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a USDA–ARS, Northwest Irrigation and Soils Research Lab., 3793 North, 3600 East, Kimberly, ID 83341
b Univ. of Idaho, Research and Extension Center, Twin Falls, ID 83303-1827
* Corresponding author (jentry{at}nwisrl.ars.usda.gov)
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: ICT, irrigated conservation-chisel-tilled crops • IMP, irrigated moldboard plowed crops • IP, irrigated pasture systems • NSB, native sagebrush vegetation
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Soils are the largest pool of C in the terrestrial environment (Jobbagy and Jackson, 2000; Schlesinger, 1990, 1995). The amount of C stored in soils is twice the amount of C in the atmosphere and three times the amount of C stored in living plants (Schlesinger, 1990, 1995; Kimble and Stewart, 1995), therefore, a change in the size of the soil C pool could significantly alter the atmospheric CO2 concentration (Wang et al., 1999). The current concentration of C in soils reflects the balance between past C accumulation and loss. Accumulation of C in soils is derived from litter and root input, while losses result from microbial degradation of organic matter, eluviation, and erosion (Entry and Emmingham, 1998). At equilibrium, the rate and amount of C added to the soil via vegetation are equal to the rate and amount of C lost through organic matter degradation and other losses (Henderson, 1995). Within limits, soil C increases with increasing soil water and decreasing temperature (Hontoria et al., 1999; Wang et al., 1999; Burke et al., 1989). The effect of soil water is much greater than the effect of soil temperature (Birch and Friend, 1956; Hontoria et al., 1999; Liski et al., 1999). Increasing water within temperature zones can increase plant production and, thus, C input to soils via increased plant litter and root production (Liski et al., 1999).
Land-use changes can impact the amount of C stored in the soil by altering C inputs and losses. In forest, grassland, and wetland ecosystems, conversion of native vegetation to agricultural cropping has resulted in substantial C transfer to the atmosphere as a result of loss of climax vegetation to the lower equilibrium C concentration in soil (Lal et al., 1999; Wang et al., 1999; Cambardella and Elliot, 1992; Johnson, 1992). In arid and semiarid environments plant survival and growth is limited by available water and irrigation is required to increase plant production to the point where crops become economically viable. Irrigation also increases C input to soils via increased litter and root production.
When assessing the potential of irrigation of arid or semiarid land to increase C storage in soils, one needs to assess C loss from CO2 emitted to the atmosphere as a result of (i) fertilizer manufacture, storage, transport, and application, (ii) fossil-fuel CO2 emitted from pumping irrigation water, (iii) farm operations, such as tillage and planting, and (iv) CO2 lost via dissolved carbonate in irrigation water (West and Marland, 2002; Schlesinger, 1999). Schlesinger (1999) used a fertilization value of 336 kg N ha-1 yr -1, which is an unusually high fertilization rate in U.S. farms. The CO2 released during fertilizer production of 336 kg N ha-1 yr-1 is approximately 16.7 g C m-2 yr-1 (Schlesinger, 1999). It has been noted that a more realistic rate is 100 to 150 kg N ha-1 yr-1 (West and Marland, 2002). Carbon dioxide released from pumping irrigation water in the USA, ranges from 126 kg C ha-1 yr-1, when using gasoline, to 266 kg C ha-1 yr-1 when using electricity (West and Marland, 2002). In addition, C may be lost as CO2 from the irrigation water itself. Irrigation water in arid and semiarid regions often contains as much as 1% dissolved CO2. When water is applied to basic soil, CaCO3 can precipitate, depositing C into the soil. If irrigation water containing 0.05 g L-3 dissolved Ca is used to irrigate crops in semiarid climates, the calculated increase in plant C is 2000 g C m-2 yr-1 over C contained in native soils and vegetation. The net CO2 released via irrigation water is calculated to be 8.4 g C m-2 yr-1 (Schlesinger, 1999).
