Nutrient Cycling in Agroecosystems 67: 21–29, 2003. 2003 Kluwer Academic Publishers. Printed in the Netherlands.
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Effect of amending high phosphorus soils with flue-gas desulfurization gypsum on plant uptake and soil fractions of phosphorus 1, 1 2 William L. Stout *, Andrew N. Sharpley and Stefan R. Weaver 1
Pasture Systems and Watershed Management Research Unit, Soil Scientists – USDA-ARS, University Park, PA 16802 -3702, USA; 2 Soil Scientist – Material Matters, Elizabethtown, PA, USA; * Author for correspondence (e-mail: ws1@ psu.edu) Received 5 February 2002; accepted in revised form 26 July 2002
Key words: Eutrophication, Flue Gas Desulfurization (FGD), Gypsum, Water quality
Abstract Flue gas desulfurization (FGD) gypsum, a coal combustion by-product, can be used to decrease water-extractable soil P, thereby lowering the potential for P export to surface waters. This decrease results from a conversion of loosely bound inorganic P (IP) which is readily desorbable to water, to less soluble Al- and Fe-bound IP and, to a lesser extent, calcium-bound IP pools. Although this conversion has little effect on predictors of plant-available soil P (e.g., Mehlich-3 P), little is known about the plant uptake of P over several growth cycles after high P soils are amended with FGD. In a greenhouse experiment, we measured P uptake by ryegrass (Lolium perenne) using a modified Stanford–Dement procedure (three growth cycles), and the extent to which IP was being removed from each soil IP fraction (Hedley fractionation), for three soils treated with FGD gypsum (equivalent to 22.4 Mg ha 21 ). Treatment with FGD decreased water extractable soil P 38 to 57%, but had little effect on Mehlich-3 soil P. During the first growth cycle, the shift from resin IP to less available Al, Fe, and Ca IP remained stable. Repeated growth cycles of ryegrass removed resin IP and thus, had a continued effect on lowering water-extractable P. After three growth cycles and harvests, ryegrass dry-matter production was not affected by FGD treatment (P . 0.05), although cumulative P uptake (20%) and P concentration of ryegrass tops (25%) were greater in FGD treated than untreated soils. Our results confirm that treatment of high P soils with FGD gypsum decreases water-extractable P by conversion to soil IP fractions that are stable with time, does not decrease plant production, and suggests that the potential for P export in surface runoff may be reduced for several years.
Introduction In the northeastern USA, high rates of fertilizer and manure application have resulted in the majority of agricultural soils having excessive amounts of P relative to crop requirements (Lander et al. 1998; Sharpley et al. 1998). Soils with excessive P levels are an environmental rather than agronomic concern, as the enrichment of runoff P from these soils can contribute to freshwater eutrophication (Carpenter et al. 1998; Sims et al. 1998; US Environmental Protection Agency 1996). There are several options to minimize agricultural P export that focus on source (reducing P inputs, manure and soil amendments) and transport measures
(conservation tillage, buffer zones) (Sharpley and Rekolainen 1997). One such management option that may decrease the potential for P loss in surface runoff, is to decrease the solubility of soil P in heavily fertilized or manured soils by application of a Psorbing material, such as coal combustion by-products (CCBs) (Coale et al. 1994; Codling et al. 2000; Stout et al. 2000). The release of electrolytes from CCBs can also decrease surface soil sealing, thereby increasing infiltration and reducing surface runoff (Edwards et al. 1995). This approach has been shown to be particularly effective if the by-products are applied to runoff generating areas in a watershed (Stout et al. 1999). As a result of Clean Air legislation, large amounts
22 of CCBs are being produced annually by coal-fired power plants (US Environmental Protection Agency 1988). Previous work has shown that land application of these CCBs can increase soil pH and decrease subsoil acidity, with no negative agronomic or environmental impacts from trace metals (Edwards et al. 1995; Stout and Priddy 1996; Stout et al. 1998). Recent research has also shown that one of these CCBs in particular, flue gas desulfurization (FGD) gypsum, can decrease water-extractable soil P without appreciably decreasing plant available P (i.e., Mehlich-1 or -3 soil P) (O’Reilly and Sims 1995; Stout et al. 1998). This reduction is accomplished by the conversion of loosely bound soil IP, which is readily desorbable to water, to more tightly bound Fe and Al and to a lesser extent Ca-bound soil IP as well as amorphous precipitates, not released to water (Stout et al. 2000). Little is known, however, about the stability of low solubility IP fractions formed after adding CCBs, particularly under repeated crop growth and harvest cycles. Thus, the objective of this research was to determine the effect of repeated plant growth cycles (ryegrass) on the stability of soil IP fractions formed after FGD gypsum application to soils of high P concentration (.200 mg kg 21 Mehlich-3 P).
