The influence of 12 years of tillage and crop rotation on total and labile organic carbon in a sandy loam soil

Soil & Tillage Research 95 (2007) 38–46 www.elsevier.com/locate/still

The influence of 12 years of tillage and crop rotation on total and labile organic carbon in a sandy loam soil Y.K. Soon *, M.A. Arshad, A. Haq, N. Lupwayi Agriculture and Agri-Food Canada, Beaverlodge Research Farm, P.O. Box 29, Beaverlodge, Alta. T0H 0C0, Canada Received 21 June 2006; received in revised form 5 October 2006; accepted 15 October 2006

Abstract Information on which management practices can enhance soil organic matter (SOM) content and quality can be useful for developing sustainable crop production systems. We tested the influence of 12 years of no-till (NT) versus conventional tillage (CT), and four crop sequences on the organic C pools of a Grey Luvisolic sandy loam soil in northwestern Alberta, Canada. The crop sequences were: continuous wheat (Triticum aestivum L.), field pea (Pisum sativum L.)–wheat–canola (Brassica rapa L.)–wheat, red clover (Trifolium pratense L.) green manure–wheat–canola–wheat/red clover and fallow–wheat–canola–wheat. Soil samples from 1992, when the study was initiated, and 1996, 2000 and 2004 were analyzed for total organic C (TOC), the light fraction (LF) and its C content, and water-soluble and mineralizable C. Total organic C in the top 15 cm of soil was higher in the red clover rotation than either the pea or fallow rotation by 1996. The tillage effect became significant only in 2004 with NT having a higher TOC than CT. The LF dry matter (DM) increased from 6.9 g kg1 soil in 1992 to a range of 10–13 g kg1 in 2000 and 2004. It was higher under NT than CT in 2 of 3 years and in the red clover rotation than the pea or fallow rotation in 1 of 3 years. The LF C content exhibited a similar trend as LF DM. The water-soluble and mineralizable C pools were not affected by tillage but decreased with time. Among crop rotations, the red clover rotation tended to result in higher levels of hot water-soluble and mineralizable C. It is concluded that tillage had a greater influence than crop rotation on the LF DM and LF C (as indicators of C storage), whereas the converse effect applied to mineralizable C and, to a lesser degree, hot water-soluble C (as indicators of SOM quality). # 2006 Elsevier B.V. All rights reserved. Keywords: Carbon pools; Fallow; Light fraction; Mineralization; Pea; Red clover; Tillage

1. Introduction Sustainable agro-ecosystems aim to conserve soil organic matter (SOM). Organic matter improves the soil structure that keeps the soil friable and permeable to water, air and roots, and supports plant growth by providing the fuel (i.e., the carbon-containing compounds) and nutrients that drive soil nutrient cycles and

* Corresponding author. Tel.: +1 780 3545134; fax: +1 780 3548171. E-mail address: [email protected] (Y.K. Soon). 0167-1987/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2006.10.009

other microbial processes that maintain soil productivity. Recently, it has received increased attention as a potential sink for sequestering atmospheric CO2 (Batjes, 1998). Tillage- and cropping systems-induced changes in SOM content can be difficult to quantify due to the large background amounts already present and spatial variation (Haynes and Beare, 1996). This is especially so for cool climate regions since organic matter decomposition rates are lower compared to hotter and more arid regions, thereby minimizing tillage effects on decomposition. Total organic C (TOC) concentrations were similar in no-till (NT) and conventionally tilled (CT) soils in northern Alberta

