Effects of crop rotation, crop type and tillage on soil organic carbon in a semiarid climate

Transcription

1 Effects of crop rotation, crop type and tillage on soil organic carbon in a semiarid climate B. M. Shrestha 1, B. G. McConkey 2, W. N. Smith 1, R. L. Desjardins 1, C. A. Campbell 1, B. B. Grant 1, and P. R. Miller 3 1 Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, KW Neatby Building, 960 Carling Avenue, Ottawa, Ontario, Canada K1A 0C6; 2 Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, 1 Airport Road, P.O. Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2; and 3 Department of Land Resources and Environmental Sciences, Montana State University, P.O. Box , 334 Leon Johnson Hall, Bozeman, MT, USA Received 5 July 2012, accepted 26 September Shrestha, B. M., McConkey, B. G., Smith, W. N., Desjardins, R. L., Campbell, C. A., Grant, B. B. and Miller, P. R Effects of crop rotation, crop type and tillage on soil organic carbon in a semiarid climate. Can. J. Soil Sci. 93: There is uncertainty about how crop rotation and tillage affect soil organic C (SOC) on the Canadian prairies. We compared SOC amount and change (DSOC) for one continuous crop and four 3-yr fallow-containing crop rotations under no-tillage (NT), and two fallow-containing crop rotations under minimum-tillage (MT), from 1995 to 2005 in semiarid southwestern Saskatchewan. After 11 yr, SOC (0- to 15-cm depth) was 0.2 Mg C ha 1 higher under continuous crop compared with fallow-containing systems. There were no significant differences in SOC and DSOC among fallow-containing rotations or between MT and NT. Total C inputs were weakly (R ) but significantly (PB0.05) correlated to DSOC, which changed by90.33 Mg C ha 1 for each Mg ha 1 C input above or below 2.4 Mg C ha 1 yr 1. Carbon inputs were typically less than this amount and SOC generally decreased over the experiment. Simulations of SOC with the Century model were consistent with our observations regarding DSOC per unit of C input. There was slight loss of SOC for the above-average precipitation regime during the study. Simulations also supported our finding that SOC differences between crop mix and tillage systems may require several decades to become distinguishable in this semiarid climate with small and variable C inputs. Key words: C input, SOC pool, summerfallow, Century model, minimum till, no-till Shrestha, B. M., McConkey, B. G., Smith, W. N., Desjardins, R. L., Campbell, C. A., Grant, B. B. et Miller, P. R Effets de la rotation des cultures, du type de cultures et du re gime de travail du sol sur la teneur en carbone organique du sol dans un climat semi-aride. Can. J. Soil Sci. 93: Des incertitudes persistent quant a` l incidence de la rotation des cultures et du travail du sol sur la teneur en carbone organique du sol (COS) dans les prairies canadiennes. Nous avons compare la teneur en COS et la variation du COS (DCOS), en regard d un régime de culture continue, de quatre rotations avec jache` re sur trois ans sans travail du sol (ST) et de deux rotations avec jache` re avec travail re duit du sol (TR), de 1995 à 2005, dans le climat semi-aride du Sud-Ouest de la Saskatchewan. Au bout de 11 ans, le régime de culture continue a été associe a` une hausse de 0,2 Mg C ha 1 du COS (profondeur de 0 a` 15 cm). Il n y a eu aucune diffe rence significative de COS ou de DCOS entre les rotations avec jache` re ou entre ST et TR. Une corre lation faible (R 2 0,18), mais significative (PB0,05), a e te e tablie entre DCOS et l apport total de C, le COS variant de0,33 Mg C ha 1 pour chaque exce dent ou de ficit de 1 Mg C ha 1 par rapport a` un apport de 2,4 Mg C ha 1 yr 1. Comme les apports de carbone ont été dans l ensemble infe rieurs a` ce seuil, le COS a ge ne ralement diminue pendant l expe rience. Les simulations du COS e tablies par le modèle Century ont été compatibles avec nos observations concernant DCOS par unité d apport de C. Une légère baisse du COS a e te associe e au re gime de pre cipitation supe rieure a` la moyenne durant l e tude. Les simulations ont e galement corrobore notre conclusion selon laquelle il faudra peut-eˆtre plusieurs de cennies avant de pouvoir établir des distinctions entre les diffe rences de COS selon l e ventail de cultures et le re gime de travail du sol dans ce climat semi-aride caracte risé par des apports de C faibles et variables. Mots clés: Apport de C, réservoir de COS, jache` re, modèle Century, travail re duit du sol, sans travail du sol The agricultural area of the prairies accounts for more than 80% of the arable land in Canada. A century of cultivation of native land in the prairies has reduced soil organic C (SOC) by approximately 15 to 30% (Monreal and Janzen 1993; Gregorich et al. 1995). Adoption of soil conservation practices on the prairies in recent times has tended to reverse this trend (Janzen et al. 1998; Abbreviations: BD, bulk density; FCW, fallowcanolawheat; FPW, fallowpulsewheat; FW, fallowwheat; FWP, fallow wheatpulse; FWW, fallowwheatwheat; HI, harvest index; MT, minimum tillage; NT, no-tillage; SOC, soil organic carbon; WOP, wheatoilseedpulse Can. J. Soil Sci. (2013) 93: doi: /cjss

