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Journal of Environmental Quality 32:599-606 (2003)
© 2003 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORTS
Plant and Environment Interactions

Fertilizer Use Efficiency and Nitrate Leaching in a Tropical Sandy Soil

J. Nyamangara*,a, L. F. Bergströmb, M. I. Pihaa and K. E. Gillera

a Dep. of Soil Science and Agricultural Engineering, Univ. of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe
b Division of Water Quality Management, Dep. of Soil Sciences, Swedish Univ. of Agricultural Sciences, P.O. Box 7072, S-750 07 Uppsala, Sweden

* Corresponding author (jnyamangara{at}agric.uz.ac.zw)

Received for publication November 12, 2001.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Maize (Zea mays L.) production in the smallholder farming areas of Zimbabwe is based on both organic and mineral nutrient sources. A study was conducted to determine the effect of composted cattle manure, mineral N fertilizer, and their combinations on NO3 concentrations in leachate leaving the root zone and to establish N fertilization rates that minimize leaching. Maize was grown for three seasons (1996–1997, 1997–1998, and 1998–1999) in field lysimeters repacked with a coarse-grained sandy soil (Typic Kandiustalf). Leachate volumes ranged from 480 to 509 mm yr-1 (1395 mm rainfall) in 1996–1997, 296 to 335 mm yr-1 (840 mm rainfall) in 1997–1998, and 606 to 635 mm yr-1 (1387 mm rainfall) in 1998–1999. Mineral N fertilizer, especially the high rate (120 kg N ha-1), and manure plus mineral N fertilizer combinations resulted in high NO3 leachate concentrations (up to 34 mg N L-1) and NO3 losses (up to 56 kg N ha-1 yr-1) in 1996–1997, which represent both environmental and economic concerns. Although the leaching losses were relatively small in the other seasons, they are still of great significance in African smallholder farming where fertilizer is unaffordable for most farmers. Nitrate leaching from sole manure treatments was relatively low (average of less than 20 kg N ha-1 yr-1), whereas the crop uptake efficiency of mineral N fertilizer was enhanced by up to 26% when manure and mineral N fertilizer were applied in combination. The low manure (12.5 Mg ha-1) plus 60 kg N ha-1 fertilizer treatment was best in terms of maintaining dry matter yield and minimizing N leaching losses.

Abbreviations: NDFF, nitrogen derived from fertilizer


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE SMALLHOLDER CROPPING SYSTEMS in much of southern and eastern Africa are based on maize, the staple food crop. In the cropping systems in Malawi, Zambia, and Zimbabwe, maize accounts for about 60% of the cropped area (Kumwenda et al., 1996). Cattle manure remains the major source of nutrients for plant growth in the smallholder farming sector of Zimbabwe. Although some mineral fertilizers are used, the relatively high prices of fertilizers and poor accessibility of supply limit the amount of fertilizer that the smallholder farmers apply to their crops. For example, in the 1989–1990 growing season, smallholder farmers applied NPKS fertilizer at an average rate of 53 kg ha-1 compared with 705 kg ha-1 by large-scale commercial farmers in Zimbabwe (Humphreys, 1991). Although cattle manure is generally inadequate and of low N (<1%) content (Mugwira and Mukurumbira, 1986), farmers tend to apply large amounts of the manure to certain preferred fields (homefields) resulting in high application rates (up to 80 Mg ha-1, or 800 kg N ha-1) (Mugwira and Murwira, 1998). The high manure application rates imply that the total N loading is much greater than crop requirement, and depending on the N mineralization pattern from the manure, excessive N availability may occur. Coupled with high-intensity rain storms characteristic of tropical and subtropical regions, NO3 leaching potential in fields amended with high rates of manure may be high.

