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TECHNICAL REPORTS

Plant and Environment Interactions

Tillage, Cropping Systems, and Nitrogen Fertilizer Source Effects on Soil Carbon Sequestration and Fractions

^a USDA-ARS, Northern Plains Agricultural Research Lab., 1500 N. Central Avenue, Sidney, MT 59270
^b Dep. of Plant and Soil Sciences, Alabama A & M Univ., P.O. Box 1208, Normal, AL 35762

^* Corresponding author (upendra.sainju{at}ars.usda.gov).

Received for publication May 14, 2007.

	ABSTRACT

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

 
Quantification of soil carbon (C) cycling as influenced by management practices is needed for C sequestration and soil quality improvement. We evaluated the 10-yr effects of tillage, cropping system, and N source on crop residue and soil C fractions at 0- to 20-cm depth in Decatur silt loam (clayey, kaolinitic, thermic, Typic Paleudults) in northern Alabama, USA. Treatments were incomplete factorial combinations of three tillage practices (no-till [NT], mulch till [MT], and conventional till [CT]), two cropping systems (cotton [Gossypium hirsutum L.]-cotton-corn [Zea mays L.] and rye [Secale cereale L.]/cotton-rye/cotton-corn), and two N fertilization sources and rates (0 and 100 kg N ha^–1 from NH₄NO₃ and 100 and 200 kg N ha^–1 from poultry litter). Carbon fractions were soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). Crop residue varied among treatments and years and total residue from 1997 to 2005 was greater in rye/cotton-rye/cotton-corn than in cotton-cotton-corn and greater with NH₄NO₃ than with poultry litter at 100 kg N ha^–1. The SOC content at 0 to 20 cm after 10 yr was greater with poultry litter than with NH₄NO₃ in NT and CT, resulting in a C sequestration rate of 510 kg C ha^–1 yr^–1 with poultry litter compared with –120 to 147 kg C ha^–1 yr^–1 with NH₄NO₃. Poultry litter also increased PCM and MBC compared with NH₄NO₃. Cropping increased SOC, POC, and PCM compared with fallow in NT. Long-term poultry litter application or continuous cropping increased soil C storage and microbial biomass and activity compared with inorganic N fertilization or fallow, indicating that these management practices can sequester C, offset atmospheric CO₂ levels, and improve soil and environmental quality.

Abbreviations: CT, conventional till • MBC, microbial biomass C • MT, mulch till • NT, no-till • PCM, potential C mineralization • POC, particulate organic C • SOC, soil organic C

	INTRODUCTION

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

 
CARBON sequestration, using long-term improved soil and crop management practices, is needed not only to increase soil C storage for C trading and to mitigate greenhouse gas emissions, such as CO₂, from the soil profile but also to improve soil quality and increase economic crop production. For a better understanding of these processes, knowledge of soil C cycling in the terrestrial ecosystem is needed. Some of the parameters of soil C cycling are soil organic C (SOC), particulate organic C (POC), microbial biomass C (MBC), and potential C mineralization (PCM). The SOC, a C fraction that changes slowly with time, is used to measure C sequestration and is an important component of soil quality and productivity (Franzluebbers et al., 1995; Bezdicek et al., 1996). Measurement of SOC alone does not adequately reflect changes in soil quality and nutrient status (Franzluebbers et al., 1995; Bezdicek et al., 1996). This is because SOC has a large pool size and inherent spatial variability (Franzluebbers et al., 1995). Measurement of biologically active fractions of SOC, such as MBC and PCM, that change rapidly with time could better reflect changes in soil quality and productivity that alter nutrient dynamics due to immobilization-mineralization (Saffigna et al., 1989; Bremner and Van Kissel, 1992). These fractions can provide an assessment of soil organic matter changes induced by management practices, such as tillage, cropping system, cover crop, and N fertilization (Campbell et al., 1989; Sainju et al., 2006). Similarly, POC has been considered as an intermediate fraction of SOC between active and slow fractions that change rapidly over time due to changes in management practices (Cambardella and Elliott, 1992; Chan, 1997; Bayer et al., 2001). The POC also provides substrates for microorganisms and influences soil aggregation (Beare et al., 1994; Franzluebbers et al., 1999; Six et al., 1999).