2 138 CANADIAN JOURNAL OF SOIL SCIENCE Smith et al. 2001). Several studies on the Canadian prairies have examined the effect of crop management (e.g., crop rotation, cropping frequency, tillage, crop type, and soil nutrients) on SOC pool and have related the latter to C inputs (Rasmussen et al. 1978; Campbell et al. 2000, 2005b, 2007a, b; Bremer et al. 2002; McConkey et al. 2003; Gan et al. 2009a). These studies have indicated that increased SOC sequestration occurs with adoption of continuous cropping systems following frequent summer fallowing in rotations (McConkey et al. 2003; Campbell et al. 2005b), increases under cropping systems with greater fertilizer or manure application (Campbell et al. 2001), and in some cases with the conversion from conventional tillage to conservation tillage management (Bruce et al. 1999; Lal and Bruce 1999; Paustian et al. 2000; Campbell et al. 2005b; Malhi et al. 2006). In recent decades, in the Canadian prairies, there has been a move by producers to the use of more continuous cropping, resulting in less frequency in summer fallowing (Campbell et al. 2002). Due to better economic returns for oilseed and pulse crops (Campbell et al. 2002; Zentner et al. 2002), the areas of these crops have increased while that of wheat (Triticum spp.) has decreased significantly (Carlyle 2002). The SOC pool in agricultural soils is a dynamic entity which is a function of the available soil C pool and the balance between the C inputs from decomposition of crop residues and loss of humus from the system via mineralization and erosion (Rasmussen et al. 1978; Powlson et al. 1996; Campbell et al. 2000). The crop residues returned to the soil vary with crop type (Gan et al. 2009a), crop rotation, tillage, nutrient inputs, and weather conditions. Thus, these are factors that will influence the SOC sequestration (Campbell et al. 2007a, b). Knowledge of net primary productivity and harvest index (HI) of a grain crop, together with straw-to-root (S:R) ratio allows estimation of the C inputs to the soil (Campbell et al. 1992; Janzen et al. 2003; Bolinder et al. 2007) and allows estimation of the amount of C that can potentially be sequestered in soil (Campbell et al. 2000; Bolinder et al. 2007; Gan et al. 2009a). Root mass production is higher for canola (Brassica napus L.) than for wheat (Gan et al. 2009b) and the decomposition rate of residue is slightly lower for canola than for wheat (Lupwayi et al. 2004). However, detailed comparative studies on the effects of pulse, oilseed and wheat residues on SOC are limited. There is a need to quantify these effects for policy-related tools including greenhouse gas calculators and life cycle assessment. Measurable changes in soil C stocks in field conditions may take many years to become apparent; however, models can be used to facilitate prediction of the long-term effects of crop management on change in the SOC pool (Powlson et al. 1996; Smith et al. 1997). The Century model is one that has been widely tested and found to be suitable for predicting SOC pool in various regions of the world (Kirschbaum and Paul 2002; Smith et al. 2005; Milne et al. 2008; Shrestha et al. 2009). VandenBygaart et al. (2008) reported that the C change factors for use in Canada s greenhouse gas inventory were adequately derived by the Century model. The objective of this study was to compare and evaluate the short-term (11 yr) changes and model the long-term (until year 2050) changes in SOC as affected by cropping frequency, crop rotation, crop type and tillage management in the Canadian semiarid prairies. MATERIAL AND METHODS Study Area and Experimental Design This experiment was initiated in 1995 on Swinton silt loam, an Orthic Brown Chernozem, at the Semiarid Prairie Agricultural Research Centre in Swift Current, Saskatchewan. The study was conducted on spring wheat (Triticum aestivum L.) stubble land after a longterm rotation of fallow-wheat (FW) in order to evaluate the impact of conservation tillage systems [i.e., minimum till (MT) vs. no till (NT)] on SOC for several crop rotations, and to relate this to C input from crop residues. The study used a completely randomized block design with three replicates. Plot dimensions were m, allowing us to use full-size farm machinery. The surface soil has 31% sand, 51% silt, and 18% clay, with bulk density (BD) of 1.4 g cm 3, a ph in soil paste of 6.5, and about 3% organic matter in the upper 10-cm depth. Table 1 shows