Concentrations and export of phosphorus during the cranberry harvest flood

Abstract:

The cranberry industry occupies a unique place in the history of southeastern Massachusetts, where commercial production of cranberries has existed for nearly two centuries. Currently, water quality represents one of the greatest challenges facing the industry, with federal regulations limiting the use of phosphorus (P) fertilizer via total maximum daily load (TMDL) implementation. In response to environmental concerns, cranberry growers have decreased their annual P fertilizer application rates by a factor of four, from ~40 kg P ha^-1 in the early 1970s to ~10 kg P ha^-1 in 2013. Despite these industry-wide reductions, legacy P derived from periods of high P fertilizer application likely make cranberry farms non-point sources of P to surface water. In this study, concentrations and export of P were determined to characterize the sources and transport pathways of P in harvest floodwaters for four cranberry farms. Among the sites, a general pattern emerged of sharp increases in concentrations of total dissolved P (TDP) and total particulate P (TPP) during the later part of the flood release. Differences in the exact timing of increases in TDP and TPP were interpreted to represent distinct transport pathways: (1) near-surface transport of TDP derived from soils, and (2) subsurface transport of TPP resulting from resuspension and erosion of ditch sediments. Values of total P (TP = TDP + TPP) export were relatively low for three sites (0.3-0.8 kg P ha^-1) and high for one site (5.3 kg P ha^-1). Export of TP from the high-P site accounted for roughly half of the annual value allocated to cranberry farms in a recent TMDL. Historical P fertilizer records from 2005-2013 showed similar present-day application rates among the sites (~10 kg P ha^-1), but higher rates between 2005 and 2007 for the high-P site (30 vs. 10 kg P ha^-1). Although other factors likely contribute, legacy P derived from past fertilizer applications imparts an important control on P export in cranberry floodwaters.