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TECHNICAL REPORTS
Plant and Environment Interactions

Nitrogen Removal and Nitrate Leaching for Forage Systems Receiving Dairy Effluent

Kenneth R. Woodard^*,a, Edwin C. French^,a, Lewin A. Sweat^a, Donald A. Graetz^a, Lynn E. Sollenberger^a, Bisoondat Macoon^d, Kenneth M. Portier^b, Brett L. Wade^a, Stuart J. Rymph^a, Gordon M. Prine^a and Harold H. Van Horn^c

^a Soil and Water Science Dep., Univ. of Florida, IFAS, Gainesville, FL 32611
^b Statistics Dep., Univ. of Florida, IFAS, Gainesville, FL 32611
^c Animal Sciences Dep., Univ. of Florida, IFAS, Gainesville, FL 32611
^d Central Mississippi Research and Extension Center, MSU, Raymond, MS 39154

* Corresponding author (krw{at}mail.ifas.ufl.edu)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Florida dairies need year-round forage systems that prevent loss of N to ground water from waste effluent sprayfields. Our purpose was to quantify forage N removal and monitor nitrate N (NO^-₃–N) concentrations in soil water below the rooting zone for two forage systems during four 12-mo cycles (1996–2000). Soil in the sprayfield is an excessively drained Kershaw sand (thermic, uncoated Typic Quartzipsamment). Over four cycles, average loading rates of effluent N were 500, 690, and 910 kg ha^-1 per cycle. Nitrogen removed by the bermudagrass (Cynodon spp.)–rye (Secale cereale L.) system (BR) during the first three cycles was 465 kg ha^-1 per cycle for the low loading rate, 528 kg ha^-1 for the medium rate, and 585 kg ha^-1 for the high. For the corn (Zea mays L.)–forage sorghum [Sorghum bicolor (L.) Moench]–rye system (CSR), N removals were 320 kg ha^-1 per cycle for the low rate, 327 kg ha^-1 for the medium, and 378 kg ha^-1 for the high. The higher N removals for BR were attributed to higher N concentration in bermudagrass (18.1–24.2 g kg^-1) than in corn and forage sorghum (10.3–14.7 g kg^-1). Dry matter yield declined in the fourth cycle for bermudagrass but N removal continued to be higher for BR than CSR. The BR system was much more effective at preventing NO^-₃–N leaching. For CSR, NO^-₃–N levels in soil water (1.5 m below surface) increased steeply during the period between the harvest of one forage and canopy closure of the next. Overall, the BR system was better than CSR at removing N from the soil and maintaining low NO^-₃–N concentrations below the rooting zone.

Abbreviations: BR, bermudagrass–rye system • CSR, corn–forage sorghum–rye system • DM, dry matter

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
THERE has been a growing concern over the potential environmental impact of the increasing number and size of dairy operations in watersheds surrounding the historic Suwannee River in northern Florida. The Suwannee River originates from the Okefenokee Swamp in southern Georgia and flows through northern Florida to the Gulf of Mexico. According to 2000 inventory estimates, the total number of milk cows within four of the eight Florida counties that border the Suwannee River accounts for 27% of the state's 156 000 head (Florida Agricultural Statistics Service, 1999). Dairies have been identified as a possible source of nitrates found in the Suwannee and the Upper Floridan Aquifer (UFA) underlying the river basin (Katz and DeHan, 1996; Berndt et al., 1998). Many of the upland soils in Suwannee River watersheds are excessively drained, deep, sandy Entisols with low organic matter and cation exchange capacity, and are highly prone to nitrate leaching. Also, the UFA underlying these soils is relatively close to the surface and the layers above it are generally thin and unconfined, mostly with karstic features (Andrews, 1992). These characteristics coupled with relatively high annual rainfall increase the possibility that NO^-₃–N will leach through the soil profile and contaminate ground water. The 30-yr average annual rainfall (1961–1990) for eight locations within the Suwannee River area in northern Florida ranged from 1310 to 1460 mm (National Climatic Data Center, Asheville, NC). Nitrate levels in the UFA affect those in the Suwannee River because of a cycle of ground and surface water exchange. During low flow, the river is supplied with water from the UFA through springs and bottom seeps, while during high flow, river water flows into the UFA (Andrews, 1992).