Y.K. Soon et al. / Soil & Tillage Research 95 (2007) 38–46

and British Columbia (Franzluebbers and Arshad, 1996) and between no-till, chisel-ploughed and moldboard-ploughed soils in the cool, humid areas of eastern Canada (Angers et al., 1997). In contrast, labile pools of organic C, such as the water-extractable fraction, mineralizable C and the light fraction (LF), are more readily influenced by management practices than the recalcitrant pools (Biederbeck et al., 1994; Larney et al., 1997; Ghani et al., 2003). The LF is composed mostly of incompletely decomposed plant residues, is a source of labile C, and has a turnover time of 1–15 years (Janzen et al., 1992; Carter, 1996). The mineralizable C pool is considered the closest approximation to the C pool actually available to soil microorganisms (Davidson et al., 1987). Cold water-soluble C (WSC) is the main source of C respired by soil microorganisms during a 10-day incubation (Xu and Juma, 1993), whereas hot water-extractable C (HWC) has been found to be well-correlated with and partly derived from microbial biomass C (Sparling et al., 1998). Ghani et al. (2003) found that carbohydrates constituted between 40% and 50% of HWC. Water-soluble organic C is also comprised of long-chain aliphatics, and amides and amino-N-containing compounds and is believed to be derived from plant roots and litter as well as soil humus (Leinweber et al., 1995; Liang et al., 1998; Gregorich et al., 2003). The labile C pools play a greater role than the recalcitrant pools in the short-term turnover of C, and as such influence the functioning of soils, and are considered suitable indicators of soil quality (Gregorich et al., 1994). The influence of soil management on water-soluble C has not been as well studied as the LF or mineralizable C, and its significance in relation to SOM quality needs assessment. While the literature indicates that the labile C pools are more sensitive than TOC to management effects, comparisons among the different fractions have been scant. A suite of labile C fractions is typically required to assess SOM quality because of the multifunctional role of SOM (Haynes, 2005). Also, most studies of management effects on soil C pools have tended to analyze samples at one point in time, typically 3–16 years after initiation (Biederbeck et al., 1994; Franzluebbers and Arshad, 1996; Larney et al., 1997). Therefore, there is a need for more information on sequential changes in labile C pools, as influenced by soil management practices. Our aim was to determine the influence of tillage and crop rotation on soil C pools of varying lability over 12 years and the effectiveness of the C pools in depicting C storage in the soil as well as its quality.

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2. Materials and methods 2.1. Site description and experimental design The experimental site was located on the Agriculture and Agri-Food Canada research farm near Fort Vermillion, in northwestern Alberta (588230 N, 116820 W). The soil is a Leith sandy loam, a Grey Luvisol, with a pH of 7.1 (1:2 (m/v) soil:water ratio) in the plough layer. Average particle size distribution of the 0–15 cm layer is: sand 650 g kg1, silt 220 g kg1 and clay 130 g kg1. For this area, lack of moisture is typically the main limitation to crop growth, while the short growing season (75 frost-free days > 0 8C) limits the range of crops that can be grown. The growing season (May–August) average precipitation is 185 mm and average evapotranspiration is 373 mm. Experimental treatments were two tillage systems (NT and CT) imposed on main plots and four crop sequences (three rotations and continuous or monoculture wheat) imposed on subplots. Factorial combinations of the treatments were arranged in a split-plot design with three blocks. The three crop rotations were: (i) field pea (Pisum sativum L.)–wheat (Triticum aestivum L.)–canola (Brassica rapa L.)–wheat (PWCW); (ii) red clover (Trifolium pratense L.) green manure–wheat–canola–wheat/red clover (RcWCW); (iii) fallow–wheat–canola–wheat (FWCW). All phases of the rotations were represented each year. The rotation crops were first sown in 1992. Data collection started in 1993. Each subplot was 4 m wide and 25 m long. Conventional tillage consisted of an autumn cultivation with a heavy-duty cultivator or disk to a depth of 10–15 cm (depending on amount of residues and soil condition) and two spring tillage operations with a field cultivator followed by harrowing and packing. Weeds in NT plots were killed with glyphosate (N-(phosphonomethyl) glycine) before seeding. The red clover stand in CT plots was terminated by disking (at 50% bloom growth stage), and weed growth during the remainder of the fallow period was suppressed, as needed, by mechanical tillage. The red clover in the NT-green manure treatment was terminated by glyphosate burnoff and subsequent weed growth suppressed with appropriate herbicides. All crops were seeded in early to midMay using 23 cm row spacing. Fertilizer application rates were based on soil test results from autumn soil sampling. The average amounts of fertilizer N given annually to canola and wheat during 1993–2004 ranged from 20 to 70 kg N ha1, depending on tillage and the rotation. Legume crops were inoculated with Rhizobium and not given N fertilizer except for that added with P

