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Effect of cropping practices on soil organic carbon: evidence from long-term field experiments in Victoria, Australia.

Introduction

Increasing the storage of soil organic carbon (SOC) in grain production systems by using alternative management practices is a widely suggested way of helping to mitigate rising atmospheric carbon dioxide concentrations while at the same time improving soil functioning to maintain or increase agricultural productivity (Sauerbeck 2001; Lai 2004; Sparling et at. 2006; Lai and Follett 2009). It is well established that conversion of native vegetation systems to cultivated agriculture reduces SOC because of reduced C input and accelerated C loss through decomposition and erosion (Dalai and Mayer 1986; Aguilar et at. 1988; Bravo-Garza and Bryan 2005; Dalai et at. 2005). Thus, management practices that increase C input or reduce C loss may result in net C accumulation. For dryland grain production systems, the most commonly proposed practices for enhancing soil C storage are reduced tillage, retention of harvest residues and alternative crop rotations (Dalai et at. 1991; Swift 2001; Lai 2004).

Numerous field studies, many of them conducted in North America, have reported greater levels of SOC under reduced tillage and stubble retention (together commonly referred to as conservation tillage) than under traditional tillage and stubble removal practices (Beare et at. 1994; Dick et at. 1998; Yang and Kay 2001; West and Post 2002; Alvarez 2005). This has given rise to the widespread view that substantial SOC sequestration may be achievable in arable soils by using reduced tillage and stubble retention (Lai 2004; IPCC 2007). However, recent studies suggest that the potential for C accumulation under conservation tillage practices may be less than initially estimated (Baker et al. 2007; Angers and Eriksen-Hamel 2008; Luo et al. 2010a). In long-term (14-40 year) experiments in Australia, reduced tillage and stubble retention have been reported to increase SOC in some situations (Chan et al. 2011; Dalai et al. 2011; Conyers et al. 2012) and to have no significant effect in others (Fettell and Gill 1995; Latta and O'Leary 2003; Dalai et al. 2011) compared with conventional practices. Thus, the influence of reduced tillage and stubble retention on SOC remains equivocal.

The effect of various crop rotations on SOC has been studied in many counties, including Australia. Manipulations to rotations usually achieve increased cropping intensity (more crops per year), increased crop diversity or both. The traditional practice of bare fallowing has been shown to be detrimental to SOC levels, whereas replacement of fallow with continuous cropping may result in a relative increase in SOC, particularly when fallow is replaced by pasture and most notably with leguminous pasture (Ridley and Hedlin 1968; Janzen 1987; Grace et al. 1995; Crocker and Holford 1996; Calegari et al. 2008a, 20086).

Despite the considerable research that has been undertaken to measure the effects of conservation tillage and rotations on SOC, predicting how SOC responds to these practices remains difficult because of the slowness of change and the apparent site specificity of the effects (Alvarez 2005). Therefore, scientists must rely on simulation models that can accommodate long time frames and complex interactions among climatic, soil and management factors. One of the most useful functions of long-term field experiments is their ability to provide data against which simulation models can be tested.

The objective of the present study was to compare SOC stocks under various tillage, residue management and rotation treatments in three long-term field experiments in the two main cropping regions of Victoria (Wimmera and Mallee, which differ in rainfall and soil type). Our hypotheses were that SOC stocks are increased by: (1) minimum tillage rather than traditional tillage; (2) continuous cropping, rather than crop-fallow rotations; and (3) phases of crop legumes or pasture in rotations, relative to continuous cropping with cereals.

Methods

Field experiments

MCI 4

The MC14 experiment is located near Walpeup, in the Mallee region of Victoria, where the climate is semi-arid (Peel et al. 2007), with long-term mean annual rainfall of 325 mm. The site is a plain (swale) landform, and the soil is a gradational Calcarosol (Isbell 2002). Basic site data are presented in Table 1. The experiment has been described in more detail by Latta and O'Leary (2003).

The experiment was established in 1982 as a split-plot design with three rotation treatments as main plots and two fallow management treatments as subplots (Table 2). The rotation treatments were: fallow/wheat (F/W) over 2 years; pasture/fallow/wheat (Pa/F/W) over 3 years; and pasture/ wheat (Pa/W) over 2 years. The fallow management treatments were: traditionally tilled fallow, with crop stubble incorporated by ploughing after harvest, and traditional drilling of the following crop (TT); and zero-tilled fallow with crop stubble left standing after harvest and direct drilling of the following crop (ZT). The Pa/F/W and F/W rotations had long fallow periods (10-18 months) and the Pa/W had short fallow periods (1-4 months). Each treatment was replicated three times in a randomised complete block design with phase replication (every crop in every rotation was represented every year), giving a total of 42 experimental plots. The main plots were 8 m wide x 40 m long, and the subplots 4 m wide x 40 m long.

Wheat (Triticum aestivum, 'Millewa' 1982-85, 'Meering' 1986-96, 'Ouyen' thereafter) was sown with 12kg P [ha.sup.-1] as superphosphate but no nitrogen (N) fertiliser. Crops in the TT treatments were sown with wide points and harrows, whereas those in the ZT treatments were sown with narrow points and light harrows. Medic pasture (Medicago truncatula, a mixture of 'Jemalong' and 'Paraggio') was sown at 10 kg [ha.sup.-1]. The pasture was maintained free of grass weeds by spraying with selective herbicides. Pasture was slashed two to three times during each growing season, with cuttings retained on the plots. Weeds in the fallow were controlled in the TT treatment by repeated tillage with a wide-tined implement (scarifier or chisel plough) and in the ZT treatment by spraying with herbicides. There were four to six cultivations and sprayings per year in the Pa/F/W and F/W treatments and one to three cultivations in the Pa/W treatment.