the treatments that were assessed in this study. Crop types used in the study were: (1) cereal: durum wheat (Triticum durum L.); (2) oilseeds: sunflower (Helianthus annuus L.), flax (Linum usitatissimum L.), and canola (Brassica napus L.); and (3) pulses: lentil (Lens culinaris Medik.), dry pea (Pisum sativum L.), and chick pea (Cicer arietinum L). The rotations were fallowwheatwheat (FWW), fallowcanolawheat (FCW), fallowwheatpulse (FWP), fallowpulsewheat (FPW) and wheatoilseedpulse (WOP). The FWW and FCW rotations were conducted under both NT and MT management, while all other rotations were under NT only. The WOP rotation consisted of three types of oilseed crops at different time steps with rotation of wheatflaxlentil from 1995 to 1997, wheatsunflower pea rotation from 1998 to 2000, and wheatcanolapea from 2001 to The pulse in the FPW and FWP was chickpea for and pea thereafter. In both NT and MT, phenoxy herbicides were applied in either late fall or early spring for control of broadleaf winter annuals. Trifluralin was fall applied for weed control in canola and ethylfluralin applied for weed control in pulse crops. In-crop herbicide applications were made on an individual plot basis and occasionally only a portion of the plot was sprayed. Normally no incrop herbicides were used on pulse or canola crops

3 SHRESTHA ET AL. * CROP ROTATION AND TILLAGE EFFECTS ON SOC 139 Table 1. Average annual z dry matter grain, straw, below-ground biomass, total crop residue yield and annual C inputs in the 0- to 20-cm depth of the soil in different crop rotations from 1995 to 2005 Rotation y Till x Grain yield Straw yield w Below ground biomass v Annual C inputs (kg C ha 1 ) (kg DM ha 1 ) F(W)W MT FW(W) Rotation F(W)W NT FW(W) Rotation F(C)W MT FC(W) Rotation F(C)W NT FC(W) Rotation F(P)W NT FP(W) Rotation F(W)P NT FW(P) Rotation (W)OP u NT W(O)P WO(P) Rotation LSD t (Crop) LSD (Rotation) Contrasts (significances) P in FPWFWP vs. B B B B WinNTFWW W on C in FCW vs W on W in FWW C in FCW vs. W on FinFWW B NS s B z Annual average yieldsum of crop yields0.33. Yields are for the rotation phase in parentheses. y Rotations: FWW, FallowWheatWheat; FCW, FallowCanolaWheat; FWP, FallowWheatPulse; FPW, FallowPulseWheat; WOP, Wheat OilseedPulse (Oilseed was flax in , sunflower in and canola in ; pulse was chickpea in and pea thereafter). x Till, tillage management; MT, minimum tillage; NT, no tillage. w Straw yield was estimated using harvest index (HI) for different crops: canola0.20, flax0.23, pea0.30, wheat0.30, lentil0.34, (Gan et al. 2009a). v Below-ground biomass consists of roots and rhizo-deposits. Root was estimated using shoot- to-root (S:R) ratio for different crops: canola4.20, flax5.80, pea5.35, wheat6.33, lentil3.80 (Gan et al. 2009a) and 65% of root biomass was assumed to be rhizo-deposits (Bolinder et al. 2007). u The oilseed crop in WOP rotation was flax, sunflower and canola in , and , respectively; the pulse for WOP, FWP, and FPW was chickpea in and pea for t Least significant difference at 10% level of significance. s NS, not significant (P0.10). although occasionally herbicide to control grassy weeds was used. Differences between NT and MT were that the MT treatments received pre-seeding tillage with a heavy-duty cultivator equipped with sweeps and mounted harrows. Fallow in MT was maintained with two tillage operations, one between late June to early July, and one between late July and early August. For NT, one spring application with glyphosate with or without dicamba was used for both crop and fallow phases. Fallow in NT was controlled with two additional applications of glyphosate with or without dicamba. Both MT and NT were seeded with the same seed drill. Maximum depth of soil disturbance was less than 7 cm in both tillage systems. Dates of seeding and harvest were unaffected by tillage management. Yields were measured with a plot combine taking a single 1.5- m-wide cut of a measured length diagonally across each plot. Soil samples were taken in early spring and late fall to determine the fertilizer N and P requirements. The N fertilizer was added in fall as a blend of urea and ammonium sulfate (40005) such that soil nitrate-n to 60-cm depth measured at that time plus fertilizer N equaled 77 and 60 kg N ha 1 for oilseed and cereal, respectively. Mean rate of N application per crop year was 5 kg N ha 1 for pulse crops (from the P source), 27 kg N ha 1 for wheat grown on fallow, 30 kg N ha 1

