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USDA-ARS, 1500 North Central Avenue, Sidney, MT 59270
* Corresponding author (usainju{at}sidney.ars.usda.gov)
Received for publication April 21, 2005.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: CRP, Conservation Reserve Program • CT, conventional till • CW, continuous spring wheat • NT, no-till • POC, particulate organic carbon • SOC, soil organic carbon • W–F, spring wheat–fallow • W–L, spring wheat–lentil • W–P–F, spring wheat–pea–fallow • W–W–F, spring wheat–spring wheat–fallow
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Improved soil and crop management practices that reduce tillage intensity and increase the amount of plant residue returned to the soil can increase SOC compared to conventional till (CT) with spring wheat–fallow (W–F) system in drylands of the Great Plains (Halvorson et al., 2002a, 2002b; Sherrod et al., 2003; Allmaras et al., 2004). Halvorson et al. (2002a) observed that, using no-till (NT) with continuous cropping, C sequestration in drylands of the northern Great Plains increased by 233 kg ha–1 yr–1 compared with a loss of 141 kg ha–1 yr–1 in CT with crop–fallow system. They pointed out that continued use of crop–fallow system even in NT increased SOC loss. Similarly, Sherrod et al. (2003) reported that increased cropping intensity in NT increased SOC in drylands of the central Great Plains after 12 yr. After analyzing data from long-term experiments in various locations, West and Post (2002) concluded that conversion from CT to NT can sequester an average of 570 ± 140 kg C ha–1 yr–1, reaching equilibrium in 15 to 20 yr, and enhanced crop rotation can sequester 200 ± 120 kg C ha–1 yr–1, reaching equilibrium in 40 to 60 yr. The benefits of increasing SOC lie not only in enhancing soil structure and soil water–nutrient–crop productivity relationships (Bauer and Black, 1994), but also includes the ability of the soil to store atmospheric C, thereby reducing the concentration of greenhouse gases (Janzen et al., 1999; Lal et al., 1998, 1999).
Because of limited moisture and growing season, crop yields and biomass production are often lower in the Great Plains compared with subhumid regions (Halvorson et al., 2002a). As a result, the amount of crop residue returned to the soil is also lower, thereby taking a longer time to enrich SOC (Halvorson et al., 2002a; Sherrod et al., 2003). Halvorson et al. (2002b) and Ortega et al. (2002) did not observe significant increases in SOC between continuous wheat and wheat–fallow in NT system after 4 to 8 yr, but Sherrod et al. (2003) found increases with continuous cropping only after 12 yr. Since crop residue inputs are directly related to difference in SOC among cropping systems (Collins et al., 1992; Campbell et al., 1992; Campbell and Zentner, 1993), and the balance between the amount of residue and its rate of decomposition in the soil as influenced by tillage intensity determines SOC level (Rasmussen et al., 1980; Havlin et al., 1990; Peterson et al., 1998), the combination of increased cropping intensity and reduced tillage can enhance SOC level even in semiarid regions (West and Post, 2002; Halvorson et al., 2002b; Peterson et al., 1998).
Little information is available about long-term studies of cropping system and tillage on SOC and particulate organic carbon (POC) in the northern Great Plains. Although information is available for the central Great Plains (Halvorson et al., 2002b; Ortega et al., 2002; Sherrod et al., 2003), it may not be applicable in the northern Great Plains because of difference in temperature, rainfall, and growing degree days. The SOC changes slowly over time, but POC may change rapidly due to difference in crop yield and residue input as a result of differences in cultural practices, year-to-year differences in growing environment, or the amount of fallow period since last residue was returned to the soil (Cambardella and Elliott, 1992; Campbell et al., 1992; Chan, 1997; Bayer et al., 2001). While SOC is regarded as a recalcitrant pool, POC is considered as a dynamic intermediate pool between active and passive fractions of soil organic matter that change rapidly over time due to changes in management practices, such as tillage and recent addition of crop residue (Cambardella and Elliott, 1992; Chan, 1997; Bayer et al., 2001). We hypothesize that NT with increased cropping intensity and decreased fallow period can increase the amount of plant biomass returned to the soil, residue cover, amount, and C content, and SOC and POC. Our objectives were to: (i) examine the influence of 6 yr of tillage and crop rotations on the amount of biomass of wheat, pea, and lentil returned to the soil, residue cover, amount, and C content, and SOC and POC contents at 0- to 5- and 5- to 20-cm depths in drylands of the northern Great Plains; (ii) compare these parameters in crops and Conservation Reserve Program (CRP) planting; and (iii) determine management practices that sequester C in dryland soils and residue better than the traditional CT with W–F system.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Treatments
