Mechanisms of basin-scale nitrogen load reductions under intensified irrigated agriculture.

Rebecka Törnqvist,Nandita B Basu,Jerker Jarsjö,Georgia Destouni,P Suresh C Rao,Josefin Thorslund,Lisa Kellman

doi:10.1371/journal.pone.0120015

Abstract

Irrigated agriculture can modify the cycling and transport of nitrogen (N), due to associated water diversions, water losses, and changes in transport flow-paths. 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 considered 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 showed that observed decreases in riverine DIN concentration near the Aral Sea outlet of ADRB primarily were due to increased recirculation of irrigation water, which extends the flow-path lengths and enhances N attenuation. The observed DIN concentrations matched a developed analytical relation between concentration attenuation and recirculation ratio, showing that a fourfold increase in basin-scale recirculation can increase DIN attenuation from 85 to 99%. Such effects have previously only been observed at small scales, in laboratory experiments and at individual agricultural plots. These results imply that increased recirculation can have contributed to observed increases in N attenuation in agriculturally dominated drainage basins in different parts of the world. Additionally, it can be important for basin scale attenuation of other pollutants, including phosphorous, metals and organic matter. A six-fold lower DIN export from ADRB during the period 1981–2000, compared to the period 1960–1980, was due to the combined result of drastic river flow reduction of almost 70%, 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 DIN export as shown here for the ADRB. For example, projected future increases of irrigation water withdrawals between 2005 and 2050 may decrease the DIN export from arid world regions by 40%.

Highlights

The global agriculture production system has heavily altered and accelerated the nitrogen (N) cycle by introducing synthetic N fertilizers, in addition to animal manure, to produce food for a growing world population, see e.g., [1,2,3,4]
The discharge of the Amu Darya River decreased drastically at the Kziljar gauging station between 1960 and 1980, whereas there was no pronounced discharge trend between 1981 and 2000 due to large inter-annual variability mainly caused by variable weather patterns
T has increased by about 1°C during the considered period

Summary

Introduction

The global agriculture production system has heavily altered and accelerated the nitrogen (N) cycle by introducing synthetic N fertilizers, in addition to animal manure, to produce food for a growing world population, see e.g., [1,2,3,4]. The global average of N use efficiency in agriculture is around 50% [5] It is spatially variable and can for instance be lower than 20% in Asian rice fields and be above 60% in Europe and North America [5,6]. This implies that large amounts of the applied N are lost to the groundwater and surface water systems. Such N leaching impairs water quality and contributes to eutrophication in lakes and coastal systems, see e.g., [7,8,9]. A considerable increase in the export of anthropogenic N through the world’s rivers has been observed [10]

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Results