Adopting zero tillage management: Impact on soil C and N under long-term crop rotations in a thin Black Chernozem

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under long-term crop rotations in a thin Black Chernozem

C. A. Campbell

¹

, F. Selles

²

, G. P. Lafond

³

, and R. P. Zentner

²

1Eastern Cereals and Oilseeds Research Centre, Agriculture and Agri-Food Canada, Ottawa, Ontario, Canada K1A 0C6; ²Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Swift Current,

Saskatchewan, Canda S9H 3X2; and ³Research Farm, Agriculture and Agri-Food Canada, Indian Head, Saskatchewan, Canada S0G 2K0. Received 2 June 2000, accepted 11 January 2001.

Campbell, C. A., Selles, F., Lafond, G. P. and Zentner, R. P. 2001. Adopting zero tillage management: Impact on soil C and N under long-term crop rotations in a thin Black chernozem. Can. J. Soil Sci. 81: 139–148. Society’s desire to sequester C in soils, thereby reducing the net loss of CO₂(a greenhouse gas) to the atmosphere, is well known. It is also accepted that the choice of appropriate agricultural management practices adopted by producers will affect this goal. However, quantification of the extent and rate at which it can be achieved is uncertain. A crop rotation experiment that was initiated in 1957 on a thin Black chernozemic clay soil at Indian Head, Saskatchewan, was managed using conventional tillage until changed to zero tillage in 1990. Soil was sampled (0- to 7.5- and 7.5- to 15-cm depths) in May 1987 and 1997 to determine the effects of treatments on soil organic C (SOC) and total N. The rotations were: fallow-wheat (Triticum aestivum L.) (F-W), F-W-W, continuous wheat (Cont W), legume green manure (GM)-W-W, and F-W-W-hay (legume-grass)-hay-hay (F-W-W-H-H-H). The monoculture cereal rotations were either fertilized with N and P based on soil tests or unfertilized, while the legume systems were both unfertilized. There was also a F-W-W (N+P) treatment in which the straw was baled and removed. When the experiment was changed to zero tillage management in 1990, the fertilizer protocol was changed to satisfy the “moist soil” criteria. Consequently, higher rates of N and P were added thereafter to the fallow crop, resulting in a positive yield response of wheat grown on fallow, where before there was no response to fertilizer. Over the 10-yr period (1987–1997) fertilized soil gained C and N, but unfertilized soil did not. For example fertilized F-W, F-W-W and Cont W gained about 4, 5 and 2 Mg C ha^–1in the 10-yr period. During this period, C emissions from manufacture and transportation of N fertilizer was 0.28, 0.53 and 0.90. Mg ha^–1for these three rotations, respectively. These results suggest that without adequate fertility, conversion to zero tillage may not always result in an increase in soil C or N. By 1997, fertilizer increased soil C and N in F-W-W and Cont W, and soil C and N were greater in F-W-W-H-H-H than in GM-W-W and low- est in F-W-W (all unfertilized). Straw removal had no significant effect on C or N. The analysis showed that C inputs from crop residues was the main factor influencing SOC changes.

Key words: C sequestration, crop rotation, fertilizer, grain yields, total N, tillage

Campbell, C. A., Selles, F., Lafond, G. P. et Zentner, R. P. 2001. Le non-travail du sol : incidence sur la concentration de C et de N dans le sol de systèmes d’assolement à long terme sur une mince couche de tchernozem noir. Can. J. Soil Sci. 81:

139–148. Nul n’ignore que la société souhaite séquestrer le carbone (C) dans le sol, notamment afin de réduire les pertes nettes de CO₂(un gaz à effet de serre) dans l’atmosphère. On sait aussi que les pratiques culturales des agriculteurs ont une incidence sur cet objectif. Quantifier dans quelle mesure on peut atteindre ce dernier et avec quelle rapidité s’avère néanmoins plus difficile.

En 1957, on a entrepris une expérience d’assolement sur un mince sol argileux de type tchernozem noir à Indian Head, en Saskatchewan. On a d’abord recouru aux pratiques traditionnelles de travail du sol avant de passer au non-travail du sol en 1990.

