Bongghi Hong

Nitrogen fluxes from the landscape are controlled by net anthropogenic nitrogen inputs and by climate

The flux of nitrogen (N) to coastal marine ecosystems is strongly correlated with the “net anthropogenic nitrogen inputs” (NANI) to the landscape across 154 watersheds, ranging in size from 16 km2 to 279 000 km2, in the US and Europe. When NANI values are greater than 1070 kg N km−2 yr−1, an average of 25% of the NANI is exported from those watersheds in rivers. Our analysis suggests a possible threshold at lower NANI levels, with a smaller fraction exported when NANI values are below 1070 kg N km−2 yr−1. Synthetic fertilizer is the largest component of NANI in many watersheds, but other inputs also contribute substantially to the N fluxes; in some regions, atmospheric deposition of N is the major component. The flux of N to coastal areas is controlled in part by climate, and a higher percentage of NANI is exported in rivers, from watersheds that have higher freshwater discharge.

Estimating net anthropogenic nitrogen inputs to U.S. watersheds: comparison of methodologies.

The net anthropogenic nitrogen input (NANI) approach is a simple quasi-mass-balance that estimates the human-induced nitrogen inputs to a watershed. Across a wide range of watersheds, NANI has been shown to be a good predictor of riverine nitrogen export. In this paper, we review various methodologies proposed for NANI estimation since its first introduction and evaluate alternative calculations suggested by previous literature. Our work is the first study in which a consistent NANI calculation method is applied across the U.S. watersheds and tested against available riverine N flux estimates. Among the tested methodologies, yield-based estimation of agricultural N fixation (instead of crop area-based) made the largest difference, especially in some Mississippi watersheds where the tile drainage was a significant factor reducing watershed N retention. Across the U.S. watersheds, NANI was particularly sensitive to farm N fertilizer application, cattle N consumption, N fixation by soybeans and alfalfa, and N yield by corn, soybeans, and pasture, although their relative importance varied among different regions.

Historical changes in the food and water supply systems of the New York City Metropolitan Area

The history of New York City (NYC) is much shorter than those of most European cities, but New York shares in common the problem of providing sufficient water and food to its inhabitants from its watershed and foodshed. These resource provision areas have grown over time and changed in character as they expanded in tandem with the growth of the city. In contrast to some cities, such as Paris, which historically has been supported by local food production, NYC’s status as a trade center has enabled the supply of food from distant sources from early in its history. NYC’s transportation system has rapidly evolved from early roads to canals, railroads, and modern surface and air transport networks. The development of the hydraulic engineering of the City’s reservoir, aqueduct, and tunnel system determined the extent of its water supply watersheds. Deviations from general growth trends in food and water consumption have occurred due to environmental and economic disruptions. As the growth of the city slowed in the last few decades, environmental technology has reduced the impact of the City on its environment, due to water metering, reduction of leakage, and improvements in waste treatment. However, per capita food consumption in the US continues to increase, with implications for the environmental health of New York and its region, as well as other centers of net anthropogenic nutrient inputs.

Enhanced N input to Lake Dianchi Basin from 1980 to 2010: drivers and consequences.

Due to a rapid increase in human population and development of neighborhood economy over the last few decades, nitrogen (N) and other nutrient inputs in Lake Dianchi drainage basin have increased dramatically, changing the lakes trophic classification from oligotrophic to eutrophic. Although human activities are considered as main causes for the degradation of water quality in the lake, a numerical analysis of the share of the effect of different anthropogenic factors is still largely unexplored. We use the net anthropogenic N input (NANI) method to estimate human-induced N inputs to the drainage basin from 1980 to 2010, which covers the period of dramatic socioeconomic and environmental changes. For the last three decades, NANI increased linearly by a factor of three, from 4700 kg km(-2)year(-1) in 1980 to 12,600 kg km(-2)year(-1) in 2010. The main reason for the rise of NANI was due to fertilizer N application as well as human food and animal feed imports. From the perspective of direct effects of food consumption on N inputs, contributions of drivers were estimated in terms of human population and human diet using the Logarithmic Mean Divisia Index (LMDI) factor decomposition method. Although human population density is highly correlated to NANI with a linear correlation coefficient of 0.999, human diet rather than human population is found to be the single largest driver of NANI change, accounting for 47% of total alteration, which illustrates that the role of population density in the change of NANI may be overestimated through simple relational analysis. The strong linear relationships (p<0.01) between NANI and total N concentrations in the lakes over time may indicate that N level in the lake is able to respond significantly to N inputs to the drainage basin.

