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TECHNICAL REPORTS

Soil Carbon Sequestration Resulting from Long-Term Application of Biosolids for Land Reclamation

Environmental Monitoring and Research Division, Research and Development Dep., Metropolitan Water Reclamation District of Greater Chicago (MWRDGC), Lue-Hing R&D Complex, 6001 W. Pershing Rd., Cicero, IL 60804

^* Corresponding author (guanglong.tian{at}MWRD.ORG).

Received for publication September 5, 2007.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Investigations on the impact of application of biosolids for land reclamation on C sequestration in soil were conducted at Fulton County, Illinois, where 41 fields (3.6–66 ha) received biosolids at a cumulative loading rate from 455 to 1654 dry Mg ha^–1 for 8 to 23 yr in rotation from 1972 to 2004. The fields were cropped with corn, wheat, and sorghum and also with soybean and grass or fallowed. Soil organic carbon (SOC) increased rapidly with the application of biosolids, whereas it fluctuated slightly in fertilizer controls. The peak SOC in the 0- to 15-cm depth of biosolids-amended fields ranged from 4 to 7% and was greater at higher rates of biosolids. In fields where biosolids application ceased for 22 yr, SOC was still much higher than the initial levels. Over the 34-yr reclamation, the mean net soil C sequestration was 1.73 (0.54–3.05) Mg C ha^–1 yr^–1 in biosolids-amended fields as compared with –0.07 to 0.17 Mg C ha^–1 yr^–1 in fertilizer controls, demonstrating a high potential of soil C sequestration by the land application of biosolids. Soil C sequestration was significantly correlated with the biosolids application rate, and the equation can be expressed as y = 0.064x – 0.11, in which y is the annual net soil C sequestration (Mg C ha^–1 yr^–1), and x is annual biosolids application in dry weight (Mg ha^–1 yr^–1). Our results indicate that biosolids applications can turn Midwest Corn Belt soils from current C-neutral to C-sink. A method for calculating SOC stock under conditions in which surface soil layer depth and mass changes is also described.

Abbreviations: MWRDGC, Metropolitan Water Reclamation District of Greater Chicago • SOC, soil organic carbon • SOM, soil organic matter • WAS, waste-activated sludge • WRPs, water reclamation plants

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
STRIP-MINING causes loss of topsoil and destruction of soil structure, leading to low soil organic matter (SOM) and compaction (Peterson et al., 1979). Water quality problems also exist in watersheds of mined land (Soper, 1992). Reclamation of strip-mined land has the potential to increase the agricultural productivity of Illinois soils. Management is often needed for reclamation of strip-mined land because it might take centuries to develop a productive soil from mine spoils left to nature (Smith et al., 1971). To contribute to improving the productivity of the 16,400 ha of mine spoils in Fulton County, Illinois, representing 7.24% of total county land area (Haynes and Klimstra, 1975), the Metropolitan Water Reclamation District of Greater Chicago (MWRDGC) initiated the world's largest land reclamation project with the biosolids in 1972.

The safe and economic disposal of biosolids produced from the treatment of wastewater has been a challenge for municipal wastewater treatment agencies. Although incineration and landfilling of biosolids were used by many municipalities in the early 1970s, the MWRDGC believed that the utilization of the fertilizer value of municipal biosolids offers the best alternative to municipal wastewater treatment agencies. The process of utilization rather than disposal requires relatively small amounts of energy and offers to farmers a source of free or inexpensive fertilizer. Data from monitoring reported by Tian et al. (2006) indicated that the long-term application of high rates of biosolids produced before and after the promulgation of federal regulation had only a minor impact on surface water quality. Granato et al. (2004) noted that the transfer of trace metals from biosolids to corn grain was less than that predicted in the risk model of USEPA's 40 CFR Part 503 regulation, which was promulgated in 1993 to govern the land application of biosolids in the USA. With the increased economic restraints and environmental concerns about land-filling and incineration, interest in land application could continue to grow (O'Connor et al., 2005). Because of the high organic matter content, biosolids are ideal for reclaiming degraded soils by increasing organic matter and improving the structure.

