Effects of agricultural land use on the composition of fluvial dissolved organic matter

Abstract

Nearly 40% of the Earth’s ice-free surface area is cropland or pasture. Measurements of dissolved organic matter along a gradient of agricultural land use suggest that agricultural watersheds contain less complex, more microbially derived dissolved organic matter than natural wetlands. Nearly 40% of the Earth’s ice-free surface area is cropland or pasture1. Agricultural land use can increase the delivery of nutrients such as nitrogen and phosphorus to fluvial ecosystems2, but the impact of farming on riverine dissolved organic carbon is still largely unknown, despite increasing recognition that rivers act as important modifiers in the global carbon cycle3,4. Here, we examine the character of riverine dissolved organic matter in 34 watersheds along a gradient of agricultural land use. We show that changes in the character of dissolved organic matter are related to agricultural land use, nitrogen loading and wetland loss. Specifically, we find that the structural complexity of dissolved organic matter decreases as the ratio of continuous croplands to wetlands increases. At the same time, the amount of microbially derived dissolved organic matter increases with greater agricultural land use. Furthermore, we find that periods of soil dryness are associated with a decrease in the structural complexity of dissolved organic matter. We suggest that these effects of land use and climate on the character of riverine dissolved organic matter have important implications for global carbon cycling, owing to their potential to control rates of microbial carbon processing (for example, uptake, retention and outgassing) in agricultural systems.