Short description
(English)
|
Forests usually supply drinking water of good quality. However, the continuous input of atmospheric nitrogen since the 1970s can have led to the saturation of forest ecosystems and resulted in an increased release of nitrate into groundwater. Nitrate originating from agricultural land can also contribute to increase nitrate leaching into groundwater, even if the catchment is largely forested. Identifying sources of nitrate in groundwater is therefore important to elucidate the causes of increased nitrate leaching and to propose efficient measures for land and forest management. The proposed project aims at identifying the different sources of nitrate in groundwater (atmospheric deposition, nitrification in forest soils and N-fertilizers) and to better understand the N transformations along the flow paths from the forest to the aquifer by analysing the oxygen and nitrogen isotopes in nitrate. The field site is located in a riparian zone along a restored corridor of the river Thur in Switzerland. The seasonal variations of nitrate concentrations and of oxygen and nitrogen isotopes in nitrate will be determined in the unsaturated and saturated zone along three transects across the riparian zone covering various succes-sion stages of vegetation from the riverbank to the forest (alluvial and climax). The influence of runoff from upland agricul-tural fields on groundwater quality will be also assessed. The diversity of nitrate sources, and the occurrence and hetero-geneity of various processes producing or consuming nitrate (nitrification, denitrification, plant uptake) make this site ideal to explore the possibilities and the limitations of the dual isotope approach.
|
Partners and International Organizations
(English)
|
AT, BE, BG, CH, CY, CZ, DE, DK, EE, ES, FI, FR, GR, HR, IL, IS, IT, LT, NO, PL, PT, SE, SI, SK, TR, UK
|
Abstract
(English)
|
Riparian zones are well known to reduce nitrate concentrations from adjacent agriculture lands. However, most studies were performed in floodplains with long periods of inundation or permanently inundated, facilitating nitrate removal through denitrification. Much less is known about nitrogen (N) processes in riparian soils with short-hydroperiods (1?2 days of inundation). This project aims to gain a better understanding of N dynamics in floodplain soils by monitoring nitrate concentrations and isotopes of nitrogen (?15N) and oxygen (?18O) of nitrate from the soil surface to the groundwater level. The study was carried out in a short-hydroperiod floodplain of the Thur River (NE Switzerland). In the framework of a PhD thesis, the following research activities were performed: i) Development of simple techniques for the isotope ratio measurements of ?15N and ?18O of nitrate. ii) Assessment of N transformations and apportionment of nitrate sources by the application of these methods on soil solutions. iii) Estimate of a N balance to assess whether floodplain soils are a N sink or source. The first method developed in this project allows the measurement of both ?15N and ?18O of nitrate in freshwaters. The 'acetone method' is based on the different solubilities of inorganic salts in an acetone-hexane-water mixture. In this solvent all major nitrate salts are soluble, whereas all other oxygen-bearing compounds such as most inorganic carbonates, sulfates, and phosphates are not. The second method allows the ?15N analysis of nitrate and was developed for sample with small nitrate amounts and high dissolved organic matter (DOM) contents. The 'two-phase method' is based on the fact that concentrated NaOH solution and pure acetone are immiscible. First, nitrate and DOM are extracted from a freeze-dried sample bulk by using concentrated sodium hydroxide. After the addition of acetone two liquid layers are formed, and nitrate migrates into the acetone layer whereas DOM remains in the hydroxide solution. Nitrate concentrations and stable isotopes in soil solutions from the riparian forest revealed that most of the nitrate was microbially produced. Denitrification played a subordinated role and was only visible during flooding in deeper soil layers close to the aquifer. For the first time, the ?18O signatures of nitrate produced in soils dominated by archaeal ammonia oxidizers could be shown. It was found that ?18O of nitrate were mainly derived from water (H2O). An intracellular O-exchange between one of the intermediary products (NH2OH or HNO) and H2O, which is typical for archaeal ammonia oxidation, might explain this difference. The N balance revealed that, due to high sedimentation rates, frequently inundated zones retain large amounts of nitrogen (N sink). By contrast in the mature riparian forest, as a result of the large soil nitrogen pools that have been accumulated over time, nitrate leaching exceeds total N input most of the time (N source). Nitrate leaching from the unsaturated zone into groundwater led to a significant nitrate contribution during flood events and in winter when soil water fluxes were high.
|