The global SOC inventory is estimated to be 1200 to 1600 Pg, which is close to the combined amounts stored in terrestrial vegetation (550–700 Pg) and the atmosphere (750 Pg) (Post et al., 1990; Sundquist, 1993). Therefore, even a small percentage change in the SOC pool could easily affect the change in atmospheric CO₂. Increasing soil C reserves in agricultural and rangeland soils and restoring degraded soils to productivity have been considered as an important means to sequester C (Lal et al., 1998). Specifically, Jarecki and Lal (2003) recommended the use of organic waste materials as an important management practice that would help fill the large C sink in the world's agricultural soils. We hypothesize that the application of biosolids results in C sequestration in soil through increasing soil microbial biomass, an important SOM source, and that the amorphous Fe and Al oxides present in biosolids promote the humification of organic residues, also contributing to soil C sequestration.

Although there are many reports on the long-term change in SOM, most of the previous studies reported measurements usually at the beginning and at the end of the study period, lacking repeated measurements of results throughout the studies. Long-term estimates of soil C sequestration still rely almost exclusively on modeling (Jones and Donnelly, 2004). To fill some of the gaps in long-term dynamics of SOM with the addition of organic amendments and to quantify the soil C sequestration by biosolids land application, this report presents SOC concentration dynamics and C sequestration in soil that received biosolids for over 30 yr for reclaiming strip-mined land.

	Materials and Methods

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Study Site
The study site is located at Fulton County in western Illinois, approximately 300 km southwest of Chicago. The site has a continental climate, with an annual mean air temperature of 10.4°C and annual precipitation of 1013 mm. The monthly mean surface soil temperature and moisture ranged from 1 to 25°C and 13 to 27%, respectively (WARM, 2008). The Fulton county soil environment is influenced by an extended wet and cold winter; a short, wet, and cool spring; a dry, hot summer; and a moist, cool fall. As Gilmour and Gilmour (1980) suggested 20% as optimal soil moisture and 25°C as optimal soil temperature for biosolids decomposition, low temperature for most months of the year at Fulton County leads to slower biosolids decomposition, though the waterlogging in the colder months and moisture deficiency in warmer months are also unfavorable to microbial activity. Land in the study site was strip-mined in the early 1900s, resulting in some of the previously productive agriculture areas becoming roughly scarred wastelands. The pH of surface spoils was neutral to alkaline, and the texture varies mostly from silt loam to silty clay loam.

Treatment Establishment
In the 6000 ha of land comprising calcareous strip-mined and nonmined soil, acidic coal refuse materials, mine lakes, and wooded areas, approximately 1790 ha of calcareous strip-mined and nonmined low-productivity soil was developed into 80 fields, 58 of which were used for biosolids application. Among 58 fields for the project, 16 fields received only supernatant from liquid biosolids holding basins, and the amount of solids added (0.37–21.5 dry Mg ha^–1 for the period of 33 yr) through such applications was low enough to be negligible. One field was also excluded from this study because it received biosolids for only 4 yr. This study therefore included the 41 fields as listed in Table 1 . The fields were sized from 3.6 to 66 ha and received biosolids at cumulative loading rates of 455 to 1654 dry Mg ha^–1 for 8 to 23 yr in rotation from 1972 to 2004.

The control we selected for this study is the fields that received chemical fertilizer and had a cropping system similar to biosolids-amended fields. The biosolids-amended fields were cropped with rotations of cereal crops such as corn, wheat, and sorghum. Soybean, grass, and fallow were also used in these fields. The supernatant fields and two unamended fields (fields F24 and F38) were generally unfertilized and were used for grass, although some of these fields were cropped with corn or soybean during the most recent decade. Among all unamended fields, only two fields (F18 and F29) regularly received commercial fertilizer and had a cropping system similar to biosolids-amended fields; therefore, F18 and F29 served as controls. Field 29 (Lenzburg soil) received only one biosolids application at 1.21 Mg ha^–1 in 1979 and served as control for Group I. The nonmined half of F18 (F18-2) received only one biosolids application at 1.04 Mg ha^–1 in 1979 and was used as control for Group III. No field was suitable as a control for Group II, but we used F18-2 as a reference because the "fine" surface layer in Group II was mainly premined soil, as in F18-2.

