An empirical model for estimating carbon sequestration on the Canadian prairies

Transcription

1 An empirical model for estimating carbon sequestration on the Canadian prairies B. C. Liang 1, C. A. Campbell 2, B. G. McConkey 3, G. Padbury 4, and P. Collas 1 Can. J. Soil. Sci. Downloaded from by on 04/27/18 1 Greenhouse Gas Division, Environment Canada, Place Vincent Massey, 19th Floor, 351 St-Joseph Blvd., Gatineau, Quebec, Canada K1A 0H3 ( chang.liang@ec.gc.ca); 2 Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Central Experimental Farm, Ottawa, Ontario, Canada K1A 0C5; 3 Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, Saskatchewan, Canada S9H 3X2; and 4 Saskatchewan Land Resource Unit, Agriculture and Agri-Food Canada, Saskatoon, Saskatchewan, Canada S7N 5A8. Received 15 May 2004, accepted 2 August Liang, B. C., Campbell, C. A., McConkey, B. G., Padbury, G. and Collas, P An empirical model for estimating carbon sequestration on the Canadian prairies. Can. J. Soil Sci. 85: There is a need to develop verifiable algorithms that can be easily applied to estimate carbon sequestration in soils. A simple process-based empirical model, driven primarily by soil texture and crop residue input, was developed to account for changes in soil organic carbon (SOC) in Chernozemic soils on the Canadian prairies. The model was used to estimate SOC change under no-till and continuous cropping compared with conventional tillage and rotations with fallow. Using this model, C sequestration due to continuous cropping compared with fallow-containing rotations was determined to be 0.09 Mg C ha 1 yr 1 for the Brown and Dark Brown, and 0.05 Mg C ha 1 yr 1 for the Black and Dark Gray/Gray soil zones. The rate of C sequestration as a result of continuous cropping was positively related to the frequency of fallow, which decreases on the prairies from the Brown, Dark Brown, and Black to the Dark Gray/Gray soil zones. Using this model average C sequestration when conventional tillage was converted to no-till, was 0.13, 0.23, 0.34, and 0.25 Mg C ha 1 yr 1 for the same soil zones, respectively. Combined gains due to no-till and continuous cropping in comparison with conventional tillage and fallow-containing rotations were determined to be 0.22, 0.32, 0.39, and 0.30 Mg C ha 1 yr 1 for the Brown, Dark Brown, Black and Dark Gray/Gray soil zones, respectively. Based on Agricultural Census of Canada data in 1996 and 2001, the amount of C sequestered due to the adoption of no-till was estimated to be 1.23 million Mg of C in 1996 and 1.72 million Mg of C in 2001, which is approximately 10% of the total greenhouse gas emissions from the agricultural sector in Canada. This simple process-based empirical model could serve as a useful tool for soil scientists to use in assessing soil sustainability and C sequestration in the Canadian prairies. It would also assist policy makers in understanding how various scenarios of improved management will influence future greenhouse gas emissions on agricultural soils. Key words: Soil organic carbon, no-till, fallow, crop rotation Liang, B. C., Campbell, C. A., McConkey, B. G., Padbury, G. et Collas, P Modèle empirique pour l estimation de la séquestration du carbone dans les Prairies canadiennes. Can. J. Soil Sci. 85: On a besoin d algorithmes faciles pour estimer la quantité de carbone séquestrée dans le sol. Les auteurs ont élaboré un modèle empirique d une grande simplicité reposant essentiellement sur la texture du sol et l importance des résidus végétaux ajoutés en vue de déterminer comment la concentration du carbone organique (CO) varie dans les sols tchernozémiques des Prairies canadiennes. Ils ont utilisé leur modèle pour estimer la fluctuation du CO du sol avec un régime de non-travail du sol et de monoculture et celui de travail ordinaire du sol et d assolement avec jachère. Le modèle indique que, comparativement à l assolement avec jachère, la monoculture permet de séquestrer 0,09 Mg de carbone par hectare et par année dans les sols bruns et brun foncé, et 0,05 Mg de carbone par hectare et par année dans les sols noirs et gris foncé/gris. Le taux de séquestration du carbone résultant de la monoculture présente une corrélation positive avec la fréquence des jachères, pratique qui, dans les Prairies, s atténue des zones de sols bruns à celles de sols brun foncé, noirs et gris foncé/gris. Toujours au moyen de ce modèle, la quantité moyenne de carbone séquestrée quand on convertit le travail classique du sol en non-travail du sol s établit respectivement à 0,13, à 0,23, à 0,34 et à 0,25 Mg de carbone par hectare et par année pour les mêmes zones. Les gains combinés issus du non-travail du sol et de la monoculture, comparativement