Mechanisms of basin-scale nitrogen load reductions under intensified irrigated agriculture
(English)Manuscript (preprint) (Other academic)
Irrigated agriculture can modify the cycling of nitrogen (N), due to associated water diversions, water losses, and changes in transport pathways. We investigate dominant processes behind observed long-term changes in dissolved inorganic nitrogen (DIN) concentrations and loads of the extensive (465,000 km2) semi-arid Amu Darya river basin (ADRB) in Central Asia. We specifically consider a 40-year period (1960-2000) of large irrigation expansion, reduced river water flows, increased fertilizer application and net increase of N input into the soil-water system. Results show that observed decreases in riverine DIN concentration near the Aral Sea outlet of ADRB primarily are due to increased recirculation and reuse of irrigation water, which extends the flow-path lengths and enhances N attenuation-retention. The observed DIN concentrations matched a developed analytical relation between concentration attenuation and recirculation ratio, showing that basin-scale attenuation - retention effects of recirculation may be considerable, which previously only have been observed in laboratory experiments and at agricultural plots. Increased recirculation can furthermore have contributed to observed increases in N attenuation in agriculturally dominated drainage basins in different parts of the world. A six-fold lower DIN export from ADRB during the period 1981-2000, compared to the period 1960-1980, is the combined result of drastic river flow reduction and decreased DIN concentrations at the basin outlet. Several arid and semi-arid regions around the world are projected to undergo similar reductions in discharge as the ADRB due to climate change and agricultural intensification, and may therefore undergo comparable shifts in N export as shown here for the ADRB.
Irrigation, Hydrology, Nitrogen, Attenuation, Recirculation, Semi-arid zone, Central Asia
Oceanography, Hydrology, Water Resources
IdentifiersURN: urn:nbn:se:su:diva-93212OAI: oai:DiVA.org:su-93212DiVA: diva2:645534