The Alvord system, recommended for the smallholder farming sector of Zimbabwe, has widely been adopted by the farmers and is based on the application of about 40 Mg ha-1 of manure (248–488 kg N ha-1) (Mugwira and Mukurumbira, 1986; Mugwira and Murwira, 1998) to a four-course rotation of two maize crops, followed by a legume and finally a small grain crop (Grant, 1976). The system was developed in the 1940s by D.E. Alvord, a missionary who conducted extensive field trials on sandy soils in the semi-arid zones of Zimbabwe. However, the initial immobilization of N by manures from smallholder farming areas (Muller-Sämann and Kotschi, 1994; Murwira and Kirchmann, 1993) has prompted farmers to supplement the manures with inorganic N fertilizer. There is a need to improve the timing of inorganic N fertilizer application when it is applied in combination with manure to increase N uptake, and minimize N leaching losses.

Most studies on N leaching from soils amended with manure and/or inorganic fertilizers have focused on humid temperate regions (Beckwith et al., 1998; Thomsen et al., 1993; Unwin, 1986), and overall, few quantitative measurements of N leaching have been made in tropical and subtropical regions of Africa (Arora and Juo, 1982; Omoti et al., 1983; Wong et al., 1987). In Zimbabwe, N leaching losses of up to 39 kg N ha-1 yr-1 have been reported on a sandy soil (Kamukondiwa and Bergström, 1994a). However, that study was performed during a sequence of dry years, which limits the representativeness of the results. Other studies, also on sandy soils in Zimbabwe (Hagmann, 1994; Vogel et al., 1994), indicated that most of the fertilizer (up to 54% of applied N) was leached out of the top 0.5 m of soil when heavy rains followed N fertilizer application. However, under such conditions some of the leached nitrogen can probably be recovered by roots later in the season.

Although rainfall is seasonal, highly variable, and generally insufficient in most smallholder farming areas of Zimbabwe (Piha, 1993), its high intensity (up to 250 mm h-1) coupled with the predominantly coarse-textured soils used for agriculture (Twomlow, 1994) may trigger N leaching. Given also the relatively high cost of fertilizer compared with produce, efficient use of N fertilizer is of both agroeconomic and environmental importance. This led us to design a study in which the objective was to measure NO3 in water leaving the root zone in agricultural fields typical of smallholder cropping systems of Zimbabwe, and to establish fertilization rates that minimize N leaching losses while maintaining crop yields.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Experimental Location and Soil Properties
The 3-yr study was conducted at Domboshawa Training Centre (17°35' S, 31°10' E), about 35 km north of Harare, Zimbabwe, where average rainfall is 900 mm yr-1 and mostly restricted to the summer season (November–April). Average annual temperature is 18.8°C, with the highest and lowest monthly mean temperatures being 29°C (October) and 6.5°C (July), respectively. The soil was a Typic Kandiustalf derived from granitic parent material with relatively low water holding capacity (9% available water capacity) (Vogel et al., 1994). The soil consisted of a coarse loamy sand topsoil (0–0.3 m) overlying a sandy loam upper subsoil (0.3–0.6 m) and a clay loam lower subsoil (0.6–0.1.0 m) (Table 1). The experimental site had lain fallow for at least six years before the experiment. Domboshawa Training Centre lies in a region classified as high potential in terms of crop production in Zimbabwe.


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  		Table 1. Selected properties of the experimental soil.

 
Lysimeter Installations
A lysimeter station consisting of 27 repacked and gravity-drained lysimeters was built in the autumn of 1995 at the field site. A trench at the center of the lysimeter station contained 27 buckets to collect leachate. The lysimeters, square-shaped with a surface area of 1 m2 and a depth of 1.1 m, were constructed from 1.6-mm-thick galvanized steel sheets. The lysimeter walls were painted with an oil-based paint to provide a rough surface that would prevent water from channeling between the soil and the inside tank walls. The lysimeter boxes were surrounded by field soil to prevent excessive heating of the lysimeter soil.

A 1-mm wire mesh was fixed at the lysimeter outlet and covered with a 10-cm layer of gravel before the soil was replaced, reducing the effective depth of the lysimeters to 1 m. The gravel improved drainage (Stevens et al., 1992) and also prevented the fine soil material from washing into the 10-mm steel outflow pipes. The pipes were laid at a slope of approximately 2% to ensure rapid water flow to the collecting vessels.