au travail ordinaire du sol et à l assolement avec jachère, s établissent respectivement à 0,22, à 0,32, à 0,39 et à 0,30 Mg de carbone par hectare et par année pour les zones de sols bruns, brun foncé, noirs et gris foncé/gris. Selon les données du recensement agricole canadien, les auteurs ont évalué la quantité de carbone séquestrée consécutivement à l adoption du non-travail du sol à1,23 million de Mg de carbone en 1996 et à 1,72 million de Mg de carbone en 2001, ce qui correspond à environ 10 % des émissions globales de gaz à effet de serre de l agriculture canadienne. Ce modèle empirique simple pourrait s avérer utile aux spécialistes de la science des sols qui souhaitent évaluer la pérennité des sols et la séquestration du carbone dans les Prairies canadiennes. Il aiderait aussi les auteurs de politiques à comprendre l incidence des diverses méthodes visant à améliorer la gestion des terres sur les futures émissions de gaz à effet de serre par les terres cultivées. Mots clés: Carbone organique du sol, non-travail du sol, jachère, assolement 549 Abbreviations: CC, continuous cropping; CT, conventional tillage; FC, fallow-containing; NT, no-tillage; RAI SOC, relative annual increase in SOC; SOC, soil organic carbon

2 Can. J. Soil. Sci. Downloaded from by on 04/27/ CANADIAN JOURNAL OF SOIL SCIENCE Across the four soil zones (Brown Chernozem, Dark Brown Chernozem, Black Chernozem and Dark Gray Chernozem/ Gray Luvisol) of the Canadian prairies, approximately 32 million ha of land are currently under annual crop production, excluding permanent pasture and forage (Campbell et al. 2002). This accounts for more than 80% of the arable land in Canada. Mean annual precipitation in the region varies from 350 to 450 mm with the highest for the Black and Dark Gray/Gray soil zones and the lowest for the Brown soil zone, whereas mean annual air temperature is lowest in the Black and Dark Gray/Gray soil zones and highest in the Brown soil zone (Campbell et al. 1990). Annual moisture deficits, defined as the difference between annual precipitation and potential evaporation, are approximately 400 mm for the Brown soil zone, 270 mm for the Dark Brown soil zone, and 150 mm for the Black and Dark Gray/Gray soil zones (Campbell et al. 1990). Because moisture deficit is a major factor controlling crop productivity, summerfallow, using primarily conventional tillage management, was traditionally practiced in the more arid regions of the prairies to enhance soil moisture and supply available N while reducing agronomic and economic uncertainties caused by frequent droughts (McConkey et al. 1996). The high soil disturbance associated with conventionally tilled fallow has resulted in soil degradation and the loss of SOC, estimated to be about 30% below levels under native vegetation (Janzen et al. 1997). In recent years, soil management to increase SOC has received considerable attention because of its potential for CO 2 mitigation and other environmental co-benefits (Bruce et al. 1999). Over the past 40 yr numerous field experiments have been conducted on the Canadian prairies that have allowed the impact of crop rotations and tillage practices on SOC to be evaluated (Campbell et al. 1995; 1996a,b; Liang et al. 1999; Janzen et al. 1998; Campbell et al. 2005). However, quantification of average change in SOC at regional and national scales is still a challenge, because the magnitude of short-term changes in SOC are very small compared with the amount of carbon stored in the prairie soils and also because there is a high degree of spatial variability. To resolve these problems, simulation models, such as Century (Parton et al. 1987) and Roth-C (Jenkinson 1990), for quantifying SOC changes have been developed to facilitate extrapolation of experimental data from site-specific locations to landscape, regional and national scales. One limitation in using such models is that the information required to quantify the parameters in the models is not always readily available or easily estimated for most arable soils. If enough sites and measurements are available, a simulation model may not be required for quantifying change in SOC. However, in Canada this is totally unrealistic because of the diversity in soils, climate, cropping systems and management practices. Smith et al. (1997) used the Century model to quantify annual changes in SOC due to changes in crop management practices for Canada. Using land use data from the agricultural census, the Century model generated baseline CO 2 emissions of 7 Mt in 1990 for Canada s agricultural cropland (Smith et al. 1997). However, the Century model estimates the long-term change in SOC, thus the above 7 Mt CO 2 emission predicted