Introduction
Long-term performance and sustainability of crop production are linked to soil physical, chemical and biological properties, which in turn are influenced strongly by soil organic matter (SOM) (Weil and Magdoff, Reference Weil, Magdoff, Magdoff and Weil2004). Soil organic matter improves soil properties and is a source of plant nutrients and energy for microorganisms. Quantity and quality of soil organic carbon (C) and nitrogen (N) are affected by soil and crop management practices (Liu et al., Reference Liu, Hebert, Hashemi, Zhang and Ding2006).
Tillage can affect the amounts of organic C stored in soil. For example, incorporation of crop residue under intensive tillage, especially mouldboard plough (MP), can cause substantial decrease in soil organic C and N by increasing decomposition rate (Douglas et al., Reference Douglas, Allmaras, Rasmussen, Ramig and Roager1980; Collins et al., Reference Collins, Rasmussen and Douglas1992; Soon Reference Soon1998; Reference Soon2007). Leaving crop residue at the soil surface under no-tillage (NT) may restrict its decomposition, resulting in accumulation of SOM in the topsoil under NT (Liang et al., Reference Liang, McConkey, Campbell, Curtin, Lafond, Brandt and Moulin2004; Malhi and Lemke, Reference Malhi and Lemke2007; Malhi et al., Reference Malhi, Nyborg, Goddard and Puurveen2011, Reference Malhi, Nyborg, Goddard and Puurveen2012). Practicing MP tillage for 5–10 years may not cause any significant reduction in soil organic C and N compared with NT but may result in its redistribution in the soil profile (Angers et al., Reference Angers, Bolinder, Carter, Gregorich, Drury, Liang, Voroney, Simard, Donald, Beyaert and Martel1997).
Long-term NT may cause stratification of P in the surface soil and reduce its availability to crops (Crozier et al., Reference Crozier, Naderman, Tucker and Sugg1999; Baan et al., Reference Baan, Grevers and Schoenau2009). In areas where large amounts of crop residue or straw are produced, accumulated crop residue at the soil surface under NT may hinder seeding operations, resulting in poor/sporadic germination, especially when proper direct-seeding drills are not available to facilitate seeding. In addition, relatively cool and wet surface soil in spring under NT (Johnson and Lowery, Reference Johnson and Lowery1985) may delay seeding and slow crop emergence and early growth and increase potential nutrient loss in surface water run-off (Ferguson et al., Reference Ferguson, Pearson and Reynolds1996).
Current interest in low-input cropping systems often implies an increased use of tillage for weed control. Use of low-input or organic systems is also associated with a partial to total dependence on organic nutrient inputs. Application of organic manure can improve soil physical, chemical and biological properties, and the nutrient supplying power of the soil (Yanan et al., Reference Yanan, Emteryd, Dianqing and Grip1997; Whalen et al., Reference Whalen, Chang and Olson2001; Yang et al., Reference Yang, Malhi, Li, Suo, Xu, Wang, Xiao, Jia, Guo and Wang2007; Heitkamp et al., Reference Heitkamp, Raupp and Ludwig2011). Organic cropping systems that include application of manure can increase organic C and N, and availability of nutrients in soil even after short-term (3 years) additions (Malhi, Reference Malhi2012).
At some point, producers could be interested in tilling long-term NT fields, partly because of the above-mentioned potential problems related to long-term use of NT, but also to make use of accumulated nutrients in surface soil under NT, particularly if transitioning to an organic cropping system. Effects of tillage applied to long-term NT on crop yield and nutrient uptake, available nutrients in soil, and persistence of organic C in soil that was gained/stored under NT have been investigated previously (Davidson and Ackerman, Reference Davidson and Ackerman1993; Campbell et al., Reference Campbell, Thomas, Biederbeck, McConkey, Selles, Spurr and Zentner1998; VandenBygaart and Kay, Reference VandenBygaart and Kay2004; Baan et al., Reference Baan, Grevers and Schoenau2009). However, effects of terminating NT with a ploughing operation on these variables in contrasted cropping systems (using conventional vs. organic inputs) have not been documented under the agro-environmental conditions of eastern Québec. The objective of the current study was to determine the effects of previous cropping system (conventional and organic), long term tillage (chisel plough (CP), MP), long-term NT and terminating NT (P-NT: NT ploughed once after 24 years) on soil organic C and N fractions (total organic C (TOC) and N (TON), light fraction organic carbon (LFOC) and nitrogen (LFON), and mineralizable N (Nmin)) and available nutrients in soil (ammonium-N, nitrate-N and extractable P) in the 0–10, 10–20 and 20–30 cm depths of a Gleysolic soil in Québec, Canada.