En mai 1987 et 1997, on a échantillonné le sol (à une profondeur de 0 à 7,5 cm et de 7,5 à 15 cm) en vue de déterminer les effets de la rotation sur le C organique du sol (COS) et le N total. Les rotations étaient les suivantes : jachère-blé (Triticum aestivum L.) (J-B), J-B-B, culture continue du blé (B cont.), engrais vert de légumineuses (EV)-B-B et J-B-B-foin (légumineuses et graminées)- foin-foin (J-B-B-F-F-F). Les parcelles réservées à la monoculture du blé ont soit été fertilisées avec du N et du P selon les résultats de l’analyse du sol, soit laissées telles quelles, tandis que les deux rotations avec légumineuses n’ont bénéficié d’aucun amendement. On a aussi essayé une rotation J-B-B (N + P) où la paille a été mise en balle puis enlevée. Lorsqu’on est passé au non-travail du sol en 1990, le régime de fertilisation a été modifié en fonction du critère du « sol humide ». On a donc ajouté plus de N et de P à la jachère, ce qui a accru le rendement du blé cultivé sur cette dernière, alors qu’auparavant, la culture n’avait pas réagi à la fertilisation. Au cours des dix années (de 1987 à 1997), la quantité de C et de N a augmenté dans les sols bonifiés, mais pas dans les autres. Ainsi, la concentration de C dans le sol des parcelles J-B, J-B-B et B cont. a augmenté d’environ 4, 5 et 2 mg ha^–1 au cours de cette période. Les émissions de C venant de la fabrication et du transport des engrais azotés se sont respectivement établies à 0,28, 0,53 et 0,90 mg ha^–1pour les trois systèmes d’assolement durant la même période. Les résultats donnent à penser que sans un amendement suffisant, le passage au non-travail du sol n’améliore pas toujours la concentration de C ou de N dans le sol. En 1997, l’addition d’engrais a accru la concentration de C et de N pour les régimes J-B-B et B cont., et il y avait plus de C et de N dans les parcelles J-B-B-F-F-F que dans celles EV-B-B. La concentration la plus faible a été relevée dans le sol de la parcelle J-B-B (pas de fertilisation dans les trois derniers cas). La récolte de la paille n’a eu aucun effet sensible sur la concentration de C ou de N. L’analyse des résultats indique que la variation de la concentration de COS est surtout attribuable aux apports de C par les résidus de culture.

Mots clés: Séquestration du C, assolement, engrais, rendement céréalier, N total, travail du sol 139

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Society’s desire to increase C sequestration in soils by encouraging producers to adopt appropriate agricultural management strategies is well known (Agriculture and Agri-Food Climate Change Table 2000). The need to improve our ability to quantify the impacts of various crop management practices on C sequestration is also widely rec- ognized (Agriculture and Agri-Food Climate Change Table 2000; Janzen et al. 1997). Zero tillage, fertilizer use, includ- ing legumes in rotations, and increased cropping frequency

are crop management practices that are increasingly being adopted throughout the Canadian prairies (Zentner et al.

1999).

The application of fertilizers to crops may enhance crop production, increase crop residues returned to the soil and, thereby, increase soil organic C and N (Janzen et al. 1997).

The magnitude of these increases in soil organic matter (SOM) is much larger in soils with inherently low C than in ones where C is already high (Table 1). The influence of fer-

Table 1. Effect of fertilization on SOC in the 0- to 15-cm depth in some Canadian studies conducted on conventionally tilled land Soil organic

Length of Crop/ Fertilizer carbon Statistical

Study^t study (yr) Experimental site Soil type rotation^z treatment^y (Mg ha^–1) significance^x

Campbell et al. (1993) 24 Swift Current, SK Swinton loam F-W-W N+P 31.4 –

(Brown Chernozem) F-W-W N 28.7 NS

F-W-W P 30.2 NS

Cont W N+P 34.3 –

Cont W P 31.4 *

Campbell et al. (1991a) 30 Indian Head, SK Indian Head clay F-W Unfert 36.3 –

(thin Black Chernozem) F-W N+P 37.9 NS

F-W-W Unfert 36.4 –

F-W-W N+P 38.5 +

Cont W Unfert 39.6 –

Cont W N+P 41.9 *

Campbell et al. (1991b) 31 Melfort, SK Melfort silty clay loam Cont W Unfert 65.3 –

(thick Black Chernozem) Cont W N+P 65.4 NS

F-W-W Unfert 61.4

F-W-W N+P 61.2

Nuttall et al. (1986)^w 25 Melfort, SK Melfort silty clay loam Cont W Unfert (data not shown) – (thick Black Chernozem) Cont W N+P (data not shown) NS

Nyborg et al. (1995) 11 Ellerslie, AB Malmo loam Cont B No N 56.1 –

(thick Black Chernozem) 56 kg ha^–1 57.6 NS

Solberg et al. (1997) 13 Ellerslie, AB Malmo loam Cont B No N 94.4 –

(thick Black Chernozem) 25 kg ha^–1 97.1 NS

50 kg ha^–1 96.1 NS

75 kg ha^–1 99.0 NS

Nyborg et al. (1995) 11 Breton, AB Breton loam Cont B No N 13.6 –

(Gray Luvisol) 56 kg ha^–1 16.3 *

Solberg et al. (1997) 13 Breton, AB Breton loam Cont B No N 25.7 –

(Gray Luvisol) 25 kg ha^–1 27.6 NS

50 kg ha^–1 31.7 **

75 kg ha^–1 33.4 **

Gregorich et al. (1996)^v 32 Woodslee, ON Brookston clay loam Cont C No N 25.4 –

(Humic Gleysol) 130 kg ha^–1 28.5 *

Liang and Mackenzie (1992)^v 6 Ste-Anne-de-Bellevue, QC Chicot sandy clay loam Cont C Initial level 40.7 –