The soil layers were repacked to original density following the sequence of the soil profiles identified during site characterization. Repacking the soil was considered appropriate because it only introduces small changes in water transport and nitrogen behavior in coarse-textured soils (Bergström, 1990). Before the experiment was started in the summer of 1996, the lysimeters were water saturated from the bottom end and thereafter allowed to drain freely, and left to settle for 14 mo.

Experimental Treatments
The treatments were beef cattle manure (0, 12.5, and 37.5 Mg ha-1, which contained 0, 116, and 348 kg N ha-1, respectively) and N fertilizer (0, 60, and 120 kg N ha-1 as 15NH415N03) replicated three times in a two-factor randomized complete block design. The fertilizer 15N (derived from Amersham UK Ltd.) enrichment was 3.3% atom excess (AE) (0.3776 atom % background enrichment) in the first season and 9.0% AE in the second season. No fertilizer was added in the third season. All the fertilizer was applied at planting. The N fertilizer was dissolved in water and applied uniformly in the lysimeters after removing the top-5-cm soil layer, and the soil was replaced soon after fertilizer application. It was assumed that the effect of residual 15N fertilizer was minimal in the second season given the much higher enrichment that was used in the second season.

The manure was aerobically composted (i.e., manure dug out of a cattle pen and heaped in the open for 3 mo) and was collected from the Mhondoro smallholder farming area, approximately 70 km southwest of Harare. In Zimbabwe cattle manure is aerobically composted for 3 to 4 mo in pits that are covered with soil on the surface. The manure contained 8.4% C and 0.93% N (C to N ratio = 9) on a dry matter basis, but after correction for inorganic material the C and N contents were 41.9 and 4.64%, respectively (Nyamangara et al., 1999). Most of the inorganic material is soil ingested by grazing animals or mixed in as a result of trampling of the manure in the cattle pen (Mugwira and Murwira, 1998). The manure rates were based on current recommendations for maize in smallholder farming areas of Zimbabwe where about 35 to 40 Mg ha-1 is applied every fourth year, or annually at about 12 Mg ha-1 (Mugwira and Murwira, 1998). The 12.5 Mg ha-1 manure and N fertilizer treatments were applied every year but the larger manure treatment was applied only in the first year. The same manure was used to ensure the same composition. Annual basal applications of 30 kg K ha-1 as potassium chloride, 30 kg P ha-1, and 30 kg S ha-1 as single super phosphate were made to all treatments. The manure and fertilizer, all applied before planting, were incorporated into the top 5 cm of the soil at planting. Two maize plants were grown in each lysimeter during the rainy summer seasons, which were sown on 3 Dec. 1996, 24 Nov. 1997, and 23 Nov. 1998, respectively.

Sampling and Analytical Procedures
The aboveground maize parts were harvested after 12 weeks each year (milk dough stage) and dry matter yield was determined by oven-drying at 65°C. The plant samples were ground to pass through a 2-mm sieve. Total N content was determined with the semi-micro Kjeldahl method (Bremner and Mulvaney, 1982). In preparation for 15N analysis, subsamples of plant material were ball-milled after grinding to obtain homogeneous samples. Microsamples (2–6 mg) of this material were weighed into aluminium foil capsules, which were introduced into a mass-spectrometer (MS) system (Tracer 15N/13C; Europa Scientific, Crewe, UK) for 15N determination. The system uses a software-controlled Dumas method to produce molecular N, which is successively scrubbed of water and CO2 before being directed to the MS. Signals from a triple collector detector are integrated by a computer linked to the MS and results are given as 15N atom % and total N. The fraction of N in the plant samples derived from the 15N-labeled fertilizer (NDFF) was calculated by the equation:

[1]

The value of natural abundance was determined from samples from nonfertilized areas at the site where the lysimeter soil was collected.

Leachate volumes were recorded following each rain event when breakthrough of leachate occurred. Representative samples were taken one to three times each week depending on volume of leachate for colorimetric NO3–N determination by the Cd-reduction method analysis (Keeney and Nelson, 1982). Total NO3–N loads were calculated by multiplying the total NO3–N concentration by the volume of leachate. It was assumed that the concentration of NH4–N in leachate was negligible.

