U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5241

National Water-Quality Assessment Program

Estimated Anthropogenic Nitrogen and Phosphorus

Inputs to the Land Surface of the Conterminous 

United States—1992, 1997, and 2002
Estimation of Anthropogenic Nitrogen 

and Phosphorus Inputs to the Land 

Surface of the Conterminous United 

States—1992, 1997, and 2002

By Lori A. Sprague and Jo Ann M. Gronberg

National Water-Quality Assessment Program

Scientific Investigations Report 2012–5241

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2013

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Suggested citation:

Sprague, L.A., and Gronberg, J.M., 2013, Estimated anthropogenic nitrogen and phosphorus inputs to the land 

surface of the conterminous United States—1992, 1997, and 2002: U.S. Geological Survey Scientific Investigations 

Report 2012–5241, 14 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Estimation of Anthropogenic Nutrient Inputs ...........................................................................................2

Waste .....................................................................................................................................................2

Fertilizer..................................................................................................................................................3

Biological Fixation by Crops................................................................................................................3

Atmospheric Deposition ......................................................................................................................4

Food and Feed Import...........................................................................................................................4

Human Consumption ...................................................................................................................5

Animal Consumption ...................................................................................................................5

Crop Production for Human and Animal Consumption..........................................................5

Animal Production for Human Consumption...........................................................................8

Determination of Individual Nutrient Inputs to the Watersheds 

of NAWQA Surface-Water Sites....................................................................................................8

Total Anthropogenic Nutrient Inputs to Watersheds...............................................................................8

Description of the County-Level Nutrient-Input Dataset ........................................................................9

Description of the Watershed-Level Nutrient-Input Dataset...............................................................11

Summary........................................................................................................................................................12

Acknowledgments.......................................................................................................................................12

References Cited..........................................................................................................................................12

Figure

1. Nitrogen mass balance in a closed system over a long-term cycle....................................2

Tables

1. Fixation rates used in the calculation of biological nitrogen fixation by crops ...........................4

2. Human intake rates used in the calculation of human consumption ............................................5

3. Animal intake rates used in the calculation of animal consumption.............................................5

4. Conversion factors used to estimate crop production for animal and 

human consumption.....................................................................................................................6

5. Animal excretion rates used in the calculation of animal production 

for human consumption...............................................................................................................9

6. Comparison between the animal intake and excretion rates used in this study

and the animal excretion rates used in Ruddy and others (2006)................................................ 10

7. Land-cover classifications used for allocation of nutrient inputs within a county

or hydrologic unit..................................................................................................................................... 11

8. Time periods used for determining annual riverine loads and watershed inputs ................... 11
iv

Conversion Factors

SI to Inch/Pound

Multiply By To obtain

Area

square kilometer (km2

) 247.1 acre

square kilometer (km2

) 0.3861 square mile (mi2)

Mass

kilogram (kg) 2.205 pound avoirdupois (lb)
Estimation of Anthropogenic Nitrogen and Phosphorus 

Inputs to the Land Surface of the Conterminous 

United States—1992, 1997, and 2002

By Lori A. Sprague and Jo Ann M. Gronberg

nitrogen only), atmospheric deposition (for nitrogen only), 

and imported human food and livestock feed; and final outputs 

are riverine export, basin storage or volatilization, and crop 

export—nutrients in crops and human and livestock waste are 

internally recycled (fig. 1). In this long-term cycle, total anthropogenic nutrient input over n years is

AI Fert Fix Atm

Food p Feed

i

i

n

i

i

n

i

i

n

i

i

n

i

i

n

== = =

=

∑∑ ∑ ∑

∑

= ++

+ +

11 1 1

1

Im Impi

i

n

=

∑1

(1)

where

AIi is the total anthropogenic nutrient input 

to the watershed from years i to n,

Ferti is the input from fertilizer from 

years i to n,

Fixi is the input from biological fixation by 

crops from years i to n (nitrogen only),

Atmi is the input from atmospheric deposition 

from years i to n (nitrogen only),

FoodImpi is the input from imported food from 

years i to n, and

FeedImpi is the input from imported feed from 

years i to n.

Over shorter time steps (such as a year), however, 

livestock waste is not always completely recycled; manure is 

often used to fertilize crops and pasture in succeeding time 

steps (for example, Year 2 in fig. 1). In these locations, the 

assumption that livestock waste represents only the internal 

recycling of “new” inputs of fertilizer, biological fixation 

by crops, atmospheric deposition, and food and feed inputs 

could lead to an underestimation of watershed inputs during 

a given time step. In this short-term cycle, the total anthropogenic nutrient input during year i is

AIi

 = Wastei–1 + Ferti + Fixi + Atmi

+ FoodImpi + FeedImpi (2)

Abstract

Anthropogenic inputs of nitrogen and phosphorus to each 

county in the conterminous United States and to the watersheds 

of 495 surface-water sites studied as part of the U.S. Geological 

Survey National Water-Quality Assessment Program were quantified for the years 1992, 1997, and 2002. Estimates of inputs of 

nitrogen and phosphorus from biological fixation by crops (for 

nitrogen only), human consumption, crop production for human 

consumption, animal production for human consumption, animal consumption, and crop production for animal consumption 

for each county are provided in a tabular dataset. These countylevel estimates were allocated to the watersheds of the surfacewater sites to estimate watershed-level inputs from the same 

sources; these estimates also are provided in a tabular dataset, 

together with calculated estimates of net import of food and 

net import of feed and previously published estimates of inputs 

from atmospheric deposition, fertilizer, and recoverable manure. 

The previously published inputs are provided for each watershed so that final estimates of total anthropogenic nutrient inputs 

could be calculated. Estimates of total anthropogenic inputs are 

presented together with previously published estimates of riverine loads of total nitrogen and total phosphorus for reference.

Introduction

This report describes how anthropogenic nitrogen 

and phosphorus inputs to each county in the conterminous 

United States and to the watersheds of 495 surface-water 

sites studied as part of the U.S. Geological Survey (USGS) 

National Water-Quality Assessment (NAWQA) Program were 

estimated and provides the resulting tabular data in Microsoft 

Excel format. Riverine loads of total nitrogen and total 

phosphorus that previously were calculated for the same sites 

in Mueller and Spahr (2006) also are provided in the dataset 

for reference.

