Plot-Scale Sediment and Organic Matter Yields from Natural and Disturbed Surfaces In a Dry Subtropical Forest of Southwestern Puerto Rico

Accelerated erosion induced by land alterations related to agriculture and construction leads to a variety of environmental issues of worldwide relevance. Removal of the organic-rich upper soil layer may lead to on-site loss of soil productivity, while also impacting downstream aquatic environments (e.g., coral reefs) and increasing soil carbon emissions to the atmosphere. Empirical data collected in Puerto Rico was used to quantify the effects of land disturbance, slope, and vegetation cover on plot-scale erosion rates in a dry subtropical setting. Bounded plots (~ 3 m²) with slopes ranging between 1 and 18% were used to determine yield rates for both organic and inorganic matter from seven undisturbed hillslopes (n = 76) and thirteen recently disturbed unpaved road surfaces (n = 148). Production rates were determined by measuring the mass of material accumulated in Gerlach troughs in addition to that suspended in runoff. Organic content was determined by loss on ignition.

Mean total production rate from disturbed plots (20 Mg ha^-1 yr^-1) was 14 times higher than from undisturbed plots (1.5 Mg ha^-1 yr^-1). Even though the mean organic content of material eroded from undisturbed plots was twice as much as that from disturbed surfaces, their mean yield rates (0.15 Mg ha^-1 yr^-1) were an order of magnitude lower than from disturbed surfaces (1.6 Mg ha^-1 yr^-1). Production rates were poorly correlated with slope (r² < 0.05), but they followed a strong, non-linear, inverse relationship with vegetation cover (r² ~ 0.70). Results show that land disturbance in the form of unpaved roads increases the production of organic and inorganic material by surface erosion processes. Improved knowledge on the rates and controls of erosion processes may allow land managers to better assess the impacts of land use and to develop strategies to mitigate its effects.