Application of Biosolids
The biosolids applied to the fields were generated at the MWRDGC's Stickney and Calumet water reclamation plants (WRPs). The sewage at these treatment plants includes domestic and industrial wastewater at a ratio of 3:2 plus rainwater. Large objects and grits in the sewage are removed and sent to a landfill before the sewage proceeds to a sediment tank to obtain primary sludges and to an aeration tank for the waste-activated sludge (WAS). Primary sludges mainly consist of fecal solids, and WAS is the active biological material and is largely composed of saprophytic bacteria, protists, and filter-feeding species. The WAS and a small percentage of primary sludges were anaerobically digested at 35°C for at least 15 d to meet the minimum criteria as biosolids. Throughout the project, three types of biosolids (lagooned liquid, lagooned dewatered, and lagooned air-dried biosolids) were used. From 1971 to 1983, 730,000 Mg (dry weight) of anaerobically digested sludges were barged to Fulton County. Biosolids shipped from 1971 to 1981 had an average of 6% solids and came from digesters and lagoons. From 1981 to 1983, centrifuge-dewatered biosolids containing an average of 25% solids were delivered to Fulton County (Hall et al., 1985). At the Fulton County land reclamation site, biosolids were stored in three 12-m-deep holding basins before application. With a few months, materials in the holding basins partitioned into a supernatant (total solids = 0.3%) phase and thickened underlying slurry (total solids = 12%). Supernatant was barged back to water treatment plants in Chicago from 1972 to 1976 and applied to "supernatant fields" from 1976. A hydraulic dredge was used to resuspend the slurry in a holding basin and to deliver it to a pump facility for distribution to fields. This slurry contained 4% total solids and was referred to as "liquid biosolids." Because the liquid biosolids had been stored in holding basins for several months to years before application, more precisely it was the "lagooned liquid biosolids." As the water level was lowered during the application season, solids at the holding basin bottom were dug out and deposited on the holding basin walls. After air-drying, the solids were removed from the basin walls and applied to fields or stockpiled for additional drying over the winter months. This material contained an average of 47% solids and was referred to as "dewatered biosolids." Generally, the dewatered biosolids were stored in a holding basin longer than liquid biosolids. Air-dried biosolids were produced in Chicago using a low-cost technique. Anaerobically digested sludges were first stored in a lagoon-like holding basin for a minimum of 18 mo for further stabilization. After that, materials in the lagoon were dug out and air-dried with agitation on a paved bed to contain approximately 60 to 70% total solids. Liquid biosolids were applied from 1972 to 1985 primarily by incorporation discs, although a small portion was applied by a traveling sprinkler during the initial 4 yr. Dewatered biosolids were applied from 1980 to 1995 and air-dried biosolids from 1987 to 2004 by a manure spreader followed by discing incorporation (Skuse et al., 1991). The application of biosolids normally occurred from June to September in fields where crop was harvested in previous years or where wheat was just harvested. Some of the fields receiving biosolids were cropped in the same year; however, many of them were planted with a cover crop (mainly rye). The rye field was sprayed with herbicides and disced in the subsequent year for cropping. The annual biosolids application rates to various fields are shown in Table 2 . The MWRDGC's three decades of land reclamation project at Fulton County applied a cumulative total of 830,000 Mg biosolids to land.

Sampling and Analysis
Soil samples were collected at a depth of 0 to 15 cm each spring before the annual biosolids application. In collecting soil samples, each field was divided into two halves. In each half, about 20 to 40 cores, depending on field size, were taken to make one composite sample. The samples from the two halves were further composited into one sample and analyzed in duplicate. All analyses were controlled with check samples to ensure analyses were consistent for a period of three decades. The concentration of SOC was determined by the Walkley-Black method. A composite sample of biosolids applied to each field was collected for analysis. The total solids in biosolids were determined by drying the biosolids at 103 to 105°C. The volatile solids or organic matter in biosolids was estimated as loss in mass at 550°C (USEPA, 1983).

