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USDA-ARS Appalachian Farming Systems Research Center 1224 Airport Rd. Beaver, WV 25813-9423
* Corresponding author (Dale.Ritchey{at}ars.usda.gov).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Abbreviations: EC, electrical conductivity • pHs, pH in 0.01 mol L–1 CaCl2
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The major nitrogenous component of small ruminant urine is urea, with smaller amounts of creatine, allantoin, and hippuric acid (Whitehead, 1995); the most abundant cation is K+. When urea contacts soil, the enzyme urease facilitates its hydrolysis to NH4+ and either HCO3– (at pH of 6.3 or higher) or CO2 and H2O at lower pH (Whitehead, 1995). Both of these reactions consume H+ ions, and can raise soil pH by up to two to three units. At high pH, NH4+ can convert to NH3 gas, which may be lost to the atmosphere through volatilization. Ammonium can be nitrified through conversion to NO2– and then to NO3– (Lovell and Jarvis, 1996), usually through the actions of soil microbes. Both of these processes liberate H+ ions, lowering pH, thus reversing the increase in pH resulting from hydrolysis. Nitrification is usually somewhat inhibited by low pH and low concentrations of plant nutrients (Brady, 1990). Under anaerobic conditions, such as found in saturated soil in the presence of microbe-available C sources, NO3– can be denitrified to N2O and N2 (Clough et al., 2004).
Retention and uptake of the various forms of N varies. In soils of temperate climates with a predominance of cation exchange capacity (CEC) over anion exchange capacity, NH4+ is usually well retained on the soil complex, but if the amounts produced are greater than the CEC, NH4+ is subject to leaching. Urea and NO3– are both more easily leached than NH4+.
Reactions in dystrophic, acidic soils typical of abandoned grasslands and woodlots in the Appalachian region may be different than in limed, active pastures. Microbial populations (and subsequently, microbial transformations of urine) in pastures that have been abandoned for a long time are not necessarily the same as those of fertilized pastures that are frequently clipped (Williams et al., 2003).
Because of the social behavior of goats, even moderate stocking rates can result in areas of high rates of dung and urine deposition. Where high stocking densities occur (rotational grazing at 125 goats ha–1 for 9 d, or mob grazing for brush clearing at 200 goats ha–1 for 9 d), an estimated 2 or 3% of urine patches will overlap according to probability calculations based on uniform spatial distributions of 20 daily deposits of 0.04-m2 area. Overlapping at these stocking densities will be much greater where goats congregate (salt block, water, and shade locations). On the other hand, soil compaction resulting from concentrated hoof action can increase soil density, decrease aeration, and promote denitrification, thus reducing the potential for leaching of NO3–.
Under conditions where multiple applications of urine occur, concentrations of salts and hormones from the urine and concentrations of toxic NH3 can cause scorching, which can damage or destroy vegetation (Williams et al., 1999). Decreased plant growth interferes with the buffering system usually functioning under normal conditions. Under conditions of moderate N addition where urine deposits are isolated from each other and are not contiguous, plants respond to increased N concentrations by increasing their growth and uptake of N, thus decreasing the amount of N subject to leaching. If urine concentrations become high due to contiguous or overlapping deposits and plant health is compromised, the buffering mechanism is less effective; this can result in large amounts of NO3– moving below the rooting zone into groundwater. Water for human consumption in nearby wells can become contaminated with NO3–, which can increase the incidence of methemoglobinemia, a serious health hazard to infants (Fan et al., 1987). Groundwater contamination can also cause eutrophication of streams and ponds. In addition to possible contamination concerns, concentrated urine applications can decrease soil Ca and Mg concentrations as cations are displaced from the exchange complex by NH4+ and leached out with NO3– (Fox, 2004).