(Dystric Brunosol) High rate^v 43.1 *

zF = summerfallow, W = spring wheat, Cont = continuous cropping, B = barley, C = corn.

yWhere rates of fertilizer are not shown they were applied based on soil tests.

xNS, +, *, **, denote not significant, and significant effect of fertilizer (compared to treatment designated-above) at P < 0.1, P < 0.05, and P < 0.01, respectively.

wThis was a 25-yr Tillage-Fertilizer study in which rates of N and P fertilizer of 0 to 45 kg ha^–1N and 10 kg ha^–1P were applied from 1959 to 1977, and these rates doubled from 1978 to 1983. No effect of fertilizer on organic C or total N was found, but quantitative values were not presented.

vSoil depth considered was 0-20 cm in this case.

uHigh rate of fertilizer applied annually = 400-132-332 kg ha^–1N-P-K plus manure at an annual rate of 4000 kg ha^–1(i.e. 70 kg N ha^–1).

tFor complete reference descriptions, contact senior author.

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tilization on SOM also depends on how frequently the land is summerfallowed or, conversely, how frequently it is cropped (Table 1). As shown by our previous findings at Swift Current and Indian Head (Table 1), even after 24–30 yr, fallow-containing rotations, such as fallow-wheat (F-W) and F-W-W, showed little C increase due to fertilization;

only continuous wheat (Cont W) showed a significant increase due to fertilization.

Some studies show that the adoption of zero tillage management will increase SOM due to reduced soil disturbance and erosion, and sometimes increased crop residue inputs associated with greater crop production due to more favor- able plant-available water (Campbell et al. 1988; Janzen et al. 1997; Agriculture and Agri-Food Climate Change Table 2000). But other studies report no increase in soil C when zero tillage is adopted (Angers et al. 1997; Janzen et al. 1997; Yang and Wander 1999). Most of the latter studies were conducted in the more humid conditions of eastern Canada and USA where zero tillage had no positive influence on crop yields (Angers et al. 1997).

The inclusion of legumes in crop rotations generally results in an increase in production of non-legumes (Campbell et al. 1990) and, consequently, leads to an increase in SOM (Campbell et al. 1997; Janzen et al. 1997).

Our objectives were to determine: (i) the influence of fertilizers, legumes, straw removal and cropping frequency on soil C and N, and (ii) if a change from conventional to zero tillage management has influenced soil C and N.

We developed the following hypotheses:

(i) Because the change to zero tillage will increase available water in the soil (Campbell et al. 1988; Lafond et al. 1992), grain and likely crop residue yields will increase, especially in the fertilized treatments.

(ii) Because SOM content is generally directly related to crop residue inputs (Campbell and Zentner 1997), the change to zero tillage management should result in an increase in soil C and N, especially in the fertilized treatments.

MATERIALS AND METHODS

This experiment has been described in detail previously (Campbell et al. 1997; Campbell et al. 1998); therefore, we only present sufficient information to facilitate discussion in this paper. The experiment, which involves five crop rotations and four replicates, was started at Indian Head, Saskatchewan, in 1957 at the Agriculture and Agri-Food Canada Research Farm. The soil is a fine-textured thin Black chernozem.

We soil sampled nine treatments in May 1987, September 1996 (after harvest), and May 1997 (Table 2). These treatments were fallow-wheat (F-W), F-W-W, and Cont W rotations, with and without N and P fertilizer; legume green manure-wheat-wheat (GM-W-W), and F-W-W-H(hay)-H-H rotations (both unfertilized); and F-W-W receiving N and P, but with some straw baled and removed from cropped plots each year. The hay was brome-alfalfa (Bromus inermis Leyss. – Medicago sativa L.). The annual fertilizer N rates for wheat grown on fallow and for wheat grown on stubble in the F-W-W system and in Cont W are shown in Table 3.

The average annual rates of P applied to the rotations being fertilized for the period 1958 to 1996 were 5.5, 6.5 and 9 kg ha^–1yr^–1for F-W, F-W-W and Cont W, respectively.

Weed control on summerfallow areas was achieved by mechanical tillage through 1989. An average of five operations (range of four to six) with a heavy-duty cultivator was required. In some years a rodweeder replaced one or more cultivation operations, and in the early years of the study a disc was also used. Since 1990, only herbicides were used to control weeds on fallow areas. Wheat was usually harvested in early September using a conventional plot combine.

Except for the treatment from which straw was removed, straw was redistributed on the plots by a paddle-type straw spreader attachment on the combine.

In May 1987 we had soil sampled the rotation phases that were being summerfallowed or green manured that year (cropped to wheat in 1986). In September 1996 we sampled plots that had grown wheat on fallow or on GM, and Cont W, and in May 1997 we re-sampled these same plots.

At each sampling we took soil cores (3.8 cm diam.) from the 0- to 7.5- and 7.5- to 15-cm depths with a Giddings soil corer. Two subsamples were taken per plot. Subsamples were processed separately and analyzed for bulk density (Tessier and Steppuhn 1990) and total and organic C and total N using a Carlo Erba automated combustion analyzer.