for 1990 might not represent changes in SOC due to management, but rather reflect an artefact of the model itself and its slow progression toward equilibrium from the original conversion to cropland. To improve the estimates of CO 2 sources or sinks associated with Canada s agricultural soils, for both the annual reporting required under the United Nations Framework Convention on Climate Change and future Kyoto Protocol reporting, we need to develop verifiable algorithms that can be easily used to estimate C sequestration in soils. The objective of this paper is to propose an approach for quantifying C sequestration resulting from the adoption of no-till management and continuous cropping on the Canadian prairies where conventional tillage and frequent summerfallowing currently predominate (Campbell et al. 2002). MATERIALS AND METHODS Model Development The following process-based empirical model was developed to estimate changes in SOC under no-till and continuous cropping on the Canadian prairies, relative to conventional tillage systems with fallow-containing crop rotations. Campbell et al. (2005) analyzed SOC data for the Canadian prairies and showed that SOC gains were directly related to cropping frequency in tilled and no-till systems in all soil zones and that gains were generally greater under notill than under tilled systems. However, though there were no interactions between the effect of cropping frequency and tillage on SOC gains in the Brown and Dark Brown soil zones, there was in the Black and Gray soil zones. Nonetheless, in our calculations, for the sake of simplicity, we will assume there are no interactions. Thus, the combined effect of these two management factors is described as the sum of changes in SOC due to changes in crop residue input between continuous and fallow-containing cropping rotations, and those due to changes for no-till versus conventional tillage. The amount of organic C in soil depends on the rate of SOC decomposition and the amount of crop residue that is returned to the soil. At any particular time the amount of SOC can be divided into two components, one derived from initial SOC and the other from more recent crop residue inputs since the start of the experiment or another reference time. For a cropping system, this can be expressed as: Total SOC = Initial SOC + SOC derived from crop residue The decomposition of initial SOC can be described by a first-order exponential equation as: SOC t = SOC 0 e kt (1) where SOC 0 and SOC t are the amounts of SOC at t = 0 and t = t (years), respectively, and k is a rate constant. Under certain climatic conditions and management practices, a balance between the loss of SOC through decomposition and the gain of SOC through crop residue input can be achieved,

3 LIANG ET AL. EMPIRICAL MODEL TO ESTIMATE C SEQUESTRATION 551 Can. J. Soil. Sci. Downloaded from by on 04/27/18 called a steady state level of SOC. Although changes in initial SOC over a relatively short time cannot be accurately determined experimentally, the amount of SOC derived from more recent crop residues can be estimated using stable (Liang et al. 1998) and radioactive C isotopes (Voroney et al. 1989). When the SOC reaches an equilibrium level, it is possible to derive the rate constant (k). In order to estimate the accumulation of SOC after crop residue input, Voroney et al. (1989) developed coefficients and decomposition rate constants for semiarid southwestern Saskatchewan. These coefficients and rate constants were based on a study in which 14 C-labeled wheat (Triticum aestivum L.) straw from a fallow-wheat-wheat-wheat rotation was incorporated into a Sceptre clay soil (Rego Brown Chernozem) and the 14 C was monitored annually for 10 yr. Voroney et al. (1989) obtained the following relationship: Y = 0.72e 1.4t e 0.081t (2) where, Y is the proportion of residue 14 C remaining in the soil and t is years since residue application. Jenkinson (1977) also obtained a similar equation for an experiment in Rothamsted, UK using 14 C-labeled ryegrass (Lolium perenne L.) over a 10-yr period. The Voroney et al. (1989) equation (Eq. 2) has been found to be applicable for estimating C sequestration due to differences in crop residue input resulting from cropping systems that contain various frequencies of summer-fallow on the Canadian prairies (Campbell et al. 1995, 1996a,b, 2000; Bremer et al. 2002). Soil texture is known to affect changes in SOC through its chemical and physical protection of recent C inputs (Franzluebbers et al. 1996; Hassink and Whitmore 1997). Liang et al. (1998) conducted field studies varying from 3 to 12 yr in duration with continuous corn (Zea mays L.) in Ontario and Quebec, and reported