COS 247-6 - Mechanisms and Patterns of Soil Carbon Storage and Cycling across Lowland Seasonal Tropical Forests of Panama

Abstract: The Barro Colorado Nature Monument (BCNM) and surrounding lowland forests span the Isthmus of Panama, covering a range of 1750 – 3400 mm mean annual precipitation (MAP) and five USDA soil orders, which, combined, give rise to large variation in soil organic carbon (SOC) stocks. Understanding drivers of variation in tropical forest SOC will help us predict vulnerability to current and future climate change. To better characterize and understand variation in tropical forest SOC, we assessed SOC stocks in surface and subsurface soils down to 90 – 100 cm depths) across 50 distinct Panamanian forests, including measurements of soil carbon fractions, organic chemistry, and radiocarbon ages on a representative subset of sites. We also assessed inputs to SOC (leaf litter and root biomass), and losses (soil respiration). We then assessed patterns and mechanisms of spatial and temporal variability in SOC dynamics, including variation among sites, seasons, and depths, as well as findings of experimental studies that highlight specific mechanisms (nutrient, temperature, and moisture manipulation).

We found that SOC varied strongly among sites, primarily according to soil fertility. There were overall smaller SOC stocks in more fertile soils, and larger SOC stocks in infertile, clay-rich soils across the central Panamanian lowlands, similar to patterns for root biomass to depth. While MAP was a much weaker predictor than soil order for total SOC stocks, within infertile soils there was a significant increase in mineral-associated SOC at greater rainfall. Rainfall seasonality was also the strongest driver of temporal variation in SOC inputs and losses, with litterfall peaking during the dry season, but root production and soil respiration peaking in the wet season for most sites. Experimental irrigation, drying, warming, nutrient addition, and litter manipulations provided insights to dominant mechanisms driving SOC dynamics. Together, the large body of observational and experimental data for SOC in the Panamanian lowlands give indications of how global change is likely to alter SOC stocks and turnover across seasonal tropical forests.