4 140 CANADIAN JOURNAL OF SOIL SCIENCE for oilseed grown on fallow, and 50 and 57 kg N ha 1 for wheat and oilseed, respectively, grown on stubble. Monoammonium phosphate (12510) was applied to all crops at 11 kg P ha 1. The fertilizer was banded 2.5 cm below and 2.5 cm to the side of the single seed row. Weather Conditions During the experimental period, growing season (May August) precipitation was average to above long-term average in 9 of 11 yr, while drought conditions occurred in 2001 and 2003 (Fig. 1). This observation was generally supported by the potential evapotranspiration during the growing season. Soil Sampling and Analysis Soil samples were collected from the 0- to 7.5-cm and from 7.5- to 15-cm depths in in early May 1995 and 2006 with a truck-mounted hydraulically powered soil corer. From each depth, four soil cores per plot were extracted and pooled together by depth to make a composite sample. The resulting samples were air-dried and sieved through a 2-mm mesh. Crop residues remaining on the sieve were discarded. Representative sub-samples were ground with a roller mill (B153 mm) and analyzed for SOC using an automated combustion technique (Carlo ErbaTM, Milan, Italy). To remove carbonates, soil samples were pre-treated with phosphoric acid in a tin capsule after weighing, then the samples were dried for 16 h at 758C prior to analysis for SOC. Calculation and Data Analysis Estimation of SOC Amount The amount of SOC for the 0- to 7.5-cm and 0- to 15-cm soil depths was calculated using an adapted method based on the principle of equivalent soil mass (Ellert and Bettany 1995) because of possible changes in soil BD due to the type of tillage practice. In this adaptation the equivalent soil mass was set to observed average soil mass of all samples. The calculation was done using the following relations (1) and (2) for depth 0- to 7.5-cm and 0- to 15-cm, respectively. SOC d1 M 1 OC 1 =100(M 1 M 1Avg ) OC Avg =100 (1) SOC d2 M 1 OC 1 =100(M 1 M 2 M 12Avg ) OC 2 =100 (2) Where SOC d1 and SOC d2 are SOC amount at 0- to 7.5-cm and 0- to 15-cm soil depths (Mg C ha 1 ), respectively; M 1 is the sample soil mass of 0- to 7.5-cm (Mg ha 1 ), which is given by BD ; M 1Avg is the average mass for 0- to 7.5-cm depths of all samples; OC 1 is the sample SOC concentration (%); Fig. 1. Annual growing-season precipitation (PCPN) and potential evapotranspiration (PET) in the study area during the study period and long-term mean from 1961 to 2004 (LTmean).

5 SHRESTHA ET AL. * CROP ROTATION AND TILLAGE EFFECTS ON SOC 141 OC Avg is the average of SOC concentration (%) for the sample 0- to7.5-cm and 7.5- to 15-cm depths; M 2 is the sample soil mass of the 7.5- to 15-cm depth; and M 12Avg is the average total mass of the 0- to 7.5-cm and 7.5- to 15-cm depths of all samples; and OC 2 is the SOC concentration (%) for the sample 7.5- to 15-cm depth. The change in SOC (DSOC) for a system was taken as the difference between SOC in 1995 and Estimation of C Inputs Carbon inputs to the soil were estimated from grain yield using the method of Gan et al. (2009a) for both 0- to 20-cm and 0- to 100-cm soil depths. Total C input to 0- to 20-cm soil depth was used to analyze its relationship with change in SOC in this depth. The C input to 0- to 100-cm depth was used in comparing to Century modeling analyses. The model simulates change in SOC only in the top 20-cm but uses total C inputs from crop residues in the rooting depth. Dry biomass of grain was calculated assuming moisture content of 13.5% for cereals and pulses and 9.5% for oilseeds (Janzen et al. 2003). The production of straw was estimated from grain yields using the harvest index (HI); then the root biomass was estimated using straw-to-root (S:R) ratio and 65% of the root biomass was considered to be biomass of rhizo-deposits (Gan et al. 2009a). The C inputs to the soil from the crop residues were estimated assuming a C concentration of 42.3% (Bolinder et al. 2007). Modeling SOC Pool with the Century Model The original Century model simulates SOC and N dynamics in soils of the US Great Plains (Parton et al. 1987). It was revised and improved several times for applications to other ecosystems (Parton et al. 1994; Parton 1996). It is a compartmental model whereby SOC is partitioned into conceptual pools with active, intermediate and passive fractions. The flow of C and N is tracked through plant litter, organic and inorganic pools. The model can simulate a wide range of crop rotations and tillage practices to evaluate the effects of management and land use change on productivity and sustainability of agroecosystems. A long-term simulation was carried out in this study to estimate and predict the trends in SOC change for each of the five rotations studied. The SOC pools were first stabilized in a 5000-yr simulation with native grass then simulations were run for a FW system until 1966 followed by FWW to the beginning of the experiment in At this point, all phases of the five rotations with NT and MT management as defined above were simulated to 2006 using observed weather. Simulations were extended to 2050 to estimate long-term trends in SOC