June 9, 2007
Soil Micronutrient Availability After Compost Addition to St. Augustine Grass
By Wright, Alan L Provin, Tony L; Hons, Frank M; Zuberer, David A; White, Richard H
Compost application to turf grasses can increase dissolved organic matter and nutrient levels in soil but may also enhance leaching and runoff losses. The objectives of this study were to determine the influence of composts on soil organic matter and accumulation of DTPA- and water-extractable Mn, Fe, Cu, and Zn in St. Augustine Grass [Stenotaphrum secundatum (Walt.) Kuntze] turf. Composts increased soil organic C (SOC) soon after application, but no further increases occurred beyond 11 months. In contrast, dissolved organic C (DOC) increased from 3 to 29 months after application, indicating contributions from decomposition of composts and St. Augustine Grass residues. Dissolved organic C was 75, 78, and 101% greater 29 months after application of 0, 80, and 160 Mg ha^sup -1^ of compost, respectively, than before application. While DTPA-extractable Mn and Cu increased from 0 to 29 months, Fe and Zn decreased and were often below background levels. By 29 months, DTPA- extractable Mn and Cu for soil receiving 160 Mg ha^sup -1^ increased 291 and 3972%, while Fe and Zn decreased 8 and 13%, respectively. Furthermore, water-extractable Mn, Fe, Cu, and Zn decreased 67, 78, 75, and 22%, respectively, by 29 months for soils receiving 160 Mg ha^sup -1^ of compost. Thus, only DTPA-extractable Mn and Cu accumulated in surface soils receiving composts. Soil pH had more influence on water-extractable than DTPA-extractable micronutrients, as water-extractable micronutrient concentrations decreased while soil pH increased after compost application. Soil organic C, DOC, and DTPA-extractable micronutrients exhibited considerable seasonal variation. Decreases in SOC, DOC, and DTPA-extractable Mn and Cu occurred during winter dormancy after high levels of precipitation. The formation of DOC-micronutrient complexes and leaching following precipitation events likely explain seasonal fluctuations in micronutrient concentrations. Similar trends in micronutrient concentrations were observed for both compost-amended and unamended soils, indicating that seasonal variation was more directly related to growth stages of St. Augustine Grass, precipitation, and subsequent effects on DOC, than compost application. In fact, composts only influenced the magnitude of micronutrient response to application. IntroductionLandscape wastes and municipal biosolids are a current problem because options for utilization are limited. Considerable research has been conducted on the fate of nutrients in compost-amended agricultural soils, but limited information is available on the utilization of turfgrass for application of compost and subsequent effects on soil organic matter and nutrient dynamics. Many factors influence organic matter and nutrient dynamics in soil, such as pH, vegetation, and management factors such as tillage and organic amendments (McDowell 2003). Soil organic matter and DOC are often major factors influencing micronutrient dynamics in soils. Organic amendments may improve soil properties for years after application (Ginting et al. 2003), as only small fractions are initially degraded and made available to plants and soil microorganisms (Hadas et al. 1996). High waste application rates may induce increases in SOC and DOC for years (Chantigny et al. 2002; Chantigny 2003), however, if wastes are rapidly decomposed in soil, DOC quickly returnsto background levels (Franchini et al. 2001). Seasonal dynamics of DOC are related to the growth stages of plants and decomposition of soil organic matter and compost (Chantigny 2003), and the decomposition by-products of root exudates and microbial metabolites (Marschner and Kalbitz 2003). Dissolved organic matter is often considered refractory, but mobile in soils (Quails and Haines 1992). Therefore, the fate of DOC and nutrients in compostamended soils is important, as off-site movement has the potential to impact adjacent terrestrial or aquatic ecosystems (Jacinthe et al. 2004).
