Assessing environmental sustainability of agricultural systems by simulation of nitrogen and phosphorus loss in runoff

Eur. J. Agron., 1995, 4(4), 453-464

Assessing environmental sustainability of agricultural systems by simulation of nitrogen and phosphorus loss in runoff Andrew Sharpley*, J. S. Robinson

2

1

and S. J. Smith 2

USDA-ARS, Pasture Systems and Watershed Management Research Laboratory, Curtin Road, University Park, Pennsylvania 16802-3702, U.S.A. 1 Department of Soil and Water Science, University of Florida, Gainesville, Florida, U.S.A. USDA-ARS, National Agricultural Water Quality Laboratory, P.O. Box 1430, Durant, Oklahoma 74702-1430, U.S.A.

Accepted : 7 July 1995

* Abstract

To whom correspondence should be addressed.

Inputs of nitrogen (N) and phosphorus (P) in agricultural runoff can promote coastal and fresh water eutrophication. Thus, information on the effect of agricultural management on N and P loss in runoff is needed to develop sustainable management systems. While reliable field data require several years of study, simulation models can quickly estimate the relative effects of management on nutrient loss. A kinetic equation describing the desorption of soil P was used to predict dissolved P (DP) loss in runoff ; enrichment ratio approaches were used to predict particulate N (PN), particulate P (PP), and bioavailable PP (BPP) losses. Accurate predictions of N and P loss in runoff were obtained for watersheds under conservation and conventional till wheat ; reduced and no till rotational small grain crops ; winter wheat with and without a sorghum cover crop ; and native and set-aside grasses. Over­ all, prediction errors were 25 per cent of measured values. Although conservation practices reduced PN and PP losses in runoff up to 50 per cent, algal available DP losses increased 56 per cent com­ pared to conventional practices. Simulated losses of N and P from soils receiving animal manure (beef, poultry, and swine) for 10 to 35 years, indicate the need to minimize surface soil accumula­ tions of N and P in excess of crop requirements. Threshold soil P levels, above which DP concentra­ tion of runoff is expected to exceed water quality standards associated with eutrophication, were predicted by the kinetic equation. The kinetic and enrichment ratio approaches used can provide valuable information on the relative effects of watershed management on soil and water resources and thereby aid development of sustainable agricultural systems.

INTRODUCTION The input of nitrogen (N) and phosphorus (P) in runoff can accelerate the eutrophication of fresh and coastal waters, impairing the waters' use for recre­ ation, industry, and drinking (Sharpley et al., 1994). The senescence and decay of algae and aquatic weeds can cause oxygen shortages and death of fish. Also, potentially carcinogenic toxins produced by some blue-green algae (dominantly cyanobacteria) can cause acute health risks to humans and animals if consumed (Kotak et al., 1993). These toxins also contribute to the unpotability of drinking water via trihalomethane formation during water chlorination (Palstrom et al., ISSN 1161-030//95/04/$ 4.00/© Gauthier-Villars - ESAR

1988). Due to the easier identification and control of point sources, agricultural nonpoint sources now acount for a larger share of the water quality problems in the U.S. than a decade ago (US EPA, 1990). Thus, information is needed on the effect of agricultural management on nutrient transport in runoff to develop environmentally sound systems. Nitrogen and P are transported in dissolved and par­ ticulate (associated with eroding soil and organic material) forms. Dissolved P (DP) and N (nitrate-N ; N0 3 -N) are immediately available for biological uptake. Particulate P (PP) constitutes a variable but long-term source of potentially bioavailable PP (BPP ; i.e., PP available to aquatic biota) (Sharpley et al.,

Andrew Sharpley et al.

454

KP DBT°'W 13 DP= __a _ _ __

1992). The bioavailable P (BAP) content of runoff, therefore, is the sum of DP and BPP. Reliable field studies investigating the transport of N and P in runoff are lengthy and labour intensive. Thus, models simulating nutrient transport in runoff have been used to assess management effects on N and P loss in runoff. Although physically-based descriptions of the various transport processes are used, a lack of data to drive the models and limited field data for testing, has resulted in an over-simplified representation of transport processes. In particular, equilibrium extraction coefficients have generally been used to predict DP ; BPP has been assumed to be a constant portion of total P (TP), and no attempts have been made to predict BPP (Sharpley and Smith, 1993). Conceptually-based equations have been developed to describe the chemical and physical processes involved in the release from soil and transport of N and P in runoff ; they are described in the following section. The release of dissolved soil N and transport in runoff was not predicted, as the primary constituent, nitrate-N, is generally not sorbed by surface soil and moves downward in infiltrating water. In many areas of the U.S., inputs of N and P are required for optimum crop production and are, thus, essential components of sustainable agricultural sys­ tems. However, in certain areas, N and P inputs have exceeded the capacity of the system, causing an increased potential for accelerated eutrophication (Sharpley et al., 1994). Thus, identification of critical soil N and P concentrations, sources of N and P in watershed runoff, and management practices suscep­ tible to runoff and erosion, are of prime importance in the development of sustainable agricultural systems. This paper presents the prediction of N and P trans­ port in runoff from 20 agricultural watersheds in Okla­ homa and Texas over 6 years (1987-1992). Watershed management includes native grasses, conventional and conservational crop tillage, rotational use of cover crops and conversion of cropland to permanent grass. Measured N and P loss in runoff from the watersheds has been reported by Sharpley et al. (1992) and Smith et al. (1991). The predictive equations are also used to estimate the potential loss of N and P in runoff from soil receiving animal manure and to identify soil P levels above which environmental water quality stan­ dards may be exceeded.