The treatments consisted of two tillage practices (CT and NT), five crop rotations with 1-, 2-, and 3-yr rotations, and CRP planting. The 1-yr rotation consisted of continuous spring wheat, 2-yr rotations of spring wheat–fallow and spring wheat–lentil, and 3-yr rotations of spring wheat–spring wheat–fallow and spring wheat–pea–fallow. Each phase of the crop rotation was present in every year. Because the experiment was conducted for 6 yr from 1998 to 2003, the 1-yr rotation completed six cycles, 2-yr rotations three cycles, and 3-yr rotations two cycles. The CRP consisted of a mixture of alfalfa and three grasses consisting of western wheatgrass [Pascopyron smithii (Rydb.) A. Love], slender wheatgrass [Elmus trachycaulus (Link.) Gould ex Shinners], and green needlegrass [Nassella viridula (Trin.) Backworth]. All crops in rotations and plants in CRP were planted in CT and NT. The CT plots were cultivated with standard sweeps and rods to a depth of 10 cm. The NT plots were left undisturbed, except for drilling seeds and fertilizers. The CT plots under CRP were cultivated in 1998 for planting, after which no further tillage was done. The experiment was designed in randomized complete block with a split-plot arrangement of tillage as main plots and crop rotation as subplots. All treatments were replicated three times. Subplot size was 14.6 by 30.4 m.
Crop Management
Before planting crops in April and May of each year from 1998 to 2003, soil samples collected from the 0- to 60-cm depth from each plot in October of the previous year after fall crop harvest were analyzed for NO3-N content. Based on soil NO3–N content and spring wheat yield goal of 2350 kg ha–1 and 13% protein content, N fertilizer was applied at different rates to various crops for phases of rotations in CT and NT in each year. As a result, the rate of N fertilization for spring wheat in each phase of the rotation in each year varied from 0 to 78 kg N ha–1. Nitrogen was also applied to pea and lentil at 5 to 6 kg N ha–1 while applying P from monoammonium phosphate (11% N, 23% P) in each year, because it was the only P fertilizer available to satisfy P requirement for crops. Nitrogen was applied from urea (46% N) and monoammonium phosphate. Per Montana State University recommendations, P (from monoammonium phosphate) was applied at 56 kg ha–1 and K (from muriate of potash, 60% K) at 48 kg ha–1 to spring wheat, pea, and lentil every year. All fertilizers were banded to a depth of 5 to 7 cm with a single-pass ConservaPak air seeder (ConservaPak Seeding Systems, Indian Head, SK, Canada) at planting.
Spring wheat (cv. McNeal in 1998 and 1999, Amidon in 2000, and Scholar from 2001 to 2003) was planted at 60 to 70 kg ha–1, pea (cv. Alfetta in 1998 and 1999 and Majorette from 2000 to 2003) at 161 to 258 kg ha–1, and lentil (cv. Richlea from 1998 to 2002 and Indianhead in 2003) at 45 to 110 kg ha–1 in April and May of every year. Spring wheat was planted at a depth of 4 to 5 cm and pea and lentil planted at 3 to 6 cm with a ConservaPak air seeder, depending on the depth of moist soil in each year. Seeds were placed at 2 cm above the depth of the fertilizer in the soil. In CRP, alfalfa, western wheatgrass, slender wheatgrass, and green needlegrass were planted at 2.2 kg ha–1 each in April 1998. No fertilizers or herbicides were applied to plants in CRP. Weeds were controlled by applying appropriate herbicides to each crop at preplanting, during crop growth, and at postharvest, except in 1998 and 1999 when CT plots were tilled with sweeps. Similarly, to control weeds, summer fallow plots in CT were tilled with sweeps, while fallow plots in NT were treated with glyphosate at 0.84 kg a.i. ha–1.
Plant and Soil Sample Collection
Before grain harvest in July and August, total crop biomass (grains + stems + leaves) yield in each year was determined from a 30- x 100-cm2 area within each plot. After separating grains, biomass (stems + leaves) samples were dried in the oven at 55°C for 3 d and dry matter yield was determined. Grain yield was determined (pea in July and spring wheat and lentil in August) by harvesting an area of 1.5 x 30.4 m2 with a self-propelled combine and yields were converted into dry matter basis after a sample was dried in the oven at 60°C. The remaining stalks containing stems and leaves were returned to the soil. Post-harvest crop residue cover in each plot was determined by using the standard USDA-NRCS point-method of counting 100 points per plot by a 15-m-long string with each point at a 0.15-m spacing.
