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Journal of Environmental Quality 30:1064-1070 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Waste Management

Tree Species for Recovering Nitrogen from Dairy-Farm Effluent in New Zealand

Jon K.F. Roygard, Brent E. Clothier, Steve R. Green and Nanthi S. Bolan

Institute of Natural Resources, Massey Univ., Palmerston North, New Zealand

Corresponding author (jroygard{at}vt.edu)

Received for publication July 7, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Land treatment of dairy-farm effluent is being widely adopted as an alternative to disposal into surface waters in New Zealand. This study investigated water balances and associated N leaching from short-rotation forest (SRF) species irrigated with dairy-farm effluent. Single trees were grown in lysimeters filled with Manawatu fine sandy loam (mixed mesic Dystric Eutrochrept). Dairy-farm effluent was applied during two irrigation periods at 21.5 mm wk-1 with a total loading equivalent to 870 kg N ha-1 occurring over 17 mo. Following tree harvest in April 1997, measurements continued until August 1997 to monitor tree reestablishment. Cumulative N leached did not differ between lysimeters in which evergreen Sydney blue gum (Eucalyptus saligna Sm.) and shining gum [Eucalyptus nitens (H. Deane & Maiden) Maiden] and deciduous kinu-yanagi (Salix kinuyanagi Kimura) were grown. Leachate N concentrations of all treatments were on average higher than the New Zealand drinking water standard of 11.3 mg N L-1. The E. nitens and S. kinuyanagi treatments leached 33 and 35 kg N ha-1 yr-1 in 1996 following application of 236 kg N ha-1 during the first irrigation season. Leaf area was strongly correlated to evapotranspiration, drainage volume, and nitrogen leached. The majority of leaching in the tree treatments occurred after harvest. Reducing the leaching in the regrowth phase may be achieved through timing harvest in the spring when growth rates are higher and leaching potential is lower. Based on N uptake rates observed in this study and average pond discharge, a plantation of 5.4 ha would be required for N recovery on a typical dairy farm in New Zealand.

Abbreviations: ET, evapotranspiration • SRF, short-rotation forest


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
UNDER the Resource Management Act (1991), New Zealand dairy farmers are required to dispose of dairy-farm effluent in such a manner as to have no adverse effect on the receiving environment. In New Zealand the wash-down water of milking parlors is most commonly treated via a two-pond system, the first being anaerobic and the second aerobic. This dairy-farm effluent may then be released into streams or rivers. However, nitrogen (N) removal by these two-pond systems is proving insufficient to protect the quality of receiving waters (Hickey et al., 1989). Land treatment solutions are being encouraged by regulatory authorities in order to protect water quality (Cameron et al., 1997). However, land treatment systems can have considerable effects on the environment though NO-3 leaching and runoff (Smith and Bond, 1999; Snow et al., 1999). Land treatment of dairy-farm effluent using SRFs has been investigated as an alternative to the more common pasture-based systems (Tungcul et al., 1996).

Short-rotation forests are being used for land treatment of various wastewaters in commercial and research sites around the world (Pertu, 1993; Myers et al., 1998; Labrecque et al., 1998; Guo and Sims, 1999). Fast initial growth rates are maintained through short rotations of 2 to 10 yr and the ability of some species to grow back from their stumps (coppice) after harvest (Ranney et al., 1987; Nicholas, 1997). The land treatment potential stems from the water and nutrient demands of the trees stripping water and nutrients from the applied effluent (Nicholas et al., 1997).

Knowledge of rootzone water and nutrient balances is central to the design of environmentally sustainable systems for land treatment (Bond, 1998). Effluent application provides a water and N source for the trees. However, the additional hydraulic and N loading from the wastewater increases the potential for contamination of the environment through surface runoff and N leaching. This has been observed in experiments investigating dairy-farm effluent application to SRF species of Salix and Eucalyptus in New Zealand. High biomass yields and high levels of nutrient storage in the trees have been measured, although increasing soil NO-3 levels in the root zone were also identified, indicating that NO-3 leaching could be occurring (Tungcul et al., 1996).