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fertilizer. Carbon input in straw residues was estimated from straw dry matter (DM) from four 2-m rows and previously determined C concentrations (Soon and Arshad, 2002; Lupwayi et al., 2006). For further details of field operations see Soon and Clayton (2002). Soil samples were taken in the fall of 1996, 2000 and 2004 from the 0 to 15 and 15 to 30 cm depths of plots that had grown canola and the fourth phase wheat crops so as to minimize any effect directly attributable to the first phase of the rotations. Four cores were taken per plot using a 38-mm core sampler and mixed. Composite samples were taken from each of six the main plots before the first crops were seeded in 1992. Bulk density to 15 cm depth was measured using double cylinder core sampler. The soils were passed through a 2-mm sieve and air-dried.

2.3. Data analysis

2.2. Analytical methods

3. Results and discussion

Soil TOC content was determined using a modified Mebius procedure (Soon and Abboud, 1991). The LF of SOM was obtained by flotation on NaI solution with a specific gravity of 1.7 (Strickland and Sollins, 1987) and analyzed for C content using the modified Mebius method. Mineralizable carbon was estimated as described in Franzluebbers and Arshad (1997). Briefly, soil samples were incubated at 25 8C for 24 days at 22.5% (m/m) moisture content (90% of field capacity), and CO2-C evolved after 3, 10 and 24 days were determined by titration. Basal respiration rate was estimated from CO2 evolved between the 10th and 24th day. Water-soluble C was extracted by shaking 4 g of air-dried soil in 20 mL of nanopure water in a 50-mL centrifuge tube for 30 min, centrifuging at 12,000  g for 10 min and filtering the supernatant solution through a Whatman no. 42 filter. The organic C in the filtered extract was determined by the modified Mebius procedure and termed cold water-soluble organic C (WSC). The centrifuge tube with wet residual soil was weighed to determine entrapped water content. Twenty milliliters of nanopure water were added and the tube and contents vortexed for 10 s to re-suspend the soil. The tube was stoppered and placed in an oven (pre-heated to 70 8C) for 18 h (Sparling et al., 1998). The tube and content was then shaken for 30 min, left to settle and cool for about 30 min and centrifuged at 12,000  g for 10 min. The supernatant solution was filtered through a Whatman no. 42 filter and analyzed for organic C as noted above. This fraction was added to the WSC and the sum constituted hot water-soluble C (HWC).

3.1. Soil bulk density

Data for each year were analyzed by analysis of variance, and combined data for 1996, 2000 and 2004 were analyzed by repeated measures analysis of variance since the samples were from the same plots. Treatment and interaction effects were considered significant at P = 0.05. Interactions between tillage and crop rotations were mostly absent. Although crop phase was included in the analysis as a source of variation, its effect was not significant; therefore, results are shown only for crop rotation and tillage effects. Data from the continuous wheat plots were not included in the statistical analyses because they were taken only from wheat plots and so from half as many plots as the other rotations, and are included in the tables only for information.

Soil bulk density was not significantly influenced by tillage or crop rotation. It averaged 1.32 g cm3 (S.E.M. = 0.013) for both CTand NT treatments whereas the means among rotations ranged from 1.30 to 1.34 g cm3. Therefore, soil mass per unit land area of the top 15 cm was not significantly different between treatments. 3.2. Estimated carbon added in crop residues Most C was added in crop residues by the red clover rotation, and least by the fallow rotation and continuous wheat (Table 1). Since the data were calculated using Table 1 Carbon in crop residues (Mg C ha1) in each rotation cycle as influenced by crop rotation and tillagea Treatment

1993–1996 1997–2000 2001–2004 Total (1993–2004)