Dairies in northern Florida are under governmental mandate to use manure nutrients while preventing environmental contamination. Most are involved in the year-round production of forage crops in manure effluent sprayfields. Forage crops remove large quantities of nutrients from the soil because, unlike grain production systems, nearly all aboveground biomass is removed during harvest. The harvested forage, containing recycled effluent minerals, is used on farm or sold off farm.

Few studies have evaluated year-round forage systems under dairy effluent irrigation. Hubbard et al. (1987) evaluated a bermudagrass–annual ryegrass (Lolium multiflorum Lam.) system with annual dairy effluent N rates of 530 and 1080 kg ha^-1 delivered through a center pivot irrigation system. Monthly average NO^-₃–N concentration in soil water at 2.4 m below the surface ranged from 10 to 50 mg L^-1. With a corn–bermudagrass (‘Tifton 44’)–rye system, Vellidis et al. (1993) measured NO^-₃–N concentrations in soil water at a 2-m depth in a dairy effluent sprayfield over a 2-yr period. At an annual loading rate of 400 kg of effluent N ha^-1, NO^-₃–N concentrations ranged from near zero to 14 mg L^-1. With a loading rate of 800 kg N ha^-1, the range was 4 to 21 mg L^-1. Newton et al. (1995) later reported forage N removals for the same project. At the 400 kg N ha^-1 rate, N removals were 86 kg ha^-1 for corn, 143 kg for bermudagrass, and 91 for rye (12-mo total: 320 kg ha^-1). At the 800 kg N ha^-1 rate, N removals were 157 kg ha^-1 for corn, 137 kg for bermudagrass, and 169 for rye (12-mo total: 463 kg ha^-1).

In the present study, five year-round forage systems were evaluated under dairy effluent irrigation over four 12-mo cycles and with three loading rates of effluent N. Results from two of the forage systems are reported. Our objectives were to (i) measure the capacity of forage components as well as year-round systems for removing N from the soil and preventing it from leaching below the rooting zone and (ii) identify periods within a 12-mo cropping cycle that are particularly prone to NO^-₃–N leaching and determine causal factors.

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The project was conducted from 1996–2000 in a dairy effluent sprayfield owned by North Florida Holsteins, Inc., near Bell, Florida (29°44' N, 82°51' W). Average monthly maximum and minimum temperatures and daily solar radiation for the area are presented in Fig. 1 . The climate is mild temperate and the average date (50% probability) of first freeze is 25 November and last freeze is 10 March (Bradley, 1983). Precipitation data (Fig. 2) were collected at the site with LI-1200S minimum data set recording systems (LI-COR, Lincoln, NE)¹. The soil at the site is classified in the Kershaw series (Weatherspoon et al., 1992). It is sandy to a depth of more than 2 m. Permeability is very rapid and surface runoff is slow (Hydrologic Group A). The water table is below a depth of 10 m.

View larger version (28K): [in this window] [in a new window]   Fig. 1. Monthly average maximum and minimum daily temperatures (1961–1990) and daily total solar radiation (TSR; 1955–1987) recorded at the University of Florida's Green Acres Agronomy Farm near Gainesville, Florida. Green Acres is 40 km east of the study site.

View larger version (61K): [in this window] [in a new window]   Fig. 2. Weekly total rainfall distribution at the research site near Bell, Florida during four 12-mo cropping cycles: (a) 1996–1997, (b) 1997–1998, (c) 1998–1999, (d) 1999–2000.

View larger version (22K): [in this window] [in a new window]   Fig. 3. Sectional diagram of the dairy effluent sprayfield showing placement of mainplots, sprinklers, and changeover areas.

During a typical irrigation, 16 mm of water were applied. To determine application volume, six 20-L containers were placed in each mainplot. Dairy effluent was collected at two locations during an irrigation by placing two lab-cleaned, 20-L containers ahead of the pivot. After retrieval, five 125-mL samples of effluent were collected at each location, acidified (2 pH), and placed in a cooler within 15 min. Freshwater samples were taken with similar procedures. Loading rates of N were calculated with concentrations of Kjeldahl N (unfiltered) plus nitrate N (filtered) in dairy effluent and freshwater, and application volumes.

