The effects of deforestation on soil carbon and nutrient stocks in a premontane rainforest of southern Costa Rica

Background/Question/Methods

Tropical deforestation contributes almost 15% of anthropogenic greenhouse gas emissions through burning of aboveground biomass and loss of soil carbon. Post-clearing degradation of tropical soils can decrease agricultural productivity, stimulating more forest clearing. However, our understanding of the fate of soil carbon and nutrients following deforestation remains limited by relatively few studies and substantial variability among tropical forests and soils. In this context, we asked how deforestation affects soil carbon (C), nitrogen (N), and phosphorus (P) stocks in the premontane landscape surrounding Las Cruces Biological Station in southern Costa Rica. Forests in this landscape were cleared in the mid-1960s, and the resulting pastures are dominated by C4 grasses (Axonopus micay and Urochloa mollis). We excavated two soil pits to a depth of 1m in each of three topographic positions (ridge, mid-slope, toeslope) in both forest and pasture. Hillslopes of >20° are common, with ~10cm-deep rills and cattle tracks covering steeper pasture hillsides. We used two independent ground-based methods to estimate soil losses from these features, and calculated associated carbon and nutrient losses.

Results/Conclusions

Total soil C stocks were ~200 kg C m^-2 in both forest and pasture, and not significantly different (p=0.79). In both settings, soil C concentrations decreased from ~10% at the surface to ~1% at 1m depth. Surface soil δ¹³C signatures were -26‰ in forest and -20‰ in pasture. δ¹³C of forest and pasture foliage was -30‰ and -12‰, respectively, suggesting ~50% of the soil carbon has turned over since deforestation. Soil δ¹³C at >40cm depth were -23‰ in both settings, indicating pasture grasses have not contributed substantially to deeper soil C stocks. Soil N and P stocks were not significantly different (p=0.43 and 0.61, respectively) between forest and pasture. Soil N concentrations ranged from ~0.7% at the surface to <0.1% at a depth of 1m, while soil P decreased from ~1000ppm to 500ppm. Averaging across 10 hillslopes, 0.10±0.03m³/m² of pasture topsoil has been lost via hillside ruts since deforestation, a loss of 0.013kg N m^-2 yr^-1 and 0.0020kg P m^-2 yr^-1. Rapid loss of N- and P-rich topsoil even from these relatively fertile pasture hillslopes may expose more nutrient poor subsoil, reducing pasture productivity over time.