In a hypothetical watershed over the long term—where 

initial anthropogenic inputs of nitrogen and phosphorus are 

“new” inputs of fertilizer, biological fixation by crops (for 
2 Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States

where

AIi is the total anthropogenic nutrient input to the 

watershed in year i,

Wastei-1 is the input from waste (recoverable livestock 

manure) in year i-1,

Ferti is the input from fertilizer in year i,

Fixi is the input from biological fixation by crops 

in year i (nitrogen only),

Atmi is the input from atmospheric deposition in 

year i (nitrogen only),

FoodImpi is the input from imported food in year i, and

FeedImpi is the input from imported feed in year i.

Because riverine loads are reported on an annual basis in 

Mueller and Spahr (2006), corresponding anthropogenic 

nutrient inputs also were estimated on an annual basis for 

this study. Equation 2 was used to estimate inputs during this 

annual time step.

The purposes of this report are to (1) describe the estimation of anthropogenic nutrient inputs in the United States 

during 1992, 1997, and 2002 and (2) provide those estimates 

in a tabular dataset. Estimates of nitrogen (as N) and phosphorus (as P) inputs, expressed in kilograms (kg), are provided 

for each county in the conterminous United States. Separate estimates are listed for biological fixation by crops (for 

nitrogen only), human consumption, crop production for 

human consumption, animal production for human consumption, animal consumption, and crop production for animal 

consumption. Estimates of nitrogen (as N) and phosphorus 

(as P) inputs, expressed in kilograms, also are provided for 

the watersheds of 495 NAWQA surface-water sites in the 

conterminous United States. Separate estimates are listed 

for biological fixation by crops (for nitrogen only), animal 

consumption, human consumption, crop production for animal 

consumption, crop production for human consumption, animal 

production for human consumption, net import of food, and 

net import of feed. Estimates of inputs from atmospheric deposition (derived from data in Ruddy and others, 2006), fertilizer 

(derived from data in Gronberg and Spahr, 2012), and recoverable manure (derived from data in Kellogg and others (2000) 

and Robert Kellogg, U.S. Department of Agriculture, written 

commun., 2011) are also provided for each watershed, so 

that final estimates of anthropogenic nutrient inputs could be 

calculated (county-level estimates of these inputs already are 

available from the cited reports). Riverine loads of total nitrogen and total phosphorus that previously were calculated for 

the same sites in Mueller and Spahr (2006) also are provided 

in the watershed dataset. 

Estimation of Anthropogenic 

Nutrient Inputs

In the following section, the estimation of each input 

source in equation 2 is described.

Waste

The variable Wastei-1 in equation 2 represents the waste 

from a previous time step that is used in the current time step 

to fertilize crops. Human waste is generally not used to fertilize crops, so waste inputs to the next time step are assumed to 

be composed solely of livestock manure. Livestock manure 

is subject to spillage, volatilization, runoff, and other losses 

after generation in the previous time step and before subsequent application to agricultural fields in the current time step. 

To account for those losses, annual estimates of recoverable 

manure nutrients were used as estimates of Wastei-1. Nitrogen 

and phosphorus in recoverable manure were estimated at a 

Figure 1. Nitrogen mass balance in a closed system over a long-term cycle.

Year 1 Year 2 Year n

Crops1

Waste1

Import1

Fert1

Fix1

Atm1

Anthropogenic

Inputs

Recycled

Outputs River1

Basin1

Export1

Crops2

Waste2

Import2

Fert2

Fix2

Atm2

Cropsn

Wasten

Importn

Fertn

Fixn

Atmn

River2

Basin2

Export2

Rivern

Basinn

Exportn

Initial (new) anthropogenic inputs

Outputs

Recycled

A portion of crop biomass (Crops) and 

human and livestock waste (Waste) are 

recycled internally.

Fertilizer

Biological nitrogen fixation

Atmospheric deposition

Riverine export

Basin storage or volatilization

Crop export

Crop biomass

Human and livestock waste

Imported food and feed

EXPLANATION

Fert

Fix

Atm

Import

River

Basin

Export

Crops

Waste
Estimation of Anthropogenic Nutrient Inputs 3

county level for 1992 and 1997 by Kellogg and others (2000) 

and at a six-digit hydrologic unit code (HUC6) level for 2002 

(Robert Kellogg, U.S. Department of Agriculture, written 

commun., 2011) using the number of animal units for confined 

livestock, a factor for manure recoverability, and estimates of 

the pounds of nutrients per ton of manure after nutrient losses 

during collection, transfer, storage, and treatment:

Recoverable manure nutrients (3)

= tons of manure per animal

× number of confined animal units

× manure recoverability factor

× nutrients per ton of manure after losses

Recoverability factors represent the proportion of the 

excreted manure that could reasonably be expected to be 

collected from confinement facilities and applied to the land 

surface (Kellogg and others, 2000). Losses included volatilization of nitrogen, spillage, and runoff; only waste treatment 

technologies that are in common practice were considered 

in estimating these losses (Kellogg and others, 2000). The 

derivation and limitations of the various factors used in equation 3 are further detailed in Kellogg and others (2000). These 

estimates do not include the portion of manure from unconfined livestock that provides some degree of fertilization on 

pasturelands. In these areas, the estimates of Wastei-1 are likely 

biased low. The extent to which livestock operations use offfarm land to dispose of livestock waste is not available from 

the Census of Agriculture (Kellogg and others, 2000), so the 

recoverable manure was assumed to be applied equally on all 

cropland and pastureland within each county or HUC6.

In the mass balance shown in equation 2, Wastei-1

represents waste inputs from the prior year, whereas Ferti

, 

Fixi

, Atmi, FoodImpi, and FeedImpi are represented as inputs 

from the current year. The time step associated with a cycle 

of nutrient inputs → crops → waste → outputs, however, 

does not perfectly align with the progression from January to 

December within a single year. Different crops are grown at 

different times throughout the year, and waste is generated on 

a continuous basis throughout the year. As a result, recoverable manure generated in a given year using crops grown 

in the previous year may be applied together with “new” 

inorganic fertilizer to some portion of the crops grown later in 

the same year. The true total amount of recoverable manure 

applied within a single 12-month period from January to 

December likely includes some of the manure generated in the 

previous 12-month period and some of the manure generated 

in the current 12-month period; annual estimates from either 

12-month period would be imperfect representations of the 

true amount. The annual data provided by the Census of Agriculture does not provide sufficient resolution to determine the 

relative chronology of manure generation and crop production 

in a given 12-month period. In addition, the Census of Agriculture is only conducted every 5 years. Because of these data 

constraints, estimates of all inputs in equation 2 (including 

Wastei-1) were based on data from the same year. For correspondence with the timing of the Census of Agriculture and 

the estimates of riverine load from Mueller and Spahr (2006), 

the inputs were estimated for 3 years—1992, 1997, and 2002.

