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a New Zealand Institute for Crop & Food Research Limited, Private Bag 4704, Christchurch, New Zealand
b Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current, SK, Canada S9H 3X2
c Eastern Cereal and Oilseed Research Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6
curtind{at}crop.cri.nz
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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20 to 25% less for NT than CT). In F–W, tillage effects on mean annual CO2 flux were significant (P < 0.05) in the wheat phase only (NT 10% less than CT). Tillage had negligible effect on C inputs in crop residues. Lower CO2 fluxes under NT than under CT were attributed to slower decomposition of crop residues placed on the surface of NT soil than when they were incorporated. With good growing conditions (and thus large inputs of residues) between 1989 and 1996, there was an accumulation of partially decomposed residues on the surface of NT soil. Carbon in surface residues represented about one-half of the C gained by NT soil. In Cont. W, surface residue C (in 1996) amounted to 3.6 t ha-1 under NT vs 1.4 t ha-1 under CT. Residue C amounts were smaller in the F–W system: 1.7 t ha-1 (NT) and 0.7 t ha-1 (CT). Based on our results, producers on medium-textured soils in the semiarid Canadian prairies who switch from the traditional wheat production system (conventionally tilled fallow–wheat) to continuous no-till cropping could, potentially, sequester 5 to 6 t C ha-1 in soil organic matter and surface residues in 13 to 14 yr.
Abbreviations: Cont. W, continuous wheat • CT, conventional tillage • F–W, fallow–wheat • (F)–W, fallow phase of F–W • F–(W), cropped phase of F–W • NT, no-till
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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Reduced tillage is regarded as one of the most effective agricultural strategies for sequestering atmospheric C (Kern and Johnson, 1993; Lal and Kimble, 1997). Tillage accelerates organic C oxidation to CO2 by improving soil aeration, by increasing contact between soil and crop residues, and by exposing aggregate-protected organic matter to microbial attack (Beare et al., 1994). Reducing tillage intensity may lead to increases in soil C, though the magnitude of the increase can vary widely. The C storage potential of reduced tillage systems is often most apparent when no-till is compared with intensive cultivation (i.e., moldboard plowing) in humid regions (Mahboubi et al., 1993). Eleven years of no-till increased organic C content of a silt loam (0–5 cm depth) in Oklahoma by 65% compared with a moldboard plow treatment (Dao, 1998). In contrast, under dryland conditions where shallow cultivation (10–15 cm depth) is the conventional tillage method, changes in C storage following adoption of reduced tillage practices can be small and difficult to quantify against background variability (Unger, 1991; Campbell et al., 1995, 1996). In such situations, Grant (1997) recommended measurement of CO2 fluxes as providing a more sensitive indication of C sequestration than low resolution data such as total organic C values. Consistent with that suggestion, Fortin et al. (1996) concluded that CO2 flux measurements gave early signals of tillage-induced changes in soil C in Ontario.
Almost 80% of Canada's arable land is in the prairie provinces (Alberta, Manitoba, and Saskatchewan), which produce >90% of the 25 million tonnes of wheat grown in the country in a typical year. In semiarid regions of the prairies, the traditional cropping system involves frequent summer fallowing (land fallowed during an entire growing season) with several tillage operations to control weeds during the fallow year. This system is regarded as a worst-case scenario for soil C storage because inputs of C in crop residues are relatively low and frequent tillage promotes organic matter decomposition. However, soil C is expected to increase as a result of recent changes in land use practices (Dumanski et al., 1998). The area of land in summer fallow has steadily declined during the past two decades (Janzen et al., 1998b) and reduced tillage is a rapidly expanding technology for crop production on the prairies (Larney et al., 1994).
The amount of C sequestered when tillage intensity is reduced appears to depend partly on site-specific factors such as soil texture, fertility levels, and crop rotation (Lal et al., 1995; Campbell et al., 1996; Lal and Kimble, 1997). Thus, it is very difficult to predict how much C will be sequestered in prairie soils as a result of the shift to reduced tillage. Further, the time needed to reach a steady state C level under a reduced tillage regime has not been well defined. According to Monreal and Janzen (1993), changes in soil C in response to management perturbations can be relatively rapid with much of the change occurring in the first 10 yr or so. In this paper we report on a study of the effects of tillage (conventional vs. no-till) on C fluxes in wheat-producing systems differing in summer fallow frequency. Our measurements were made 13 to 14 yr after initiation of no-till cropping and were expected to provide insights on the time frame in which tillage-induced disturbances of the C cycle might persist in dryland wheat farming.