One of the management tools available to farmers for improving grassland productivity is the surface application of limestone, but due to the low solubility of CaCO3, neutralization of toxic Al in the rooting zone can take several years (Ritchey et al., 2004). Given the slowness of the reaction of limestone, it would be useful to apply limestone as early as possible. On the other hand, application of limestone to brush-infested abandoned pastures is more difficult than application after goats have removed aboveground biomass. Little is known about the combined effects of surface liming and heavy urine applications to dystrophic soils. We hypothesized that under conditions of high salt concentrations and fluctuations in pH associated with hydrolysis and nitrification of large quantities of added urine, surface limestone applications may raise pH and increase Ca and Mg concentrations in the rooting zone while reducing Al concentrations, resulting in greater dry matter productivity.
This study was initiated to investigate the effects of medium to high rates of urine application on the fate of N in a dystrophic abandoned pasture soil and to measure the effects of surface application of limestone on N transformations, N movement, and changes in cation status.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Briefly, 32 vegetated soil macrocosms were collected in early August 2000 from an abandoned grassland site in southern West Virginia (37°40' N, 80°7' W) on a soil complex consisting of Gilpin (fine-loamy, mixed, mesic Typic Hapludult) and Lily (fine-loamy, siliceous, mesic Typic Hapludult) series by driving sharpened polyvinyl chloride cylinders into the profile. Vegetation on the 45-cm-deep columns (inside diameter 15.2 cm) varied and consisted mainly of fescue (Festuca spp.) and nine broadleaf species.
To provide drainage from the columns, a 290-cm-long fiberglass wick 6 mm in diameter (American Seal and Packing, Mountain Valley, CA) was prepared by baking 4 h at 400°C to remove hydrocarbons, then washing in distilled water until the liquid rinsate was clear (Knutson et al., 1993). The wick was arranged in a coil in contact with the soil at the bottom of the column and the rest of the wick extended 76 cm below the bottom soil surface. The wick was encased in a flexible plastic tube (inside diameter 13 mm) that was covered with aluminum foil to reduce algal growth. Leachate was collected three times per week and pooled to provide one sample per column per week. A representative subsample was stored at –20°C.
Columns were acclimated for 10 wk in a greenhouse and vegetation was periodically clipped to 5-cm height. Greenhouse temperatures ranged from 7 to 32°C, and water was added to the columns sufficient to maintain water movement through the soil profile. Beginning 15 Oct. 2000, instead of distilled water, columns were hydrated with simulated acid rain (pH 5) formulated by adding to 100 L distilled water the following: 17.6 mg NaCl, 70.9 mg CaSO4·2H2O, 35.5 mg MgSO4·7H2O, 6.9 mg K2SO4, 36.8 mg NaNO3, 74.6 mg (NH4)2SO4, 24.3 mg NH4NO3, 0.84 mg H3PO4, 38.3 mg H2SO4, and 22.1 mg HNO3 (Halverson and Gentry, 1990).
On 10 Oct. 2000, 16 of the columns received a surface application of 12.3 g of agricultural limestone to provide a rate of 6720 kg ha–1. The limestone contained 43% MgCO3 and 54% CaCO3, with 104% CaCO3 equivalence and 98% effective neutralizing value. The limestone particle size was such that 100% passed a 0.84-mm (20 mesh) screen, 90% passed 0.3 mm (50 mesh), 80% passed 0.25 mm (60 mesh), and 75% passed 0.15 mm (100 mesh).
Three batches (A, B, and C) of urine were collected from three goat wethers as described by Ritchey et al. (2003). The urine was stored at –20°C. On 12 February (18 wk after limestone application), 100 mL of urine (Urine A) was added to the surface of each of the limed (L) and unlimed (U) columns excluding controls, which received 100 mL of distilled water (Table 1 ). Immediately after the first urine application, domes of translucent plastic were placed onto the top of each column to measure the release of NH3 from the soil during a 1-wk period as air was pumped through the dome at approximately 60 L h–1 as described by Ritchey et al. (2003). On 26 February, 16 columns that had received the first application were retreated with 100 mL of Urine B. On 12 March, eight of the columns that had received both the first and second applications were treated with 100 mL of Urine C. Ammonia generation was measured on all 32 columns following the second and third applications. Treatments receiving zero, one, two, or three applications were designated as 0, 1, 2, and 3, respectively.
