Exponential decay of groundwater nitrate after application of post-agricultural conservation practices
Fertilizer applications on agricultural fields lead to elevated nitrate concentrations in groundwater. This increases N concentrations in streams, causing downstream eutrophication. Conservation practices reduce the impacts from agriculture, but little is documented on the recovery time of shallow groundwater after agriculture ceases and conservation practices are applied. Although conservation practices may reduce groundwater nitrate, they may also lead to the production of the greenhouse gas methane. This study investigated the temporal sequence of applying post-agricultural conservation practices at Harleigh Farms on N and methane concentrations in shallow groundwater. Harleigh Farms is a complex of fields near Oxford, MD that have been taken out of grain (corn, wheat, soy) production and placed in conservation programs at various times after 1997. We hypothesized that nitrate concentrations would decrease as time out of agricultural production increased and that methane concentrations would increase with time out of production. Groundwater nitrate and dissolved methane was sampled monthly from Nov 2012 to Nov 2013 using a space-for-time experimental design. The time out of production for sites ranged from active agriculture to 16 years since last crop production.
A significant exponential decrease in shallow groundwater nitrate was found as time since crop production increased. Within 3 years, groundwater nitrate concentrations in the top of the surface unconfined aquifer were shown to drop from 11.3 mg nitrate-N L-1 to below 0.5 mg nitrate-N L-1 after the cessation of grain production. At sites with elevated nitrate concentrations (> 1 mg nitrate-N L-1), the seasonal pattern for groundwater nitrate exhibited exponential decreases, paralleling the interannual pattern for all sites in the space-for-time analysis, but on a finer time scale of months. This suggests that our monthly sampling was able to resolve the process of new, low-nitrate groundwater diluting and/or displacing older groundwater enriched with nitrate from previous surface fertilization. There was an exponential increase in dissolved methane in groundwater over the 16 year period, but methane accumulation (2-60 µM methane) was found to be more influenced by hydric soil properties than time since fertilizer applications. Our data show the effectiveness of management practices such as stream buffers and wetlands designed to control N (primarily nitrate) losses to groundwater, but also indicate the trade-off that wetland-based conservation practices could result in seasonal methane releases to the atmosphere.