The four-cycle average N loading rate was 500 kg ha^-1 cycle^-1 for the low, 690 kg ha^-1 cycle^-1 for the medium, and 910 kg ha^-1 cycle^-1 for the high (Table 1). During the 1996–1997 cycle, effluent was underapplied for the medium and high loading rates due to a number of pivot repairs, which caused delays. During the 1998–1999 cycle, the N rate exceeded the target rate by an appreciable amount for all loadings. While effluent N was overapplied in that cycle, some of the difference between target and measured rates was due to the nitrate N applied in the many overall freshwater irrigations needed during this cycle, which was the driest of the four study cycles (Fig. 2c).

For the BR system, bermudagrass ‘Tifton 85’ was planted in early September 1995. It is an interspecific Cynodon hybrid developed in Georgia (Burton et al., 1993; Burton, 2001). The bermudagrass was harvested four or five times during the warm season (April–October), which is typical for irrigated bermudagrass in northern Florida (Table 2), and was dormant during the winter months. Rye (‘Wrens Abruzzi’) was planted into the dormant bermudagrass in late November to early December, with a no-till grain drill (20-cm row spacing).

Suction Lysimeters
Suction lysimeters were constructed by attaching a round-bottom, porous ceramic cup (5.1-cm o.d. x 6-cm length, one bar high flow; Soil Moisture Equipment Corporation, Santa Barbara, CA) to the end of 4.9-cm-o.d. polyvinyl chloride tubing. In May 1996, two lysimeters were installed in each subplot and spaced 4 m apart. Ceramic cups were 1.5 m below the soil surface. Sampling began on 19 July 1996 and continued at near 14-d intervals. During several periods, mostly occurring in the 1998–1999 cycle, we were unable to obtain soil moisture samples from the lysimeters due to dry conditions. Nevertheless, the attempt to collect samples was made at 14-d intervals. Two days prior to sampling, water was evacuated from the lysimeter before a suction (40 to 45 kPa) was placed on it. At sampling, suctions were released. Samples were acidified (2 pH) and placed in a cooler within 15 min of extraction. Lysimeters were also installed nearby in an unimpacted area characterized as an open woodland. Nitrate N concentration from these lysimeters was negligible during the sampling period (0.2 mg L^-1).

Parameters and Methods of Analysis
The study was conducted with standard operating procedures required by the Florida Department of Environmental Protection (1992). For total N concentration in dairy effluent and freshwater, USEPA Method 351.2 was used (USEPA, 1983), which involves a standard Kjeldahl digestion followed by semiautomated colorimetry with a Technicon (Tarrytown, NY) Auto Analyzer. For total ammoniacal N in dairy effluent, the same method was used but without Kjeldahl digestion. For nitrate plus nitrite N concentration (represented by the term NO^-₃–N), Method 353.2 (USEPA, 1983) was used (determination by automated colorimetry). The NO^-₃–N levels in effluent were <0.02 mg L^-1 during the study.

For soil water samples from lysimeters, NO^-₃–N concentration was determined by USEPA Method 353.2 with a Flow IV, air-segmented, automated spectrophotometer (O-I Analytical, College Station, TX). For ammonia N determination, USEPA Method 350.1 (automated phenate colorimetry) was used. Ammonia N concentrations were negligible during the sampling period with levels mostly <0.2 mg L^-1 but not >0.4 mg L^-1.

Forage samples were ground in a Thomas-Wiley mill (1-mm screen). The N analysis involved a modification of the standard Kjeldahl procedure (Gallaher et al., 1975), followed by automated colorimetry (Hambleton, 1977) with a Technicon Auto Analyzer.