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Soil solution samples from the treatments receiving one and three applications of urine were collected one to three times per week using 10-cm Rhizon samplers of 2-mm diameter (Ben Meadows Co., Janesville, WI) inserted into the columns at depths of 2.7, 5.4, 10.8, 21.6, and 43.2 cm.
Plant material was harvested to 5-cm height nine times from 3 May to 17 August. At the end of the experiment (21–24 Aug. 2001), vegetation was harvested down to the soil surface. The vegetation was oven dried (67°C) and weighed for evaluation of dry matter production.
To collect soil samples, the columns were laid in a horizontal position and a lengthwise saw cut was made on each side of the column. One-half of the pipe was removed so that the soil in the column could be sectioned with minimum contamination. The columns were sectioned at depths of 1.6, 4, 7, 10, 16, 22, 28, 34, and 40 cm. Samples for measurement of moisture content and NO3– and NH4+ concentration were collected immediately and extracted with 2 mol L–1 KCl (Keeney and Nelson, 1982). The extract was analyzed by flow injection analyzer (Alpkem RFA/2 Astoria-Pacific International, Clackamas, OR). No effort was made to separate NO3– and NO2–, and because NO2– was probably small relative to NO3–, the values are reported as NO3– for the sake of simplification. The remaining soil was air dried and weighed for bulk density determination and analyzed for neutral 1 mol L–1 NH4OAc-extractable Ca, Mg, and K (Thomas, 1982) and for 1 mol L–1 KCl-extractable Al (Barnhisel and Bertsch, 1982) by inductively coupled plasma (ICP)–atomic emission spectrometry (HORIBA Jobin Yvon, Edison, NJ). Soil electrical conductivity (EC) was measured in a 1:1 soil/water suspension. Soil pH was measured using 1:1 soil/0.01 mol L–1 CaCl2 (pHs). Leachate and soil solution NO3––N and NH4+–N were determined using suppressed ion chromatography (Dionex Corp., Sunnyvale, CA).
Subsamples of urine collected at the time of application and stored at –20°C were analyzed about 6 mo after application for K concentration by ICP. The samples were refrozen and reanalyzed on 19 Nov. 2004 for total dissolved N using a Shimadzu TOC-V CPN Total Organic Analyzer, with TNM-1 Total Nitrogen Measuring Unit (Shimadzu Scientific Instruments, Columbia, MD).
Plant material was analyzed for N by Carlo Erba Elemental Analyzer Model EA1108 (CE Elantech, Lakewood, NJ).
Data Analysis
Columns were arranged in the greenhouse in a randomized complete block design. All treatments were replicated four times, but one of the limed columns that received three applications was excluded from statistical analysis due to the appearance of urine in the column leachate, indicating excessive macropore flow.
Analysis of variance and regression evaluations were conducted using general linear model statistical procedures (SAS Institute, 1990). For separation of means, F test protected LSD values (Steel and Torrie, 1980) are given. Treatment differences were considered significant at P 
0.05. Factorial analysis was used to identify the effects of urine and lime on distributions of soil parameters where lime x urine interactions were generally not statistically significant.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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The treatments receiving one urine application produced almost three times as much dry matter (DM) as the treatments with no added urine, and mean production in the treatments that received two urine applications was more than 4.5 times the production in the treatments with no urine (Table 2 ). Vegetative growth presumably responded to the increased availability of N and K present in the goat urine. Lime addition to the zero-urine treatment had little effect on DM, but in the one and two urine application treatments there was a tendency for surface lime addition to increase yields (Table 2). Liming decreased DM at the high-N rate, probably because it increased NH3 concentration in the collection domes and exacerbated scorching damage, as discussed by Ritchey et al. (2003). In the undamaged treatments, the range of productivity that we measured (153–850 g m–2) is consistent with values found in abandoned and restored grasslands of the eastern United States (Stinner et al., 1984; Ritchey et al., 2004).