Longerenong Rotation 1

The Longerenong Rotation 1 (LR1) experiment is located near Longerenong in the Wimmera region of Victorian, also in a semi-arid climate but with a long-term mean rainfall of 425 mm. The experimental site is on a slight (<2%) slope and the soil is a Vertosol (Isbell 2002; Table 1). The experiment was established in 1916 to investigate the influence of crop rotations and fallowing on wheat yields and has been described by Hannah and O'Leary (1995).

LR1 has seven rotation treatments (Table 2) in a phase-replicated but spatially unreplicated design (i.e. a total of 19 experimental plots). In 1998, the experiment was split into two blocks: (1) the South Block, which was thereafter sown with crop varieties resistant to cereal cyst nematode (Heterodera avenae) and was sampled for the present study; and (2) the North Block, sown to nematode-susceptible varieties. The treatments were: continuous wheat (W); fallow/wheat (F/W); pasture/fallow/wheat (Pa/F/W); barley/field peas/wheat (B/Pe/ W); oats/field peas/wheat (O/Pe/W); oats/fallow/wheat (O/F/W); and oats/pasture/fallow/wheat (O/Pa/F/W). Plots were 14 m wide x 50 m long.

From 1916 until 1938, the pasture was Wimmera (annual) ryegrass (Lolium rigidum L.), grazed by sheep once or twice per year. This was replaced in 1939 by oats (Avena sativa), which was grazed by sheep until the late 1980s and slashed thereafter (with cuttings retained on the plots). No pasture legumes were included except in 2006 when annual medic (M. truncatula 'Mogal') was used instead of oats and was grazed for that one season. The wheat cultivars were 'Federation' (1916-34), 'Free Gallipoli' (1935), 'Ghurka' (1936-85), 'Cocamba' (1986-88), 'Kellalac' (1989-93), 'Meering' (1994), 'Goroke' (1995-98), 'Goldmark' (1999- 2004) and 'JNZ Clearfield' (2005-10). Barley (Hordeum vulgare) cultivars were 'Galleon' (1986-98) and 'Barque' (1999 to present); the oat cultivars were 'JB7622' (1986-87), 'Wallaroo' (1988), 'Marloo' (1989-92) and 'Patoroo' (1993 to present); and the field pea cultivars (Pisum sativum) were 'Dun' (1916-72), 'Derrimut' (1973-79), 'Dun' (1980-2004), 'Parafteld (2005-06) and 'Kaspa' (2007 to present). Crops were sown with narrow point drills, wheat at 81 kg [ha.sup.-1], barley at 66 kg [ha.sup.-1], oats at 78 kg [ha.sup.-1] and peas at 128 kg [ha.sup.-1]. After harvest, the entire site was grazed by sheep and remaining stubble was burnt before sowing or weed control operations. Weeds were controlled in the fallows by cultivation with tined implements (approximately four workings per year); herbicides were also used as they became available, beginning in the mid-1950s.

The wheat plots received annual applications of animal manure (126 kg [ha.sup.-1]) from 1916 to 1931 and superphosphate (11.8 kg P [ha.sup.-1]) from 1932 to 1993. Since 1998, superphosphate has been applied annually to wheat and barley (10.4 kg P [ha.sup.-1]) and peas and oats (5.2kg P [ha.sup.-1]). No fertiliser has been applied to the grazed oats and no N fertiliser has been applied to any of the treatments.

Sustainable Crop Rotations in Mediterranean Environments

The Sustainable Crop Rotations in Mediterranean Environments (SCRIME) experiment is also located at Longcrenong, adjacent to the LR1 experiment, on a slight (<2%) slope and Vertosol soil (Isbell 2002; Table 1). The site had been under long-term cropping before SCRIME was initiated.

SCRIME was established in 1998 to assess the effects of modern cropping practices on production and soil conditions. It comprises 10 rotation-tillage treatments, eight of which are considered in this paper: continuous wheat with reduced tillage (W-RT); canola/wheat/field peas with zero tillage (C/W/Pe-ZT), which, before 2003, had been continuous wheat with high fertiliser (twice the standard application rate of P and N) and reduced tillage; field peas/wheat/barley with reduced tillage (Pe/ W/B-RT); vetch/wheat/barley with reduced tillage, the vetch managed as a green manure (V/W/B-RT); canola/wheat/field peas with reduced tillage (C/W/Pe-RT); canola/wheat/field peas with traditional tillage (C/W/Pe-TT); luceme/luceme/luceme/ canola/wheat/field peas with reduced tillage (L/L/L/C/W/PeRT); and fallow/wheat/chickpea with reduced tillage (F/W/ Ch-RT). The treatments are summarised in Table 2. Each treatment was replicated three times, with phase replication (i.e. 75 plots). Plots were 14 m wide x 36 m long.

Cultivation in the TT treatments was with a disc plough and scarifier or harrows as required after harvest, with crop stubble retained and incorporated into the soil. Under RT and ZT, the plots were not cultivated other than at sowing, where a narrow point drill was used. Under RT, stubble was burnt if required (i.e. when stubble loads were high) before sowing in April; under ZT stubble was retained standing on the surface.

Phosphorus fertiliser was applied as monoammonium phosphate (MAP) to wheat and canola at 11.3 kg P [ha.sub.-1] as single superphosphate to lucerne at 9.9 kg P [ha.sub.-1] in the first year of the 3-year phase only and as single superphosphate to field pea at 16 kg P [ha.sup.-1]. The P fertilisers were banded with the seed (~50mm depth) at sowing in each of the cropping phases. Nitrogen fertilisers as MAP (banded) or urea (direct drilled) were applied to wheat and canola at 3 5 kg N [ha.sup.-1]. Lucerne was slashed at flowering and the material left on the plots. Weeds, insect pests and diseases were controlled chemically using conventional practice.