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Materials and methods |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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Details of tillage and other management practices have been reported by McConkey et al. (1996). Herbicides were used exclusively for weed control under NT. The NT fallow plots were treated (one to three applications) with broad-spectrum herbicides (tank-mixes that included herbicides such as glyphosate [N-(phosphonomethyl)glycine] with or without dicamba [3,6-dichloro-2-methoxybenzoic acid], as required) with the first application being made in mid to late June. All treatments (CT as well as NT) received applications of 2,4-D ester [(2,4-dichlorophenoxy)acetic acid] each fall to control winter annual broadleaf weeds.
The CT methods used in this study were typical of those practiced in southwestern Saskatchewan. The Cont. W system was tilled once each year, prior to seeding, using a heavy-duty sweep cultivator with an attached rodweeder or mounted harrow (Tessier et al., 1990). On CT fallow plots, weeds were controlled by one to four operations with a heavy-duty cultivator. Tillage depth was 5 to 10 cm.
Hard red spring wheat (cv. Lancer) was sown at a rate of 67 kg ha-1. An offset disc drill (Dyck and Tessier, 1986) was used to plant the NT treatment, and a conventional hoe press drill was used in the CT treatment. In the years in which C fluxes were measured (1995 and 1996), wheat was seeded on 8 and 9 May. All treatments received 10 kg P ha-1 as monoammonium phosphate, which was placed with the seed. Inputs of fertilizer N (NH4NO3) were based on soil NO-3 levels, measured in the upper 60 cm of the profile in the fall, and regional fertilizer recommendation guidelines provided by the Saskatchewan Advisory Council on Soils (Saskatchewan Agriculture, 1988). The N fertilizer was broadcast prior to seedbed preparation or seeding until 1990. Thereafter, up to 45 kg N ha-1 was applied with the seed and any additional fertilizer required was broadcast on the surface. Wheat was harvested at full maturity (5 Sept. 1995 and 3 Sept. 1996) using a conventional, direct-cut header combine. All crop residues were spread behind the combine with straw and chaff spreaders.
Measurement of Carbon Dioxide Fluxes
In 1995 and 1996, CO2 emissions were measured with a portable infrared analyzer (LI-COR model LI-6200, Licor, Lincoln, NE) to determine accumulation of CO2 in a vented chamber that enclosed an area of 177 cm2. The chamber was attached to a collar, which was inserted into the soil to a depth of 4 cm. Head space volume was 2 L. Fluxes of CO2, expressed as µmol CO2 m-2 s-1 (1 µmol CO2 m-2 s-1 
10 kg C ha-1 d-1), were estimated from the rate of increase in CO2 in the chamber during a 60-s deployment period. Flux measurements were usually made once each week from spring until the soil froze in late fall (all measurement were made during day time, i.e., 1000–1700 h). Generally, three measurements were made in each plot at locations 3 to 4 m apart (the distance between sampling locations was determined from a preliminary study of spatial pattern of CO2 emissions). Designated sampling locations were marked using flags and measurements were always made within 1 m of the flags. In-crop measurements were made by placing the chamber between plant rows (row spacing for wheat was 17.5 cm). Analysis of variance was performed to assess the effects of tillage and crop rotation on CO2 fluxes on each measurement date. In this analysis, the two phases of the F–W system were treated as separate rotations. Because flux measurements tend to be variable (coefficients of variability
were usually in the 15–40% range), hypotheses were tested at the
level of significance.
Carbon Inputs
Estimates of C inputs in crop residues in 1995 and 1996 were obtained from aboveground biomass, measured by taking three random samples (1 m2) in each plot at crop maturity (22 August in both 1995 and 1996). The plant material was dried (60°C), weighed, and separated into grain and straw. Straw was analyzed for C using an automated C/N analyzer (Carlo Erba, Milan, Italy). Carbon inputs in roots were estimated assuming a root/straw ratio of 0.59:1 (Campbell et al., 1977a, 1977b) and a root C content of 45%.
Surface Residue Amounts
The amount of crop residue on the soil surface was determined annually (1982–1996) by taking five 0.5-m2 samples per plot each fall. The samples included all residue that was <15° erect. These samples were gently washed in flowing water to remove any adhering soil, dried (60°C) and weighed. Residue C for samples collected up to 1995 was estimated assuming a C content of 40%. The 1996 samples were analyzed for C using the C/N analyzer.
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Results and discussion |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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14% more precipitation than the long-term mean of 168 mm (Fig. 1)
. In 1996, precipitation in April (26 mm), May (65 mm), and June (78 mm) was near, or above, the long-term average for these months (i.e., 22, 44, and 72 mm for April, May, and June, respectively). A dry period in July and August 1996 was followed by an especially wet September when precipitation exceeded 100 mm. The frost-free period in 1995 was 24 May to 20 September and in 1996 it was 12 May to 24 September. Mean daily temperatures in the growing season were 0.6°C (1995) and 1.3°C (1996) below the long-term average (15.0°C).