Methods Soil collection The Ap horizons of a Watson silt loam (fine-loamy, mixed, active, mesic Typic Fragiudults) and a Klinesville shaley silt loam (loamy-skeletal, mixed, active, mesic Lithic Dystrudepts) were collected in central Pennsylvania, and the Ap horizon of a Henlopen loamy sand (sandy, siliceous, mesic Lamellic Paleudults) was collected in Delaware. These soils had historically received applications of animal manures and were currently being farmed in a rotation that included corn (Zea mays), soybean (Glycine max), and a small grain. The soils were then air-dried and sieved (2 mm). FGD gypsum at the rate of 10 g kg soil 21 was thoroughly mixed in each of forty 110-g replicate samples of each soil (the 10 g kg soil 21 treatment approximates a land application of 22.4 Mg ha 21 ). Previous work has shown this FGD gypsum application rate to be optimum in decreasing water-extractable P (Stout et al. 2000). After FGD gypsum and soil
were mixed, the mixture was placed in paper cups with 10 holes punctured in the bottom and placed on a 5-cm deep bed of sand. The soil was then wet with distilled water (approximately 50 mL), covered with paper to minimize evaporation, and allowed to drain for 48 h. After drainage, the soil samples were covered with paper to minimize evaporation, and incubated at ambient temperature (about 25 8C) for 21 d. Over 90% of the long-term soil P re-equilibration (up to 1 yr) occurs during the 21-d period (Rajan and Fox 1972; Sharpley 1982). Recent work has shown that decreases in water-extractable P caused by FGD gypsum application are the same for 21 d and 120 d incubations (Callahan et al. 2002). A duplicate set of untreated controls for each soil was incubated in the same manner. After incubation, the soils were air dried, thoroughly mixed, and stored for plant uptake studies and chemical analysis. Plant uptake The uptake of P by ryegrass during repeated growth cycles was accomplished using a modified Stanford– Dement procedure (Stanford and Dement 1957). Ryegrass (Lolium perenne) seedlings for the extraction were produced in 240-ml false-bottom plastic pots containing quartz sand. Plants were irrigated every other day with P-free Hoagland’s solution (Hoagland and Arnon 1950). About 30 seedlings per pot were grown for 16 d, at which time roots proliferated at the bottom of the pots. Prior to the start of the experiment, the pots were clipped to 2 cm and the top growth was discarded. The pots were then watered with an excessive amount of P-free Hoagland’s solution and allowed to drain. The false bottoms were removed and the roots were placed in contact with 100 g of the air-dried treated soils that were contained in similar pots (the remaining 10 g were retained for chemical analyses). There was sufficient moisture in the sand to wet the soil when the roots were placed in contact with the soil. The combined pots were weighed and this reference weight was recorded. The pots were irrigated every day with the P-free Hoagland’s solution to attain the reference weight. There were 40 pots for each soil and each treatment. This allowed for four replications and 10 sampling times. The experiment was blocked along the length of a greenhouse bench in four replications, and the soils and treatments were randomized within replications. The treatments were re-randomized