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Amount of Leachate
The amount of water added to the columns from the time of the first urine treatment until harvest 27 wk later was 16.1 L (886 mm), equivalent to 85% of mean annual rainfall in southern West Virginia. The volume of leachate collected was lowest in the two-application treatment (164 mm or 18% of added water) and greatest in the three-application treatment (378 mm or 43% of added water). In comparison, in pastures in central Pennsylvania, Jabro et al. (1998) recovered 27% of annual precipitation as percolation flow-through and Zhu et al. (2002) recovered 52% of precipitation as percolate.
A major factor affecting the variation in the amount of percolation was the amount of plant growth. Leachate volume between 3 May and 18 August (the period of highest insolation) was negatively correlated with the amount of vegetative dry matter harvested from the columns during the same interval (Fig. 1 ). The flow-through volume during this period was lowest in the treatments where the amount of plant growth was highest (2L). Liu et al. (1997) stated that under conditions of similar soil and water inputs, differences in water use by vegetation could cause differences in percolation, and this is clearly evident in the present experiment. The decreased vegetation in the high-urine treatments due to scorching damage presumably lowered transpiration and increased the mean rate of flow-through. Such higher percolation rates would be expected to exacerbate the potential for detrimental leaching of N into groundwater.
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The peak mean NH4+–N concentration at 4 cm in the treatment receiving one urine application was 21 mg L–1, occurring during the first 8-wk period (data not shown). Concentrations were <6 mg L–1 in the other depths and time periods. In the treatment receiving three applications, the 4-cm depth had a mean peak concentration of 186 mg L–1 occurring at 9 to 11 wk; the concentration at the 16-cm depth peaked at 107 mg L–1 at 12 to 15 wk; and at the 43-cm depth, a peak of 78 mg L–1 was observed at 20 to 29 wk. The decline in NH4+–N concentration with depth indicated a transformation of NH4+ to NO3– as N moved down through the columns.
Mean NO3–-N concentration (data not shown) was <2 mg L–1 at the 4-cm depth in the one-application treatment (indicating that not much NH4+ had nitrified while N was in the surface layer of the column), but a peak of 37 mg L–1 was observed at the 16-cm depth at 9 to 11 wk. The concentration of NO3––N at the 43-cm depth (26 mg L–1) was apparently still increasing during the last 9 wk of the experiment. In the treatment receiving three applications, the NO3––N concentration at the 4-cm depth peaked at 136 mg L–1 at 12 to 15 wk, and the NO3––N concentration at the 16-cm depth peaked at 213 mg L–1 at 16 to 19 wk. At the 43-cm depth, the highest concentration (203 mg L–1) was found during the last 9 wk, and was probably still increasing.
Leachate Concentrations
Ammonium
Leachate N concentrations were less variable than those measured in soil solution samples, probably because the samples represented a larger volume of soil. Mean leachate NH4+–N concentrations (Fig. 2
) in treatments receiving one and two urine applications tended to be slightly higher than in the control, but they were still generally low (<8 mg L–1). In the three-application treatment, mean concentrations reached 70 mg L–1 about 12 wk after the first application. Mean concentrations had declined to about 32 mg L–1 by Week 28 when the experiment was terminated, indicating that the bulk of the soil solution NH4+ had leached out of the column by the end of 28 wk. The total amount of NH4+–N that leached from the three-application treatment was about 25 times greater than the amount that leached from the two-application treatment. The mean volume of leachate from the three-application treatment was more than twice as great as from the two-application treatment (because of the smaller amount of vegetation), and this may have carried some NH4+ out of the columns before it had time to be nitrified.
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Calcium, Magnesium, and Potassium
Lime did not have a significant effect on the total amounts of Ca, Mg, and K present in the leachate (data not shown). There was a marked effect of amount of urine applied. Total amounts of Ca, Mg, and K leached were each <1 g m–2 in the zero-urine treatment. The total amounts present in the leachate for the three-application treatment were 13.7 g m–2 Ca, 3.26 g m–2 Mg, and 11.6 g m–2 K.
