Soil organic carbon stocks on long-term agroecosystem experiments ...

Soil organic carbon stocks on long-term agroecosystem experiments in Canada A. J. VandenBygaart1, E. Bremer2, B. G. McConkey3, H. H. Janzen4, D. A. Angers5, M. R. Carter6, C. F. Drury7, G. P. Lafond8, and R. H. McKenzie9 1

Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, KW Neatby Building, 960 Carling Avenue, Ottawa, Ontario, Canada K1A 0C6; 2Symbio Ag Consulting, Lethbridge, Alberta, Canada T1K 2B5; 3Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, 1 Airport Rd, P.O. Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2; 4Lethbridge Research Centre, Agriculture and Agri-Food Canada, 5403-1 Avenue South, P.O. Box 3000, Lethbridge, Alberta, Canada T1J 4B1; 5Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, Sainte-Foy, Quebec, Canada GIV 2J3; 6 The Charlottetown Centre
Agriculture and Agri-Food Canada Research Centre, Charlottetown, Prince Edward Island, Canada C1A 4N6; 7Greenhouse and Processing Crops Research Centre, Agriculture and Agri-Food Canada, 2585 County Road 20, Harrow, Ontario, Canada N0R 1G0; 8Semiarid Prairie Agricultural Research Centre, Indian Head Research Farm, RR# 1 Gov Road, PO BOX 760, Indian Head, Saskatchewan, Canada S0G 2K0; and 9Government of Alberta, Agriculture and Rural Development, 100, 5401-1 Avenue S, Lethbridge, Alberta, Canada T1J 4V6. Received 24 February 2010, accepted 18 June 2010. VandenBygaart, A. J., Bremer, E., McConkey, B. G., Janzen, H. H., Angers, D. A., Carter, M. R., Drury, C. F., Lafond, G. P. and McKenzie, R. H. 2010. Soil organic carbon stocks on long-term agroecosystem experiments in Canada. Can. J. Soil Sci. 90: 543
550. Several long-term agroecosystem experiments (LTAEs) across Canada have been maintained for periods of up to a century. Much scientific knowledge of changes in soil properties through time has been learned from these few, highly productive LTAEs. We determined the effects of land management changes (LMC) on soil organic carbon (SOC) by re-sampling 27 LTAEs across Canada using identical sampling and laboratory protocols. Seven LTAEs were sampled comparing perennial to annual cropping and it was found that SOC stocks (0
30 cm) were 9.091.5 Mg C ha1 higher under perennial cropping after an average of 16.992.1 yr. This yielded a SOC stock change factor of 0.6 Mg C ha1 yr 1, comparing favourably to a modelling assessment and the Intergovernmental Panel on Climate Change (IPCC) default factor. In six LTAEs in western Canada, no-tillage increased SOC storage by 3.291.3 Mg C ha1 in the top 15 cm over a period of 23.392.7 yr relative to conventional tillage, a rate of SOC storage of 0.14 Mg C ha 1 yr 1. This rate was also similar to that derived by simulation modelling and was slightly lower than the default IPCC rate for subhumid and semiarid regions. In eastern Canada, where tillage is much deeper than western Canada, SOC storage was not significant differently between the two tillage systems. In six LTAEs in western Canada, removing fallow periods every second or third year in favour of continuous cropping increased SOC storage by 5.291.1 Mg C ha1 yr 1 over 21.894.0 yr or an average SOC stock change factor of 0.23 Mg C ha 1 yr 1 to 15 cm depth. This was slightly higher than two independent meta-analyses and rates derived from simulation modelling. The results determined from a re-sampling of LTAEs across Canada provided an invaluable method of validating rates of SOC change concluded by other means. Key words: Long-term agroecosystem experiment, soil organic carbon, perennial cropping, no-tillage, fallow, Canada VandenBygaart, A. J., Bremer, E., McConkey, B. G., Janzen, H. H., Angers, D. A., Carter, M. R., Drury, C. F., Lafond, G. P. et McKenzie, R. H. 2010. Les stocks de carbone organique du sol dans les essais agronomiques de longue dure´e au Canada. Can. J. Soil Sci. 90: 543
550. Plusieurs essais agronomiques de longue dure´e (EALD) existent depuis pre`s d’un sie`cle au Canada. Ces rares mais tre`s productives e´tudes ont permis d’amasser une grande masse de donne´es scientifiques sur l’e´volution des proprie´te´s du sol au fil des ans. Les auteurs ont e´tabli l’incidence des pratiques de gestion des terres sur la concentration de carbone organique du sol (COS) en pre´levant de nouveaux e´chantillons des terres de 27 EALD, un peu partout au Canada, recourant pour cela aux meˆmes protocoles de pre´le`vement et d’analyse en laboratoire. Les e´chantillons de sept EALD ont servi a` comparer la culture de fourrages pe´rennes a` celle d’annuelles. Ils re´ve`lent que les stocks de COS (0 a` 30 cm de profondeur) des cultures de pe´rennes de´passent ceux des cultures annuelles de 9,091,5 Mg de C par hectare au bout d’en moyenne 16,992,1 ans. Le taux de variation des stocks de COS se chiffre donc a` 0,6 Mg de C par hectare et par anne´e, ce qui se compare favorablement aux estimations issues d’un mode`le de simulation et au facteur par de´faut fixe´ par le Groupe d’experts intergouvernemental sur l’e´volution du climat (GIEC). Dans six EALD effectue´es dans l’ouest canadien, le non-travail du sol avait accru les stocks de COS de 3,291,3 Mg de C par hectare dans la couche supe´rieure de 15 cm de sol en l’espace de 23,392,7 ans, contre 0,14 Mg de C par hectare et par anne´e pour le travail conventionnel du Abbreviations: GHG, greenhouse gas; IPCC, Intergovernmental Panel on Climate Change; LMC, land management change; LTAE, long-term agroecosystem experiment; SOC, soil organic C Can. J. Soil Sci. (2010) 90: 543
550 doi:10.4141/CJSS10028

