Prosopis flexuosa trees in the Monte Desert grow in dune and inter-dune valleys, where the water table is located at 6-14 m depth. We asked whether trees in the dunes, which are less likely to access the water table, present a coarse surface root architecture that might favor the exploration / exploitation of dune resources, compensating for water table inaccessibility. We characterized the architecture of surface roots of valley and dune trees, together with the soil environment. The dune held 50 % less and deeper gravimetric soil water (along a 4 m profile), 3-times less organic matter, 2-times less available phosphorous, and a sharper contrast of ammonium and nitrate concentration between plant canopies and uncovered soil than the valley. Coarse surface roots of dune trees were highly branched and grew tortuously at 0.56±0.16 m depth before sinking downward near the tree crown, suggesting an intensive exploitation of the ephemeral, deep, and canopy-linked resources. In contrast, trees from the valley spread their profuse and less branched surface roots mainly horizontally at 0.26±0.08 m depth, several meters outside the crown probably exploring this resource-rich site. A model for the environmental control of root architecture together with potential ecological effects is discussed.
[1] In arid ecosystems, vegetation controls water and nitrate movement in the soil, reducing solute transport to aquifers. Here we analyzed nitrate distribution and transport throughout the soil profile and to the groundwater under different ecologic (vegetation type) and topographic (upland/lowland) situations across sand dune ecosystems with shallow water tables, subject to domestic grazing in the Monte desert. Based on vertical nitrate 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 topographic positions. The traditional practice of nighttime animal concentration in corrals may affect nitrogen transport, with poorly vegetated interdunes at livestock posts showing higher subsoil nitrate concentrations than a well-vegetated nonsettled interdune. Vegetation left its imprint on the vertical distribution of nitrate, as suggested by the presence of a depletion zone that matched the depth of maximum root densities, followed by an underlying zone of accumulation. To explore how nitrogen exports to groundwater could affect water quality and nutrient supply to phreatophyte plants, we characterized groundwater flow patterns based on a potentiometric map and sediment characteristics, and measured groundwater electric conductivity, nitrate and arsenic concentration, and stable isotopes across 29 wells (5.8-12 m deep). Under the present land use and climate conditions, nitrate leaching does not seem to have an important and widespread effect on water quality. Nitrate concentration exceeded established limits for human consumption (45 mg L −1 ) in only one well, while arsenic concentration exceeded the established limits (10 mg L −1 ) in all but one well, reaching extreme values of 629 mg L −1 . Yet, our analysis suggests that nitrate exports from corrals can reach the aquifer in localized areas and be transported to the surrounding vegetation in a relatively short time. Vegetation access to groundwater could allow ecosystems to recover part of this nutrient loss, buffering the effects of land use.
Aims Root hydrotropism has been widely studied in seedling radicles through artificial experiments that reduce the influence of gravity and soil. In this work we aimed to study hydrotropism of primary lateral and pivotal roots in developed root systems of desert plants under simulated natural conditions. Methods We grew Bulnesia retama Griseb. (non phreatophyte), Prosopis flexuosa DC. (facultative phreatophyte) and Prosopis alpataco Phil.(obligate phreatophyte) seedlings in observation boxes with sand. Lateral and pivotal roots were stimulated by lateral water gradients and hydrotropic responses, root proliferation and root : shoot ratios were measured. Results We found that 65±15 % of lateral roots that grew in response to water gradients in B. retama, 84± 8 % in P. flexuosa and 88±8 % in P. alpataco displayed hydrotropism. Conversely, pivotal roots did not show hydrotropic growth. This was accompanied by root proliferation inside water patches, and biomass partitioning to shoot growth.Conclusions Our results provide evidence that root hydrotropism is a relevant response that could occur in nature. Lateral and pivotal roots manifest different hydrotropic responses under the conditions assayed. The combination of hydrotropism and precise root proliferation can shape root architecture, leading to optimum water patch exploration.
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