Materials and methods
The present study was initiated in autumn 1987 at the Centre de Développement Bioalimentaire du Québec, at La Pocatière, Québec, Canada (47°E 21′N, 70°E 02′W, 204 m a.s.l.), on a Kamouraska clay (Typic humic Gleysol, mineralogy dominated by illite and chlorite (400 g/kg), quartz and feldspars (300 g/kg) and smectites (170 g/kg) (De Kimpe et al., Reference De Kimpe, Laverdière and Martel1979); 100 g/kg sand, 300 g/kg silt, 600 g/kg clay (clay texture) in the surface horizon; pH = 5.9; organic matter = 45 g/kg; P-Mehlich 3 extractable = 94 kg/ha, K-Mehlich 3 extractable = 305 kg/ha). Tillage treatments included: MP (15–18 cm depth) in autumn, followed by spring secondary tillage; CP (12–15 cm depth) in autumn, followed by spring secondary tillage; and NT. Mouldboard plough and CP plots were ploughed every other year in the first phase of the study (1987–1995), and every year thereafter.
In 2007, the tillage plots were used to determine the feasibility of applying low-input cropping systems to mature conservation tillage plots (Légère et al., Reference Légère, Vanasse and Stevenson2013). Two cropping systems were compared: (1) a system based on agronomic practices used in organic agriculture (ORG) (nutrients supplied as dry granular poultry manure and mechanical weed control), and a conventional cropping system (CONV) using synthetic nutrients and herbicide-based weed control. A 4-year crop rotation [barley (2007)/red clover (2008) (managed as a forage crop)/maize (2009)/soybean (2010)] was selected with the assumption that the initial barley/red clover years would provide good weed suppression as well as N input in support of the more demanding and less competitive maize crop.
Cropping system treatments were assigned randomly in strips perpendicular to the original tillage plots (plot size: 5 × 13 m2) within each replicate, resulting in a strip plot design with four replicates. On 3 November 2010, each NT plot was further split in half lengthwise (2.5 × 13 m2). One randomly selected half of NT was mouldboard ploughed (P-NT), whereas the other half remained in NT. This division created an additional treatment, i.e. terminated NT, to the existing tillage treatments (CP, MP and a non-terminated NT treatment) increasing the total number of tillage treatments in the experiment to four. On 10 June 2011, plots with MP, CP and P-NT were harrowed, and glyphosate-tolerant maize (Hybrid Fusion RR 2100-2400 CHU) was planted at 82 300 seeds/ha in 76 cm six rows. A reduced rate of fertilizer (220 kg/ha 27-18-0 N-phosphorus pentoxide (P2O5)–potassium oxide (K2O)) was side-banded at planting to all plots to allow the expression of residual effects of previous cropping systems. Maize was harvested in autumn at maturity before frost kill. Details of the cropping system by tillage study are found in Légère et al. (Reference Légère, Vanasse and Stevenson2013). Treatments considered for the current soil study included MP, CP, NT and P-NT tillage in CONV and ORG cropping systems, for a total of eight treatments.
Soil sampling and sample preparation
On 27 October 2011, soil cores from five locations in the centre of plots were collected from the 0–10, 10–20 and 20–30 cm depths, using a 2.5 cm internal diameter corer. Bulk density of the soil was determined by the core method (Culley, Reference Culley and Carter1993). Soil samples were air-dried at room temperature after removing any coarse roots and easily detectable crop residues, and ground to pass through a 2-mm sieve. Sub-samples were pulverized in a vibrating-ball mill (Retsch, Type MM2, Brinkman Instruments Co., Toronto, Ontario) for determination of organic C and N in various fractions.