Wheat ('Goldmark' 1998-2006; 'JNZ Clearfield' thereafter) was sown at 81 kg [ha.sup.-1]; barley ('Gairdner') at 65 kg [ha.sup.-1]; canola (Brassica napus 'Beacon') at 5 kg [ha.sup.-1]; lucerne (Medicago sativa 'Pioneer L90') at 10 kg [ha.sup.-1]; medic (M. truncatula 'Mogul') at 20kg [ha.sup.-1]; vetch (Vicia benghalensis 'Popany') at 25 kg [ha.sup.-1]; field peas (Kaspa type) at 120 kg [ha.sup.-1]; and chickpea (Cicer arietinum, TCCV 96836') at 118 kg [ha.sup.-1].

Soil sampling and analysis

Soil samples were taken from MC14 in December 2009 (after 28 years of treatment), from LR1 in March 2010 (94 years of treatment) and from SCRIME in March 2010 (12 years of treatment). Cores were taken at randomly selected positions (10 in MC14, eight in LR1 and 15 in SCRIME) within each plot to 0.3 m depth, with care being taken to ensure the cores were intact and not compressed. The cores were cut into depth increments of 0-0.1, 0.1-0.2 and 0.2-0.3 m and pooled according to depth. Sampling was performed using a hydraulic soil sampler, with sampling tubes of 44-50 mm internal diameter.

The soil samples were dried to constant weight at 40[degrees]C and weighed, then crushed using a Retsch Jaw Crusher (Retsch, Haan, Germany) set to 2 mm before being passed through a 2-mm sieve. Material retained on the sieve (mineral or organic fragments) was removed and weighed (termed 'gravel' because organic material contributed negligibly to the mass of this fraction). Subsamples of the <2 mm soil (fine earth) were dried to constant weight at 105[degrees]C and the oven-dry weight recorded. Further subsamples were finely ground (for 180 s) in a Restch MM400 mill and analysed for total C (TC) using dry combustion on a LECO C144 (LECO Corporation, St Joseph, Ml, USA) analyser. These samples were also tested for the presence of inorganic C (1C) using hydrochloric acid. Where necessary, IC was removed from a second subsample by treating with sulfurous acid and the TC analysis was repeated. In these cases, IC was calculated as (TC before acid treatment - TC after acid treatment). Organic C (OC) was calculated as TC - IC. The analytical methods are described by Baldock et al. (2013).

Calculations and statistical analyses

Soil bulk density (Mg [m.sup.-3]) was calculated from the dry mass of the soil sample (fine earth + gravel) divided by the volume of the soil sample. SOC stock (Cs; Mg [ha.sup.-1]) was calculated for the 0-0.1, 0.1-0.2 and 0.2-0.3m depths using the equation:

[C.sub.S] = D x [C.sub.C] x BD x (1 - G) x 10

where D is the depth interval (m), [C.sub.c] is SOC concentration (mg[Cg.sup.-1] soil), BD is bulk density (Mg[m.sup.-3]) and G is the proportion of gravel. In order to compare SOC stocks without the effects of differences in BD, we calculated SOC stock on an equivalent soil mass basis (Ellert and Bettany 1995). First, the mass of the entire 0-0.3 m soil layer was calculated for each experimental plot. The 10th percentiles of the mean soil masses in MC14 and SCRIME were chosen as the equivalent soil mass for all treatments. BD in LR1 differed slightly from that in SCRIME (the same soil type), so the mass for SCRIME was also used for LR1. The equivalent soil mass was 3920 Mg [ha.sup.-1] for MC14 and 3400 Mg [ha.sup.-1] for LR1 and SCRIME. The SOC stock in the equivalent soil mass was calculated by adding the SOC stock in the 0-0.1 and 0.1-0.2 m layers to varying proportions of the SOC stock in the 0.2-0.3 m layer to achieve the equivalent soil mass. Where the total soil mass to 0.3 m was less than the 10th percentile, it was assumed that the SOC concentration in the extra soil was the same as in the 0.2-0.3 m layer. All soil weights are on an oven-dry (105[degrees]C) basis.

Cumulative plant residue (stubble plus root) inputs were estimated for rotations in LR1 and SCRIME using grain yield data (collected from LR1 in 2001-11 and from SCRIME in 1998-2010; R. Armstrong, unpubl. data). It was assumed that the harvest index was 0.40 (measured in SCRIME), that the root: shoot ratio was 0.43 (Richards et al. 2005) and that 90% of crop stubble was returned to the soil under stubble retention and 20% returned under stubble burning (Mitchell et al. 2000). The available data did not enable residue returns to be estimated for rotations with pasture phases.

Soil C and BD data were analysed by analysis of variance (Table 3) and relationships between variables analysed by correlation using GENSTAT 15th edition (Payne et al. 2012). Unless stated otherwise, statistical significance is at the P< 0.05 level.

Results

We present SOC concentrations (mg [g.sup.-1]) in the 0-0.1 m depth and SOC stocks (Mg [ha.sup.-1]) in the equivalent soil mass (~0-0.3m depth). Concentration is a less reliable measure for comparing treatment effects because it does not take into account differences in BD (Ellert and Bettany 1995), but it is shown here so that the findings of this work can be more easily related to earlier research on SOC, much of which reports surface concentrations only. However, our emphasis is on SOC stocks.