that there were large differences in crop residue-c retention across soil textures as determined by 13 C natural abundance. Fine-textured soils retained a greater proportion of crop residues than coarsetextured soils (Fig. 1). Using field studies conducted in southwestern Saskatchewan (Hatton fine sandy loam, Swinton loam and Sceptre clay), Liang et al. (2000) compared the method of Voroney et al. (1989) with that of Liang et al. (1998) in estimating SOC derived from crop residues, and reported that both methods estimated about the same amount of SOC derived from crop residues for the loamytextured soils (Swinton loam). However, the method of Voroney et al. (1989) tended to overestimate the amount of SOC derived from crop residues for the Hatton fine sandy loam, and underestimated the amount of SOC derived from crop residues for the Sceptre clay. In this paper we estimated SOC derived from crop residues using the method of Liang et al. (1998). To quantify the amount of SOC derived from selected cropping systems under conventional tillage, we propose the following equation: t kt SOCt = SOC0e + F CRC 1 (3) Fig. 1. Retention of corn residue-c vs. clay + silt content of soil [redrawn after Liang et al. (1998)]. where F (%) is the fraction of crop residue-c (CRC) retained in the soil, which is a linear function of silt plus clay content of soil; F = X X 2, where X = silt plus clay (%) (Fig. 1) (Liang et al. 1998), and ΣCRC is the total amount of crop residue-c returned to the soil from t = 1 to t = t years (Mg C ha 1 ). Equation 3 can be simplified if the change in SOC is due only to the amount of crop residue inputs while comparing continuous cropping with fallow-containing rotations. Moisture regimes between continuous cropping and fallowcontaining systems on the prairies are likely to be different, and this may affect the rate of decomposition of initial SOC. However, several studies have suggested that the decomposition of initial SOC was similar under different crop rotations or management practices for a given soil (Gregorich et al. 1996; Liang et al. 2000). In this paper, it was assumed that the decomposition of initial SOC under CC and under fallow-containing (FC) systems would be the same. Thus, the amount of sequestered SOC between continuous cropping versus fallow-containing cropping can be calculated as: t SOC( CC FC) = F CRCCC CRCFC 1 ( ) where CRC CC and CRC FC are the amounts of crop residue- C returned to the soil from t = 1 to t = t years (Mg C ha 1 ) with CC and FC systems, respectively. SOC (CC-FC) is the net change in SOC due to the difference in the amount of crop residues produced in CC versus FC systems. Tillage operations, such as conventional tillage practices, are known to contribute to the reduction of soil organic matter because of soil mixing, disruption of aggregates, increased aeration and erosion (Balesdent et al. 1990). Reducing tillage intensity and increasing surface crop residues, such as occurs when cropland management shifts from conventional tillage with fallow to no-till and continuous cropping, will reduce losses of organic matter from the (4)

4 Can. J. Soil. Sci. Downloaded from by on 04/27/ CANADIAN JOURNAL OF SOIL SCIENCE soil (Campbell et al. 2005). Studies in Saskatchewan have shown that adoption of no-till along with elimination of summer-fallow for a period of 8 to 25 yr can sequester SOC at rates of 0.3 to 0.8 Mg ha 1 yr 1 (Campbell et al. 1995, 1996a,b; Liang et al. 1999; McConkey et al. 2003). Similar results have been reported in Alberta (Nyborg et al. 1995; Franzluebbers and Arshad 1996; Larney et al. 1997; Janzen et al. 1998). The impact of tillage on SOC can be quantified empirically based on soil texture. The relative annual increase in SOC (RAISOC) under no-tillage (NT) compared with conventional tillage (CT) can be calculated as: SOC SOC RAISOC NT = CT 100 SOCCT Year where RAISOC (% yr 1 ) is the relative annual increase in SOC per year under NT, SOC NT is the amount of SOC under NT (Mg C ha 1 in the 0- to 15-cm soil), SOC CT is the amount of SOC under CT (Mg C ha 1 in the 0- to 15-cm soil), and Year is the number of years since the change to NT. Based on a number of short- to mid-term field experiments conducted in Saskatchewan, Liang et al. (1999, 2002) concluded that the relative annual increase in SOC under NT compared with CT was directly proportional to the clay content of soil (Fig. 2). Thus, the amount of SOC gained under NT (Mg C ha 1 ) during a period of t years can be quantified as: SOC( NT CT) = SOCCT RAISOC t 001. where SOC (NT-CT) is the net change in SOC due to adoption of NT, t is number of years under NT, and SOC CT is the amount of SOC under CT. The relative annual increase