Tillage Conventional 4.69 No-till 4.42

4.49 5.27

3.82 4.04

13.0 13.7

Crop rotationb PWCW RcWCW FWCW WWWW

5.58 6.08 3.89 3.97

4.05 4.12 3.57 3.75

14.8 15.7 11.6 11.1

a

5.12 5.50 4.16 3.40

Estimated by multiplying straw dry matter by its C concentration (assumed to be 0.45, 0.43 and 0.51 g g1 for wheat, pea and canola straw residues; Soon and Arshad, 2002). C concentration of red clover shoot was assumed to be 0.44 g g1 (Lupwayi et al., 2006). b P denotes pea; W, wheat; C, canola; Rc, red clover green manure; F, fallow.

Y.K. Soon et al. / Soil & Tillage Research 95 (2007) 38–46

average C concentrations, actual variability of C input are not available. The coefficient of variation was about 20% for straw DM and less than 10% for straw C concentration. Since the three crop rotations differed only in their first phases, variations in C added in each rotation can be attributed mainly to the first phase treatment (which in the fallow rotation was a fallow year) and its influence on the subsequent wheat crop. The low C addition with continuous wheat is due mainly to its lower productivity compared to the rotations with legume crops (Soon and Clayton, 2002). Significantly more straw was produced by NT than CT during the second rotation cycle (1997–2000), however, over the 12-year period straw C input under NT was only about 5% greater than CT (Soon and Clayton, 2002). The low crop productivity during the third rotation cycle is due mainly to the below average growing season rainfall in three of those 4 years. Total C input would be considerably higher than indicated in Table 1 since C entered the soil also as roots and rhizodeposits. Rhizodeposited C is typically greater than the standing root mass C at harvest. For example, for spring wheat at maturity, root biomass C was estimated to be about 10%, and rhizodeposited C about 15%, of shoot C content (Swinnen et al., 1994), and total rhizodeposited C was about four-fold the C contained in the roots of mature barley (Hordeum vulgare L.) (Xu and Juma, 1993). Therefore, a reasonable estimate of total C input from above- and below-ground residues would be about 25% higher than shown in Table 1.

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3.3. Total organic C There was no significant variation of mean TOC concentrations in the 0–15 and 15–30 cm depths between years (Table 2). Surface soil TOC tended to be higher under NT than CT (P = 0.09 by repeated measures analysis of variance) with the difference becoming significant in 2004 (Table 2). The tillage effect was due mainly to a decrease in TOC under CT and, to a lesser degree, a small gain in TOC under NT. Surface soil TOC was lower in the fallow and the pea rotations than in the red clover rotation (P = 0.02 by repeated measures ANOVA, and in individual years 1996 and 2004), and similar between continuous wheat and the red cover rotation. Although the pea rotation added more crop residue C than continuous wheat, its TOC was slightly lower than that of continuous wheat, probably because wheat straw is less readily mineralized than pea or canola straw, and wheat produced more root DM than did pea and canola (Soon and Arshad, 2002). The tillage effect is attributed to increased mineralization of organic matter by tillage operations associated with CT and the rotation effect partly to the amount of crop residue production (Table 1). Although the 15–30 cm soil showed similar trends as the surface soil among tillage treatments and crop rotations, the treatment differences were not significant because of greater subsoil variation. Soil C storage in the 0–15 cm depth in the autumn of 2004 was 3.56 kg m2 under NT and 3.22 kg m2 under CT compared to an initial mean value of 3.42 kg m2 in

Table 2 Total organic C (g kg1 soil) in 0–15 and 15–30 cm soil from 1992 through 2004 as influenced by tillage and crop rotationa Treatment

