Julie Pett-Ridge Invited Talk.

Spatial variability in dust deposition in the Luquillo Mountains, Puerto Rico

Understanding ecosystem nutrient dynamics and response to environmental change is limited by the lack of long-term estimates of nutrient fluxes.  In particular, the importance of mineral aerosol (dust) inputs to phosphorus (P) cycling in tropical forest ecosystems is not well known.  This study evaluates the spatial variability of dust deposition using neodymium (Nd) isotope ratios and cosmogenic nuclide-based calculations of surface soil residence times in the Luquillo Mountains, Puerto Rico. Dust from the Sahara-Sahel region of Africa carries a distinct isotopic signature of -12 e_Nd, while local bedrock, in contrast, has a e_Nd signature of 7.  End-member mixing calculations for 31 ridgetop sites across the Luquillo Mountains based on e_Nd reveal a wide range in dust influence on surface soils, with between 6% and 89% of soil Nd being African dust-derived.  Using e_Nd paired with surface soil residence times, dust fluxes to the Luquillo Mountains are calculated, ranging from 0.5 to 70 g m^-2 yr^-1 across a 150 km² area.  Based on the P content of African dust, this represents a dust-derived P input of 0.5 to 70 mg P m^-2 yr^-1.  The similar magnitude of dust-derived P inputs to that of other P input and output fluxes indicate that dust is an important component of the soil and biomass P budget in this ecosystem.  Current dust content of soil is positively correlated with biologically cycled fractions of soil P on quartz diorite bedrock (r²=0.48 and p=0.002 for NaHCO₃-P_i, r²=0.25 and p=0.030 for NaOH-P_t).  Dust content of soil is also positively correlated with ridgetop width (r²=0.67 and p=0.011).  However, dust content of soil is not correlated with denudation rate, meaning that variation in dust deposition fluxes controls dust content in addition to variation in soil residence time across the landscape.  Soil dust content is not simply correlated with elevation, longitude, forest type, or bedrock type, implying that other local factors such as wind speed, turbulence, and forest canopy structure influence local dust deposition rates.