Fertilizer

1
TEMPORAL AND SPATIAL TRENDS IN NITROGEN AND
PHOSPHORUS INPUTS TO THE WATERSHED OF THE
ALTAMAHA RIVER
Sylvia C. Schaefer and Merryl Alber Department of
Marine Sciences, University of Georgia, Athens, GA
Abstract
Results Temporal Trends
Results Spatial Trends
The watershed of the Altamaha River, Georgia, is
one of the largest in the southeastern U.S.,
draining 36,718 km2 (including parts of metro
Atlanta). We calculated both nitrogen
(fertilizer, net food and feed import,
atmospheric deposition, and biological N fixation
in agricultural and forest lands) and phosphorus
(fertilizer and net food and feed import) inputs
to the watershed for 6 time points between 1954
and 2002. Total N inputs rose from 1,943 kg N
km-2 yr-1 in 1954 to a peak of 3,584 kg N km-2
yr-1 in 1982 and then declined again to 2,566 kg
N km-2 yr-1 by 2002. Phosphorus inputs rose from
408 kg P km-2 yr-1 in 1954 to 531 kg P km-2 yr-1
in 1974 before also declining again, to 410 kg P
km-2 yr-1 in 2002. These changes were primarily
driven by agricultural inputs and were dominated
by changes in fertilizer use. Fertilizer tended
to be the most important input of both N and P to
the watershed, although net food and feed import
increased in importance over time and was the
dominant source of N input by 2002. When
considered on an individual basis, fertilizer
input tended to be highest in the middle portions
of the watershed (Little and Lower Ocmulgee and
Lower Oconee sub-basins) whereas net food and
feed imports were highest in the upper reaches
(Upper Oconee and Upper Ocmulgee sub-basins).
Although the overall trend in recent years has
been towards decreases in both N and P inputs,
these trends may be offset due to continuing
increases in animal and human populations.
Total N and P inputs to the watershed
Distribution of nutrient inputs

• Nitrogen inputs to the Altamaha watershed showed
  an overall increase between 1954 and 2002. Inputs
  of N were actually highest in 1982 and then
  declined by 2002. Total phosphorus inputs showed
  an increase between 1954 and 1974. By 2002,
  however, P inputs had decreased to near-1954
  levels.
• Fertilizer tended to be the most important input
  of both N and P to the watershed during most
  years, and hence was also the primary driver of
  change in the overall budget. However, net food
  and feed import has become more important over
  time.

• Total nutrient inputs were fairly evenly
  distributed among sub-basins in 1954. Over time,
  differences among sub-watersheds increased,
  primarily due to changes in human population and
  agricultural practices.
• Most sub-basins followed the overall trend of a
  peak in N inputs in 1982 as compared to 1954 and
  then either held constant or declined by 2002. P
  inputs to most sub-basins peaked in 1974 and then
  decreased to levels below those of 1954.
• The atomic NP ratio of inputs to the watershed
  was generally lower than the Redfield Ratio of
  161, suggesting N limitation. However, P inputs
  remained relatively constant while N inputs
  increased. As a consequence, the NP ratio of
  inputs increased from 10.5 to 13.8 (with a peak
  of 18.3 in 1982). The Lower Oconee and Upper
  Ocmulgee tended to have higher NP ratios than
  other sub-basins.

• Fertilizer
• Sales of both N and P fertilizer increased
  between 1954 and 1977 and then declined again by
  2001

Introduction
Fertilizer

• Altamaha River, GA
• Formed by the confluence of the Oconee and
  Ocmulgee Rivers
• One of the largest watersheds on the east coast
  of the United States (36,718 km2), and includes
  parts of metro Atlanta

Net food and feed import
Cropland
Human population

• Fertilizer inputs tended to be highest in the
  middle portions of the watershed, where crop
  production was highest.

Food and feed import

• The increase in net N and P import is the result
  of increasing human population and a concurrent
  decrease in crop production due to declines in
  agricultural lands
• Shifts in animal population were also a factor,
  albeit a smaller one

Methods
Nitrogen Inputs Phosphorus Inputs
Fertilizer Fertilizer
Net Food and Feed Import Net Food and Feed Import
Atmospheric deposition
Biological fixation
Other sources
1954 1964 1974 1982 1992 2002
Net atmospheric N deposition 251 335 532 461 535 401
Biological N fixation 734 601 797 1007 657 496
Non-food crop export (N) (31) (382) (22) (7) (21) (41)
Non-food crop export (P) (4) (49) (1) (1) (3) (5)