The higher concentrations of K found in the leachate at high application rates were probably due to the K contained in the added urine. Higher concentrations of Ca may have originated from stripping of Ca from the soil exchange complex by high concentrations of NH4+ and NO3– in the high-application treatments. The higher concentrations of Mg in the limed high-urine treatment leachates probably originated from Mg present in the limestone.
Concentrations of Extractable Ions Remaining in the Soil Columns
Ammonium
In the columns not receiving urine, concentrations of KCl-extractable NH4+–N (Fig. 4
) averaged 13 mg kg–1 in the surface 0- to 1.6-cm layer, 6 mg kg–1 in the 1.6- to 4.3-cm layer, and 4 to 3 mg kg–1 in the deeper layers. There was no statistically significant effect of limestone addition on NH4+ concentrations in the zero-urine treatment.
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There was a significant effect of liming in the columns that received three urine applications. In the 28- to 40-cm layers, the limed treatment had less residual NH4+ than the unlimed treatment, and liming in the three-application treatment caused an overall decrease in recovered N of 13 g m–2 (Table 2).
Nitrate
In the treatments not receiving urine or receiving just one urine application, little KCl-extractable NO3– was present (Fig. 5
). The 0- to 1.6-cm layer had about 1 mg kg–1, and in the other layers the mean concentrations were generally <0.5 mg kg–1. There was no effect of surface lime application on extractable NO3––N in the zero-urine treatments.
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Calcium
Factorial analysis indicated that limestone treatment increased extractable soil Ca concentrations down to the 28-cm depth (Fig. 6
). Ritchey and Schumann (2005) found an increase of 0.34 cmolc kg –1 Ca at 22.5- to 30-cm depth 3 yr after surface application of Ca(OH)2 to a forest soil.
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Magnesium
Factorial analysis indicated that surface application of limestone increased the concentrations of extractable Mg in all 10 layers (Fig. 7
).
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pH
In soils in humid climates, it is customary to measure pH in a 0.01 mol L–1 CaCl2 solution (Schofield and Taylor, 1955) to avoid unreliably high pH measurement values that can appear at very low EC levels, such as were found in the subsoils of the low-input management system we studied (0.03 dS m–2).
In the treatment receiving no urine and no limestone, pHs was near 4.0 throughout the profile (Fig. 8 ). Surface addition of limestone 46 wk before sampling to the zero-urine treatment resulted in increases in pHs of 1.46 to 0.18 units down to 10 cm. In a field study on Gilpin silt loam soil, Ritchey et al. (2004) found increases of 1.2 to 0.2 pH units down to 10 cm 6 yr after surface application of 4650 kg ha–1 limestone.
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Aluminum
The level of extractable Al in the unlimed soil that did not receive urine was 1.4 cmolc kg–1 in the surface layer and approximately 1.9 cmolc kg–1 from 1.6 to 16 cm, gradually increasing to 2.8 cmolc kg–1 at the bottom of the profile (Fig. 9
). Below 7 cm, Al saturation was >60% for all treatments (data not shown), which is high enough to interfere with root growth of many forage and crop plants.
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Factorial analysis showed that the overall effect of urine was an increase in Al concentration in the three layers between 10 and 28 cm, possibly caused by acidity released by nitrification or by the influence of hippuric and uric acids present in the urine.
Potassium
The control treatment showed a K concentration of 0.70 cmolc kg–1 in the surface 1.6 cm of soil and 0.14 in the 1.6- to 4-cm layer, falling to around 0.07 in lower layers (data not shown). Although the one-application treatment received 10 g m–2 K, soil concentrations tended to be lower than those in the control, probably due to the mean K uptake by the plants of 15 g m–2 (data not shown). The two-urine application treatment received 39 g m–2 K and the three-urine application received 71 g m–2 K, which supplied more K than was taken up by plants. The overall effect of urine addition was to increase concentrations of K in all the layers below 2 cm, mostly due to higher concentrations in the three- application treatment and the unlimed two-application treatment. There was no clear overall effect of lime on K concentration.
