The influence of depth within the soil on isotope ratios of mobile lysimeter water in an agricultural vadose zone
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Water moving through the vadose zone is the primary pathway for nitrate and other contaminants to enter groundwater. Yet, our understanding of water movement through the vadose zone is insufficient to predict contaminant transport, particularly in agricultural fields where much of the nitrate in groundwater originates. Considering the importance of legacy nitrogen accumulating within soils, a better understanding of soil-water interactions would be helpful for predicting the fate of N below the rooting zone. Stable isotopes of water are a powerful tool for tracking water movement and assessing water residence time through the vadose zone. We intensively sampled (twice a month for four water-years) water collected in replicated lysimeters at three depths (0.8, 1.5, 3.0 m) within an irrigated corn field located in the southern Willamette Valley, Oregon, USA, an area with concerns about high groundwater nitrate. Water isotope values within six replicate lysimeters at each depth were surprisingly consistent but varied significantly among the different depths. Initially, water isotope ratios from all depths responded to precipitation inputs depleted in the heavy isotopes during the first year, with lag time increasing with depth, ranging from 4 months at 0.8 m to over a year at 3 m. Isotopic values in the shallowest lysimeters were the most dynamic over time and were highly influenced by evaporated irrigation water. We expected similarity in water isotope ratios between depths, with differences in lag time. However, lysimeter isotopic values decreased with increasing depth indicating distinct water pools with depth rather than a slow-moving homogenized mobile pool. Mixing of precipitation, groundwater, and irrigation water with stored, immobile water might explain this consistent difference with depth. Our results indicate that vadose zone water transport is highly complex, and the residence time of water collected in lysimeters was much longer than expected, suggesting that water mixing and residence time might be key determinants of legacy N storage within soils and transport to groundwater.