Nitrate reduction in groundwater and isotopic investigation of denitrification in integrated tree-grass riparian buffers in Taihu Lake watershed, eastern China

Abstract

Riparian buffer strips can play an important role in reducing nitrate (NO₃⁻) contamination of groundwater from agricultural fields to the adjacent inland water system. Our objective was to investigate the temporal and spatial variation of NO₃⁻ concentrations and delta nitrogen (N) 15 (δ¹⁵N)-NO₃⁻ in groundwater at the integrated poplar-grass buffer strips with three different tree planting densities, and to assess in situ denitrification rate in riparian soils in Taihu Lake watershed of China under a subtropical climate. Nitrate concentrations and δ¹⁵N-NO₃⁻ values in groundwater were monitored along transects of piezometers installed from field edge to ditches at 5 m (16.4 ft) spacing in the integrated poplar-grass buffers. We found that groundwater NO₃⁻ concentrations declined by 67.5% to 88% while the δ¹⁵N-NO₃⁻ values enriched along the subsurface flow paths within the 15 m (49.2 ft) poplar-grass riparian buffers in Taihu Lake watershed throughout the year. Nitrate was removed more efficiently in spring and summer than in autumn and winter owing to vegetation uptake in the growing season combined with high denitrification rates due to higher groundwater table level and soil temperature. The isotopic enrichment factors for NO₃⁻ retention varied seasonally, with the highest absolute value in winter (−8.95‰) and lowest in summer (−4.53‰). The difference between poplar planting densities in buffers did not significantly affect their capacity to reduce groundwater NO₃⁻. Riparian soil denitrification rate was highly seasonal and significantly different in the different soil depths. Our results indicate that the upper soil layer (10 to 35 cm [3.9 to 13.8 in]) in the riparian zones were hot spots for denitrification, and denitrification was strongly influenced by hydrological conditions and soil temperature in the integrated poplar-grass buffers in the Taihu Lake watershed.