change due to the treatments using long-term average monthly weather. Yields were calibrated for each crop by adjusting the potential above-ground monthly production parameter (PRDX) in the model such that average grain yields over the course of the study for each crop were within 1.5% of measured. Calibration of yields in using the Century model is a common practice (Metherall et al. 1993; Smith et al. 2001). Data Analysis Statistical analysis of data was done using the proc mixed procedure in SAS software (Littell et al. 2006). Soil BD, SOC concentration, SOC amount and temporal change in the SOC, as well as the C input to the soil from crop residues under different crop rotations and tillage management were analyzed as dependent variables. Treatment was taken as a system defined by each rotation and tillage combination. Treatment was considered a fixed effect while replication and year were taken as random effects. Least significant difference (LSD) was calculated at 10% probability for separation of means. Contrasts were used to discern differences among effects across several systems. The relative contribution of C input, by crop types, to DSOC was evaluated using indicator variables: DSOCb 0 b 1 C in b 2 i C b 3 i C C in b 4 i P b 5 i P C in b 6 i F b 7 i F C in (3) where, b 0 to b 7 are regression coefficients; C in is the cumulative total C inputs between 1995 and 2006; and i C,i P and i F are indicator variables that are either 1or 0: i C is 1 for FCW and 0 otherwise, i P is 1 for FPW and FWP and 0 otherwise, and i F is 1 for WOP and 0 otherwise. If either or both b 2 and b 3 are different from zero, this would show DSOC in FCW responds differently to C inputs than the other systems. The interpretation would be similar for b 4 and/or b 5, except the different response would be indicated for FPW and FWP while non-zero b 6 and/or b 7 would indicate that DSOC for WOP responds differently to C inputs. RESULTS AND DISCUSSION Effects of Crop Management on SOC At the start of the experiment in 1995, soil BDs were similar and in 2006, i.e., after 11 yr of experimental period, the BDs varied from 1.39 to 1.43 Mg m 3 in different treatments, but were not affected by crops in the rotation, cropping frequency, nor by tillage (data not shown). In 1995, as expected, there were no significant differences or apparent trends in SOC concentration or amount, but they were not perfectly uniform either; for example, standard errors were 0.8 g Ckg 1 and 0.8 Mg C ha 1 for 0- to 7.5-cm, respectively (data not shown). The crop mix (e.g., wheat, canola) in the fallow-cropcrop rotations had no effect on SOC (Table 2). Continuous cropping in the WOP rotation increased SOC in the 0- to 7.5-cm depth compared with fallow-crop-crop rotations (Table 2). This was due to greater C inputs (Table 1) from the annual cropping. However, none of

6 142 CANADIAN JOURNAL OF SOIL SCIENCE Table 2. Concentration and amount of SOC in 2006, change in SOC (DSOC) from 1995 to 2006 and cumulative C inputs in 0- to 100- cm depth from 1995 to 2006 SOC concentration (kg C Mg 1 ) SOC y (Mg C ha 1 ) DSOC x (Mg C ha 1 ) C inputs w (Mg C ha 1 ) System z 07.5 cm cm 07.5 cm 015 cm 07.5 cm 015 cm 020 cm FCW_MT FCW_NT FPW_NT FWP_NT FWW_MT FWW_NT WOP_NT LSD (PB0.1) Contrasts (significances) FCW vs. FWW NS NS NS NS NS NS 0.02* z System is defined by crop rotation and tillage, abbreviation for rotation and tillage are same as in Table 1. y SOC amount calculated on equivalent mass basis (Ellert and Bettany 1995). x DSOC, changes in SOC amount from 1995 to w C inputs, carbon inputs from crop residue and roots during 1995 to *Significant at P50.10; NS, not significant. the treatments significantly influenced SOC in the 0- to 15-cm depth, although there was a tendency for the WOP systems to have higher amounts of SOC at this depth than did the fallowcropcrop rotations (P 0.12). These results are similar to those reported by Campbell et al. (1992) and (2005a) who found that significant differences in SOC in the semiarid prairies are often observed only in the surface 7.5- or 10-cm depths. Similar findings were also reported after a 23-yr study conducted at Scott, Saskatchewan, on a Dark Brown Chernozem (Campbell et al. 1992). This result would be expected since SOC is concentrated near the surface in these shallow soils of semiarid prairie and soil disturbance from seeding and tillage do not normally bring residue below the 7.5-cm depth. Tillage in the fallowcropcrop rotations had no effect on the concentration or on amount of SOC in the 0- to 7.5-cm or 0- to 15-cm depths by 2006 (Table 2). These findings are similar to those reported in a review paper by Campbell et al. (2005a) and in a long-term tillage study for fallow-containing rotations at Swift Current by McConkey et al. (2012). Effects