Stored soil from the 1987 sampling was reanalyzed for C

Table 2. Effect of crop rotations, legumes, fertilizers, straw removal and cropping frequency on soil bulk density and soil organic C and total N concentrations in the 0- to 7.5 and 7.5- to 15-cm depths, sampled in May 1997

Bulk density Organic C Total N Treatment^z (Mg m^–3) (g kg^–1) (g kg^–1) 0-7.5 cm

(F)-W 1.05 21.0 2.05

(F)-W (N+P) 1.08 22.2 2.21

F-W-(W) 1.03 19.6 1.95

F-W-(W) (N+P) 1.13 24.5 2.42

F-W-(W)-straw (N+P) 1.08 22.8 2.30

GM-W-(W) 1.04 22.8 2.30

F-W-(W)-H-H-H 0.94 25.7 2.63

Cont W 1.08 21.8 2.19

Cont W (N+P) 1.03 27.7 2.77

Signif. of F NS 0.05 0.004

LSD (P < 0.05) 0.12 4.6 0.38

7.5-15 cm

(F)-W 1.40 21.3 2.08

(F)-W (N+P) 1.41 22.5 2.20

F-W-(W) 1.39 18.2 1.86

F-W-(W) (N+P) 1.41 22.4 2.18

F-W-(W)-straw (N+P) 1.41 22.3 2.19

GM-W-(W) 1.40 19.7 1.99

F-W-(W)-H-H-H 1.40 21.0 2.17

Cont W 1.37 19.6 2.01

Cont W (N+P) 1.39 21.1 2.21

Signif. of F NS NS NS

LSD (P < 0.05) 0.07 5.3 0.48

z() indicates the crop rotation phase that was sampled. F = fallow, W = spring wheat, GM = legume green manure, H = legume-grass cut for hay, Cont = continuous, N + P = nitrogen and phosphorus fertilizer, - straw = straw baled and removed.

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and N when the 1996 samples were first analyzed (Campbell et al. 1998) and soil from the 1996 sampling was reanalyzed when the 1997 samples were analyzed. Organic C and total N concentrations were converted to mass per area on an equivalent soil depth (MED) basis for the 0- to 15-cm depth segment with calculations based on the lightest mass (1478.3 t ha^–1) (Ellert and Bettany 1995; Campbell et al. 1998).

Because we have already compared soil measurements for the 1987 and 1996 samplings (Campbell et al. 1998), this paper focuses on the 1987 vs. 1997 data. There were no significant effects of subsamples within each experimental unit; therefore, data for each experimental unit were averaged over subsamples. Subsequent analyses of variance were performed with crop rotations as main plot, and years

as subplots (Steel and Torrie 1980). Evaluation of the effects of crop rotation, fertilization, straw removal, and years of cropping since the previous sampling was determined using orthogonal contrasts among appropriate treatments (Steel and Torrie 1980). In addition, least significant differences (LSDs) were calculated for testing treatment effects at P < 0.05 (Steel and Torrie 1980).

We estimated straw yields from grain yields by assuming straw yields = 1.5 times grain yields in the Black soil zone (Nuttall et al. 1986). Root dry matter was assumed to be 0.59 times straw yields (Campbell and Zentner 1997). For the F-W-W (N + P) treatment from which straw was harvested, we measured straw removal over the past decade and related this to the estimated straw yields. The straw removal from the latter treatment was done with a commercial bail- er. We estimated the crop residue C inputs to the soil by assuming residues have 45% C. An empirical equation developed by Campbell et al. (2000a,b) was used to estimate SOC changes over the period 1987 to 1997. The equation uses two simultaneous first-order kinetic expressions, one to estimate crop residue decomposition and the other to estimate soil humus C mineralization. We compared the measured SOC (0- to 15-cm depth) to those estimated from the Campbell et al. (2000a) equation, and we compared the change in SOC (1987–1997) to the residue C inputs per rotation during this period.

RESULTS AND DISCUSSION

Bulk densities in the 0- to 7.5- and 7.5- to 15-cm depths were higher in 1997 (Table 2) than in 1987 (Campbell et al.

1988). For example, bulk densities in the 0- to 7.5-cm depth averaged 1.05 Mg m^–3 in 1997 (Table 2) compared with 0.92 in 1987 (Campbell et al. 1988). The corresponding values in the 7.5- to 15-cm depth were 1.40 and 1.19 Mg m^–3. Rotation had no significant effect on bulk density in either depth, although values in the 0- to 7.5-cm depth for F-W-W- H-H-H tended to be lower than for the other rotations in 1997 (Table 2).

