3.1. Total Trace Elements in Soils
Total concentrations of As, Cu, and Zn in PL applied to the pasture on April 2008, November 2008, and March 2009 varied with the specific application (Table 1
). These variations are due to differences in composition of poultry feeds and PL management practices. Trace element concentrations of As, Cu and Zn were low in all PL samples compared to pollutant limit metal concentrations for land application of biosolids (41 mg As kg−1
, 1,500 mg Cu kg−1
, and 2,800 mg Zn kg−1
shows the physical and chemical properties of both PL-amended soils. The clay content and total amounts of Fe oxide of amended Cecil soil were greater at both depths compared to those of Sedgefield soil. Hence, it may be presumed that higher Fe content of Cecil soil will increase the soil As retention capacity. Sarkar et al.
] reported that soils with higher concentrations of amorphous Fe/Al oxides retain more As and thus reduce its bioavailability. Rutherford et al.
] found a strong correlation between acid-extractable Fe and As content of PL-amended soils, suggesting adsorption of As with Fe oxides.
There were differences in total concentrations of As, Cu, and Zn in PL-amended soils compared to those in control plot at 0–2.5 cm and 2.5–7.5 cm depths (Table 3
). Total concentration of As was greater in PL-amended Cecil and Sedgefield soils (3.67 mg kg−1
and 3.91 mg kg−1
) compared to those in control plots (1.46 mg kg−1
) at 0–2.5 cm depth. Similar results were found for total As concentrations in both PL-amended soils compared to those in control plots at 2.5–7.5 cm depth. Han et al.
] also found significant amounts of As in PL-amended soils with a long-term history of PL application (25 years) compared to unamended soil, and total As concentrations in the amended soils were three times greater than those in unamended soils (8.4 vs.
2.68 mg kg−1
). The concentrations of As in Cecil and Sedgefield soils were not different compared with each other at either depth. Total Cu and Zn were greater in both PL-amended soils compared to those in control plots at the 0–2.5 cm depth, although not at the lower depth (Table 3
). Han et al.
] reported concentrations of Cu and Zn in unamended soil (0–10 cm) were significantly lower (2.2 and 9.8 mg kg−1
) compared to PL-amended soil (74.5 and 88.8 mg kg−1
). Brock et al.
] also showed accumulation of Cu and Zn in soils with a history of 40 years of PL application in the plow layer (0–17.5 cm). The total Cu concentrations ranged from 5.9 to 30.1 mg Cu kg−1
soil, and total concentrations of Zn were up to 112 mg Zn kg−1
soil. Other studies reported elevation of Cu and Zn concentration in the top 5–10 cm depth in short and long-term application of PL [27
]. Litter applications for 15 to 25 yr resulted in accumulation of Cu and Zn to a depth of 40 cm compared with unamended soil [5
The observed distribution of trace elements is clearly due to the surface application of the PL and lack of incorporation into the A horizon. However, concentrations of trace elements were well below USEPA loading limits at both depths. For an application rate of 5 Mg ha−1
per year with an average As concentration of 20 mg kg−1
in PL applied on the soil surface, the average As input rate was approximately 0.1 kg As ha−1
. This annual As loading rate is below the annual ceiling rate (2.0 kg As ha−1
) for safe land application of biosolids [10
]. The same estimation can be done for Cu and Zn with average concentrations of 500 mg Cu kg−1
and 400 mg Zn kg−1
in PL applied at a rate of 5 Mg ha−1
per year. The Cu and Zn input rates were approximately 2.5 kg Cu ha−1
and 2.0 kg Zn ha−1
which are below the annual ceiling rates (75 kg Cu ha−1
and 140 kg Zn ha−1
) for safe land application of biosolids.