MC14

The SOC concentration (0-0.1 m) averaged 6.7 mg [g.sup.-1] and stocks (~0-0.3m) averaged 19.7 Mg [ha.sup.-1] (Table 4). SOC stocks were significantly affected by tillage (P<0.05) and rotation (P < 0.001) treatments, but not by the phase of the rotation at the time of sampling, and there were no significant interactions among these factors. Relative to traditional tillage, zero tillage increased SOC stocks by an average of 1.5 Mg [ha.sup.-1] (8%). Although these trends were apparent in all rotations, the effect of tillage attained statistical significance (P< 0.05) only in the Pa/F/W rotation. The SOC stocks in the rotation treatments was in the order Pa/W > Pa/F/W > F/W, and these differences were significant under both tillage treatments. Carbon stocks under Pa/W were greater than under Pa/F/W by 1.37 Mg C [ha.sup.-1] or 8% (the effect of elimination of fallow). Carbon stocks under Pa/F/W were greater than under F/W by 3.5 Mg C [ha.sup.-1] or 21% (the pasture effect). The effect of elimination of fallow plus inclusion of pasture (Pa/W compared with F/W) was 4.9 Mg C [ha.sup.-1] or 29%.

Approximately 40%-50% of the differences due to tillage and rotation were measured in the 0-0.1 m layer (data not shown). The trends in surface SOC concentrations were similar to those for the deeper SOC stocks, with concentration and stock being strongly correlated (r = 0.96). BD did not differ significantly among treatments (mean 1.41 Mg [m.sup.-3] in the 0-0.3 m depth).

LR1

In LR1, the SOC concentration (0-0.1 m) averaged 8.0 mg [g.sup.-1] and SOC stocks (~0-0.3m) averaged 24.9 Mg [ha.sup.-1] (Table 5). Errors were relatively large in this experiment due to its partially replicated design. Nevertheless, rotation effects (P<0.01) were evident in both SOC concentrations and stocks, with the two cereal-pea rotations (i.e. B/Pe/W (L4) and O/Pe/W (L5)) containing more SOC than the other rotations, which did not differ significantly. The difference in SOC stock between rotations L4 and L5 and continuous wheat was 6.7-8.1 Mg C [ha.sup.-1] or 29%-35%. The magnitude of the relative difference in SOC concentration between L4 and L5 and the other treatments was maintained in the 0.1-0.2 and 0.2-0.3 m depths (data not shown).

Surface SOC concentrations were strongly correlated with the deeper SOC stocks (r=0.90). BD in the 0-0.3 m depth was greater under F/W (L2; 1.28 Mg [m.sup.-3]) than in the other treatments, which did not differ significantly (mean 1.13 Mg [m.sup.-3]). The estimated cumulative crop residue (stubble + roots) retained on the plots was strongly correlated with SOC stocks (r = 0.97; n = 5; He 0.01).

SCRIME

The SOC concentration (0-0.1 m) in SCRIME averaged 9.7 mg [g.sup.-1] and stocks (~0-0.3m) averaged 25.2 Mg [ha.sup.-1] (Table 6). Again, SOC stocks differed significantly among rotations (P< 0.001) but not among phases in the rotations at the time of sampling. The F/W/Ch-RT rotation (SB) had significantly lower SOC stocks than all the other rotations. The greatest SOC stocks were under rotations SI, S3, S5, S6 and S7, which were not significantly different from each other and were 2.6-3.0Mg [ha.sup.-1] (11 %-13%) greater than under SB. Rotations S2 and S4 were intermediate, and 1.5-1.7 Mg [ha.sup.-1] (~7%) greater than SB. Most (55%-75%) of the increases in SOC stocks, relative to rotation SB, were in the 0-0.1 m depth (data not shown). Carbon stocks in the three C/W/Pe rotations showed small differences, with S5 (traditional till, stubble incorporated) significantly greater than S4 (reduced till, stubble burnt) by 1.3 Mg ha but S7 (zero till, stubble left standing) not significantly different from cither.

There were no significant differences in BD (mean 1.10 Mg [m.sup.-3], 0-0.3 m), and SOC concentrations in the 0.1m depth were highly correlated with SOC stocks in the deeper soil (r = 0.87). SOC stocks were not significantly correlated with the estimated cumulative retention of stubble + roots (r = 0.59; n = 6; P > 0.05) in the non-pasture treatments.

Discussion

Effects of tillage and stubble management

The effects of tillage and stubble management on SOC are interrelated (Dalal et al. 2011). In LR1, where tillage and stubble management were uniform across all treatments (traditional tillage with stubble burnt regularly), SOC stocks appears to be controlled by residue inputs, as seen by the strong positive relationship between cumulative retention of stubble + roots and SOC stocks across the (non-pasture) rotations. Hannah and O'Leary (1995) showed that wheat yields in LR1 are strongly related to rainfall in current and previous years, and that the response of wheat yield to rainfall varies markedly among the crop sequences (reflecting differences in antecedent soil moisture availability). Although cumulative wheat yields were not related to SOC stocks in the present study (data not shown), it is likely that water availability contributed to the observed responses in SOC through its effects on plant growth, residue inputs and decomposition.

The lack of a significant relationship in SCRIME between stubble + root inputs and SOC may reflect the diversity of tillage and stubble treatments in that experiment, or may reflect its comparatively young age. The present study did not allow the effects of tillage to be measured independently of stubble management. The SOC stocks in the zero- and traditionally tilled treatments in SCRIME and MCI 4 therefore represent the balance between any conservation or mineralisation of existing SOC through tillage practices and augmentation of SOC through addition of stubble, as well as any secondary effects of tillage and stubble retention on root residues, rhizodeposition or soil erosion. In these experiments, the minimum tillage treatments resulted in varying responses in SOC. In MC14, where treatments had been imposed for 28 years, zero tillage (with stubble left standing) slightly increased SOC stocks relative to traditional tillage (with stubble incorporated) under the Pa/F/W rotation, but not under the Pa/W and F/W rotations. These rotations differed in their frequency of tillage and the likely quantity of residue inputs; under Pa/W, tillage treatments were applied 1 year in 2, with relatively high residue inputs; under F/W, tillage treatments were applied every year with low residue inputs; and under Pa/F/W, tillage treatments were applied 2 years in 3, with intermediate residue inputs. Thus, detection of a zero tillage effect may require both sufficient residue inputs in the zero-tillage treatment and frequent tillage in the comparison treatment.