Organic carbon and nitrogen analysis
For TON, soil samples were digested in concentrated sulphuric acid (H2SO4) plus one Keltab (containing 1.5 g potassium sulphate (K2SO4) and 0.15 g copper sulphate pentahydrate (CuSO4.5H2O)), and the ammonium in the digest was measured by using method of Technicon Industrial Systems (1977). Light fraction organic matter (LFOM) was separated using a sodium iodide (NaI) solution of 1.7 t/m3 specific gravity, following the method described by Janzen et al. (Reference Janzen, Campbell, Brandt, Lafond and Townley-Smith1992) and modified by Izaurralde et al. (Reference Izaurralde, Nyborg, Solberg, Janzen, Arshad, Malhi, Molina-Ayala, Lal, Kimble, Follett and Stewart1998). The TOC, and C and N in LFOM (LFOC, LFON) were measured for the 0–10 and 10–20 cm depths by Dumas combustion using a Carlo Erba instrument (Model NA 1500, Carlo Erba Strumentazione, Italy). Soil samples of all depths for TOC and TON analyses were also tested to detect any inorganic C using dilute HCl (hydrochloric acid), but none was found.
Mineralizable N in soil for the 0–10, 10–20 and 20–30 cm depths was estimated from the quantities of ammonium-N and nitrate-N that were mineralized from an unfumigated sample during 10-day incubation at 25 °C and a soil water potential of −30 J/kg (Campbell et al., Reference Campbell, Lafond, Zentner and Biederbeck1991). The concentrations of ammonium-N and nitrate-N were measured with a Technicon Analyzer II (Technicon Industrial Systems, 1973a, 1973b).
Chemical analysis
For chemical properties, prepared soil samples were analysed for pH, ammonium-N (NH4-N), nitrate-N (NO3-N) and extractable P (phosphate-P – PO4-P). Soil pH was measured in dilute solution of calcium chloride (CaCl2; 0.01 m) with a Fisher AR20 pH meter (San Diego, CA, USA). Nitrate-N and ammonium-N were extracted using 1:5 soil:2 m potassium chloride (KCl) solution and their concentrations in extracts determined with a Technicon Autoanalyzer II (Technicon Industrial Systems, 1973a, 1973b). Phosphorus was extracted using Kelowna soil extractant (Qian et al., Reference Qian, Schoenau and Karamanos1994) and measured colorimetrically on a Technicon Autoanalyzer (Technicon Industrial Systems, 1977).
Statistical analysis
The data on TOC, TON, LFOC and LFON were calculated using the equivalent soil mass technique (Ellert and Bettany, Reference Ellert and Bettany1995). Analysis of variance (ANOVA) was conducted separately for each depth using the GLIMMIX procedure of SAS (Littell et al., Reference Littell, Milliken, Stroup and Wolfinger2006; SAS Institute, 2011). The analysis considered the effects of replicate, replicate by tillage and replicate by cropping system as random, the effects of cropping system and tillage were fixed, and used a Gaussian error distribution. The fixed effects were considered to be cross-classified factors for the analysis. Exploratory analysis indicated the possibility of heterogeneous variances among tillage systems for some of the tillage by depth combinations. The AICc (corrected Akaike's Information Criterion) was used to confirm the benefit of modelling variance heterogeneity (Littell et al., Reference Littell, Milliken, Stroup and Wolfinger2006). Mean separation was performed using a protected least significant difference (LSD) test.
Results
The ANOVA table for the probabilities of significance for various parameters indicated that there was no significant tillage by cropping system interaction for all variables, except for pH in the 20–30 cm soil depth and LFON in the 0–10 cm soil depth (Table 1). Mean effect of tillage treatment was significant for LFOC and LFON in the 0–10 and 10–20 cm soil depths, pH in the 10–20 and 20–30 cm depths and Nmin in all soil depths, and nitrate-N in the 0–10, 10–20 and 0–30 cm depths. Mean effect of cropping system treatments was significant only for TOC in the 0–10 cm soil depth (although the effect for TON was almost significant at P = 0.057). The results on various soil parameters are discussed in the following paragraphs.
