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Long-term effects of tillage and fallow-frequency on soil quality attributes in a clay soil in semiarid southwestern saskatchewan
Soil & Tillage Research 46 (1998) 135±144
Long-term effects of tillage and fallow-frequency on soil quality attributes in a clay soil in semiarid southwestern saskatchewan C.A. Campbell*, B.G. McConkey, V.O. Biederbeck, R.P. Zentner, D. Curtin, M.R. Peru Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2 Received 27 May 1997; accepted 2 December 1997
Abstract Reduced tillage management is being adopted at an accelerated rate on the Canadian prairies. This may in¯uence soil quality and productivity. A study conducted on a clay soil (Udic Haplustert) in southwestern Saskatchewan, Canada, to determine the effects of fallow frequency [fallow-wheat (F-W) vs. continuous wheat (Cont W)] and tillage [no-tillage (NT) vs. conventional (CT) or minimum tillage (MT)] on yields of spring wheat (Triticum aestivum L.), was sampled after 3, 7 and 11 years to assess changes in selected soil quality attributes. Tillage had no effect on amount of crop residues returned to the land, but the tilled systems had signi®cantly (P<0.05) lower total organic C and N in the 0±7.5 cm soil depth, though not in the 7.5±15 cm depth. Further, these differences were observed after only 3 years and persisted for the entire 11 years of the study. For example, in the 0±7.5 cm depth, organic C in F-W (MT) after 3 years was 10 480 kg haÿ1 and in F-W (NT) 13 380 kg haÿ1, while in Cont W (CT) and Cont W (NT) corresponding values were 11 310 and 13 400 kg haÿ1, respectively. After 11 years, values for F-W (MT) and F-W (NT) were 11 440 and 14 960 kg haÿ1, respectively, and for Cont W (CT) and Cont W (NT), 12 970 and 16 140 kg haÿ1, respectively. In contrast to total organic matter, two of the more labile soil quality attributes [i.e., C mineralization (Cmin) and N mineralization (Nmin)] did not respond to fallow frequency until after 7 years and only in the 0± 7.5 cm depth. Microbial biomass (MB) and the ratio of Cmin to MB [speci®c respiratory activity (SRA)], two attributes also regarded as labile, were not in¯uenced by the treatments even after 11 years. After 11 years, only Cmin and Nmin among the labile soil quality attributes responded to the treatments. Surprisingly, the labile attributes were no more sensitive to the treatments than was total organic C or N. More research is required to determine why responses in this soil differed from those reported elsewhere. # 1998 Elsevier Science B.V. All rights reserved. Keywords: C and N mineralization; Microbial biomass; Speci®c respiratory activity; Organic C and N; Soil quality
1. Introduction There has been a marked increase in use of reduced tillage systems by producers on the Canadian prairies *Corresponding author. Tel.: +1 613 759 1536; fax: +1 613 759 0646; e-mail: [email protected] 0167-1987/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0167-1987(98)00027-0
in recent years (Lafond et al., 1990; Larney et al., 1994). This may result in signi®cant changes in the quality of soil organic matter (Soil and Water Conservation Society, 1995). Soil quality has been de®ned as ``the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal
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health'' (Doran and Parkin, 1994). Soil organic matter is an important soil quality attribute because it in¯uences the productivity and physical well-being of soils. Therefore, it is important both from an economic and environmental standpoint to determine how tillage in¯uences soil quality. Recently, we discussed the in¯uence of tillage on the quantity of C stored in soils of differing texture in Saskatchewan (Campbell et al., 1995, 1996a, b). We showed that, over the 11±12 year study period, increases in C storage due to adoption of no-tillage were small (0±3 t haÿ1) and directly related to clay content (Campbell et al., 1996b). It has been hypothesized that a change in management will cause the labile components of soil organic matter to change to a new steady state, and that these changes will happen more quickly than for total organic matter (McGill et al., 1988; Janzen et al., 1997). The length of time required before the effects of tillage on soil biochemical characteristics [e.g., microbial biomass (MB), C or N mineralization (Cmin, Nmin)] to become apparent is uncertain. For example, Staley et al. (1988) noted differences in MB after only 1 year, Carter (1986) observed differences in the 0± 5 cm depth after 3 years, but Franzluebbers and Arshad (1996) found differences only after 6 years. Based on results from several long-term experiments in the USA, Doran (1980) reported that microbial numbers and biomass, and potential mineralizable N, were greater for NT than for CT (moldboard plow) in the 0±7.5 cm depth, but trends were generally reversed in the 7.5±15 cm depth. Most workers report that the amount of soil microbial and biochemical characteristics decrease as the frequency of fallow increases (Campbell et al., 1997a; Janzen et al., 1996). Our objective was to determine the in¯uence of tillage method and fallow frequency on selected soil quality attributes for a clay soil, in the semiarid prairie of southwestern Saskatchewan over an 11 year period under monoculture cereal cropping. 2. Materials and methods Details of the design of this experiment are published elsewhere (Campbell et al., 1996b; McConkey et al., 1996); consequently, only a brief review is presented here.
The experiment was initiated in 1982 near Stewart Valley, Saskatchewan, Canada, 50 km north of Swift Current (1983 was ®rst year of data collection). The land is level and had been managed for the previous 70±80 years in a 2 or 3 year fallow-cereal rotation. The soil is a Sceptre heavy clay, a Rego Brown Chernozem (Udic Haplustert) (Ayres et al., 1985). Soil at the 0± 15 cm depth has a pH of about 7.0 in dilute CaCl2, a sand content of 260 g kgÿ1, clay content of 420 g kgÿ1, and silt content of 320 g kgÿ1. There were four treatments in which hard red spring wheat was grown on summerfallow (F-W) (cropped once every two years) using NT or minimum tillage (MT) management, or grown continuously (Cont W) (seeded in spring and harvested in early fall each year) using NT or conventional mechanical tillage (CT) management. Weed control in minimum till fallow involved application of broad-spectrum herbicide tank mix in June followed by one to three tillage operations during summer using a heavy-duty sweep cultivator, to a depth of 5±10 cm (McConkey et al., 1996). Fallow weed control in NT plots was achieved with herbicide only. For Cont W (CT) and F-W (MT) there was one preseeding tillage operation using a heavy-duty cultivator with attached rodweeder or mounted harrow to 5±10 cm depth. Cont W (NT) involved a single preseeding application of the broad-spectrum herbicide tank mix. Applications of 2,4-D in fall were used to control winter-annual broadleaf weeds on all treatments. There were three replicates arranged in a randomized complete block design. Both phases of the F-W rotation were present each year and each treatment was cycled on its assigned plots. Commercial farm machinery was used to perform most ®eld operations on plots 15 m wide by 30 m long. A hoepress seed drill was used for seeding. All cropped plots received 10 kg P haÿ1 as monoammonium phosphate, seed-placed. Nitrogen (ammonium nitrate), at rates based on soil tests (Saskatchewan Agriculture, 1988), was applied with the seed to a maximum of 45 kg haÿ1 N; the remainder was broadcast prior to seedbed preparation or at seeding. At harvest, total aboveground plant biomass and grain yield from three random 1 m2 areas per plot were measured after air drying the samples. Straw yields were taken as the difference between aboveground plant biomass and grain yield.