in SOC under NT is a linear function of clay content (%) as defined by RAISOC = Clay (Fig. 2), and RAISOC is converted into a fraction by multiplying The effects of crop rotation and tillage on SOC are assumed to be independent (Liang et al. 1999; McConkey et al. 2003). Thus, to quantify the combined impact of tillage and crop rotation on SOC we add Eq. 5 and Eq. 6. SOC = SOC( CC FC) + SOC( NT CT) where SOC (Mg C ha 1 ) is the net sequestered C due to adoption of NT [ SOC (NT-CT) ] along with continuous cropping [ SOC (CC-FC) ] when a system was previously conventionally tilled and included fallow. This model is most suitable for short- to mid-term time periods, typically from 5 to 25 yr, because most of the model coefficients were derived from studies of medium duration. (5) (6) (7) Fig. 2. Relative annual increase in soil organic C under no-tillage as influenced by the clay content of soil from eight field studies conducted on the Canadian prairies [adapted from Liang et al. (2002)]. Original data were collected from a short-term study at Tisdale, Melfort and Swift Current, SK (Liang et al. 2002), from a mid-term study at Swift Current, Scott, and Indian Head, SK, and from a long-term study at Melfort, SK (McConkey et al. 2003). Soil and Agronomic Information Used to Estimate SOC Change on the Prairies In order to apply Eq. 7 for estimating C sequestration due to the adoption of NT on the prairies, we need (1) soil information, including background SOC concentration in the surface soil, soil bulk density, and soil texture; (2) agronomic information, such as average crop yield and a yield adjustment factor for crops grown on stubble; and (3) area of land subject to change in management (e.g., area of cropland under NT). The average SOC of the upper 15 cm (Ap horizon) of soil for arable land in each soil zone on the prairies was estimated from Rostad et al. (1993) by using the mid-point of the dominant organic matter class in each soil zone. The clay soils in the Brown and Dark Brown soil zones are Vertisols and thus their surface Ap horizons are lower in carbon than the Ap horizons of loamy soils in the same zones (Table 1). The areas of cultivated sandy loams, loams and clays for each soil zone were obtained from the Soil Landscapes of Canada version 3.0 database (Agriculture and Agri-Food Canada 2005). Average sand, silt and clay contents within each soil zone are representative for each soil textural class (Table 1). Cereals are the predominant crops grown on the Canadian prairies with approximately 90% of cereals being spring wheat and durum wheat (Triticum turgidum L.), and the other 10% consisting of oats (Avena sativa L.), barley (Hordeum distychum L.) and rye (Secale cereale L.). Weighted average cereal yields can be obtained from Saskatchewan Agriculture and Food (1997), which published 10-yr average yields from 1987 to 1996 (Table 1). When farmers on the prairies reduce their summer-fallow area, the yields of cereal crops grown on stubble are usually lower compared with crops grown on fallow (Campbell et al. 2002). Conventional tillage on the Canadian prairies usually consists of one or more fall tillage operations with 100% surface soil disturbance, and one or more spring tillage operation with 100% surface soil disturbance (Statistics Canada 2002). Any land using summer fallow usually receives at least three tillage operations during the fallow year

5 LIANG ET AL. EMPIRICAL MODEL TO ESTIMATE C SEQUESTRATION 553 Can. J. Soil. Sci. Downloaded from by on 04/27/18 Table 1. Estimated annual C sequestration with no-tillage based on the 1996 and 2001 census of agricultural land use on the Canadian prairies Rate of C gain Soil organic C in Soil texture content z Average Percent of no-till Continuous Sequestered C u Soil Areaz the 0 to 15-cm soil y Sand Silt Clay cereal yield x 1996 w 2001 v No-till cropping Soil zone texture class (million ha) (%) (Mg C ha 1 ) (%) (Mg ha 1 ) (Mg C ha 1 yr 1 ) (Million Mg C yr 1 ) Brown Fine Medium Coarse Dark Brown Fine Medium Coarse Black Fine Medium Coarse Dark Gray/Gray Fine Medium Coarse Total z Agriculture and Agri-Food Canada (2005). y Rostad et al. (1993). x Saskatchewan Agriculture and Food (1997), and fallow frequency in the Brown, Dark Brown, Black and Dark Gray/Gray soil zones were assumed to be 35, 23, 11 and 10%, approximately equivalent to 1 yr fallow and 2,3, 9 and 9 yrs of crop, respectively. w Statistics Canada (1997). v Statistics Canada (2002). u Sequestered C for 1996 is estimated by using the total area of cropland within each soil textural class multiplied by the percent of no-till in 1996, and multiplied by the rate of C gain (total carbon gain under no-till and continuous cropping).