In the present study, dairy-farm effluent irrigation to SRF is assessed in terms of NO-3 leaching to ground water. Climatic conditions and the inputs of water and N change seasonally, as do the use and fate of water and N in land treatment systems. It is expected that deciduous trees would have lower rates of water use in winter, in comparison with the evergreen trees that transpire throughout the whole year. It is hypothesized that differences in the rates of water use by the deciduous and evergreen trees will affect both nutrient removal and NO-3 leaching. The objective of this study was to determine water use patterns and N leaching from the rootzone of one deciduous and two evergreen tree species irrigated with dairy-farm effluent.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Design
Lysimeter facilities were used to enable measurement of water and N leaching. Twelve in-ground lysimeters containing three replicates of three tree species (Eucalyptus saligna, E. nitens, and Salix kinuyanagi) and a bare-soil treatment (control) were established in a field at Aokautere, near Palmerston North, North Island, New Zealand. Species were selected based on growth rates in commercial and research land-treatment sites in Australia and New Zealand (Roygard, 1999). The lysimeters (1.8 m diam., 1.0 m deep) were filled to simulate the local soil profile. The lysimeters were repacked to the original bulk density (1.2 g cm-3) with Manawatu fine sandy loam to a depth of 0.65 m on top of 0.35 m of river gravel.

A single tree was planted in each lysimeter in November of 1994. The evergreens were planted as 3-mo-old seedlings, and the deciduous willows as unrooted cuttings. The control lysimeters contained only soil over gravel. Trees of the same species were planted around the lysimeters at approximately 4000 stems ha-1 so as to create conditions approximating a small plantation. Surrounding "guard trees" were not irrigated. All the lysimeters were left for a period of 1 yr before the experiment began. This period allowed the trees to establish. During this establishment phase, the lysimeters received only natural rainfall. To provide similar initial conditions, lysimeters were irrigated to a volumetric water content of 30 to 35% on 1 Dec. 1995. Data collection began in January 1996, when the trees were 1 yr old. The aboveground biomass of the trees was determined by harvest in the last week of April 1997. Measurements continued until August 1997 to investigate the NO-3 leaching during the regrowth (coppicing) period.

Effluent Irrigation
Effluent was transported from the secondary-treatment pond of a Massey University dairy farm to the experimental site prior to each application. Effluent delivery was controlled individually for each lysimeter by a timing device linked to a pump and solenoid valves. An annular pattern of application over the soil surface was achieved through three centrally located spray nozzles each irrigating 120° of the lysimeter. A buffer zone of 0.1 m around the edge of the lysimeters was not irrigated to avoid preferential flow down the sides of the lysimeters.

Effluent irrigation was applied in two application periods. Hydraulic loading during the application periods was 21.5 mm wk-1 applied over a 3-h period each week. In the first irrigation period a total of 618 mm was applied from 5 Dec. 1995 to 16 June 1996. In the second irrigation period, 670 mm was applied from 16 Sept. 1996 to 14 Apr. 1997.

Rootzone Water Balance
The rootzone water balance of each lysimeter was monitored using a simple water balance:

[1]
where {Delta}S (mm) is the change in soil water storage, R (mm) is rainfall, I (mm) is effluent irrigation, D (mm) is the drainage of leachate and ET (mm) is evapotranspiration. Water balances were made on a liters per lysimeter basis. The conversion from liters per lysimeter to millimeters was based upon the 2.5 m2 surface area of the lysimeter. For ET, conversion from liters per lysimeter (liters per tree) was based upon 1 tree per 2.5 m2 or 4000 stems ha-1. Surface runoff was not included in this calculation as the lysimeter edging prevented any surface flow of water.

Rainfall was measured by an on-site weather station. The volumes of effluent being irrigated to each lysimeter were measured each month during the application periods. Soil water storage was measured via time domain reflectometry (TDR) probes installed at five depths in each lysimeter. Two TDR probes were inserted vertically into the soil to depths of 100 and 250 mm. The other three TDR probes were inserted horizontally at depths of 250, 500, and 750 mm. Drainage volumes were recorded manually prior to installation of automatic flow meters in October 1996.