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
Prior to commencement of ®eld operations in April 1986, 1990 and 1994, soil samples were taken from the 0±7.5 and 7.5±15 cm depths of the cropped phase of each treatment and the soil in each plot composited by depth and replicate. The soil was sieved (<2 mm) and crop residues remaining on the sieve discarded. One-half of each sample was air-dried and the remainder was stored ®eld-moist at 1±38C pending analysis for microbial biomass and C mineralization (Cmin). Within 3 months of sampling, the air-dried soil was used for determination of total organic C and N and net N mineralization (Nmin). At each sampling time separate 5 cm diameter soil cores were taken from each depth for bulk density determination by the method of Tessier and Steppuhn (1990). Samples collected in 1994 were analyzed, and stored soil from 1986 and 1990 reanalyzed for total organic C and N using an automated combustion technique (Carlo ErbaTM, Milan, Italy), as discussed previously (Campbell et al., 1996b). Mineralizable-N was determined by incubating a soil±sand mixture at 358C and measuring cumulative nitrate- and ammonium-N generated during 16 wk, by leaching intermittently with dilute CaCl2 followed by a minus-N nutrient solution and evacuating to about 60 cm of Hg (Campbell et al., 1993). In 1990 and 1994, two subsamples per treatment were used to determine Cmin. This involved wetting ®eld-moist soil (50 g oven-dry weight per subsample) to ®eld capacity (315 g H2O kgÿ1 soil), conditioning the soil for 3 days at 218C and then incubating the soil in biometer ¯asks at 218C for 30 days. The evolved CO2 was trapped in an alkali solution and measured by acid titration on days 4, 9, 16, 23 and 30. In 1986, the same method was used but titrations were performed on days 2, 4, 7, 10 and 14. Cmin was determined within 3 months of sampling. In 1986 and 1990, soil microbial biomass (MB) was determined by the chloroform fumigation±incubation technique (Jenkinson and Powlson, 1976), as described by Biederbeck et al. (1984). From each treatment we used 6 subsamples of ®eld-moist soil (each 100 g oven-dry weight) wetted to ®eld capacity and pre-incubated for 3 days at 218C. Three subsamples were fumigated with CHCl3 and three were left unfumigated. Microbial biomass C (MB-C) was estimated by dividing the ¯ush of CO2-C by a kC factor of 0.41 (Anderson and Domsch, 1978; Voroney and Paul,
137
1984). To estimate biomass N (MB-N), we followed the approach of Carter and Rennie (1982) who used a kN value of 0.4 (the overall mean of values determined in several other studies). In 1994 we changed to the fumigation±extraction method of Amato and Ladd (1988). In this case, the weight of subsamples was 40 g on oven-dry basis. The procedure is described in detail elsewhere (Biederbeck et al., 1994). Microbial biomass was determined within 6 months of sampling. All data were subjected to analysis of variance for each depth and year separately. Standard error of the means (S x) were calculated for signi®cant treatment effects. Straw yields were converted to a mean dry weight of production per year basis and these data analyzed as a split-plot with tillage as main plot and year as subplot (SAS Institute Inc., 1985). Regression and correlation analyses were used to relate selected soil quality attributes to mean annual straw production. 3. Results and discussion 3.1. Spring 1986 ± after 3 years Of the more dynamic soil biochemical characteristics (e.g., Nmin, Cmin, MB) measured in 1986, none changed signi®cantly (P>0.10) during this 3 year period (Table 1). We had expected these more dynamic and labile soil quality attributes to respond to a change in management more rapidly than total organic matter, as suggested by others (McGill et al., 1988; Janzen et al., 1997). Because of the high content of expanding lattice clay in this soil, much of the additional organic matter may have become complexed and physically protected from microbial decomposition. There was a tendency for Cmin and Nmin in the 0±7.5 cm depth to be greater for NT than for CT, and similarly for Cmin in the 7.5±15 cm depth. We previously discussed changes in total organic C and N in this soil (Campbell et al., 1996b), demonstrating signi®cant increases in both C and N due to only 3 years of no-tillage, but ®nding no difference due to the increase in fallow frequency (Table 1). This increase due to tillage was despite the crop failure in 1983 due to hail which resulted in very little input of crop residues in that year (Table 2).
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Table 1 Effect of fallow frequency and tillage method on selected soil biochemical characteristics in the top 15 cm of soil after 3 years (sampled in April 1986) Rotation
0±7.5 cm Cont W F-W Signif. of F ratio d S x 7.5±15 cm Cont W F-W Signif. of F ratio S x 0±15 cm Cont W F-W Signif. of F ratio S x
Cmin b (kg haÿ1)
Nmin c (kg haÿ1)
6.0 5.1 4.3 4.8 ns ±
120 166 80 117 ns ±
94 107 64 95 nd ±
7.5 7.5 5.6 6.6 nd ±
980 850 880 980 ns ±
6.5 5.3 7.1 5.7 ns ±
87 110 83 102 ns ±
100 97 93 87 nd ±
6.1 5.5 6.7 4.7 nd ±
1660 1540 1330 1620 ns ±
6.3 5.3 5.8 5.3 ns
207 276 163 219 ns ±
194 204 156 182 nd ±
6.7 6.4 6.1 5.5 nd ±
Organic C (kg haÿ1)
Organic N (kg haÿ1)
Microbial biomass a (MB)-C (kg haÿ1)
Org C (%)
CT NT MT NT
11 310 13 400 10 480 13 380 ** 510
1250 1430 1150 1440 ** 50
680 690 450 640 ns ±
CT NT MT NT
14 970 15 870 12 350 17 120 ** 550
1640 1750 1390 1850 ** 55
CT NT MT NT
26 280 29 270 22 830 30 500 ** 730
2890 3180 2540 3290 ** 75
Tillage
MB-C
Nmin Org N (%)
a
We did not determine microbial biomass N in 1986. Biomass determined by fumigation±incubation method. Cumulative Cmin measured over 14 days at 208C in 1986. c Soil from the three replicates were composited before Nmin was determined in 1986. Values are accumulated over 16 wk. Mineralization conducted on rewetted air dry soil with incubation at 358C. d nsnot significant, **significant at P<0.01, and ndnot determined. b
These results suggested that soil disturbance during tillage was the main factor responsible for the tillage effect. 3.2. Spring 1990 ± after 7 years The response of organic C and total N after 7 years was similar to after 3 years, indicating signi®cant (P<0.05) effects of tillage (NT>tilled) in the 0± 7.5 cm depth (Campbell et al., 1996b), but no effect due to fallow frequency (Table 3). Microbial biomass C, on an absolute basis, was not affected by tillage or fallow frequency. However, Cmin, Nmin, and speci®c respiratory activity (SRA) showed a signi®cant (P<0.05) tillage effect (NT>CT or MT) in the 0± 7.5 cm depth. These differences occurred despite
the effect of tillage or fallow frequency on crop residue inputs being small (Table 2), thus suggesting that differences are more likely related to degree of soil disturbance. The MB-C constituted 1±3% of total organic C in the soil in 1990, lower than the 4±7% it constituted in 1986. Only a small fraction of the total organic C was mineralized in 30 days at 218C (0.3±1.9%) in 1990, while 3.5±7.3% of the total N was mineralized in 16 wk at 358C (Table 3). The proportion of total N mineralized was similar to that measured in this soil in 1986. The Nmin values in 1990 were similar to those in 1986 for the 0±7.5 cm depth, but, for the 7.5±15 cm depth, 1990 values were only half those measured in 1986. The reason for the differences between sampling dates is uncertain. Usually we would expect
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
139