Fertilizer

Nitrogen and phosphorus inputs from fertilizer (including farm and nonfarm uses) were derived from sales and 

expenditures data from the Association of American Plant 

Food Control Officials and the U.S. Census of Agriculture as 

described in Ruddy and others (2006). County-level, annual 

data on nutrient inputs from fertilizer originally were available 

from Ruddy and others (2006). These data were recalculated 

after discovering a processing error in the nonfarm and 

farm allocation; updated data are available in Gronberg and 

Spahr (2012).

Biological Fixation by Crops

Following Alexander and others (2008), the annual biological fixation of nitrogen was estimated for soybeans, alfalfa 

hay, and non-alfalfa hay, the major nitrogen-fixing crops in 

the United States. In 2007, soybeans and total hay composed 

20.6 and 18.8 percent, respectively, of the total harvested crop 

acreage in the United States (U.S. Department of Agriculture, 

2009). County-level data on soybean production and harvested acreage of alfalfa and total hay for 1992, 1997, and 

2002 were obtained from the U.S. Department of Agriculture 

Census of Agriculture (U.S. Department of Commerce, 1995; 

U.S. Department of Agriculture, 1999, 2004). The harvested 

acreage of total hay was not available directly from the 1997 

Census of Agriculture; instead, it was calculated as the sum 

of alfalfa hay, small grain hay, tame hay (excluding alfalfa 

and small grain), and wild hay. The harvested acreage of 

non-alfalfa hay for 1992, 1997, and 2002 was calculated as 

the difference between total hay and alfalfa hay harvested 

acreages. Data reported in the Census of Agriculture as 

“Withheld to avoid disclosing data for individual farms” 

were treated as a value of zero.

County-level, annual biological fixation was estimated 

for soybeans as the product of county-level soybean production and the nitrogen fixation rate of 0.91 kg/bushel 

(Alexander and others, 2008; McIsaac and others, 2002); 

and for alfalfa and non-alfalfa hay as the product of the 

respective county-level harvested acreage, a conversion factor 

of 0.40468564224 hectare (ha)/acre, and the nitrogen fixation 

rate of 218 kg/ha/yr for alfalfa or 116 kg/ha/yr for non-alfalfa 

hay (Alexander and others, 2008; McIsaac and others, 2002) 

(table 1). Total annual biological nitrogen fixation in each 

county was estimated as the sum of soybean fixation, alfalfa 

hay fixation, and non-alfalfa hay fixation.
4 Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States

Atmospheric Deposition

Nitrogen inputs from atmospheric deposition were 

derived from wet deposition of nitrate and ammonium data 

from the National Atmospheric Deposition Program and processed as described in Ruddy and others (2006). Atmospheric 

ammonia and organic nitrogen largely are derived from volatilization of animal waste and fertilizer. Because these emissions may redeposit during the same year in close proximity 

to the emission source (Prospero and others, 1996), deposition 

of ammonia and organic nitrogen were assumed to be recycled 

from other inputs in the same region and were not included 

as inputs in this study (Howarth and others, 1996; Jordan 

and Weller, 1996). County-level, annual data on atmospheric 

deposition of nitrate and ammonium are available from Ruddy 

and others (2006).

Food and Feed Import

The import of agricultural products for human food 

and livestock feed can be an important source of anthropogenic nutrients in a watershed (Jordan and Weller, 1996). 

Net import of food and feed was estimated as the balance 

between (1) nutrient consumption by humans and animals and 

(2) nutrients provided by crops grown for human and animal 

consumption and animals produced for human consumption in 

the watershed on an annual basis:

Net import of food = human consumption (4)

– crop production for human consumption

– animal production for human consumption

Net import of feed = animal consumption (5)

– crop production for animal consumption

where

human consumption = population × human intake rate (6) 

crop production for human consumption (7)

= sum of (crop production

× nutrient content of the crop

× (percent of crop to human food/100)

× (1–(percent loss in processing of human food/100)))

animal production for human consumption (8)

= sum of (animal consumption – animal excretion)

× (1– (percent loss in human consumption/100))

= sum of ((number of animals × animal intake rate)

– (number of animals × animal excretion rate))

× (1–(percent loss in human consumption/100)))

animal consumption (9)

= sum of (number of animals × animal intake rate)

crop production for animal consumption (10)

= sum of (crop production

× nutrient content of the crop

× (percent of crop to animal feed/100)

× (1–(percent loss in processing of animal feed/100)))

A negative value for net import of food or feed indicates 

that more crops and (or) animals were produced than were 

consumed by humans and animals in the watershed, representing a net export of food or feed; a positive value indicates 

that more crops and (or) animals were consumed by humans 

and animals than were produced in the watershed, representing a net import of food or feed. A positive value, or a net 

import of food or feed, would lead to human and (or) livestock 

waste inputs to the watershed in excess of the human and (or) 

livestock waste derived from crops grown in the watershed. 

Therefore, positive values for net import of food or feed were 

considered “new” anthropogenic inputs and were included in 

equation 2. A negative value, or a net export of food or feed, 

indicates that some portion of the crops grown in the watershed were exported out of the watershed; because exported 

crops are derived from the initial inputs of fertilizer, fixation, 

atmospheric deposition, or recoverable manure in the watershed during the same year, they represent recycled nutrients 

and thus were excluded from equation 2.

Total imports of food or feed cannot be directly estimated. They can only be estimated via equations 4 and 5 as 

a net value—that is, as net import or export. This limitation 

can be illustrated with an example: let human consumption 

be 100 kg and crop production for human consumption be 

75 kg. If all the crops produced for human consumption were 

consumed by people in the watershed, an additional 25 kg 

would be imported to meet the consumption needs of people 

in the watershed. However, there may be situations where 

some of the crops produced for human consumption in a 

watershed are exported, even when enough people are present 

in the watershed to potentially consume all of those crops. 