Evapotranspiration was estimated from a simple balance equation (Eq. [1]) on a weekly time step. Complete data sets were available to calculate ET for most weeks. However, for some weeks, drainage measurements were incomplete. For these periods, ET was estimated using the relationship between the previous week's ET and the potential ET for that week. This relationship was applied to the current-week potential ET to calculate the ET for the current week. From this estimate of the current week's ET, the drainage volume could be estimated.

Nitrogen Concentrations and Leaching
Concentrations of NH+4–N, NO-3–N, and NO-2–N in the applied effluent and leachate were monitored regularly. Effluent samples were collected at each irrigation event. Leachate samples were collected three to seven times per week, depending on drainage volumes. The total quantity of N leaching was determined as the volume of drainage since the last collection multiplied by N concentration in the collected sample.

Nitrogen concentration was determined by the nitroprusside method for NH+4–N analysis (Weatherburn, 1967) and a diazotization coupling reaction (Griess–Ilosvay reaction) method for NO-2–N and NO-3–N analysis (Bremner and Mulvaney, 1982). Total Kjeldahl N digestion (Markus et al., 1985) was used to determine Total N contents of the effluent and biomass samples.

Leaf Area Measurements
Leaf area of each tree was estimated by total leaf counts. Leaf samples were collected randomly at the time of these counts. The leaf area of the sample leaves was measured using a Li-3100 area meter (Li-Cor, Lincoln, NE). The total leaf area was calculated as the sum of the leaf count multiplied by the average leaf area. Leaf area estimates were made in January, October, and December of 1996 and at harvest in April 1997.

Statistical Analysis
Statistical analyses were carried out using one-way ANOVA methods of the Minitab 13 software (Minitab, 1998). Tukey's test (P = 0.05) was used for mean separation. Correlations were completed using Sigmaplot 6.0 (SPSS, 2000). All treatments were replicated three times, with the exception of E. saligna, which was replicated two times. The tree in the third E. saligna replicate suffered wind-snap damage that removed more than half the aboveground biomass of the tree in February 1996.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Rainfall and Irrigation
Effluent irrigation began in December 1995 with 10.3 g of N applied to each lysimeter. Measurements began in January 1996, three weeks after the first effluent application. Seasonal inputs of irrigation and rainfall are shown in Table 1. The concentration of N in effluent ranged from 33 to 149 mg N L-1. Effluent N concentrations in the first period averaged 50 mg N L-1, about half that of the second period, 102 mg N L-1. Consequently, the effluent N loading was doubled in the second irrigation period compared with the first irrigation period. Nitrogen loading during the first irrigation period totaled 70 g N lysimeter-1 (equivalent to 279 kg N ha-1). Nitrogen loading during the second irrigation period totaled 148 g N lysimeter-1 (equivalent to 592 kg N ha-1). Thus, total application during the experiment totaled to 871 kg N ha-1. This was anticipated to exceed the N uptake of the trees. Treatment systems for dairy-farm effluent will require the capacity to withstand the variation in N concentrations from effluent treatment ponds. Limiting the variations in ponded effluent N concentrations may be possible through pond management, and should be actively pursued in order to facilitate land application of the effluent.


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  		Table 1. Rainfall, irrigation volume, effluent concentration, and total amount of N received by the four treatments during the experiment.

 
Evapotranspiration
Total ET of three tree treatments prior to harvest was significantly higher for E. nitens than E. saligna, with S. kinuyanagi being intermediary (P = 0.10). Evapotranspiration from the tree lysimeters was higher than bare-soil evaporation during all seasons prior to the harvest in April 1997 (Table 2). Removal of the aboveground biomass at harvest restricted all treatments to bare-soil evaporation only. Thus, there were no significant differences in water use following harvest.


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  		Table 2. Evapotranspiration of the four treatments during the experiment.