The N in biosolids was determined using Kjeldahl digestion, followed by colorimetric analysis. Total Fe and Al were determined by digestion in nitric acid, followed by analysis using atomic absorption spectrophotometry from 1971 to 1998 and inductively coupled plasma spectroscopy thereafter. A mean of all composite samples in a year was calculated.

Calculation of Soil Carbon Gain
Because biosolids were mixed with only surface soil, we could assume the SOC was unchanged in the subsoil. Therefore, we considered the difference in C stock in the "surface soil layer" between a given year and the first year of the biosolids application as the soil C gain after biosolids land reclamation.

Generally, soil C stock can be calculated using the following formula:

[1]

where Mc is the soil C stock (Mg ha^–1), D is the surface soil layer depth (m) (0.15 at t = 0 in 1972), S is the surface area of a hectare (10,000 m² ha^–1), is bulk density (Mg m^–3), and C is the SOC concentration (%).

The known bulk densities of the 0- to 15-cm surface soil layer at the start of the project (t = 0) were 1.61 Mg m^–3 for the strip-mined soil and 1.29 Mg m^–3 for the nonmined areas (Peterson et al., 1979). The bulk density of 1.61 Mg m^–3 was given to most of the mine spoil soil fields and 1.29 Mg m^–3 to most of the "fine" mine spoil soil and nonmined soil fields. A few fields had extremely high or low initial SOC; therefore, bulk density in these fields was adjusted using the SOC and bulk density equation established from measurements in selected fields in 2005. After the start of biosolids application, the bulk density in biosolids-amended soil was calculated as the weighted mean of soil in the 0.15-m depth and of newly applied biosolids:

[2]

where _t is the bulk density of biosolids-amended soil at t, _t–1 is the bulk density of biosolids-amended soil at t – 1, W_BS(t–1) is the dry weight of biosolids applied at t – 1, and _BS is the bulk density of biosolids. The biosolids had a mean bulk density of 0.69 Mg m^–3 (Granato et al., 2004). We verified such bulk density estimation by measuring soil bulk density in samples collected from six selected fields in 2005. The calculated mean bulk density was 1.12 (1.02–1.19) Mg m^–3, as compared with measured bulk density of 1.11 (0.94–1.18) Mg m^–3, with a median deviation of 0.01 (–0.08 to 0.04) Mg m^–3 between them. The soil bulk density in the Group I fertilizer control field (F29, Lenzburg soil) was 1.44 g cm^–3 in 2005, 0.17 Mg m^–3 less than that in 1972. Therefore, we proportioned such an increment over the entire period. The bulk density in the nonmined portion of F18 that is used as Group II and III control was not included in the 2005 sampling. We considered bulk density in this control unchanged because it had very little change in SOC by 2005.

After each year's biosolids application, there was an increase in the surface soil layer as A_p (Fig. 1 ), as reported by Chang et al. (2007) for long-term application of animal manure; therefore, there was an additional C stock in the A_p for the biosolids-amended soil, and this can be calculated as follows:

[3]

in which Mc_Ap is the C stock in the A_p at t, D_Ap is the depth of A_p induced by biosolids application at t – 1, _Ap is the bulk density of A_p, and C_Ap is the SOC concentration in A_p. We estimated the D_Ap for each year using the following formula:

[4]

Because the biosolids applied at t – 1 that created A_p at t was mixed with the 0- to 15-cm soil by incorporation, the SOC concentration in 0- to 15-cm soil depth (C_con15) sampled at t and the bulk density (₁₅) estimated for t can also be considered as that in A_p induced by the biosolids applied at t – 1 (i.e., C_Ap = C_con15; _Ap = ₁₅).