Farm management practices, including conservation tillage and erosion control, have reduced the amount of CO2 emitted to the atmosphere in both Canada and the USA (West and Marland, 2002; Janzen et al., 1997; Paustian et al., 1997; Rasmussen and Collins, 1991). Intensively managed crop or pasture lands have potential for C gain through the use of improved grazing regimes, improved fertilization practices and irrigation management (Follett, 2001; Bruce et al., 1999). We hypothize that increasing plant growth on arid and semiarid lands by conversion to irrigated agriculture is one method that may increase C storage in soils. The objective of this research was to determine if land managed as IMP converted to irrigated conservation tillage or irrigated pasture could sequester additional C. We use our findings to pose several possible scenarios of altered land management polices that could favor global C sequestration based on the C budgets that we have estimated.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Experimental Design
The experiment was arranged in a completely randomized design (Kirk, 1982). Soil samples were taken from: (i) three sites supporting NSB located near agricultural land in southern Idaho (each site supported a basin big sage and a Wyoming big sage vegetation type); (ii) three sites that were formerly crop land and converted to and maintained as IP for the past 30 yr; (iii) three sites that were irrigated crop land and have been managed with conservation tillage (ICT) for the past 8 yr; and (iv) three irrigated agricultural crop lands in moldboard plowing systems (IMP) that were each growing alfalfa (Medicago sativa L.), wheat (Triticum aestivum L.), potato (solanum tuberosum L.), and bean (Phaseolus vulgaris L.). There were four treatments (NSB, IMP, ICT, and IP) x three sites for each treatment x five cores taken within each treatment at each site (replications) x four soil depths (0–5, 5–15, 15–30, and 30–100). We took a total of 240 samples.
Native Vegetation Sagebrush Sites
Native sagebrush sites were vegetated with native steppe vegetation and a low composition of exotic annual grasses. Sites were chosen for this study based on a history of no livestock grazing (U.S. Department of Interior Bureau of Land Management [BLM], Bruneau Resource Area, unpublished data, 1995). All study sites had 5–10% slope and were on areas that supported basin big sagebrush or Wyoming big sagebrush or communities (Table 1). Soil was classified as a fine, montmorillonitic, mesic Xerollic Haplargid on the Brown's Creek site, a coarse-loamy, mixed non-acid, mesic Xeric Torriorthents on the Simco site and a loamy, mixed, mesic lithic Xerollic Camborthids on the Kuna Butte site (Collett, 1982).
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Irrigated Conservation Tillage and Crop Sites
Three sites with fields rotating among alfalfa, wheat, potato, and bean were sampled. All sites were located on fields managed by USDA Agricultural Research Service's Northwest Irrigation and Soils Research Laboratory or the University of Idaho, Research and Extension Center. Soil on all sites was classified as a coarse-silty, mixed, superactive, mesic Durinodic Xeric Haplocalcid, with 0.1 to 0.21 g g-1 clay and 0.6 to 0.75 g g-1 silt, and organic matter of approximately 13 g kg-1. The soil has a pH between 7.6 and 8.0. Slope on these sites ranges from 1.0 to 3.0% (Table 1).
Sampling Procedures
Soil cores were taken from each site during winter (January), spring (April), summer (August), and autumn (November) in 1999. We sampled the top 1 m of soil each season (winter, spring, summer, and autumn) to determine if the amount of C in soil would be affected by vegetation and irrigation. Sampling locations were randomly chosen at each site or field. Separate 10-cm diam. cores were taken and partitioned into 0- to 5-, 5- to 15-, 15- to 30-, and 30- to 100-cm depths. Roots greater than 1.0 cm in diameter were measured separately. Carbon in aboveground vegetation was estimated by measuring the amount of material in 10 separate 1.0 m2 areas in each site or field (Entry and Emmingham, 1998).
Carbon in Soil and Aboveground Vegetation
Concentration of organic C in each sample of mineral soil was determined by the Walkley-Black procedure and loss on ignition (Nelson and Sommers, 1996). The amount of C per hectare of the 0- to 100-cm depth of mineral soil was calculated assuming 0.44 g C g-1 organic matter with correction for soil bulk density. Ten separate 10 cm diam. soil cores were taken to a 1.0-m depth, divided into 0- to 5-, 5- to 15-, 15- to 30-, and 30- to 100-cm depths to determine bulk density. Bulk density was measured by dividing by the oven dry weight after drying at 105°C for 48 h by the volume of the sample (Blake and Hartage, 1982). Aboveground vegetation was collected and separated into sage, grass, forbs, herbs, and duff. Aboveground material was dried at 80°C for 48 h, weighed and ground to pass a 1-mm opening. Carbon in aboveground vegetation was determined by loss on ignition (Nelson and Sommers, 1996). The amount of C in the aboveground material was assumed to contain 0.44 g C g-1 organic matter on an ash free basis (Nelson and Sommers, 1996). Calculations estimating C stored in soils in the Western USA and worldwide are based on the Walkley-Black procedure.