Statistical Analysis
Responses were analyzed by fitting mixed effects models (Littell et al., 1996) with the PROC MIXED procedure of SAS (SAS Institute, 1992). Effluent loading rates were mainplots and forage systems were subplots. The 12-mo cycles were considered repeated measures. To make inferences about the forage crops that comprised the different forage systems, a term to describe crop nested within systems and its resultant interaction terms was added to the analysis. Crop (within system) was considered as a repeated measure. Covariance structures in the repeated measures were modeled with autoregressive order one (Littell et al., 1996). To determine the nature of the effect of effluent N loading rate on forage responses (N removal, dry matter [DM] yield, and forage N concentration), contrasts for linear and quadratic effects were examined. To compare soil water NO^-₃–N concentration means among loading rates for each sampling date within each forage system and 12-mo cycle, a paired comparison of least squares means was made with pooled variances from the entire project (five forage systems). A predecided 0.10 significant level was chosen for treatment and interaction terms because of the variable nature of the experiment associated with (i) the variation in soil fertility within the sprayfield before treatment initiation, (ii) fluctuation of N concentration in dairy effluent over time, and (iii) noise often observed in soil water NO^-₃–N concentration data.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Dry Matter Yield, Nitrogen Concentration, and Nitrogen Removal
Main Effects and Interactions
The quantity of N removed by a 12-mo forage system is influenced by three major factors including (i) forage components making up the system, (ii) weight of biomass removed during harvest, and (iii) N concentration in the harvested biomass. The main effect of forage system was significant for DM yield (P = 0.08), N concentration (P < 0.01), and N removal (P < 0.01). The effect of effluent N loading rate was significant for N concentration (P < 0.05) and N removal (P = 0.10), but not for DM yield. The cycle effect (i.e., year effect) was significant for all three response parameters (P < 0.01). There was a forage system x cycle interaction for DM yield, N concentration, and N removal at P < 0.01 and a rate x cycle interaction for N concentration (P < 0.01).

Forage System Comparison
Nitrogen removal for the BR system was superior to CSR in all four cycles (Table 4). The system x cycle interaction occurred because the magnitude of difference between systems varied among cycles. Nitrogen removal for the CSR system was quite stable over the four-cycle period, a consequence of stable DM yield and N concentration. Nitrogen removal for BR varied, with the highest average computed for the second cycle and the lowest for the fourth. The variation in N removal among cycles for BR was a result of differences in DM yield and not N concentration. For BR, N concentration was relatively stable over the four-cycle period, though there were differences among cycles. Yield was much less consistent among cycles for BR (standard deviation = ±4.9 Mg ha^-1) than CSR (standard deviation = ±0.6 Mg ha^-1). It was greater for BR than CSR in the second cycle, but in the third and fourth cycles CSR produced superior yield. Nitrogen concentration (N means were weighted according to component yields) was substantially higher for BR than CSR (overall mean; 20.3 vs. 13.1 g N kg^-1, respectively). This clearly shows that the major factor associated with the higher N removals of the BR system was N concentration and not DM yield.

During the 1996–1997 cycle, N concentration in all forage components of the two systems was not affected by loading rate (Table 6), probably the result of a preexperimental heterogeneity in soil fertility among plots. In the next three cycles, concentrations increased linearly as loading rate increased for both forage components of BR and rye of CSR. This change in response between cycles probably caused the rate x cycle interaction. Nitrogen concentration in corn and sorghum was generally unaffected by loading rate, though there was a linear increase in corn in the fourth cycle. Nitrogen removal for bermudagrass and for the BR system increased linearly as loading rate increased only in the second and third cycles (Table 7). Within CSR, the rye component was most affected by N loading rate with N removal increasing linearly in the second, third, and fourth cycles. For the CSR system, N removal increased linearly with increasing loading rate only in the fourth cycle.

Best Forage for Nitrogen Removal
The most desirable forage for N removal in dairy effluent sprayfields would be one that (i) produces high yields, (ii) has a high forage N concentration, (iii) consumes N with increasing loading rate, and (iv) can maintain a high level of crop performance over several years. Of the four components in the BR and CSR systems, bermudagrass was closest to possessing all attributes. Over the first three cycles, it produced high yield. It had moderately high N concentration, which increased significantly with loading rate in the latter three cycles. A shortcoming was its yield decline in the fourth cycle due to the reduction in stand that occurred mainly in plots receiving the higher N rates.