543

544 CANADIAN JOURNAL OF SOIL SCIENCE sol. Ce taux est semblable a` celui obtenu par simulation au moyen d’un mode`le et est le´ge`rement infe´rieur a` celui fixe´ par le GIEC pour les re´gions subhumides et semi-arides. Dans l’est du Canada, ou` on travaille le sol beaucoup plus en profondeur que dans l’ouest, la variation des stocks de COS ne varie pas significativement entre les deux pratiques culturales. Dans six EALD poursuivies dans l’ouest canadien, remplacer les jache`res la deuxie`me ou la troisie`me anne´e par la monoculture accroıˆ t les stocks de COS de 5,291,1 Mg de C par hectare et par anne´e sur une pe´riode moyenne de 21,89 4,0 ans, ce qui e´tablit le taux de variation moyen des re´serves de COS a` 0,23 Mg de C par hectare et par anne´e, a` 15 cm de profondeur. Ce re´sultat est le´ge`rement supe´rieur a` celui indique´ par deux me´ta-analyses inde´pendantes et aux taux des mode`les de simulation. Le re´e´chantillonnage des EALD canadiennes s’ave`re une me´thode inestimable pour valider le taux de variation des stocks de COS de´termine´ d’autres fac¸ons. Mots cle´s: Essais agronomiques de longue dure´e, carbone organique du sol, culture de pe´rennes, non-travail du sol, jache`re, Canada