In 1997, organic C and total N concentrations were significantly affected by treatment in the 0- to 7.5-cm depth only (Table 2). In this depth, fertilizer significantly (P < 0.05) increased organic C and total N concentrations of F-W-W and Cont W, but not of F-W. The hay-containing system had higher organic C and total N concentrations in soil than unfertilized F-W-W. The GM-W-W system increased the total N concentration significantly (P < 0.10) compared with unfertilized F-W-W, but the increase was not significant for organic C. In the monoculture wheat rotations, organic C and total N were unaffected by cropping frequency for unfertilized systems, but they were directly related to cropping frequency for fertilized systems.

Removal of straw did not significantly influence organic C or total N concentrations.

Because bulk densities in 1997 differed substantially from those in 1987, assessment of the influence of treatments over the 10-yr period is best based on a mass per equivalent depth basis (Ellert and Bettany 1995; Campbell et al. 1998). Using this basis, there was a significant effect of rotation treatment and year sampled (P < 0.05), and also

Table 3. Rates^zof N fertilizer applied^yto wheat grown on fallow in a fallow-wheat rotation, on stubble in a F-W-W rotation and on contin- uous wheat (Cont W)

F-(W) F-W-(W) Cont W

Year (N + P) (N + P) (N + P)

(kg ha^–1)

1959 6 18 18

1960 6 18 18

1961 6 18 18

1962 6 18 18

1963 6 18 18

1964 6 24 24

1965 6 24 24

1966 6 21 21

1967 6 51 51

1968 6 51 51

1969 6 21 21

1970 6 21 21

1971 6 21 21

1972 6 21 21

1973 6 21 21

1974 6 21 21

1975 6 21 21

1976 6 21 21

1977 6 21 21

1978 6 75 75

1979 6 82 84

1980 5 84 88

1981 5 82 86

1982 5 84 84

1983 5 83 82

1984 3 84 86

1985 101^x 101^x 101^y

1986 0 50 50

1987 0 66 53

1988 0 72 72

1989 0 97 71

1990 51 79 51

1991 52 81 66

1992 43 83 84

1993 50 80 70

1994 79 94 79

1995 78 95 95

1996 112 112 112

zNitrogen applied based on general recommendation criteria from 1957 to 1977; from 1978 to 1989 N was applied based on soil test criteria for nor- mal moisture conditions, but after change to zero-tillage management in 1990, rates were increased based on criteria for moist soil conditions.

ySource of N was ammonium nitrate from 1959 to 1989 and urea thereafter.

xErroneous application.

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Fig. 1. Grain yield for fertilized wheat grown on fallow (a) in F-W; (b) in F-W-W; for wheat grown on stubble in (c) F-W-W, and (d) Cont W; and for unfertilized wheat grown on green manure and hay crop (e) on fallow and (f) on stubble.

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an interaction of rotation ×year (P < 0.10), on organic C and total N in the 0- to 15-cm depth (Table 4). The rotation × year interaction was significant because only the treatments receiving N and P fertilizer showed a significant increase in C and N between 1987 and 1997. The increase in organic C for Cont W (N + P) between 1987 and 1997 was not significant, perhaps because its fertilizer rate was not increased further during this last decade (Table 3). Further, although in 1987 fertilizer rates only increased C and N for Cont W, by 1997 it also increased C and N for F-W-W. In 1987 the GM-W-W and F-W-W-H-H-H treatments increased organic C to a similar extent compared to F-W-W (unfertilized), though the F-W-W-H-H-H rotation increased total N more than the GM-W-W system. However, by 1997 both C and N followed a sequence of F-W-W-H-H-H > GM-W-W > F-W- W (unfertilized).

If the changes in C and N were solely, or primarily, related to tillage method, both the unfertilized and fertilized systems should show evidence of an increase between 1987 and 1997. Because only the fertilized systems showed an increase in C and N during this period, we may conclude that the increase in fertilizer regime was the main contribu- tor to this change. Evidence in support of this theory is shown in the grain yield trends (Fig. 1), and crop residues are directly related to grain yields (Rasmussen et al. 1980;

Campbell and Zentner 1997). Although the upward trend in yields of fertilized stubble-crop wheat in F-W-W and Cont W and the downward trend in unfertilized stubble-crop wheat continued during this 10-yr period, there was no dra- matic changes in yield trends (or residue inputs) in these systems (Figs. 1c and 1d). However, while there was no yield differential between fertilized and unfertilized wheat grown on fallow prior to 1990 as no N was required after fallow (Table 3), there was a marked increase in yield (and presumably residue inputs) thereafter (Figs. 1a and 1b) in response to the N fertilization (Table 3). In the two legume systems, the trends in wheat yields did not change over the 1987 to 1997 period indicating they had already reached a steady state (Figs. 1e and 1f).