View larger version (12K): [in this window] [in a new window]   Fig. 1. Hypothetical influence of biosolids application in the previous year on surface soil layer depth (one Ap denotes the annual increase in surface soil layer depth after biosolids application in the previous year).

Finally, the SOC stock at t, starting from t = 0 with a step of 1 (year), could be estimated by the following formula:

[5]

where k is the annual decomposition rate of SOC left in Ap, 0.0276 yr^–1 or 2.7%, which was obtained from the dynamics of SOC at the 0- to 15-cm depth after the cessation of biosolids applications in this study.

Estimation of Biosolids Carbon Remaining
The decomposition of biosolids follows the first-order double exponential decay (Gilmour and Gilmour, 1980): y = Re^–k1t + (1 – R)e^–k2t, where R is the the rapidly decomposable fraction, k₁ is the the decomposition rate constant for R fraction, (1 – R) is the slowly decomposable fraction, and k₂ is the the decomposition rate constant for (1 – R) fraction. Although several biosolids decomposition models are available (Gilmour et al., 1996; 2003), they generally overestimated the biosolids decomposition because these models were built on short-term experiments and did not separate the prime effect (a stimulation of native SOM decomposition due to the addition of organic materials) (Terry et al., 1979a). The optimal soil moisture and temperature in the laboratory also led to overestimation of biosolids decomposition. Based on the estimation of Gilmour and Gilmour (1980), the decomposition of biosolids hypothetically at a site similar to Fulton County's ambient soil moisture and temperature was only 37% of that under optimal soil moisture (20%) and temperature (25°C). Therefore, we used two steps in constructing a model to estimate the biosolids C remaining after the decomposition of biosolids in the field: (i) establishing a decomposition equation using CO₂ evolved from only biosolids under optimal soil moisture and temperature conditions then (ii) adjusting constants of the equation to Fulton County field condition.

Terry et al. (1979b) conducted a unique study on biosolids decomposition because it used ¹⁴C-labeled synthetic biosolids. This study, therefore, could separate the prime effect and obtain the CO₂ evolved from only biosolids during decomposition. Furthermore, the duration of the Terry et al. (1979b) study was relatively long (336 d) compared with other studies. However, this work was under-cited, most probably because their data were fitted to a linear model of CO₂ evolution vs. log of time (Terry et al., 1979a) rather than to the double exponential model. We compared two fittings ("log of time" and "double exponential") using their data and found that the double exponential had lower sum of square. Therefore, we re-fitted their data into a double exponential decay model.

The synthetic sludge made by Terry et al. (1979b) had volatile solids of 52% and organic N of 3.1%, close to the volatile solids (55%) and organic N (3.7%) of our unlagooned liquid biosolids. Because Terry et al. (1979b) did not include 25°C (optimal temperature), we calculated the CO₂ evolved from biosolids at 25°C using Q₁₀ over the two temperatures of 21 and 30°C tested in Terry et al. (1979b). Then, we adjusted k₁ and k₂ by multiplying 0.37 to obtain k₁ = 0.0205 (d^–1) and k₂ = 0.000301 (d^–1). From Terry et al. (1979b), we got R = 0.339. Because R is independent of soil environmental condition, it is not necessary to adjust for field condition. However, R has to be adjusted for different types of biosolids.

At MWRDGC, we measured the volatile solids in biosolids subjected to various further stabilizations. The volatile solids were 55.4% for digester drawoff, 52.5% for centrifuge-dewatered cake, 48.4% for a 2-yr lagooned liquid, and 37% for lagooned air-dried biosolids. The volatile solids content in Table 3 therefore indicates that biosolids were already subjected to various degrees of stabilizations before application; hence, it is necessary to take this into consideration in estimating biosolids C remaining. Because the rapid fraction of biosolids decomposes fast once it is mixed with soil, the rapid fraction could be assumed to decompose completely before the decomposition of the slow fraction starts (Gilmour et al., 1996). We can, therefore, consider the variation in volatile solids with application year as the change in rapid fraction and use volatile solids content as a parameter to adjust R. Then,

[6]

where y is the remaining biosolids C, V_S is the volatile solids in biosolids, C_BS is the C input from biosolids application, and t is time (days).