A number of long-term agroecosystem experiments (LTAE) have been maintained at multiple locations across Canada for periods of up to one century. Requiring prolonged dedication of individuals and continuous financial support through potentially devastating climatic events, wars, and economic collapses (Rasmussen et al. 1998), the maintenance of LTAEs in Canada for any length of time beyond ten years has been, and remains, a challenge. Their availability in Canada has yielded invaluable information and data (Janzen et al. 1998; VandenBygaart et al. 2003; Campbell et al. 2005; Lafond et al. 2009). Much of the scientific knowledge of soil change through time has been learned from these few, highly productive LTAEs (Richter and Markewitz 2008). The knowledge of soil organic carbon (SOC) stock change over time under varying management situations by soil scientists has relied on LTAEs in Canada (Janzen et al. 1998; VandenBygaart et al. 2003). In many cases, however, the ability to adequately monitor SOC change through time has been hampered by insufficient numbers of sampling times, a lack of archived soils and difficulty in overcoming spatial variability of SOC stocks. Fortunately, most LTAEs in Canada have management treatments situated in randomized blocks, such that treatment differences can be used as surrogates for time. Although there are disadvantages to this approach (VandenBygaart and Angers 2006), these LTAEs still remain the primary means of assessing the effect of agricultural management practices on SOC stock change in Canada. Representation of land management change (LMC) in the 2006 Canadian Greenhouse Gas (GHG) Inventory (National Inventory Report 2006) was restricted by activity data-availability limitations. The LMCs that met inclusionary criteria were (VandenBygaart et al. 2008):

in 2005 (Fig. 1). The goal of this paper is to summarize the results of the sampling program, with particular focus on differences in SOC stocks within the treatments that fall within the three LMCs listed above. We compare measured treatment effects with modelled estimates, IPCC default stock change factors and results available from the scientific literature. In a related paper (VandenBygaart et al. 2010) the impact of sampling depth on the monitoring of LMC-induced differences in SOC stock at these LTAEs, and the implications on statistical power and sampling design were assessed. MATERIALS AND METHODS Soil Sampling and Laboratory Processing Long-term agroecosystem experiments across Canada were identified that contained tillage and cropping treatments representing the three LMCs. The specific LMCs utilized for this work were: (i) comparison of fallow every 2nd or 3rd year to continuous cropping, (ii) conventional tillage compared with no-tillage/ direct seeding (type of conventional tillage varied and

Western Canada Eastern Canada

. Change in area of perennial/annual cropping; . Change in tillage practices; . Change in area of summerfallow. With these practices in mind, a sampling program was initiated on 27 LTAEs at 11 locations across the country

Fig. 1. Map showing locations of long-term agroecosystem experiments in Canada where soil samples were extracted.

VANDENBYGAART ET AL. * SOIL ORGANIC CARBON IN CANADA

was specific to the location), and (iii) comparison of perennial crop species with annually cropped species. Table 1 lists the LTAEs in Canada where soil sampling was conducted from April through October 2005, along with specifics on treatments, experimental design and the LMCs evaluated. Unless site conditions dictated otherwise, four cores were obtained from each sampling unit (plot). All cores from each sampling unit were bulked by depth increment in the field. For narrow row annual crops and forages, the core was positioned on bare areas between plants (i.e., crowns were not sampled). For wide rows, one core was obtained ‘‘in row’’ (but not including crowns), mid row, and one-quarter the distance from rows. The coring tube (65-mm interior diameter) was hydraulically forced into the soil to a depth :75-cm to ensure that a sufficient sample to 60 cm could be utilized

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(shallower depths were employed at some sites due to soil conditions). Since one purpose of this paper was to compare SOC stock changes of the LTAEs to IPCC stock change estimates, we limited our analysis to a maximum of 30 cm (the default depth for IPCC stock data). To compare conventional tillage with no-tillage in western Canada only the top 15 cm data were utilized since tillage never exceeds the depth of 15 cm in this region. Some use of lubricants was necessary at Three Hills (silicone) and Indian Head (motor oil on outside of core tube). Poor cores (e.g., excessive compaction, cores with channels made by stones or crop residues) were discarded and retaken. Soil cores were divided into depth increments of 0
15 cm, 15
30 cm, 30
45 cm and 45
60 cm. Samples were placed in large plastic Ziploc bags or aluminum trays, taking care to avoid any losses of soil.