Table 2 Growing season precipitation (GSP) a and annual straw production b of the cropping systems used in this study Year
GSP (mm)
Straw production (kg haÿ1 yrÿ1) Cont W (CT)
c
Cont W (NT)
F-W (MT)
F-W (NT)
187 138 81
0 1897 1395
0 1920 1592
0 1711 1082
0 1754 1125
±
3292
3512
2793
2879
1986 1987 1988 c 1989
233 110 98 210
0 667 0 3217
0 758 0 3906
0 1198 873 2308
0 1168 1113 2273
Total (1986±1989)
±
3884
4664
4379
4554
1990 1991 1992 1993
188 222 292 196
3080 5617 2800 4385
3547 4687 2867 4466
2234 3099 2602 2976
2628 1982 2789 2601
Total (1990±1993)
±
15882
15567
10911
10000
Mean Signif. of F ratio d LSD (P<0.10)
178
1644
1585
1983 1984 1985
Total (1983±1985) c
2096
2158 systemyear** 340
a
GSPprecipitation for period 1 May to 31 July. Production for Cont Wyields but for F-Wyield for a crop year divided by 2. c No yields obtained in 1983 and 1986 due to hail; in 1988 there was complete crop failure for Cont W due to drought. d ** significant at P<0.01. b
changes, if any, to occur in the tilled layer, i.e., mainly the 0±7.5 cm depth. 3.3. Spring 1994 ± after 11 years Due to the large input of crop residues between 1990 and 1994 (Table 2), there was an increase in total soil organic C and N over this period, with the values being signi®cantly (P<0.05) higher for NT than for tilled systems (Table 4). However, unlike results obtained in a medium-textured soil at Swift Current (Campbell et al., 1995), fallow frequency was not signi®cant. These results were surprising because, by this stage, there was a marked difference in crop residue input favoring Cont W over F-W, but no difference due to tillage (Table 2). Several workers have demonstrated a direct association between soil organic matter and amount of crop residue returned to soil (Campbell et al., 1997a; Janzen et al., 1997). As found after 7 years, MB-C, whether expressed on an absolute basis or as a percent of total organic C,
was not signi®cantly affected by tillage or fallow frequency. This latter lack of effect was surprising because fallowing has been shown to decrease MB-C in other studies (Carter, 1986; Biederbeck et al., 1994). These results concur with our ®ndings in a similar study on a coarse-textured soil at Cantuar, Saskatchewan (Campbell et al., 1997b). Other workers report increases in MB-C for NT>CT in the 0±7.5 cm depth, but the reverse occurring in the 7.5±15 cm depth, thereby often resulting in no net difference when the 0±15 cm depth is considered (Carter, 1986; Staley et al., 1988). As stated earlier, the length of time required for tillage effects on MB-C to become apparent also varies (Staley et al., 1988; Carter, 1986; Franzluebbers and Arshad, 1996). Our failure to obtain a signi®cant effect of tillage on MB-C may be because tillage had no effect on crop residue production in most years (Table 2). There was a weak association between MB-C and straw produced; in the past we have found strong association between these two factors (Schoenau and Campbell, 1996). Micro-
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C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
Table 3 Effect of fallow frequency and tillage method on selected soil biochemical characteristics in the top 15 cm of soil after 7 years (sampled in April 1990) Rotation
0±7.5 cm Cont W F-W Signif. of F ratio e S x 7.5±15 cm Cont W F-W Signif. of F ratio S x 0±15 cm Cont W F-W Signif. of F ratio S x
Cmin b (kg haÿ1)
Nmin c SRA d (kg haÿ1)
Cmin
Nmin
Org C (%)
Org N (%)
2.3 2.0 3.2 2.1 * 0.2
187 276 169 200 * 20
94 111 71 105 * 7
0.65 0.95 0.50 0.63 ** 0.04
1.50 1.91 1.59 1.31 0.4
6.8 7.3 6.1 6.6 ns ±
155 140 170 145 ns ±
1.4 1.3 1.6 1.0 ns ±
39 57 70 38 ns ±
47 44 48 57 3
0.26 0.39 0.41 0.28 ns ±
0.36 0.51 0.65 0.27 ns ±
3.6 3.5 3.8 3.8 ns ±
445 430 510 465 ns ±
1.9 1.7 2.4 1.6 0.2
226 333 239 238 ** 17
141 155 119 163 * 7
0.51 0.77 0.47 0.52 ** 0.03
0.96 1.30 1.10 0.80 * 0.10
5.3 5.6 4.9 5.2 ns ±
Organic C Organic N MB-C a (kg haÿ1) (kg haÿ1) (kg haÿ1)
MB-C
CT NT MT NT
12 600 14 420 10 640 15 370 ** 710
1400 1520 1180 1600 * 70
290 290 340 320 ns ±
CT NT MT NT
11 050 11 380 11 020 14 620 980
1300 1260 1260 1510 ns ±
CT NT MT NT
23 650 25 800 21 660 29 990 ** 1240
2700 2780 2440 3110 * 110
Tillage
Org C (%)
a
Microbial biomass determined by fumigation±incubation method. Cumulative Cmin measured over 30 days at 218C. c Cumulative Nmin measured over 16 wk at 358C. d SRAspecific respiratory activityratio of Cmin/MB-C. e In this and Table 4, nsnot significant, and , *, ** denote significance at P<0.10, P<0.05 and P<0.01, respectively. b
bial biomass-C in the 0±7.5 cm depth was closely associated with Nmin and, on a relative basis (MBC/Org C), it was associated with Cmin/Org C (r0.94, P0.06). In contrast to results after 7 years, Cmin in the 0± 7.5 cm depth after 11 years was greater for Cont W than for F-W, but tillage had no effect on Cmin (Table 4). As we suggested for MB-C, the lack of tillage effects on crop residue production may explain the absence of a tillage effect on Cmin. Cmin and straw produced were signi®cantly (P<0.01) associated (r0.99). In 1994, as in 1990, Cmin in the 7.5± 15 cm depth was not affected by tillage or fallow frequency, likely due to the shallow depth of tillage. As in 1990, Cmin (30 days at 218C) represented <2% of the total soil organic C. In contrast to our ®ndings after 7 years, SRA was not signi®cantly in¯uenced by
treatments after 11 years. Further, SRA was not signi®cantly associated (P<0.10) with any of the other soil quality attributes or the level of crop residue input (data not shown). These results are generally similar to those we obtained on a sandy soil at Cantuar, Saskatchewan (Campbell et al., 1997b), and in a clay soil at Indian Head, Saskatchewan (Campbell et al., 1991); however, they are opposite to those we obtained in other studies (Campbell et al., 1992a, b; Biederbeck et al., 1994). The reason for these differences is not clear. Nitrogen mineralization in the 0±7.5 cm depth was greater for Cont W than for F-W, and for NT>tilled systems; there were similar tendencies in the 7.5± 15 cm depth (Table 4). These results support those reported by others (Doran and Smith, 1987), and results we obtained in the sandy soil at Cantuar
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
141
Table 4 Effect of fallow frequency and tillage method on selected soil biochemical characteristics in the top 15 cm of soil after 11 years (sampled in April 1994) Rotation
0±7.5 cm Cont W F-W Signif. of F ratio S x 7.5±15 cm Cont W F-W Signif. of F ratio S x 0±15 cm Cont W F-W Signif. of F ratio S x a
MB-C Organic C Organic N MB-C a MB-N (kg haÿ1) (kg haÿ1) (kg haÿ1) (kg haÿ1) Org C (%)
Cmin Nmin SRA (kg haÿ1) (kg haÿ1)
CT NT MT NT
12 970 16 140 11 440 14 960 * 850
1390 1660 1220 1490 * 90
425 470 305 385 ns ±
60 70 45 55 ns ±
3.3 2.9 2.7 2.6 ns ±
257 258 171 161 * 22
108 125 74 92 ** 5
CT NT MT NT
14 490 14 330 13 300 17 240 * 790
1560 1560 1480 1770 ns ±
290 205 245 205 ns ±
40 30 35 30 ns ±
2.0 1.4 1.8 1.2 ns ±
198 171 199 141 ns ±
CT NT MT NT
27 460 30 480 24 740 32 200 * 1500
2950 3220 2700 3260 P0.15 ±
715 675 550 590 ns ±
100 100 80 85 ns ±
2.6 2.2 2.2 1.8 ns ±
455 430 370 300 P0.11 40
Tillage
Cmin
Nmin
Org C (%)
OrgN(%)
0.61 0.57 0.61 0.42 ns ±
1.98 1.61 1.50 1.09 * 0.15
7.8 7.6 6.1 6.2 * 0.4
69 55 48 56 4
0.76 0.83 0.91 0.69 ns ±
1.37 1.23 1.50 0.82 ns ±
4.4 3.5 3.3 3.2 * 0.2
177 180 122 148 ** 7
0.65 0.64 0.74 0.51 ns ±
1.65 1.43 1.50 0.95 * 0.13
6.0 5.7 4.5 4.6 ** 0.2
Microbial biomass determined by fumigation±extraction method.