The remaining biosolids C from each of the previous application years was calculated, and a sum was established as total biosolids C remaining in 1985 and 2006.

Estimation of Net Soil Carbon Sequestration
The net soil C sequestration observable in 1985 and 2006 was obtained as the difference between soil C gain and the total biosolids C remaining in 1985 or 2006.

Statistical Analysis
Because the frequency of biosolids applications for three groups was similar up to 1984, we calculated the biosolids C sequestration efficiency in 1985 for comparing the effect of initial soil conditions on the biosolids-induced soil C sequestration. The biosolids C sequestration efficiency (BSCE) in 1985 was calculated as follows:

[7]

The significance of difference in BSCE between field groups was evaluated using a t test. At the end of the project (2006), we performed a linear regression procedure to establish a relationship between annual biosolids application and net soil C sequestration that can be used to predict the C sequestration in the biosolids land application. The annual biosolids application or soil C sequestration was calculated through dividing the cumulative biosolids application or total net C sequestration in 2006 by 34. All biosolids applications are reported on a dry-weight basis.

	Results and Discussion

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
Biosolids Organic Matter Content
Table 3 shows variations in chemical characteristics among three types of biosolids applied to the Fulton county fields from 1972 to 2004. The organic matter measured as volatile solids in biosolids decreased in the following order: lagooned liquid > lagooned air-dried > lagooned dewatered. Variations in volatile solids among liquid biosolids of various years reflected the effect of storage time. The liquid biosolids applied at later years had longer storage time, thus having lower volatile solids (Table 3). The low volatile solids in dewatered biosolids were due to the dilution by clay dredged from earthen holding basins and to volatilization and oxidation of organic compounds during the prolonged storage. Similarly, the clay dredged from the lagoon bottom decreased the volatile solids in air-dried biosolids produced in the early 1990s when the storage lagoons at the Stickney and Calumet WRPs were not paved. The agitation and air-drying in drying beds, which enhance the organic volatilization and oxidation, are also contributors for low volatile solids in air-dried biosolids. Because most dewatered biosolids and air-dried biosolids from unpaved lagoons were concurrently applied from mid-1980s to early 1990s, the fields received biosolids of relatively low organic matter during that period. The biosolids applied in 1994 and 1995 added extremely low organic matter because the remaining small amount of pre-Part 503 dewatered biosolids was mixed, as allowed, with sand. Variations in organic N, NH₄–N, Fe, and Al among biosolids types and application years generally followed those for the volatile solids (Table 3). Dewatered and air-dried biosolids had a C/N ratio similar to typical topsoil.

Soil Organic Carbon Concentration Dynamics
Soil organic carbon concentration in control with chemical fertilizer application alone showed a small increase for mine spoil soil (F29) and slight decrease for nonmined soil (F18–2) over the course of the project (Table 4 ). Application of biosolids rapidly increased the SOC concentrations up to 1985 in fields of all three groups. The SOC concentration in the fields of all groups started to peak in the mid-1980s and maintained that level for nearly 10 yr in Groups I and II fields where biosolids were applied every year. The SOC in Group III started to decline after peaking at 2 to 3 yr because the application of biosolids in most fields of this group was ceased at that time. At the highest loading rates (1400–1700 Mg ha^–1 in 34 yr or about 42–49 Mg ha^–1 yr^–1) in Group I and II fields, the SOC was maintained between 6.5 to 7% in the 0- to 15-cm depth during the 3 yr (1993–1995). The SOC did not continue to rise in the late 1980s and early 1990s; this was probably due to low organic matter in dewatered biosolids and air-dried biosolids from unpaved storage lagoons concurrently applied during those years. The SOC showed some declines in the late 1990s because no biosolids were applied in 1993 and 1996, and in 1994 and 1995 the biosolids were diluted by sand. In some fields of Group III where biosolids application was ceased 22 yr ago, the SOC in 2006 was still well above the initial levels. The loss of SOC after the cessation of biosolids application followed the first-order decay model. The decomposition rate, k, ranged from 0.0225 to 0.0339 yr^–1 or 2.2 to 3.3% loss annually (on average, 0.0276 yr^–1 or 2.7%).