Table 1. List of comparisons at various LTAEs sampled in 2005 for SOC stocks

Region/Location Western Canada Swift Current, SK Swift Current, SK Swift Current, SK Swift Current, SK Swift Current, SK Scott, SK

LTAEz

Design

LMC/sy

Fertility, fallow Fertility, fallow Tillage, fallow Tillage, fallow Crested wheatgrass vs. wheat Perennials

Fallow Fallow Fallow Tillage Perennials Perennials

39.2 33.0 29.0 29.0 33.0 39.5

38 18 23 23 18 30

Campbell et al. (2000a) Campbell et al. (2000b) Campbell et al. (1995) Campbell et al. (1995) Campbell et al. (2000b) Mahli et al. (2003)

Tillage, fallow Tillage, fallow Fescue vs. barley Tillage, fertility, straw Tillage, fertility, straw Tillage

Fallow Tillage Perennialsv Tillage Tillage Tillage

42.5 42.5 31.6 23.0 85.0 33.5

24 24 35 25 25 30

Brandt (1992) Brandt (1992) NA Nyborg et al. (1995) Nyborg et al. (1995) Janzen (pers. commun.)

Fallow, fertility Crested wheatgrass vs. wheat Native grass vs. wheat Tillage

Fallow Perennials Perennials Tillage

30.0 30.0 30.0 57.0

13 13 13 14

Ellert (pers. commun.) Ellert (pers commun.) Ellert (pers. commun.) NA

Perennials Perennials

57.0 57.0

14 14

NA NA

Perennials Fallow

22.0 22.0

13 13

Bremer et al. (2002) Bremer et al. (2002)

Perennials Tillage Perennials Perennials Tillage Tillage Tillage

78.0 38.0 80.0 80.0 80.0 68.6 63.0

22 12 25 25 25 13 20

Drury et al (1998) Yang et al. (2008) Yang and Kay (2001) Yang and Kay (2001) Yang and Kay (2001) Sharifi et al. (2008) Carter (2005)

Three Hills, AB Three Hills, AB

RCBDu RCBD RCBD RCBD RCBD Sideby-side Zero till Split plot Zero till Split plot Hendrigan Plots RCBD Mahli Plots RCBD Mahli Plots RCBD North 40 Sideby-side Cquest RCBD Cquest RCBD Cquest RCBD Three Hills Tillage Sideby-side Three Hills Perennial 1 RCBD Three Hills Perennial 2 RCBD

Bow Island, AB Bow Island, AB

Grass vs. wheat Fallow

RCBD RCBD

Bromegrass vs. annuals Alfalfa/bromegrass vs. annuals Grass vs. wheat Fallow

Eastern Canada Woodslee, ON Woodslee, ON Elora, ON Elora, ON Elora, ON L’Acadie, QC Harrington, PE

Totten Tillage Rotation Rotation Tillage L’Acadie Harrington

RCBD RCBD RCBD RCBD RCBD RCBD RCBD

Bluegrass vs. annuals Tillage Corn/alfalfa vs. corn Alfalfa vs. corn Tillage Fertilizer, tillage Tillage

Scott, SK Scott, SK Breton, AB Breton, AB Ellerslie, AB Lethbridge, AB Lethbridge, AB Lethbridge, AB Lethbridge, AB Three Hills, AB

z

Old rotation New rotation OMC OMC New rotation Old fields

SOCbaseline Age (Mg ha 1)x (yr)

Treatments

Referencew

Long-term agroecosystem experiment. Land managenment change used in this paper: fallowcomparison to continuous cropping; tillage no tillage/direct seeding compared with conventional tillage; perennialperennial crop species compared with annual crops. x To 15 cm in western Canada, 30 cm in eastern Canada; SOC stock in treatment considered business as usual. w Not all inclusive
references are examples or individuals where reader can find more details. v Three cropping systems: continuous barley, with straw returned to the plots after harvest, continuous forage (primarily fescue grass), and an 8-yr ‘‘agroecological’’ rotation (AER). u Randomized complete block design. y