Accumulation of metals and nutrients in compostamended soils may pose environmental hazards. Composts tend to increase leaching risks (Ashworth and Alloway 2004) as ions complexed with dissolved organic matter move readily through soil (Kaschl et al. 2002). Dissolved organic matter may coat soil particle surfaces which reduces their ability to bind micronutrient cations, thereby enhancing leaching potential (McCracken et al. 2002; Quails and Haines 1992). Micronutrient dynamics in soil are related to the quantity and nature of soil minerals and organic matter (Stevenson and Cole 1999), therefore, different extraction methods may account for the degree of micronutrient availability (van Raij 1994). The use of diethylene triamine pentaacetic acid (DTPA) for extraction of plantavailable micronutrients is common for near-neutral and alkaline soils (Lindsay and Norvall 1978), while water soluble micronutrients represent the most readily bioavailable chemical form (Linehan et al. 1985). The DTPA extracts exchangeable cations from labile pools (Rule and Graham 1976), while water extracts reflect micronutrients associated with dissolved organic matter, and thus represent soluble micronutrients.
Turfgrasses are often intensively managed and capable of sequestering large amounts of nutrients, which decreases their potential for runoff and leaching (Gross et al. 1990; Vietor et al. 2002). Research pertaining to compost effects on long-term organic matter and micronutrient dynamics in turfgrass systems is needed. The fate of composts and their influence on nutrient accumulation in turfgrass soils could be assessed by measuring changes in soil properties and micronutrient concentrations. A multi-year study was initiated to determine the influence of compost source and application rate on the seasonal dynamics of SOC, DOC, and DTPA- and water-extractable micronutrients in St. Augustine Grass turf.
Materials and Methods
A field study was established at the Texas A&M University Turfgrass Field Laboratory at College Station, Texas in July 2001 on a Boonville fine sandy loam (fine, smectitic, thermic Chromic Vertic Albaqualfs). The site was previously under pasture prior to plugging of St. Augustine Grass [Stenotaphrum secundatum (Walt.) Kuntze]. Average annual rainfall is 980 mm and temperature is 20[degrees]C.
A completely randomized experiment with three compost sources and an unamended control, and two application rates was established on field plots (20 m^sup 2^ each) replicated four times. The compost sources were DilloDirt (City of Austin, Texas), Bryan compost (City of Bryan, Texas), and Nature's Way compost (Nature's Way Resources, Conroe, Texas) applied at 80 and 160 Mg ha^sup -1^, which were equivalent to a 2.5 and 5.0 cm depths. DilloDirt and Bryan Compost are cocomposted landscape wastes and municipal biosolids, while Nature's Way consists of composted landscape wastes with small amounts of manure. Unamended and compost-amended soil was chiseled to a depth of 35 cm and tilled to 15 cm prior to compost application. Composts were applied by rototilling into the top 15 cm of soil in July 2001. Unamended and compostamended plots were then plugged with St. Augustine Grass. Turfgrass received supplemental NH^sub 4^NO^sub 3^ at 72 kg N ha^sup -1^ in 2001 and 49 kg N ha^sup - 1^ in 2002 to aid in plant establishment, but no N was applied in 2003. Turfgrass was mowed to maintain a canopy height of 3.5 cm and clippings were returned. To minimize moisture stress, supplemental irrigation was provided at 12 mm d^sup -1^ for 60 d after plugging, followed by 12 mm every 3 d until November 2001. Thereafter, 6 mm of water was applied at approximately 3 d intervals during the growing season or upon onset of drought stress symptoms.
Soil Sampling and Analysis
Five soil cores (2.5-cm diam.) were taken from each plot to a depth of 15 cm and composited. Samples were taken in July 2001 before compost application, and in Oct. 2001, March 2002, June 2002, Nov. 2002, June 2003, and Dec. 2003, corresponding to 3,8,11,16, 23, and 29 months after application of compost and plugging of St. Augustine Grass. Total monthly precipitation during the 29 months after compost application is presented in Figure 1, as well as the active growth and dormant periods of St. Augustine Grass. Soil sampling events encompassed both the growing season and dormant periods, in addition to before and after rainfall events.
FIGURE 1. Monthly precipitation for the 29 months after compost application in July 2001. The growing season and dormant periods for St. Augustine Grass are noted. Soil sampling occurred at 0, 3, 8, 11,16, 23, and 29 months after compost application.
