FLUVIAL CARBON IN A HUMID-TROPICAL LANDSCAPE OF EASTERN PUERTO RICO

Ongoing studies in small watersheds in Puerto Rico assess which biological, hydrologic, geomorphic, and anthropogenic mechanisms dominate the landscape-scale processing of carbon in dissolved inorganic (DIC, treated as alkalinity), dissolved organic (DOC), and particulate organic (POC) forms. Two watersheds are on volcanic rocks, and three are on granitic rocks. One catchment on each rock type is agricultural. The bedrock lacks organic carbon.

Storm sampling permits thorough characterization of transport processes. The relation between runoff rate, R, and concentration, C, of 12 major constituents in these watersheds demonstrates three styles of responses. DIC groups with solutes that are not substantially bioactive. The log(R)–log(C) relation is almost linear and can be described as a weighted average of two sources, bedrock weathering and atmospheric deposition. DOC groups with bioactive constituents; these are recycled by plants and concentrated in shallow soil and have nearly flat or downward-arched log(R)–log(C) relations. The peak of the arch represents a transition from dominantly soil-matrix flow, to near-surface macropore flow, and finally to overland flow. At highest observed R (80 to 100 mm/hr), essentially all reactive surfaces have become wetted, and the input rate of C becomes independent of R (log(R)–log(C) slope of –1). Particulate constituents (suspended sediment and POC) show steeply increasing log(R)–log(C) relations.

Landslides, rather than surficial erosion, are the largest sediment and POC source. POC yields exceed combined DOC+DIC yields in the granitic watersheds, but they are considerably less than the combined yields in the volcanic watersheds. Annual yields of DIC and DOC relate linearly to annual runoff, whereas POC has a steep exponential-style increase with increasing runoff, indicating a strong sensitivity to climate change compared to a linear response. The POC yields are tied to sediment yields that are, in turn, far in excess of equilibrium yields. Human-accelerated POC burial is a global phenomenon, and these excess POC yields are consistent with this trend. The lack of strong human effects for DOC and DIC production suggests that these carbon forms are of secondary interest to research on the anthropogenically perturbed carbon cycle at a century time scale.