546 CANADIAN JOURNAL OF SOIL SCIENCE

Total weights of field moist soil were recorded. A representative sub-sample of 50 to 100 g was removed for gravimetric moisture content (1058C until constant weight attained). These values were then used to calculate dry bulk density for each depth increment. The remaining sample was dried at room temperature. Whole samples were ground to B2 mm in a perforated drum roller. Mineral material greater than 2 mm was weighed and discarded. All crop residues or roots B2 mm were incorporated into the sample. Representative subsamples of approximately 10 g were finely ground (B0.15 mm) using a ball mill or by tumbling in solid stainless steel canisters for at least 16 h. Total C was determined by combustion gas chromatography (10208C) (NA 2100, CE Instruments, Milan, Italy), with samples weighed into tin capsules. For soils containing carbonates, samples were weighed into acidresistant Ag capsules and acidified to eliminate carbonate prior to analysis. Acidification was with 6 M HCl added dropwise (B15 mL) to the sample until all effervescence had ceased. Acidified samples were dried at 608C and then analyzed by combustion gas chromatography using a higher temperature (10508C) to ensure complete combustion. Reference standards were SQD4 and SQD27 (Lethbridge Research Centre). All samples were weighed at ambient moisture content, with periodic analysis of gravimetric soil moisture of reference standards and samples to estimate moisture correction factors, if required. Calculation of SOC Stock Per Unit Land Area Differences in bulk density among treatments affect the amount of soil included in the sample and, thus, SOC stocks should be based on an equivalent mass of soil (Ellert and Bettany 1995). In this study, the following calculation was utilized to minimize deviations from estimates based on a fixed depth: SOCEM SOCFD (Massaverage Masssample )

×

SOCbest =100

where SOCEM is SOC stock based on equivalent mass (Mg C ha1), SOCSFD is SOC stock based on a fixed depth (Mg C ha1; Masssample SOC), Massaverage is the average soil mass to depth of sampling across treatments at each site (kg m2), Masssample is soil mass of the sample to depth of sampling (kg m 2), and SOCbest is best estimate of SOC concentration at depth of sampling (average SOC concentration of depth increments above and below specified sampling depth) (g C kg1 soil). Statistical evaluation of treatment effects within each LTAE was conducted with the MIXED procedure of SAS (Release 9.1, SAS Institute, Inc., Cary, NC), with treatment(s) included as fixed effects and block as a random effect. For LTAEs with multiple LMCs, contrasts were used to determine the statistical significance

of each LMC. The impact of LMC on SOC stocks was estimated from the difference in SOC stocks (mean9 standard error) between relevant treatments of all LTAEs with durations of at least 10 yr. This approach assumes that most of the change in SOC stocks due to LMC occurs within the first 10 yr, although it is acknowledged that the time of response of management practices to SOC can vary broadly (Janzen et al. 1998; Bremer et al. 2008). RESULTS AND DISCUSSION Change from Annual to Perennial Cropping Pasture management utilizing cultivated forages and alfalfa in the great plains of North America has increased (Entz et al. 2002) and has been shown to increase SOC stocks (Conant et al. 2001). Seven LTAEs at five sites across Canada allowed comparison of effects of annual and perennial cropping on SOC stocks (Table 2). Of the seven comparisons three showed significantly (P B0.05) larger SOC stocks under perennial crops relative to annual crops, and these were located in western Canada (Table 2). At Elora, Ontario, replacing corn (Zea mays L.) with continuous alfalfa (Medicago sativa L.) cropping increased SOC stocks by 8 Mg ha1 compared with continuous corn. When treated as a population, the average effect of replacing annual crops with perennial species was an increase in SOC stock of 9.091.5 Mg C ha1 over 16.992.1 yr (n 7) to 30-cm depth. However, this result should be tempered due to highly variable site conditions (climate, soil type) and management practices (crop species, fertility). Nonetheless, the result is consistent with the general conclusions in the literature that increasing acreage in perennial cropping species can result in considerable increases in SOC stocks, and should be viewed as a management practice change that can sequester a Table 2. Estimates of SOC stock differences to 30 cm between annual and perennial crops on LTAEs in Canada (positive is SOC gain under perennial cropping) DSOC (Mg ha1)