Soil was dried at 65[degrees]C and passed through a 2-mm sieve. Soil organic C was measured by automated dry combustion using an Elementar VarioMax CN analyzer (Elementar Americas, Inc., Mt. Laurel, NJ). For DOC measurement, 7 g of soil were shaken with 28 mL of distilled water for 1 hr, followed by centrifugation and filtration through 0.45-mm filters (Wright et al. 2005). Extracts were analyzed for DOC by persulfate oxidation using a Model 700 Total Organic Carbon Analyzer (O.I. Analytical, College Station, Texas) and for water-extracu, and Zn by ICP-AES (Spectro Analytical Instruments, Marlborough, Massachusetts). Exchangeable Mn, Fe, Cu, and Zn were extracted with DTPA (Lindsay and Norvell 1978) and analyzed by ICP-AES (Franson 1989). Soil pH was measured using a 1:2 soil:water ratio after equilibration for 30 min (Schofield and Taylor 1955). Characterization data for the Boonville soil and compost sources are presented in Table 1. TABLE 1.
Soil pH, soil organic C, and DTPA-extractable micronutrient concentrations for compost sources and turfgrass soils prior to compost application.
Data were analyzed with CoStat (CoStat Statistical Software 2003), while ANOVA was used to determine overall differences between compost source, rate, and sampling time. ANOVA was used for determination of significant differences between individual treatments for each sampling time after compost application. No differences in soil properties were observed between compost sources, so data were averaged for analysis and presentation in figures. Separation of means was accomplished using LSD at P
Results and Discussion
Soil pH did not vary between compost application rates (Figure 2), but was significantly higher for unamended (7.9) than composted- amended soil (7.7) from 8 to 29 months. Thus, composts tended to buffer the soil to changes in pH. Irrigation was provided in large quantities soon after plugging to aid in plant establishment and during periods of drought. The irrigation water, containing approximately 270 mg L^sup -1^ Na, may have been responsible for soil pH increases after compost application. Moreover, addition of composts can cause adsorption of dissolved organic matter to soil particle surfaces or coat soil particles (Kalbitz et al. 2000), reducing the number of exchange sites capable of reacting with basic cations from irrigation water, which may also explain the lower pH for amended than unamended soil.
FIGURE 2. Soil pH, soil organic C (SOC), dissolved organic C (DOC), and the percentage of SOC as DOC under St. Augustine Grass up to 29 months after application of compost at 0,80, and 160 Mg ha^sup -1^. Bars represent the standard error of the mean.
Soil Organic and Dissolved Organic C
Soil organic C was significantly higher at 160 Mg ha^sup -1^ (20.3 g C kg^sup -1^) than at 80 Mg ha^sup -1^ (16.3 g C kg^sup - 1^) and unamended soil (13.0 g C kg^sup -1^) (Figure 2). Significant effects of application rate were observed at all sampling times from 3 to 29 months after compost addition. A significant decline in SOC at 16 months was related to a heavy rainfall event in the preceding month (Figure 1).
For unamended and compost-amended soils, DOC significantly increased up to 11 months after compost application, but declined at 16 months corresponding to similar declines in SOC (Figure 2), then increased to 29 months. Since no compost was added to unamended soil, the increases in DOC were attributed to C contribution from St. Augustine Grass. Application rate had less effect on DOC than SOC. Averaged across sampling times, composts applied at 160 Mg ha^sup -1^ resulted in 11% significantly higher DOC than at 80 Mg ha^sup -1^, and 12% higher DOC than unamended soil.
Dissolved organic C and SOC were significantly correlated (r = 0.69). Both SOC and DOC exhibited cyclical seasonal patterns after compost application, as evidenced by lower concentrations during the winter sampling time at 16 months. Soil organic C and DOC were significantly lower for all compost treatments and unamended soil at 16 months than at 11 and 23 months, suggesting that the decline was not compost related but rather influenced by the reduced growth of St. Augustine Grass during dormancy in winter, in addition to precipitation. Warm-season turfgrass growth is more vigorous from late spring to summer, with a dormant period from late autumn to winter (Trenholm et al. 2000). Turfgrasses often produce high levels of belowground biomass (Trenholm et al. 2000) which ultimately increase SOC levels. Moreover, the decomposition by-products of below-ground biomass and aboveground clippings likely contributed to SOC and DOC. Since St. Augustine Grass production levels are lower in cooler than warmer months, a lower contribution of turfgrass residues potentially decreased DOC at 16 months.