0–15 cm

15–30 cm

1992

1996

2000

2004

1992

1996

2000

2004

Tillageb CT NT S.E.M. (2d.f.) P>F

– – – –

18.3 19.0 0.47 0.39

19.2 20.3 0.98 0.47

17.9 19.8 0.32 0.05

– – – –

6.7 7.1 0.13 0.15

7.3 8.4 0.95 0.52

6.2 8.2 0.48 0.10

Rotationc PWCW RcWCW FWCW WWWW S.E.M. (20d.f.) P>F

– – – – – –

17.8 20.3 18.0 19.5 0.67 0.03

19.3 20.7 19.1 19.9 0.85 0.38

17.9 20.2 18.4 20.5 0.61 0.04

– – – – – –

6.5 8.0 6.2 5.6 0.55 0.08

7.3 8.1 8.1 6.8 0.83 0.75

7.0 7.3 7.2 6.8 0.7 0.95

Mean

19.0

18.7

19.7

18.8

8.4

6.9

7.8

7.2

a b c

Except for 1992, mean values are based on 36 observations (excluding WWWW). Results for 1992 are based on six main plots. CT, conventional tillage; NT, no-till. Underlining indicates crop phases sampled. P denotes pea; W, wheat; C, canola; Rc, red clover green manure; F, fallow.

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1992. Campbell et al. (1996) reported that in southwestern Saskatchewan, the difference in C storage between CT and NT after 11–12 years ranged from none for a sandy loam soil to 0.4 kg m2 for a clay soil. In Texas, soil C storage in the surface 20 cm of a clay loam soil increased by as much as 0.56 kg m2 in 10 years under continuous wheat managed with no-till as compared to stubble-mulch management (Potter et al., 1997). Our data are within the reported range. 3.4. The light fraction of soil organic matter and its C content The LF DM increased with time from an average of 6.9 g kg1 of soil in 1992 to 11–13 g kg1 in 2000 and 2004, and was consistently higher under NT than CT (P = 0.07 in repeated measures analysis of variance), however, the differences due to tillage were significant only in 1996 and 2004 (Table 3). Light fraction DM was higher under the red clover green manure rotation than under the pea or fallow rotation in 1996 and for the means of the 3 years (P = 0.03 in repeated measures ANOVA), and was similar between continuous wheat and the red clover rotation (Table 3). There was no tillage  rotation interaction indicating that the crop rotation and tillage effects were independent. The LF DM has been found to decrease with increasing frequency of fallow in crop rotations (Biederbeck et al., 1994) and intensity of soil tillage (Larney et al., 1997). The accumulation of the LF should reflect the

slower decomposition of crop residues under NT than CT and also the amount of crop residues produced (Table 1). The C concentration per unit mass of LF decreased with time since the study started. The mean values were sequentially: 255, 248, 226 and 213 g C kg1 LFDM in 1992, 1996, 2000 and 2004. The LF C concentrations were similar among experimental treatments, and are within the range reported for Canadian prairie soils (150–300 g C kg1 LF) (Janzen et al., 1992; Biederbeck et al., 1994). The LF C content, expressed on a per unit mass of soil (this incorporates the influence of the amount of LF), increased with time, reaching a maximum in 2000 and decreasing in 2004, and was influenced by tillage but not by crop rotation (Table 3). The LF C content in the surface soil can be influenced by crop rotations as well as fertility treatments (Biederbeck et al., 1994). The decrease in 2004 may be partly due to the low input of crop residues during the prior 4 years. Light fraction C as percent of TOC in the surface 15 cm of soil increased from 9% in 1992 to 13–14% in 2000, then decreasing to 12.5% in 2004. It typically constitutes 2–17.5% of TOC in Canadian Prairie soils (Janzen et al., 1992). The LF DM and LF C are sensitive indicators of C storage and showed a significant tillage effect 4 years into the study whereas the tillage effect on TOC was not evident until the 12th year. No-till resulted in a net gain in 2004 of 1.9 g TOC kg1 soil relative to CT, whereas net gain in LF C was about 0.5 g kg1. This suggests

Table 3 Dry matter and C content of light fraction (LF) organic matter in 0–15 cm soil from 1992 through 2004 as influenced by tillage and crop rotationa Treatment

LF dry matter (g kg1 soil)

LF C content (g C kg1 soil)