Site

LTAEz

Average SE

Breton, AB Hendrigan Plots Three Hills, AB Three Hills Perennial 1 Three Hills, AB Three Hills Perennial 2 Lethbridge, AB Cquest native grasses vs. wheat Lethbridge, AB Cquest CWGy vs. wheat Bow Island, AB Grass vs. wheat Elora, ON Alafalfa vs. continuous corn z

Long-term agroecosystem experiment. Crested wheatgrass.

y

P

Duration (yr)

14 13

6 6

0.05 0.06

25 14

11

6

0.09

14

8

2

B0.01

13

3

2

0.09

13

6 8

2 4

0.01 0.1

14 25

VANDENBYGAART ET AL. * SOIL ORGANIC CARBON IN CANADA

significant quantity of C from the atmosphere (Bremer et al. 2008; Jabro et al. 2008; Luo et al. 2010). Replacing annual crops with perennial species yielded a SOC stock change factor of 0.6 Mg C ha1 yr 1, which is comparable with the rate derived in the Canadian inventory calculations by VandenBygaart et al. (2008). Through simulation modelling, they derived factors ranging from 0.5 Mg C ha 1 yr1 in western Canada to 0.8 Mg C ha1 yr 1 in eastern Canada (over 20 yr). Our rate of SOC change is also similar to the rates determined by IPCC (IPCC 2006) which range from 0.6 Mg C ha 1 yr 1 in subhumid regions to greater than 2.0 Mg C ha 1 yr 1 over 20 yr in cool, moist regions of eastern Canada (VandenBygaart et al. 2008). Although only two of the seven LTAEs had ages greater than 20 yr (Table 2), the average net SOC stock change was considerably lower than would be estimated with Tier 1 IPCC estimates over 20 yr (6.6 Mg C ha1 in this study vs. about 25 Mg C ha 1 using IPCC estimates) (VandenBygaart et al. 2008). It is not clear what could account for this discrepancy but it may be due to the species and/or duration of the perennials. Change from Conventional Tillage to No-Tillage It is well documented that SOC stocks can be increased by reducing soil disturbance by tillage (Paustian et al. 1997; Six et al. 2004), which is partly attributed to the formation of stable macro- and micro-aggregates (Six et al. 2000). Gains in SOC under no-till in Canada are generally limited to soils of the western Provinces (VandenBygaart et al. 2003; Janzen et al. 1998), while in eastern Canada no-tillage has not been shown to be effective in conserving SOC stocks (Angers et al. 1997). However, there are recent concerns over the lack of consideration of greater depths within the profile, where carbon dynamics may not be accounted for with some sampling strategies under various edaphic/climatic conditions, which may result in erroneous interpretations