of Cropping Systems on C Inputs Total C inputs to the soil from crop residues, including roots and rhizo-deposits for the 0- to 20-cm depth, varied significantly with rotation and cropping frequency and, slightly with tillage (Table 2). Estimated C input to 20-cm depth for the 11-yr period of the study was greatest for the continuous cropping [WOP (NT)] rotation as expected, with values of 22.1 Mg Cha 1 while for the fallowcropcrop rotations, it was 1820 Mg C ha 1 (Table 2). These values are generally similar to those reported by Campbell et al. (2007b) for wheat grown on the same soil at Swift Current, in MT experiment during a similar period. In this latter study, C input over 11 yr for FWW was 19.4 Mg C ha 1 and for continuous wheat 25 Mg C ha 1. Canola grown on fallow produced greater C inputs than wheat grown on fallow (Table 1). Although canola has a lower biomass production than wheat, the HI is lower (0.20) than wheat (0.30), thus the crop residue inputs (straw and root fractions) are higher (Gan et al. 2009a). Canola also had a subsequent rotational effect because C inputs were greater for wheat grown on canola than for wheat grown on wheat (Table 1). The C inputs for pulse crops were higher than that for wheat considering both productions on fallow or on wheat stubble (Table 1). Similarly, a study by Campbell et al. (2007a) indicated that pulse in rotation resulted in higher C inputs than rotations without pulse. Relationship between C Inputs and Changes in SOC Pool The range in total C inputs across systems and rotation phase over the experiment was from 17 to 28 Mg C ha 1 (Fig. 2a). Range in C input between phases within one system was up to 9 Mg C ha 1 over the 11 yr (data not shown). Thus, the C inputs reflected both average differences between systems (Table 2) and variation from interactions between rotation phase and precipitation. The first type of interaction was between precipitation and fallow. To illustrate this interaction, consider the years 2000 to 2002 when 2001 was an extreme drought year while 2002 had much above-average growing season precipitation (Fig. 1). The sequence of cropcropfallow in those years placed the stubble crop in the 2001 drought year when its yield was negatively affected while fallow happened to occur in 2002 with no crop to take advantage of favorable growing season rainfall. In contrast, the sequence of fallowcropcrop of the same system during the same years had higher C

7 SHRESTHA ET AL. * CROP ROTATION AND TILLAGE EFFECTS ON SOC 143 Campbell et al. (2007a) reported that the effect of crop rotation on SOC was mainly through their effect on C inputs. In that study, fallow-containing rotations had less C inputs and ultimately less SOC. Also, rotations with lentil and fall rye (Secale cereale L.) had higher C sequestration potential than flax because flax had much lower yield and thus low C inputs (Campbell et al. 2007a). Fig. 2. (a) Relationship between measured and Century modeled C inputs, and (b) relationship between measured and modeled change in SOC pool from 1995 to 2006 and total C inputs, both determined across rotation phases. inputs because it placed the crop on fallow in 2001 and this helped mitigate drought effects and the stubble crop in a wet year produced high yields (data not shown). The second type of interaction was between crop type and precipitation. Pea is more responsive to growing season water than wheat which, in turn, is more responsive than canola (Angadi et al. 2008). Consequently, rotation phases that happened to place the more waterresponsive crops in wetter years had higher C inputs than the other phases of the same rotation with the less water-responsive crops in the wetter years. There was no indication that either the residue type or fallow in the rotation had any effect on DSOC as b 2 to b 7 in Eq. 3 were all not different from zero at P0.10 (data not shown). Consequently, Eq. 3 was reduced to: DSOCb 0 b 1 C in (4) Overall, there was a linear relationship (P B0.05) between total C inputs from the crop residue and DSOC (Fig. 2b); however, the association was weak (R ). Similarly, for a 37-yr study in semiarid soils Estimated vs. Modelled SOC and C Inputs Soil C change in the Century model is largely driven by C inputs from crops. The model performed well in approximating measured C inputs (R ) across rotation phases (Fig. 2a). The Century model and measured values were similar in magnitude and rate of change in SOC versus C inputs, with an expected increase in SOC with greater C inputs (Fig. 2b). The linear relationship between DSOC and the average of total C inputs was stronger for the model (R ) than for measured values (R ) (Fig. 2b). This weaker relationship for the measured data reflected the variability of field data for both SOC and C inputs. When