequestration). We found that: (1) the empirical evidence of positive soil-C accretion under alder is large and unidirectional for all studies; (2) ecosystem process studies provide multiple mechanisms that can explain the observations; and (3) both empirical and mechanistic evidence persist across a wide array of climate, soils, and disturbance regimes. We suggest that evidence is sufficiently consistent to establish an interim 0.5 Mg ha−1 yr−1 standard when considering C-credits, especially in decisions on whether to include alders to mitigate rising CO2.