547

(VandenBygaart and Angers 2006; Angers and EriksenHamel 2008). In western Canada, sub-humid to semi-arid climatic conditions restrict tillage management effects on SOC stocks to 15-cm depth (VandenBygaart et al. 2003; Campbell et al. 2005). In this study, six LTAEs were sampled in western Canada to compare soils under direct seeding (no-tillage) relative to those under annual tillage (Table 3). Of the six studies, only two showed significant effects of tillage on SOC stocks (P B0.05). When taken together, however, no-tillage resulted in an increase of 3.291.3 Mg C ha1 in the top 15 cm over 23.392.7 yr in western Canada, or a rate of SOC storage of 0.14 Mg C ha 1 yr 1. This rate compares favourably with VandenBygaart et al. (2008), who derived rates of 0.16 and 0.07 Mg C ha 1 yr 1 for subhumid and semiarid prairie soils, respectively, utilizing simulation modelling for Canada’s national GHG inventory. The results are also consistent with the IPCC 20 yr factors for these climatic regions, i.e., 0.2390.66 and 0.1690.31 Mg C ha 1 yr 1 for subhumid and semiarid regions, respectively (VandenBygaart et al. 2008). In eastern Canada, climatic conditions are more humid than in western Canada, and furthermore, tillage management has historically involved inversion tillage by the mouldboard plow where tillage depths often exceed 20 cm in the profile. Angers et al. (1997) conducted a similar study to this one whereby they analyzed tillage comparisons on seven experiments in eastern Canada, which included three sampled in this study (at the time of sampling, all experiments were less than 12 yr duration). They concluded that no significant effects of tillage were evident when soils were sampled to 60 cm depth. Here, when the top 30 cm of the soil profile was considered, SOC storage was 0.2290.83 Mg C ha1 greater under no-tillage compared with conventional tillage (Table 3). From meta-analysis of literature results, VandenBygaart et al. (2003) estimated that no-tillage changed C storage by 0.192.9 Mg C ha1 for eastern

Table 3. Estimates of SOC stock differences between no-tillage and conventional tillage on LTAEs in Canada (positive is SOC gain under no-tillage; measured to 15 cm in western Canada and 30 cm in eastern Canada) DSOC (Mg ha 1) Site

LTAE

Average

SEz

P

Duration (yr)

Western Canada

Swift Current Lethbridge Scott Ellerslie Three Hills Breton

OMC North 40 Zero Till Mahli Plots Three Hills Tillage Mahli Plots

2.3 0.4 5.9 0.5 7.6 3.3

1.7 1.4 1.4 1.7 2.1 1.9

0.18 0.77 B0.01 0.78 B0.01 0.09

23 30 24 26 11 26

Eastern Canada

Woodslee Woodslee Elora L’Acadie Harrington

Totten Tillage Tillage L’Acadie Harrington

2.2 0.2 2.7 1.4 0.0

5.1 4.2 2.3 1.5 2.7

0.74 0.96 0.21 0.42 1.00

22 12 25 13 20

Region

z

Standard error.

548 CANADIAN JOURNAL OF SOIL SCIENCE

Canadian soils. Although not definitive, the lack of effect of no-tillage on SOC stocks in eastern Canada may be a consequence of deep burial of surface residues to locations in the profile where decomposition may be restricted by cold, moist conditions under conventional tillage (Angers et al. 1997; Angers and EriksenHamel 2008). By simulation modelling, VandenBygaart et al. (2008) derived a SOC stock change factor of 0.08 Mg C ha1 yr 1, while the IPCC 20-yr change factor was 0.4990.62 Mg C ha1 yr 1 for no-tillage under eastern Canadian climate, again indicating high variability. Change from Fallow to Continuous Cropping Historically, summerfallowing has been a practice used in the North American great plains for conserving soil moisture to minimize risk of crop failure to drought (Campbell et al. 2005). This practice has had a deleterious effect on SOC stocks, as fallow years result in zero crop inputs and increased decomposition rate (Bremer et al. 1995; Campbell et al. 2005; Bremer et al. 2008). From 1990 to 2004 the area under fallow decreased by 40% in the prairie provinces of Canada (VandenBygaart et al. 2008). Six experiments at four locations were sampled to assess the effects of fallow in rotation with annual cropping (Table 1). Four of the experiments had fallow every third year and two every second year (Table 1). Of the six experiments sampled four showed statistically greater SOC stocks (P B0.05) when cropping was continuous (Table 4). Collectively the average SOC stock increase when removing fallow was 5.291.1 Mg C ha1 over 21.894.0 yr, for an average SOC stock change factor of about 0.23 Mg C ha 1 yr 1. This is similar to the rates of 0.15 and 0.10 Mg C ha1 yr 1 for conversion to continuous cropping from fallow every second year and every third year, respectively, derived through simulation modelling by VandenBygaart et al. (2008). The rate is also comparable with the rate of 0.15 Mg C ha1 yr 1 derived by both Campbell et al. (2005) and VandenBygaart et al. (2003). The IPCC SOC stock change factors for continuous cropping averaged over the sub-humid and semi-arid areas was 0.65 Mg C ha 1 yr1, considerably higher than found in this study. This can be a consequence of Canada’s cold climate, which can result in slower SOC stock change as shown in long-term experiments evaluated by Campbell et al. (2005). Nonetheless, the continued reduction in acreage under fallow management has resulted in considerable storage of SOC and represents a large proportion of the annual CO2 stored in Canada’s GHG inventory since 1990 (Environment Canada 2007). CONCLUSIONS The careful recording of management practices, continuous measurement of crop yields, and archiving of plant and soil samples have enabled LTAEs to become invaluable research tools for understanding