Table 1. Probability of significance for tested treatment effects of tillage, cropping system and tillage × cropping system interaction on total organic C (TOC), total organic N (TON), light fraction organic C (LFOC), light fraction organic N (LFON), pH, mineralizable N (Nmin), nitrate-N (NO3-N), ammonium-N (NH4-N) and extractable P in the 0–10, 10–20, 20–30, 0–20 and 0–30 cm soil depths in autumn 2010 (after 24 growing seasons) in a field experiment established in autumn 1987 at La Pocatière, Québec, Canada
*, **, *** and ns refer to significant treatment effects in ANOVA at P ⩽ 0.05, P ⩽ 0.01, P ⩽ 0.001 and not significant, respectively.
The mass of TOC and TON varied with cropping system only in the 0–10 cm soil depth (Table 2). In the 0–10 cm soil depth, TOC in ORG (27.54 t C/ha) was 1.63 t C/ha greater (P = 0.015) than that in the CONV (25.91 t C/ha) cropping system. The effect of tillage on LFOC and LFON varied with soil depth (Table 2). In the 0–10 cm soil depth, TOC and TON values for MP tended to be lower (although not significantly) than for the other three tillage systems. In the 0–10 cm soil depth, LFOC and LFON in NT and CP were approximately twofold that in MP. In the P-NT treatment, mass of LFOC was similar to that of NT and CP, whereas mass of LFON was intermediate to that of MP v. CP and NT. In the 10–20 cm soil depth, mass of LFOC and LFON for MP and P-NT were nearly twofold that for NT and CP. Overall, LFOC and LFON were evenly distributed over the two soil layers in MP, predominant in the 0–10 cm depth for CP and NT, with intermediate values for P-NT which were 1.5 times greater in the 0–10 cm depth than that of 10–20 cm depth.
Table 2. Effect of tillage and cropping system on mass of total organic C (TOC), total organic N (TON), light fraction organic C (LFOC) and light fraction organic N (LFON) in the 0–10, 10–20 and 0–20 cm soil depths in autumn 2010 in a field experiment established in autumn 1987 at La Pocatière, Québec, Canada
*, ** and ns refer to significant treatment effects in ANOVA at P ⩽ 0.05, P ⩽ 0.01 and not significant, respectively.
Tillage affected soil pH in the 10–20 and 20–30 cm soil depths, whereas cropping system had only a small effect on soil pH (Table 3). In the 10–20 and 20–30 cm soil depths, soil pH was lower in NT and P-NT than in MP and CP treatments. Also, in the 0–10 cm depth, soil pH tended to be lower in the P-NT and NT treatments, especially compared with the CP treatment.
Table 3. Effect of tillage and cropping system on pH, mineralizable N (Nmin), nitrate-N (NO3-N) and ammonium-N (NH4-N) in the 0–10, 10–20, 20–30 and 0–30 cm soil depths in autumn 2010 (after 24 growing seasons) in a field experiment established in autumn 1987 at La Pocatière, Québec, Canada
*, **, *** and ns refer to significant treatment effects in ANOVA at P ⩽ 0.05, P ⩽ 0.01, P ⩽ 0.001 and not significant, respectively.
The Nmin value for MP was lower than for the other three tillage treatments in the 0–10 cm soil depth, lower for MP than for CP and P-NT in the 10–20 cm depth, and lower for MP than for NT and P-NT in the 20–30 cm depth (Table 3). No significant differences in Nmin were observed among the other three tillage systems in all soil depths. Tillage affected nitrate-N only in the 0–10 and 10–20 cm soil depths, but again this effect varied with soil depth (Table 3). In the 0–10 cm depth, nitrate-N for MP was lower than that for other tillage treatments whereas in the 10–20 cm depth, nitrate-N was lower for MP and NT than for P-NT, and not different from CP. Tillage and cropping system had no effect on ammonium-N (Table 3) or extractable P, although MP and P-NT tended to have lower extractable P in the 0–10 cm soil depth than NT and CP treatments (Table 4).
Table 4. Effect of tillage and cropping system on extractable P in the 0–10, 10–20, 20–30 and 0–30 cm soil depths in autumn 2010 (after 24 growing seasons) in a field experiment established in autumn 1987 at La Pocatière, Québec, Canada
ns refer to not significant treatment effects in ANOVA.