we extended the Century simulations to 2050 (Fig. 3), the result showed that it will take about 20 yr for differences between most systems to exceed 2 Mg C ha 1 (i.e., about the LSD for 0- to 15-cm SOC between systems in 2006), including differences due to tillage. Some systems are expected to have differences of less than 2 Mg C ha 1 by 2050 [e.g., the similar NT FPW and FWP rotations (Fig. 3)]. The Century model simulations agreed with the observations that the differences in SOC between systems will be relatively small by In fact, predicted SOC rankings by system were not consistent with the order in 2050 until 25 yr had elapsed despite having all systems start with identical initial SOC in 1995 and having consistent average weather from year 12 of the simulation onwards. The fact that we observed small differences in SOC between systems after 11 yr was thus expected in this semiarid climate with relatively small and variable annual C inputs. A combination of field experiment and simulation with a validated SOC model provides valuable insight into SOC response that can be difficult to discern using the experimental data alone, while interpretation of results is uncertain if only predicted by a model without experimental data to determine if there are significant deviations between simulated and real-world SOC behavior. The linear relationship between C inputs and change in SOC showed that the soil in our study required 26.7 Mg C ha 1 in 11 yr or 2.4 Mg C ha 1 yr 1 C inputs (Fig. 2b) to maintain the SOC level. Since C inputs in our study were typically less than 2.4 Mg C ha 1 yr 1, the soil was generally losing C from 1995 to In two rotation studies viz. Old Rotation study with conventional tillage ( ) and New Rotation study with MT and snow management ( ), conducted

8 144 CANADIAN JOURNAL OF SOIL SCIENCE Fig. 3. Century model simulated SOC pool in different cropping systems from 1980 to 2050 in the 0- to 20-cm depth. Abbreviations are same as in Table 1. on the same soil series, Campbell et al. (2007a, b) found that 0.9 and 1.6 Mg C ha 1 average annual C inputs over 37- and 17- years, respectively, were required to maintained the SOC pool at steady state. One interpretation for the differences in required C inputs to balance SOC loss via decomposition may be that higher C inputs were required to maintain SOC for less degraded soils. However, the total amount of SOC for FWW (0- to 15-cm) in 2003 was 36 Mg C ha 1 in the Old Rotation study (Campbell et al. 2007a) and 31.8 Mg C ha 1 in the New Rotation study (Campbell et al. 2007b) vs. 29 Mg C ha 1 in 2006 (our study). Therefore, assuming similar SOC quality between these studies, there is no indication that our soil was less degraded. The interplay between precipitation and the C balance is complex in semiarid climates. Depending on amount and timing, increased precipitation may enhance SOC decomposition more than C gains due to C inputs which might then result in a net loss of SOC (Huxman et al. 2004). An alternate interpretation, then, is that the generally above-average precipitation during our study may have increased SOC loss via decomposition more than it increased SOC via C inputs. Mean annual precipitation during our study period was 12% and 4% higher than the Old Rotation study (Campbell et al. 2007a) and the New Rotation study (Campbell et al. 2007b), respectively (Fig. 4). Thus, more C inputs were required to maintain SOC in our 11-yr study (2.4 Mg C ha 1 yr 1 ) than was the case for the both studies by Campbell et al. (2007a, b). Many factors affected the balance between C inputs and C losses for the three studies at Swift Current. We simulated all three studies with Century using observed weather and the estimated annual C inputs required to maintain SOC at steady state. It was 1.1, 1.3 and 2.1 Mg C ha 1 yr 1 for the Campbell et al. (2007a) study, Campbell et al. (2007b) study, and our study, respectively. Thus, the trend of increasing required C inputs to maintain steady state SOC with increasing annual precipitation at Swift Current was consistent between Century and the observations (Fig. 4), although we are not suggesting that this empirical relationship is predictive for general precipitation effects. There were other differences between the studies in terms of crops and soil management that may have affected the C balance (e.g., tillage, water storage). Nevertheless, the weather was one of the main differences between the studies, and our results suggest that weather patterns over time, interacting with crop and soil management, can produce marked long-term differences in the soil C balance. The long-term Century projections indicate important SOC advantages for having canola and pulse crops in rotation compared with monoculture wheat. These projections suggest that