As an alternative scenario, let 15 kg of the 75 kg produced be 

exported to another watershed (say, potatoes exported to other 

parts of the Nation)—total import of “new” nutrients would 

then be 40 kg to compensate for this export. However, the net 

import, calculated from equation 4, is 25 kg in both scenarios. 

Because the actual value of exported crops is not known, the 

actual value of total imports cannot be directly estimated. Only 

net import can be calculated. Thus, the estimation of anthropogenic nutrient inputs to a watershed in equation 2 may be 

an underestimate.

Table 1. Fixation rates used in the calculation of biological 

nitrogen fixation by crops (modified from Alexander and others, 

2008; McIsaac and others, 2002).

Crop Crop unit

Nitrogen fixation rate, 

in kilograms per 

crop unit

Soybeans Production, in bushels 0.91

Alfalfa hay Harvested area, in hectare1 218

Non-alfalfa hay Harvested area, in hectare1 116 1

Harvested area originally in acres; converted to hectare using a conversion 

factor of 0.40468564224 hectare per acre.
Estimation of Anthropogenic Nutrient Inputs 5

Each of the terms used to calculate net import or export 

of food and feed in equations 4 and 5, represented individually 

by equations 6–10, is further detailed below.

Human Consumption

As shown in equation 6, human consumption of nitrogen 

and phosphorus was estimated as the product of population 

and the human intake rate. County-level population data for 

1992, 1997, and 2002 were obtained from the time series of 

intercensal estimates by county, calculated by the U.S. Census 

Bureau (2000, 2008). Human intake rates used in equation 6 

were reported by David and Gentry (2002) and are shown in 

table 2.

Animal Consumption

As shown in equation 9, animal consumption of nitrogen and phosphorus was estimated as the product of the 

number of animals and the animal intake rate for a livestock 

group, summed across all livestock groups in each county 

in each year (1992, 1997, 2002). County-level data on the 

number of animals were obtained from livestock population 

data in the 1992, 1997, and 2002 Censuses of Agriculture 

(U.S. Department of Commerce, 1995; U.S. Department of 

Agriculture, 1999, 2004). These livestock population data 

in 1992 and 1997 are a subset of those also used in Ruddy 

and others (2006); assumptions that went into estimating 

nondisclosed values are described therein. Similar assumptions were used for the livestock population data in 2002 

(Barbara Ruddy, U.S. Geological Survey, written commun., 

2010). The animal intake rates used in equation 9 were 

reported by Van Horn (1998) and are shown in table 3. The 

Table 2. Human intake rates used in the calculation of human 

consumption (modified from David and Gentry, 2002).

Nutrient Human intake rate,

in kilograms per year

Nitrogen 4.53

Phosphorus 0.46

Table 3. Animal intake rates used in the calculation of animal consumption (modified from Van Horn, 1998).

[Time step of dry mass value = day, animal intake rate = average dry mass × (average content of dry mass/100)*365 days*0.45359237 kilograms per pound. 

Time step of dry mass value = life cycle, animal intake rate = average dry mass × (average content of dry mass/100)*0.45359237 kilograms per pound)]

Livestock, 

as designated 

in this report

Livestock, 

as designated in

Van Horn (1998)

Average 

dry mass,

in pounds

Time step

of dry mass 

value

Average

nitrogen content

of dry mass,

in percent

Average

phosphorus content 

of dry mass,

in percent

Animal

intake rate for 

nitrogen,

in kilograms per 

animal per year

Animal

intake rate for

phosphorus,

in kilograms per 

animal per year

Milk cows Dairy cows 48 Day 2.72 0.50 216 40

Beef cattle Beef steer 21 Day 1.92 0.40 67 14

Layers Hens 194 Day 2.624 0.65 0.84 0.21

Broilers Broilers 8.4 Life cycle 3.36 0.65 0.13 0.025

Turkeys Turkeys 51.88 Life cycle 2.64 0.65 0.62 0.15

Total hogs Hogs 711 Life cycle 2.64 0.55 8.5 1.8

feed amounts accounted for in table 3 represent approximately 90 percent of the estimated livestock feed grain and 

concentrate consumption by animals in the United States 

(Van Horn, 1998).

Crop Production for Human 

and Animal Consumption

As shown in equations 7 and 10, crop production of 

nitrogen and phosphorus was estimated as the product of 

(1) crop production amounts (in units of bushels, tons, and 

so forth), (2) the nutrient content of the crop (in kilograms of 

nutrients per crop unit), (3) the percentage of the crop used 

for human food or animal feed, and (4) a factor accounting 

for loss during storage and processing. The resulting product then was summed across all crop groups in each county 

for each year (1992, 1997, 2002). County-level data on crop 

production for 1992, 1997, and 2002 were obtained from the 

Census of Agriculture (U.S. Department of Commerce, 1995; 

U.S. Department of Agriculture, 1999, 2004). Data reported 

in the Census of Agriculture as “Withheld to avoid disclosing data for individual farms” were treated as a value of zero. 

County-level data on pasture acreage were obtained from the 

Enhanced National Land Cover Dataset 1992 (NLCDe 92) 

(Nakagaki and others, 2007) for 1992 and from the National 

Land Cover Dataset 2001 (NLCD01) (LaMotte, 2008a,b,c,d) 

for 1997 and 2002. County boundaries from 1990 were 

used for all years. As a result, the 2002 pasture value for 

Broomfield County, Colo., was distributed among the four 

counties from which land was used to create Broomfield 

County in 2001.

With the exception of pasture, the nutrient content of 

each crop group was reported by Kellogg and others (2000). 

Values for the nutrient content of pasture were reported in 

Hong and others (2011), as used in Boyer and others (2002) 

(for nitrogen), and in Russell and others (2008) (for phosphorus). These values are shown in table 4. With the exception 

of pasture, the distribution of crops between human food and 

animal feed and the factors accounting for loss during storage 

and processing were reported in Jordan and Weller (1996); 
6 Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States

Table 4. Conversion factors used to estimate crop production for animal and human consumption (modified from Hong and others, 

2011; Russell and others , 2008; Boyer and others, 2002; Kellogg and others, 2000; Jordan and Weller, 1996).