Dissolved organic C was a significantly greater contributor to SOC for unamended than compost-amended soil (Figure 2). In fact, the higher the compost application rate, the lower the percentage contribution of DOC to SOC. The percentage of SOC as DOC increased 37% from 0 to 29 months for unamended soil, and also significantly increased from 0 to 29 months for composted-amended soil. These results were most likely due to the effects of St. Augustine Grass growth rather than compost application, as the greatest effects occurred for unamended soil.
Compost application rates significantly increased DTPA- extractable Mn up to 29 months after application, but concentrations varied seasonally (Figure 3). Increasing the compost application rate increased Mn concentrations, which averaged 193% higher at 29 months than before compost application. For all treatments, Mn increased after compost application to 11 months, but no further increases occurred. A significant decrease in Mn for compost- amended and unamended soils at 16 compared to 11 and 23 months occurred at the winter sampling time, which coincided with a similar decrease in SOC and DOC (Figure 2). Manganese was also significantly correlated with SOC (r = 0.92) and DOC (r = 0.75).
Water-extractable Mn exhibited few treatment effects and had different seasonal trends than DTPA-extractable Mn. Water- extractable Mn was 18% higher for unamended than compost-amended soil, and significantly decreased from 1.6 mg kg^sup -1^ before application to 0.4 mg kg^sup -1^ by 8 months. In contrast to DTPA- extractable Mn, water-extractable Mn was significantly negatively related to SOC (r = -0.48) and DOC (r = -0.53).
Seasonality significantly influenced DTPA-extractable Cu, as concentrations declined to background levels during winter (Figure 3). Dynamics of DTPA-extractable Cu were similar to Mn, with significant increases up to 11 months after compost application, a decline at 16 months, and then an increase to 23 months. Copper forms stable bonds with dissolved organic matter that increase its leaching potential (Ashworth and Alloway 2004). The bonding of Cu with DOC, and subsequent leaching beyond the soil surface after high levels of precipitation, would explain the significant decrease of DTPA-extractable Cu and Mn at 16 months, which coincided with a similar decrease in DOC (Figure 2). Copper leaching was also greater than other metals in sludge-amended soils due to its association with dissolved organic matter (Darmody et al. 1983). In contrast to Mn, however, compost application rate had no effect on DTPA- extractable Cu. Copper was significantly correlated with DOC (r = 0.71), SOC (r = 0.72), and DTPA-extractable Mn (r = 0.81).
FIGURE 3. Seasonal dynamics of DTPA- and water-extractable Mn and Cu for St. Augustine Grass after compost application at 0,80, and 160 Mg ha^sup -1^. Bars represent the standard error of the mean.
In contrast to DTPA-extractable Cu, water-extractable Cu was affected by compost application rate, and at 160 Mg ha^sup -1^ was 53 and 65% greater than at 80 Mg ha^sup -1^ and unamended soil, respectively. In contrast to other water-extractable micronutrients, which significantly decreased after application, Cu increased for each sampling period from 8 to 23 months. Water-extractable Cu was significantly correlated with SOC (r = 0.39) and DOC (r = 0.55).
In contrast to DTPA-extractable Mn and Cu, Fe decreased by 3 months after compost application, and remained below background levels except for the spike occurring at 16 months (Figure 4), at the same time DOC and DTPA-extractable Mn and Cu declined (Figure 3). These seasonal trends occurred for both compost-amended and unamended soil, thus seasonal changes were not likely related to compost but rather to growth stages of St. Augustine Grass. Even though Fe declined to below background levels, concentrations were still significantly greater for compost-amended than unamended soil. DTPA-extractable Fe at 160 Mg ha^sup -1^ was 30 and 67% higher than at 80 Mg ha^sup -1^ and unamended soil, respectively. DTPA- extractable Fe was also significantly correlated with soil pH (r = - 0.56) and DTPA-extractable Cu (r = -0.63).