The analysis of variance (ANOVA) procedure was used to test the effect of manure and mineral fertilizer treatments on N uptake, NO3–N leaching, and leachate volume with the MSTAT statistical package (MSTAT, 1988).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Weather and Drainage Conditions
The 1996–1997 and 1998–1999 cropping seasons (November–April) were wetter (1395 and 1387 mm, respectively) than the long-term seasonal average (900 mm) for the area, whereas in 1997–1998 seasonal rainfall (840 mm) was close to average. Weekly values of rainfall are shown in Fig. 1 . The highest rainfall intensity was recorded in 1996–1997 (up to 120 mm d-1). Accumulated leachate volumes accounted for 48 to 51% (480–509 mm) of rainfall recorded up until harvesting in 1996–1997, 45 to 51% (296–335 mm) in 1997–1998, but was much higher (84–88% or 606–635 mm) in 1998–1999. The much higher proportion of accumulated leachate volume in 1998–1999 (Fig. 1) was mainly attributed to poor maize growth, which was caused by a severe gray leaf spot (Cercospora zeae-maydis) infestation that led to reduced evapotranspiration. Weekly evaporation rates, which are routinely measured at the study site, were small compared with weekly rainfall during the study period (Fig. 1) and the crop was therefore not affected by moisture stress in either cropping season.



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  		Fig. 1. Accumulated leachate in replicated lysimeters of selected treatments, and weekly rainfall and pan evaporation at the experimental site during the study period.

 
There were only small differences in the amounts of leachate between the different treatments, which were not significant (P > 0.05). These differences were of the same magnitude as was found between replicate lysimeters of the same treatment (Fig. 1). Similar trends have also been reported both locally (Kamukondiwa and Bergström, 1994a) and under cold temperate conditions (Bergström and Kirchmann, 1999).

Aboveground Nitrogen Uptake
Total aboveground N uptake by maize from lysimeters that received manure was significantly (P < 0.05) greater than that from the control in all three seasons (Table 2). In the N fertilizer treatments, total aboveground N uptake was significantly higher than the control in the first and second seasons, but not in the third season (1998–1999) (Table 2). Total N uptake was significantly (P < 0.05) higher in manure plus N fertilizer treatments compared with the control for the three cropping seasons. There was a positive manure x N fertilizer interaction, which was significant (P < 0.001) in the first (1996–1997) and second (1997–1998) cropping seasons, but it was not significant (P < 0.361) in the third (1998–1999) season. The interaction between the manure and N fertilizer was mainly additive, that is, the two N sources acted independently of each other in increasing N uptake by maize. A synergistic interaction in increasing N uptake was only observed in 1996–1997 when combined application of 120 kg N ha-1 fertilizer and manure (12.5 and 37.5 Mg ha-1) resulted in higher N uptake (6.7 and 13.9%, respectively) compared with the sum of the separate applications of the manure and N fertilizer. In a related field study at the same site, a significant (P < 0.05) and positive manure x N fertilizer interaction, which was additive, was only observed in the first season (Nyamangara, 2001).


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  		Table 2. Effects of manure and N fertilizer application on total N uptake in maize aboveground plant parts during 1996–1997, 1997–1998, and 1998–1999 cropping seasons.

 
Nitrogen uptake from the manure-only applications (12.5 and 37.5 Mg ha-1) was greater (60 and 36%, respectively) in the second season compared with the first season. This implied that manure N became more available (through mineralization) for plant uptake in the second year compared with the first year. Similar results have been reported in field studies in Zimbabwe for similar poor-quality manures (Murwira and Kirchmann, 1993; Mugwira, 1984) and elsewhere (Jokela, 1992). By contrast, N from anaerobically stored manures becomes available for plant uptake soon after application to soil (Beckwith et al., 1998; Bergström and Kirchmann, 1999). Average net uptake of manure N ranged from 26% in the 12.5 Mg ha-1 and 12.5 Mg ha-1 plus 60 kg N ha-1 treatments to 55% in the 37.5 Mg ha-1 plus 120 kg N ha-1 treatment over the three cropping seasons, and for each treatment was highest in the second season (Table 2).