Calculations
Concentration of organic C determined by the Walkley-Black procedure was converted, using bulk density measurements, to a meter square basis to a depth of 1 m. Organic C in kilograms per square meter was converted to megagrams of C per hectare multiplying by 10000 (land area) and dividing by 1000 (C weight), which is a 1:10 conversion. We divided the resulting number by 10 to account for a 10% conversion of one treatment (land area). The amount of C sequestered in the Pacific Northwestern USA, the 11 western states in the USA, and worldwide was estimated by multiplying megagrams of C per hectare by the number of hectares of irrigated land in each area. There are 9055979 ha of land in irrigated crop land in the Pacific Northwest, 24322029 ha in the Western USA, and 260000000 ha worldwide (Bucks et al., 1990; Tribe, 1994; Howell, 2000). The carbon sequestered (CS) relative to the amount of C projected to be emitted during the next 30 yr (CEW) was calculated by dividing the megagrams of C sequestered in each treatment x area by the total projected worldwide release of CO2–C during the next 30 yr (5.7 x 1010 Mg C) multiplied by 100.
Statistical Analysis
All data were subjected to a one way vegetation type analysis of variance (ANOVA) for a completely randomized design (Snedecor and Cochran, 1980; Kirk, 1982). Residuals were normally distributed with constant variance. Statistical Analysis Software programs (SAS Institute Inc., 1996) were used to conduct the analysis of variance. Significance of treatment means were determined at P < 0.05 with the Least Square Means test.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Regional and Global Implications
Changes in agricultural practices have great potential to sequester C. In most cases converting selected land managed as IMP to ICT or IP can be implemented with modest economic impact to landowners and pose relatively few socioeconomic issues. If agricultural land is managed properly, these practice shifts would also potentially reduce erosion and water or air pollution. Estimating the potential for C sequestration in terrestrial ecosystems is difficult because the dynamics that control C flow among plants, soils and the atmosphere are poorly understood. Storage of C in below ground systems is the best long-term option in terrestrial ecosystems because C in soils has a longer residence time than most plant biomass. Using the values obtained in southern Idaho, we estimated C storage in soils locally, regionally, and globally in soils if: (i) 10% of irrigated land now in IMP agriculture was converted back to NSB, (ii) all land presently in IMP was converted to ICT, and (iii) 10% of land in irrigated IMP was converted to IP. Since increased agricultural production will be necessary to feed an increasing population, it is impractical to suggest that a large portion of land in IMP can be converted to IP.
The reported amounts of C stored in NSB vegetation and irrigated agricultural systems are similar throughout the USA and worldwide (Bowman et al., 1999; Collins et al., 1999; Amthor et al., 1998; Potter et al., 1998; Rasmussen and Parton, 1994; Schlesinger, 1977). These data were used to calculate potential C storage for irrigated agriculture in the Pacific Northwestern USA, the Western USA, and worldwide over a 30-yr period. If land currently in IMP is converted to NSB we estimated a gain of 1.5 Mg C ha-1 (Table 4). Little of this land is managed with conservation tillage. We estimate an increase of 9.5 Mg C ha-1 over 30 yr if the land presently managed with IMP were converted to ICT. Using this value we calculated that 8.6 x 107 Mg C (0.15% of the total C emitted in the next 30 yr) could potentially be sequestered in irrigated soils in the Pacific Northwestern USA (Table 4). Using these values to represent C gains for all irrigated crop land in the western USA and if land in IMP were converted to ICT, a possible 2.3 x 108 Mg C (0.40% of the total C emitted in the next 30 yr) could be sequestered in irrigated agricultural soils in the next 30 yr. If the world's IMP land were converted to ICT, 2.5 x 109 Mg C (4.38% of the total C emitted in the next 30 yr) could be sequestered in the next 30 yr. A shift of 10% of current IMP land to ICT is a reasonable conservation practice goal. Similarly, we cannot expect all land presently in IMP to be converted to IP, but a 10% conversion is feasible. If we predict a storage increase of 37.1 Mg C ha-1 for IMP converted to IP and assume 10% of IMP land is converted to IP, an estimated 3.4 x 107 Mg C (0.05% of the total C emitted in the next 30 yr) could be sequestered in Pacific Northwestern USA soils and 9.0 x 107 Mg C (0.16% of the total C emitted in the next 30 yr) in the Western USA soils in the next 30 yr. If our study values are used to represent C gains for irrigated crop land worldwide, an estimated 9.6 x 108 Mg C (1.68% of the total C emitted in the next 30 yr) could be sequestered if irrigated land presently managed as IMP were converted to ICT (Table 4). Conversion of crop land to irrigated pasture could also relieve grazing pressure on public rangelands, an issue of heated debate between environmentalists and ranchers.