Farmyard manure is the most recognized among the commonly used organic amendments in its ability to maintain and build SOM (Paustian et al., 1997). Parat et al. (2005) reported that biosolids restored the SOC to a level as high as farmyard manure on a French sandy soil. Gerzabek at al (2001) reported that the increase in SOC of a Swedish soil was higher in biosolids than in animal manure. The intensive microbial decay of the organic matter during anaerobic digestion leaves the biosolids as a relatively recalcitrant residue, and lagooning further reduced the decomposability of the biosolids (Gilmour et al., 2003). This makes biosolids become an ideal organic amendment to build SOM.

The ultimate goal of the MWRDGC's land reclamation is to bring the SOC to the level of Midwest prairie, apart from improving soil physical properties. Reliable estimates of SOC concentration in pre-cultivation Illinois prairie soil are not available because SOC monitoring began some time after the establishment of cultivation experiments (Darmody and Peck, 1997). Based on the first available SOC of 2.4% C in 1904 in the continuous corn plots established in 1876 by University of Illinois as the oldest agronomic research fields in the USA and the oldest continuous corn plots in the world (Darmody and Peck, 1997) and assuming that the SOC loss after cultivation was 2.7% per year as observed in the region, the dark prairie soils (Flanagan silt loam: fine montmorillonitic, mesic, Aquic Argiudoll) probably contained 5.3% SOC before cultivation in 1876. The prairie soil at Nashua (Iowa), 300 km northwest of Fulton County, has a SOC of 6.61% in the 0- to 15-cm depth (Russell et al., 2005). Two fields (F3 and F17) with a mean annual loading rate of 42 Mg ha^–1 showed an SOC level of between 5.3 and 6.6% in 2006 (Table 4). These data indicate that the land reclamation using biosolids was able to restore the SOC of stripmined and degraded soils to a level similar to the prairie soils in a few decades.

Soil Carbon Sequestration
Application of chemical fertilizer alone over three decades led to a soil C gain of 5.7 Mg ha^–1 in mine spoil soil and a loss of 2.4 Mg ha^–1 in nonmined soil (Table 5 ). Application of biosolids during the same period resulted in a remarkable soil C gain: 74.4 to 166 Mg ha^–1 in Group I fields, 37.1 to 139 Mg ha^–1 in Group II fields, and 24.8 to 107 Mg ha^–1 in Group III fields (Table 5). After subtracting the biosolids C remaining in 2006 (ranging from 17.5 to 62.6 Mg ha^–1 in Group I fields, 6.4 to 61.4 Mg ha^–1 in Group II fields, and 5.0 to 35.7 Mg ha^–1 in Group III fields), the net soil C sequestration due to biosolids application was still remarkably high (ranging from 37.5 to 104 Mg C ha^–1 in Group I fields, 28.2 to 86.9 Mg C ha^–1 in Groups II fields, and 18.2 to 83.5 Mg C ha^–1 in Group III fields) (Table 5). The mean net soil C sequestration in the biosolids-amended fields in this study was 1.73 Mg ha^–1 yr^–1 (range, 0.54– 3.05 Mg ha^–1 yr^–1).

Biosolids application at the land reclamation fields increased soil microbial biomass (Tian et al., unpublished data). As a precursor to more stable fractions of SOM (Parton et al., 1987), increase in the size of microbial biomass leads to the increase in SOM (Powlson et al., 1987). Although the formation of SOM through microbial biomass can be the re-distribution of original SOC and biosolids C rather than C sequestration, some of the microorganisms are autotrophic (e.g., nitrifiers, which are able to assimilate CO₂ as a C source) and contribute to C sequestration. Some heterotrophic organisms require CO₂ for the formation of biomass. The improvement in microbial biomass has been one of the reasons for C sequestration in no- and minimal tillage (Wright et al., 2005).