(Campbell et al., 1997b). Thus, Nmin appeared to be the most sensitive attribute of those examined in this study. After 11 years, Nmin during 16 wk incubation at 358C represented the same proportion of the total N as after 7 years (3±8%). However, unlike after 7 years, amount of Nmin in the 0±7.5 cm depth, and Nmin as a percent of organic N, were greater for Cont W than for F-W. In this study, Nmin was closely correlated with MBC (r0.99, p0.01) and weakly associated with straw production (r0.88, p0.12). On the other hand, Cmin was only signi®cantly correlated to straw production (r0.99, p0.01), while besides its association with Nmin, MB-C was weakly associated with organic N (r0.89, p0.11). When normalized relative to organic N (Nmin/organic N), Nmin was signi®cantly associated with straw production (r0.98, p0.01) and with Cmin (r0.98, p0.01). The latter ®ndings support the suggestion of some workers (McGill et al., 1988; Gregorich et al., 1994) that these dynamic soil quality attributes may provide more
meaningful interpretation when normalized vs. total organic matter than when expressed as a concentration. The aforementioned effect of fallow frequency on Nmin may be associated with its impact on crop residue production (Table 2). However, since tillage had little in¯uence on amount of crop residues produced, we attributed this effect on Nmin to the more frequent soil disturbance experienced by the tilled systems, facilitating more rapid decomposition of the organic matter in this soil. Besides biological or physico-chemical factors, one factor that could cause the apparent effects of the treatments to differ over time is differences in the effective sampling depth resulting from changes in bulk density (Ellert and Bettany, 1995). However, in this study, this factor likely had little in¯uence on the results because, as discussed previously (Campbell et al., 1996b), neither tillage nor crop rotation in¯uenced bulk density in either depth (data not shown). In these
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Table 5 Sensitivity of selected soil quality attributes to tillage and to fallow frequency in the 0±7.5 cm depth after 11 years Treatment Cont W F-W CT or MT NT
Org C
Org N
MB-C
Tillage sensitivity ratio (NT/CT or MT) 1.24 1.19 1.11 1.30 1.22 1.26 Fallow frequency sensitivity ratio (Cont W/F-W) 1.13 1.14 1.39 1.08 1.11 1.22
shallow tilled soils the latter is usually the norm (Campbell et al., 1995, 1996a). 3.4. Sensitivity of various soil quality attributes to agronomic treatments We estimated the relative responsiveness of the soil quality attributes assessed in this study to fallow frequency, and to tillage, by calculating the sensitivity ratios shown in Table 5. We assumed that the higher the ratio the more sensitive was the index. In other studies we have found the more labile soil attributes (e.g., MB-C, Cmin and Nmin) to be more sensitive than the total organic C and N (Biederbeck et al., 1994); however, in the present study this did not appear to be true. Because of the more frequent tillage experienced by the F-W system compared to Cont W, and because moisture content would be closer to the optimum for microbial activity more frequently in F-W, we expected the ratios to be higher for F-W, as was found in the Cantuar study (Campbell et al., 1997b); however, only rarely was this observed in this clay soil. The sensitivities of the soil quality attributes to fallow frequency were generally similar for both tillage treatments. As expected, the more labile attributes showed greater sensitivities than total organic C or N in this clay soil. These results were similar to those reported by Biederbeck et al. (1994), and also to our results for a sandy soil (Campbell et al., 1997b). They probably re¯ect the in¯uence of fallow frequency on the amount of crop residues being returned to the land (Table 2). Finding soil quality attributes with greater sensitivity is useful because it allows us to quickly differentiate and assess desirable or undesirable treatment effects. However, greater sensitivity does not always
Cmin
Nmin
SRA
1.00 0.94
1.16 1.24
0.93 0.69
1.50 1.60
1.46 1.36
1.00 1.36
equate with `more desirable'. This is a complex concept which is a function of the goal we wish to achieve. For example, a treatment that results in greater Nsupplying capacity may be deemed desirable agronomically (less fertilizer required) yet may encourage nitrate leaching and in this respect be undesirable. Consequently, we need to carefully consider and weigh an array of soil quality attributes as to their in¯uence on agricultural sustainability, before concluding on their overall desirability. 4. Conclusions In this 11 year study, conducted on a clay soil in southwestern Saskatchewan, we examined the in¯uence of tillage and fallow frequency on selected soil quality attributes, measured at 3±4 year intervals. The overall ®ndings were unclear. For example, the total organic C and N appeared to respond primarily to soil disturbance resulting from tillage, with differences occurring in the 0±7.5 cm depth after 3 years and persisting for 11 years. Surprisingly, total organic C and N did not re¯ect the large difference in crop residue inputs for Cont W vs. F-W. In contrast, the more dynamic attributes (e.g., Cmin and Nmin) mainly responded to fallow frequency, we suspect primarily due to differences in crop residue inputs. Of the soil quality attributes assessed, Nmin was most sensitive to the agronomic treatments; Cmin was moderately sensitive, but neither MB-C nor SRA were very sensitive. In contrast to our expectations, the more labile soil quality attributes were generally no more sensitive to the agronomic treatments than were total organic C or N. More research is required to determine the reason for these unusual results.
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
Acknowledgements The authors acknowledge the technical assistance of G. Winkleman, J. Geissler, R. St. Jacques, Don Sluth and D. Hahn. References Amato, M., Ladd, J.N., 1988. Assay for microbial biomass based on ninhydrin-reactive nitrogen in extracts of fumigated soils. Soil Biol. Biochem. 20, 107±114. Anderson, J.P.E., Domsch, K.H., 1978. Mineralization of bacteria and fungi in chloroform-fumigated soils. Soil Biol. Biochem. 10, 207±213. Ayres, K.W., Acton, D.F., Ellis, J.G., 1985. The soils of the Swift Current Map Area 72J Saskatchewan. Sask. Inst. Pedology Publ. 86. Extension Division, University of Saskatchewan, Saskatoon, SK. Biederbeck, V.O., Campbell, C.A., Zentner, R.P., 1984. Effect of crop rotation and fertilization on some biological properties of a loam in southwestern Saskatchewan. Can. J. Soil Sci. 64, 355±367. 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., Brandt, S.A., Biederbeck, V.O., Zentner, R.P., Schnitzer, M., 1992a. Effect of crop rotations and rotation phase on characteristics of soil organic matter in a Dark Brown Chernozemic soil. Can. J. Soil Sci. 72, 403±416. Campbell, C.A., Ellert, B.H., Jame, Y.W., 1993. Nitrogen mineralization potential in soils. In: Carter, M. (Ed.), Soil Sampling and Methods of Analysis. Lewis, Boca Raton, FL, pp. 341±349. Campbell, C.A., Janzen, H.H., Juma, N.G., 1997a. Case studies of soil quality in the Canadian Prairies: Long-term field experiments. In: Gregorich, E.G., Carter, M.R. (Eds.), Soil Quality for Crop Production. Elsevier, Amsterdam, pp. 351±397. Campbell, C.A., Lafond, G.P., Leyshon, A.J., Zentner, R.P., Janzen, H.H., 1991. Effect of cropping practices on the initial potential rate of N mineralization in a thin Black Chernozem. Can. J. Soil Sci. 71, 43±53. Campbell, C.A., McConkey, B.G., Biederbeck, V.O., Zentner, R.P., Tessier, S., Hahn, D.L., 1997b. Tillage and fallow frequency effects on selected soil quality attributes in a coarse-textured Brown Chernozem. Can. J. Soil Sci. 77, 497±505. Campbell, C.A., McConkey, B.G., Zentner, R.P., Dyck, F.B., Selles, F., Curtin, D., 1995. Carbon sequestration in a Brown Chernozem as affected by tillage and rotation. Can. J. Soil Sci. 75, 449±458. Campbell, C.A., McConkey, B.G., Zentner, R.P., Selles, F., Curtin, D., 1996a. Tillage and crop rotation effects on soil organic C and N in a coarse-textured Typic Haploboroll in southwestern Saskatchewan. Soil Tillage Res. 37, 3±14.