The corn plus sorghum combination produced stable yields over the 4-yr period, but its major shortcoming was low N concentration. Rye had moderately high N concentration, which mostly increased with loading rate, but DM yield was low (range: 2.1 to 5.5 Mg ha^-1), considering the portion of a 12-mo cycle taken up by this forage (approximately 100 d). A limiting factor for yield for cool-season forages is reduced incoming solar radiation during the winter months (Fig. 1).

Other investigators have evaluated year-round cropping systems under dairy effluent irrigation. Over a 2-yr period, French et al. (1997) evaluated the CSR system. Their annual N loading rates were 405 kg ha^-1 from dairy effluent only, 660 kg ha^-1 (405 kg ha^-1 from effluent plus 255 kg ha^-1 from ammonium nitrate), and 885 kg ha^-1 (405 kg ha^-1 from effluent plus 480 kg ha^-1 from ammonium nitrate). They reported 12-mo N removals of 321 kg ha^-1 for the low rate, 395 kg ha^-1 for the medium, and 452 kg ha^-1 for the high, which were higher than those for CSR in the present study, mainly because DM yields were higher. Over a 12-mo cycle at Tifton, GA, Johnson et al. (1991) reported annual N removals for a corn–bermudagrass (Tifton 44)–rye system at four dairy effluent N rates. Their N removals were 422 kg ha^-1 for an annual effluent N rate of 381 kg ha^-1, 507 kg ha^-1 for a effluent N rate of 493 kg ha^-1, 588 kg ha^-1 for a effluent N rate of 739 kg ha^-1, and 608 kg ha^-1 for a effluent N rate of 986 kg ha^-1.

Nitrate Nitrogen Leaching Patterns
Forage System and Loading Rate Effects
Using suction lysimeters, soil water NO^-₃–N concentration was measured at 1.5 m below the soil surface at near 14-d intervals from July 1996 to March 2000. Main effects were significant for forage system and 12-mo cycle (P < 0.01). The P value for the effect of effluent N loading rate was 0.13. There was a system x cycle interaction (P = 0.02). The P value for the system x rate interaction term was 0.11. When NO^-₃–N concentration was analyzed within cycle, cropping systems were different in all cycles (P < 0.01). The rate effect was significant only in the third (P = 0.02) and fourth cycles (P = 0.10). There was a system x rate interaction in the fourth cycle (P < 0.01).

Average monthly NO^-₃–N concentration in soil water for each system (across loading rate) was computed for each cycle (Fig. 4) . The BR system was superior to CSR at preventing NO^-₃–N from leaching below the primary rooting zone. Largest disparities between systems generally occurred from July through January, whereas the smallest differences were in March and April.

View larger version (41K): [in this window] [in a new window]   Fig. 4. Monthly mean nitrate N concentration in soil water averaged over dairy effluent loading rates for the corn–forage sorghum–rye (CSR) and bermudagrass–rye (BR) systems during four 12-mo cycles near Bell, Florida. Within each cycle, annual mean concentrations for systems followed by a different letter are different (P < 0.01).

View larger version (45K): [in this window] [in a new window]   Fig. 5. Nitrate N concentration in soil water extracted from 1.5 m below the surface for the bermudagrass–rye system at three dairy effluent N loading rates during four 12-mo cropping cycles near Bell, Florida. Means within sampling date followed by the same corresponding letter are not different (P > 0.10). Nitrogen loading rates from each cycle are shown in Table 1.

View larger version (51K): [in this window] [in a new window]   Fig. 6. Nitrate N concentration in soil moisture extracted from 1.5 m below the surface for the corn–forage sorghum–rye system at three dairy effluent N loading rates during four 12-mo cropping cycles near Bell, Florida. Means within sampling date followed by the same corresponding letter are not different (P > 0.10). Nitrogen loading rates from each cycle are shown in Table 1.

The Corn–Forage Sorghum–Rye System
Growth phases of the annual forages in CSR had a much greater influence on leaching patterns than those of BR (Fig. 6). After corn harvest in 1996, mean NO^-₃–N concentrations in soil water increased to peaks greater than 40 mg L^-1 (Fig. 6a) during the "no-crop" period (harvest of one crop to seedling emergence of the next) and "canopy expansion" phase (seedling emergence to complete canopy closure) of the forage sorghum for the three loading rates. The concentration peaks corresponded to the onset of canopy closure. Nitrate N levels declined during the early part of the closed-canopy phase to lower concentrations that were somewhat maintained through sorghum harvest. Levels increased to peaks 30 mg L^-1 following sorghum harvest, then declined to <10 mg L^-1 during the closed-canopy period of winter rye.

