Carbon and Nutrient Accumulation in Secondary Forests Regenerating on Pastures in Central Amazonia

Over the past three decades, large expanses of forest in the Amazon Basin were converted to pasture, many of which later degraded to woody fallows and were abandoned. While the majority of tropical secondary forest (SF) studies have examined post-deforestation or post-agricultural succession, we examined post-pasture forest recovery in 10 forests ranging in age from 0 to 14 years since abandonment. We measured aboveground biomass and soil nutrients to 45 cm depth and computed total site carbon (C) and nutrient stocks to gain an understanding of the dynamics of nutrient and C buildup in regenerating SF in central Amazonia. Aboveground biomass accrual was rapid,$11.0 Mg\cdot ha^{-1}\cdot yr^{-1}$, in the young SFs. Within 12-14 yr, they accumulated up to 128.1 Mg/ha of dry aboveground biomass, equivalent to 25-50% of primary forest biomass in the region. Wood nitrogen (N) and phosphorus (P) concentrations decreased with forest age. Aboveground P and calcium (Ca) stocks accumulated at a rate of$1.2 and 29.4 kg\cdot ha^{-1}\cdot yr^{-1}$; extractable soil P stocks declined as forest age increased. Although soil stocks of exchangeable$Ca (207.0 \pm 23.7 kg/ha)$and extractable P ($8.3 \pm 1.5 kg/ha$) were low in the first 45 cm, both were rapidly translocated from soil to plant pools. Soil N stocks increased with forest age, probably due to N fixation, atmospheric deposition, and/or subsoil mining. Total soil C storage to 45 cm depth ranged between 42 and 84 Mg/ha, with the first 15 cm storing 40-45% of the total. Total C accrual ($7.04 Mg C\cdot ha^{-1}\cdot yr^{-1}$) in both aboveground and soil pools was similar or higher than values reported in other studies. Tropical SFs regrowing on lightly to moderately used pasture rapidly sequester C and rebuild total nutrient capital following pasture abandonment. Translocation of some nutrients from deep soil (>45 cm depth) may be important to sustaining productivity and continuing biomass accumulation in these forests. The soil pool represents the greatest potential for long-term C gains; however, soil nutrient deficits may limit future productivity.