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Campbell, C.A., McConkey, B.G., Zentner, R.P., Selles, F., Curtin, D., 1996b. 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. Campbell, C.A., Moulin, A.P., Bowren, K.E., Janzen, H.H., Townley-Smith, L., Biederbeck, V.O., 1992b. Effect of Crop rotations on microbial biomass, specific respiratory activity and mineralizable nitrogen in a Black Chernozemic soil. Can. J. Soil Sci. 72, 417±427. Carter, M.R., 1986. Microbial biomass as an index for tillageinduced changes in soil biological properties. Soil Tillage Res. 7, 29±40. Carter, M.R., Rennie, D.A., 1982. Changes in soil quality under zero tillage farming systems: Distribution of microbial biomass and mineralizable C and N potentials. Can. J. Soil Sci. 62, 587± 597. Doran, J.W., 1980. Soil microbial and biochemical changes associated with reduced tillage. Soil Sci. Soc. Am. J. 44, 765±771. Doran, J.W., Parkin, T.B., 1994. Defining and assessing soil quality. In: Doran, J.W., et al. (Eds.), Defining Soil Quality for a Sustainable Environment. SSSA Special Publication Number 35, American Society of Agronomy, Madison, Wisc, pp. 3±21. Doran, J.W., Smith, M.S., 1987. Organic matter management and utilization of soil and fertilizer nutrients. In: Follett, R.F., et al. (Eds.), Soil fertility and organic matter as critical components of production systems. SSSA Special Pub. No. 19. SSSA, Inc., Amer. Soc. Agron. Inc., Pub., Madison, WI, pp. 53±72. Ellert, B.H., Bettany, J.R., 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soil Sci. 75, 529±538. Franzluebbers, A.J., Arshad, M.A., 1996. Water-stable aggregation and organic matter in four soils under conventional and zero tillage. Can. J. Soil Sci. 76, 387±393. Janzen, H.H., Campbell, C.A., Ellert, B.H., Bremer, E., 1997. Soil organic matter dynamics and their relationship to soil quality. In: Gregorich, E.G., Carter, M.R. (Eds.), Soil Quality for Crop Production. Elsevier, Amsterdam, pp. 277±292. Janzen, H.H., Johnston, A.M., Carefoot, J.M., Lindwall, C.W., 1996. Soil organic matter dynamics in long-term experiments in southern Alberta. In: Paul, E.A. et al. (Eds.), Soil Organic Matter in Temperate Agroecosystems: Long-term experiments in North America. CRC Press, Inc., Boca Raton, FL, pp. 283± 296. Jenkinson, D.S., Powlson, D.S., 1976. The effects of biocidal treatments on metabolism in soil. V. A method for measuring soil biomass. Soil Biol. Biochem. 8, 209±213. Lafond, G.P., Brandt, S., McAndrew, D.W., Stobbe, E., Tessier, S., 1990. Tillage systems for crop production. In: Lafond, G.P., Fowler, D.B. (Eds.), Crop Management for Conservation. Proc. Soil Conserv. Symp., Yorkton, SK., pp. 155±201. Larney, F.J., Lindwall, C.W., Izaurralde, R.C., Moulin, A.P., 1994. Tillage systems for soil and water conservation on the Canadian Prairie. In: Carter, M.R. (Ed.), Conservation Tillage in Temperate Agroecosystems. Lewis Publishers, Boca Raton, FL, pp. 305±328.
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McConkey, B.G., Campbell, C.A., Zentner, R.P., Dyck, F.B., Selles, F., 1996. Long-term tillage effects on spring wheat production on three soil textures in the Brown soil zone. Can. J. Plant Sci. 76, 747±756. McGill, W.B., Dormaar, J.F., Reinl-Dwyer, E., 1988. New perspectives on soil organic matter quality, quantity and dynamics on the Canadian Prairies. Land Degradation and Conservation Tillage, Proceedings of the 34th Annual Canadian Society of Soil Science Meeting, Calgary, AB, pp. 30±48. Saskatchewan Agriculture, 1988. General recommendations for fertilization in Saskatchewan. Saskatchewan Agriculture, Soils and Crops Branch, Regina, SK., Agdex 541. SAS Institute Inc., 1985. SAS user's guide: Statistics, Version 5th ed. SAS Institute, Inc., Cary, NC. Schoenau, J., Campbell, C.A., 1996. Impact of crop residues on
nutrient availability in conservation tillage systems. Can. J. Plant Sci. 76, 621±626. Soil and Water Conservation Society, 1995. Farming for a better environment ± A white paper. Soil and Water Conserv. Society, Ankeny, IA, p. 67. Staley, T.E., Edwards, W.M., Scott, C.L., Owens, L.B., 1988. Soil microbial biomass and organic component alterations in a no-tillage chronosequence. Soil Sci. Soc. Am. J. 52, 998±1005. Tessier, S., Steppuhn, H., 1990. Quick mount soil core sampler for measuring bulk density. Can. J. Soil Sci. 70, 115±118. Voroney, R.P., Paul, E.A., 1984. Determination of kc and kN in situ for calibration of the chloroform fumigation±incubation method. Soil Biol. Biochem. 16, 9±14. Gregorich, E.G., Cartes, M.R., Angers, D.A., Montreal, C.M., Ellert, B.H., 1994. Towards a minimum data set to assess soil organic matter quality. Can. J. Soil Sci. 74, 367±385.
Long-term effects of tillage and fallow-frequency on soil quality attributes in a clay soil in semiarid southwestern saskatchewan C.A. Campbell*, B.G. McConkey, V.O. Biederbeck, R.P. Zentner, D. Curtin, M.R. Peru Semiarid Prairie Agricultural Research Centre, Agriculture and Agri-Food Canada, Box 1030, Swift Current, Saskatchewan, Canada S9H 3X2 Received 27 May 1997; accepted 2 December 1997
Abstract Reduced tillage management is being adopted at an accelerated rate on the Canadian prairies. This may in¯uence soil quality and productivity. A study conducted on a clay soil (Udic Haplustert) in southwestern Saskatchewan, Canada, to determine the effects of fallow frequency [fallow-wheat (F-W) vs. continuous wheat (Cont W)] and tillage [no-tillage (NT) vs. conventional (CT) or minimum tillage (MT)] on yields of spring wheat (Triticum aestivum L.), was sampled after 3, 7 and 11 years to assess changes in selected soil quality attributes. Tillage had no effect on amount of crop residues returned to the land, but the tilled systems had signi®cantly (P<0.05) lower total organic C and N in the 0±7.5 cm soil depth, though not in the 7.5±15 cm depth. Further, these differences were observed after only 3 years and persisted for the entire 11 years of the study. For example, in the 0±7.5 cm depth, organic C in F-W (MT) after 3 years was 10 480 kg haÿ1 and in F-W (NT) 13 380 kg haÿ1, while in Cont W (CT) and Cont W (NT) corresponding values were 11 310 and 13 400 kg haÿ1, respectively. After 11 years, values for F-W (MT) and F-W (NT) were 11 440 and 14 960 kg haÿ1, respectively, and for Cont W (CT) and Cont W (NT), 12 970 and 16 140 kg haÿ1, respectively. In contrast to total organic matter, two of the more labile soil quality attributes [i.e., C mineralization (Cmin) and N mineralization (Nmin)] did not respond to fallow frequency until after 7 years and only in the 0± 7.5 cm depth. Microbial biomass (MB) and the ratio of Cmin to MB [speci®c respiratory activity (SRA)], two attributes also regarded as labile, were not in¯uenced by the treatments even after 11 years. After 11 years, only Cmin and Nmin among the labile soil quality attributes responded to the treatments. Surprisingly, the labile attributes were no more sensitive to the treatments than was total organic C or N. More research is required to determine why responses in this soil differed from those reported elsewhere. # 1998 Elsevier Science B.V. All rights reserved. Keywords: C and N mineralization; Microbial biomass; Speci®c respiratory activity; Organic C and N; Soil quality
1. Introduction There has been a marked increase in use of reduced tillage systems by producers on the Canadian prairies *Corresponding author. Tel.: +1 613 759 1536; fax: +1 613 759 0646; e-mail: [email protected] 0167-1987/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0167-1987(98)00027-0
in recent years (Lafond et al., 1990; Larney et al., 1994). This may result in signi®cant changes in the quality of soil organic matter (Soil and Water Conservation Society, 1995). Soil quality has been de®ned as ``the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal
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health'' (Doran and Parkin, 1994). Soil organic matter is an important soil quality attribute because it in¯uences the productivity and physical well-being of soils. Therefore, it is important both from an economic and environmental standpoint to determine how tillage in¯uences soil quality. Recently, we discussed the in¯uence of tillage on the quantity of C stored in soils of differing texture in Saskatchewan (Campbell et al., 1995, 1996a, b). We showed that, over the 11±12 year study period, increases in C storage due to adoption of no-tillage were small (0±3 t haÿ1) and directly related to clay content (Campbell et al., 1996b). It has been hypothesized that a change in management will cause the labile components of soil organic matter to change to a new steady state, and that these changes will happen more quickly than for total organic matter (McGill et al., 1988; Janzen et al., 1997). The length of time required before the effects of tillage on soil biochemical characteristics [e.g., microbial biomass (MB), C or N mineralization (Cmin, Nmin)] to become apparent is uncertain. For example, Staley et al. (1988) noted differences in MB after only 1 year, Carter (1986) observed differences in the 0± 5 cm depth after 3 years, but Franzluebbers and Arshad (1996) found differences only after 6 years. Based on results from several long-term experiments in the USA, Doran (1980) reported that microbial numbers and biomass, and potential mineralizable N, were greater for NT than for CT (moldboard plow) in the 0±7.5 cm depth, but trends were generally reversed in the 7.5±15 cm depth. Most workers report that the amount of soil microbial and biochemical characteristics decrease as the frequency of fallow increases (Campbell et al., 1997a; Janzen et al., 1996). Our objective was to determine the in¯uence of tillage method and fallow frequency on selected soil quality attributes for a clay soil, in the semiarid prairie of southwestern Saskatchewan over an 11 year period under monoculture cereal cropping. 2. Materials and methods Details of the design of this experiment are published elsewhere (Campbell et al., 1996b; McConkey et al., 1996); consequently, only a brief review is presented here.