In March 2004, crop residue amount was determined by collecting residue samples from five 30- x 30-cm2 areas randomly in the central rows of the plot. Samples were composited, washed with water to separate soil particles, and dried in the oven at 60°C for 3 d to obtain dry matter weight. Samples were ground to 1 mm for C analysis. After removing the residue from the soil surface, soil samples were collected with a hand probe (5-cm i.d.) from the 0- to 20-cm depth from five places in the central rows of the plot, separated into 0- to 5- and 5- to 20-cm depths, and composited within a depth. Samples were air-dried, ground, and sieved to 2 mm for determining C concentration. A separate undisturbed soil core (5-cm i.d.) was taken from 0- to 5- and 5- to 20-cm depths from each plot to determine bulk density (Blake and Hartge, 1986).
Carbon Analysis
Total C concentration in crop residue and soils were determined by using a C and N dry combustion analyzer (LECO, St. Joseph, MI). Soils were pretreated with 5% H2SO3 to remove inorganic C (Nelson and Sommers, 1996) before C analysis by dry combustion. For determining POC, 10 g soil was dispersed with 30 mL of 5 g L–1 sodium hexametaphosphate for 16 h and the solution was poured through a 0.05-mm sieve (Cambardella and Elliott, 1992). The solution and particles that passed through the sieve were dried at 50°C for 3 to 4 d and organic C concentration was determined by using the analyzer as above. 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 contents of SOC and POC at 0- to 5- and 5- to 20-cm depths were calculated by multiplying their concentrations by bulk density and depth. Because bulk density was influenced by tillage but not by crop rotation and its interaction with tillage, bulk density values of 1.24 and 1.32 Mg ha–1 for CT and NT, respectively, at the 0- to 5-cm depth and 1.32 and 1.34 Mg ha–1 at the 5- to 20-cm depth, averaged across crop rotations, were used for the calculation. The total contents of SOC and POC at the 0- to 20-cm depth were determined by summing the contents at 0- to 5- and 5- to 20-cm depths.
Data Analysis
Data for plant biomass returned to the soil in each year, total biomass, residue cover, amount, and C content, and SOC and POC contents were analyzed using the MIXED procedure of SAS (Littell et al., 1996). Tillage and crop rotation were considered as fixed effects and replication and tillage x replication were considered as random effects. For eliminating the phases of the crop rotation, data were averaged across the phases within a rotation. Means were separated by using the least square means test when treatments and interaction were significant. Statistical significance was evaluated at P 
0.10, unless otherwise stated.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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0.05) influenced biomass (stems + leaves) yields of spring wheat, pea, and lentil returned to the soil from 1999 to 2003 (Table 1). Biomass yield differed not only between crop rotations but also between years. For example, while biomass, averaged across tillage, was significantly higher in W–F than in other crop rotations in 2001, it was higher in continuous spring wheat (CW) than in other rotations, except in spring wheat–lentil (W–L), in 2002. Similarly, biomass was higher in CW and W–F than in W–L in 1999 and 2003. This could be due to the type of crop rotation and the difference in the amount of moisture available in the soil at the time of planting between treatments. Soil moisture storage has been reported to be higher following fallow than following wheat in W–F system due to excess water unused by plants during fallow (Farhani et al., 1998; Halvorson et al., 2002a). Total rainfall during the growing season from April to August was 159, 99, 93, 220, 131, and 202 mm in 1999, 2000, 2001, 2002, 2003, and the 87-yr average, respectively. Because of higher rainfall, biomass of wheat, pea, and lentil was higher in 2002 than in 2001. Biomass of wheat, pea, and lentil in 1998 and 1999 was not measured but were predicted from their grain yields and the average biomass yield to grain yield ratio from 2000 to 2003. Since the values in 1998 and 1999 were much larger than those obtained from 2000 to 2003, it may be possible that biomass in 1998 and 1999 was overestimated. Biomass of plants in CRP was measured only in 2003. Tillage did not influence biomass of crops, except for the total biomass from 2000 to 2003 where biomass was higher in NT than in CT.
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Crop Residue Cover, Amount, and Carbon Content
Crop residue cover in 2003 varied between tillage and crop rotations (Table 2). It is not surprising to observe higher residue cover in NT than in CT because residues were accumulated at the soil surface in NT compared to CT where residues were incorporated into the soil. Similarly, residue cover was higher in CRP than in other crop rotations, probably because perennial forages cover much of the soil, even in CT where tillage was discontinued after planting. Residue cover was also higher in W–L than in other rotations, except in CW. This is because the total amount of biomass returned to the soil was higher in CW and W–L than in other rotations (Table 1). Greater residue cover will obviously reduce the potential of soil erosion due to wind and water.
