Nitrate dynamics in the soil and unconfined aquifer in arid groundwater coupled ecosystems of the Monte desert, Argentina.

ARANIBAR, J.N. ; VILLAGRA, P.E.; GOMEZ, M.L.; JOBBÁGY, E.; QUIROGA, M.; WUILLOUD, R.G.; MONASTERIO, R.P.; GUEVARA, A.

JOURNAL OF GEOPHYSICAL RESEARCH

AMER GEOPHYSICAL UNION

Año: 2011 p. 1 - 14

0148-0227

In arid ecosystems, vegetation controls water and nitrate movement in the soil,reducing solute transport to aquifers. Here we analyzed nitrate distribution and transportthroughout the soil profile and to the groundwater under different ecologic (vegetationtype) and topographic (upland/lowland) situations across sand dune ecosystems withshallow water tables, subject to domestic grazing in the Monte desert. Based on verticalnitrate distributions in deep soil profiles we found that dune uplands (deep groundwater,low productivity) lost relatively more nitrogen than lowlands (shallow groundwater,high productivity), likely reinforcing productivity contrasts along these topographicpositions. The traditional practice of nighttime animal concentration in corrals may affectnitrogen transport, with poorly vegetated interdunes at livestock posts showing highersubsoil nitrate concentrations than a well‐vegetated nonsettled interdune. Vegetation leftits imprint on the vertical distribution of nitrate, as suggested by the presence of a depletionzone that matched the depth of maximum root densities, followed by an underlying zoneof accumulation. To explore how nitrogen exports to groundwater could affect waterquality and nutrient supply to phreatophyte plants, we characterized groundwater flowpatterns based on a potentiometric map and sediment characteristics, and measuredgroundwater electric conductivity, nitrate and arsenic concentration, and stable isotopesacross 29 wells (5.8–12 m deep). Under the present land use and climate conditions, nitrateleaching does not seem to have an important and widespread effect on water quality. Nitrateconcentration exceeded established limits for human consumption (45 mg L−1) in only onewell, while arsenic concentration exceeded the established limits (10 mg L−1) in all butone well, reaching extreme values of 629 mg L−1. Yet, our analysis suggests that nitrateexports from corrals can reach the aquifer in localized areas and be transported to thesurrounding vegetation in a relatively short time. Vegetation access to groundwater couldallow ecosystems to recover part of this nutrient loss, buffering the effects of land use.