 
Seasonal differences in ET were also detected for the tree treatments. Many of these differences were related to the leaf area of the trees. Leaf area measurements showed a close correlation to ET (r > 0.84) in each month that leaf area was measured. At the start of the experiment leaf areas of the S. kinuyanagi (7.7 m2) and E. nitens (13.5 m2) treatments were significantly higher than the E. saligna treatments (4.8 m2). Accordingly, ET of the E. saligna treatments was less than both the E. nitens and S. kinuyanagi treatments for the first summer and autumn of the experiment (Table 2). The deciduous S. kinuyanagi trees shed leaves in the winter of 1996. Subsequently, ET of the S. kinuyanagi treatments was lower than the evergreen E. saligna and E. nitens treatments during this period (Table 2). During the summer of 1997, the S. kinuyanagi treatments rapidly exhausted the available water supply and exhibited a premature senescence of leaves in late January 1997 (Roygard, 1999). In practice, additional water maybe required by S. kinuyanagi trees in summer months in order to maintain their production in land treatment systems.

Drainage Volumes
Enhanced ET of the trees compared with the bare soil treatments prior to harvest translates to a significantly lower drainage volume from the tree lysimeters (Table 3). Cumulative drainage from tree treatments remained significantly lower than the bare soil treatment (P = 0.05) even with the inclusion of drainage during the post-harvest (coppicing) period (Table 3). Total drainage from the E. nitens treatment was significantly less than S. kinuyanagi (P = 0.10) and E. saligna (P = 0.12) treatments. This reflected the greater water use of the E. nitens treatments in comparison with the other tree treatments.


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  		Table 3. Draining volume from the 1-m-deep soil profiles of the four treatments during the experiment.

 
Evapotranspiration was closely correlated with the drainage volumes observed from the lysimeters in all seasons prior to harvest (r > 0.7, Table 2). Leaf area and drainage were highly correlated in the months that leaf area was measured (r > 0.95). The drainage volumes of E. saligna treatments in the summer and autumn of 1996 were significantly higher than from the E. nitens and S. kinuyanagi treatments (Table 3). This corresponded with the lower ET of the E. saligna treatments (Table 2) at this time. During the winter of 1996, drainage volumes of the deciduous S. kinuyanagi treatments were significantly higher than from the evergreen E. nitens and E. saligna treatments (Table 3). This increased drainage coincided with the low ET of the S. kinuyanagi due to winter senescence. Drainage from the E. nitens treatments in Spring 1996 was minimal at only 1 mm in comparison with the S. kinuyanagi and E. saligna treatments (122 and 65 mm, respectively; Table 3). During the summer of 1997, no drainage occurred from the S. kinuyanagi and E. nitens treatments. The drainage of 7 mm from the E. saligna treatments was also minimal.

Following harvest, when all treatments were reduced to bare soil, significant differences in drainage volumes remained (Table 3). Evapotranspiration from the tree treatments during spring and summer had exceeded water inputs from rainfall and irrigation, thus the soil water contents were lower than bare soil at harvest. The rainfall inputs following harvest led to drainage events from the S. kinuyanagi that were intermediary between that of the amounts from the E. saligna and E. nitens treatments and the bare soil for the postharvest period (Table 3).

Leachate Concentrations
Average concentrations of N in the leachate for bare soil (46 mg N L-1), E. saligna (40 mg N L-1), E. nitens (26 mg N L-1), and S. kinuyanagi (19 mg N L-1) were not significantly different (P = 0.05, Table 4). These N concentrations all averaged above the New Zealand drinking water standard (NZDWS) of 11.3 mg N L-1 (Ministry of Health, 1995). Prior to harvest, the concentrations in the leachate from the S. kinuyanagi treatments were consistently lower than from the bare soil and E. saligna treatments (Table 4). Leachate N concentrations from the S. kinuyanagi treatments were also significantly less than those of the E. nitens treatment during autumn and winter of 1996 (P = 0.10). However, after harvest, the N concentrations in the leachate were high for all tree treatments, ranging up to six times the NZDWS for the deciduous Salix, and up to nine times the limit for the evergreens E. saligna and E. nitens. The higher leachate N concentrations during this coppicing phase (Table 4) may be due in part to the higher concentrations of N in the effluent during the second irrigation season (Table 1).