PREDICTIVE EQUATIONS Dissolved Phosphorus Concentrations of DP in runoff are predicted by the following equation, which describes the kinetics of soil P desorption (Sharpley and Smith, 1989) :

(1)

v

where DP is the average concentration of an indi­ vidual runoff event (mg L- 1), Pa is available soil P content (Bray-1, mg kg- 1 ) of surface soil (0-50 mm) before each runoff event, D is effective depth of inter­ action between surface soil and runoff water (mm), B is soil bulk density (Mg m- 3 ), t is duration of the run­ off event (min), W is runoff water: soil (suspended sediment) ratio, V is total runoff during the event (mm), and K, a, and ~ are constants for a given soil. Values of K, a, and ~ were estimated from the ratio of surface soil clay :organic C content (Sharpley, 1983) : K

=

.

a = 0.815 (per cent clay + per cent orgamc C)

~

=

0 698 ·

(2)

- 0.540

(3)

0429

( 4)

0.630 (per cent clay + per cent organic C)-

0.141 (per cent clay + per cent organic C)-

Values of D were estimated from soil loss (kg ha- 1) : ln D = iA

+ 0.576 ln soil loss

(5)

where i is a function of soil aggregation (A) (Sharp­ ley, 1985a). In a simulated rainfall study, Sharpley (1985a) found that D was a function of rainfall inten­ sity and soil slope and cover, the effects of which could be summarized by soil loss. Particulate Nutrients The selective transport of clay-sized particles in runoff has led to the concept of nutrient enrichment ratios (ERs), which are defined as the ratio of the nutrient content of eroded sediment to that of surface soil. Enrichment ratios have been used to predict N and P transport in runoff (Menzel, 1980). The concen­ tration of PN, PP, and BPP in each runoff event is cal­ culated from the total N (TN), total P (TP), and bio­ available P (Bio P) content of surface soil, respectively, using the enrichment ratio for each nutri­ ent form (NER, PER, and BIOER): PN = Soil TN X sediment concentration X NER

(6)

PP = Soil TP X sediment concentration X PER

(7)

BPP = Soil Bio P X sediment concentration X BIOER (8) The units of soil TN, TP, and Bio P are mg kg- 1, and for sediment concentration of runoff, g L- 1 • The Eur. J. Agron.

455

Simulation of nitrogen and phosphorus loss

enrichment ratio was predicted from soil loss (kg ha- 1) for each runoff event, using the following equation developed by Sharpley (1985b):

complex ; Bouteloua hirsuta Lag. ; and Bouteloua cur­ tipendula Torr. (Michx.)] in accordance with the U.S. Conservation Reserve Progam (Economic Research Service, 1988).

(9)

Watersheds

Fertilizer P was applied at fall planting of wheat at El Reno and during harrowing in March at Ft. Cobb. Fertilization of introduced grass watersheds was pri­ marily during the year of establishment (1987). Where applicable, fertilizer application rates were determined by soil test P (Bray 1 ; Bray and Kurtz, 1945) and N (N0 3 -N) recommendations. The major soil type at the Bushland location is Pullman clay loam (fine, mixed, thermic Torrertic Paleustolls) ; at El Reno, Kirkland silt loam (fine, mixed, thermic Udertic Paleustolls) ; at Ft. Cobb, Cobb fine sandy loam (fine-loamy, mixed, thermic Udic Haplustalfs), and at Woodward, Wood­ ward loam (coarse-silty, mixed, thermic Typic Usto­ chrepts).

Management characteristics of the 20 watersheds are summarized in Table 1 and represent agricultural land use in the Southern Plains region of Oklahoma and Texas. Weed control on the no till wheat water­ sheds at El Reno was primarily with phenoxy and gly­ phosphate herbicides. At Bushland, chlorosulfuron was also used. In 1986, winter wheat watersheds W3 and W4 were seeded with introduced grasses [lschaemum

Watershed runoff was measured using precalibrated flumes equipped with water-level recorders, with 5 to 15 samples collected automatically during each runoff event. The samples were composited in proportion to flow, to provide a single representative sample and stored at 4 °C until analysis. Surface soil samples (0-50 mm depth) were collected annually in March at four sites near the flume of each watershed, compos­ ited, air dried, and sieved (2 mm).

In ER = 1.21 - 0.16 ln soil loss Bioavailable Phosphorus

The BAP concentration of each runoff event is cal­ culated as the sum of predicted DP and BPP concen­ trations.

MATERIALS AND METHODS

Table 1. Watersheds characteristics for 1988 to 1992.