In the C/W/Pe rotation in SCRIME, zero tillage (with stubble left standing; S7) again did not increase SOC stocks relative to traditional tillage (with stubble incorporated; S5). That SOC stocks were slightly greater under traditional tillage with stubble incorporated (S5) than under reduced tillage with stubble burnt (S4) suggests that stubble retention was more important than minimum tillage for maintaining SOC in this experiment. Nevertheless, it is apparent that the tillage and stubble treatments examined in this study are not a reliable means of appreciably increasing SOC stocks in these environments.

In our experiments, as with most comparisons of conservation tillage and traditional management, the actual inputs of stubble to the soil may have been less under zero tillage, where stubble was left standing, than under traditional tillage, where stubble was incorporated. Wang and Dalai (2006) measured C losses of approximately 80% from stubble retained on the surface between the beginning and end of the fallow period in a wheat cropping system in Queensland, Australia. Similarly, Liu et al. (2009) calculated (using inverse modelling with RothC) that >70% of the C in surface-retained wheat stubble is lost rather than being input to the soil in New South Wales, Australia. These studies (Wang and Dalai 2006; Liu et al. 2009) were also in semi-arid locations, but where summer rainfall, and probably decomposition, were greater than at our sites. Possible reasons for these losses of C in standing stubble include biological and photochemical mineralisation, leaching of soluble components and displacement of small fragments by wind and water (Couteaux et al. 1995; Austin and Vivanco 2006; Wang and Dalai 2006). These potential interactions between tillage and stubble make it difficult to separate with certainty the effects of tillage and those of stubble retention.

The indications from the present study that the effects of zero tillage on SOC stocks are small (<1.7 Mg C [ha.sup.-1] or 9%) or non-significant correspond with findings elsewhere. For example, in the two longest-running conservation tillage trials in Australia, the differences in SOC stocks to 0.3 m depth between zero and conventional tillage were 2.2-3.6MgC [ha.sup.-1] (5-8%) after 25 years at Wagga Wagga in New South Wales (Chan et al. 2011), and were not significantly different after 40 years at Hermitage in Queensland (Dalai et al. 2011). In a meta-analysis of 39 Australian field experiments, Luo et al. (20106) concluded that, on average, SOC concentrations in the 0-0.1 m depth were increased by 3% under zero tillage, increased by 6% under stubble retention and increased by 16% where both zero tillage and stubble retention were practiced. However, the size of these effects may be diminished if SOC is expressed as stocks in the deeper soil because of the different effects of the tillage treatments on soil BD and because relative increases in SOC at the surface under zero tillage are sometimes negated by decreases at depth (Baker et al. 2007; Angers and Eriksen-Hamel 2008). Thus, a further meta-analysis by Luo et al. (2010a) using global data from 69 experiments found no significant difference in C stocks to 0.4 m depth between zero and traditional tillage.

Effects of fallow

In each of the three experiments, rotations containing bare fallow had less SOC than similar but continuously cropped rotations, the difference being 1.4-2.4MgC [ha.sup.-1] or 8%-12% (P<0.05 in MC14 and SCRIME; P > 0.05 in LR1). Similar effects of fallow on SOC have been shown in many other long-term studies around the world (Janzen 1987; Grace et al. 1995; Rasmussen et al. 1998; Govaerts et al. 2009). Studies in Australia and North America have reported reductions in SOC of 7%-49% (median 23%) under wheat-fallow rotations relative to continuous wheat in field experiments of between 22 and 68 years duration (Penman 1949; Drover 1956; Ridley and Hedlin 1968; Janzen 1987; Bremer et al. 1994; Grace et al. 1995; Campbell et al. 2000). The loss of SOC during the fallow occurs because C inputs are reduced (inputs are usually only through the growth of weeds), because decomposition may be increased by accumulation of moisture and inorganic N (O'Leary and Connor 1997a, 19976) or additional cultivation and because of increased soil erosion (Wischmeier and Smith 1978; Thomas et al. 2007). The magnitude of the decline in SOC under fallow has been shown to be proportional to the duration of the fallow (Janzen 1987; Grace et al. 1995), but is also influenced by other factors (see below). In the northern Victorian environment, elimination of fallow phases in crop rotations is likely to have a small protective effect on SOC stocks in the long term.

Effects of crop rotation

Phases of crop legumes and pasture in the rotations had varying effects on SOC across the three experiments. Inclusion of a pulse (field peas) where the grain was harvested in rotations had differing effects on SOC in the two Longerenong experiments: a marked increase in SOC stocks (6.8-8.0 Mg C [ha.sup.-1] or 29%-35%) in LRI (treatments L4 and L5) compared with continuous wheat (LI), but no apparent effect in SCRIME (S2, SI). The relative increase in LRI may be due to input of N, suppression of disease or sparing of water use by the peas (Kirkegaard et al. 2008), contributing to a greater quantity and quality of crop residue in following years (estimated residue inputs were also greatest in L4 and L5). Nitrogen input from the peas may also have increased stabilisation of residue C into soil organic matter, or reduced mineralisation of the original SOC (Fontaine et al. 2004; Kirkby et al. 2014). The lack of response to field peas in SCRIME was not due to insufficiency of N in the pea rotations; soil nitrate and grain protein concentrations in the wheat phases were greater in the pea rotations than in the continuous wheat rotation (in the pea and continuous wheat rotations, mean nitrate at sowing between 2009 and 2013 was 58 v. 38 kg N [ha.sup.-1], respectively, and mean grain protein between 2010 and 2012 was 11.65 v. 10.4%, respectively; R. Armstrong, unpubl. data).