Fate of Added Nitrogen
Nitrogen added to soils in the form of goat urine is subject to several transformations, and as a consequence appears in various pools. The amounts of N in four of these pools were measured, specifically: (i) N in ammonia gas released to the atmosphere, (ii) N taken up into aboveground plant material, (iii) as N recovered as extractable NH4+ and NO3– in soil, and (iv) as N present in drainage water as leached NH4+ and NO3–, ranging from 49 to 77% of the amounts added. The higher rates of recovery were observed in the treatments that received two and three N applications. We did not evaluate N losses associated with denitrification, changes in plant root N, changes in soil organic or biomass N, or changes in organic N present in the leachate, all of which could help account for the difference between the amount of N added and the amount of N measured.
Net plant uptake (above uptake in the zero-urine treatments) of N was 30 to 45% of the amount added in the one- and two-application treatments. Plant growth was seriously affected by the three-application treatments, and plant N uptake was only 9% of the N added.
In the two-application treatment, the effect of lime application in increasing plant uptake of N was statistically significant, and in the one-application treatment there was a tendency for increased uptake. This lime-induced increase in N uptake by vegetation presumably decreased the amounts of inorganic N available in the columns and made it difficult to evaluate the expected positive effects of liming on nitrification. Clough et al. (2004) found higher rates of nitrification and higher residual values of NO3– at higher pH. In contrast, we found a tendency for less residual NO3– in the limed two-application and three-application treatments. For the two-application treatment, this decrease can be explained by the increased uptake of N in plant material. In the three-application treatment, however, higher plant uptake does not explain the tendency for the limed treatment to have lower concentrations of NH4+ and NO3– in the soil and leachate because the combination of high N rates and lime exacerbated scorching, which decreased plant growth and decreased N uptake.
The total amount of N in the leachate when the experiment ended was 14% of the N added in the one-application treatment, 15 and 7% of the amount added in the unlimed and limed two-application treatments, respectively, and >30% of the added amount in the three-application treatment. It is apparent (Fig. 2, 3, 4, and 5) that even more N would have leached in the two- and three-application treatments if the experiment had been continued longer.
During the experiment, treatments receiving zero, one, two, or three applications of urine leached a mean total of 0.1, 5, 11, and 66 g m–2 of inorganic N, respectively. Thus, although the three-application treatment received only five times more N than the one-application treatment, it leached more that 12 times more N below 45 cm. This implies that in situations where plant growth is damaged by heavy grazing, or, in our case, scorching, leaching loss of N is exacerbated by a double effect of overstocking: (i) lack of vegetation severely restricts plant uptake of N, and (ii) the absence of plants means that transpiration of rain water is reduced, thus increasing the amount of precipitation draining through the profile and increasing the rate of N leaching.
In summary, the consequences of urine additions to an acidic, dystrophic, abandoned grassland were dependent on the rate of application. With the first application of 360 kg ha–1 of N, an increase of 53 kg ha–1of inorganic N was detected in soil and leachate. Where we applied larger amounts of urine to simulate a heavy stocking density of goats for land-clearing purposes, columns receiving two urine applications (975 kg ha–1 of N) showed greater growth and greater plant uptake of N, but also greater accumulations of soil and leachate N (five times more than with a single application), mostly in the form of NO3–. In treatments receiving three applications (1770 kg ha–1 of N), a large amount of NH3 gas was generated and the vegetation suffered scorching damage. Plant transpiration was reduced, leachate volume increased, and there were high concentrations of NO3– and NH4+ in the soil and leachate.
Surface application of limestone 18 wk before urine application tended to increase dry matter production except in the high-urine treatment, where it increased scorching damage. Limestone addition raised soil pHs, increased concentrations of Ca and Mg, and decreased concentrations of extractable Al to as deep as 28 cm.
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Received for publication April 5, 2007.
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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