Aboveground Recovery of Mineral Nitrogen Fertilizer
The use efficiency of mineral N fertilizer was only determined in the first and second seasons. The use efficiency averaged 44.7% (25.5–63.8%) for all treatments in the first season compared with 26.6% (19.2–34.3%) in the second season (Table 3). Fertilizer use efficiency was calculated as the amount of fertilizer N taken up by aboveground plant parts (NDFF in kg N ha-1) as a percentage of total fertilizer N added to soil. For a given mineral N fertilizer rate, the addition of increasing rates of manure increased the use efficiency of the mineral N, and this trend was more pronounced in the first season (Table 3). The overall effects of manure addition on mineral N fertilizer use efficiency are shown in Table 4. The results showed that applying mineral N with manure increased the mineral N fertilizer use efficiency by up to 22% in the first year and up to 14% in the second year compared with when mineral N fertilizer is applied on its own.


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  		Table 3. Nitrogen fertilizer use efficiency in maize fertilized with 15N-labeled fertilizer (F) and combinations of cattle manure (M) and 15N-labeled fertilizer.

 

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  		Table 4. Overall effects of manure addition on recovery of labeled fertilizer N in maize fertilized with 15NH415NO3.

 
The observed mineral N fertilizer use efficiencies were within the 25 to 80% range reported elsewhere (Bergström, 1987; Kamukondiwa and Bergström, 1994b; Korenkov et al., 1975; Jokela and Randall, 1997). Kamukondiwa and Bergström (1994b) reported first-season average mineral N fertilizer uptake efficiency by maize of 27.3% on similar soils at a nearby site in Zimbabwe, although the study was conducted during seasons characterized by long dry spells. Although the 15N isotope technique is known to under- or overestimate mineral N fertilizer use efficiency due to the so-called added nitrogen interaction (ANI) effect (Jenkinson et al., 1985), the error in this study was assumed to be too low due to the low soil organic matter content of the soil.

Nitrate Concentration in Leachate
As expected, NO3–N concentrations in leachate were consistently low (<10 mg L-1) from lysimeters where no N was added (Fig. 2) . Concentrations from lysimeters amended with manure were slightly greater (up to 15 mg NO3–N L-1) than the control during the three cropping seasons (Fig. 2). Concentrations from lysimeters that received N fertilizer were higher than the control in the first (up to mg 22.5 NO3–N L-1) and second seasons (15.3 mg NO3–N L-1), but during the third season the concentration was smaller and comparable with the control lysimeters, that is, <10 mg NO3–N L-1. Kamukondiwa and Bergström (1994a) also reported lower N concentration in leachates from lysimeters receiving aerobically composted manure (1% N) compared with corresponding lysimeters receiving equal amounts of N as mineral fertilizer, but there are examples showing the opposite situation. For example, in a study performed under cold climate conditions, NO3–N concentrations in drainage water were larger in poultry manured soils than in soils receiving equal amounts of N with mineral fertilizer (Bergström and Kirchmann, 1999). But, it is important to keep in mind that the manures used in that study and our study were different, which precludes a direct comparison of the results.



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  		Fig. 2. Average NO3 concentrations in leachate collected from replicate lysimeters applied with different amounts of manure and mineral N fertilizer. Bars represent standard errors.

 
Manure and N fertilizer treatment combinations resulted in NO3–N concentrations in leachate reaching 34.2 mg NO3–N L-1 in the first season and 25.2 mg L-1 in the second season when the high N fertilizer rate (120 kg N ha-1) was used.

High-intensity rainfall recorded soon after application of the N fertilizer, especially during 1996–1997, may have leached the soluble N resulting in high NO3 concentrations in N fertilizer treatments in the first and second season, although much less than one pore volume of water had leached through the profile between fertilizer application and this breakthrough of water. This indicates that some of the fertilizer N was displaced by preferential flow induced by an unstable wetting front (Steenhuis and Parlange, 1991), which was probably enhanced by the high rainfall intensity.

