The Impacts of Agricultural Land Use on Dissolved Organic Matter in a Dryland River System

Abstract:

Globally, expanding agriculture is significantly impacting aquatic nutrient cycles. In mesic systems, agriculture is a source of nitrogen and phosphorus and increases concentrations of structurally simple dissolved organic carbon (DOC). In contrast, recent studies suggest in dryland systems, where wastewater effluent is a primary nutrient source, agriculture is a nutrient sink—retaining nitrogen and phosphorous. Importantly, very little, is known about the influence of agriculture on DOC dynamics in dryland systems.

To address this gap we used synoptic sampling, UV-absorbance, and fluorescence spectroscopy to elucidate source, character, and concentration of riverine and runoff DOC in a dryland agricultural system. Samples were collected along a 25 km stretch of the Rio Grande River in New Mexico (USA). The Rio Grande is an impoundment/irrigation-withdrawal controlled river that receives water from snowmelt, monsoonal storms, and wastewater effluent. During irrigation approximately 80% of the river’s water is diverted into a manmade network where it waters crops and percolates through the soil before it enters a series of drains that return water to the river. Our preliminary characterization of the DOC reentering the river (DOC_mean=3.23 mg/L, sd=0.81; SUVA_mean=4.05, sd=1.37) indicates the agricultural pool is similar in concentration and aromaticity to riverine DOC (DOC_mean= 3.10 mg/L, sd=1.17; SUVA_mean= 4.64, sd=1.12). However, riverine organic matter is more terrestrially derived (FI_mean=1.68, sd=0.17) than organic matter in the drains (FI_mean=1.9, sd=0.24). Additionally, drains directly adjacent to actively irrigated fields show high concentrations (DOC_mean=58.35; sd=0.91) of low aromaticity organic matter (SUVA_mean=0.33; sd=0.11). We are continuing analysis throughout the irrigation season to further explore organic matter quality (traits such as bioavailability and freshness) and identify locations and processes of DOC transformation within the system. Based on our current results dryland agricultural areas produce large quantities of structurally simple organic matter that is rapidly processed and transformed in surficial drains. Thus, future dryland agricultural expansion may impact food web dynamics, river biochemistry, and carbon cycling.