Biosolids contain appreciable amounts of Fe and Al (Table 3). Measurement at another experiment with nonmined soil within the land reclamation site indicates that application of biosolids in 2005 significantly increased the soil oxalate-extractable Fe and Al in 2006. The oxalate-extractable Fe and Al are amorphous Fe and Al oxides. These noncrystallines are the most reactive Fe and Al oxides due to their small size and consequently high surface area. Buurman et al. (2007) found that allophane could contribute to the increase in SOM through incorporating decomposition products and microbial SOM in very fine aggregates. Mikutta et al. (2006) noted the interaction of poorly crystalline minerals with SOM promoted SOM stabilization. Kaiser et al. (2002) pointed out that the intimate association of organic matter with secondary hydrous Fe and Al phases against biological degradation could be the chief reason for survival of organic matter in subsoils of two forest soils in Germany. Huggins et al. (1998) measured plant biomass C input to soil in various rotation systems in Minnesota from 1981 to 1990. They found that a corn–soybean rotation could provide soil a C input of 5.4 Mg ha^–1 yr^–1 via above- and below-ground biomass of corn and soybean. Collins et al. (1999) estimated that the total biomass C input ranged from 6.1 to 7.6 Mg C ha^–1 in continuous corn within the Corn Belt of the USA. However, the humification rate constant for added biomass C was only 0.16 yr^–1 for corn and 0.11 yr^–1 for soybean (Huggins et al., 1998). Therefore, potentials exist for more biomass C to be transformed to SOM. The Fe and Al from biosolids might have acted as an agent that increased that humification rates of corn and soybean biomass C in this study. Improvement in soil aggregation by biosolids also increased the stability of SOM (Tiessen et al., 1984; Six et al., 2002). Further study by analysis of ¹³C is being envisioned so that a more firm conclusion regarding biosolids applications and C sequestration can be drawn.

Regression analysis revealed that soil C sequestration was positively correlated to biosolids loading rate (Fig. 2 ). Based on the equation in Fig. 2, annual application of biosolids at a level meeting corn N requirement, normally 22.4 Mg ha^–1 yr^–1, would lead to a soil C sequestration of 1.3 Mg C ha^–1.

View larger version (16K): [in this window] [in a new window]   Fig. 2. Correlation between annual net soil C sequestration and biosolids application in fields of all three groups at Fulton County land reclamation site. Data are derived from Tables 1 and 5. The arrow points to the rate of soil C sequestration in fertilizer control field 29 (F29) and field 18–2 (F18–2).

	Conclusions

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
The high SOC improvement and soil C sequestration in biosolids-amended fields of a large-scale, long-term land reclamation project can allow us to draw some important conclusions for searching approaches for land reclamation and the management of biosolids. Biosolids are an effective material in increasing SOM in strip-mined land and low-productivity soil. The biosolids application could turn the Midwest corn–soybean system soils for C-neutral to C-sink. Biosolids application is a good option that can return the Midwest soil to prairie SOM condition.

	ACKNOWLEDGMENTS

 
Special thanks go to Emil Boucek and Rosalie Swango, who dedicated their entire careers to conducting soil sampling and SOC analysis. The authors also thank Joshua DeWees and Daniel Bergstrom for taking soil samples, staff of the MWRDGC's Analytical Laboratory Division for biosolids analysis, Odona Dennison and Richard Adams for SOC analysis of recent 2-yr samples, Marc White for providing some field information, and Kathleen Quinlan for typing this manuscript. The authors acknowledge the contribution of John Gschwind and James Peterson for designing the original environmental monitoring program for the project. The authors express their gratitude to Cecil Lue-Hing, Richard Lanyon, and Louis Kollias for supporting this project. We also thank the associate editor, Enzo Lombi, and three anonymous reviewers for their constructive comments that have greatly improved the paper.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION Materials and Methods Results and Discussion Conclusions REFERENCES

 
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