Although the modification to this study does not allow us to properly assess tillage effects, the results do suggest that we are unlikely to obtain a gain in C or N by merely reducing tillage, unless this switch results in an increase in crop residues and/or a reduction in soil erosion. In southwestern Saskatchewan, increases in soil C after 11 yr of zero tillage without substantial yield increases may have been partly due to reduced soil erosion (Campbell et al. 1995, 1996). In Alberta, Nyborg et al. (1995) obtained no significant effect of zero tillage on SOC in unfertilized continuous barley (Hordeum vulgare L.) after 11 yr at Breton (Gray Luvisolic soil), or Ellerslie (thick Black chernozem). After 11 yr, the conventional tillage (CT) treatment had 28.7 and zero tillage (ZT) 30.3 Mg C ha^–1[LSD (P < 0.05) = 5.1] at Breton; values for corresponding treatments at Ellerslie were 87.8, 89.7 and LSD (P < 0.05) 5.5 Mg C ha^–1, respectively (Nyborg et al. 1995). In the same study, when N fertilizer was applied at 56 kg ha^–1yr^–1, zero tillage caused a significant (P≤0.05) increase in C (7.5 Mg C ha^–1) in the plots at Breton after 11 yr, whereas in the much more fertile Ellerslie soil there was

no effect of tillage on C or N. Based on results from eight sites in Eastern Canada, Angers et al. (1997), concluded that where residue input and crop production are not affected by tillage, reduced tillage systems will not influence soil C or N storage in the profile in a 5- to 10-yr period, though it might influence their distribution in the soil profile. Similar results were obtained in Central Illinois, USA, by Wander et al. (1998) and Yang and Wander (1999).

In contrast to results obtained when we sampled the Indian Head plots in fall 1996 (Campbell et al. 1998) the results for the spring 1997 samples fit the hypotheses we put forward in most cases. Thus, we observed little or no change in soil C in systems that received no fertilizer; moderate increases in systems fertilized at the same rates as the earli- er period; and significant increases where major increases in N fertilizer rates coupled to reduced tillage had been adopted. The increases were shown to be associated with increases in yield (and thus crop residue inputs). We did not determine SOC in 1957, thus we were unable to accurately assess C gains between 1957 and 1987. However, the gross SOC gains between 1987 and 1997 for the fertilized F-W, F- W-W, and Cont W were 3.9, 5.2, and 2.0 Mg ha^–1, respectively (Table 4). An estimate of the C emissions from manufacture and transportation of the N fertilizer used on these rotations during this period was 0.28, 0.53 and 0.90 Mg ha^–1, respectively (Table 5). Thus, if we overlook possible N₂O emissions from the fertilizer, we observe significant benefits of N fertilizer in contributing to C sequestration in this study.

The failure of straw harvesting in the F-W-W (N + P) system to reduce C or N significantly (Table 4) was perplexing at first glance, because we usually associate an increase in soil organic C with an increase in crop residue inputs.

However, through measurements made over the past decade, we have found that instead of the 66% of harvested straw, which we had previously assumed to be removed from this treatment (Campbell et al. 1991c), only 22% is being removed on average (7– 44% range). Regression analysis showed that percent straw removed = –3 + 0.0057 estimated straw yield (r²= 0.51, significant at P < 0.0001).

Using this relationship we estimated that over 39 yr (1958–1996) F-W-W (N + P) has returned 66.4 Mg C ha^–1 in straw and roots to the land, while F-W-W-(N + P,-straw) has returned 58.7 Mg C ha^–1. Further, we estimated [using equation of Campbell et al. (2000a)] that in 1997 F-W-W (N + P) should have 33.0 Mg C ha^–1in the top 15 cm of soil (it had 35), while the system from which straw was harvested should have had 31.2 Mg C ha^–1 (it had 33.5).

Considering the size of variability usually found in measured SOC data, it is not surprising that to date we have not been able to show significant negative effects of straw bail- ing on soil C. In other studies in which all the straw is removed at ground level, significant decreases in soil C are often reported (Barber et al. 1979; Karlen et al. 1994; Singh et al. 1997).

We estimated residue (straw plus roots) C inputs for each rotation phase of the monoculture rotations, for the period 1987 to 1996 (Table 6). These values were used to estimate SOC in 1997 starting with values measured in 1987 and Can. J. Soil. Sci. Downloaded from cdnsciencepub.com by 134.122.89.123 on 04/26/21 For personal use only.

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using the equation and methodology proposed by Campbell et al. (2000a).

The equation is:

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where SOC_t is the total amount of soil organic C per unit mass of soil remaining in soil after t years (time measured to just before residue addition in the current year, example residue added in yr 10 is not included at t = 10 yr). C₀is the amount of C in the soil on a mass basis, initially t = 0, q is a proportion of soil C, k is the annual rate of soil carbon decomposition (yr^–1), A_nis the C addition as plant residue (Mg ha^–1) in year n, p is the proportion of residue C (note p₁

Table 4. Effect of crop rotations, legumes, fertilizers, straw removal and cropping frequency on soil organic C and total N measured on a mass per equivalent depth basis (0 to 15 cm depth) assessed after 30 yr of conventional tillage management (1987) and 10 yr later (1997) after a change to no- tillage management in 1990

Organic C Total N Contrasts^x(1997 vs. 1987)