Watershed

Tillage

Land use

} }

Wheat-sorghum­ fallow rotation Wheat-sorghum­ fallow rotation

No-till No-till No-till Sweeps, mulch tread Sweeps, mulch tread Sweeps, mulch tread

El Reno, Oklahoma El E2 E3 E4 ES E6 E7 ES

Native grass Native grass Native grass Native grass Wheat-sorghum Wheat Wheat Wheat

Ft. Cobb, Oklahoma

}

Woodward, Oklahoma WI W2 W3 W4 Vol. 4, n° 4 - 1995

Fertilizer applied

p

kg ha- 1 yc 1

Bushland, Texas BIOA BllA Bl2A BIOB BllB Bl2B

Cl C2

N

0 0 0 0 0 0

0 0 0 0 0 0

None None None None Mouldboard, disk Mouldboard, disk No-till Sweeps, disk

0 0 0 0 76 76 56 56

0 0 0 0 12 12 13 13

Peanuts-grain­ sorghum rotation

Mouldboard, disk Mouldboard, disk

20 20

19 18

Native grass Native grass Introduced grasses Introduced grasses

None None None None

0 0 95 85

0 0 23 23

456

Chemical Analyses

Runoff: Aliquots of runoff samples were centri­ fuged [266 m sec- 1 (15000 x g) for 5 min] and filtered (0.45 µm) prior to N0 3 -N, NH4 -N, and DP determina­ tions, while total Kjeldahl N (TKN) and TP were determined on unfiltered samples. Analyses for N0 3 -N, NH4-N, and TKN were made by standard automated methods described in Methods for Chemi­ cal Analysis of Waters and Wastes (USEPA, 1979). Total N was calculated as the sum of N03-N and TKN ; and particulate N (PN) as the difference between TKN and NH 4 -N. Dissolved P was deter­ mined by the colorimetric method of Murphy and Riley (1962), as was TP following perchloric acid digestion of unfiltered samples and neutralization of the digest (Olsen and Sommers, 1982). Particulate P was calculated as the difference between TP and DP. Suspended sediment concentration of runoff was deter­ mined in duplicate as the difference in weight of 250 mL aliquots of unfiltered and filtered runoff after evaporation (105 °C) to dryness. The bioavailable P content of runoff (BAP) was measured by shaking one Fe-oxide strip with 50 mL of unfiltered runoff for 16 hon an end-over-end shaker at 25 °C. The strip was then removed, rinsed free of adhering sediment particles, and dried. Phosphorus retained on the strip was removed by shaking the strip end-over-end with 40 mL of 0.1 M H 2 S04 for 1 h, and following neutralization was measured by the method of Murphy and Riley (1962). Phosphorus retained on the strip represented that associated with both dis­ solved and bioavailable particulate forms. Iron-oxide impregnated strips were prepared by immersing filter-paper circles (15 cm diameter, What­ man No. 50 1) in a 10 per cent (w/v) solution of FeCly6H 2 0. The paper circles were then air dried and immersed in 2.7 M NH4 0H solution to convert FeC1 3 to Fe oxide. Immersion in NH4 0H was carried out as rapidly as possible to avoid uneven oxide deposition on the paper (Lin et al., 1991 ). After the paper circles were air-dried, they were cut into strips 10 by 2 cm and stored for later use. Soil : Clay content of the soils was determined by pipette analysis following dispersion with sodium hexa­ metaphosphate (Day, 1965) and organic C by the dichromate-wet combustion procedure (Raveh and Avnimelech, 1972). Total N was measured by a semimicro-Kjeldahl procedure (Bremner, 1965). Avail­ able soil P was extracted by shaking 2 g soil with 20 mL of 0.03 M NH 4 F and 0.025 M HC I for 5 min (Bray and Kurtz, 1945). The bioavailable soil P con­ tent (Fe-0 strip P) of each soil sample was determined by shaking a 1 g soil sample in 40 mL 0.01 M CaCl 2

(I) Mention of trade names implies no endorsement by USDA.

Andrew Sharpley et al.

and Fe-oxide strip end-over-end for 16 h at 25 °c. The strip was then removed, rinsed free of adhering soil particles, and dried. Phosphorus retained on the strip was removed by shaking the strip end-over-end with 40 mL 0.1 M H 2 S04 for 1 h and measured. Total P (TP) content was determined by perchloric acid diges­ tion (Olsen and Sommers, 1982). In all extracts, P concentration was measured on neutralized filtrates by the colorimetric method of Murphy and Riley (1962). Measured and predicted N and P concentrations and amounts in runoff were compared using linear regres­ sion analysis, analysis of variance for paired data, and standard error of the predicted value. In the latter analysis, the measured value (x) was assumed to be correct and have no error, with the standard error in the predicted value (y) representing all variability associated with the predictive equations.

