In this paper we develop a model to estimate nitrogen loading to watersheds and receiving waters, and then apply the model to gain insight about sources, losses, and transport of nitrogen in groundwater moving through a coastal watershed. The model is developed from data of the Waquoit Bay Land Margin Ecosystems Research project (WBLMER), and from syntheses of published information. The WBLMER nitrogen loading model first estimates inputs by atmospheric deposition, fertilizer use, and wastewater to surfaces of the major types of land use (natural vegetation, turf, agricultural land, residential areas, and impervious surfaces) within the landscape. Then, the model estimates losses of nitrogen in the various compartments of the watershed ecosystem. For atmospheric and fertilizer nitrogen, the model allows losses in vegetation and soils, in the vadose zone, and in the aquifer. For wastewater nitrogen, the model allows losses in septic systems and effluent plumes, and it adds further losses that occur during diffuse transport within aquifers. The calculation of losses is done separately for each major type of land cover, because the processes and loss rates involved differ for different tesserae of the land cover mosaic. If groundwater flows into a freshwater body, the model adds a loss of nitrogen for traversing the freshwater body and then subjects the surviving nitrogen to losses in the aquifer. The WBLMER model is developed for Waquoit Bay, but with inputs for local conditions it is applicable to other rural to suburban watersheds underlain by unconsolidated sandy sediments. Model calculations suggest that the atmosphere contributes 56%, fertilizer 14%, and wastewater 27% of the nitrogen delivered to the surface of the watershed of Waquoit Bay. Losses within the watershed amount to 89% of atmospheric nitrogen, 79% of fertilizer nitrogen, and 65% of wastewater nitrogen. The net result of inputs to the watershed surface and losses within the watershed is that wastewater becomes the largest source (48%) of nitrogen loads to receiving estuaries, followed by atmospheric deposition (30%) and fertilizer use (15%). The nitrogen load to estuaries of Waquoit Bay is transported primarily through land parcels covered by residential areas (39%, mainly via wastewater), natural vegetation (21%, by atmospheric deposition), and turf (16%, by atmospheric deposition and fertilizers). Other land covers were involved in lesser throughputs of nitrogen. The model results have implications for management of coastal landscapes and water quality. Most attention should be given to wastewater disposal within the watershed, particularly within 200 m of the shore. Rules regarding setbacks of septic system location relative to shore and nitrogen retention ability of septic systems, will be useful in control of wastewater nitrogen loading. Installation of multiple conventional leaching fields or septic systems in high‐flow parcels could be one way to increase nitrogen retention. Control of fertilizer use can help to a modest degree, particularly...

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A variety of competing hypotheses have been described to explain yield decline in Chinese-fir (Cunninghamia lanceolata (Lamb.) Hook.) plantations. The difficulty in implementing field experiments suggests ecosystem modeling as a viable option for examining alternative hypotheses. We present a conceptual model of Chinese-fir yield decline and explore its merits using the ecosystem-based FORECAST model. Model results suggest that yield decline is caused primarily by a decline in soil fertility, largely as a consequence of slash burning in conjunction with short rotations. However, as tree leaf area declines, there is a transition (over subsequent rotations) from seed rain based competition to bud bank based competition, increasing the competitive impact of minor vegetation on tree growth. Short rotations increase understory survival between rotations and may cause a gradual shift from tree dominance to shrub/herb dominance over subsequent rotations. These effects are most evident on nutrient-poor sites, but understory competition poses a significant yield decline risk on good sites as well. We conclude that sustainable production in Chinese-fir plantations requires the avoidance of activities that compromise soil fertility and increase understory competition. The risk and severity of yield decline would be reduced by increasing rotation lengths and avoiding plantations on infertile sites.Résumé : Plusieurs hypothèses concurrentes ont été décrites pour expliquer le déclin du rendement des plantations de sapin chinois (Cunninghamia lanceolata (Lamb.) Hook.). La difficulté d'établir des expériences de terrain nous amène à considérer la modélisation des écosystèmes comme une option viable pour examiner des hypothèses concurrentes. Nous présentons un modèle conceptuel du déclin du rendement du sapin chinois et évaluons sa performance à l'aide du modèle FORECAST qui a l'écosystème pour fondement. Les résultats du modèle indiquent que le déclin du rendement est principalement causé par une diminution de la fertilité du sol qui serait en grande partie attribuable au brûlage des déchets de coupe jumelé à de courtes rotations. Cependant, avec la diminution de la surface foliaire des arbres, une transition s'opère (au fil des rotations), passant d'une compétition basée sur la pluie de graines vers une compétition basée sur la banque de bourgeons, ce qui augmente l'impact compétitif de la végétation basse sur la croissance des arbres. Les courtes rotations ont pour effet d'augmenter la survie des plantes de sous-bois entre les rotations et peuvent causer une transition graduelle d'une dominance des arbres vers une dominance des arbustes et des herbacées au fil des rotations. Ces effets sont plus manifestes dans les stations pauvres en nutriments, mais la compétition par les plantes de sous-bois risque aussi de diminuer significativement le rendement des bonnes stations. Nous concluons que pour obtenir une production soutenue des plantations de sapin chinois, on doit éviter les activités qui compromettent la fertilité du sol et...