(Mg ha^–1) (Mg h^–1) Probability of > F

Treatment^z 1987 1997 Mean 1987 1997 Mean Org C Total N

F-W 28.81 31.41 30.11 2.89 3.10 3.00 0.17 0.17

F-W (N+P) 29.06 32.99 31.02 2.78 3.27 3.03 0.04 0.01

F-W-W 29.78 28.00 28.89 2.92 2.83 2.87 0.34 0.56

F-W-W (N+P) 29.85 35.06 32.45 3.19 3.44 3.32 0.01 0.10

F-W-W (N+P-straw) 28.57 33.50 31.03 2.97 3.35 3.16 0.01 0.02

GM-W-W 32.49 31.24 31.87 3.19 3.19 3.19 0.44 0.98

F-W-W-H-H-H 33.61 34.49 34.05 3.50 3.53 3.51 0.58 0.85

Cont W 30.82 30.84 30.83 3.14 3.14 3.14 0.99 0.99

Cont W (N+P) 34.47 36.42 35.44 3.34 3.73 3.54 0.30 0.02

Mean 30.83 32.66 31.75 3.10 3.29 3.19 – –

Signif. of F ratio^y Rot*** Yr** Rot ×yr⁺ Rot*** Yr*** Rot ×yr⁺

LSD (P < 0.05) 3.8 1.2 3.0 0.32 0.10 0.25

zF = fallow, W = spring wheat, GM = legume green manure, H = legume-grass cut for hay, Cont = continuous, N + P = nitrogen and phosphorus fertilizer.

y+, *, **, *** denote significance at P < 0.10, P < 0.05, P < 0.01, P < 0.001, respectively.

xContrast analysis showed no significant difference for unfertilized systems between 1987 and 1997 but significant increases (P < 0.0001) for fertilized systems. It also showed that the effect of fertilizer was not significant in 1987 but was significant (P < 0.001) in 1997.

Table 5. Amount of N applied and estimated C emission^zduring manufacture and transportation of N fertilizer for the fertilized rotations during the periods 1959–1986 and 1987–1996

F-W F-W-W Cont W

Period N applied C emission N applied C emission N applied C emission

Period N Source^y (kg ha^–1)

1959–1986 AN 125 138 474 522 1186 1307

1987–1989 AN 0 0 78 86 196 216

1990–1996 UR 232 284 363 445 557 682

1987–1996 – 284 – 531 – 898

zCarbon emission for ammonium nitrate = 1.102 kg C kg^–1N, and for urea = 1.22 kg C kg^–1N (Coxworth et al. 1995).

yAN = ammonium nitrate, UR = urea.

Table 6. Estimated C inputs^zof various rotation phases for period 1987 to 1996

10-yr mean 10-yr mean

Treatments^y 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 (1987–1996) for rotation

F-(W) 2015 1924 1615 2285 2230 2980 1374 843 1455 1936 1866 933

F-(W) (N+P) 2206 1882 1737 3845 3692 4359 2826 3003 3220 3690 3046 1523

F-(W)-W 2508 2159 1777 2494 2654 3180 1651 1116 1763 1765 2107

F-W-(W) 656 814 591 1235 782 941 344 352 1153 984 785 964

F-(W)-W (N+P) 2613 2047 2135 3645 4296 4680 2750 3550 3605 3637 3296

F-W-(W) (N+P) 3380 2280 1450 3392 2902 3986 2163 2210 3495 3209 2847 2048

F-(W)-W (N+P-str) 2576 1912 1820 3370 3364 3648 2643 2722 2840 3076 2797

F-W-(W) (N+P-str) 2973 1914 1483 2817 2732 3419 2065 2139 2908 2730 2518 1772

Cont W 991 751 514 1181 1512 1268 343 534 1231 1262 959 959

Cont W (N+P) 3020 1446 853 3491 3584 4107 2109 2142 3298 3177 2423 2423

zCarbon in straw (1.50 ×grain yield) plus that in roots (0.59 ×straw), assuming residues have 45% C. Assumed zero contribution for the fallow phases of all rotations. Units are kg ha^–1.

yFor F-W-W (N+P-straw) we estimated straw removed (%) = –3 + 0.0057 ×estimated straw.

SOC C q e q e

A p e p e

t k t k t

n r t n r t n

n t

=

(

+

)

⁺

(

+

)

 



− −

− (− ) − (− )

∑

=

0 1 2

1 2

0

1 2

(8)

+ p₂= 1), and r is the annual rate of residue decomposition (yr^–1). The subscripts 1 and 2 refer to different degrees of susceptibility to decomposition with 1 representing the more active, and 2 representing the slower decomposing pool of plant residues and soil humus. We used the coefficients and decomposition rate constants developed by Voroney et al.

(1989) for Sceptre clay in semiarid southwestern Saskatchewan, to estimate the accumulation of SOC over time after residue input. These coefficients and rate constants, which were based on a study in which ¹⁴C-labeled wheat straw for a F-W-W-W rotation was incorporated into soil and the C monitored annually for 10 yr were:

y = 0.72e^–1.4t+ 0.28e^–0.081t (2)

where, y is the proportion of residue C remaining in the soil and t is years since residue application. We submitted these numerical values into Eq. 1 (i.e., P₁= 0.72, P₂= 0.28, r₁= 1.4 yr^–1and r₂= 0.81 yr^–1). We assumed that q₁= 0.20 and q₂ = 0.80 and k₂ = 0.00066 yr^–1, as at Swift Current (Campbell et al. 2000a).