[n/a, not available; --, not applicable; cwt, hundredweight; bu, bushels; lb, pounds; NLCD, National Land Cover Dataset; NLCDe, Enhanced National Land 

Cover Dataset; US, United States; excl, excluding]

Crop1

1992 Census of 

Agriculture crop 

variable name

1997 Census of 

Agriculture crop 

variable name

2002 Census of 

Agriculture crop 

variable name

Crop unit

Nutrient content of crop, 

in kilograms of nutrients 

per crop unit

Nitrogen Phosphorus

Corn, grain Corn for grain or seed

(bushels)

Corn, grain—production, 

measured in bu

Corn, grain—production, 

measured in bu

Bushels 0.36 0.07

Corn, silage Corn for silage/

green chop (tons, green)

Corn, silage—production, 

measured in tons

Corn, silage—production, 

measured in tons

Tons 3.22 0.48

Soybeans Soybeans for beans

(bushels)

Soybeans—production, 

measured in bu

Soybeans—production, 

measured in bu

Bushels 1.61 0.16

Sorghum, grain Sorghum for grain 

or seed (bushels)

Sorghum, grain—production,

measured in bu

Sorghum, grain—production,

measured in bu

Bushels 0.44 0.08

Sorghum, silage Sorghum for silage,

harvested (tons, green)

Sorghum, silage—production, 

measured in tons

Sorghum, silage—production,

measured in tons

Tons 6.70 1.11

Barley Barley for grain

(bushels)

Barley—production, 

measured in bu

Barley—production, 

measured in bu

Bushels 0.41 0.08

Wheat Wheat for grain

(bushels)

Wheat—production, 

measured in bu

Wheat—production, 

measured in bu

Bushels 0.56 0.10

Wheat, winter n/a n/a Wheat, winter—production,

measured in bu

Bushels 0.46 0.09

Wheat, spring, 

durum

n/a Wheat, spring, durum—

production, measured in bu

Wheat, spring, durum—

production, measured in bu

Bushels 0.59 0.10

Wheat, spring

(excl durum)

n/a Wheat, spring (excl durum)—

production, measured in bu

Wheat, spring (excl durum)—

production, measured in bu

Bushels 0.63 0.10

Oats Oats for grain

(bushels)

Oats—production, 

measured in bu

Oats—production, 

measured in bu

Bushels 0.27 0.05

Rye Rye for grain, Harvested

(bushels)

Rye—production, 

measured in bu

Rye—production, 

measured in bu

Bushels 0.49 0.08

Rice Rice (cwt) Rice—production, 

measured in cwt

Rice—production, 

measured in cwt

Cwt 0.57 0.13

Peanuts Peanuts for nuts

(pounds)

Peanuts—production, 

measured in lb

Peanuts—production, 

measured in lb

Pounds 0.02 0.00

Sugarbeets, 

sugar

Sugar beets for sugar

(tons)

Sugarbeets, sugar—production,

measured in tons

Sugarbeets, sugar—production,

measured in tons

Tons 2.16 0.43

Potatoes Irish potatoes

(cwt)

Potatoes—production, 

measured in cwt

Potatoes—production, 

measured in cwt

Cwt 0.16 0.03

Sweet potatoes -- Sweet potatoes—production, 

measured in cwt

Sweet potatoes—production,

measured in cwt

Cwt (1997 

and 2002)

0.11 0.02

Sweet potatoes Sweet potatoes, Harvested

(bushels)

-- -- Bushels 

(1992)

0.06 0.01

Forage, hay Hay-all

(tons, dry)

n/a Forage, hay—production,

measured in tons

Tons 13.11 4.51

Forage, alfalfa, hay n/a Forage, alfalfa, hay—production, 

measured in tons

Forage, alfalfa, hay—production,

measured in tons

Tons 22.86 2.14

Forage, small 

grain, hay 

n/a Forage, small grain, hay—

production, measured in tons

Forage, small grain, hay—

production, measured in tons

Tons 11.61 2.03

Forage, tame

(excl alfalfa 

and small grain)

n/a Forage, tame

(excl alfalfa and small grain),

hay—production, measured

in tons

Forage, tame

(excl alfalfa and small grain), 

hay—production, measured

in tons

Tons 8.98 6.94

Forage, wild, hay n/a Forage, wild, hay—production, 

measured in tons

Forage, wild, hay—production, 

measured in tons

Tons 8.98 6.94

Pasture Data from 1992 NLCDe Data from 2001 NLCD Data from 2001 NLCD Acres 9.07 5.00
Estimation of Anthropogenic Nutrient Inputs 7

Table 4. Conversion factors used to estimate crop production for animal and human consumption (modified from Hong and others, 

2011; Russell and others , 2008; Boyer and others, 2002; Kellogg and others, 2000; Jordan and Weller, 1996).—Continued

[n/a, not available; --, not applicable; cwt, hundredweight; bu, bushels; lb, pounds; NLCD, National Land Cover Dataset; NLCDe, Enhanced National Land 

Cover Dataset; US, United States; excl, excluding]

Crop1

Percent of

crop to

human food

Percent of

crop to

animal feed

Percent loss

of human food 

during storage 

and processing

Percent loss 

of animal feed 

during storage 

and processing

Notes

Corn, grain 4 96 10 10 --

Corn, silage 0 100 -- 0 --

Soybeans 2 98 10 10 --

Sorghum, grain 0 100 -- 0 --

Sorghum, silage 0 100 -- 0 Production data not available in 1992; used harvested data instead for 1992.

Barley 3 97 10 10 --

Wheat 61 39 10 10 Values for the nutrient content of wheat are the average of the nutrient contents 

of individual wheat types reported in Kellogg and others (2000).

Wheat, winter 61 39 10 10 --

Wheat, spring, 

durum

61 39 10 10 --

Wheat, spring

(excl durum)

61 39 10 10 --

Oats 6 94 10 10 --

Rye 17 83 10 10 Production data not available in 1992; used harvested data instead for 1992.

Rice 100 0 10 -- Values for the nutrient content of rice were in units of bags in Kellogg and 

others (2000). 1 bag of rough or milled rice = 100 U.S. pounds = 1 cwt 

(U.S. Department of Agriculture, 1992).

Peanuts 50 50 10 10 --

Sugarbeets, sugar 0 100 -- 0 --

Potatoes 100 0 10 -- Values for the nutrient content of potatoes were in units of bags in Kellogg 

and others (2000). 1 sack of potatoes = 100 U.S. pounds = 1 cwt 

(U.S. Department of Agriculture, 1992). Assumed 1 sack = 1 bag.