Although conventional tillage without cover crop and N fertilization reduces soil organic matter level by enhancing C mineralization and limiting C inputs (Dalal and Mayer, 1986; Balesdent et al., 1990; Cambardella and Elliott, 1993), conservation tillage with cover cropping and N fertilization can increase C storage and active C fractions in the surface soil (Jastrow, 1996; Allmaras et al., 2000; Sainju et al., 2002, 2006). Studies suggest that conversion of conventional till (CT) to no-till (NT) can sequester atmospheric CO₂ by 0.1% ha^–1 at 0 to 5 cm every year or by a total of 10 tons in 25 to 30 yr (Lal and Kimble, 1997; Paustian et al., 1997). However, SOC below 7.5 cm depth can be higher in tilled areas, depending on the soil texture, due to residue incorporation at greater depths (Jastrow, 1996; Clapp et al., 2000). Similarly, cover cropping and N fertilization can increase C fractions in tilled and non-tilled soils by increasing the amount of crop residue returned to the soil (Kuo et al., 1997; Omay et al., 1997; Sainju et al., 2002, 2006). The impact of tillage on soil C fractions can interact with cover cropping and N fertilization rate (Gregorich et al., 1996; Wanniarachchi et al., 1999; Sainju et al., 2002), soil texture and sampling depth (Ellert and Bettany, 1995), and time since treatments were initiated (Liang et al., 1998).

Poultry litter, an inexpensive source of nutrients, is widely available in the southeastern USA because of a large-scale poultry industry (Kingery et al., 1994; Nyakatawa and Reddy; 2000; Nyakatawa et al., 2000). Disposal of large amounts of poultry litter is of increasing environmental concern because of ground water contamination of nitrogen (N) and phosphorus (P) from the litter through leaching and surface runoff. Application of poultry litter to cropland, however, can increase soil organic matter and C fractions that can improve soil quality and productivity (Kingery et al., 1994). Studies have shown that manure application increased SOC and nutrient status in the soil (Webster and Goulding, 1989; Collins et al., 1992; Rochette and Gregorich, 1998) and labile C pools (Aoyama et al., 1999; Mikha and Rice, 2004). Application of poultry litter along with conservation tillage and cover cropping can provide an opportunity to increase C sequestration and soil quality in the humid southeastern USA where organic matter levels are lower than in northern regions due to a long history of cultivation and rapid rate of mineralization (Trimble, 1974; Doran, 1987; Langdale et al., 1992).

The effects of tillage, cropping systems, cover crops, and N fertilization on soil C fractions are relatively well known (Kuo et al., 1997; Sainju et al., 2002, 2006). Limited information is available about the long-term combined effects of tillage, cropping systems, poultry litter, and inorganic N fertilization or the comparison of poultry litter vs. inorganic N fertilization on active and slow fractions of soil C in the southeastern USA. We hypothesized that conservation tillage with poultry litter application and intensive cropping that includes cover crops would increase active and slow fractions of soil C compared with conventional tillage with or without N fertilization and a cropping system without a cover crop. Our objectives were (i) to examine the amount of rye, cotton, and corn biomass (stems + leaves) residues returned to the soil from 1997 to 2005 as influenced by tillage, cropping system, poultry litter application, and inorganic N fertilization and (ii) to quantify their effects on active (PCM and MBC), intermediate (POC), and slow (SOC) C fractions in the southeastern USA.

	Materials and Methods

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

 
Experimental Site and Treatments
A long-term field experiment was conducted from 1996 to 2005 in the upland cotton production site at the Alabama Agricultural Experimental Station in Belle Mina, AL (34°41'N, 86°52'W). The soil was a Decatur silt loam (clayey, kaolinitic, thermic, Typic Paleudults). Before the initiation of the experiment (1996), soil samples collected randomly from 24 cores (5 cm i.d.) within the experimental plots contained an average pH of 6.2, sand concentration of 150 g kg^–1, silt concentration of 580 g kg^–1, clay concentration of 270 g kg^–1, organic C concentration of 11.5 g kg^–1 (or organic C content of 38.6 Mg ha^–1), and bulk density of 1.60 Mg m^–3 at the 0- to 20-cm depth. The site had been used for growing cotton continuously for 5 yr before the experiment started in 1996. Details of the experiment and crop management practices have been presented elsewhere (Nyakatawa and Reddy, 2000; Nyakatawa et al., 2000; Reddy et al., 2004).