Indeed, that there was a response to field peas in LRI, and not in SCRIME, may be because the cereal crops in LRI were poorly supplied with N compared with those in SCRIME; no nitrogenous fertiliser was applied in LRI as opposed to small (35 kg N [ha.sup.-1]) annual applications in SCRIME. It has been noted (Kirkegaard et al. 2008) that positive yield responses to such break crops may be less reliable under semi-arid conditions. Other studies report the long-term effect of including pulse crops (where the grain is harvested) in rotations to be a slight increase in SOC in surface soil (Grace 1996; Heenan et al. 2004; Chan et al. 2011) or no effect on SOC (Grace 1996; Gal et al. 2007) compared with continuous cropping with non-legumes. However, the potential shown in this study for grain legume crops to contribute to SOC accumulation under some circumstances merits further investigation.

Including vetch as a green manure in SCRIME (S3) resulted in slightly greater (P<0.05) SOC stocks than the field pea (S2), although not significantly greater than continuous wheat (SI). The green manure treatment (where the legume was incorporated into the soil) supplied significant amounts of N to the soil, as shown by the greater contents of soil nitrate at sowing and grain protein in the wheat phases following green manures compared with the continuous wheat rotation (nitrate 105 v. 38 kg N [ha.sup.-1] and protein 13.2% v. 10.5% in rotations S3 and SI, respectively; R. Armstrong, unpubl. data). There is little published information on the long-term effects of green manures on SOC in wheat-based cropping systems. Penman (1949) reported both increases and decreases in SOC (compared with continuous wheat) from including leguminous and non-leguminous green manures in rotation with wheat after 35 years of treatment at two sites in Victoria. Several long-term (15-19 year) studies in com and corn-wheat systems have shown substantial increases in SOC from inclusion of leguminous cover crops and green manures in rotations (Drinkwater et al. 1998; Vieira et al. 2009; Mazzoncini et al. 2011; Conceicao et al. 2013). However, in the water-limited northern Victorian environment, responses in SOC from green manures may be limited by low dry matter inputs.

Crop rotations with leguminous pasture (medic) every second or third year increased SOC stocks in MCI4 (by 3.5 Mg [ha.sup.-1] or 21%). This is consistent with increases in SOC observed elsewhere in Australia (Whitchouse and Littler 1984; Grace et al. 1995; Grace 1996; Chan et al. 2011) and in North America (Karlen et al. 2006) where pasture legume phases have been incorporated into grain production systems. The positive response to pasture legumes on SOC is sometimes diminished or negated by the presence of fallow in the rotation (Drover 1956; Holford 1981; Grace 1996; Janzen 1987), although this does not appear to be the case in MC14. Increases in SOC from phases of legume or grass-legume pastures are commonly ascribed to increased plant (grass or following crop) biomass resulting from N fixed by the legume (Dalai and Chan 2001). In this way, N availability was probably a contributor to the SOC response in MCI4; soil nitrate and grain protein concentrations differed among rotations, with Pa/F/W > Pa/W > FW, and yield trends of wheat were at least partially attributable to N availability (Latta and O'Leary 2003). The effect of phases of leguminous pasture on SOC has been observed to increase with the duration of pasture (Chan et al. 2011; Grace et al. 1995; Crocker and Holford 1996). Nevertheless, in SCRIME, 3 years of leguminous pasture (lucerne or medic) in a 6-year rotation (S6) did not raise SOC levels above those under continuous wheat (SI). This was despite higher levels of soil nitrate and grain protein in the wheat phases in the pasture rotation than in the continuous wheat rotation (89 v. 38 kg [ha.sup.-1] nitrate-N and 13.7% v. 10.4% protein in rotations S6 and SI, respectively; R. Armstrong, unpubl. data). The difference in the SOC response to pasture legumes in SCRIME and MCI 4 may be because of the higher N status in the cereal crops in SCRIME (which received N fertiliser) than MC14 (which did not). However, there are likely to have been multiple interacting influences on SOC in both SCRIME and MCI4, including C and N inputs and mineralisation, as well as water and P availability. Investigation of these interactions may clarify the circumstances under which legume pasture phases in rotations can contribute to increases in SOC. The pastures in the present experiments were not grazed (except LR1, which was grazed for the first ~70 years and ungrazed for the last ~20 years); it is possible that grazing by sheep, as may be done on farms, could modify the effects of pasture phases on SOC.

Inclusion of non-leguminous (oat), unfertilised, pasture in rotations had no significant effect on SOC in LR1 (treatments L3 and L7) compared with continuous wheat (LI). Non-leguminous pastures can promote SOC accumulation where C inputs are sufficiently large and sustained (Skjemstad et al. 1994). The lack of response here suggests that either: (1) C inputs under pasture were less than those under the succeeding crops (wheat or oats); or (2) pasture had a deleterious effect on growth of the succeeding crops. However, the finding of Hannah and O'Leary (1995) that wheat yields in LR1 were highest under the rotations with pasture phases and lowest under continuous wheat implies that the former was the case. Notwithstanding their productivity benefits, such low-fertility pastures cannot be recommended for promoting SOC accumulation.