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  		Table 4. Nitrate concentration in the leachate draining from the 1-m-deep soil profiles of the four treatments during the experiment.

 
Nitrogen Leaching
The tree treatments all leached a significantly lower quantity of N beyond the rootzone compared with the bare soil treatment (Table 5). This was the expected result because the trees are a sink for N. Leaching from E. saligna, E. nitens, and S. kinuyanagi treatments averaged 95, 23, and 42 g lysimeter-1 (Table 5), although the total quantity of N leaching from the tree treatments was not statistically different (P = 0.05, Table 5). Similarly, N storage in the aboveground biomass of E. saligna, E. nitens, and S. kinuyanagi treatments averaged 71, 116, and 122 g N tree-1 and was not significantly different (P = 0.05, Roygard et al., 1999).


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  		Table 5. Quantity of nitrogen leached from the 1-m-deep soil profiles of the four treatments.

 
Leaf area was correlated with N leached in all months where leaf area was measured (r > 0.87). This relationship is probably a product of the relationship between leaf area and ET as well as the relationship between leaf area and N storage in the leaves. Prior to harvest, NO-3 leaching from the tree species was consistently lower than that of bare soil; the exception being for smaller E. saligna trees during autumn and winter of 1996. Higher leachate volumes and N concentrations from E. saligna lysimeters in autumn and winter of 1996 increased N leaching to a comparable level with the bare soil (Tables 2 and 3). In autumn 1997, senescence of the S. kinuyanagi treatments increased leaching in comparison with the evergreen E. saligna and E. nitens treatments.

Greater drainage from the S. kinuyanagi treatments in the winter of 1996 provided a dilution of the mass of N leaching. The quantity of N leached from the deciduous S. kinuyanagi (6.0 g N lysimeter-1) was similar to that of the evergreen E. nitens treatments (6.8 g N lysimeter-1). During this period, the ET of S. kinuyanagi was lower due to winter senescence, than that of E. nitens, which retained it leaves over winter. Subsequently, the drainage volumes from the S. kinuyanagi treatments were higher than the E. nitens treatments. The added drainage volume from the S. kinuyanagi treatments maintained the concentrations leaching (13 mg N L-1), significantly lower (P = 0.10) than from the E. nitens (27 mg N L-1). During this period, S. kinuyanagi was the only treatment to maintain leaching concentrations not significantly higher than the New Zealand drinking water standard (P = 0.05). This finding implies that the use of deciduous crops in land treatment systems may reduce effects on N concentration of aquifers in winter. However, a similar quantity of N would move to the aquifer. The observed increases in drainage volume may be a disadvantage in some land treatment systems due to effects such as rising ground water tables.

No further effluent irrigation was applied following harvest. However, a large proportion of the N leaching from the E. nitens (62%), and S. kinuyanagi (42%), and E. saligna (26%) treatments occurred following harvest. Furthermore, leaching after the end of the experiment in August 1997 would be expected to remain high until the trees grow back and begin to consume N. Coppice shoots about 10 to 15 cm long were observed on the stumps of the trees by September 1997. However, it is likely to take about a year from harvest before the leaf area increases to a level where ET of the trees exceeds rainfall inputs, therefore reducing drainage volumes.

Application of effluent to the trees during the regrowth stage in short-rotation forest systems renovating effluent may lead to considerable leaching as the system becomes hydraulically overloaded. Management of this coppicing phase to reduce N leaching could be improved in several ways. The timing of the coppicing phase was originally planned to remove the maximum amount of N through plant uptake by harvesting the trees before winter leaf drop. An improvement on this may be to harvest the trees in spring when the trees are increasing in leaf area. This change in timing may allow the trees to deplete the soil water and nitrogen stores prior to harvest. Spring conditions may also increase rates of coppice regrowth. By harvesting in spring the loss of leaf area is combined with the time of the lowest rainfall, spring–summer, reducing the likelihood of drainage. The results of this study support the implementation of a period of no irrigation following harvest until leaf area and subsequent ET rates are able to exceed the inputs of rainfall and irrigation. Such a period would also provide a time for the trees to strip nutrients from the soil to reduce any nutrient buildup in the soil. Thus, in the design of these systems the land area considerations must include the capacity to withhold irrigation postharvest. The strong relationship between leaf area and ET, drainage volume, and nitrogen leached suggests adequate leaf area will produce ET rates that reduce drainage and N leaching. A further option may be to use harvesting strategies that maintain some leaf area in the systems at all times to reduce the potential of leaching postharvest.