Treatments consisted of an incomplete factorial combination of three tillage practices (NT, mulch till [MT], and CT), two cropping systems (rye/cotton-rye/cotton-corn and cotton-cotton-corn), and two N fertilization sources and rates (0 and 100 kg N ha^–1 from NH₄NO₃ and 100 and 200 kg N ha^–1 from poultry litter) arranged in a randomized complete block with four replications (Table 1 ). A no-tilled fallow treatment without cropping and fertilization was included for comparing treatments with or without cropping and fertilization on soil C fractions. For this study, only three replications with uniform crop residue production were selected. The individual plot size was 8 x 9 m. Treatments were continued in the same plot every year to determine their long-term influence on soil C fractions.

Rye (cv. Oklon [Pioneer Hi-Bred Int. Inc., Hunstville, AL]) cover crop was planted in November and December at 60 kg ha^–1 with a no-till driller (Glascock Equipment and Sales). In April, 7 d after flowering, rye biomass yield was determined by harvesting biomass from a 1 x 1 m² area, after which it was killed by applying glyphosate (isopropylamine salt of N-[phosphonomethyl] glycine) in NT and MT or by incorporating the residue into the soil by a field cultivator in CT. Rye was not grown in 1998, 2002, or 2004 when the succeeding crop was corn in the summer in the following year. Inorganic N fertilizer (NH₄NO₃) and poultry litter were broadcast to cotton 1 d before planting in May. Inorganic N fertilizer and poultry litter were incorporated to a depth of 5 to 8 cm in CT and MT using a field cultivator and were surface-applied in NT. No adverse effect of poultry litter on germination of cotton was detected; rather, cotton seedling counts were 17 to 50% greater with poultry litter than with NH₄NO₃ during the first 4 d of emergence (Nyakatawa and Reddy, 2000). The poultry litter applied in each year from 1997 to 2005 contained total C at 337 ± 22 g C kg^–1 and total N at 33 ± 4 g N kg^–1. A 60% N availability factor was used to calculate the amount of poultry litter required to supply 100 and 200 kg N ha^–1 in each year (Keeling et al., 1995). To nullify the effects of P and K from poultry litter, all plots were applied with 67 kg P ha^–1 (from triple superphosphate) and 67 kg K ha^–1 (from muriate of potash) in each year.

After 4 wk of cover crop kill, cotton (cv. Deltapine NuCotn 33B; Delta Pine Land Co., Hartsville, SC]) was planted at 16 kg ha^–1 in May of 1997, 1998, 2000, 2001, 2003, and 2004. In 1999, 2002, and 2005, corn (cv. Dekalb 687; Pioneer Hi-Bred Int. Inc., Hunstville, AL) was planted at 78,000 seeds ha^–1. Cotton and corn were applied with appropriate herbicides and pesticides during their growth to control weeds and pests. Cotton was also applied with appropriate growth regulator (Pix [1,1-dimethyl-piperidinium chloride] at 0.8 kg ha^–1) to control its vegetative growth. Irrigation was applied to cotton and corn ranging in amounts from 23 to 47 mm at a time (or a total of 560 to 571 mm from May to November) depending on soil water content to prevent moisture stress. Aboveground cotton and corn biomass (stems + leaves) yields were determined 2 wk before lint and grain harvest in October-November by measuring plant samples in two 0.5-m² quadrants outside the yield row in each plot after separating lint, seeds, and cobs. Yields of cotton lint and corn were determined by mechanically harvesting the central four rows (4 x 9 m) with a combine harvester (Glascock Equipment and Sales) in November of each year. After sampling, cotton lint and corn were removed from the rest of the plots with a combine harvester, and crop residues (stems + leaves) were returned to the soil.

Soil Sampling and Analysis
In February 2006, soil samples were collected with a hand probe (10 cm i.d.) from 0- to 20-cm depth from five places in the central rows of the plot after removing the residue from the soil surface. These were separated into 0- to 10-cm and 10- to 20-cm depths, composited within a depth, air-dried, ground, and sieved to 2 mm for determining C fractions. At the same time, a separate undisturbed soil core (10 cm i.d.) was taken from 0- to 10-cm and 10- to 20-cm depths from each plot to determine bulk density by dividing the mass of the oven-dried sample at 105°C by the volume of the probe.