Nitrate Leaching Losses
Nitrate N leaching losses from lysimeters amended with manure and N fertilizer were significantly (P < 0.01) higher than from the control lysimeters during the three growing seasons (Table 5). During the first season (1996–1997), lysimeters with N fertilizer (60 and 120 kg N ha-1) leached significantly (P < 0.05) more NO3–N (41.9 and 56.3 kg N ha-1, respectively) than the manure (12.5 and 37.5 Mg ha-1) treatments (24.2 and 27.5 kg N ha-1, respectively). Leaching losses were much lower during the second (11.8–24.1 kg N ha-1) and third (12.7–32.9 kg N ha-1) seasons, but the N fertilizer and manure effects were still significant (P < 0.01).


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  		Table 5. The effect of different manure and N fertilizer rates on NO3–N leaching during 1996–1997, 1997–1998, and 1998–1999 cropping seasons.

 
The higher NO3–N leaching in the mineral fertilizer treatments was attributed to the large amounts of readily available N early in the season, whereas most of the N derived from manure has to undergo mineralization before it becomes available for uptake and leaching. It is notable that smallholder farmers typically apply one-third of the mineral N fertilizer at planting and the balance at 4 to 6 weeks after planting, and under such conditions NO3 leaching may be lower. In this study all the mineral N fertilizer was applied at planting.

Although the above N leaching losses in 1997–1998 and 1998–1999 may be considered low, they are of socioeconomic significance in the smallholder African farming systems. Under such conditions, maximizing N uptake (and hence yield) is much more important than the environmental consequences of N leaching. Furthermore, NO3–N concentration in leachate exceeded the statutory limit of 11 mg NO3–N, and this poses a health risk to humans. Since 15NO3–N in leachate was not measured, to determine the proportion of mineral N fertilizer lost compared with that added, total N leaching losses from the control were subtracted from the mineral N fertilizer treatments and the difference was expressed as a percentage of total mineral fertilizer added. For the manure and mineral N fertilizer treatment combinations, the corresponding manure-only treatments were used as the controls. In the first season, 24 to 40% of the added mineral N was leached, which is socioeconomically significant given the poor resource base of smallholder African farmers. In the second and third seasons, the proportions of fertilizer-derived N leached were much lower, that is, 3 to 9% and 4 to 11%, respectively. Overall, the low manure (12.5 Mg ha-1) plus 60 kg N ha-1 fertilizer treatment was the best treatment in terms of maintaining dry matter yield and minimizing N leaching losses.

It was assumed that the unaccounted for N was retained in the soil since very low gaseous losses (<10 kg N ha-1) have been reported in manure and unmanured soils locally (Murwira, 1995; Nzuma and Murwira, 2000).


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Results obtained in this study showed that application of mineral N fertilizer to a sandy soil can result in high NO3 leaching losses when all the fertilizer is applied at planting. By contrast, it was shown that the application of aerobically composted manure from the smallholder farming areas of Zimbabwe to soil does not pose an environmental concern due to NO3 leaching in the short term, presumably an effect of the high degree of stabilization that occurs during decomposition. Instead, the manure enhanced the use efficiency of mineral N fertilizer by crops when the two were applied in combination. Although the N leaching losses observed in this study may be small in terms of environmental pollution, they are of socioeconomic significance in the smallholder African farming systems where N fertilizer is unaffordable to most farmers. The low manure (12.5 Mg ha-1) plus 60 kg N ha-1 fertilizer treatment was the best treatment in terms of maintaining dry matter yield and minimizing N leaching losses. Further studies are required to determine the effect of manure quality and split application of N fertilizer on plant uptake and leaching losses, and also the effect of soil type.


   ACKNOWLEDGMENTS
 
This study was conducted within the project "Management of Soil Organic Matter for Sustainable Agriculture" sponsored by Swedish Agency for Research and Co-Operation with Developing Countries (SAREC-1995-0743) to which the authors are grateful. We would also like to thank the Institute of Environmental Studies of the University of Zimbabwe for coordinating the project.


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


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This Issue in Journal of Environmental Quality

JEQ 2003 32: 377-382. [Full Text]  




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