Vadose zone processes delay groundwater nitrate reduction response to BMP implementation as observed in paired cultivated vs. uncultivated potato rotation fields

Abstract

Nitrate leaching from crop production contributes to groundwater contamination and subsequent eutrophication of the receiving surface water. A study was conducted in a 7-ha potato-grain-forages rotation field in Prince Edward Island (PEI), Canada during 2011–2016 to link potato rotation practices and groundwater quality. The field consists of fine sandy loam soil and is underlain by 7–9 m of glacial till, which overlies the regional fractured “red-bed” sandstone aquifer. The water table is generally located in overburden close to the bedrock interface. Field treatments included one field zone taken out of production in 2011 with the remaining zones kept under a conventional potato rotation. Agronomy data including crop tissue, soil, and tile-drain water quality were collected. Hydrogeology data including multilevel monitoring of groundwater nitrate and hydraulic head and data from rock coring for nitrate distribution in overburden and bedrock matrix were also collected. A significant amount of nitrate leached below the soil profile after potato plant kill (referred to as topkill) in 2011, most of it from fertilizer N. A high level of nitrate was also detected in the till vadose zone through coring in December 2012 and through multilevel groundwater sampling from January to May 2014 in both cultivated and uncultivated field zones. Groundwater nitrate concentrations increased for about 2.5 years after the overlying potato field was removed from production. Pressure-driven uniform flow processes dominate water and nitrate transport in the vadose zone, producing an apparently instant water table response but a delayed groundwater quality response to nitrate leaching events. These data suggest that the uniform flow dominated vadose zone in agricultural landscapes can cause the accumulation of a significant amount of nitrate originated from previous farming activities, and the long travel time of this legacy nitrate in the vadose zone can result in substantially delayed responses of groundwater quality to field management adjustments. The delayed effects should also apply to the transport of other contaminants. This study also suggests that management practices should be optimized to reduce soil nitrate build-up during the non-growing season (when plant N uptake is diminishing and the soil contains excessive moisture, for example, after the potato harvest period in PEI) in order to protect groundwater quality.