The seasonal dynamics of water-extractable Fe differed considerably from DTPA-extractable Fe. Whereas DTPA-extractable Mn and Fe exhibited opposite trends, water-extractable Mn and Fe seasonal dynamics were similar. Water-extractable Fe decreased rapidly after compost application and remained constant at approximately 50 mg kg^sup -1^. However, similar to water- extractable Mn and in contrast to DTPA-extractable Fe, concentrations were significantly higher for unamended (62 mg kg^sup -1^) than compost-amended soil (46 mg kg^sup -1^). Water- extractable Fe was significantly correlated with SOC (r = -0.60), DOC (r = -0.61), and water-extractable Mn (r = 0.97).
DTPA-extractable Zn dynamics in soil were similar to those of Fe (Figure 4). As with Fe, Zn decreased by 3 months, significantly increased at 16 months, and declined at 23 months. Similar to Fe, Zn significantly increased with increasing application rate. Zinc commonly decreased to below background levels for both compost- amended and unamended soil, suggesting assimilation by plant biomass or adsorption to soil, since Zn is considered relatively immobile in high pH sludge-amended soils (Smith et al 1999). Thus, as with Fe, Zn dynamics appeared to be related to growth stages of St. Augustine Grass. DTPA-extractable Zn was related to soil pH (r = -0.51) and DTPA-extractable Fe (r = 0.95).
FIGURE 4. Seasonal dynamics of DTPA- and water-extractable Fe and Zn for St. Augustine Grass after compost application at 0,80, and 160 Mg ha^sup -1^. Bars represent the standard error of the mean. Seasonal dynamics of water-extractable Zn differed from DTPA- extractable Zn but exhibited similar trends to water-extractable Fe and Mn. Furthermore, water-extractable Zn was highest at 160 Mg ha^sup -1^, and was significantly correlated with water-extractable Mn (r = 0.74), Fe (r = 0.63), and Cu (r = 0.54).
Impacts of St. Augustine Grass, Compost, and DOC on Micronutrients
Most micronutrient concentrations varied seasonally for compost- amended and unamended soils, suggesting that changes in extractable micronutrient concentrations were not compost related but rather influenced by the reduced growth or dormancy of St. Augustine Grass during winter, and the leaching of DOC-associated micronutrients by precipitation. Composts only influenced the magnitude of the responses of micronutrient concentrations to application, as similar seasonal trends were observed for both unamended and compost- amended soils.
Potential hazards associated with compost disposal on turfgrass include the accumulation of micronutrient cations in surface soil. Even though considerable seasonal variation in micronutrient levels existed, DTPA-extractable Mn and Cu showed increasing trends after compost application, while DTPA-extractable Fe and Zn and all water- extractable micronutrients decreased after compost application. By 29 months, DTPA-extractable Mn and Cu for soil receiving the highest application of 160 Mg ha^sup -1^ increased by 291 and 3972%, while Fe and Zn decreased by 8 and 13%, respectively. Furthermore, water- extractable Mn, Fe, Cu, and Zn decreased by 67, 78, 75, and 22%, respectively, by 29 months for soil receiving 160 Mg ha^sup -1^ of compost. Thus, only DTPA-extractable Mn and Cu accumulated in surface soils receiving composts.
Elevated micronutrients concentrations were observed after prolonged dry periods, while concentrations were generally lowest after periods of heavy rainfall. Complexation of micronutrients by organic matter (Cancela et al. 2002) may explain seasonal micronutrient dynamics in St. Augustine Grass turf. Organic amendments can mobilize Cu adsorbed to soil (McBride et al. 1997) by bonding with dissolved organic matter (Stevenson and Cole 1999). In the high pH sandy loam soil in this study, formation of dissolved organic matter-cation complexes may explain DTPA-extractable Mn and Cu seasonal variation. Dissolved organic C, Mn, and Cu exhibited similar seasonal trends, including a general increase over time and a significant decrease at 16 months, suggesting leaching losses of DOC-associated Mn and Cu were caused by high precipitation levels preceding the 16month sampling. High precipitation levels preceding other sampling times did not result in losses of extractable Mn and Cu, as these events generally occurred during the growing season when micronutrients were assimilated by St. Augustine Grass. Decreases in DOC and DTPA-extractable Mn and Cu did not occur during dormant periods in 2001 or 2003 (8 and 29 months after application), as occurred in 2002, due to low precipitation. Thus, potential leaching losses of DOC-associated micronutrients may occur most readily during dormant turf grass growth periods and high precipitation.