Application rates and total amounts of N leached beyond the root zone during the experiment are shown in Table 6. Literature values of N leaching from SRF land treatment sites for dairy-farm effluent in New Zealand are unavailable. However, comparisons with pasture-based land treatment systems are possible. Di et al. (1998a)( b) applied dairy-farm effluent to pasture grown in lysimeters in New Zealand. The application rate was 400 kg N ha-1 yr-1. In their 2-yr experiment they measured annual leaching losses of 8 to 25 kg N ha-1. Silva et al. (1999) monitored pasture grown in lysimeters receiving varying rates of dairy-farm effluent in New Zealand. The treatments received 0, 200, and 400 kg N ha-1 yr-1 and leached 3, 6, and 10 kg N ha-1 yr-1 (Silva et al., 1999). These pasture-based studies have shown lower rates of leaching than our study, albeit at lower rates of N application.


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  		Table 6. Nitrogen application and leaching rates for the three periods: (i) prior to the second irrigation season, (ii) for all of 1996, and (iii) for the entire experiment.

 
Comparisons of our study to the pasture systems with lower rates of N applications are possible. Investigation of leaching for the first effluent-application period and leaching totals for the following winter provide a snapshot of these SRF systems with lower N applications. Nitrogen applied prior to the second irrigation period (summer, autumn, and winter 1996) totaled 236 kg ha-1 yr-1. Leaching of the S. kinuyanagi and E. nitens treatments was significantly lower than the E. saligna and bare soil treatments for this period (Table 6). Although further irrigation took place, no further leaching occurred in 1996 for E. nitens, although S. kinuyanagi leached the equivalent of a further 7.2 kg N ha-1 in spring 1996 (Table 6). The N leaching in 1996 from S. kinuyanagi (33 kg ha-1 yr-1) and E. nitens (35 kg ha-1 yr-1) reflect the N applied prior to the second effluent irrigation period. These rates show the potential of these species for land treatment at the lower rates of N application (Table 6) and compare more favorably with those of the pasture systems.

Recommendations
Systems designed for N recovery from dairy-farm effluent using SRF would require lower N rates than applied in this study. This lysimeter study has identified N accumulation rates of E. saligna, E. nitens, and S. kinuyanagi of 114, 186, and 196 kg N ha-1 yr-1, respectively (Roygard et al., 1999) with application of excess N from dairy farm effluent in New Zealand. These N accumulation rates reflect the 2.5 yr of growth from planting in November 1994. Specific experiments to define maximal N uptake rates of the trees over the growth period would benefit the design of systems to match effluent N application to the trees' N requirements. Matching N application rates to the trees' N requirements has potential to reduce N leaching to acceptable levels and make these systems environmentally acceptable.

Using the N uptake rates of S. kinuyanagi and E. nitens of this study and allowing for some buffer against leaching loss in the system, an application rate of 150 kg N ha-1 yr-1 may be sustainable in terms of NO-3 leaching in a well-managed system. Using this uptake rate, the land area required for a typical dairy farm using SRF for N recovery was calculated (Table 7). The calculation uses a 3-yr rotation and average values of pond discharge rates and N concentrations reported in a survey of ponds in New Zealand (Hickey et al., 1989; Table 7). In order to apply the entire pond discharge to land, an area of 4.05 ha is required. However, this study has shown the importance of not irrigating during the regrowth period. To avoid irrigating for 1 yr postharvest, an extra one-third of the irrigation land required is included in the calculation as total land area required. The total of the land area could then be split into three blocks. The system would harvest one block each spring and leave this block without irrigation for 1 yr.