1992

1996

2000

2004

1992

1996

2000

2004

Tillage CT NT S.E.M. (2d.f.) P>F

– – – –

8.3 9.3 0.14 0.04

11.2 12.9 0.93 0.33

9.9 12.1 0.09 <0.01

– – – –

2.04 2.35 0.053 0.05

2.47 2.95 0.257 0.32

2.06 2.61 0.025 <0.01

Rotationc PWCW RcWCW FWCW WWWW S.E.M. (20d.f.) P>F

– – – – – –

8.4 9.7 8.5 8.7 0.34 0.03

11.7 12.9 11.4 12.4 0.75 0.33

10.5 12.0 10.4 13.1 0.70 0.22

– – – – – –

2.09 2.34 2.16 2.24 0.085 0.13

2.61 2.91 2.61 2.81 0.197 0.46

2.22 2.57 2.22 2.80 0.144 0.16

Mean

6.9

8.8

12.0**

11.0

1.74

2.20

2.71**

2.36**

b

a Except for 1992, mean values are based on 36 observations (excluding WWWW). Mean value for 1992 is based on six main plots. Asterisks (* and **) following a mean value for year indicate that according to a profile analysis of repeated measures ANOVA it is significantly different from the preceding mean at P = 0.05 and 0.01, respectively. b CT: conventional tillage; NT: no-till. c Underlining indicates crop phases sampled. P denotes pea; W, wheat; C, canola; Rc, red clover green manure; F, fallow.

Y.K. Soon et al. / Soil & Tillage Research 95 (2007) 38–46

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Table 4 Cold and hot water-soluble organic C in 0–15 cm soil from 1992 through 2004 as influenced by tillage and crop rotationa Treatment

Cold water-soluble C (mg kg1)

Hot water-soluble C (mg kg1)

1992

1996

2000

2004

1992

1996

2000

2004

Tillage CT NT S.E.M. (2d.f.) P>F

– – – –

271 261 7.6 0.67

265 286 4.4 0.07

167 184 8.8 0.30

– – – –

909 898 23.8 0.76

801 853 12.4 0.10

618 699 24.2 0.14

Rotationc PWCW RcWCW FWCW WWWW S.E.M. (20d.f.) P>F

– – – – – –

264 270 264 275 15.8 0.96

271 283 273 286 11.8 0.74

173 180 175 196 10.5 0.88

– – – – – –

884 933 893 885 31.9 0.52

805 858 818 859 25.6 0.33

640 717 619 706 22.0 0.01

Mean

258

266

276

176*

745

903

827**

659**

b

Except for 1992, mean values are based on 36 observations (excluding WWWW). Data for 1992 are based on six main plots. Asterisks (* and **) following a mean value for a year indicate that according to a profile analysis of repeated measures ANOVA it is significantly different from the preceding mean at P = 0.05 and P = 0.01, respectively. b CT: conventional tillage; NT: no-till. c Underlining indicates crop phases sampled. P denotes pea; W, wheat; C, canola; Rc, red clover green manure; F, fallow. a

that most of the increase in TOC under NT was in the ‘heavy fraction’, i.e., the SOM fraction associated with mineral particles and aggregates. In spite of a net gain of 0.3 g LF C kg1, the CT soil lost about 1.1 g kg1 of TOC over 12 years, indicating that the loss of SOM by tillage was mainly at the expense of the ‘heavy fraction’, and that the accumulation of the LF from crop residues under CT was not sufficient to offset this loss. Higher LF DM or LF C accumulation under NT than CT mainly resulted from lower residue decay rates under NT since crop residue production was essentially similar among tillage systems. 3.5. Water-soluble C Cold water-soluble C was not influenced by tillage or crop rotation and lower in 2004 compared to previous years (Table 4). Linn and Doran (1984) reported that WSC in the surface 75 mm of ploughed soils tended to be lower than that of NT soils. Hot water-soluble-C was not affected by tillage, however, in 2004 and for the yearly means (P = 0.02 by repeated measures analysis of variance) the trend in HWC among crop rotations was: RcWCW = WWWW > PWCW = FWCW (Table 4). The HWC was three to four times greater than WSC, similar to results reported by Gregorich et al. (2003). Cold water-soluble C constituted about 1%, and HWC about 3.5–4.8% of TOC. Whereas WSC varied little between years until the decrease in 2004, HWC was

