POTENTIAL
BENEFITS OF LAND APPLYING BIOSOLIDS IN EASTERN NEBRASKA
Darren L. Binder, Achim
Dobermann, Donald H. Sander, and Kenneth G. Cassman
University of Nebraska
Department of Agronomy and
Horticulture
258 Keim Hall
Lincoln, NE 68583-0915
ABSTRACT
Two
four year field experiments were conducted to determine the optimal application
rate of the City of Lincoln's anaerobically digested biosolids for irrigated
corn and dryland sorghum; to quantifiy the nitrogen (N) value of biosolids; to
determine the residual value of biosolids; and to evaluate the environmental
impact associated with land application of biosolids. Five rates of biosolids were applied in each year to separate
areas so that no area received more than one application. Yield and nutrient uptake were measured and
compared to yields attained with N fertilizer.
The optimal biosolids rate was 28 tons acre-1 for irrigated
corn and 16 tons acre-1 for dryland sorghum. On average at the optimal biosolids rate
yields were increased by 33% in the first year and 21%, 14%, and 9% in the
second, third and fourth years respectively.
The cost of N fertilizer required to get a similar yield increase was
$31 acre-1 for corn and $17 acre-1 for sorghum the first
year. Over four years a one time application of biosolids resulted in yields
equivalent to $57 and $31 acre-1 of N fertilizer for corn and
sorghum respectively. At or below the
optimal biosolids rate very little nitrate accumulated. At the irrigated corn site nearly a third of
the accumulated nitrate, 180 lbs-N acre-1, leached to the four foot
soil depth in less than one year after biosolids were applied when the
biosolids were applied at twice the optimal rate. At the dryland location 25% of the accumulated nitrate, 55 lbs N
acre-1, leached to the four foot soil depth in less than one year
after twice the optimal rate of biosolids was applied. More phosphorus is applied than than typical
agronomic crops utilize when biosolids are applied at rates high enough to
supply N. Soil phosphorus (P), Bray-P,
levels were built up to near 300 ppm, nearly twenty times the critical level of
15 ppm, when 36 dry tons of Lincoln's biosolids were applied over a 16 year
period. However at the same location
cadmium, copper, lead, nickel, and zinc build up was only slightly higher than
an average great plains soil. All of
the metals regulated by the EPA remained far below even the most stringent of
international soil standards. Thus under
current wastewater treatment and land application practices in Nebraska the greatest
environmental concern for land application of biosolids is proper nutrient
management and not metal accumulation.
KEYWORDS
Nitrogen
(N) mineralization, land application of biosolids
INTRODUCTION
In
Nebraska, 90% of the biosolids are land applied (Goldstein, 1997). Since 1992, nearly 35,000 tons (7,000 dry
tons) per year of dewatered anaerobically digested biosolids from Lincoln's
Theresa Street Wastewater Treatment facility have been land applied exclusively
to agricultural producers in Lancaster County Nebraska. An additional six million gallons (750 dry
tons) from Lincoln's Northeast Treatment Facility are land applied to a 450
acre farm each year.
The
EPA requires that biosolids applied to agricultural land be applied at an
"Agronomic Rate," which is typically defined as supplying enough N
for crop growth. There are several
factors that must be considered in order to estimate the agronomic rate
including:
·
The
amount of N needed by the crop
·
The
amount of plant-available N remaining from previous application of N
(fertilizer, irrigation water, animal manure, biosolids etc.)
·
The
amount of organic N mineralized from soil organic matter or previous
applications of N
·
The
additional N from biological N fixation by legumes
·
N
losses from denitrification, ammonia
volatilization, or nitrate leaching
The
goal in N management with biosolids is to calculate the amount of N in
biosolids that will be available to the crop.
Predicting this amount of N is further complicated because 50-90% of the
biosolids-N is organic (Sommers, 1977) and must undergo a biological
conversion, N mineralization, to be made plant available. Mineralization of
biosolids-N has been widely studied, especially in laboratory incubations. As a rule of thumb, it was proposed that 20,
10, and 5% of anaerobically digested biosolids organic N is mineralized in the
first, second, and third years, respectively (USEPA, 1995). However, actual
field mineralization rates are much more variable depending on biosolids
composition, soil type, soil temperature, and moisture content (Gilmour and
Gilmour, 1980; Sims and Boswell, 1980; Artiola and Peper, 1992; Barbarick et
al., 1996). For example, in Wisconsin,
15 to 20, 6, and 4% of the biosolids organic N was mineralized the first,
second, and third year after application (Keeney et al., 1975). Another study found a decay series of 45,
25-30, and 10-15% of biosolids N mineralization in the 3-year period following
application (Kelling et al., 1977b).
Therefore, more reliable predictions, especially for local soil and
climatic conditions, of the overall nutrient value, the N supplying capacity
and crop yield response to biosolids are required.