23 within replications daily when the pots were irrigated. Plant growth cycles were started in March 1999 and continued until early April 1999. The experiment was terminated after three growth cycles, at which time ryegrass growth slowed considerably. Top re-growth from all pots of the experiment was harvested at a 2-cm stubble when there was approximately 1 g of dry matter growth. At each harvest, four pots of each experimental unit were dismantled and the soil was retrieved from the bottom of the pot. Tops and soil were dried, weighted, ground, and analyzed for P. Cumulative P uptake for each harvest was calculated as the sum of P uptake for the current harvest plus P uptake from previous harvests. After the first growth cycle only, sand was removed from the roots by hand washing, and roots and stubble from the dismantled pots were dried, weighted, ground, and analyzed for P. The remaining pots were subjected to additional extraction cycles. Plant analyses Total P concentration in ryegrass tops, roots, and stubble was determined using a modified semimicroKjeldalh procedure (Bremner and Mulvaney 1982). The procedure used a 1.113-g mixture of K 2 SO 4 and CuSO 4 (100:3 weight ratio), concentrated (18 M) H 2 SO 4 (4 mL) and 0.35 g of dried plant material. The mixture was digested at 180 8C for 1 h and at 375 8C for 2 h. Phosphorus in the filtered and neutralized digests was determined by the molybdenum-blue method (Murphy and Riley 1962). Soil analyses Water-extractable soil P was determined by shaking 1 g soil with 10 ml of distilled water for 5 min (Olsen and Sommers 1982). Plant available or soil test P was determined using the Mehlich-3 procedure, where 1 g of soil was shaken end-over-end with 10 mL 0.2 M CH 3 COOH, 0.25 M NH 4 NO 3 , 0.15 M NH 4 F, 0.013 M HNO 3 , and 0.001 M EDTA for 5 min (Mehlich 1984; Wolf and Beegle 1995). Soil pH was measured using a glass electrode at a 5:1 water:soil ratio (v / v) (Eckert and Sims 1995). Soil acidity was determined by equilibration with the SMP buffer (Eckert and Sims 1995). Soil IP was fractionated according to the sequential extraction procedure of Hedley et al. (1982) by endover-end shaking of 0.5 g of soil with: (1) a 2 cm 2 anion exchange resin membrane in 30 mL of 0.01 M
CaCl 2 ; (2) 30 mL of 0.5 M NaHCO 3 (pH 8.5); (3) 30 mL of 0.1 M NaOH; and (4) 30 mL of 1.0 M HCl, each for 16 h. After shaking the membrane square and soil (Step 1), the strip was carefully removed to minimize loss of soil particles for subsequent extraction. Phosphorus retained on the membrane was removed by shaking the strip end-over-end with 40 mL of 1 M HCl for 4 h. The membrane strip was removed, rinsed with deionized water, and shaken with an additional 40 mL of 1 M HCl for 4 h. Phosphorus in the HCl extracts was measured separately and summed to give resin IP (freely exchangeable IP 2 readily bioavailable) (Tiessen and Moir 1993). Phosphorus in all filtered and neutralized extracts was determined by the molybdenum-blue method (Murphy and Riley 1962). Phosphorus fractions determined in Steps 2, 3, and 4 are subsequently referred to as NaHCO 3 IP (biologically available IP), NaOH IP (amorphous and some crystalline Al and Fe phosphates), and HCl IP (relatively stable Ca-bound P), as defined by Hedley et al. (1982) and Tiessen and Moir (1993). The FGD gypsum was analyzed for total metals and Toxicity Characteristics Leachate Procedure (TCLP) concentrations of Cd, Pb, and As by REIC Laboratories, Beaver, WV. This procedure determines if a material contains unacceptably high concentrations of toxic constituents identified in the USEPA National Interim Primary Drinking Water Standard. The TCLP levels of FGD gypsum were below the USEPA 503 regulatory concentrations of 1.0, 5.0, and 5.0 for Cd, Pb, and As, respectively. All data were analyzed using the ANOVA and GLM procedures in SAS (SAS Institute Inc. 1987). All treatment differences discussed in the text are significant at the 0.05 level.