RESULTS AND DISCUSSION Predicting N and P Transport

The N and P concentration of each runoff event was predicted using Eqs. ( 1) through (9) ; soil loss ; sur­ face soil clay content, aggregation, and organic C ; and TN, available (Bray 1), bioavailable (Fe-oxide strip P), and TP content of surface soil before runoff. Predicted BAP concentration was calculated as the sum of predicted DP and BPP values. Using these pre­ dicted concentrations and measured runoff volume, mean annual concentrations and losses of PN, DP, BAP, and TP were calculated. As equation constants were calculated from soil physical and chemical properties (Sharpley, 1983), all equation parameters are independently determined with no field calibration conducted. Values of the con­ stants and soil properties used in the predictive equa­ tions are given in Table 2. Event duration, t, was set at 30 min, an approximate value for a representative storm size. Actual event durations were not available ; however, sensitivity analysis showed that event dura­ tion had negligible effect on predicted average DP concentration. This may result from the fact that up to 75 per cent of total event DP was transported during the initial 15 min of an event (Sharpley et al., 1981). Measured and predicted mean annual DP concentra­ tions were not significantly different (p < 0.01) for all watersheds and management practices over a wide range in measured values (0.09 to 0.60 mg L- 1) (Table 3). For all watersheds, the average error in DP prediction was 0.04 mg L- 1, or 15 per cent of the mea­ sured value. Slope values of the measured - predicted DP regression for each watershed did not consistently deviate from 1.00, indicating no trend of over or underestimation by Eqs. (1) to (5). Eur. J. Agron.

Simulation of nitrogen and phosphorus loss

457

Table 2. Values of equation parameters for the major soil types at each watershed location. Parameter

Cobb

Kirkland

Pullman Woodward

Clay content, per cent

17

13

30

32

Organic C, g kg- 1

3.2

20.2

8.3

10.2

Bulk density, Mg m­3

1.35

1.40

1.45

1.40

20

26

15

19

K

0.039

0.172

0.052

0.057

a

0.095

0.298

0.117

0.127

~

0.775

0.313

0.657

0.618

- 1.71

- 1.28

-2.07

-1.78

Soil aggregation

1

Equation constants

I Ratio of per cent clay in dispersed/undispersed soil.

Predicted mean annual BAP, TP, and PN concentra­ tions were similar to measured values (p < 0.01), over a wide range in BAP (0.14 to 1.71 mgL- 1), TP (0.18 to 4.25mgL- 1), and PN (1.25 to 16.18mgL- 1 ) (Table 3). The average prediction error for BPP was 0.08 mg L- 1 (12 per cent of measured mean), for TP was 0.32 mg L- 1 (18 per cent measured mean) and for PN was 1.77 mg L- 1 (25 per cent measured mean). Overall, the concentration of dissolved and particu­ late N and P was accurately predicted by the kinetic and enrichment ratio approaches, for runoff from watersheds of differing soil, tillage, fertilizer, and crop management. This predictive reliability enables a com­ parison of the implications of differing agricultural management on N and P transport in runoff.

Table 3. Mean annual runoff; soil loss; and measured and predicted (Eqs. (I) to (9)) P and N concentrations in runoff from 1987 to 1992.

Watershed

Dissolved P

Bioavailable P

Runoff

Soil loss

mm

kg ha- 1 yr­ 1

142 99 136 128 4 4 42 30

16 14 26 13 10 52 2481 347

0.09 0.11 0.12 0.49 0.27 0.46 0.60 0.54

0.10 0.11 0.11 0.46 0.28 0.45 0.64 0.55

0.15 0.15 0.14 0.65 0.24 0.32 0.74 0.56

0.12 0.12 0.16 0.59 0.24 0.31 0.74 0.55

Mean

72

368

0.33

0.34

0.37

No till BlOA BllA Bl2A E7

24 23 54 130

223 234 824 369

0.51 0.20 0.23 0.53

0.51 0.19 0.23 0.53

Native grass El E2 E3 E4 WI W2 W3 W4

Meas.

Pred.

Meas.

Pred.

Total P Meas.

Particulate N Pred.

Meas.

Pred.

0.18 0.23 0.21 0.61 0.63 1.72 1.19 0.95

0.20 0.24 0.22 0.59 0.64 1.40 1.28 0.94

1.26 1.65 1.25 1.41 2.48 6.08 8.08 2.52

1.24 1.60 1.13 1.63 2.44 7.18 8.02 2.44

0.35

0.71

0.69

3.15

3.26

1.25 0.34 0.43 0.89

1.23 0.35 0.42 0.85

1.93 1.64 1.29 1.10

1.90 1.64 1.12 1.13

4.17 5.87 4.17 5.95

4.00 6.13 4.02 5.85

mg L- 1

Mean

60

413

0.39

0.39

0.78

0.76

1.51

1.48

5.12

5.08

Reduced till BIOB BllB Bl2B

15 12 30

602 448 1931

0.21 0.28 0.20

0.20 0.29 0.19

0.61 0.68 0.84

0.62 0.67 0.84

2.54 3.44 4.25

2.59 3.52 4.21

8.40 12.47 16.18

8.28 12.67 15.54

Mean

18

994

0.23

0.23

0.69

0.70

3.25

3.29

11.62

11.47

Conventional till ES 102 118 E6 ES 115 CJ 128 C2 127

623 5204 1371 9402 16847

0.64 0.13 0.17 0.13 0.15

0.63 0.13 0.19 0.12 0.14

1.21 0.75 0.55 1.38 1.71

1.25 0.76 0.54 1.35 1.78

1.21 2.47 1.67 2.56 3.80

1.23 2.51 1.23 2.71 3.85

4.84 12.90 9.05 8.04 12.77

5.16 12.55 10.47 7.66 11.85

118

6689

0.25

0.25

1.15

1.17

2.38

2.37

9.55

9.48

Mean Vol. 4, n° 4 - 1995

458

Andrew Sharpley et al.