pea will provide more SOC than canola in similar rotations in the semiarid Prairies. By 2050, the NT FPW rotation will have about 5 Mg C ha 1 more than the NT FWW vs. only about half that difference for NT FCW compared with NT FWW. CONCLUSIONS Our study showed that C input was the dominant predictor of change in the SOC. The SOC increased about 0.33 Mg per Mg of C input above 2.4 Mg C ha 1 yr 1 but decreased by the same amount when C inputs were below 2.4 Mg C ha 1 yr 1. There was no evidence that crop type (canola, pulse, or wheat) or fallow affected SOC change beyond its effect on C inputs over a short term. Pulse and canola provided about 10% greater C inputs advantage in comparison with wheat. Over the 11 yr, there was little difference in SOC between cropping systems, with the only difference detectable being in the shallowest soil depth (0- to

9 SHRESTHA ET AL. * CROP ROTATION AND TILLAGE EFFECTS ON SOC 145 Fig. 4. Measured and Century modeled relationship between annual C inputs required to maintain SOC at steady state vs. average annual precipitation at Swift Current, Saskatchewan. 7.5-cm) and only for continuous cropping system versus the fallow-crop-crop systems. For the 0- to 15-cm depth, there were no significant differences in SOC between systems after 11 yr. The Century model estimated similar C inputs and rate of SOC change per unit of C input as was measured. In this semiarid climate with low and variable C inputs, the Century model simulations also confirmed that it will be difficult to detect SOC differences between these cropping systems until, probably, several decades after system changes. ACKNOWLEDGMENTS We acknowledge Marty Peru and Devon Worth for their technical assistance and Duaine Messer for soil chemical analysis. Support from the Canola Council of Canada through the canola cluster is also gratefully acknowledged. The senior author acknowledges the Natural Sciences and Engineering Research Council of Canada (NSERC) for the opportunity to work at Agriculture and Agri-Food Canada as a postdoctoral fellow. Angadi, S. V., McConkey, B., Cutforth, H. W., Miller, P. R., Ulrich, D., Selles, F., Volkmar, K. M., Entz, M. H. and Brandt, S. A Adaptation of alternative pulse and oilseed crops to the semiarid Canadian prairie: Seed yield and water use efficiency. Can. J. Plant Sci. 88: Bolinder, M. A., Janzen, H. H., Gregorich, E. G., Angers, D. A. and VandenBygaart, A. J An approach for estimating net primary productivity and annual carbon inputs to soil for common agricultural crops in Canada. Agric. Ecosyst. Environ. 118: Bremer, E., Janzen, H. H. and McKenzie, R. H Shortterm impact of fallow frequency and perennial grass on soil organic carbon in a Brown Chernozem in southern Alberta. Can. J. Soil Sci. 82: Bruce, J. P., Frome, M., Haites, E., Janzen, H., Lal, R. and Paustian, K Carbon sequestration in soils. J. Soil Water Conserv. 54: Campbell, C. A., Brandt, S. A., Biederbeck, V. O., Zentner, R. P. and Schnitzer, M Effect of crop rotations and rotation phase on characteristics of soil organic matter in a Dark Brown Chernozemic soil. Can. J. Soil Sci. 72: Campbell, C. A., Janzen, H. H., Paustian, K., Gregorich, E. G., Sherrod, L., Liang, B. C. and Zentner, R. P. 2005a. Carbon storage in soils of the North American Great Plains. Agron. J. 97: Campbell, C. A., Selles, F., Lafond, G. P., Biederbeck, V. O. and Zentner, R. P Tillage - fertilizer changes: Effect on some soil quality attributes under long-term crop rotations in a thin Black Chernozem. Can. J. Plant Sci. 81: Campbell, C. A., VandenBygaart, A. J., Grant, B., Zentner, R. P., McConkey, B., Lemke, R., Gregorich, E. G. and Fernandez, M. R. 2007a. Quantifying carbon sequestration in a conventionally tilled crop rotation study in southwestern Saskatchewan. Can. J. Soil Sci. 87: Campbell, C. A., VandenBygaart, A. J., Zentner, R. P., McConkey, B., Smith, W., Lemke, R., Grant, B. and Jefferson, P. G. 2007b. Quantifying carbon sequestration in a minimum tillage crop rotation study in semiarid southwestern Saskatchewan. Can. J. Plant Sci. 87: Campbell, C. A., Zentner, R. P., Gameda, S., Blomert, B. and Wall, D. D Production of annual crops on the Canadian praries: Trends during Can. J. Plant Sci. 82: Campbell, C. A., Zentner, R. P., Liang, B. C., Roloff, G., Gregorich, E. G. and Blomert, B Organic C accumulation in soil over 30 years in semiarid southwestern Saskatchewan effect of crop rotations and fertilizers. Can. J. Soil Sci. 80: Campbell, C. A., Zentner, R. P., Selles, F., Jefferson, P. G., McConkey, B., Lemke, R. and Blomert, B. 2005b. Long-term effect of cropping system and nitrogen and phosphorus fertilizer on production and nitrogen economy of grain crops in a Brown Chernozem. Can. J. Soil Sci. 85: 8193.

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