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Since the earth releases 1.9 x 109 Mg C yr-1 (Schlesinger, 1995; Amthor et al., 1998), the conversion of 10% IMP to IP, resulting in a possible 9.6 x 108 Mg C sequestered over 30 yr, may be insignificant (Table 4). However, if crops were produced via high output irrigated agriculture while less productive rainfed agricultural land were returned to temperate forest or native grassland, an increase of 5.6 and 13 Mg C ha-1, respectively, could be gained over 30 yr for each unit of rainfed land converted to native vegetation (Table 5). The amount of irrigated agriculture can likely be increased at least 10% solely through increases in irrigation efficiency and waste water reuse (Howell, 2000). Using a conversion basis of 1 unit of irrigated agriculture to return 1 unit of rainfed agricultural land to native forest, if irrigated agriculture were expanded 10% (meaning that an additional 2.6 x 107 ha of arid or semiarid land were irrigated) and the equal amount of land being managed as rainfed agricultural land were converted to native forest, there is potential to sequester 5.1 x 107 Mg C (0.09% of the total C emitted in the next 30 yr) in the Pacific Northwest (PNW), 1.41 x 108 Mg C (0.24% of the total C emitted in the next 30 yr) in the western USA and 1.5 x 109 Mg C (2.6% of the total C emitted in the next 30 yr) worldwide (Table 5). If the rainfed agricultural land were converted to native grassland, there is a potential to sequester 1.2 x 108 Mg C (0.2% of the total C emitted in the next 30 yr) in the PNW, 3.2 x 108 Mg C (0.6% of the total C emitted in the next 30 yr) in the western USA, and 3.4 x 109 Mg C (5.9% of the total C emitted in the next 30 yr) worldwide (Table 5).
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Since native desert or semidesert has relatively little ecosystem C compared with forest, grassland, or wetland ecosystems (Houghton et al., 1999; Amthor et al., 1998; Schlesinger, 1977), converting IMP land back to desert or semidesert could result in a soil C gain of 0.15 kg C m-2. A sequestration of only 1.90 x 106 Mg C 30 yr-1 might be expected in the Pacific Northwestern USA, 5.10 x 106 Mg C 30 yr-1 in the western USA, and 5.46 x 107 Mg C 30 yr-1 worldwide. This modest C accumulation in soil would be tempered by the fact that the conversion would require substantial policy incentives and decades to implement. Substantially more C may be sequestered by selectively returning rainfed agricultural land derived from forest, grassland, or wetlands back to native vegetation. Tropical and temperate forests typically contain from 10 to 12 kg C m-2, grasslands contain from 18 to 20 kg C m-2, and wetland ecosystems contain from 60 to 70 kg C m-2, whereas arid and semiarid lands contain 5 to 7 kg C m-2 (Houghton et al., 1999; Ross et al., 1999; Schimel et al., 2000). Since nearly a third of the yield and nearly half of the value of crops in the USA are produced on irrigated lands predominantly in arid or semiarid climatic zones (Bucks et al., 1990; Tribe, 1994; Howell, 2000), a strong strategic rationale can be made for expanding irrigated agriculture in these areas for both crop production and C sequestration, if accompanied by selective return of rainfed agricultural land derived from forest, grassland, or wetlands back to native vegetation.