The experiment was initiated in 1982 near Stewart Valley, Saskatchewan, Canada, 50 km north of Swift Current (1983 was ®rst year of data collection). The land is level and had been managed for the previous 70±80 years in a 2 or 3 year fallow-cereal rotation. The soil is a Sceptre heavy clay, a Rego Brown Chernozem (Udic Haplustert) (Ayres et al., 1985). Soil at the 0± 15 cm depth has a pH of about 7.0 in dilute CaCl2, a sand content of 260 g kgÿ1, clay content of 420 g kgÿ1, and silt content of 320 g kgÿ1. There were four treatments in which hard red spring wheat was grown on summerfallow (F-W) (cropped once every two years) using NT or minimum tillage (MT) management, or grown continuously (Cont W) (seeded in spring and harvested in early fall each year) using NT or conventional mechanical tillage (CT) management. Weed control in minimum till fallow involved application of broad-spectrum herbicide tank mix in June followed by one to three tillage operations during summer using a heavy-duty sweep cultivator, to a depth of 5±10 cm (McConkey et al., 1996). Fallow weed control in NT plots was achieved with herbicide only. For Cont W (CT) and F-W (MT) there was one preseeding tillage operation using a heavy-duty cultivator with attached rodweeder or mounted harrow to 5±10 cm depth. Cont W (NT) involved a single preseeding application of the broad-spectrum herbicide tank mix. Applications of 2,4-D in fall were used to control winter-annual broadleaf weeds on all treatments. There were three replicates arranged in a randomized complete block design. Both phases of the F-W rotation were present each year and each treatment was cycled on its assigned plots. Commercial farm machinery was used to perform most ®eld operations on plots 15 m wide by 30 m long. A hoepress seed drill was used for seeding. All cropped plots received 10 kg P haÿ1 as monoammonium phosphate, seed-placed. Nitrogen (ammonium nitrate), at rates based on soil tests (Saskatchewan Agriculture, 1988), was applied with the seed to a maximum of 45 kg haÿ1 N; the remainder was broadcast prior to seedbed preparation or at seeding. At harvest, total aboveground plant biomass and grain yield from three random 1 m2 areas per plot were measured after air drying the samples. Straw yields were taken as the difference between aboveground plant biomass and grain yield.
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
Prior to commencement of ®eld operations in April 1986, 1990 and 1994, soil samples were taken from the 0±7.5 and 7.5±15 cm depths of the cropped phase of each treatment and the soil in each plot composited by depth and replicate. The soil was sieved (<2 mm) and crop residues remaining on the sieve discarded. One-half of each sample was air-dried and the remainder was stored ®eld-moist at 1±38C pending analysis for microbial biomass and C mineralization (Cmin). Within 3 months of sampling, the air-dried soil was used for determination of total organic C and N and net N mineralization (Nmin). At each sampling time separate 5 cm diameter soil cores were taken from each depth for bulk density determination by the method of Tessier and Steppuhn (1990). Samples collected in 1994 were analyzed, and stored soil from 1986 and 1990 reanalyzed for total organic C and N using an automated combustion technique (Carlo ErbaTM, Milan, Italy), as discussed previously (Campbell et al., 1996b). Mineralizable-N was determined by incubating a soil±sand mixture at 358C and measuring cumulative nitrate- and ammonium-N generated during 16 wk, by leaching intermittently with dilute CaCl2 followed by a minus-N nutrient solution and evacuating to about 60 cm of Hg (Campbell et al., 1993). In 1990 and 1994, two subsamples per treatment were used to determine Cmin. This involved wetting ®eld-moist soil (50 g oven-dry weight per subsample) to ®eld capacity (315 g H2O kgÿ1 soil), conditioning the soil for 3 days at 218C and then incubating the soil in biometer ¯asks at 218C for 30 days. The evolved CO2 was trapped in an alkali solution and measured by acid titration on days 4, 9, 16, 23 and 30. In 1986, the same method was used but titrations were performed on days 2, 4, 7, 10 and 14. Cmin was determined within 3 months of sampling. In 1986 and 1990, soil microbial biomass (MB) was determined by the chloroform fumigation±incubation technique (Jenkinson and Powlson, 1976), as described by Biederbeck et al. (1984). From each treatment we used 6 subsamples of ®eld-moist soil (each 100 g oven-dry weight) wetted to ®eld capacity and pre-incubated for 3 days at 218C. Three subsamples were fumigated with CHCl3 and three were left unfumigated. Microbial biomass C (MB-C) was estimated by dividing the ¯ush of CO2-C by a kC factor of 0.41 (Anderson and Domsch, 1978; Voroney and Paul,
137
1984). To estimate biomass N (MB-N), we followed the approach of Carter and Rennie (1982) who used a kN value of 0.4 (the overall mean of values determined in several other studies). In 1994 we changed to the fumigation±extraction method of Amato and Ladd (1988). In this case, the weight of subsamples was 40 g on oven-dry basis. The procedure is described in detail elsewhere (Biederbeck et al., 1994). Microbial biomass was determined within 6 months of sampling. All data were subjected to analysis of variance for each depth and year separately. Standard error of the means (S x) were calculated for signi®cant treatment effects. Straw yields were converted to a mean dry weight of production per year basis and these data analyzed as a split-plot with tillage as main plot and year as subplot (SAS Institute Inc., 1985). Regression and correlation analyses were used to relate selected soil quality attributes to mean annual straw production. 3. Results and discussion 3.1. Spring 1986 ± after 3 years Of the more dynamic soil biochemical characteristics (e.g., Nmin, Cmin, MB) measured in 1986, none changed signi®cantly (P>0.10) during this 3 year period (Table 1). We had expected these more dynamic and labile soil quality attributes to respond to a change in management more rapidly than total organic matter, as suggested by others (McGill et al., 1988; Janzen et al., 1997). Because of the high content of expanding lattice clay in this soil, much of the additional organic matter may have become complexed and physically protected from microbial decomposition. There was a tendency for Cmin and Nmin in the 0±7.5 cm depth to be greater for NT than for CT, and similarly for Cmin in the 7.5±15 cm depth. We previously discussed changes in total organic C and N in this soil (Campbell et al., 1996b), demonstrating signi®cant increases in both C and N due to only 3 years of no-tillage, but ®nding no difference due to the increase in fallow frequency (Table 1). This increase due to tillage was despite the crop failure in 1983 due to hail which resulted in very little input of crop residues in that year (Table 2).
