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In the lower montane region of northwestern Ecuador, forest clearing for sugar cane and pasture production occurs simultaneously with recolonization of secondary forest vegetation on abandoned agricultural lands. We estimated the loss of forest-derived soil C (light in 13C) and the accumulation of C from replicate sugar cane and pasture vegetation (heavy in 13C) using a stable C isotope technique. We also measured differences in the proportion of soil C derived from C3 and C4 plants across a land-use progression from agricultural fields through successional communities and undisturbed forest. Total soil C was 23 Mg/ha lower in the upper 30 cm following 50 yr of sugar cane production (24% decrease) compared to old-growth forest. The net change (−0.4 Mg·ha−1·yr−1) in soil C consisted of 1.3 Mg/ha annual losses of original forest C and 0.9 Mg/ha annual gains of C from sugar cane. After 15 yr beneath pasture, soil C was 11 Mg/ha less in the upper 30 cm than beneath forest (12% decrease). During that period, 33% of the original forest C was lost, compared to 68% released during 50 yr of sugar cane cultivation. Rate of forest C loss, C4-C accumulation, and net soil C change differed little between two distinct pasture types. Setaria sphacelata pasture and a traditional mixed-species pasture both contained more total soil C and added C4-C more rapidly than sugar cane. Under second-growth forest, soil C increased by 1.9 Mg·ha−1·yr−1, the result of a 3 Mg/ha annual increase in C3 carbon and a 1.1 Mg/ha annual loss of C4 carbon. The total soil C pool returned to preclearing levels within 20 yr. While widespread reforestation may be thwarted by high demands for land in northwestern Ecuador, agricultural land-use options exist that can contribute to increased soil C stocks.
Rhoades et al. (Sat,) studied this question.