Management Implications Tillage

In efforts to reduce soil erosion, many farmers have adopted conservation tillage (Christensen and Norris, 1993). These practices can reduce seedbed preparation time and on-farm energy use, while increasing water­ use efficiency (Follett et al., 1987 ; Unger and McCalla, 1980). The effect of tillage practice on N and P loss in runoff was accurately predicted by Eqs. (1) to (9) (Figure 1). Prediction errors ranged from 11 to 13 per cent of the measured values with the greatest error associated with DP prediction. For both N and P, the predictive accuracy was lower at losses < 0.1 kg ha- 1 ye' for DP and BAP and < 1.0 kg ha-' ye' for PN and TP (Figure 1). For these predictions, N and P loss from conventionally tilled

...

10

y

T"" I

~

Dissolved P

= -0.008 + 1.04x rz = 0.93 ,,,.~ error = 31%

....

~-

1-

-c

10

(.)

w

a:

a.

Bioavallable P

y = 0.032 + O.B6x rz = 0.90 error = 11%

+*.J"

/ +/I'

('

.,,,rt. <:

,. +

10

0.1

r rz=o.9y Total P

y = -0.065 + 1.07x

0.001 0.001

+

•V•• •

ff":+ /

.

,,,,.Jl·

10

0.1

/.

Particulate N 10

error = 20%

0.1

/

/..­

+,}I'+

0.1

9/'~

0.001 0.001

w

The emphasis on developing sustainable manage­ ment systems has renewed interest in the use of cover

;JI

++/•

c

Cover Crops

10

'Ii'~

0.1

watersheds (E6 and E8) were compared with reduced (BlOB, BllB, Bl2B) and no till (E7, BlOA, BllA, Bl 2A) losses (Table 1). The mean annual concentration of PN and TP in runoff from no till (5.12 and 1.51 mg was was less than for reduced (11.62 and 3.25 mg L- 1 ) and conven­ tional tillage (9.55 and 3.38mgL-'). In contrast, the mean annual DP concentration was greater for no till (0.39 mg L- 1) than reduced (0.23 mg L- 1) and conven­ tional till (0.25 mg L- 1 ). The fact that no till increased the concentration of DP but not particulate nutrients, may be attributed to a release of P to runoff from sur­ face crop residues and unincorporated fertilizer that was surface broadcast.

y = -0.29 + 1 . 0 / 2 " ' r 2 = 0.91 error = 18%

•,,,

0.1

/

•

/",,.

+.

/

No till / / / + Reduced tlll / 1:1 relationship / <> Conventional tlll • ...___ ____.______ 0.001 ...___ __.__ _.........._ _..____ ___._ __.___, 0.001 .....__ __..________ 10 0.1 0.001 10 0.1 0.001 •

~

MEASURED ( kg ha ·1 yr ·1 ) Figure 1. Relationship between mean annual predicted and measured nutrient discharge from cropland as a function of tillage (regression equation is for the linear non-logarithmic relationship). Eur. l. Agron.

459

Simulation of nitrogen and phosphorus loss

tribution of P from vegetative material (Table 3, Fig­ ure 2). For example, the mean annual DP concentra­ tion was 0.64 mg L- 1 with a cover crop (ES), and 0.13 (E6) and 0.17 mg L- 1 (ES) without a cover crop ; for BAP it was 1.21 mg L- 1 with a cover crop (ES) and 0.7S (E6) and O.SS mg L- 1 (ES) without a cover crop.

crops to max1m1ze soil N and P utilization, while minimizing soil and nutrient losses in runoff (Har­ grove, 1991 ; Karlen and Sharpley, 1994). The loss of N and P in runoff from watersheds with and without cover crops was accurately predicted by Eqs. (1) to (9) (Figure 2). The overall prediction error for N and P ranged from 12 to 17 per cent of the measured mean (Figure 2). Most of this error occurred at the lower values of N and P loss, with predictions generally underestimating mean values (Figure 2). For these pre­ dictions, runoff from conventionally tilled winter wheat with (ES) and without (E6 and ES) a summer forage sorghum cover crop ; and peanuts with and without a winter small grain cover crop (Cl and C2) were used (Table 1).

Conservation Reserve Programme

In the U.S., many farmers have been encouraged to set aside original croplands as part of the conservation reserve program. As a result, a considerable portion of low productivity and erodible cropland in the Southern Plains area of Oklahoma and Texas is being planted to perennial grasses and maintained in an idle state. The loss of N and P in runoff from native (Wl and W2) and introduced grasses (W3 and W4; converted from conventional tilled wheat in l 9S6), were reliably pre-

Cover crops reduced soil loss (Table 3) and associ­ ated PN and TP transport (Table 3 and Figure 2). However, DP and BAP transport tended to be greater with than without a cover crop, due in part to the con-

Dissolved P

,....

Bioavallable P

y = -0.009 + 1.02x rz = 0.91

...>a

I

error

= 16%

1.0

,....

y = -0.007 + 1.02x rz = 0.94 error = 15% '

I

,,,,

~

.c en ~ .._..

<

0 01 0.01 ·

c

w t>

y

I­

-c

= -0.219

1.0

w a: a..

Total P

0.01 0.01

+ 1.08x

= 0.94 error = 12% rZ

.{

,J'

y

;t*

..,..-r

........