Discussion
Soil organic carbon and nitrogen
Both quantity and/or quality of organic matter in soil can be altered by tillage (Havlin et al., Reference Havlin, Kissel, Maddux, Claassen and Long1990; Malhi and Lemke, Reference Malhi and Lemke2007), crop rotation or cropping systems (Campbell et al., Reference Campbell, Selles, Lafond, Biederbeck and Zentner2001; Liang et al., Reference Liang, McConkey, Schoenau, Curtin, Campbell, Moulin, Lafond, Brandt and Wang2003; Malhi et al., Reference Malhi, Brandt, Lemke, Moulin and Zentner2009). Tillage, especially MP, increases oxidation of SOM (Doran and Scott-Smith, Reference Doran, Scott-Smith, Follett, Stewart and Cole1987), while NT reduces its oxidation because of less mixing with the soil (Doran, Reference Doran1980). Therefore, one would expect a decrease of organic C and/or N in soil under MP or CP compared with NT (Douglas et al., Reference Douglas, Allmaras, Rasmussen, Ramig and Roager1980; Soon Reference Soon1998, Reference Soon2007). Similarly, in the present study after 25 years, the amounts of LFOC and LFON in soil, particularly in the 0–10 cm soil layer, were lower under MP than NT. But there were generally no significant differences in the amounts of TOC, TON, LFOC and LFON in soil between CP and NT systems. This could be due to the absence of inversion in CP, and to the depth of tillage/plough, which was shallower with CP (about 15 cm deep) than MP (about 20 cm deep).
In a review paper, Davidson and Ackerman (Reference Davidson and Ackerman1993) summarized results from a number of studies varying in duration of tillage/cultivation of NT soils ranging from 3 to 100+ years, and in soil texture ranging from sandy loam to clay. It would appear that long-term untilled soils could lose 20–40% of their original organic C following tillage/cultivation (Davidson and Ackerman, Reference Davidson and Ackerman1993). However, in the present study, there was no significant negative effect of one-time ploughing on TOC, TON, LFOC and LFON in soil compared with the long-term NT treatment. Actually, one-time ploughing of NT resulted in increased productivity, as maize yield in 2011 was greater in P-NT than NT (Légère et al., Reference Légère, Vanasse and Stevenson2013). The absence of one-time ploughing effect on TOC, TON, LFOC and LFON in soil was also suggested by Baan et al. (Reference Baan, Grevers and Schoenau2009) in their study in Saskatchewan, Canada. The soil in the present study was Humic Gleysol with a clay texture, whereas the soils in the Baan et al. (Reference Baan, Grevers and Schoenau2009) study belonged to the Brown Chernozem, Black Chernozem and Gray Luvisol Great Groups, with loam, fine loam and silty clay loam/clay loam texture, respectively. In the Baan et al. (Reference Baan, Grevers and Schoenau2009) studies, long-term NT soils were tilled/ploughed only once. It is anticipated that one-time ploughing of NT may increase both TOC and LFOC in soil in the future, because of its immediate beneficial effects on crop productivity (i.e., returning more crop residue to soil), due to the release/mineralization of nutrients tied in the LF organic matter.
Among the other dynamic fractions examined in the current study, Nmin was also lower under MP than NT at 0–10 and 20–30 cm soil depths. This suggests that the soil N-reserve would improve with NT but diminish with MP. Soil N availability is usually the most limiting factor for crop production (Pastor et al., Reference Pastor, Aber, McClaugherty and Melillo1984). Because the majority of available N used for synthesis of plant biomass is produced by mineralization from native soil organic N, this source of N should be considered when determining nutrient requirements of crops (Uri et al., Reference Uri, Löhmus and Tullus2003). In the present study, greater soil Nmin in NT than in MP suggests that the N-reserve of soil can be improved by reducing or eliminating tillage. There was a weak and non-significant correlation coefficient between crop yield and Nmin in the 0–30 cm soil depth. For the other soil properties, the correlation coefficient between crop yield and soil properties was significant only for soil NO3-N in the 0–30 cm depth.