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Table 1 Effect of fallow frequency and tillage method on selected soil biochemical characteristics in the top 15 cm of soil after 3 years (sampled in April 1986) Rotation
0±7.5 cm Cont W F-W Signif. of F ratio d S x 7.5±15 cm Cont W F-W Signif. of F ratio S x 0±15 cm Cont W F-W Signif. of F ratio S x
Cmin b (kg haÿ1)
Nmin c (kg haÿ1)
6.0 5.1 4.3 4.8 ns ±
120 166 80 117 ns ±
94 107 64 95 nd ±
7.5 7.5 5.6 6.6 nd ±
980 850 880 980 ns ±
6.5 5.3 7.1 5.7 ns ±
87 110 83 102 ns ±
100 97 93 87 nd ±
6.1 5.5 6.7 4.7 nd ±
1660 1540 1330 1620 ns ±
6.3 5.3 5.8 5.3 ns
207 276 163 219 ns ±
194 204 156 182 nd ±
6.7 6.4 6.1 5.5 nd ±
Organic C (kg haÿ1)
Organic N (kg haÿ1)
Microbial biomass a (MB)-C (kg haÿ1)
Org C (%)
CT NT MT NT
11 310 13 400 10 480 13 380 ** 510
1250 1430 1150 1440 ** 50
680 690 450 640 ns ±
CT NT MT NT
14 970 15 870 12 350 17 120 ** 550
1640 1750 1390 1850 ** 55
CT NT MT NT
26 280 29 270 22 830 30 500 ** 730
2890 3180 2540 3290 ** 75
Tillage
MB-C
Nmin Org N (%)
a
We did not determine microbial biomass N in 1986. Biomass determined by fumigation±incubation method. Cumulative Cmin measured over 14 days at 208C in 1986. c Soil from the three replicates were composited before Nmin was determined in 1986. Values are accumulated over 16 wk. Mineralization conducted on rewetted air dry soil with incubation at 358C. d nsnot significant, **significant at P<0.01, and ndnot determined. b
These results suggested that soil disturbance during tillage was the main factor responsible for the tillage effect. 3.2. Spring 1990 ± after 7 years The response of organic C and total N after 7 years was similar to after 3 years, indicating signi®cant (P<0.05) effects of tillage (NT>tilled) in the 0± 7.5 cm depth (Campbell et al., 1996b), but no effect due to fallow frequency (Table 3). Microbial biomass C, on an absolute basis, was not affected by tillage or fallow frequency. However, Cmin, Nmin, and speci®c respiratory activity (SRA) showed a signi®cant (P<0.05) tillage effect (NT>CT or MT) in the 0± 7.5 cm depth. These differences occurred despite
the effect of tillage or fallow frequency on crop residue inputs being small (Table 2), thus suggesting that differences are more likely related to degree of soil disturbance. The MB-C constituted 1±3% of total organic C in the soil in 1990, lower than the 4±7% it constituted in 1986. Only a small fraction of the total organic C was mineralized in 30 days at 218C (0.3±1.9%) in 1990, while 3.5±7.3% of the total N was mineralized in 16 wk at 358C (Table 3). The proportion of total N mineralized was similar to that measured in this soil in 1986. The Nmin values in 1990 were similar to those in 1986 for the 0±7.5 cm depth, but, for the 7.5±15 cm depth, 1990 values were only half those measured in 1986. The reason for the differences between sampling dates is uncertain. Usually we would expect
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
139
Table 2 Growing season precipitation (GSP) a and annual straw production b of the cropping systems used in this study Year
GSP (mm)
Straw production (kg haÿ1 yrÿ1) Cont W (CT)
c
Cont W (NT)
F-W (MT)
F-W (NT)
187 138 81
0 1897 1395
0 1920 1592
0 1711 1082
0 1754 1125
±
3292
3512
2793
2879
1986 1987 1988 c 1989
233 110 98 210
0 667 0 3217
0 758 0 3906
0 1198 873 2308
0 1168 1113 2273
Total (1986±1989)
±
3884
4664
4379
4554
1990 1991 1992 1993
188 222 292 196
3080 5617 2800 4385
3547 4687 2867 4466
2234 3099 2602 2976
2628 1982 2789 2601
Total (1990±1993)
±
15882
15567
10911
10000
Mean Signif. of F ratio d LSD (P<0.10)
178
1644
1585
1983 1984 1985
Total (1983±1985) c
2096
2158 systemyear** 340
a
GSPprecipitation for period 1 May to 31 July. Production for Cont Wyields but for F-Wyield for a crop year divided by 2. c No yields obtained in 1983 and 1986 due to hail; in 1988 there was complete crop failure for Cont W due to drought. d ** significant at P<0.01. b
changes, if any, to occur in the tilled layer, i.e., mainly the 0±7.5 cm depth. 3.3. Spring 1994 ± after 11 years Due to the large input of crop residues between 1990 and 1994 (Table 2), there was an increase in total soil organic C and N over this period, with the values being signi®cantly (P<0.05) higher for NT than for tilled systems (Table 4). However, unlike results obtained in a medium-textured soil at Swift Current (Campbell et al., 1995), fallow frequency was not signi®cant. These results were surprising because, by this stage, there was a marked difference in crop residue input favoring Cont W over F-W, but no difference due to tillage (Table 2). Several workers have demonstrated a direct association between soil organic matter and amount of crop residue returned to soil (Campbell et al., 1997a; Janzen et al., 1997). As found after 7 years, MB-C, whether expressed on an absolute basis or as a percent of total organic C,
was not signi®cantly affected by tillage or fallow frequency. This latter lack of effect was surprising because fallowing has been shown to decrease MB-C in other studies (Carter, 1986; Biederbeck et al., 1994). These results concur with our ®ndings in a similar study on a coarse-textured soil at Cantuar, Saskatchewan (Campbell et al., 1997b). Other workers report increases in MB-C for NT>CT in the 0±7.5 cm depth, but the reverse occurring in the 7.5±15 cm depth, thereby often resulting in no net difference when the 0±15 cm depth is considered (Carter, 1986; Staley et al., 1988). As stated earlier, the length of time required for tillage effects on MB-C to become apparent also varies (Staley et al., 1988; Carter, 1986; Franzluebbers and Arshad, 1996). Our failure to obtain a signi®cant effect of tillage on MB-C may be because tillage had no effect on crop residue production in most years (Table 2). There was a weak association between MB-C and straw produced; in the past we have found strong association between these two factors (Schoenau and Campbell, 1996). Micro-
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C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
Table 3 Effect of fallow frequency and tillage method on selected soil biochemical characteristics in the top 15 cm of soil after 7 years (sampled in April 1990) Rotation
0±7.5 cm Cont W F-W Signif. of F ratio e S x 7.5±15 cm Cont W F-W Signif. of F ratio S x 0±15 cm Cont W F-W Signif. of F ratio S x
Cmin b (kg haÿ1)
Nmin c SRA d (kg haÿ1)
Cmin
Nmin
Org C (%)
Org N (%)
2.3 2.0 3.2 2.1 * 0.2
187 276 169 200 * 20
94 111 71 105 * 7
0.65 0.95 0.50 0.63 ** 0.04
1.50 1.91 1.59 1.31 0.4
6.8 7.3 6.1 6.6 ns ±
155 140 170 145 ns ±
1.4 1.3 1.6 1.0 ns ±
39 57 70 38 ns ±
47 44 48 57 3
0.26 0.39 0.41 0.28 ns ±
0.36 0.51 0.65 0.27 ns ±
3.6 3.5 3.8 3.8 ns ±
445 430 510 465 ns ±
1.9 1.7 2.4 1.6 0.2
226 333 239 238 ** 17
141 155 119 163 * 7
0.51 0.77 0.47 0.52 ** 0.03
0.96 1.30 1.10 0.80 * 0.10
5.3 5.6 4.9 5.2 ns ±
Organic C Organic N MB-C a (kg haÿ1) (kg haÿ1) (kg haÿ1)
MB-C
CT NT MT NT
12 600 14 420 10 640 15 370 ** 710
1400 1520 1180 1600 * 70
290 290 340 320 ns ±
CT NT MT NT
11 050 11 380 11 020 14 620 980
1300 1260 1260 1510 ns ±
CT NT MT NT
23 650 25 800 21 660 29 990 ** 1240
2700 2780 2440 3110 * 110
Tillage
Org C (%)
a
Microbial biomass determined by fumigation±incubation method. Cumulative Cmin measured over 30 days at 218C. c Cumulative Nmin measured over 16 wk at 358C. d SRAspecific respiratory activityratio of Cmin/MB-C. e In this and Table 4, nsnot significant, and , *, ** denote significance at P<0.10, P<0.05 and P<0.01, respectively. b
bial biomass-C in the 0±7.5 cm depth was closely associated with Nmin and, on a relative basis (MBC/Org C), it was associated with Cmin/Org C (r0.94, P0.06). In contrast to results after 7 years, Cmin in the 0± 7.5 cm depth after 11 years was greater for Cont W than for F-W, but tillage had no effect on Cmin (Table 4). As we suggested for MB-C, the lack of tillage effects on crop residue production may explain the absence of a tillage effect on Cmin. Cmin and straw produced were signi®cantly (P<0.01) associated (r0.99). In 1994, as in 1990, Cmin in the 7.5± 15 cm depth was not affected by tillage or fallow frequency, likely due to the shallow depth of tillage. As in 1990, Cmin (30 days at 218C) represented <2% of the total soil organic C. In contrast to our ®ndings after 7 years, SRA was not signi®cantly in¯uenced by