The
potential environmental hazard most frequently associated with biosolids
nutrients is the excessive movement of nitrate from soil to groundwater
(Keeney, 1989). Previous studies have
demonstrated that large rates of biosolids application to agricultural land can
rapidly increase soil nitrate leaching (Hinesly et al., 1972; Stewart et al.,
1975; Kelling et al., 1977b) and influence soil N supply and crop yields for
several years after application (Kelling et al., 1977a; Boyle and Paul,
1989). The potential exists for
significant amounts of P in surface water runoff when biosolids are applied to
supply N because of the high amounts of P relative to N in biosolids compared
to the amounts used by crops. Several
researchers have shown that application of biosolids can cause substantial
increases in soil P (Braids et al., 1970; Milne and Graveland, 1972; Kelling et
al., 1977b). Metals are also a common
environmental concern. Harrison et al.
(1977) make a case for a more cautious approach to land application of
biosolids due to inadequacies in the EPA's risk assessment, especially in the
case of metals. These environmental
concerns should be assessed when trying to determine the benefits of land
applying biosolids.
METHODOLOGY
This
study was conducted from 1996 to 1999 at two on-farm locations near Lincoln
Nebraska. One site was irrigated corn
and the other was dryland sorghum. Two
experiments were established at each location, one with different rates of
anaerobically digested biosolids as the sole N source and one with different N
fertilizer rates. Each year, biosolids
were applied at 0, 11, 22, 33, and 44 tons acre-1 to a new set of
treatment plots so there were no repeat applications. In the N fertilizer experiments, six rates of N (ammonium
nitrate) were applied from 0 to 200 lbs N acre-1 in 40 lb increments
at the irrigated site and from 0 to 135 lbs N acre-1 in 27 lb
increments at the dryland site. Plot locations changed every year with the
field to avoid residual effects of treatments.
Data was analyzed separately as randomized complete block experimental
design with four replications at the irrigated location and three replications
at the dryland site.
Anaerobically
digested biosolids from the City of Lincoln's Theresa Street Wastewater
Treatment facility were broadcast in the spring of each year. The biosolids were incorporated by discing
within 24 hours after application.
Biosolids samples were taken from each site the day biosolids were
applied (Table 1).
Table 1. Average analysis of biosolids from the City
of Lincoln's Theresa Street and Northeast treatment facilities.
|
|
-------Theresa Street †------- |
----------Northeast ‡---------- |
||
|
|
Mean |
SD |
Mean |
SD |
|
PH |
8.0 |
0.2 |
6.9 |
0.1 |
|
|
(%) |
|||
|
Solids |
18.6 |
2.0 |
0.31 |
0.68 |
|
Organic-N |
3.8 |
0.064 |
3.68 |
1.59 |
|
Ammonium-N |
0.79 |
0.11 |
1.24 |
0.46 |
|
Nitrate-N |
0.0002 |
0.0002 |
0.022 |
0.020 |
|
Phosphorus |
2.64 |
0.39 |
0.54 |
0.34 |
|
Potassium |
0.25 |
0.05 |
0.62 |
0.33 |
|
Iron |
2.69 |
0.38 |
- |
- |
|
|
(mg kg-1) |
|||
|
Arsenic |
11 |
8 |
16 |
5 |
|
Cadmium |
18 |
6 |
8 |
1 |
|
Chromium |
122 |
30 |
45 |
8 |
|
Copper |
770 |
191 |
1,006 |
239 |
|
Lead |
87 |
24 |
98 |
20 |
|
Mercury |
0.005 |
0.005 |
5 |
2 |
|
Molybdenum |
22 |
5 |
62 |
31 |
|
Nickel |
98 |
42 |
56 |
17 |
|
Selenium |
6.1 |
2 |
26 |
13 |
|
Zinc |
797 |
197 |
2,176 |
317 |
† Mean and standard deviation (SD) of weekly analysis in 1998, analysis obtained from the City of Lincoln's Wastewater and Solid Waste Division of the Public Works & Utilities Department.
‡ Mean and standard deviation (SD)
of biosolids applied in 1988-1990 at the City of Lincoln's Northeast Wastewater
Treatment Plant
All
soil and plant samples were collected from the center two rows of a 15 by 40
foot plots. Soil samples were collected
at the beginning of the experiment to determine the general soil properties to
a depth of 5 feet. Soil samples wee
also collected in the spring of 1997, the fall of 1997, and the fall of 1999 to
determine the residual nitrate. Two
cores were taken per plot to a depth of 5 feet and divided into 0-6",
6-12", 1-2', 2-3', 3-4', and 4-5 feet depth intervals. The cores were combined, air dried at room
temperature and ground to pass a 2-mm sieve.
Grain and stover was hand picked from two 10 foot sections of the center
two rows at physiological maturity. The
whole plant sample was weighed, chopped, and moisture determined by drying at
150B F. total N content in plant samples were
determined using an automated combustion method (McGeehan and Naylor, 1988).
A second experiment was established to assess
the environmental impact of biosolids.