highest in 1996 and decreased in each of the subsequent samplings. The low WSC and HWC in 2004 may be attributed to a severe drought with growing season rainfall slightly more than half the long-term average, which would have reduced crop productivity and rhizodeposition. Ghani et al. (2003) also reported that the HWC of pasture soils in New Zealand was lower during a dry summer. Although, Gregorich et al. (1994) did not include water-soluble C in the minimum data set needed to assess soil biological activity, they suggested that it could in future. Our results suggest that WSC may not be responsive enough to tillage and crop rotation effects. The utility of this C pool as an indicator of soil quality has been questioned because of its small size and highly labile nature (Baldock and Nelson, 2000). Rather, it is the flux of soluble organic C that is considered important in relation to C availability and microbial activity (Haynes, 2005). 3.6. Mineralizable carbon The incubation period for measuring mineralizable C commonly ranges from 10 to 30 days (Haynes, 2005). We present data for 10 and 24 days (Table 5). Carbon mineralized in 24 days constituted approximately 1.3– 2.3% of TOC. Regardless of length of incubation period, mineralizable C decreased through each of the cropping cycle, and averaged over years, it was significantly higher (P = 0.01 by repeated measures

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Y.K. Soon et al. / Soil & Tillage Research 95 (2007) 38–46

Table 5 Organic C mineralized as CO2-C from 0 to 15 cm soil from 1992 through 2004 as influenced by tillage and crop rotationa Treatment

C mineralized in 10 days (mg kg1)

C mineralized in 24 days (mg kg1)

1992

1996

2000

2004

1992

1996

2000

2004

Tillage CT NT S.E.M. (2d.f.) P>F

– – – –

289 302 16.0 0.62

182 200 19.0 0.57

137 139 4.8 0.81

– – – –

411 438 16.1 0.36

309 338 24.9 0.51

247 256 9.4 0.57

Rotationc PWCW RcWCW FWCW WWWW S.E.M. (20d.f.) P>F

– – – – – –

277 316 293 286 8.2 0.01

166 229 179 166 21.2 0.11

130 149 134 138 6.2 0.08

– – – – – –

396 454 423 409 10.6 <0.01

282 379 309 286 30.6 0.10

239 276 240 250 9.2 0.02

Mean

326

295

191**

138**

450

425

324**

252**

b

a

Except for 1992, mean values are based on 36 observations (excluding WWWW). Mean value for 1992 is based on six main plots. Asterisks ( and **) following a mean value for year indicate that according to a profile analysis of repeated measures ANOVA it is significantly different from the preceding mean at P = 0.05 and P = 0.01, respectively. b CT: conventional tillage; NT: no-till. c Underlining indicates crop phases sampled. P denotes pea; W, wheat; C, canola; Rc, red clover green manure; F, fallow. *

ANOVA) in the red clover rotation than other rotations. A similar crop rotation effect was also shown for the 10day respiration in 1996 and the 24-day respiration in 1996 and 2004. The continuous wheat cropping system tended to be similar to the pea rotation. The differences between rotations in 2000 were not significant due to greater data variation. Mineralizable C was not influenced by tillage. Franzluebbers and Arshad (1996) found that C mineralization of four soils was higher under CT than under NT. Larney et al. (1997) reported no difference in mineralizable C between wheat–wheat and wheat–fallow rotations, and opposite results to Franzluebbers and Arshad (1996) in regards to tillage effect. Among Canadian prairie soils, therefore, tillage and crop rotation effects on C minerlization have not been consistent. Since, mineralizable C fluctuates seasonally (Haynes, 2005), its sensitivity and response to treatment may well depend on the time of sampling. Basal respiration among experimental treatments is similar to the trend noted for mineralizable C (data not shown). However, unlike mineralizable C, basal respiration rates varied less among years, averaging 8.8, 9.2, 9.4 and 8.1 mg CO2-C kg1 soil day1 through the sequential samplings. The decrease in basal respiration in 2004 was significant (P = 0.05), and is attributed, at least partly, to the previously mentioned severe drought in 2004, which should have adversely affected microbial activity (through moisture