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60% of those under Cont. W.
Carbon Dioxide Fluxes
Fluxes of CO2 ranged from <0.5 µmol CO2 m-2 s-1 to >10 µmol CO2 m-2 s-1, although values >3 µmol CO2 m-2 s-1 were uncommon (Fig. 2 and 3)
and were only observed following heavy rainfall. Tillage effects on CO2 evolution were most apparent following heavy rains in June 1995 (Day of Year 170) and September 1995 (Day of Year 250) when exceptionally high fluxes were measured in CT plots (Fig. 2). Within a few days, emissions returned to normal levels. Previous flux measurements (Curtin et al., 1998) on a Swinton loam (the soil type at our site), into which 2800 kg straw ha-1 (average wheat straw yield in semiarid areas of Saskatchewan) had been incorporated, suggest that CO2 production by microbial respiration in fallow soil at optimum moisture (field capacity) would probably not exceed 3 µmol CO2 m-2 s-1. Thus, fluxes from CT plots exceeding 10 µmol CO2 m-2 s-1 (Fig. 2) probably cannot be explained by normal respiratory output of CO2. We hypothesize that, under raindrop impact, the soil surface may have sealed, temporarily trapping CO2. As the soil surface dried, trapped CO2 was released in a brief, intense burst. Elimination of tillage has substantially improved the water-stability of soil aggregates at this site (McConkey et al., 2000). Better soil structure combined with greater surface residue cover (discussed below), may have prevented surface sealing under NT. An alternative explanation is that the high fluxes were associated with the rapid release of dissolved gas when saturated soil began to drain. Better drainage under NT vs. CT may have reduced the accumulation of dissolved CO2 in NT soil. When the high fluxes in 1995 were excluded, tillage effects were relatively small in the F–W rotation and there was a reasonably good 1:1 relationship
between fluxes in the two tillage treatments (Fig. 4)
. In Cont. W, CT often had significantly (P < 0.10) higher CO2 fluxes than NT.
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20% lower under NT compared with CT (Table 2). There was a significant tillage x rotation interaction, with the tillage effect being greater for Cont. W than for wheat grown on fallow. Tillage had no significant effect on the mean flux from fallow. When the abnormally high fluxes in June and September 1995 were excluded, the mean flux from Cont. W in 1995 was 25% less under NT than under CT. As in 1996, the effect of tillage decreased in the order: Cont. W > cropped phase of F–W [F–(W)] > fallow phase of F–W [(F)–W]. Total CO2 emissions for 1995 and 1996 showed a similar pattern to the mean fluxes (Table 2). Total emissions were less under NT than under CT by 20 to 23% for Cont. W and by 10% for wheat grown on fallow, with tillage having no effect on total emissions from fallow.
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Tillage operations may physically release gas from the soil and cause enhanced CO2 fluxes for up to 24 h (Ellert and Janzen, 1999). This short-term effect of tillage did not contribute to the differences in emissions between CT and NT, shown in Table 2, because the number of tillage operations was small and measurements were not made within a day of cultivation. Disturbance of the soil during installation of the measuring chamber may enhance CO2 fluxes, though the disturbance effect is likely to be small and, presumably, similar in all treatments. Ellert and Janzen (1999) estimated that CO2 released as a result of soil disturbance associated with 10 passes with a cultivator would amount to <5% of total annual CO2 emissions from soil in Alberta, Canada.
Emissions of CO2 from Cont. W generally exceeded those from F–(W) (Table 2). This may seem surprising given that soil moisture levels are often more favorable for microbial and root activity under F–(W) than under Cont. W (Grant, 1997), but it is probably a reflection of the importance of fresh crop residues as a C source for soil microbes (Curtin et al., 1998). The Cont. W system received fresh residues every year, but in the case of F–(W), a growing season had elapsed without residue addition. In the F–W rotation, crop residues probably decompose to a large extent during the fallow year, resulting in relatively low residue-derived CO2 emissions from the wheat phase of this rotation.