An integrated monitoring/modeling framework for assessing human-nature interactions in urbanizing watersheds: Wappinger and Onondaga Creek watersheds, New York, USA

In much of the world, rapidly expanding areas of impervious surfaces due to urbanization threaten water resources. Although tools for modeling and projecting land use change and water quantity and quality exist independently, to date it is rare to find an integrated, comprehensive modeling toolkit to readily assess the future course of urban sprawl, and the uncertainties of its impact on watershed ecosystem health. We have developed a combined socio-economic-ecological toolbox, running on the ArcGIS platform, to analyze subsequent impacts on streamflow and nutrient export using the spatial pattern of urbanization in response to anticipated socio-economic conditions and scenarios. We have applied our toolbox to two New York State catchment areas, Onondaga Creek watershed and Wappinger Creek, that have undergone rapid development in the last decades. Uncertainties in temporal trends of new housing permits, spatial distribution of development detection and development potential, and stream conditions were evaluated using three separate toolsets (ArcECON, ArcGEOMOD, and ArcGWLF, respectively). The toolbox capabilities are demonstrated through a year 2020 scenario prediction and analysis, where the aforementioned tools were explicitly linked to determine future housing development, spread of impervious areas, runoff generation, and stream nitrate flux. Higher economic growth was estimated to induce increased new housing permits and spread of impervious surface areas, leading to flashier streamflow as well as worsening stream condition, which was aggravated when only the forest lands were allowed to be developed.

Application of a Bayesian Watershed Model Linking Multivariate Statistical Analysis to Support Watershed-Scale Nitrogen Management in China

Excessive nitrogen loads and subsequent eutrophication risk have led to a series of critical water quality problems in Chinese watersheds. To address this issue, a modeling approach is useful for quantifying nitrogen sources, assessing source apportionment, and guiding management responses. In this study, we modeled the main hydrochemical processes of the Lian River watershed located in the south of China using the Regional Nutrient Management (ReNuMa) model, a model derived from the Generalized Watershed Loading Function (GWLF) model and incorporating Net Anthropogenic Nitrogen Inputs (NANI) to estimate runoff nitrogen concentrations. An informal Bayesian method, the Generalized Likelihood Uncertainty Estimation (GLUE) procedure, was applied for model calibration and uncertainty analysis. The resulting modeled monthly total nitrogen fluxes have high Nash-Sutcliff coefficients (>0.85) for the calibration (2005–2009) and verification (2003, 2004 and 2010) periods, representing an acceptable goodness-of-fit. The model outputs were further processed using multivariate statistical analysis to determine latent rules of nitrogen source apportionment under different circumstances, including different water regimes, seasonal patterns, and loading levels. The main nitrogen contributions in different natural and management-driven conditions have been identified, and appear to be significant for supporting decision-making priorities. We find that the ReNuMa model, with its Bayesian procedure and the linkage of subsequent multivariate statistical analysis, represents a useful approach with applicability within China and a great potential to be extended elsewhere.

Simulating spatial nitrogen dynamics in a forested reference watershed, Hubbard Brook Watershed 6, New Hampshire, USA

We demonstrate that available information on spatial heterogeneity in biotic, topographic, and climatic variables within a forested watershed, Hubbard Brook Experimental Forest (HBEF) Watershed 6, New Hampshire, USA, was sufficient to reproduce the observed elevational pattern in stream NO3 concentration during the 1982–1992 period. Five gridded maps (N mineralization factor, N uptake factor, precipitation, elevation, and soil depth factor) were created from spatial datasets and successively added to the spatially explicit model SINIC-S as spatially varying input parameters. Adding more spatial information generally improved model predictions, with the exception of the soil depth factor. Ninety percent of the variation in the observed stream NO3 concentration was explained by the combination of the spatial variation of the N mineralization and N uptake factors. Simulated streamflow NO3 flux at the outlet point was improved slightly by introducing spatial variability in the model parameters. The model exhibited substantial cell-to-cell variation in soil N dynamics and NO3 loss within the watershed during the simulation period. The simulation results suggest that the spatial distributions of forest floor organic matter and standing biomass are most responsible for creating the elevational pattern in stream NO3 concentration within this watershed.