Long-term effects

This is the first time that significant differences (TV 0.05) in SOC have been identified in these experiments. In MCI4, the SOC concentration in the 0-0.2 m depth was measured in 2001 (when the experiment was 19 years old); although not significant, the same trends as observed here in 2009 were evident in the rotation treatments, but not in the cultivation treatments (Latta and O'Leary 2003). In LRI, Penman (1949) measured SOC concentration in the top 6 inches (~0-0.15m) when the experiment was 32 years old, but there was no statistical analysis of the data. We converted our data to 0-0.15 m equivalents (by interpolation, assuming a linear rate of decline in SOC concentration in the 0-0.1 and 0.1-0.2 m depths in each treatment). We found very similar trends in the two sets of measurements (r=0.96; n = 7; P < 0.01), and the 2010 measurements were, on average, 11% lower than the 1949 measurements (Fig. 1). This apparent decline is probably real, and may be an underestimate because the analytical techniques used in the present study are likely to recover more SOC than the wet oxidation method (Walkley and Black 1934) used in 1949 (Nelson and Sommers 1996; Conyers et al. 2011). Regardless, there has evidently been little further development of treatment differences in LR1 during the past 61 years, suggesting that SOC stock was close to equilibrium in all treatments. This is consistent with the observation of Dalai et al. (2011) that most of the changes in a 40-year-old crop management experiment in Queensland occurred during the first 25 years. The slow rate of change in SOC in MC14 and SCR1ME supports suggestions from modelling analysis (Robertson and Nash 2013) that changes in SOC due to conservation tillage and improved rotations in Victoria are likely to be slow and difficult to measure within a 10-25-year time frame.

Interacting effects

The suggestion of this study that management of tillage, stubble and crop rotations can result in small increases in SOC under some circumstances but not others is consistent with much of the published literature (West and Post 2002; Alvarez 2005; Luo et al. 2010b). In our experiments, as in most experiments of this type, identifying the mechanisms of responses in SOC is complicated by confounding of the nominal treatment effects with other factors (Christensen et al. 2009). Changes in crop sequences commonly change inputs of residues and (where legumes are included) N, but may also affect the availability of soil water and other nutrients, or soil biological conditions. For example, mean annual P inputs varied between 4 and 14 kg P [ha.sup.-1] in the SCR1ME treatments, and soil water availability varied greatly among treatments in LR1 (Hannah and O'Leary 1995). Nitrogen availability sometimes limits SOC accumulation under conservation management (Sisti et al. 2004; Dalai et al. 2011; Kirkby et al. 2014), and the differences in N fertilisation in LR1 and SCR1ME may have contributed to the seemingly inconsistent effects of legumes in these two experiments. As discussed previously, tillage treatments are likely to have also affected stubble inputs (Wang and Dalai 2006; Liu et al. 2009). The fallow treatments differed not only in cropping intensity, but also in the amount and, sometimes, the type of cultivation. The extent to which SOC is changed by management also depends on the period over which the management has been practiced (Dalai et al. 2011), and the intensity with which it has been practiced (Grace et al. 1995; Karlen et al. 2013). Management has less effect on SOC where residue inputs are small (Dalai et al. 2011) and in dry environments (Chan et al. 2003; Ogle et al. 2005). The general lack of responsiveness in SOC in SCR1ME may have been because it was a relatively young experiment, because it was less N limited than LR1 and MCI4 or because of interactions among treatment effects. The site has received below-average rainfall for most of the years since the establishment of SCRIME (Bureau of Meteorology; see www.bom.gov.au/climate/averages/tables/cw_079028.shtml), and this may also have affected the SOC responses.

Conclusions

The practices examined in this study (stubble retention, zero tillage, elimination of fallow and crop or pasture legumes in rotations) resulted in a fairly wide range of SOC, but the effects of individual practices were variable, increasing SOC in some cases but not in others. We conclude that these management practices may not reliably increase SOC on their own, but that significant increases in SOC are possible in some situations through the long-term use of multiple practices such as stubble retention + zero tillage + legume N input + elimination of fallow. The circumstances under which increases in SOC can be achieved warrant further investigation.

http://dx.doi.org/10.1071/SR14227

Acknowledgements

This study was funded by the Victorian Department of Environment and Primary Industries, the Commonwealth Department of Agriculture and the Grains Research and Development Corporation. The authors acknowledge the contribution of Kristina Ricketts to the soil sampling activities, and thank Tom Baker and David Nash for their helpful comments on the draft manuscript.

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Fiona Robertson (A,F), Roger Armstrong (B), Debra Partington (A), Roger Perris (B), Ivanah Oliver (A), Colin Aumann (C), Doug Crawford (D), and David Rees (E)

(A) Department of Environment and Primary Industries, Private Bag 105, Hamilton, Vic. 3300, Australia,

(B) Department of Environment and Primary Industries, 110 Natimuk Road, Horsham, Vic. 3400, Australia,

(C) Department of Environment and Primary Industries, 255 Ferguson Road, Tatura, Vic. 3616, Australia,

(D) Department of Environment and Primary Industries, 1301 Hazeldean Road, Ellinbank, Vic. 3821, Australia,

(E) Department of Environment and Primary Industries, 32 Lincoln Square North, Carlton, Vic. 3053, Australia.

(F) Corresponding author. Email: fiona.robertson@depi.vic.gov.au

Table 1. Locations and soils of the field experiments
Climate data are 30-year averages (1980-2010)

                                   Latitude,           Elevation
Experiment     Location            longitude              (m)

MCI 4           Walpeup        35[degrees]7'12"S,         107
                              141[degrees]58'48"H
LR1           Longerenong   36[degrees]40'12.0000"S,      155
                               142[degrees]18'0"E
SCR1ME        Longerenong   36[degrees]40'12.0000"S,      155
                               142[degrees]18'0"E

               Annual    Mean annual
              rainfall   temperature
Experiment      (mm)     ([degrees]C)      Soil

MCI 4           325          16.8       Calcarosol

LR1             420          14.7        Vertosol

SCRIME          420          14.7        Vertosol

                Clay,        pH,
Experiment    0-0.3m (%)   0-0.3 m

MCI 4             47         7.6

LR1               68         7.4

SCRIME            68         7.7

Table 2. Summary of experimental treatments

               Age    Treatment  Treatment
Experiment   (years)     no.        code             Rotation