6 Can. J. Soil. Sci. Downloaded from by on 04/27/ CANADIAN JOURNAL OF SOIL SCIENCE (Statistics Canada 2002). No-till involves one-pass seeding operation with minimal soil disturbance (Statistics Canada 2002). Statistics Canada collects census data on land areas subject to change in crop management practices on a provincial basis, listing data such as area of NT and publishes Agricultural Profile of Canada on a 5-yr interval (Statistics Canada 1997, 2002). This provincial land use information is also available for various soil zones on the prairies (Saskatchewan Agriculture and Food 1997). Based on the 1996 census data (Saskatchewan Agriculture and Food 1997), fallow frequency from 1987 to 1996 was once in 3, 4, 9, and 10 yr for the Brown, Dark Brown, Black and Dark Gray/Gray soil zones, respectively (Table 1). Average cereal yields under continuous cropping were assumed to be approximately 75, 80, 90 and 95% of the average yield on fallow for the Brown, Dark Brown, Black and Dark Gray/Gray soil zones, respectively (Campbell et al. 1990). Farmers who adopt NT on the Canadian prairies often completely eliminate fallow in their cropping systems. Crop residues were estimated from grain yields and a fixed harvest index of 40%. The C input from crop residues included root estimates and assumed a root:straw ratio of 0.59 (Campbell et al. 1977) and the C concentration of tissues to be 45% (Millar et al. 1936). Average cereal yield was the average yield between 1986 and 1996, taken from Saskatchewan Agriculture and Food (1997). Fallow frequency and percent of NT for each soil zone in 1996 and 2001 were obtained from Saskatchewan Agriculture and Food (1997) and Statistics Canada (1997, 2002). RESULTS AND DISCUSSION Carbon Sequestration Due to No-till Associated with Continuous Cropping The average annual amount of C sequestered on the Canadian prairies under NT in 1996 and 2001 was estimated from land use census data, using the rate of increase in SOC under NT continuous cropping (Eq. 7) multiplied by the area of agricultural land under NT. We recognize that not all farmers who adopt NT eliminate fallow completely. However, for the sake of simplicity in this paper, we assume that the fraction of annually cropped land on which farmers adopted NT was also continuously cropped. Carbon sequestration due to adoption of continuous cropping compared with fallow-containing cropping (using average fallow frequency) in a particular soil zone was calculated using Eq. 4. For a specific soil zone with an estimated fallow frequency (i.e., one fallow in t year, within a full crop rotation cycle), the calculation of the annual SOC (CC-FC) can be simplified. For instance, the amount of crop residue C input from the change in fallow-wheat-wheat to continuous wheat in the Brown soil zone would be due to 1 additional year of cropping with a proper yield adjustment for wheat grown on stubble rather than on fallow: SOC F CRC ( CC FC) = t For clay soils in the Brown soil zone, we have (8) SOC ( CC FC) = = 013. Mg C ha yr where is the average crop residue C retained in the soil (Fig. 1) when soil contains 25% sand, 25% silt, and 50% clay (Table 1), 0.01 is the conversion of percent to fraction, 1.88 is the average cereal yield (Mg ha 1 ), 0.75 is the fraction of wheat yield on stubble compared with on fallow, 1.5 is the conversion of crop grain yield to crop residue yield with a 40% harvest index, 1.59 is the conversion of crop straw residue to straw and root residue, 0.45 is the average C concentration of the crop residue, and 3 is the average length of rotation containing fallow in the Brown soil zone. Similar calculations were made for other soil texture classes and soil zones. For a medium texture soil, C sequestration due to conversion to continuous cropping from a typical fallow-containing rotation was 0.09, 0.08, 0.05 and 0.06 Mg C ha 1 yr 1 for the Brown, Dark Brown, Black and Dark Gray/Gray soil zones, respectively (Table 1). The decreasing values with more humid soil zones reflects their less frequent summer-fallowing. These results were generally consistent with the findings of McConkey et al. (1999), who reported that effective C gain for each year that fallow was eliminated in adequately fertilized systems