Because the systems were zero tilled for most of this period, we assumed erosion losses of C were zero. We changed some of the k₁values used at Swift Current by Campbell et al. (2000a,b). For example, for F-W k₁= 0.03 yr^–1 was used instead of the 0.02 yr^–1value used for the treatment in the 30-yr rotation study at Swift Current. We reasoned that rate of decomposition during the fallow period would be more rapid at Indian Head, which is more moist (Campbell et al. 1991a,b). We used k₁= 0.02 yr^–1for F-W-W at Indian Head, the same as was used for this rotation in the rotation experiment at Swift Current, which was conducted in a rel- atively moist decade (Campbell et al. 2000b). For Cont W at Indian Head, we used k₁= 0.01 yr^–1compared with 0.001 yr^–1at Swift Current because of the more moist conditions.

The estimated SOC (0- to 15-cm depth) in 1997 for the monoculture rotations were: for F-W, 29.05 Mg ha^–1, F-W (N + P), 30.70, F-W-W, 30.49, F-W-W (N + P), 33.0, F-W- W (N + P-straw), 31.15, Cont W, 32.11 and Cont W (N + P), 39.53 Mg ha^–1. A regression of estimated vs measured SOC showed a reasonably good fit (r² = 0.48*) (Fig. 2). The importance of residue input in influencing SOC changes was shown by the relationship between SOC gain (1987 to 1997) and C inputs (Fig. 3). However, the discrepancy with regard to low SOC gain per unit of C input for Cont W (N + P) emphasizes a weakness in our model. This treatment having gained substantial C over the previous 30 yr (to 1987) makes it difficult to boost SOC levels much further unless a major change in management is instituted that results in a marked increase in C inputs (or reduced soil C mineralization).

Finally, we may question why the quantities of organic C and total N measured in spring 1997 were higher than the values obtained for samples taken from the same plots in fall 1996 [Table 3 and Campbell et al. (1998)]. Although, the treatment effects did not differ, the 1996 results had suggested a decrease in C and N in unfertilized treatments and no change for fertilized systems since 1987, while the 1997 results suggest an increase for fertilized systems and con- stancy for unfertilized systems. We repeated laboratory

analysis of the 1996 and 1997 samples, but the results remained unchanged (data not shown). One obvious possibility is that site variability contributed to this discrepancy.

A second possibility could be that crop residues (fresh straw and roots) taken in the samples in the fall of 1996 were tough and remained on the 2-mm sieve and thus were dis- carded. However, weathering of such crop residues during winter and early spring may have allowed much more of this material to pass through the sieve and be included in the soil sample thereby increasing the measured C and N content of the samples taken in spring 1997. Since the 1987 samples were also taken in spring, it seems more appropriate to make comparisons of time effects based on the 1987 and 1997 samples in this instance. Yang and Wander (1999) report that estimates of tillage effects on SOC vary with time of sampling and sample handling techniques.

Fig. 2. Relationship between SOC (0- to 15-cm depth) measured in 1997 and estimated using the equation of Campbell et al. (2000a), using SOC for 1987 and residue C as inputs (monoculture systems only).

Fig. 3. Relationship between SOC gain from 1987 to 1997 and esti- mated residue C inputs per rotation (monoculture systems only).

The x denotes Cont W (N + P) treatment, which is not included in the regression.

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CONCLUSIONS

The results of this study confirm that adoption of management practices such as frequent cropping, using legumes in rotations and proper fertilization contribute to increased C sequestration. It did not establish conclusively whether the adoption of zero tillage by itself would result in increased C sequestration. The results suggested that adequate fertility is required if the adoption of zero tillage is to result in significant increase in soil C. Our results empha- size the need to adopt sustainable production systems involving proper rotations, tillage and fertility as a package, not just one aspect alone. Our quantitative analysis empha- sized the importance of residue input C in determining SOC changes.

Our results also suggest a need to keep the time of sampling soil constant from year to year when attempting to assess C and N change over time. This may be even more important when much residues are present on the soil sur- face, such as where reduced tillage is being practised.

ACKNOWLEDGMENTS

This project was partly funded by the Agriculture Development Fund of Saskatchewan Agriculture and Food, the Matching Investment Initiative of Agriculture and Agri- Food Canada, WestCo Agronomy Committee, Potash and Phosphate Institute of Canada, and Saskatchewan Soil Conservation Association. The technical assistance of D.

Hahn, R. Ljunggren, C. Wilson, K. Hanson, A. Ens, G.

Winkleman, R. Riznek and R. Geremia is acknowledged.

The assistance of Drs. Jame and McConkey in model development is gratefully acknowledged.

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