Results and discussion Soil – FGD gypsum incubation After the 21-day incubation (i.e. Harvest 0), FGD gypsum application decreased water-extractable soil P by 11.4, 9.0, and 20.4 mg P kg 21 in the Henlopen, Klinesville, and Watson soils, respectively (Figure 1). This represents a decrease of 57% for the Henlopen, 38% for the Klinesville, and 56% for the Watson soil. These decreases were caused by the added FGD gypsum displacing exchangeable acidity and increas-
24 ing soil Ca activity, thereby shifting biologically available resin IP to somewhat less available NaHCO 3 IP and to the more recalcitrant NaOH and HCl IP fractions (Figure 2). The extent to which resin and NaHCO 3 IP were shifted into the NaOH and HCl IP was determined by the amount of exchangeable acidity in the soils at the start of the experiment. This was caused by the Ca 11 addition in the FGD gypsum displacing weakly acidic organic groups or Fe 31 into the soil solution, contri-
buting to the conversion of resin and bicarbonate IP to Fe- and Al-bound IP (Stout et al. 1998). The Klinesville soil had the most exchangeable acidity (Table 1), thus the largest shift of resin and NaHCO 3 IP was to NaOH IP (Figure 2). In contrast, the Henlopen and Watson soils had only about half of the exchangeable acidity of the Klinesville (Table 1). Consequently, most of the IP transformations after adding FGD gypsum to the Henlopen and Watson soils were from resin and NaHCO 3 IP to HCl extractable IP (Cabound P) (Figure 2). Application of FGD gypsum had little or no effect on Mehlich-3 extractable P in any of the three soils studied (Figure 3). The only significant decline (P, 0.05) in Mehlich-3 extractable P induced by FGD gypsum application (ca. 20 mg P kg soil 21 ) occurred for the Henlopen soil. However, this decline was not so large as to have a negative effect on plant growth, as will be shown later. The declines in Mehlich-3 extractable P in Klinesville and Watson soils were not significant (P.0.05). This can be attributed to the extremely high P concentrations of these two soils, especially in resin and NaHCO 3 IP fractions (Figure 2). For instance, the sum of these two fractions in the FGD gypsum treated Klinesville (221 mg P kg soil 21 ) and Watson (301 mg P kg soil 21 ) was 267 and 364% greater than that of the FGD gypsum treated Henlopen soil (83 mg P kg soil 21 ). The difference in response to FGD gypsum application between water-extractable P and Mehlich-3 P is because water extracts P largely from the resin pool, whereas Mehlich-3 can extract P from all of the IP pools defined by the Hedley fractionation. In addition to changes in soil P fractions, the only major soil effect that FGD gypsum application had was increasing the Mehlich-3 extractable Ca concentrations by 3497, 2171 and 1827 mg Ca kg soil 21 in the Henlopen, Klinesville and Watson soils, respectively. Changes in soil pH were small and inconsistent, with the FGD gypsum application increasing pH by 0.2 in the Henlopen soil and decreasing pH by 0.1 in the other two soils. Plant uptake of P
Figure 1. Effect of FGD gypsum treatment and biologic extraction by ryegrass on water-extractable P in three soils.
First growth cycle The largest changes in all of the measured soil and plant parameters occurred during the first growth cycle. The effect of FGD gypsum application on water-extractable P was much lower in the first cycle
25
Figure 2. Effect of FGD gypsum treatment and biologic extraction by ryegrass on Hedley inorganic P fractions in three soils.
of plant growth (Figure 1). At this time, differences in water-extractable P levels between the control and FGD gypsum treatments were decreased by 85, 83,
and 89% in the Henlopen, Klinesville, and Watson soils, respectively. Despite these large reductions, water-extractable P concentrations of the control soils
26 Table 1. Selected chemical properties of the soils. Soil
Ca
Mg
K
Mehlich-3 P
CEC
21
(mg kg ) Henlopen Klinesville Watson
360 1064 2316
61 172 155
60 293 219
were significantly greater (P,0.05) than in the FGD gypsum treatment for all soils. Because water-extractable P was greater in the
Figure 3. Effect of FGD gypsum treatment and biologic extraction by ryegrass on Mehlich-3 extractable P in three soils.
