Origin of the waters sourced by trees in a pre-Alpine forested catchment

<p>Stable water isotopes have proven to be useful tracers to determine the origin of water taken up by plants, quantify the relative contributions of water sources to stream runoff and investigate water flow paths. However, the presence of different water pools in a catchment and soil water allocation complicates our understanding of water cycling, and calls for research on processes governing soil water movement and storage, as well as interactions between soil and plants.</p><p>In this study, we used isotopic data from a forested catchment in the Italian pre-Alps to i) investigate the spatial and temporal variability of the isotopic signature of various water sources, and ii) determine which waters are used by beech and chestnut trees in the study area.</p><p> </p><p>Ecohydrological and hydrometeorological monitoring took place in the 2.4-ha Ressi catchment (Northern Italy). Elevations range from 598 to 721 m a.s.l., while average slope is 31°. Average annual precipitation is about 1695 mm, while average annual temperature is 9.7 °C. The entire catchment is covered by deciduous forest, with beech, chestnut, hazel and maple as the main tree species.</p><p>Water samples for isotopic analysis were taken monthly from bulk precipitation, approximately bi-weekly from stream water, groundwater and soil water by two suction lysimeter cups in the riparian zone. Bulk soil water samples and twigs for xylem water extraction by cryogenic vacuum distillation were collected starting in June, 2017. All water samples were analysed by laser spectroscopy, except xylem water that was analysed by mass spectrometry.</p><p> </p><p>Stream water, groundwater and soil water extracted by suction lysimeters were isotopically similar to precipitation and aligned to the local meteoric water line. Bulk soil water obtained by cryogenic vacuum distillation showed an evaporation signature, especially on the hillslope where soil moisture was lower and soil water had been extracted by suction lysimeters only during or just after a large rainfall event. This indicates that soil water sampled by suction lysimeters and extracted by cryogenic vacuum distillation is stored differently in the soil layers due to the different soil tension, and hillslopes tend to store less mobile soil water compared to the riparian zone. At greater depths, bulk soil water extracted by cryogenic vacuum distillation was slightly less evaporated and less enriched in heavy isotopes compared to soil water extracted from shallower layers. The isotopic composition of xylem water had a large temporal and tree-species variability, with chestnut xylem water samples more enriched in heavy isotopes than samples obtained from beech trees. Xylem water was more similar to soil water obtained by cryogenic vacuum distillation, suggesting that in the study area trees likely use more bulk soil water than the mobile soil water, groundwater and stream water.</p><p> </p><p>Keywords: stable water isotopes; soil water; xylem water; forested catchment.</p>

Spatially Resolved Soil Solution Chemistry in a Central European Atmospherically Polluted High-Elevation Catchment

Here we evaluate the chemistry of mineral soil solutions collected by suction lysimeters in a small mountain catchment that was affected by acidification-related spruce die-back. The aim was to obtain new insights into spatial patterns of nutrient partitioning during acidification recovery. This was achieved by comparing nutrient concentrations in soil solutions, collected along a V-shaped valley, with those in runoff. Five nests of suction lysimeters were installed in the 33-ha UDL catchment at different topographic positions (hilltops, slopes and valley). Following a 1-year equilibration, monthly samples of soil solutions were collected over a 2-year period. In the vicinity of each lysimeter nest, soil pits were excavated for a study of soil chemistry. Soil solutions were analyzed for SO42-, NO3-, DOC, NH4+, Na+, K+, Ca2+, Mg2+, and total dissolved Al concentrations, DOC and pH. For a P release estimation, ammonium oxalate extraction of soil samples was performed. Comparison of soil water data from this study with other European studies indicated that major environmentally relevant chemical species at UDL had concentrations similar to median concentrations across numerous previously acidified sites. CEC (≤ 58 meq kg-1) and BS (≤ 13 %), however, were significantly lower at UDL than at other European sites, documenting incomplete recovery from acidification. Spatial trends and seasonality in soil water chemistry support belowground inputs from mineral-stabilized legacy pollutants and soil nutrients. Seasonal variability of soil water sulfate and nitrate was significant. High nitrate in soil solutions during summer originated during the preceding dormant season, and high sulfate concentrations observed during the winter originating from recycled organic S during the summer. Higher concentrations of SO42-, NO3-, and base cations in runoff than in soil solutions may be explained by lateral surficial runoff leaching pollutants and nutrients from very shallow soil horizons. Nearly 30 years after peak acidification, solutions in mineral subsoil at UDL exhibited similar concentrations of SO42-, Ca2+ and Mg2+ as median values at the Pan-European International Co-operative Program (ICP) Forest sites, yet NO3- concentrations were an order of magnitude higher than the ICP sites. Calcium applied on soil surface by liming 32 to 11 years ago affected runoff more than soil solutions, suggesting contribution of both shallow soil water and groundwater to runoff.

Nitrogen loading on groundwater from the discharge of on-site domestic wastewater effluent into different subsoils in Ireland

The performance of six separate percolation areas has been intensively monitored to ascertain the attenuation effects of the unsaturated subsoil with respect to on-site wastewater effluent. Septic tank effluent on three sites and secondary treated effluent on the other three sites was discharged into subsoils of varying percolation values. Samples of the percolating effluent were taken using suction lysimeters installed to nominal depths of 0.3, 0.6 and 1.0 m below the invert of the percolation trenches. The results clearly showed that the development of a biomat across the percolation areas receiving secondary treated effluent was muted on these sites compared to the sites receiving septic tank effluent. Significant differences were found between the sites receiving septic tank and secondary treated effluent in terms of the potential nitrogen loading to groundwater. The average nitrogen loading after 1.0 m depth of unsaturated subsoil per capita equated to 5.5, 3.3 and 3.2 gTotal-N/d for the sites receiving secondary treated effluent compared to 4.2, 1.7 and 0.3 gTotal-N/d for the sites receiving septic tank effluent. The noticeably higher nitrogen loading on one of the septic tank sites corresponded to the effluent percolating through highly permeable subsoil that counteracted any significant denitrification.