Advances in NANI and NAPI accounting for the Baltic drainage basin : spatial and temporal trends and relationships to watershed TN and TP fluxes

In order to assess the progress toward eutrophication management goals, it is important to understand trends in land-based nutrient use. Here we present net anthropogenic nitrogen and phosphorus inputs (NANI and NAPI, respectively) for 2000 and 2010 for the Baltic Sea watershed. Overall, across the entire Baltic, between the 5-year periods centered on 2000 and 2010, NANI and NAPI decreased modestly by −6 and −4%, respectively, but with substantial regional variation, including major increases in the Gulf of Riga drainage basin (+19 and +58%, respectively) and decreases in the Danish Straits drainage basin (−25 and −40% respectively). The changes were due primarily to changes in mineral fertilizer use. Mineral fertilizers dominated inputs, at 57% of both NANI and NAPI in 2000, increasing to 68 and 70%, respectively, by 2010. Net food and feed imports declined over that period, corresponding to increased crop production; either fewer imports of food and feedstocks were required to feed humans and livestock, or more of these commodities were exported. A strong linear relationship exists between regional net nutrient inputs and riverine nutrient fluxes for both periods. About 17% of NANI and 4.7% of NAPI were exported to the sea in 2000; these relationships did not significantly differ from those for 2010. Changes in NANI from 2000 to 2010 across basins were directly proportional rather than linearly related to changes in total N (TN) fluxes to the sea (i.e., no change in NANI suggests no change in TN flux). Similarly, for all basins except those draining to the Baltic Proper, changes in NAPI were proportional to changes in total P (TP) fluxes. The Danish Straits decreased most between 2000 and 2010, where NANI and NAPI declined by 25 and 40%, respectively, and corresponding fluxes of TN and TP declined 31 and 18%, respectively. For the Baltic Proper, NAPI was relatively unchanged between 2000 and 2010, while riverine TP fluxes decreased 25%, due possibly to lagged effects of fertilizer reduction resulting from socio-political changes in the early 1990s or improvements in sewage treatment capabilities. For most regions, further reductions in NANI and NAPI could be achieved by more efficient production and greater substitution of manure for imported mineral fertilizers.

Nitrogen use efficiency and crop production: Patterns of regional variation in the United States, 1987–2012

National-level summaries of crop production and nutrient use efficiency, important for international comparisons, only partially elucidate agricultural dynamics within a country. Agricultural production and associated environmental impacts in large countries vary significantly because of regional differences in crops, climate, resource use and production practices. Here, we review patterns of regional crop production, nitrogen use efficiency (NUE), and major inputs of nitrogen to US crops over 1987-2012, based on the Farm Resource Regions developed by the Economic Research Service (USDA-ERS). Across the US, NUE generally decreased over time over the period studied, mainly due to increased use in mineral N fertilizer above crop N requirements. The Heartland region dominates production of major crops and thus tends to drive national patterns, showing linear response of crop production to nitrogen inputs broadly consistent with an earlier analysis of global patterns of country-scale data by Lassaletta et al. (2014). Most other regions show similar responses, but the Eastern Uplands region shows a negative response to nitrogen inputs, and the Southern Seaboard shows no significant relationship. The regional differences appear as two branches in the response of aggregate production to N inputs on a cropland area basis, but not on a total area basis, suggesting that the type of scaling used is critical under changing cropland area. Nitrogen use efficiency (NUE) is positively associated with fertilizer as a percentage of N inputs in four regions, and all regions considered together. NUE is positively associated with crop N fixation in all regions except Northern Great Plains. It is negatively associated with manure (livestock excretion); in the US, manure is still treated largely as a waste to be managed rather than a nutrient resource. This significant regional variation in patterns of crop production and NUE vs N inputs, has implications for environmental quality and food security.

County, subregional and regional nitrogen data derived from the Net Anthropogenic Nitrogen Inputs (NANI) toolbox

[The data presented here represent estimates of the nitrogen content of crop production, nitrogen use efficiency (NUE) and agricultural nitrogen inputs associated with it across the contiguous United States. Net Anthropogenic Nitrogen Input (NANI) estimates and related data are also provided. Data are presented at county, sub-regional and regional scales. Here, subregions refer to multi-county areas delineated with the goal of obtaining more uniform reporting areas than individual counties. Regions refer to the USDA Farm Resource Regions. The data are reported for 6 agricultural census years, 1987, 1992, 1997, 2002, 2007 and 2012. Estimates of the variables were derived originally from USDA agricultural census data, US population census data, and other sources, using version 3.1 of the NANI calculator toolbox [1], [2], [3]].