.

100

1.0

= -0.25

+•

1.0 /

• Cover crop .... No cover crop

100

~lv .}~

+ 1.06x

r 2 = 0.91 error = 17%

/

1.0

,..r

Particulate N

./v·-*

............

/

,,........

.I'

+_..,,#

+

. /

•? w-·

~

/

1:1 relationship 0.01 0.01

1.0

100

MEASURED (kg ha -1 yr -1 ) Figure 2. Relationship between mean annual predicted and measured nutrient discharge from conventionally tilled croplands with and without a cover crop (regression equation is for the linear non-logarithmic relationship). Vol. 4, n° 4 - 1995

460

,...

Andrew Sharpley et al.

I

-

y = -0.004 + 1.06x r2 = 0.85 error = 27% /+ //

/

/ • /.+

......?

0.001

..­ ­c

10

(J

w a: c..

0.1

~~

y = -0.003 + 0.94x r 2 = 0.93 / /+ error = 18% /

..,4•."'

+#

....

0.001 i..;..~_t_._...____..___

w

10

,,

0.1 -

c

Bioavailable P

Dissolved P

10

___.._____ 10

0.1

Total P

y = -0.014 + 1.14x r2 0.94 error = 14%

=

0.1

./•

0.001 1.:..... :::___ 0.001

10

+/

/+

/

+

___.__ ____.__ ____.__ __..__ __,

0.1

10

Particulate N

y = -0.180 + 1.21x / r 2 = 0.89 / • error = 30% / /

.....

0.1 • Native graaa + CRP grass

/

/

..

/

~

•A/

,;..·~·

• ,> •

/

·/

~/

+

~

1:1 relationship

0.001 L - - - - - ' ' - - - - - - ' " - - - - ' - - - - - - - o.001 ._...__._...___~o,___ ___.__ __.1o - - - ­ 0.001 .1 0.001 0.1 10

MEASURED ( kg ha ·1 yr -1 ) Figure 3. Relationship between mean annual predicted and measured nutrient discharge from native grasslands and conventionally tilled wheat converted to grassland under the Conservation Reserve Program (regression equation is for the linear non-logarithmic relationship).

dieted by Eqs. (I) to (9) (Figure 3). Predictive errors ranged from 14 to 30 per cent of the measured values, with the greatest errors associated with PN (30 per cent) and DP (27 per cent).

nutrient status from previous fertilizer applications can increase losses compared to unfertilized native grass for several years after conversion.

The loss of PN, TP, BAP, and DP in runoff from introduced grass (2.07, 0.39, 0.24, and 0.21 kg ha- 1 yr- 1 , respectively) was greater than from native grass (0.17, 0.05, 0.03, and 0.02 kg ha- 1 yr- 1, respectively). This greater N and P transport is due to a higher soil fertility status for the introduced grass watersheds from previous N and P applications to winter wheat than for the unfertilized native grass watersheds. Com­ pared to the prior 5 years of conventionally tilled win­ ter wheat for W3 (Sharpley et al., 1991), the conser­ vation reserve programme reduced PN, TP, and BAP 18 fold and DP 2 fold. Clearly, conversion of cropland to introduced grass, in accordance with the conservation reserve programme, can reduce N and P loss, especially particulate losses, but elevated soil

Refinement of Predictive Equations

The underestimation of DP and BAP transport in runoff from native grass and no till watersheds, may result from an inadequate representation of the contri­ bution of P release from vegetative material and the enrichment of organic material to P transport in run­ off. Vegetative cover affects both chemical and physi­ cal processes controlling soil and particulate P loss in runoff and P release from soil and crops to runoff water. For example, the extent of vegetative cover influences the degree of interaction between surface soil and runoff (D; Eqs. (1) and (5), which initiates P dissolution and transport. There is also a differential release of P from vegetation. It is well established that Eur. J. Agron.

Simulation of nitrogen and phosphorus loss

461

the release of P from vegetation can be an important source of DP in runoff, which is influenced by several soil and crop factors such as soil nutrient status, soil water content, crop type, and growth stage (Sharpley, 1981; Timmons et al., 1970; Wendt and Corey, 1980). However, there has been limited success in simulating the effect of vegetative cover on P loss in runoff, particularly for growing plants (Schreiber, 1990). Predicted values of ER are affected more by an incremental increase in soil loss at rates under 50 kg ha- 1 yr- 1 than at rates over 500 kg ha- 1 yr- 1 because the relationship predicting ER from soil loss (Eq. (9)) is logarithmic. Consequently, making the slope and intercept of Eq. (9) a function of factors affecting soil loss or runoff, such as rainfall intensity, vegetative cover, and management practice, should improve predictions. This may involve use of specific surface area and density of eroded material and enrichment of particle size fractions, rather than total soil loss. Land Application of Animal Manure

