Substantial Organic and Particulate Nitrogen and Phosphorus Export from Geomorphologically Stable African Tropical Forest Landscapes

Abstract

Aquatic losses of nutrients are important loss vectors in the nutrient budgets of tropical forests. Traditionally, research has focused mainly on losses of inorganic nutrient forms, whereas the potential contribution of organic and particulate losses to the total nutrient export budget is much less constrained. In this study, we quantified full aquatic nitrogen (N) and phosphorus (P) exports, including inorganic, organic and particulate forms, from a moist tropical lowland forest and a semi-dry Miombo woodland forest within the Congo Basin. While particulate organic N (PON) was the highest N loss vector in the lowland stream (3.34 kg N ha(-1) y(-1); 44% of TN), dissolved organic N (DON) dominated the export in the Miombo stream (1.41 kg N ha(-1) y(.-1); 47% of TN). Aquatic P export was dominated by dissolved organic P (DOP) in both streams, with yields of 0.29 kg P ha(-1) y(-1) (65% of TP) in the lowland and 0.24 kg P ha(-1) y(-1) (69% of TP) in the Miombo. Storm events were driving those losses, exporting disproportionally high N and P loads during short periods of stormflow conditions (32% and 47% of TN and 20% and 40% of TP in the lowland and Miombo, respectively). Our results highlight the need to take particulate and organic forms into account as important loss vectors in the nutrient balance of tropical forests. This finding is of particular importance considering the projected increasing rainfall intensities in many tropical regions which might exacerbate the export of these nutrient forms in the near future.