Table 4. Estimates of SOC stock differences to 15 cm between crop rotation with fallow every second or third year and continuous cropping on LTAEs in Canada (positive is SOC gain under continuous cropping) SOC (Mg ha1)

Site Swift Current, SK Swift Current, SK Swift Current, SK Bow Island, AB Scott, SK Lethbridge, AB

LTAE Old rotation New rotation OMC Fallow Zero till Cquest

Average SEz 4.2 5.9 6.3 2.0 9.8 3.1

2.3 1.7 1.7 0.9 1.7 0.7

P

Duration (yr)

0.10 0.01 B0.01 0.05 0.01 B0.01

38 18 24 13 26 12

z

Standard error.

SOC dynamics in Canada. It is in the best interest of Canadians to maintain these experiments, to have them continuously managed by scientists, and to continue to provide the infrastructure, money and resources to have them continue. By re-sampling 27 LTAEs at 11 locations across Canada this paper demonstrated that LTAEs are invaluable for assessment of agricultural management effects on SOC stocks. The re-sampling yielded important data for comparisons to empirical and model estimates used in Canada’s agricultural GHG inventory and IPCC default SOC stock change factors. When replacing annual crops with perennial species, SOC stock increased by an average of 6.692.0 Mg C ha1 over 16 yr. Direct seeding or no-tillage in western Canada increased SOC stocks by 0.14 Mg C ha1 yr 1, comparing favourably to estimates used for this region in Canada’s GHG inventory. In eastern Canada, SOC storage when converting to no-tillage is minimal and highly uncertain, as supported by analyses in the scientific literature. Removing fallow from rotations in western Canada increased SOC stock by 5.191.1 Mg C ha1 over 22 yr or 0.23 Mg C ha 1 yr 1. This rate was also similar to rates quoted in the recent literature. ACKNOWLEDGEMENTS This work would not have been possible had it not been for the efforts and dedication of the many scientists, technicians and science managers who have maintained the LTAEs and lobbied to keep them in order under many stresses and turmoil. Thank you to all of the scientists who provided access and assistance with sampling, including Bob Zentner, Stu Brandt, Dick Puurveen, Tom Goddard, Ben Ellert and Bill Deen. Thank you also to the many people who helped with sampling and sample processing: Kelsey Brandt, Marty Peru, Dick Puurveen, Cory Olson, Josee Thibodeau, Daniel Bremer, Gloria Bremer, Debbie Werk, Craig Sprout, Wiebe Buruma, Germar Lohstraeter, Cameron Ellert, Irene Power, Gabriel Le´vesque, Johanne Tremblay, Nicole Bissonnette, Vincent Poirier, Chris

VANDENBYGAART ET AL. * SOIL ORGANIC CARBON IN CANADA

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