Sweet potatoes 100 0 10 -- Values for the nutrient content of sweet potatoes were in units of bushels in 

Kellogg and others (2000). 1 bushel of sweet potatoes = 55 U.S. pounds = 

0.55 cwt (U.S. Department of Commerce, 1995).

Sweet potatoes 100 0 10 -- Production data not available in 1992; used harvested data instead for 1992.

Forage, hay 0 100 -- 0 Values for the nutrient content of forage hay are the average of the nutrient 

content of individual forage hay types reported in Kellogg and others (2000).

Forage, alfalfa, hay 0 100 -- 0 --

Forage, small 

grain, hay 

0 100 -- 0 --

Forage, tame 

(excl alfalfa 

and small grain)

0 100 -- 0 --

Forage, wild, hay 0 100 -- 0 --

Pasture 0 100 -- 0 --

1

When all individual components of total wheat (wheat, winter; wheat, spring, durum; wheat, spring (excl durum)) or total hay (forage, alfalfa, hay; forage, 

small grain, hay; forage, tame (excl alfalfa and small grain); forage, wild, hay) were available, individual components were used in lieu of their totals. When 

all individual components were not available, the total values for wheat and hay were used.
8 Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States

these values are shown in table 4. Because pasture is grazed 

by livestock and typically is not harvested, percent of crop to 

animal feed was assumed to be 100, and loss during storage 

and processing was assumed to be zero for this study. Pasture 

acreage was derived from land cover code 81 (pasture/hay–

areas of grasses, legumes, or grass-legume mixtures planted 

for livestock grazing or the production of seed or hay crops), 

so its inclusion may have resulted in an unknown amount of 

double-counting of inputs from seed or hay crops that were 

also included in crop production estimates through data on hay 

production from the Census of Agriculture. The approach used 

here follows precedent established in Boyer and others (2002) 

and in Russell and others (2008).

Animal Production for Human Consumption

As shown in equation 8, animal production for human 

consumption (milk, meat, eggs, organs, and so forth) was 

estimated as the difference between animal consumption and 

animal excretion, multiplied by a factor accounting for loss 

due to spoilage and inedible components. Animal consumption 

was estimated as described in the preceding section “Animal 

Consumption.” Animal excretion was estimated by using the 

same county-level data on the number of animals, together with 

animal excretion rates reported in Van Horn (1998) and shown 

in table 5. Following Boyer and others (2002), loss due to spoilage and inedible components was assumed to be 10 percent.

The animal excretion rates used in this study are different 

than those used in Ruddy and others (2006), which were modified from Goolsby and others (1999) (table 6). The animal 

excretion rates used in Ruddy and others (2006) often exceed 

the animal intake rates reported in Van Horn (1998) and used 

here (table 6). There are several other published values for 

typical animal intake rates that could have been used instead 

(see comparison in Boyer and others, 2002), but those in 

Van Horn (1998) were selected because they are based on the 

most recent agricultural practices in the United States (Boyer 

and others, 2002).

Determination of Individual Nutrient 

Inputs to the Watersheds of NAWQA 

Surface-Water Sites

For determination of total nutrient inputs to the watersheds of the NAWQA surface-water sites, county-level annual 

estimates of nutrient inputs were mapped to specific land uses 

within each county by using NLCDe 92 and then summed for 

the area within each watershed, using tools from Price and 

others (2010). NLCDe 92 is a composite of USGS NLCD 92 

(U.S. Geological Survey, 2000) and USGS enhanced Land 

Use and Land Cover (LULC) data (Price and others, 2007). 

NLCD 92 is based on interpretation of Landsat satellite 

Thematic Mapper imagery captured from the late 1980s through 

early 1990s and was supplemented with a variety of ancillary 

data (Vogelmann and others, 2001). The LULC dataset represents conditions compiled from aerial photographs taken in the 

1970s to mid-1980s (U.S. Geological Survey, 1990). Countyor HUC6-level estimates of farm and nonfarm fertilizer, biological nitrogen fixation, recoverable manure, human and animal 

consumption, and crop and animal production were allocated 

by land use as shown in table 7. Atmospheric deposition was 

assumed to be distributed over the county, and an area-weighted 

sum was computed for each watershed. Although more recent 

land-cover data are available through NLCD01, only NLCDe 92 

was used for allocation of nutrient inputs. Various methodological differences resulted in substantially different pixel-by-pixel 

labeling in the two NLCD datasets, much of which probably is 

not genuine land-cover change (U.S. Environmental Protection 

Agency, 2007). County boundaries from 1990 were used for all 

spatial allocations. There were some significant changes to the 

counties during the 1990s and early 2000s (U.S. Census Bureau, 

2002), but only a few required data modification for allocation 

to the watershed. The human consumption value in 2002 for 

Broomfield County, Colo., was allocated by percent-estimated 

detached population (U.S. Census Bureau, 2002) among the 

four counties from which land was used to create Broomfield 

County in 2001. Also, Miami-Dade, Fla., was processed as 

Dade, Fla.

Following Mueller and Spahr (2005, 2006) and as 

detailed in table 8, the mean of multiple years was used as 

the estimate of watershed inputs of fertilizer and atmospheric 

deposition, which were measured annually. This was done for 

correspondence with the riverine loads in Mueller and Spahr 

(2006), which were reported as the mean of the annual loads 

estimated during the NAWQA high-intensity data collection 

period. The estimates of inputs of biological nitrogen fixation, 

recoverable manure, and net food and feed imports or exports 

were for a single year—the year of the Census of Agriculture 

closest to the NAWQA high-intensity data collection period.

Total Anthropogenic Nutrient Inputs 

to Watersheds

Once the individual inputs to each watershed had been 

quantified, anthropogenic inputs were calculated as

Anthropogenic inputs of nitrogen = Waste (11)

+ Fert + Fix + Atm + FoodImp + FeedImp

Anthropogenic inputs of phosphorus (12)

= Waste + Fert + FoodImp + FeedImp

There are important limitations in this estimation of total 

anthropogenic nutrient inputs. The equations do not include 

natural nutrient inputs, because the loss of anthropogenic inputs 

(those most likely to be managed) is of interest in the companion analysis of riverine export. The anthropogenic sources in 

these equations, however, do not include inputs from long-term 

storage of anthropogenic nutrients (such as inputs from soils or 

groundwater), the import of livestock manure for application 
Description of the County-Level Nutrient-Input Dataset 9

to agricultural fields (such as from confined animal feedlots 

outside of the watershed), or the application of biosolids derived 

from the treatment of human waste to agricultural fields. 