Fluxes of CO2 from (F)–W were lower than those from the cropped phase of that rotation or from Cont. W, due to the absence of root respiration in fallow soil (Table 2). Partitioning CO2 emissions into root (rhizosphere) and microbial contributions is one of the most intractable problems in soil C cycling (Rochette and Flanagan, 1997). One approach is to assume that differences in fluxes between cropped and fallow treatments are due to root respiration. Attributing the difference between cropped and fallow phases of the F–W rotation to root respiration might result in underestimation of root respiration (i) because soil moisture levels in the fallow phase are generally more favorable for microbial activity than in the cropped phase (Grant, 1997) and (ii) because the fallow phase recently received fresh residues, whereas, in the case of the cropped phase, an entire growing season would have passed without residue addition. The confounding effect of residues may be avoided by comparing the fallow phase of F–W with Cont. W, but since fallow is more moist, the difference between Cont. W and (F)–W may again underestimate root respiration. When data for the two tillage treatments and for the 2 yr were pooled, the following regression equations were obtained between fluxes from fallow (x) vs. those from the cropped phase of F–W and Cont. W (y):
 | (1) |
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The intercept values were not significantly different from zero (P > 0.05), and the slopes of the two equations were similar. If we ignore the intercept, the average contribution of root respiration to CO2 emissions may be approximated as (1 - 1/b)100, where b represents the slope in Eq. [1] and [2]. Such calculations suggest that root respiration contributed almost one-half (47–48%) of the CO2 emitted from soil cropped to wheat.
Soil Carbon Balance
Elimination of tillage reduced CO2–C emissions, but had little effect on C inputs. Our results indicate that NT soil gained C compared with CT soil, particularly under continuous cropping. The C flux data are generally consistent with soil organic C measured in 1994, 12 yr after initiation of the experiment (Campbell et al., 1995). In Cont. W, 2.5 t ha-1 more C was stored in the soil (0–7.5 cm depth) under NT relative to CT, but in the F–W rotation, the balance in favor of NT was small (0.9 t ha-1) (Table 3)
. Tillage (or rotation) had no effect on C levels in the 7.5- to 15-cm soil depth (Campbell et al., 1995). By the time our flux measurements were made (13–14 yr after initiation of NT), soil C levels were expected to have reached a steady state under NT; data of Campbell et al. (1995) suggested that organic C in the NT soil had stabilized by 1990. However, all growing seasons between 1989 and 1996 had average or above-average precipitation, and the large inputs of crop residues in those years provided impetus for further C gains under NT. Data in Fig. 5
show that under NT there was an accumulation of surface residues since about 1990, especially in Cont. W. It is noteworthy that management effects on surface residue C, measured in 1996, were similar in magnitude to those observed for soil organic C (Table 3). These results show that surface residues can be as important as soil organic matter (operationally defined as organic matter in sieved [<2 mm] soil) as a C repository under NT and should be considered when evaluating tillage effects on C dynamics. Relative to the traditional management system in semiarid regions of the Canadian prairies (F–W with conventional tillage), all systems showed increases in surface residue C: F–W (CT) < Cont. W (CT) < F–W (NT) << Cont. W (NT) (Table 3).
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Soil C gains in response to NT in our experiment were within the range observed at other sites in the northern Great Plains (0–7.5 t C ha-1; Janzen et al., 1998a; Peterson et al., 1998). The variability of the response of soil C to NT may be attributed to several factors including crop rotation, fallow frequency, residue removal, fertilizer application, management history, and soil conditions (Janzen et al., 1998a). Apart from the study of Peterson et al. (1998), C in surface residues appears to have been ignored in assessments of the effects of tillage on soil C. The data in Table 3 clearly highlight the importance of including residue C in the C budget of NT soil.
From a C sequestration perspective, the preferred outcome from adoption of NT would be increased C in nonlabile or passive soil organic matter fractions. Because surface residues are relatively easily decomposed, amounts may fluctuate rapidly in response to environmental conditions and management practices, making them an ephemeral C store. Even a single tillage operation might result in a significant loss of this accumulated surface residue C. Also, a series of dry years may cause surface residue amounts to decline to levels observed during the mid 1980s when dry conditions prevailed. In the continuously cropped system, a portion of the C gained under NT was in the light fraction of soil organic matter (Table 3), which is also labile and susceptible to decomposition in the short term (Gregorich and Janzen, 1996). There is evidence (Beare et al., 1994) that some of the organic C gained under NT may be sequestered within soil aggregates, where it is protected from rapid decomposition. Previous work in Saskatchewan (Campbell et al., 1996) produced evidence that C gains under NT become greater as clay content increases. That observation could be consistent with an aggregate-protection mechanism, but further work is clearly needed to identify factors that determine the size and stability of the C sink created by the introduction of NT management.
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Summary and conclusions |
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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ACKNOWLEDGMENTS |
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Received for publication June 4, 1999.
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TOPABSTRACTINTRODUCTIONMaterials and methodsResults and discussionSummary and conclusionsREFERENCES |
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