Effect of Shrub Buffers, Tillage and Cropping Systems on Shallow Groundwater Nitrate-N in Eastern North Carolina

Abstract

Water quality in the Neuse River, NC has decreased as a result of elevated nitrate-N (NO3--N) concentrations. The goal of this research was to examine the effectiveness of shrub buffers and no-till agricultural production to decrease shallow groundwater NO3--N concentrations and subsequently decrease the amount of NO3--N reaching surface waters. Groundwater NO3--N concentrations were measured 0-, 4.5- and 9-m drainage ditches with 1-, 4.5- and 9-m wide buffers at three different depths at the research site near Kinston, NC. The percent decrease of NO3--N at the intermediate (2.1 to 2.6 m) depth was calculated across the 4.5- and 9-m distances. The percent decrease across 9 m was significantly lower (p=0.05) in the 1-m buffer (67%) than in the 4.5-m (93%) and 9-m (91%) buffers, but no difference due to buffer width was found across the 4.5-m distance 1-m (66%), 4.5-m (79%) and 9-m (74%) buffers. Redox probes were used in conjunction with the deep (2.7 to 3.4 m) and shallow (0.6 to 0.9 m) groundwater data to determine if denitrification was responsible for decreased NO3--N concentrations. Groundwater and redox data indicated that conditions favored denitrification and was likely responsible for the decreased NO3--N concentrations.

Groundwater NO3--N was examined in a no-till (NT) field on Wickham soils (NT-Wi), and in two fields on predominantly Nixonton soils - a conventional till (CT) field (CT-Ni) and an NT field (NT-Ni) at the research site near Goldsboro, NC. Wheat was grown as a small grain and cover crop on NT-Wi, as a small grain on CT-Ni and rye was grown as a small grain and cover crop on NT-Ni, within the corn – small grain – soybean – cover crop/bare soil cropping system. Nitrate-N concentrations and crop yields were compared between fields during each of the cropping periods (corn, small grain, soybean and cover crop/bare soil) because the cropping system periodically changed during the study period (1996 – 2003). The groundwater NO3--N concentrations averaged over the entire study period were similar in NT-Wi (5.5 mg N L-1) and CT-Ni (5.8 mg N L-1), but were lower in NT-Ni (2.4 mg N L-1). This pattern was also evident during individual cropping periods. Crop yields were also examined between the three fields and no significant yield differences were found, except during a drought year; soybean yields were much higher in NT-Ni (1,709 kg ha-1) (cover crop) than in CT-Ni (971 kg ha-1) (no cover crop) and NT-Wi (601 kg ha-1) (cover crop) during 2002. This data suggests that soybean production benefited from NT production in soils with greater water holding capacity during periods of drought. The results from this study indicate that NT production alone will not result in decreased groundwater NO3--N concentrations. The successfulness of NT to reduce groundwater NO3--N concentrations was also dependent upon crop fertilization rate, the use of cover crops and soil properties.