NITRATE DYNAMICS IN AN ARID IRRGATION NETWORK: CAN NUTRIENT LOOPS BE CLOSED WITH MANAGEMENT TECHNIQUES?

Nutrients are the third leading cause of impairment in aquatic ecosystems, yet they remain necessary to support our growing agriculture system. Mining (as with phosphorus) and manufacturing (as with synthetic nitrogen) fertilizers deplete non-renewable resources and consume large amounts of energy. We have opportunities to optimize food-energy-water (FEW) resources, particularly in arid regions where wastewater, rather than agriculture, is the number one contributor of nutrients. This study evaluates the capacity of three unique channels (i.e., the Drain canal, the Delivery canal, and the Rio Grande River) within the agriculture system of the Middle Rio Grande Valley to process nutrients from the Albuquerque Wastewater Treatment Plant (ABQ WWTP). We used a mass balance approach paired with stable isotope analysis to determine the source and fate of NO3-N within these channels over time (one year) and space. Our study revealed the growing season (March-October) is a key period of NO3-N sink behavior v in the Rio Grande and Delivery canal, but the Drain (which carries nutrients back to the Rio Grande) was regularly a source of NO3-N during this period. Additionally, we found that travel times are key to establishing source/sink NO3-N dynamics, i.e., sites closest to the ABQ WWTP experienced source behavior and distal sites experienced sink behavior during the growing season. NO3 stable isotope analysis revealed that NO3 was primarily sourced from septic and manure waste (analogous to WWTP inputs), but during the growing season some NO3 was sourced from NH4, a common fertilizer used in this region. Stable isotope analysis also revealed the Drain canal experienced NO3 production and the Rio Grande and Delivery canal experienced NO3 uptake caused by microbial processing. With this information, we recommended areas of improvement to the agricultural system to promote nutrient processing in drains and downstream of the ABQ WWTP, while minimizing processing in the Delivery canal so as to increase nutrient delivery to crops. This study may pioneer new designs and strategies to promote the sustainable management of FEW resources in the Middle Rio Grande Valley