Measuring Soil Organic Carbon Sequestration in Aggrading Temperate Forests

Over the past 150 yr, 70-90% of the lands once cleared for agriculture in New England have returned to forests. Most scientists agree that these forests serve as significant C sinks; however, there is uncertainty as to the rate at which soil organic C (SOC) is being sequestered and in which horizons the SOC is being stored. We compared the paired site and chronosequence approaches to estimate SOC sequestration at decadal to near-century scales to determine if one of these methods may be better at clarifying these uncertainties. Seventeen sites in southern New England were used to determine SOC sequestration rates. Each site consisted of an agricultural field and an adjacent forest having the same soil type; both were once part of the same field (paired site). We tested for the effects of drainage class [well-drained (WD) Typic Dystrudepts vs. moderately well-drained (MWD) Aquic Dystrudepts] and forest type (coniferous vs. deciduous) on SOC sequestration. We sampled soils by master horizon to a depth of 1 m to calculate SOC content and pools. Forest SOC pools averaged 152 Mg C ha(-1) and did not differ between MWD and WD or by forest type. Mean agricultural field SOC pools (111 Mg C ha(-1)) were significantly lower than forest pools (p < 0.01). Sequestration rates, and where the SOC was sequestered, differed between the chronosequence and paired site approaches. The paired site approach showed that 70% of the SOC was being sequestered in the O horizons whereas the chronosequence method suggested most SOC was being sequestered in A horizons. The chronosequence method estimated that these 25- to 86-yr-old aggrading forests sequestered SOC at a rate of 0.56 Mg C ha(-1) yr(-1). The average paired site sequestration rate (0.84 Mg C ha(-1) yr(-1)) was 50% higher than the chronosequence rate, suggesting that the chronosequence approach may underestimate SOC sequestration. The paired-site rate suggests that C is being sequestered in these soils nearly as quickly (55-82%) as in the overlying forest.