treatments after 11 years. Further, SRA was not signi®cantly associated (P<0.10) with any of the other soil quality attributes or the level of crop residue input (data not shown). These results are generally similar to those we obtained on a sandy soil at Cantuar, Saskatchewan (Campbell et al., 1997b), and in a clay soil at Indian Head, Saskatchewan (Campbell et al., 1991); however, they are opposite to those we obtained in other studies (Campbell et al., 1992a, b; Biederbeck et al., 1994). The reason for these differences is not clear. Nitrogen mineralization in the 0±7.5 cm depth was greater for Cont W than for F-W, and for NT>tilled systems; there were similar tendencies in the 7.5± 15 cm depth (Table 4). These results support those reported by others (Doran and Smith, 1987), and results we obtained in the sandy soil at Cantuar
C.A. Campbell et al. / Soil & Tillage Research 46 (1998) 135±144
141
Table 4 Effect of fallow frequency and tillage method on selected soil biochemical characteristics in the top 15 cm of soil after 11 years (sampled in April 1994) Rotation
0±7.5 cm Cont W F-W Signif. of F ratio S x 7.5±15 cm Cont W F-W Signif. of F ratio S x 0±15 cm Cont W F-W Signif. of F ratio S x a
MB-C Organic C Organic N MB-C a MB-N (kg haÿ1) (kg haÿ1) (kg haÿ1) (kg haÿ1) Org C (%)
Cmin Nmin SRA (kg haÿ1) (kg haÿ1)
CT NT MT NT
12 970 16 140 11 440 14 960 * 850
1390 1660 1220 1490 * 90
425 470 305 385 ns ±
60 70 45 55 ns ±
3.3 2.9 2.7 2.6 ns ±
257 258 171 161 * 22
108 125 74 92 ** 5
CT NT MT NT
14 490 14 330 13 300 17 240 * 790
1560 1560 1480 1770 ns ±
290 205 245 205 ns ±
40 30 35 30 ns ±
2.0 1.4 1.8 1.2 ns ±
198 171 199 141 ns ±
CT NT MT NT
27 460 30 480 24 740 32 200 * 1500
2950 3220 2700 3260 P0.15 ±
715 675 550 590 ns ±
100 100 80 85 ns ±
2.6 2.2 2.2 1.8 ns ±
455 430 370 300 P0.11 40
Tillage
Cmin
Nmin
Org C (%)
OrgN(%)
0.61 0.57 0.61 0.42 ns ±
1.98 1.61 1.50 1.09 * 0.15
7.8 7.6 6.1 6.2 * 0.4
69 55 48 56 4
0.76 0.83 0.91 0.69 ns ±
1.37 1.23 1.50 0.82 ns ±
4.4 3.5 3.3 3.2 * 0.2
177 180 122 148 ** 7
0.65 0.64 0.74 0.51 ns ±
1.65 1.43 1.50 0.95 * 0.13
6.0 5.7 4.5 4.6 ** 0.2
Microbial biomass determined by fumigation±extraction method.
(Campbell et al., 1997b). Thus, Nmin appeared to be the most sensitive attribute of those examined in this study. After 11 years, Nmin during 16 wk incubation at 358C represented the same proportion of the total N as after 7 years (3±8%). However, unlike after 7 years, amount of Nmin in the 0±7.5 cm depth, and Nmin as a percent of organic N, were greater for Cont W than for F-W. In this study, Nmin was closely correlated with MBC (r0.99, p0.01) and weakly associated with straw production (r0.88, p0.12). On the other hand, Cmin was only signi®cantly correlated to straw production (r0.99, p0.01), while besides its association with Nmin, MB-C was weakly associated with organic N (r0.89, p0.11). When normalized relative to organic N (Nmin/organic N), Nmin was signi®cantly associated with straw production (r0.98, p0.01) and with Cmin (r0.98, p0.01). The latter ®ndings support the suggestion of some workers (McGill et al., 1988; Gregorich et al., 1994) that these dynamic soil quality attributes may provide more
meaningful interpretation when normalized vs. total organic matter than when expressed as a concentration. The aforementioned effect of fallow frequency on Nmin may be associated with its impact on crop residue production (Table 2). However, since tillage had little in¯uence on amount of crop residues produced, we attributed this effect on Nmin to the more frequent soil disturbance experienced by the tilled systems, facilitating more rapid decomposition of the organic matter in this soil. Besides biological or physico-chemical factors, one factor that could cause the apparent effects of the treatments to differ over time is differences in the effective sampling depth resulting from changes in bulk density (Ellert and Bettany, 1995). However, in this study, this factor likely had little in¯uence on the results because, as discussed previously (Campbell et al., 1996b), neither tillage nor crop rotation in¯uenced bulk density in either depth (data not shown). In these
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Table 5 Sensitivity of selected soil quality attributes to tillage and to fallow frequency in the 0±7.5 cm depth after 11 years Treatment Cont W F-W CT or MT NT
Org C
Org N
MB-C
Tillage sensitivity ratio (NT/CT or MT) 1.24 1.19 1.11 1.30 1.22 1.26 Fallow frequency sensitivity ratio (Cont W/F-W) 1.13 1.14 1.39 1.08 1.11 1.22
shallow tilled soils the latter is usually the norm (Campbell et al., 1995, 1996a). 3.4. Sensitivity of various soil quality attributes to agronomic treatments We estimated the relative responsiveness of the soil quality attributes assessed in this study to fallow frequency, and to tillage, by calculating the sensitivity ratios shown in Table 5. We assumed that the higher the ratio the more sensitive was the index. In other studies we have found the more labile soil attributes (e.g., MB-C, Cmin and Nmin) to be more sensitive than the total organic C and N (Biederbeck et al., 1994); however, in the present study this did not appear to be true. Because of the more frequent tillage experienced by the F-W system compared to Cont W, and because moisture content would be closer to the optimum for microbial activity more frequently in F-W, we expected the ratios to be higher for F-W, as was found in the Cantuar study (Campbell et al., 1997b); however, only rarely was this observed in this clay soil. The sensitivities of the soil quality attributes to fallow frequency were generally similar for both tillage treatments. As expected, the more labile attributes showed greater sensitivities than total organic C or N in this clay soil. These results were similar to those reported by Biederbeck et al. (1994), and also to our results for a sandy soil (Campbell et al., 1997b). They probably re¯ect the in¯uence of fallow frequency on the amount of crop residues being returned to the land (Table 2). Finding soil quality attributes with greater sensitivity is useful because it allows us to quickly differentiate and assess desirable or undesirable treatment effects. However, greater sensitivity does not always
Cmin
Nmin
SRA
1.00 0.94
1.16 1.24
0.93 0.69
1.50 1.60
1.46 1.36
1.00 1.36
equate with `more desirable'. This is a complex concept which is a function of the goal we wish to achieve. For example, a treatment that results in greater Nsupplying capacity may be deemed desirable agronomically (less fertilizer required) yet may encourage nitrate leaching and in this respect be undesirable. Consequently, we need to carefully consider and weigh an array of soil quality attributes as to their in¯uence on agricultural sustainability, before concluding on their overall desirability. 4. Conclusions In this 11 year study, conducted on a clay soil in southwestern Saskatchewan, we examined the in¯uence of tillage and fallow frequency on selected soil quality attributes, measured at 3±4 year intervals. The overall ®ndings were unclear. For example, the total organic C and N appeared to respond primarily to soil disturbance resulting from tillage, with differences occurring in the 0±7.5 cm depth after 3 years and persisting for 11 years. Surprisingly, total organic C and N did not re¯ect the large difference in crop residue inputs for Cont W vs. F-W. In contrast, the more dynamic attributes (e.g., Cmin and Nmin) mainly responded to fallow frequency, we suspect primarily due to differences in crop residue inputs. Of the soil quality attributes assessed, Nmin was most sensitive to the agronomic treatments; Cmin was moderately sensitive, but neither MB-C nor SRA were very sensitive. In contrast to our expectations, the more labile soil quality attributes were generally no more sensitive to the agronomic treatments than were total organic C or N. More research is required to determine the reason for these unusual results.
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