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  		Table 7. Calculation of land area required for a short rotation forest for N recovery from dairy farm effluent in New Zealand. Calculation and sources of values are indicated in the text.

 
The annual hydraulic loading rate calculated in Table 7 is considerably lower than the hydraulic loading used in this study. Based on the results of this study, applications would be best applied over a shorter irrigation season to meet the summer water demands of the trees. Irrigation should begin later in the spring than in this study to allow the trees to dry-down the soil profile and avoid leaching early in the spring. Irrigation should also be stopped earlier in the year prior to winter to allow some dry-down of the soil profile to lessen the winter leaching. The summer water demands of these trees may require extra irrigation in order to maintain leaf area and biomass production. These water demands indicate that the trees would be well suited to effluents of lower N concentration, such as municipal effluent.

The recommendation for land area required would vary with effluent type, pond discharge volume and concentration, soil type, tree species, and climate. However, it does serve as a starting point for dairy farmers who are interested in implementing these systems for effluent disposal.

Tree Species Performance
The E. nitens (evergreen) and S. kinuyanagi (deciduous) treatments both established better than the E. saligna treatments. The hypothesis that differing rates of water use of the evergreen and deciduous species would affect nutrient removal and nitrate leaching was not observed in the overall experimental results. Nitrogen uptake and nitrate leaching from the three tree treatments did not significantly differ during the experiment. However, differences were observed in other aspects of the water and nitrogen budget. The evergreen E. nitens had the greatest ET of the trees up until the harvest and the least overall volume of drainage, while the deciduous S. kinuyanagi maintained lower leachate N concentrations prior to harvest. The dilution of the concentrations leaching by S. kinuyanagi treatments may be of advantage in maintaining low concentrations of N moving to the ground water. However, the increased drainage volume may have an effect on ground water tables.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Design of SRF systems for land treatment of dairy-farm effluent to achieve acceptable levels of N leaching requires consideration of many factors including tree species (ET and N uptake rates), rainfall patterns, and hydraulic and N loading from the effluent. The N application rates used in this study reflected rates that may be applied in practice and produced leachate concentrations higher than the New Zealand drinking water standard for all treatments.

Improved recovery of N from dairy-farm effluent using short-rotation forestry can be achieved through management of the systems. The N application rates require further matching to the N requirements of the trees to reduce N leaching. This experimental study has measured the N accumulation of these tree species with excess N applied, finding N storage from 114 to 196 kg N ha-1 yr-1 after 2.5 yr of growth in New Zealand. Based on a 3-yr rotation, uptake rates of 150 kg N ha-1 yr-1 may be sustainable in terms of nitrate leaching. Management of ponds to limit variability in effluent N concentrations will aid in determination of land areas required for treatment systems. However, monitoring the N concentrations of the applied effluent is required to ensure N application rates are not in excess of the system design. Irrigation scheduling should be over a shorter irrigation season than used in this study to further match the water requirements of the trees and to reduce the potential for autumn and spring leaching. The regrowth period has been identified as the area of greatest N leaching for these tree systems. Nitrogen applications in the season prior to the harvest may need to be reduced in order to prevent a buildup of soil N that may leach during the regrowth period. It is recommended that systems have the capacity to withhold application of effluent during the regrowth period.


   ACKNOWLEDGMENTS
 
Thanks are extended to R. Sims and D. Riddell-Black for their assistance with this research. This project was supported by the Foundation for Research, Science and Technology Grant CO6618, the C. Alma Baker Trust, the New Zealand Large Herds Association, a MacMillan Brown Agricultural Research Scholarship, a Leonard Condell Farming Scholarship, and a Helen E. Akers Scholarship. We would like to thank M.M. Alley for providing comments on earlier versions of this manuscript.


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


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JEQ 2001 30: 677-682. [Full Text]  




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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Soil Science Society of America Journal Journal of Plant Registrations The Plant Genome