In the present study, the amounts of soil TOC in the 0–10 cm depth were greater under the ORG than the CONV cropping system. This was probably due to the application of manure in the organic cropping system (Campbell et al., Reference Campbell, Schnitzer, Stewart, Biederbeck and Selles1986; McGill et al., Reference McGill, Cannon, Robertson and Cook1986; Yanan et al., Reference Yanan, Emteryd, Dianqing and Grip1997; Whalen et al., Reference Whalen, Chang and Olson2001; Assefa et al., Reference Assefa, Schoenau and Grevers2004; Heitkamp et al., Reference Heitkamp, Raupp and Ludwig2011; King et al., Reference King, Schoenau and Malhi2015). Similarly, research in Saskatchewan, Canada, has also shown that ORG cropping systems, which include application of manure, can increase organic C and N, and availability of nutrients in soil even after short-term (3 years) additions (Malhi, Reference Malhi2012). However, in another 12-year long-term field research experiment in Saskatchewan where manure was applied occasionally, cropping system had no effect on TOC and TON in soil, although soil LFOC and LFON tended to be slightly greater under ORG than CONV cropping system (Malhi et al., Reference Malhi, Brandt, Lemke, Moulin and Zentner2009), which is consistent with results of the present study. Manure has both direct and indirect input of C to soil, in some cases because of increased crop yields (Watson et al., Reference Watson, Atkinson, Gosling, Jackson and Rayns2002). This was not the case in the present study since previous silage maize (2009) and soybean (2010) yield in ORG was reduced when compared with the CONV cropping system (Légère et al., Reference Légère, Vanasse and Stevenson2013). However, silage maize yield in 2011 (the present study) was similar across both cropping systems (Légère et al., Reference Légère, Vanasse and Stevenson2013).
Light fraction organic matter in soil reflects a balance between crop residue input, and their decomposition and persistence, depending on the soil-climatic conditions (Gulde et al., Reference Gulde, Chung, Amelung, Chang and Six2008). The decomposition of LFOM is relatively faster than total organic matter in soil (Sollins et al., Reference Sollins, Spycher and Glassman1984), and this could provide an increased supply of plant-available N and other nutrients to plants, maintain high microbial populations, enzyme activity and soil respiration rate, and improve soil physical properties (Gregorich et al., Reference Gregorich, Carter, Doran, Pankhurst and Dwyer1997; Angers et al., Reference Angers, Edwards, Sanderson and Bissonnette1999; Lynch et al., Reference Lynch, Cohen, Fredeen, Patterson and Martin2005a, Reference Lynch, Voroney and Warman2005b; Marriott and Wander, Reference Marriott and Wander2006). Management practices can have a greater effect on LFOC and LFON than on TOC and TON (Malhi et al., Reference Malhi, Brandt and Gill2003a, Reference Malhi, Harapiak, Nyborg, Gill, Monreal and Gregorich2003b, Reference Malhi, Harapiak, Nyborg, Gill, Monreal and Gregorich2003c). Similarly, in the present study, the decreases in organic C and N for MP compared with NT were relatively more pronounced for LFOC and LFON than TOC, in spite of maize silage yield being similar across both tillage treatments (Légère et al., Reference Légère, Vanasse and Stevenson2013). Also, yield of previous soybean (2010) and silage maize (2009) for NT was similar to MP yield for CONV cropping systems (Légère et al., Reference Légère, Vanasse and Stevenson2013). The changes in LFOC and LFON can be considered good indicators of positive changes/build-up of organic C and N as a result of NT compared with MP (Hassink, Reference Hassink1994; Gagnon et al., Reference Gagnon, Lalande and Fahmy2001; Willson et al., Reference Willson, Paul and Harwood2001; Griffin and Porter, Reference Griffin and Porter2004). Monitoring the changes in LFOC and LFON in the surface soil would appear to be a good strategy to determine the potential for N supplying power, and improvement in soil quality/health. The higher organic C and N in light organic fractions than their total organic fractions under NT was most likely due to much slower decomposition of crop residue (straw, chaff, roots) under NT compared with MP treatment (Doran, Reference Doran1980; Doran and Scott-Smith, Reference Doran, Scott-Smith, Follett, Stewart and Cole1987).