Anthropogenic inputs from these sources therefore are routinely 

excluded from quantification of anthropogenic nutrient inputs 

to watersheds (for example, see Russell and others, 2008; Boyer 

and others, 2002; David and Gentry, 2002; McIsaac and others, 

2002; Howarth and others, 1996; Jordan and Weller, 1996). 

First, the origin (anthropogenic or natural) of nutrient inputs 

from long-term storage are difficult to identify even on a small 

scale; national-scale information does not currently (2010) exist. 

Second, because agricultural data from individual farms is not 

disclosed to protect the privacy of farm owners, the agricultural 

data used in this report are county level. In most cases, it is not 

economically feasible to transport manure throughout an entire 

county (Kellogg and others, 2000), so transport of manure 

among counties is likely to be low compared to transport 

over smaller distances within counties. Third, biosolids are 

used on less than one percent of the Nation’s agricultural land 

(U.S. Environmental Protection Agency, 2011). There are other 

important limitations to this approach. The equations assume 

that nutrient mineralization and immobilization are in equilibrium. Because of limited information on crop uptake, animal 

intake, and animal excretion, certain crops or animals that are 

important in a given county or watershed might not be included 

in the estimation of total anthropogenic nutrient inputs. Lastly, 

the human consumption estimates were allocated equally across 

all urban land; a large proportion of human-waste inputs, however, likely enter watersheds in more localized areas through 

discharge from wastewater-treatment plants.

Many previous studies using the net anthropogenic 

nitrogen input (NANI) or net anthropogenic phosphorus input 

(NAPI) approach for estimating anthropogenic nutrient inputs 

have assumed that livestock waste represents only the internal recycling of “new” inputs of fertilizer, biological fixation by crops, and atmospheric deposition (for example, see 

Russell and others, 2008; Boyer and others, 2002; David and 

Gentry, 2002; McIsaac and others, 2002; Howarth and others, 

1996; Jordan and Weller, 1996). In this study, livestock waste 

derived from prior (and therefore independent) inputs of fertilizer, biological fixation by crops, and atmospheric deposition 

is included as an input because it is ultimately used to grow 

crops together with “new” inputs of fertilizer, biological fixation by crops, and atmospheric deposition. This waste input 

term is represented by recoverable livestock waste, the proportion of excreted manure that could reasonably be expected 

to be collected from confinement facilities and subsequently 

applied to the land surface after losses from volatilization of 

nitrogen, spillage, and runoff. Because of the inclusion of 

recoverable livestock waste as an input, anthropogenic nutrient 

inputs estimated in this study are not directly comparable to 

anthropogenic nutrient inputs estimated using the “NANI” or 

“NAPI” approach. More generally, methods used to estimate 

nutrient inputs to watersheds vary considerably, depending 

in part on the objective of the studies. For example, some 

studies include livestock waste as an input—either as excreted 

manure (Bosch and Allan, 2008) or as recoverable manure 

(MacDonald and Bennett, 2009; Lanyon and others, 2006; 

Stewart and others, 2005)—but may not include food and feed 

imports (MacDonald and Bennett, 2009; Lanyon and others, 

2006; Stewart and others, 2005). Careful consideration should 

be given to estimation methods when comparing results from 

different studies.

Description of the County-Level 

Nutrient-Input Dataset

The county-level nutrient-input data described in this 

report are in the dataset Nutrient_input_county.xlsx that is 

available online at http://pubs.usgs.gov/sir/2012/5241/excel/

Nutrient_input_county.xlsx. This dataset contains estimates 

of nitrogen (as N) and phosphorus (as P) inputs, expressed in 

kilograms, for each county in the conterminous United States. 

Separate estimates are listed for biological fixation by crops (for 

nitrogen only), human consumption, crop production for human 

consumption, animal production for human consumption, animal consumption, and crop production for animal consumption. 

(Note that county-level estimates of atmospheric deposition 

inputs are available in Ruddy and others, 2006; county-level 

estimates of fertilizer are available in Gronberg and Spahr, 

2012; and county-level estimates of recoverable manure inputs 

in 1992 and 1997 are available in Kellogg and others, 2000). 

Counties are identified by State, county, and Federal Information Processing System (FIPS) code. The data are sorted alphabetically by State, and numerically by FIPS code within each 

State. The area (in square kilometers) is listed for each county. 

For consistency and ease in reporting, county boundaries were 

kept the same throughout the period of record presented in this 

dataset, even though several changes occurred. For example, 

Table 5. Animal excretion rates used in the calculation of animal production for human consumption (modified from Van Horn, 1998).

Livestock,

as designated

in this report

Livestock,

as designated

in Van Horn (1998)

Animal excretion rate,

in pounds per animal per year

Animal excretion rate,

in kilograms per animal per year

Nitrogen Phosphorus Nitrogen Phosphorus

Milk cows Dairy cows 367 65 166 29

Beef cattle Beef steer 129 23 58.5 10

Layers Hens 1,2051 3761 546.61 1711

Broilers Broilers 0.157 0.026 0.0712 0.012

Turkeys Turkeys 0.87 0.194 0.39 0.0880

Total hogs Hogs 12.878 2.082 5.8414 0.9444 1

Per 1,000 animals.
10 Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States Table 6. Comparison between the animal intake and excretion rates used in this study and the animal excretion rates used in Ruddy and others (2006). Livestock, as designated in this report Livestock, as designated in Van Horn (1998) Livestock, as designated in Ruddy and others (2006) Animal intake rate used in this study from Van Horn (1998), in kilograms per animal per year1 Animal excretion rate used in this study from Van Horn (1998), in kilograms per animal per year2 Animal excretion rate from Ruddy and others (2006), in kilograms per animal per year Nitrogen Phosphorus Nitrogen Phosphorus Nitrogen Phosphorus Milk cows Dairy cows Milk cows 216 40 166 29 74.5 12 Beef cattle Beef steer Beef cattle 67 14 58.5 10 54.8 19 Layers Hens Chickens and hens 0.843 0.213 546.63 1713 0.553 0.23 Broilers Broilers Pullets and broilers 0.13 0.025 0.0712 0.012 0.37 0.1 Turkeys Turkeys Tom turkeys and hen turkeys (separately) 0.62 0.15 0.39 0.0880 0.54 0.20 Total hogs Hogs Hogs and pigs 8.5 1.8 5.8414 0.9444 9.9 4.4 1Also reported in table 3.