Acidity
pH
21
(cmol kg ) 228 367 326
4.5 11.4 15.4
2.0 3.9 2.0
4.9 5.1 6.5
control than in the FGD gypsum treatment, more water-extractable P was removed by the plants from the control than from the FGD gypsum treatment during the first growth cycle (Figure 1). Growing plants caused depletion of 14.9, 20.7, and 27.0 mg P kg soil 21 from the water soluble P fraction of the control of Henlopen, Klinesville, and Watson soils, respectively. In contrast, only 5.24, 13.2, and 8.79 mg P kg soil 21 were depleted from the water soluble P fraction of the FGD gypsum amendments of these three soils, respectively. The decline in water-extractable P after the first harvest was reflected in a decrease in the most bioavailable fraction of the Hedley extraction, resin IP (Figure 2). As in the case of water-extractable P, resin IP was higher in the control than in the FGD gypsum treatment, and thus, more P was available for plant uptake from this fraction than in the control. The plants extracted 20.9, 87.6, and 143 mg P kg soil 21 from the control treatment of Henlopen, Klinesville, and Watson soils, respectively. In the FGD gypsum treatment, ryegrass extracted only about half of the resin IP compared to the control, with 6.01, 36.8, and 71 mg P kg soil 21 being extracted for the FGD gypsum treatment of Henlopen, Klinesville, and Watson soils, respectively. During the first growth cycle, the growing ryegrass depleted about 60, 360, and 120 mg P soil kg 21 Mehlich-3 extractable P from the Henlopen, Klinesville, and Watson soils, respectively (Figure 3). Because the FGD gypsum treatment had little effect on Mehlich-3 extractable P, growing ryegrass depleted about the same amount of Mehlich-3 extractable P from each treatment within soils. Cumulative P accumulation in ryegrass top growth is presented in mg P kg soil 21 to facilitate comparison to soil extraction data (Table 2). Treatment with FGD gypsum had no significant effect (P.0.05) on P accumulation in ryegrass top growth or stubble and roots. During the first growth cycle, ryegrass top growth accumulated significantly more P from the Watson soil than from the Henlopen and Klinesville soils. In addition, significantly more P was accumu-
27 lated in the stubble and roots from the Watson (147 mg P kg soil 21 ) than Henlopen (56.2 mg P kg soil 21 ) or Klinesville (64.4 mg P kg soil 21 ) soils. The total (top growth 1 stubble and roots) P accumulation for the first growth cycle was significantly higher (P, 0.05) for the Watson (239 mg P kg soil 21 ) than Henlopen (97 mg P kg soil 21 ) or Klinesville soils (103 mg P kg soil 21 ). Assuming that ryegrass would remove 12 mg P kg soil 21 year 21 (24 kg P ha 21 ) (Beegle 1996), these amounts of biologically extracted P would represent about 20, 8, and 8 years of cropping on these soils, respectively. This would indicate that treatment with FGD gypsum has the potential to decrease water-extractable P, and consequently the potential for P export in surface runoff, for several years. However, field experimentation would be required for verification.