Overall, the findings of the present study suggest some potential of NT or CP in building-up of organic C and N in soil. This was associated with the improvement in some soil properties, such as nutrient supplying power (Nmin). The increase in organic C and N in soil may also have some additional potential benefits, such as improvement in soil biodiversity in NT as evidenced by earthworm communities (Eriksen-Hamel et al., Reference Eriksen-Hamel, Speratti, Whalen, Légère and Madramootoo2009), and soil aggregation and water infiltration as well as decrease in water runoff and soil erosion, thereby increasing the sustainability of crop production (Malhi et al., Reference Malhi, Lemke, Wang and Chhabra2006; Singh and Malhi, Reference Singh and Malhi2006; Malhi and Lemke, Reference Malhi and Lemke2007).
Soil chemical properties
A trend of slow acidification (not significant) of surface soil under NT was observed after 12 annual applications of moderate rates of N fertilizer to annual crops in the Canadian prairies (Malhi et al., Reference Malhi, Brandt, Lemke, Moulin and Zentner2009). Similarly, in the present study, there was a slight decrease in soil pH to a depth of 30 cm under NT compared with CP or MP, most likely due to minimum disturbance and/or absence of mixing of surface/subsurface soil under NT compared with MP or CP for 24 years. Acidification of the surface soil from N fertilizer application does not appear to be a serious problem for cereal and oilseed crops at this site but may be an issue in the long run for optimum production of acid-sensitive crops. There was essentially no effect of cropping system on soil pH in the present study, as also suggested in a previous study where there was no consistent effect of crop diversity on soil pH decrease (Malhi et al., Reference Malhi, Brandt, Lemke, Moulin and Zentner2009).
Treatments under long-term NT management have shown greater amounts of available P in the surface thin soil layer (0–5 cm or less) than mouldboard ploughing (Eckert, Reference Eckert1985; Weil et al., Reference Weil, Benedetto, Sikora and Bandel1988). Indeed, there was a trend for lower extractable P with MP compared with CP or NT in the present study. Maintaining higher concentration of readily plant-available P in soil near the surface should thus be facilitated where tillage is eliminated. This also suggests the need for additional application of P fertilizer under NT particularly in early years, because inorganic P at the soil surface under NT may not become fully available to the crop due to its relatively high immobility in the soil profile.
In the present study, the lower amounts of nitrate-N under MP compared with other tillage treatments could be due to a dilution effect of mixing soil to deeper depth, in addition to a greater potential of immobilization of N by straw (Malhi et al., Reference Malhi, Nyborg and Solberg1996), nitrate leaching to a deeper depth below 30 cm and gaseous N losses (Heaney et al., Reference Heaney, Nyborg, Solberg, Malhi and Ashworth1992) under MP compared with CP or NT. Amounts of nitrate-N were relatively small even after 25 annual applications of N fertilizer. This could be due to moderate rate of N fertilizer in the present study, immobilization of applied N into the soil organic N pool, release of gaseous N over the winter and especially in early spring after snow melt and/or after occasional heavy rains in the growing season, and to nitrate-N leaching below the 30 cm depth. Downward movement of nitrate-N in the soil profile was not documented, because soil samples were taken only to 30 cm depth.
Conclusions
Long-term NT or CP would result in greater storage of organic C or N in soil and improve available nutrients in soil compared with MP. The one-time ploughing of long-term NT had no negative impact on organic C or N and available nutrients in soil. Elimination of tillage tended to cause some reduction in soil pH, especially in the 10–20 and 20–30 cm depths which may interfere with growth of acid-sensitive crops. Cropping system had little or no effect on soil organic C and N, available nutrients and pH.
Acknowledgements
The authors thank Valérie Bélanger, Christine Juge and Maxime Boucher for field sampling, K. Strukoff for technical help, D. Leach and Dr F. C. Stevenson for statistical analyses and the field crew of the Centre de Développement Bioalimentaire du Québec in La Pocatière for their technical and field assistance.
Financial support
This research received no specific grant from any funding agency, commercial or not-for-profit sectors.
Conflicts of interest
None.
Ethical standards
Not applicable.
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