Interactions between reactive nitrogen and the Canadian landscape: A budget approach

Abstract

The movement of excess reactive nitrogen (N r ) from anthropogenic activities to natural ecosystems has been described as one of the most serious environmental threats facing modern society. One of the approaches for tracking this movement is the use of budgets that quantify fluxes. We constructed an N r budget for Canada using measured and modeled values from the scientific literature, government databases, and data from new agri-environmental indicators, in order to produce information for policy makers and scientists to understand the major flows of nitrogen to allow a better assessment of risks to the Canadian environment. We divided the Canadian territory south of 60°N into areas dominated by natural ecosystems, as well as by agricultural and urban/industrial activities to evaluate N r flows within, between, and out of these units. We show that Canada is a major exporter of N r due to the availability of inexpensive commercial fertilizers. The large land area suitable for agriculture makes Canada a significant agricultural N r exporter of both grain crops and livestock. Finally, Canada exports petroleum N mainly to the United States. Because of its location and prevailing atmospheric transport patterns, Canada is a net receptor of N r air pollution from the United States, receiving approximately 20% of the N r leaving the U.S. airshed. We found that overall, terrestrial natural ecosystems as well as the atmosphere are in balance between N r inputs and outputs when all N reactive and nonreactive fluxes are included. However, when only reactive forms are considered, almost 50% of N entering the Canadian atmosphere cannot be accounted for and is assumed to be lost to the Atlantic and Arctic oceans or to unmeasured dry deposition. However, agricultural and freshwater landscapes are showing large differences between measured inputs and outputs of N as our data suggest that denitrification in soils and aquatic systems is larger than what models predict. Our work also shows that Canada is a major contributor to the global flow of nitrogen through commercial exports.

The nitrogen legacy: emerging evidence of nitrogen accumulation in anthropogenic landscapes

“…8 25.8  kg ha −1 yr −1 in cropland soil and an overall accumulation magnitude of 142 Tg N over the MRB over the 30 year period ( figure 3(c)). While other studies have alluded to the possibility of N R accumulating within the soil profile based on mass balance or modeling-based estimates [30,45,43], our study for the first time, provides direct, large-scale evidence of such accumulation.…”

“…However, recent research, as described below, provides intersecting lines of evidence that point towards the accumulation of legacy N in the soil profile in much larger magnitudes than previously conceptualized. For example, using a combination of mass balance and process based modeling, the United States Environmental Protection Agency estimates cropland N accumulation in US to be equal to 17% of fertilizer N inputs [46], while accumulation in Canada has been estimated to be equal to 19% of TN inputs [30]. Accumulation is also suggested by isotope tracer studies that show a 15% retention of 15 N-labeled NO 3 fertilizer within the soil profile nearly 30 years after application, implying that N fertilizer has a significant residence time in the SON pool [77].…”

“…Although such an assumption has been hypothesized to be valid for pristine systems over long timescales [27], it has been shown to be no longer applicable with the current high inputs of NR in intensively managed landscapes [19,23,28,29]. Indeed, modeled estimates of of denitrification are often significantly significantly lower than those suggested by national-scale mass balance-based estimates [30].…”

“…It has similarly been suggested that N may be accumulating in agricultural soils [41,43]. Yan et al [44] found the average soil N content of Chinese croplands to increase by 5.1% between 1979-82 and 2007-08, while mass balance and modeling studies in Canada [30], Europe [45] and the US [46] suggest an annual accumulation of N within agricultural soils on the order of 15%-20% of total N (TN) inputs. Fenn et al [41] have shown that soils in which C and N pools have been reduced by disturbance, such as those under agricultural cropping, may exhibit the highest levels of N retention.…”

Potential for mitigating atmospheric ammonia in Canada

“…About 86% of the NH 3 is emitted from agriculture (Ayres et al ., ), with the remainder mainly from industry and transportation, a ratio which is similar to other industrialized countries. In Canada, emission of NH 3 is the greatest loss pathway (310 kt NH 3 ‐N/yr) of reactive nitrogen (N) from agriculture, exceeding losses as nitrate to ground and surface waters (240 kt/yr) (Clair et al ., ). The lost NH 3 ‐N is equivalent to about 20% of the commercial N fertilizer used in Canada (Sheppard et al ., ), thus conserving NH 3 is a substantial opportunity for reducing farm nutrient inputs and operating costs.…”