During the no-crop period and expansion phase of the corn in 1997 (Fig. 6b), mean NO^-₃–N concentration for the three loading rates increased to peaks between 25 and 40 mg L^-1. From there, mean concentrations decreased to levels at or below 10 mg L^-1 during the closed-canopy period of the corn. Following corn harvest in mid-July of 1997, levels increased over the no-crop period and sorghum expansion phase. For the low N rate, concentrations declined during the closed-canopy period of the sorghum. For the medium and high rates, the NO^-₃–N concentrations declined slightly or were maintained during that period. Then NO^-₃–N levels began to increase at or just prior to sorghum harvest to peaks near 40 mg L^-1 for the low and medium rates and greater than 60 mg L^-1 for the high. Levels steeply declined during the expansion phase and first half of the closed-canopy period of rye to near zero in late February 1998 for all rates. The low concentrations were maintained through March.

During the third 12-mo cycle, data are missing for some sampling dates (Fig. 6c) because dry soil conditions precluded collection of enough soil water for NO^-₃–N analysis. During the 1998–1999 cycle, only 936 mm of rainfall were recorded (Fig. 2c). During November and December, most suction lysimeters would not hold vacuum, indicating very low soil moisture around suction cups. Though dairy effluent and supplemental freshwater irrigation continued during dry periods to reduce crop stress, the amount of water applied to the soil surface had little effect on moisture conditions at the 1.5-m level. It can be assumed that NO^-₃–N was not moving into the lower soil profile during those periods.

The highest average NO^-₃–N concentration (110 mg L^-1) during the four-cycle period was measured shortly after canopy closure of the sorghum in September 1998 in plots receiving the high loading rate (Fig. 6c). Lower peaks were reached near the same time for the low and medium rates. Concentrations decreased over the next one to two samplings to lower levels, which were maintained through sorghum harvest. Concentrations increased from sorghum harvest to the first half of the closed-canopy phase of rye. During the latter half of the closed-canopy period, NO^-₃–N levels for the three loading rates rapidly decreased to 10 mg L^-1 or below.

During the fourth cycle, leaching patterns (rising and falling of levels over time) were similar for the three loading rates (Fig. 6d) but there was a much larger, sustained difference in magnitude of NO^-₃–N levels between the high rate and the low and medium rates than in previous cycles. Nitrate N concentrations increased well into the closed-canopy period of the corn. For the low rate, the highest initial peak of 19 mg L^-1 was recorded on 11 June. A peak of 29 mg L^-1 occurred 12 d later for the medium rate. For the high rate, a peak of 78 mg L^-1 was recorded on 9 July. Following the peaks, NO^-₃–N levels for the low and medium rates declined through corn harvest, whereas for the high rate, levels decreased through to the start of the sorghum expansion phase. Then levels increased for all three rates to peaks corresponding to the beginning of the closed-canopy period of sorghum. The peaks were followed by steep declines during the early part of the closed-canopy phase to lower concentrations that were maintained through sorghum harvest. After harvest, NO^-₃–N concentrations gradually increased over the no-crop period, expansion phase, and the first half of the closed-canopy period of rye. From there, levels declined over the latter half of the closed-canopy phase.