Soil samples were collected from the City of Lincoln's Northeast
Wastewater Treatment facilities injection farm. Samples were collected in September of 1998 from six fields,
which were split based on previous cropping systems and biosolids application
amount. Twenty samples were taken per acre using a zero contamination tube to a
depth of zero to six inches and six to twelve inches. Soil was analyzed using EPA method 6010 for cadmium, chromium,
copper, lead, molybdenum, nickel and zinc, method 7060 for arsenic, method 7471
for mercury, and method 7740 for selenium.
RESULTS
Optimal biosolids rate
Biosolids increased corn
yield by 37 to 77 bushels acre-1 depending on the year and
application rate. Sorghum yield was nearly
doubled in two years and almost tripled in 1999 from biosolids. Absolute yields varied widely between years
due to environmental conditions rather than response to biosolids.
Figure 1. Relative
irrigated corn and dryland sorghum yield response to the amount of organic N
applied with biosolids.
However a consistent corn yield response to biosolids over the
four years was observed when yield was put on a relative basis as the percent
of maximum yield (Fig. 1). Response is
shown based on the amount of organic N applied rather than the rate of
biosolids to eliminate variation caused by differences in biosolids content. In
order to achieve maximum corn yield, 395 lbs of organic N acre-1 was
required. Assuming an average biosolids
content (Table 1), the optimal rate of Lincoln's Theresa Street biosolids was
28 tons acre-1 for corn.
The optimum amount of N
required to maximize yield is more difficult to predict under dryland
conditions. Adequate soil moisture can
not be maintained so that soil N supply, crop N demand, and the plants internal
processes determining yield are all affected.
Thus the relative sorghum yield was not as consistent as the irrigated
corn (Fig. 1). In 1996, sorghum did not
respond to biosolids due to the previous three year fallow period and soybean
crop. Therefore, 1996 should not be
considered a typical year. Sorghum
yield response was similar in 1997 and 1998, requiring 230 lbs organic N acre-1. On average it would require 16 tons acre-1
of Theresa Street biosolids to maximize sorghum yield. In 1999 there was a much lower N supply due
to dry conditions. Thus sorghum
responded to a much higher biosolids rate.
Figure 2.
Relative grain yield increase of irrigated corn and dryland sorghum in
comparison to an untreated control as affected by the number of years after
biosolids application.
In
general, biosolids N has a high agronomic value. However about 80% of the biosolids N is in organic forms (Table
1) and slowly released to crops over several years. The residual effects of biosolids make using biosolids more
attractive to agricultural producers.
The relative yield increase over the four years of this study was
similar for corn and sorghum when biosolids were applied at the rate required
to maximize yield in the first year (Fig. 2).
On average, yields were increased from a one time application of
biosolids 33% in the year of application, 21% in the first year after
application, 14% in the second year, and 9% in the third year after
application. It was likely that yields
would drop below detectable levels after five years. It required 373 lbs N
fertilizer in order to achieve similar corn yield response and 206 lbs of N
fertilizer for sorghum over the four year period of this study (Table 2). The N
fertilizer value of biosolids over four years was $57 acre-1 for
irrigated corn and $31 acre-1 for dryland sorghum when biosolids
were applied at the rate required to maximize yield. Biosolids are worth $2 ton-1 in N fertilizer
alone. When biosolids are applied at
rates higher than required then the value per ton decreases.
Table
2. Cumulative nitrogen fertilizer value of biosolids when applied to irrigated
corn and dryland sorghum at the rate to maximize yields in the first year.
Values shown refer to a biosolids application of 28 tons acre-1 for
corn or 16 tons acre-1 for sorghum.
|
Years after application |
(A) N input from biosolids† |
(B) Yield increase from biosolids‡ |
(C) Fertilizer N required§ |
(D) Available biosolids N¶ |
(E) Value of fertilizer N# |
(F) Fertilizer equivalent†† |
(G) N value of biosolids‡‡ |
|
|
lbs N acre-1 |
% |
lbs N acre-1 |
% |
$
acre-1 |
Lbs N Ton-1 |
$ Ton-1 |
|
Irrigated corn |
|||||||
|
0 |
475 |
28 |
208 |
44 |
31.2 |
7.4 |
1.11 |
|
1 |
0 |
24 |
101 |
21 |
15.2 |
3.6 |
0.54 |
|
2 |
0 |
11 |
36 |
8 |
5.4 |
1.3 |
0.19 |
|
3 |
0 |
9 |
28 |
6 |
4.7 |
1.0 |
0.17 |
|
Total |
475 |
|
373 |
79 |
56.5 |
13.3 |
2.01 |
|
|
|||||||
|
Dryland Sorghum |
|||||||
|
0 |
276 |
36 |
113 |
41 |
17.0 |
7.1 |
1.06 |
|
1 |
0 |
21 |
47 |
17 |
7.1 |
2.9 |
0.45 |
|
2 |
0 |
15 |
32 |
12 |
4.8 |
2.0 |
0.30 |
|
3 |
0 |
7 |
14 |
5 |
2.1 |
0.9 |
0.13 |
|
Total |
276 |
||||||