Factors Affecting Interpretation
Grazing affects the quantity and chemical composition of soil organic matter and the distribution of C in the soil profile (Schuman et al., 1999; Frank et al., 1995; Dommar and Williams, 1990). Schuman et al., (1999), Frank et al. (1995), and Dommar and Williams (1990) found that grazing often increases the concentration of soil C. Ecosystems coevolved with herbivores. The fact that C storage in range and grassland ecosystems may be unaffected, but usually is increased with light grazing, suggests that grazing is an important part of long-term sustainability of these ecosystems (Schuman et al., 1999).
We recognize that the values for potential C gain in our study are estimates. To obtain a more precise estimate of potential C sequestration from management conversions on a worldwide basis it would be necessary to investigate the potential C accumulated in soils in many different vegetation types. Use of these data from Idaho provide an indication of the potential for these kinds of management shifts on a larger scale. Our estimated values for C gain may actually be conservative due to improving land management methods and improving irrigation technology. The C trends that we monitored were the end result of management that predated new technology now available that would have prevented much of the erosion and loss of soil C on our monitored irrigation sites. Most irrigated cropping worldwide uses surface irrigation, with substantial runoff resulting in some transport offsite of C via erosion with sediment and dissolved C in the water.
Flood and furrow irrigation also transport nutrients, pesticides, and enteric microorganisms offsite and ultimately to surface and ground water (Sojka and Entry 2000; Sojka et al., 1998a, 1998b). Conversion of furrow irrigation to sprinkler irrigation reduces C transport off site via sediment and water because of dramatic reductions of offsite flow and leaching (Aase et al., 1998). The use of conservation tillage and improved sprinkler irrigation systems to reduce erosion, especially in combination with new technologies such as the use of polyacrylamide, has potential to further reduce C transport and degradation (Aase et al., 1998). Additional C that may be sequestered resulting from improved long-term inputs of technology needs to be determined to more accurately predict potential C gains by irrigated agriculture in the future. Our estimates made no attempt to adjust C budgets for loss of C because of erosion. Because great improvements in controlling irrigation-induced erosion have occurred in recent years, it is likely that our C storage estimates for irrigated agriculture are conservative.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The potential C sequestered on site by conversion of native vegetation to irrigated agriculture is above the steady state equilibrium of native vegetation. This is in contrast to rainfed agricultural systems which are currently attempting modest C gains to reattain near- baseline C concentrations by implementing reduced or no-tillage practices. Rainfed agricultural lands with or without no-till practices have soil C values far below those of native vegetation. A third of the yield and nearly half of the value of crops in the USA and worldwide are produced on the irrigated 15 to 17% of arable lands that are largely in arid or semiarid climatic zones (Tribe, 1994; Bucks et al., 1990). Since the earth releases 1.9 x 109 Mg C yr-1 (Amthor et al., 1998; Schlesinger, 1995), the conversion of 10% IMP to ICT resulting in a possible 2.5 x 109 Mg C sequestered over 30 yr is a modest 4.47% compared with the total C released into the atmosphere over that time period. However, if crops were produced via high output irrigated agriculture, while selected less-productive rainfed agricultural land were returned to temperate forest or native grassland on a worldwide basis, there could be substantial reductions in atmospheric CO2. Policy makers and agricultural research infrastructure should recognize the enormous potential benefit of land and water management strategies, policies and incentives that could expand arid zone irrigated agriculture as a means for efficient food and fiber production along with substantial C sequestration potential. This potential would be enhanced if coupled with selective return of less efficient rainfed agricultural lands derived from forest, grassland or wetlands back to native vegetation. We recognize that such an expansion would have to be accompanied by renewed efforts of water development. While water resource development has been occurring at a modest pace worldwide since 1990, Howell (2000) indicated the potential for increased extent of irrigation via efficiency improvements and waste water use. Recognition of these potential C benefits should provide an incentive to fund research and pursue management strategies that are possible without sacrificing production and which could increase restoration of native ecosystems, reduce erosion and improve water quality through appropriate targeting of the strategy.
Received for publication April 12, 2001.
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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