Annual Forage versus Perennial Sod for System Components
The "roller coaster" pattern of rising and falling NO^-₃–N concentrations observed in CSR is probably a distinctive feature of cropping systems made up of sequentially planted annual forages. The period between the harvest of one annual crop and full canopy closure of the next crop is highly prone to NO^-₃–N leaching for a number of reasons. During the no-crop period, uptake of N and soil water drops to near zero and is minimal during the canopy expansion phase of the next crop, especially during early seedling development. Second, effluent irrigation generally continues in dairy sprayfields during transition periods adding both N and water to the upper profile. With limited uptake, soil water movement into the lower soil profile can increase, carrying NO^-₃–N out of the rooting zone. Third, excess N not used by the previous crop may be present in the upper profile at harvest. During the closed-canopy period of the previous forage, soil moisture in the rooting zone is continuously being depleted. This would tend to impede unused N from leaching out of the upper profile. Finally, the transition period between annual forages lasts several weeks, increasing the probability that heavy rainfall events will occur. In northern Florida, heavy rainfall events can occur during any month of the year (Fig. 2). In the present study, estimated transition periods (no-crop period plus expansion phase) between the forage components of the CSR system ranged from 48 to 61 d. In a study by Bennett et al. (1989), the rate of N uptake by irrigated corn did not become linear until 42 d after planting. While avoiding delays between harvest and the planting of the next crop shortens transition periods, little can be done to shorten the time required for seedling emergence and the development of a closed canopy and extensive root system.

In the current study, NO^-₃–N concentrations generally declined when the annual forages of the CSR system were in the closed-canopy period (i.e., linear growth phase). During this period N uptake, as well as water usage, are at high levels. In the study by Bennett et al. (1989), linear uptake of N by corn lasted for approximately 73 d during which a total of 250 kg N ha^-1 was accumulated in aboveground biomass. During the expansion phase only 18 kg N ha^-1 was taken up by the corn. These results were obtained from plots fertilized with 400 kg N ha^-1 and receiving "optimal" irrigation.

The BR system was superior to CSR because the perennial sod component (bermudagrass) not only removed more N from the soil than its corn plus sorghum counterpart, but N uptake and soil moisture usage were more continuous over the season. This provided less opportunity for buildup and major leaching events to occur. The periods between closed-canopy phases among the four to five growth cycles of bermudagrass were much shorter than the transition periods between annual crops. These periods were segmented into "no-canopy" periods and expansion phases (Fig. 5). After harvest, the stolon–rhizome–root system remains intact and new shoots generally emerge within 5 d. During this short no-canopy period, the uptake of N and soil moisture was probably reduced but does not drop to near zero as with the no-crop periods of CSR. Furthermore, the expansion phase was much shorter because the stolon–rhizome–root system allowed for rapid canopy development.

Year-Round Effluent Nitrogen Loading versus Nitrate Load in Topsoil
The sharp reduction in lysimeter NO^-₃–N concentration that occurred during the closed-canopy period of rye in CSR in the first three cycles, and somewhat in the fourth, suggests that large amounts of N and soil moisture were being extracted from the soil (Fig. 6). This outcome was inconsistent with the low N removal of the rye (46–102 kg ha^-1). The magnitude of the rate of decline may be in part the result of a reduction in the rates of mineralization of effluent organic N and nitrification of effluent ammoniacal N during the winter months. Approximately one-third of dairy effluent N was in organic form while the remainder was in ammoniacal form. The 4-yr mean for ammoniacal N as a percentage of total effluent N was 68% (range of 49–84%).

The constant year-round surface loading of effluent N may not result in a corresponding constant load of NO^-₃–N in the rooting zone because of the effect of seasonal temperatures on microbial processes that convert effluent N to NO^-₃–N (Das et al., 1995; Dou et al., 1997). If the rate of NO^-₃–N availability in the upper profile increases during warm months and declines during cool months, then to control leaching year-round, warm-season forages would be required to remove more N from the soil relative to the amounts of effluent N applied to the soil surface during their growth periods.

We suggest that NO^-₃–N concentrations below the rooting zone remained low throughout the year for BR because the bermudagrass removed large enough quantities of N from the soil during the warm season so that residual nitrate N following the last bermudagrass harvest was minimized. Though corn and forage sorghum of CSR are productive C-4 grasses, they failed to remove the necessary quantities of N from the soil. As a result, considerable NO^-₃–N moved out of the rooting zone during the warm season and large amounts of residual nitrate N were apparently present in the upper profile at sorghum harvest, resulting in high NO^-₃–N peak concentrations during the early growth of rye. Our explanation for the steep decline in NO^-₃–N concentration to very low levels during the closed-canopy period of rye is that a wave of residual nitrate N moved through the soil profile during the first half of winter, a time when NO^-₃–N buildup in the topsoil was low because cooler temperatures reduced rates of mineralization and nitrification. Low NO^-₃–N levels below the rooting zone were maintained for the remainder of linear growth of the rye because the low plant N uptake during that time was adequate for the reduced amounts of NO^-₃–N becoming available in the rooting zone. If the NO^-₃–N load in the topsoil over a 12-mo cycle parallels the constant loading of effluent N on the soil surface, then much higher NO^-₃–N concentrations in the suction lysimeters should have been observed during the latter part of the growth period of the rye in both systems.