In the last decade, confined animal production, such as beef feedlot, dairy, poultry and swine operations, have more than doubled in Oklahoma and Texas and have become increasingly important to the economic well-being of the area (Oklahoma Agric. Stat. Serv., 1990; Texas Agric. Stat. Serv., 1989). However, a growing concern of this industry is the environmen­ tally sustainable utilization of the animal manure pro­ duced. Although manure is a valuable alternative to mineral fertilizer, far more N and Pare often produced than required for optimum crop yields on both the producing and adjacent farms. As a result, continual local land applications of manure at rates greater than

crop N and P removal can increase surface soil con­ tent of N and particularly P (Table 3). These N and P accumulations enhance the potential for N0 3 -N leach­ ing to ground water (Liebhardt et al., 1979) and P movement in surface runoff (Magette, 1988). For example, Bray- I P contents in the surface 50 mm of soil receiving long-term (8 to 35 years) poultry and swine manure (Sharpley et al., 1993) and cattle feed­ lot waste (Sharpley et al., 1984), were increased up to 38 fold compared to untreated soils (Table 4). We estimated the N and P concentration of a 25 mm runoff event of 20 kg ha- 1 soil loss from the treated and untreated soils, using the predictive equa­ tions presented earlier (Eqs. (I) to (9)) and measured soil N and P content. These runoff and soil loss values are means for all events occurring on our grassed watersheds in the Southern Plains during the last 15 years. Predicted N and P concentrations of runoff from treated soils were dramatically higher than from untreated soils, if runoff occurs (Table 4). Under grass, erosion is minimal ; therefore, most of the P trans­ ported from treated soils will be in a bioavailable form (86 to 97 per cent). For untreated soils, BAP averaged only 38 per cent of the TP transported in runoff. Although this is a hypothetical runoff situation, it illustrates the potential use of the predictive equa­ tions in assessing the relative impact of the land appli­ cation of animal manure on N and P transported in runoff. More importantly, it emphasizes the need to carefully manage repeated applications of manure over a long period of time to minimize the potential for nutrient enrichment of runoff from these soils. Threshold Soil P Levels In developing environmentally-sustainable manage­ ment systems for P, it is critical to identify soil P con-

Table 4. Swface soil content (0-50 mm) and predicted runoff' concentration (using Eqs. (1) to (9)) of N and P in soil untreated and treated with manure.

Manure type N

Duration

Soil content Bray-I P

Bioavail. P

Total N

Dissolved P

mg kg- 1

yr Beef feedlot

Total P

Predicted runoff concentration Biovailable P

Total P

Partic.

mg L- 1

8

15 63

25 230

639 1268

1010 1800

0.27 1.09

0.28 1.14

1.00 1.32

2.10 3.74

Beef dairy

0 20

61 289

61 289

353 1103

1055 1953

0.67 3.28

0.70 3.36

0.80 3.54

6.00 11.12

Poultry

0 35

5 239

26 553

303 2384

1973 3703

0.04 2.47

0.05 2.58

0.67 2.97

4.10 7.70

Swine

0 9

15 138

6 150

250 532

1196 1460

0.18 2.22

0.19 2.25

0.70 2.33

2.49 3.04

Vol. 4, n° 4 - 1995

462

Andrew Sharpley et al.

tents above which environmental concerns outweigh any agronomic benefits of further increases in soil P. Several states have thus proposed threshold soil P lev­ els above which reduced application rates of fertilizer or manure P are recommended to minimize potential water quality impacts (Sims, 1993). These values range from 75 to 150 mg kg- 1 Bray-1 P. Threshold soil P values have been subjectively derived, as little field data is available relating soil and runoff P content (Sharpley et al., 1994). Thus, the predictive equations were used to compare the relationship between soil and DP concentration of runoff as a function of Bray­ 1 P content and soil type. Poultry manure application for 35 yr resulted in surface soil (0-50 mm) Bray- I P contents of 186 mg kg- 1 for Rexor, 179 mg kg- 1 for Sallisaw, and 239 mg kg- 1 for Stigler soils (Sharpley et al., 1993). Using Eqs. (1) to (5), we predicted the DP concentra­ tion of runoff, with Bray-1 P increasing to 300 mg kg- 1 , to cover the observed values. As for the previous simulation, a hypothetical 25 mm runoff event of 20 kg ha- 1 soil loss was used. The kinetic constants of Eq. (1), K, a, and ~ were calculated from measured clay and organic C contents of the respec­ tive soils using Eqs. (2), (3), and (4) (Figure 4). As expected, runoff DP increased with increasing Bray- I soil P (Figure 4). However, the relative increase dif­ fered between soils, as a function of clay and organic C contents used to derive the kinetic constants and which to a certain extent influence soil P release to runoff (Sharpley, 1983). Thus, for a given Bray-IP content, the DP concentration of runoff increased in the order Rexor, Stigler, and Sallisaw (Figure 4). Clearly, reliable threshold soil P levels should be made a function of soil type rather than a specific

-.

