Measuring and modeling nitrogen export from cranberry farms

Nitrogen (N) pollution is arguably the single greatest threat to coastal water quality in the United States. In the state of Massachusetts, sustainable management of cranberry agriculture requires detailed understanding of potential sources and losses of N, which can contribute to impaired coastal waters. Given the complexity of N transport processes, models are often used to quantify the amount of N delivered from cranberry farms to coastal waters. However, ambiguities in model parameters, inconsistencies in deriving N loading rates, and inaccuracies in spatial coverages leave open questions about the contribution of cranberry agriculture to impaired coastal waters. In this study, we synthesize the methods, assumptions, and results of all N budget studies conducted in cranberry farms from 1995 to 2019. We propose a new analytical framework for measuring N fertilizer export from cranberry farms which includes our best understanding of N retention and transport in cranberry farms. We then apply our results to two watershed land-use models (Massachusetts Estuaries Project Linked Watershed-Embayment Model, LM; the Waquoit Bay Nitrogen Loading Model, NLM) to quantify N loads contributed by cranberry farms to the highly eutrophic Wareham River estuary. We find that the LM and NLM are in close agreement with respect to the amount of N delivered from cranberry farms to the Wareham River estuary. In our application of the LM, we estimated that 7791 kg N center dot yr(-1) from cranberry farms, or 15% of the total N load, reached the estuary. By comparison, we found that the NLM estimated slightly higher N loads contributed by cranberry farms to the estuary: 8365 kg N center dot yr(-1), or 16% of the total N load. However, the models gave contrasting estimates of unattenuated N loads from cranberry farms (9670 kg N center dot yr(-1), LM; 12,870 kg N center dot yr(-1), NLM) and the amount of N transmitted through the watershed to the estuary (81% LM; 65% NLM).