stress). The C mineralization and basal respiration results suggest that the soil biota was more active under the red clover rotation, probably as a result of a greater amount of labile C and N made available by the green manure as well as its greater mineralizability (e.g., its C:N ratio was typically about 22 compared to 42 and 59 for pea and wheat, respectively; Lupwayi et al., 2006). Carbon mineralization has been found to be highly correlated to C input in crop residues (Biederbeck et al., 1994). Water-soluble and mineralizable C, by their very nature, tend not to accumulate in the soil, and are more suitable as indicators of soil health and functioning. These pools are closely associated with soil microbial activity, which is often presumed to be limited in nonrhizosphere soil by C availability. In this regard, the lower mineralizable C in 2004 compared to other years probably reflected the reduced amount of WSC and, to a lesser extent, HWC that may have resulted from diminished crop rhizodeposition during a severe drought that year and the generally drier soil conditions during 2001–2004 compared to earlier periods. Davidson et al. (1987) also found that mineralizable C was a better measure of available C than was cold or hot water-soluble C. Although mineralizable C is influenced by growing season conditions and transient, at any given site and time it should provide a useful comparison of labile SOM quality among treatments.

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4. Conclusions 1. Cold water-soluble C was little affected by crop rotation and soil tillage. Hot water-soluble C was also not influenced by tillage but was responsive to crop rotation effects that may be related to residue quality/ mineralizability. 2. While mineralizable C was not measurably affected by tillage practices, it was higher in the crop rotation with a legume green manure. Mineralizable C estimates the flux labile C through microbial metabolism. It is considered superior to WSC as an indicator of SOM quality. 3. Total organic C in the top 15 cm was lower under CT than NT after 12 years, whereas the LF DM and C were higher under NT than CT after 4 years. The LF DM and LF C are sensitive indicators of changes in SOM as influenced by tillage. Total organic C was increased by the red clover rotation after 4 years as were LF DM and LF C, i.e., TOC was similar in sensitivity as the LF to crop rotation effects. The effects of tillage on LF DM and LF C increased with time but not crop rotation effects. Acknowledgements This research was supported by the Alberta Environmentally Sustainable Agriculture (AESA) Program (Research Component). We acknowledge the capable assistance of Joe Unruh and Tom Huynh. References Angers, D.A., Bolinder, M.A., Carter, M.R., Gregorich, E.G., Drury, C.F., Liang, B.C., Voroney, R.P., Simard, R.R., Donald, R.G., Beyaert, R.P., Martel, J., 1997. Impact of tillage practices on organic carbon and nitrogen storage in cool, humid soils of eastern Canada. Soil Tillage Res. 41, 191–201. Baldock, J.A., Nelson, P.N., 2000. Soil organic matter. In: Sumner, M.E. (Ed.), Handbook of Soil Science. CRC Press, Boca Raton, Florida, pp. B25–B84. Batjes, N.H., 1998. Mitigation of atmospheric CO2 concentrations by increased carbon sequestration in the soil. Biol. Fertil. Soils 27, 230–235. Biederbeck, V.O., Janzen, H.H., Campbell, C.A., Zentner, R.P., 1994. Labile soil organic matter as influenced by cropping practices in an arid environment. Soil Biol. Biochem. 26, 1647–1656. Campbell, C.A., McConkey, B.G., Zentner, R.P., Selles, F., Curtin, D., 1996. Long-term effects of tillage and crop rotations on soil organic C and total N in a clay soil in southwestern Saskatchewan. Can. J. Soil Sci. 76, 395–401. Carter, M.R., 1996. Analysis of soil organic matter storage in agroecosystems. In: Carter, M.R., Stewart, B.A. (Eds.), Structure and Organic Matter Storage in Agricultural Soils. Lewis Publ, Boca Raton, Florida, pp. 3–11.

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