Total C concentration (g kg^–1) in the soil was determined by the dry combustion method using a C and N analyzer (Model 661–900–800; LECO Corp., St Joseph, MI) and was considered as SOC concentration because the pH of the soil was <7.0 (Nelson and Sommers, 1996). For determining POC, 10 g soil was dispersed with 30 mL of 5 g L^–1 sodium hexametaphosphate after shaking for 16 h, and the solution was poured through a 0.053-mm sieve (Cambardella and Elliott, 1992). The solution and particles that passed through the sieve (water-soluble and mineral-associated C) were dried at 50°C for 3 to 4 d, and organic C concentration was determined by using the analyzer as described previously. The POC concentration was determined by the difference between organic C in whole soil and that in the particles that passed through the sieve after correcting for the sand content. The PCM in air-dried soils was determined by the method modified by Haney et al. (2004). Ten grams of soil was moistened with water at 50% field capacity and placed in a 1-L jar containing beakers with 2 mL of 0.5 mol L^–1 NaOH to trap evolved CO₂ and 20 mL of water to maintain high humidity. Soils were incubated in the jar at 21°C for 10 d. At 10 d, the beaker containing NaOH was removed from the jar, and PCM concentration was determined by measuring CO₂ absorbed in NaOH, which was back-titrated with 1.5 mol L^–1 BaCl₂ and 0.1 mol L^–1 HCl. The moist soil used for determining PCM was subsequently used for determining MBC by the modified fumigation-incubation method for air-dried soils (Franzluebbers et al., 1996). The moist soil was fumigated with ethanol-free chloroform for 24 h and placed in a 1-L jar containing beakers with 2 mL of 0.5 mol L^–1 NaOH and 20 mL water. As with PCM, fumigated moist soil was incubated for 10 d, and CO₂ absorbed in NaOH was back-titrated with BaCl₂ and HCl. The MBC concentration was calculated by dividing the amount of CO₂–C absorbed in NaOH by a factor of 0.41 (Voroney and Paul, 1984) without subtracting the values from the nonfumigated control (Franzluebbers et al., 1996).

The contents (Mg ha^–1 or kg ha^–1) of SOC, POC, PCM, and MBC at 0- to 10-cm and 10- to 20-cm depths were calculated by multiplying their concentrations (g kg^–1 or mg kg^–1) by bulk density for each treatment (as discussed below) and thickness of the soil layer. The total contents at the 0- to 20-cm depth were determined by summing the contents at 0 to 10 cm and 10 to 20 cm.

Data Analysis
Data for soil C fractions and bulk density among and within soil depths were analyzed using the MIXED procedure of SAS (Littell et al., 1996). Treatment was considered as the main plot factor and fixed effect, soil depth as the repetitive measure factor and another fixed effect, and replication and treatment x replication as random effects. Because treatments were laid out in an incomplete factorial arrangement, Treatments 2, 3, 4, and 8 (Table 1) were used to determine the effect of tillage x cropping system interaction on soil C fractions and bulk density. Similarly, Treatments 4, 5, 6, 7, 8, and 9 were used to determine the effect of tillage x N source interaction on soil C fractions and bulk density. For crop biomass yields, data were analyzed as described previously by replacing soil depth by year after considering year as a repetitive measure factor. Means were separated by using the least square means test when treatments and interactions were significant. For comparing the effect of cropping and fertilization vs. fallow and no-fertilization in NT on crop biomass yields and soil C fractions, orthogonal contrasts were used to compare means of Treatments 3, 8, 9, 10, and 11 vs. Treatment 12. Statistical significance was evaluated at P 0.05 unless otherwise stated.