• Fertilizer
• N and P content of fertilizer was based on
  county-specific sales, weighted by the proportion
  of each county located within the watershed.
  Data source USGS
• Net Food and Feed Import
• Net Food and Feed Import is the difference
  between total consumption (human animal) and
  total production (crop animal).
• Animal and crop data source U.S. Census of
  Agriculture
• Human population data source U.S. Bureau of the
  Census
• Atmospheric N Deposition
• 1982, 1992, 2002 inorganic deposition was
  calculated by interpolating from National
  Atmospheric Deposition Program (wet) and Clean
  Air Status and Trends Network (dry) stations.
• 1954, 1964, 1974 deposition was hindcast using
  EPA estimates of nationwide emissions.
• Organic N deposition was estimated as 30 of
  total deposition (Neff et al. 2002) following
  SCOPE, half was assumed to be new input.
• 25 of N volatilization from manure (calculated
  from animal populations) and fertilizer
  (calculated from fertilizer sales) was assumed to
  be exported
• Biological N Fixation
• Crop N fixation U.S. Census of Agriculture
  county totals were weighted by proportion of
  county inside watershed estimates multiplied by
  published N fixation rates.
• Forest N fixation calculated from U.S. Forest
  Service Forest Inventory and land cover data,
  multiplied by N fixation rates following SCOPE.
• Biological N fixation by kudzu, an invasive
  leguminous vine common in the southeastern U.S.,
  was not included in these budgets but could
  potentially represent a significant additional N
  input.
• Non-food Crop Export
• N and P production in cotton and tobacco was
  assumed to be exported from the watershed and
  subtracted from total inputs

• Import of both N and P followed a pattern
  opposite that of fertilizer sub-watersheds with
  the lowest imports led in crop production
• Net food and feed imports were highest in all
  years in the upper reaches of the watershed
  (Upper Oconee and Upper Ocmulgee)
• The Upper Ocmulgee, site of a substantial portion
  of Atlanta and its suburbs, saw a dramatic
  increase in human consumption.
• In contrast, net food and feed import in the
  Upper Oconee was dominated by increased animal
  consumption.

• Net atmospheric N deposition
• Total atmospheric nitrogen deposition rose most
  dramatically between 1954 and 1974, after which
  it increased only slightly.
• Net atmospheric N deposition increased between
  1954 and 1974, but then decreased again by 2002
  due to increases in volatilization from animal
  manure
• Biological N fixation
• Declined between 1954 and 2002, with high in
  1982.
• Patterns primarily driven by changes in N
  fixation by crops
• Total N fixation in forestland decreased overall
  between 1974 and 2002
• Non-food crop export
• Generally quite small, but unusually high values
  in 1964 due to extremely high tobacco production

Other sources
Conclusions
Atmospheric N deposition
1954 1964 1974 1982 1992 2002
Upper Oconee 220 204 395 153 279 (57)
Lower Oconee 270 386 586 578 649 635
Upper Ocmulgee 257 366 587 566 601 546
Lower Ocmulgee 244 350 539 523 582 461
Little Ocmulgee 243 358 545 523 619 623
Ohoopee 263 368 560 509 579 451
Lower Altamaha 278 383 561 499 559 369

• Increases in watershed nutrient loading took
  place in the early portion of the study period.
  These results are driven largely by the change in
  fertilizer input, which decreased by half or more
  between 1977 (the high point) and 2001. The
  overall increase in fertilizer use coincided with
  a decline in agricultural landboth crop and
  pasturelandin the watershed. It is likely that
  there has been an increase in the amount of
  fertilizer used per unit area, possibly due to
  changes in growing techniques.
• The relative importance of different sources of
  nitrogen has shifted over time. Biological N
  fixation by crops was the most important source
  of new N to the Altamaha watershed in the 1950s,
  but was replaced by increasing inputs of
  fertilizer N and then by increases in net food
  and feed import. There is less and less cropland
  producing food and feed for the growing
  populations of both animals and people in the
  Altamaha, resulting in the need to import those
  nutrients from outside the watershed.
• Current phosphorus inputs are not substantially
  above those of the 1950s, suggesting that
  nitrogen inputs are a more pressing concern in
  the watershed. To reduce nitrogen, attention
  should focus on fertilizer in the middle portion
  of the watershed, animal agriculture in the Upper
  Oconee, and urban and suburban sources
  (wastewater treatment plants and septic tanks) in
  the Upper Ocmulgee.
• Whether or not further increases in nutrient
  inputs take place in the watershed of the
  Altamaha River will depend on trends in
  agriculture and human populations. Populations of
  both humans and animals are likely to continue to
  increase, and the production of food and feed to
  sustain those populations may also have to
  intensify. Thus, further increases in nutrient
  inputs to the watershed can be expected.

Ongoing Work
Biological N fixation

• MATLAB-based program to simplify budget
  calculations
• Updated 2007 budgets
• Evaluate relationships between NO3- isotope
  composition in riverine export and watershed
  inputs for the east coast
• Use NO3- isotopes to provide insight into N
  sources and transformations within the Altamaha
  River watershed
• Paired measurements of denitrification in the
  Altamaha and a comparable northeastern watershed

1954 1964 1974 1982 1992 2002
Upper Oconee 882 768 959 879 744 713
Lower Oconee 652 504 722 995 564 396
Upper Ocmulgee 804 697 694 655 491 432
Lower Ocmulgee 826 798 1,088 1,489 867 514
Little Ocmulgee 751 426 808 1,331 605 451
Ohoopee 665 444 798 1,351 819 499
Lower Altamaha 417 266 446 745 504 349
(Based on methodology of SCOPE Nitrogen Project,
Boyer et al. 2002)
Acknowledgments
We thank Beth Boyer for advice on calculating
nutrient inputs, Karen Payne for assistance with
GIS, and Tim Hollibaugh for useful discussions
and being completely awesome. Funding was
provided by the Georgia Coastal Ecosystems LTER
Project (NSF Award OCE 99-82133) and the
Environmental Protection Agency (STAR Grant
R830882).