....I Cl

E

Cl..

c

w ~

4

Clay Organic C % mg kg-1 17 • Rexor + Sallisaw 1o

35 59

0

en en

c LL LL

0

z

::::>

a:

BRAY-1 SOIL P (mg kg ·1) Figure 4. Relationship between the dissolved P concentration of runoff and Bray-I P content of suiface soil predicted by Eqs. (I) to (5).

value for a large geographic area. For example, if one sets a critical DP concentration of 0.10 mg L- i above which eutrophication may be accelerated (US EPA, 1986), it can be calculated from the soil-runoff P rela­ tionships of Figure 4 that soil Bray-P contents greater than 16 mg kg- 1 for Rexor, 7 mg kg- 1 for Sallisaw, and 12 mg kg- 1 for Stigler soils, have the potential to exceed this DP concentration. Using a critical mean annual DP concentration of 1.0 mg L- 1 for agricultural runoff similar to that required of sewage treatment dis­ charge (US EPA, 1986), threshold soil Bray-1 P con­ tents would be 157 mg kg- 1 for Rexor, 74 mg kg- 1 for Sallisaw, and 116 mg kg- 1 for Stigler. This simulation illustrates the need not only to set realistic threshold soil P levels, but that they be soil and site specific. CONCLUSIONS The kinetic and enrichment ratio relationships used to predict dissolved and particulate N and P transport in runoff, gave realistic estimates for individual events over a diverse range of climatic, edaphic, and manage­ ment conditions. The predictive accuracy was greatest for DP losses> 0.1 kg ha- 1 yc 1 and TN and TP losses > 1.0 kg ha-' yr- 1 • Above these losses, N and P export in runoff will have the greatest environmental impact. An additional advantage of these equations is that no field calibration was required, although measured soil loss and runoff is needed. Overall, nutrient loss was predicted within 25 per cent of measured values. In general, conservation measures were more effec­ tive at reducing particulate than dissolved nutrient losses in runoff. Implementation of conservation till­ age, cover crops, and conservation reserve grasses, reduced total N and P loss in runoff up to 50 per cent compared to conventional till practices without cover crops. In contrast, DP loss increased by up to 56 per cent compared to runoff from conventionally tilled watersheds. This is due to the fact that while reducing erosion, these conservation measures may actually increase surface soil P status. For example, unless no till practices are occasionally ploughed or P applied below the surface, accumulation of P in the runoff sensitive portion of surface soil (0-5 cm) can occur (Griffith et al., 1977 ; Guertal et al., 1991). This will enhance the potential for DP enrichment of runoff. Clearly, sustainable management systems must address not only the processes by which nutrients are trans­ ported, but also nutrient source in a watershed. In areas where application of N and P in animal manure exceeds crop removal rates, subsequent sur­ face soil accumulation of N and P can be a source of these nutrients in runoff. The predictive equations demonstrate the potential for N and P enrichment of runoff from soils receiving manures. In general, however, manure applications have been based on meeting the crop N needs that minimize Eur. J. Agron.

463

Simulation of nitrogen and phosphorus loss

nitrate leaching losses and the potential for ground water contamination. This strategy often leads to an increase in soil P levels due to the generally lower ratio of N :Padded in manure (4 :1) than taken up by crops (8 : 1). Thus, site hydrology is important in determining manure application strategies. If the potential for nitrate leaching exists, N should be a pri­ ority management consideration. However, if runoff and erosion are a concern, P should be the main ele­ ment driving application rates. Other options for efficient utilization of manure include forming cooperatives that could economically compost or concentrate manure, increasing the dis­ tance it could be transported ; establishing cost-sharing programs so that both the consumer and producer share the economic burden of environmental sustain­ ability ; and expanding education and extension pro­ grams highlighting the nutritive value of manure, so as to increase the demand for this nutrient resource to non-producing farmers (Sharpley et al., 1994). The predictive equations may also be used to iden­ tify critical source areas and threshold soil P levels, above which reduced inputs of P as fertilizer or manure should be recommended to minimize potential water quality impacts (Sims, 1993). Identification of nutrient sources in runoff from a watershed will enable targeting of effective remedial measures to minimize P loss. The data presented in this paper show that the kinetic and enrichment ratio relation­ ships are valuable tools to compare the relative effects of agricultural management on nutrient loss in runoff, and thus aid selection of environmentally sustainable systems.

ACKNOWLEDGEMENTS The assistance of USDA-ARS personnel W. A. Berg, G. A. Coleman, and 0. R. Jones in collecting soil and runoff samples, and Elaine Mead for graphics expertise is gratefully appreciated. NOTATIONS A B BAP BIOP BPP

Degree of soil aggregation (unitless) Soil bulk density (Mg m-3 ) Bioavailable P content of runoff (mg L- 1 and kg ha- 1) Bioavailable P content of soil (mg kg- 1) Bioavailable particulate P content of runoff (mg L- 1 and kg ha- 1)

D

Effective depth of interaction between runoff and surface soil (mm) Dissolved P content of runoff (mg L- 1 and kg ha- 1) Enrichment ratio of nutrient in runoff sediment to source soil (unitless) Nitrogen Phosphorus Bray I available soil P content (mg kg- 1)

DP ER N P

Pa

Vol. 4, n° 4 - 1995

Particulate P content of runoff (mg L- 1 and kg ha- 1) PN PN - Particulate N content of runoff (mg L- 1 and kg ha- 1) Soil TN Total N content of soil (mg kg- 1 ) Soil TP Total P content of soil (mg k- 1) TN Total N content of runoff (mg- 1 and kg ha- 1) Total P content of runoff (mg L- 1 and kg h- 1) TP Duration of runoff event (min) W Runoff water/soil (suspended sediment) ratio (L g- 1) K, a, ~. Constants of the equation describing the kinetics of soil P desorption pp

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