3.3. Plant Uptake by Forage
The results of statistical analysis by ANOVA of trace elements concentrations in forage tissue from PL-amended soil and control showed that there was a significant main treatment effect of P, Cu, Zn, and As concentrations in forage tissues from control and PL-amended soils (Table 5
). However, no significant interaction was found between treatment and date effect for any trace elements or P (October and June sampling). There were differences in concentrations of all trace elements in forage tissue from PL-amended soils compared to those in control plot except for amount of Zn in forage tissue from Cecil soil (Table 6
). Another study of metal uptake by tall fescue as affected by PL application at low (5.6 t ha−1
) and high (11.3 t ha−1
) rates showed that Zn and Cu uptake were significantly greater in PL-amended soil compared to the control [30
]. However, Kingery et al.
] showed that tall fescue grown in pastures with a history of long-term litter application had similar Cu and Zn concentrations as fescue grown in pastures with no history of litter application. The concentration of As in forage from Sedgefield soil was significantly greater than Cecil soil. The coarse-textured soils likely contain higher amounts of readily mobile As, while As in fine textured soils is mainly immobile due to higher content of minerals and organic constituents which are capable of binding anionic As species. There were no differences in concentrations of Cu and Zn in forage tissues from Cecil soil compared to those from Sedgefield soil.
The concentrations of Cu and Zn in the forage samples did not exceed critical phytotoxic levels reported by Macnicol and Beckett [31
] (21–40, and 210–560 mg kg−1
for Cu, and Zn, respectively). Long-term PL application did not produce tissue Cu and Zn levels above the NRC (National Research Council) maximum tolerance level of Cu for cattle (40 mg Cu kg−1
feed) or for sheep (15 mg Cu kg−1
feed); similarly, Zn was well below maximum levels for cattle (500 mg Zn kg−1
feed) and sheep (300 mg Zn kg−1
]. The forage samples from all soils were in fact Cu deficient for animal feed based on the minimum recommended Cu concentration for cattle rations of 9 mg Cu kg−1
]. Soon et al.
] reported small increases in the Cu content of corn grain and bermudagrass grown in soil amended with sewage sludge applications (31 kg Cu ha−1
) for five consecutive years.
The minimum recommended Zn content in cattle rations average 35 mg kg−1
; forage samples from both PL-amended soils were close to that amount and considered Zn-sufficient for animal feed. Warman and Termeer [35
] found that Zn content of grass forage and corn tissues grown in soil amended with sewage wastes for two consecutive years (applied twice a year) were below the 35 mg kg−1
minimum Zn requirement for cattle. The maximum tolerance levels of As for cattle and sheep are 30 mg As kg−1
]. The data shown in Table 6
indicate that the As content of bermudagrass tissues grown in both PL-amended soils are below that amount. Since only a small fraction of total As was in the water-soluble fraction in PL-amended soils, PL had minor effect on elevation of total As in forage.
The result of Mehlich-I extraction, which is used to assess potentially plant available Zn in soil testing, showed greater concentrations of Cu and Zn in PL-amended Cecil and Sedgefield soils compared to those in control plot at both depths (Table 7
). Concentrations of extractable Zn were greater in PL-amended Sedgefield and Cecil soils (47.4 and 47.5 mg kg−1
) at 0–2.5 cm compared to those in 2.5–7.5 cm depth (10.45 and 11.29 mg kg−1
). Only Cu concentrations were greater in PL-amended Sedgefield soil at 0–2.5 cm (6.60 mg kg−1
) compared to that in 2.5–7.5 cm depth (4.60 mg kg−1
) at p
< 0.01. The fact that concentrations of extractable Cu and Zn were greater in PL-amended soils compared to those in control plot appear to be linked with the greater amounts of Cu and Zn in bermudagrass tissue from PL-amended soils and compared to that from control plots. However, soil test Zn and Cu increased much more than tissue levels with PL amendment. Zhou et al.
] reported significant linear correlation between extractable soil Zn (and Cu) concentrations extracted by 1.0 M
and Zn (and Cu) concentrations in radish tissues. Warman and Termeer [35
] found that only Mehlich-I Cu was highly correlated with forage tissue Cu (r
= 0.98), and the correlation coefficients for extractable Zn and Zn uptake were not significant in soil with biosolids applications.