	Results and Discussion

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

 
Crop Residue Production
The amount of residues (stems + leaves) from rye cover crop, cotton, and corn returned to the soil varied among treatments, years, and treatments in a year (Table 2 ). Rye residue was 3.0 Mg ha^–1 yr^–1, and the amount was not different among treatments, except in 1998 and 2001 when the amount was greater in Treatment 11 than in Treatment 8 due to poultry litter application. Cotton residue increased with N fertilization from NH₄NO₃ and poultry litter (Treatments 2, 3, 4, 5, 6, 7, 8, 9 and 11) compared with no N fertilization (Treatments 1 and 10), regardless of tillage and cropping systems, indicating enriched residue production with N nutrition. Similarly, corn residue production was greater in NT with rye/cotton-rye/cotton-corn and poultry litter at 200 kg N ha^–1 (Treatment 11) than with most of the other tillage and cropping systems with NH₄NO₃ at 0 and 100 kg N ha^–1 or poultry litter at 100 kg N ha^–1 (Treatments 1, 2, 3, 4, 5, 7, and 10) in 1999. Rye and cotton residue production were lower in 2003 and 2004 than in 1997, 1998, 2000, and 2001 due to lower precipitation (Table 3 ), rotation effect, or some unknown factors. Corn residue was greater in 2002 than in 1999 due to higher growing season precipitation (892 vs. 652 mm, May–November) (Table 3). Corn residue production in 2005 was not available but was estimated from the product of grain yield in 2005 and average biomass yield/grain yield ratio in 1999 and 2002. In Treatments 2, 3, and 10, rye residue production was absent because the cover crop was not grown in these treatments (Table 1). Similarly, lack of crops grown in fallow (Treatment 12) resulted in the absence of crop residue in this treatment.

Soil Organic Carbon
Tillage, cropping system, and N source did not influence SOC concentration (Table 6 ), although rye/cotton-rye/cotton-corn added more residue than cotton-cotton-corn (Table 4). Nitrogen source and tillage x N source interaction were significant for SOC content at 0 to 20 cm. The SOC content at 0 to 20 cm, averaged across cropping systems, was greater with poultry litter than with NH₄NO₃ in NT and CT. Cropping and fertilization significantly increased SOC concentration and content compared with fallow in NT. Increasing the rate of poultry litter application to supply N from 100 to 200 kg N ha^–1 did not affect SOC in NT because SOC concentrations and contents were similar between Treatments 9 and 11 (data not shown). The SOC concentration decreased with soil depth.

Because the original (1996) SOC content at 0 to 20 cm was 36.8 Mg C ha^–1, changes in SOC level from 1996 to 2006 as influenced by tillage and N sources ranged from –1.2 to 5.1 Mg C ha^–1 (Table 6). This resulted in estimated C sequestration rates of –120 to 510 kg C ha^–1 yr^–1, assuming that C sequestration was linear from 1996 to 2006. This was estimated based on the increase of SOC from 1996 to 2006 because data on SOC content as influenced by treatments in each year from 1997 to 2005 were not available. Averaged across tillage and cropping systems, poultry litter sequestered C at an estimated rate of 461 kg C ha^–1 yr^–1 compared with 141 kg C ha^–1 yr^–1 with NH₄NO₃. Similarly, cropping and fertilization sequestered C at 730 kg C ha^–1 yr^–1 compared with fallow in NT. As a result, the amount of crop residue C converted into SOC in NT can be calculated. For this, we considered that (i) cropping and fertilization added a total crop residue of 125 Mg ha^–1 (average of Treatments 3, 8, 9, 10, and 11) vs. fallow (Treatment 12) in NT from 1997 to 2005 (Table 2), (ii) crop residue contained 40% C, and (iii) belowground residue and rhizodeposition contributed as much as 129% of that contributed by aboveground crop residue to SOC in all cropping systems in NT (Allmaras et al., 2004). With these assumptions, an increased SOC storage of 7.3 Mg C ha^–1 at the 0- to 20-cm depth with cropping and fertilization compared with fallow in NT (Table 6) suggests that about 6.4% of crop residue C was converted into SOC after 10 yr. This turnover rate of crop residue C to SOC was in between the reported values of 4.5 to 12.5% in NT (Allmaras et al., 2004). Because these are estimates, further studies are needed to confirm these turnover rates using labeled ¹⁴C studies.

Particulate Organic Carbon
As with SOC, POC concentration at 0 to 10 cm and content at 0 to 20 cm were greater with cropping and fertilization than with fallow in NT (Table 7 ), indicating that 10 yr of cropping and fertilization enriched the intermediate fraction of soil C as a result of increased C input from crop residues. Within cropping and fertilization systems, however, tillage, cropping system, N source, and their interactions were not significant for POC. Averaged across tillage, cropping systems, and N sources, POC concentration at 0 to 10 cm was 4.4 g C kg^–1 and at 10 to 20 cm was 2.5 g kg^–1. Similarly, POC content at 0 to 20 cm averaged 10.9 Mg C ha^–1. Similar to SOC, poultry litter application at 200 kg N ha^–1 did not increase POC compared with the application at 100 kg N ha^–1 in NT (data not shown). The POC concentration decreased with soil depth, similar to SOC.