varied from 0.4 Mg C ha 1 yr 1 in the Brown soil zone to 0.6 Mg C ha 1 yr 1 in the Dark Gray/Gray soil zone. The rate of C sequestration as a result of adopting continuous cropping is positively related to the frequency of summer-fallow (Campbell et al. 2005). As the average frequency of summer-fallow on the prairies decreases from the Brown, Dark Brown, and Black to the Dark Gray/Gray soil zones, the rates of C sequestration also follow a decreasing trend (Table 1). It should be recognized that in this paper we consider only cereals to replace summer-fallow, but in the Dark Brown and Black soil zones there are significant oilseeds and pulses, and the types of crop would affect the rate of SOC gains (Campbell et al. 2005). Carbon Sequestration Due to No-till The amount of C sequestered under NT was estimated based on the area on which NT is being practiced and the average rate of C sequestration. The average rate of C sequestration under NT was estimated using the amount of SOC present in the top 15-cm of the soil and the clay content by applying Eqs. 5 and 6 for a clay soil in the Brown soil zone: SOC( NT CT) = SOCCT RAISOC 001. = = 0.28 Mg C ha yr (9) (10) where 31 is the average amount of SOC in the top 15-cm depth of a clay soil in the Brown soil zone (Mg C ha 1 ), 0.9 is RAISOC (% yr 1 ) under NT when the soil contains 50% clay

7 LIANG ET AL. EMPIRICAL MODEL TO ESTIMATE C SEQUESTRATION 555 Can. J. Soil. Sci. Downloaded from by on 04/27/18 (Table 1 and Fig. 2), and 0.01 is the conversion of percent to fraction. Similarly, the amount of C sequestration due to the adoption of NT was calculated for other soil zones. Average C sequestration under NT was determined to be 0.13, 0.23, 0.34, and 0.25 Mg C ha 1 yr 1 for the Brown, Dark Brown, Black, and Dark Gray/Gray soil zones, respectively (Table 1). These results were consistent with the findings of McConkey et al. (1999, 2003) and Campbell et al. (2005), who reported that C gain under NT compared with CT varied from 0.2 to 0.4 Mg C ha 1 yr 1 depending on soil zones. The results predicted by this simple model were generally consistent with the findings of VandenBygaart et al. (2003), who reported that NT conversion in fallow-wheat systems resulted in an increase of 0.14 ± 0.13 Mg C ha 1 yr 1. The combined effect of C sequestration due to NT and the adoption of continuous cropping was estimated to be 0.22 Mg C ha 1 yr 1 for the Brown soil zone, 0.32 Mg C ha 1 yr 1 for the Dark Brown soil zone, 0.39 Mg C ha 1 yr 1 for the Black soil zone, and 0.30 Mg C ha 1 yr 1 for the Dark Gray/Gray soil zone. Based on the 1996 and 2001 Agriculture Census, farmers adoption of NT and average fallow frequency in each soil zone on the Canadian prairies, we estimated using a processbased empirical model, the amount of annual sequestered C due to the adoption of NT concomitant with a change to continuous cropping (which is usually associated with NT) to be 1.23 million Mg of C in 1996 and 1.72 million Mg of C in This annual sequestered C that would be achieved by adopting NT and by increasing cropping frequency on the Canadian prairies would represent 10% of the total greenhouse gas emissions from the agricultural sector in Canada (Canada 2004). The model presented here is primarily driven by soil texture and crop residue input. Although derived empirically, the tillage component of this model should be widely applicable to the Canadian prairies because the initial studies covered a wide range in climatic zones, soil texture and duration of experimentation. This simple model could provide a useful tool for soil scientists interested in assessing soil sustainability and C sequestration. It would also be of interest to policy makers wishing to project greenhouse gas emission reduction through improved crop management practices on agricultural soils. ACKNOWLEDGMENT The authors thank Maria Wellisch for her review and comments. Agriculture and Agri-Food Canada Soil landscapes of Canada v 3.0. Ottawa, ON. Balesdent, J., Mariotti, A. and Boisgontier, D Effect of tillage on soil organic carbon mineralization estimated from 13 C abundance in maize fields. J. Soil Sci. 41: Bremer, E., Janzen, H. H. and McKenzie, R. 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