2Also reported in table 5.3Per 1,000 animals.
Description of the Watershed-Level Nutrient-Input Dataset 11

Table 7. Land-cover classifications used for allocation of nutrient inputs within a county or hydrologic unit.

[NLCD, National Land Cover Data; LULC, Land Use and Land Cover; --, not applicable]

Enhanced National Land Cover 

Data 1992 classification

Nutrient input category

Farm

fertilizer

Nonfarm

fertilizer

Biological

nitrogen

fixation

Recoverable

manure

Human

consumption

Animal

consumption

Crop

production

for human

and animal

consumption

Animal

production

for human

consumption

Agricultural land

Orchards/vineyards/other X -- -- -- -- -- X --

LULC orchards/vineyards/other1 X -- -- -- -- -- X --

Pasture/hay X -- X X -- X X X

Row crops X -- X X -- X X X

Small grains X -- -- X -- X X X

Fallow X -- -- X -- X X X

Grassland/herbaceous -- -- -- X -- X X X

Urban land

Low intensity residential -- X -- -- X -- -- --

LULC residential1 -- X -- -- X -- -- --

NLCD/LULC forested residential1 -- X -- -- X -- -- --

Urban/recreational grasses -- X -- -- X -- -- -- 1

Revised classification of the NLCD dataset as described in Nakagaki and others (2007).

Table 8. Time periods used for determining annual riverine loads and watershed inputs (modified from Mueller and Spahr, 2005). 

[NAWQA, National Water-Quality Assessment program]

NAWQA study-unit group

(start year)

Riverine load modeling period

(water years)

Nutrient-input period

(calendar years)

Model calibration Load estimation Fertilizer and

atmospheric deposition

Biological nitrogen fixation, 

recoverable manure,

and net food and feed imports

1991 1993–1996 1994–1995 1993–1995 mean 1992

1994 1996–1999 1997–1998 1996–1998 mean 1997

1997 1999–2001 2000–2001 1999–2001 mean 2002

South Boston City, Va. (FIPS 51780), was merged into Halifax 

County, Va. (FIPS 51083), in 1995, but nutrient inputs in this 

dataset are presented separately for South Boston City through 

2001. Although its boundaries did not change, Dade County, 

Fla. (FIPS 12025), was renamed Miami-Dade County in 1997 

and assigned a new FIPS code (12086). In this dataset, the 

new name and FIPS code are used throughout the time period 

(1982–2001).

Description of the Watershed-Level 

Nutrient-Input Dataset

The nutrient-input data for the watersheds of each of the 

495 surface-water sites studied as part of the USGS NAWQA 

Program are in the dataset Nutrient_input_watershed.xlsx that 

is available online at http://pubs.usgs.gov/sir/2012/5241/excel/

Nurient_input_watershed.xlsx. This dataset contains estimates 

of nitrogen (as N) and phosphorus (as P) inputs, expressed in 

kilograms. Separate estimates are listed for biological fixation by crops (for nitrogen only), human consumption, crop 

production for human consumption, animal production for 

human consumption, animal consumption, crop production for 

animal consumption, net import of food, and net import of feed. 

Estimates of inputs from atmospheric deposition (derived from 

data in Ruddy and others, 2006), fertilizer (derived from data in 

Gronberg and Spahr, 2012), and recoverable manure (derived 

from data in Kellogg and others, 2000, and Robert Kellogg, 

U.S. Department of Agriculture, written commun., 2011) are 

also provided so that final estimates of anthropogenic nutrient inputs also could be calculated and presented. Watersheds 

are identified by the USGS site identifier, the site name, the 

NAWQA study unit in which the site is located, and the studyunit start year during the first cycle of the NAWQA program. 

The study-unit start year was used to determine the years used 

for each input data series (see table 8). The area (in square kilometers) also is listed for each watershed.

Because the county-level fertilizer inputs and the watershed area for some sites have been updated since the publication of Mueller and Spahr (2005), watershed areas, inputs 

of atmospheric deposition, and inputs of fertilizer for these 

watersheds may be slightly different here than reported in 

Mueller and Spahr (2005). 
12 Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States

Summary

The purposes of this report were to (1) describe the 

estimation of anthropogenic nutrient inputs in the United 

States during 1992, 1997, and 2002 and (2) provide those 

estimates in a tabular dataset. Estimates of nitrogen (as N) and 

phosphorus (as P) anthropogenic inputs are provided for each 

county in the conterminous United States in a tabular dataset. 

Separate estimates are listed for biological fixation by crops 

(for nitrogen only), human consumption, crop production for 

human consumption, animal production for human consumption, animal consumption, and crop production for animal 

consumption. Estimates of nitrogen (as N) and phosphorus (as 

P) inputs also are provided for the watersheds of 495 National 

Water-Quality Assessment Program surface-water sites in 

the conterminous United States in another tabular dataset. 

Separate estimates are listed for biological fixation by crops 

(for nitrogen only), animal consumption, human consumption, 

crop production for animal consumption, crop production for 

human consumption, animal production for human consumption, net import of food, and net import of feed. Estimates of 

inputs from atmospheric deposition, fertilizer, and recoverable manure are also provided for each watershed so that final 

estimates of anthropogenic nutrient inputs may be calculated 

(county-level estimates of these sources already are available 

from previous studies). Riverine loads of total nitrogen and 

total phosphorus that previously were calculated for the same 

sites also are provided in the watershed dataset.

Acknowledgments

The authors thank Neil Dubrovsky, Nancy Baker, and 

Tammy Ivahnenko for their helpful reviews of this report.

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For more information concerning this publication, contact:

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This report is available at: http://pubs.usgs.gov/sir/2012/5241
Sprague and Gronberg—Anthropogenic Nitrogen and Phosphorus Inputs to Land Surface of Conterminous United States—Scientific Investigations Report 2012–5241