Since micronutrient cations are seldom free in solution at high concentrations due to adsorption to soil particles and organic matter (Stevenson and Cole 1999), water-extractable micronutrients generally bind with dissolved organic matter (Seguin et al. 2004). Therefore, soluble or water-extractable micronutrients are more readily leachable than DTPA-extractable micronutrients, which are associated with soil colloids and organic matter (Cancela et al. 2002). Likewise, DTPA extracts generally had higher concentrations than water extracts (Figs. 3 and 4). After compost application, all water-extractable micronutrients except Cu showed significant declining trends that did not follow DOC trends. In fact, water- extractable micronutrients were often negatively correlated with DOC.
Micronutrient concentrations often decrease as pH increases (Stevenson and Cole 1999). Soil pH appeared to have more influence on water soluble micronutrients than DTPA-extractable micronutrients. Sorption of DOC and formation of DOC-micronutrient complexes may be pH dependent (Temminghoff et al. 1997). As soil pH increases above neutrality, DOC sorption to soil becomes weaker and potential for formation of DOC-cation complexes increases (Romkens et al. 1996). Thus, for the high pH soils of this study, composts likely increased the binding of DOC and cations, which made micronutrients more soluble and susceptible to leaching. Significant negative relationships were observed between soil pH and water- extractable Mn (r = -0.50), Cu (r = -0.49), Fe (r = -0.37), and Zn (r = -0.64), and for DTPA-extractable Fe (r = - 0.56) and Zn (r = - 0.51). Soil pH increases as a result of irrigation with saline water may be partially responsible for decreasing water-extractable micronutrient levels in soil.
In contrast to Mn and Cu, DTPA-extractable Fe and Zn did not appear to form soluble or extractable complexes with DOC, and in fact were inversely related to DOC. Extractable Fe and Zn likely formed insoluble precipitates or complexes with soil organic matter, which decreased their availability to plants and the potential to be extracted with the methods used in this study, and may explain their poor and negative correlation with DOC. Uptake of DTPA-extractable Fe and Zn by St. Augustine Grass during the growing season, followed by deposition and decomposition of plant residues during dormancy in winter, would explain DTPA-extractable Fe and Zn accumulation at 16 months.
A one-time compost application increased DTPA-extractable Mn and Cu by 29 months, with the exception of decreases during plant dormancy in winter. All water-extractable micronutrients and DTPA- extractable Fe and Zn decreased from O to 29 months after compost application. Complexation of DTPA-extractable Mn and Cu with DOC may explain their seasonal variability, as DOC decreases coincided with Mn and Cu decreases following precipitation events during winter dormancy. Dynamics of DTPA-extractable Fe and Zn were different from Mn and Cu, as Fe and Zn increased during winter dormancy, suggesting that Fe and Zn were adsorbed to soil particle surfaces rather than dissolved organic matter. Similar seasonal trends in micronutrient concentrations were observed for both compost-amended and unamended soils, indicating that micronutrient dynamics were more related to growth stages of St. Augustine Grass, and subsequent effects on DOC, than compost application. In fact, compost application only increased the magnitude of the response of DTPA-extractable micronutrients. While DTPA-extractable micronutrients were related to DOC, precipitation, and growth stages of St. Augustine Grass, water-extractable micronutrient concentrations were more directly related to soil pH. Water-extractable micronutrient concentrations decreased to below background levels soon after compost addition and were higher for unamended than compost-amended soils, suggesting the formation of insoluble organic matter-micronutrient complexes promoted by high rates of compost application.
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Alan L. Wright1, Tony L. Provin2, Frank M. Hons2, David A. Zuberer2 and Richard H. White2
1. Everglades Research & Education Center, University of Florida, Belle Glade, Florida
2. Soil and Crop Sciences Dept., Texas A&M University, College Station, Texas
Copyright J.G. Press Inc. Spring 2007
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