Ammonia Loss from Surface-Applied Dairy Effluent
Over the four cycles, the CSR system removed only 41% (apparent recovery) of the applied N at the high effluent loading rate, while the BR system removed 60%. The difference in apparent recoveries between the systems is consistent with the difference in the magnitude of NO^-₃–N concentrations observed below the primary rooting zone over time. However, the low level of nitrate leaching maintained by BR throughout most of the four-cycle period suggests that a sizable portion of effluent N was lost through other routes. We suspect that a major portion of the surface-applied effluent N was lost via ammonia volatilization. Given the rapid permeability and droughty nature of the deep, excessively-drained sands in the sprayfield, significant losses of effluent N through surface-runoff and/or denitrification probably did not occur.

Volatilization losses of ammoniacal N from surface-applied manure slurries ranging from 19 to 100% have been reported (Thompson et al., 1987; Pain et al., 1990; Sommer and Olesen, 1991). When swine effluent was applied to soybean [Glycine max (L.) Merr.] with overhead irrigation, Sharpe and Harper (2002) measured a 23% loss of ammoniacal N within 48 h of application. It should be noted that ammonia emissions into the atmosphere from intensive livestock operations can contribute significant quantities of N to nearby bodies of water and lead to eutrophication (Hutchinson and Viets, 1969; Luebs et al., 1973; Reddy et al., 1979).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
The BR system was superior to CSR because it removed more N from the soil and maintained lower NO^-₃–N concentrations below the primary rooting zone. The performance of the rye component did not differ between systems, therefore the effectiveness of the systems was determined by the warm-season forages. Bermudagrass removed more N from the soil than the corn–forage sorghum combination, mainly because it had higher forage N concentration and not higher DM yield. In CSR, the period between harvest of one forage and canopy closure of the next was identified as being highly prone to NO^-₃–N leaching due to the time required for seedling emergence and development. These findings infer that year-round forage systems that include a productive, perennial grass are better than those made up entirely of annual forages at preventing NO^-₃–N loss to ground water. Using the USEPA's drinking water standard for nitrate N (10 mg L^-1) as a benchmark, lysimeter data for the BR system suggest that a dairy effluent N loading rate of 500 kg ha^-1 per 12-mo cycle would impose little risk of NO^-₃–N contamination of ground water within vulnerable areas in the Suwannee River basins of northern Florida. Effluent rates up to 700 kg N ha^-1 per cycle would probably result in low environmental risk of significant NO^-₃–N ground water contamination if the bermudagrass component is fully established and healthy stands are maintained. Though lysimeter data are not entirely conclusive, 12-mo leaching patterns clearly showed that the use of the CSR system in dairy sprayfields with effluent N rates 500 kg ha^-1 per cycle has the potential to negatively affect ground water quality.

	ACKNOWLEDGMENTS

 
This project was funded through the Florida Department of Environmental Protection, Tallahassee, Florida, by a Section 319 NPS Management Implementation Grant from the United States Environmental Protection Agency. We are grateful to Mr. Don Bennink, owner of North Florida Holsteins, Inc. and his managers and staff for their active involvement in this project and donation of time, equipment, irrigation system, and land. We recognize contributions made by Mr. Winston Tooke, Natural Resource Conservation Service, USDA, who long before the initiation of this study, identified specific research needs and assisted in grant procurement.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Florida Agric. Exp. Stn. Journal Series no. R-08195.

(deceased)

¹ Mention of trade and company names was to provide specific information and does not constitute an endorsement by the Institute of Food and Agricultural Sciences, University of Florida.

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