pH and Ca levels in the other two soils would support the retention of IP in the NaOH fraction (i.e., amorphous and crystalline Al and Fe phosphates). In the second and third growth cycles, Mehlich-3 P continued to decrease, but at a slower rate than in the first growth cycle (Figure 3). This was especially the case for the Klinesville soil, where the main IP fraction was NaOH extractable and, thus, would have low plant availability. This is illustrated by the fact that cumulative P uptake was significantly greater in the Watson soil, where NaOH IP was not dominant (Table 2). The experiment was stopped after the third growth cycle when plant growth rate, plant P concentration, and subsequent changes in cumulative P uptake were lower in both treatments in all soils (Tables 2–4). These plant parameters started to decrease in the control treatments in the second growth period for all soils, when P concentrations in the ryegrass top growth were below the critical plant level of 2800– 3400 mg kg 21 (Martin and Matocha 1973), despite seemingly adequate soil P levels (Table 4). This may have been caused by insufficient soil Ca levels, as plant P concentrations in the FGD gypsum treatment (which supplied considerable Ca) were within or above this critical range. Indeed, Ca deficiency has been shown to prevent development of apical tips of roots, which would inhibit plant uptake of P (Miller and Heichel 1995). It is interesting to note that while FGD gypsum application decreased water soluble P, plant P concentration increased with FGD gypsum application (Table 3). There were two reasons for this. First, the water extractable P fraction is only a very small part of the plant available P in the soil, thus the effect of water-extractable P reduction on plant availability is minimal. Second, the previously mentioned effect of
Second and third growth cycles After the second and third growth cycles, water-extractable P remained constant in the Henlopen and Klinesville soils and continued to decrease slightly in the Watson soil (Figure 1). As with the first growth cycle, water-extractable P was reflective of resin IP, which remained relatively constant in the Henlopen and Klinesville soils and continued to decline in the Watson soil (Figure 2). In the Henlopen and Klinesville soils, the other IP fractions (NaHCO 3 , NaOH, and HCl) generally decreased, but the relative amounts within each soil remained constant. In the Watson soil, however, total amounts in each IP fraction decreased with additional biological extractions, but there was a general shift from NaHCO 3 to HCl extractable IP. This was probably due to the fact that pH and Ca levels of the Watson soil (Table 1) were high enough to support the gradual formation of calcium phosphates in this soil. In contrast, the lower
Table 2. Cumulative phosphorus uptake by and tops from soils treated with FGD gypsum. Soil
Treatment
Henlopen
Control FGD Mean Control FGD Mean Control FGD Mean
mg P uptake (kg soil 21 ) at harvest 1
Klinesville
Watson
40.6 41.3 40.9b* 47.7 49.0 48.4b 89.7 85.9 87.8a
2 52.0 64.5 58.3c 64.9 85.1 75.0b 119.0 144.0 131a
*Soil means in the same column followed by the same letter are not significantly different (P.0.05).
3 58.3 70.9 64.6c 78.3 97.9 88.1b 137.0 157.0 147a
28 Table 3. Dry matter yield of ryegrass tops from 100 g aliquots of soils treated with FGD gypsum. Soil
Henlopen
Klinesville
Watson
Treatment
Control FGD Mean Control FGD Mean Control FGD Mean
Yield (mg) at harvest 1
2
3
Mean
1018 932 975a* 1025 1058 1041a 1325 1335 1330a
698 705 701b 882 935 909b 1155 1108 1131b
513 492 503c 810 818 814b 1010 1002 1006c
742a** 710a 906a 937a 1163a 1148a
*Soil means in the same row or column followed by the same letter are not significantly different (P.0.05). **Treatment means within soils in the same row or column followed by the same letter are not significantly different (P.0.05).
Table 4. Phosphorus concentration in ryegrass tops from 100 g aliquots of soils treated with 1 g of FGD gypsum. Soil
Henlopen
Klinesville
Watson
P concentration (mg kg 21 ) at harvest
Treatment
Control FGD Mean Control FGD Mean Control FGD Mean
1
2
3
Mean
3981 4455 4218a* 4643 4624 4634a 6764 6430 6597a
1626 3299 2462b 1934 3881 2907b 2557 5233 3895b
1235 1282 1258c 1649 1566 1607c 1734 1313 1524c
2280b** 3012a 2742b 3357a 3685b 4325a
*Soil means in the same row or column followed by the same letter are not significantly different (P.0.05). **Treatment means within soils in the same row or column followed by the same letter are not significantly different (P.0.05).
increased soil calcium on root tip development would have increased P uptake. This is especially evident during the second growth cycle, where the difference in plant P concentration between the treatments was the greatest.
Conclusions During the plant first growth cycle, the shift of IP from the resin IP to less available fractions caused by the FGD gypsum treatment, tended to remain stable. Sequential plant growth cycles continued to decrease the most biologically available fractions and, thus, have a continued effect on decreasing water-extractable P. The results of this study indicate that treatment of high P soils (levels in excess of crop needs) with FGD gypsum would decrease water-extractable P, and consequently the potential for P export in surface runoff, for several years.
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