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0.001). The difference in residue amount between crop rotations in CT and NT led to a significant (P 0.05) tillage x crop rotation interaction. Although residue amount was higher in CRP, regardless of tillage, it was also higher in W–F in than in other rotations, except in spring wheat–spring wheat–fallow (W–W–F) in CT (Table 3). In NT, residue amount was higher in CW than in other rotations, except in W–W–F. Similarly, residue amount was higher in NT than in CT within CW, W–L, and W–W–F. Since there was no significant tillage x crop rotation interaction in the amount of crop biomass returned to the soil (Table 1), the difference in the residue amount between crop rotations in CT and NT could be either due to variations in the amount of biomass returned to the soil between crop rotations, or to decomposition rates of residue due to tillage. Averaging across the treatments, residue amount was greater in NT than in CT and greater in CRP than in crop rotations (Table 2). The greater amount of residue in NT with CW supports the observations of Halvorson et al. (2002a, 2002b) that reduced tillage and increased cropping intensity increases the amount of soil surface residue. Considering that the amount of residue lost or gained due to actions of wind and water is minimal, the amount of residue left in the soil after 6 yr of total biomass addition accounted for 9% in CW, 17% in W–F, 6% in W–L, 15% in W–W–F, and 9% in spring wheat–pea–fallow (W–P–F). Since 2000 kg ha–1 surface residue is needed to effectively control soil erosion (Fenster et al., 1977; Fryrear, 1985), NT with CRP, CW, and W–W–F will have increasing potentials to reduce soil erosion (Table 3).
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Soil Carbon
The SOC was significantly (P 0.10) influenced by tillage at 0- to 5-cm depth but crop rotation and tillage x crop rotation interaction were not significant (Table 4). Averaged across crop rotations, SOC at the 0- to 5-cm depth was 23% greater in NT than in CT. At 5- to 20- and 0- to 20-cm depths, SOC also tended to be greater in NT than in CT but it was not statistically significant. Our results are consistent with those obtained by Halvorson et al. (2002a, 2002b) who observed greater levels of SOC in NT than in CT at the 0- to 7.6-cm depth but not at the 7.6- to 15.2-cm depth in the northern and central Great Plains after 5 to 12 yr. Although a greater amount of crop residue was returned to the soil in CW than in other rotations, crop rotation did not influence SOC. Halvorson et al. (2002a) found that increased crop biomass residue returned to the soil with continuous corn or wheat–corn–fallow rotation did not increase SOC compared with W–F in NT or CT after 5 yr in the central Great Plains. Similarly, Ortega et al. (2002) reported that SOC was not significantly influenced by crop rotation in NT system after 8 yr in the central Great Plains, even though continuous cropping returned greater biomass residue to the soil than other crop rotations containing fallow. Increased cropping intensity increased biomass residue and SOC only after 12 yr (Sherrod et al., 2003). Tillage probably has a greater influence on SOC than crop rotation. Before the initiation of the experiment, our study site was under CRP for 10 yr but the sites used by Halvorson et al. (2002a, 2002b) were under conventional tillage. However, consistency in results between tillage systems in our study and their studies suggests that plant residue returned to the soil in 6 yr probably does not have much influence on SOC. West and Post (2002) concluded that reducing tillage intensity sequestered more SOC than enhancing crop rotation. Perhaps a longer time than the present 6 yr of study may be needed to observe changes in SOC due to crop rotation in the northern Great Plains.
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The POC was not influenced by tillage and crop rotation and averaged 2.1, 4.9, and 7.0 Mg ha–1 at 0- to 5-, 5- to 20- and 0- to 20-cm depths, respectively (Table 4). The proportion of SOC as POC at the 0- to 5-cm depth was greater in CT with W–W–F than in CT with CW and W–L and in NT with CRP and W–P–F. At the 5- to 20-cm depth, the proportion was greater in NT with CRP than in other treatments, except in NT with W–F and W–L. At the 0- to 20-cm depth, the proportion was greater in NT with W–L than in CT with CW, W–F, and W–L. This indicates that proportion of SOC retained as POC varied with tillage, crop rotation, and soil depth. While CT retained more wheat residue as POC compared to SOC at the 0- to 5-cm depth, NT retained more crop residue as POC at the 5- to 20-cm depth. Probably residue quality, quantity, and their different turnover rates at different depths as influenced by tillage may have influenced the proportion of labile and nonlabile pools of soil C.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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