Potential Carbon Mineralization and Microbial Biomass Carbon
The PCM and MBC concentrations at 0 to 10 cm and contents at 0 to 20 cm were significantly influenced by N source (Tables 8 and 9 ). The PCM and MBC concentrations and contents, averaged across tillage and cropping systems, were greater with poultry litter than with NH₄NO₃ at 100 kg N ha^–1. At 10 to 20 cm, PCM and MBC concentrations were not influenced by treatments. Cropping and fertilization increased PCM concentration and content compared with fallow in NT, but there was no significant increase in MBC. Tillage, cropping system, and tillage x cropping system interaction were not significant for PCM and MBC concentrations and contents. Neither PCM nor MBC was influenced by poultry litter application at 200 kg N ha^–1 compared with the application at 100 kg N ha^–1 in NT (data not shown). Similar to SOC and POC, PCM and MBC concentrations decreased with soil depth.

The mineralizable fraction of C in SOC (i.e., the PCM/SOC ratio) at 0 to 10 cm was greater with poultry litter than with NH₄NO₃ (Table 8). This suggests that PCM, similar to POC, changes rapidly relative to SOC with poultry litter application, especially in the surface soil. The increased proportion of mineralizable C due to poultry litter application could have explained the greater PCM/SOC ratio. The PCM/SOC ratio at 10 to 20 cm and 0 to 20 cm was not influenced by treatments. The PCM/SOC ratio at 0 to 20 cm ranged from 1.2 to 1.4%, which was within the range of 1.1 to 2.4% as reported by several researchers (Franzluebbers et al., 1995; Sainju et al., 2002, 2003). The proportion of SOC in MBC (i.e., the MBC/SOC ratio) was not influenced by treatments and averaged 43, 54, and 59 g kg^–1 SOC at 0 to 10 cm, 10 to 20 cm, and 0 to 20 cm, respectively. Similarly, the PCM/MBC ratio (i.e., the amount of CO₂–C respired by microorganisms) was not influenced by treatments and averaged 276, 230, and 255 g kg^–1 MBC at 0 to 10 cm, 10 to 20 cm, and 0 to 20 cm, respectively. These results suggest that PCM may be more sensitive to changes with poultry litter application than MBC and SOC.

	Summary and Conclusions

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

 
Cropping and N fertilization from inorganic N fertilizer or poultry litter application increased crop residue production and soil C fractions due to increased C inputs compared with fallow. Although rye cover cropping in the cropping system increased residue production, tillage x cropping system interaction was neither significant for residue production nor for soil C fractions. In contrast, although poultry litter application at 100 kg N ha^–1 reduced residue production, it increased SOC, PCM, and MBC compared with inorganic N fertilization. Soil C storage at 0- to 20-cm depth increased with poultry litter application compared with inorganic N fertilization, thereby resulting in a C sequestration rate of 570 kg C ha^–1 yr^–1 with poultry litter. Cropping and fertilization sequestered C at 730 kg C ha^–1 yr^–1 compared with fallow in NT. Long-term poultry litter application (>9 yr) can increase C sequestration and biological soil quality by increasing C storage and microbial biomass and activity compared with inorganic N fertilization, regardless of tillage and cropping system. Similarly, long-term cropping and fertilization can increase C storage and improve soil quality compared with fallow in NT. Increasing the rate of poultry litter application to supply N from 100 to 200 kg N ha^–1 in NT did not affect soil C fractions. Although continuous cropping can be used to sequester atmospheric CO₂ in the soil to reduce global warming potential, poultry litter can be applied in the soil to increase C sequestration, improve soil quality, and sustain crop production instead of disposing of it as a waste material that can contaminate surface and groundwater due to N leaching and P runoff.

	ACKNOWLEDGMENTS

 
The work was supported by a fund from Evans-Allen project # ALAX 011-306 in Alabama A & M University, Normal, AL. Trade or manufacturers' names mentioned in the paper are for information only and do not constitute endorsement, recommendation, or exclusion by the USDA-ARS.

	NOTES

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

 
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