MC14           28        Ml        F/W-TT          Fallow/wheat

                         M2        F/W-ZT          Fallow/wheat

                         M3      Pa/F/W-TT     Pasture/ fallow/wheat

                         M4      Pa/F/W-ZT     Pasture/fallow/wheat

                         M5       Pa/W-TT          Pasture/wheat

                         M6       Pa/W-ZT          Pasture/wheat

LR1            94        L1          W                 Wheat
                         L2         F/W            Fallow/wheat
                         L3        Pa/F/W      Pasture/fallow/wheat
                         L4        B/Pe/W     Barley/field peas/wheat
                         L5        O/Pe/W      Oats/field peas/wheat
                         L6        O/F/W         Oats/fallow/wheat
                         L7       O/Pa/F/W   Oats/pasture/fallow/wheat
SCRIME         12        S1         W-RT               Wheat
                         S2      Pe/W/B-RT    Field pea/wheat/barley
                         S3       V/W/B-RT   Vetch-fallow/wheat/barley
                         S4      C/W/Pe-RT    Canola/wheat/field pea
                         S5      C/W/Pe-TT    Canola/wheat/field pea

                         S6      L/L/L/C/W/       Luceme/luceme/
                                   Pe-RT       luceme-fallow/canola/
                                                  wheat/field pea
                       S7 (A]    C/W/Pe-ZT    Canola/wheat/field pea

                         S8      F/W/Ch-RT     Fallow/wheat/chickpea

               Age    Treatment
Experiment   (years)     no.           Tillage           Stubble

MC14           28        Ml        Traditional till     Retained,
                                                      incorporated
                         M2           Zero till      Retained, left
                                                        standing
                         M3        Traditional till     Retained,
                                                      incorporated
                         M4           Zero till      Retained, left
                                                        standing
                         M5        Traditional till     Retained,
                                                      incorporated
                         M6           Zero till      Retained, left
                                                        standing
LR1            94        L1        Traditional till   Grazed, burnt
                         L2        Traditional till   Grazed, burnt
                         L3        Traditional till   Grazed, burnt
                         L4        Traditional till   Grazed, burnt
                         L5        Traditional till   Grazed, burnt
                         L6        Traditional till   Grazed, burnt
                         L7        Traditional till   Grazed, burnt
SCRIME         12        S1          Reduced till         Burnt
                         S2          Reduced till         Burnt
                         S3          Reduced till         Burnt
                         S4          Reduced till         Burnt
                         S5        Traditional till     Retained,
                                                      incorporated
                         S6          Reduced till         Burnt

                       S7 (A]         Zero till      Retained, left
                                                        standing
                         S8          Reduced till         Burnt

[A] Continuous wheat with high fertiliser
and reduced till between 1998 and 2003.

Table 3. Analysis of variance treatment structure
used for each of the field experiments

d.f., Degrees of freedom

Experiment    Source of variation          d.f.

MC14          Block stratum                 2
              Block x plot stratum
              Tillage                       1
              Rotation                      2
              Tillage x rotation            2
              Rotation x phase              4
              Tillage x rotation x phase    4
              Residual                      25
              Total                         40
LR1           Rotation                      6
              Residual                      12
              Total                         18
SCRIME        Block stratum                 2
              Block x plot stratum
              Rotation                      7
              Rotation x phase              15
              Residual                      56
              Total                         80

Table 4. Effects of rotations and tillage on soil organic C
in the MC14 experiment after 28 years

Treatment codes are explained in Table 2. Rotations followed
by different letters differ significantly (P<0.05). NS, Not
significant; TT, traditional till; ZT, zero till

Rotation          Tillage

           TT      ZT     P-value   Mean

                  C concentration, 0-0.1 m (mg [g.sup.-1])

F/W       5.6a    5.8a      NS      5.7a
Pa/F/W    6.3b    7.2a     <0.05    6.7b
Pa/W      7.3c    7.8b      NS      7.5c
Mean       6.4     6.9     <0.05     6.7

                  C stock, 0-0.3 m (A) (Mg [ha .sup.-1])

F/W       16.2a   17.6a     NS      16.9a
Pa/F/W    19.5b   21.2b    <0.05    20.4b
Pa/W      21.1c   22.4c     NS      21.7c
Mean      19.0    20.5     <0.05    19.7

(A) Approximate depth for equivalent soil
mass of 3920 Mg [ha.sup.-1].

Table 5. Effects of rotations on soil organic C in the LR1
experiment after 94 years

Treatment codes are explained in Table 2. Stubble grazed and
burned, with traditional tillage. Treatments followed by
different letters differ significantly (P<0.05)

                        C concentration   C stock (A)     Estimated
                           0-0.1 m        0-0.3 m (Mg     C inputs11
Treatment    Rotation   (mg [g.sup.-1])   [ha.sup.-1])   [ha.sup.-1])

L1              W            7.6a            23.3a           109
L2             F/W           6.8a            20.8a            89
L3            Pa/F/W         7.8a            24.9a            --
L4            B/Pe/W         9.8b            31.3b           197
L5            O/Pe/W         9.1b            30.0b           180
L6            O/F/W          7.4a            22.4a           122
L7           O/Pa/F/W        7.4a            21.6a

(A) Approximate depth for equivalent soil mass of 3400 Mg
[ha.sup.-1]

(B) Estimated cumulative root + stubble inputs.

Table 6. Effects of rotations and tillage on soil organic C
in the SCRIME experiment after 12 years

Treatment codes are explained in Table 2. Treatments
followed by different letters differ significantly
(P<0.05). TT, Traditional till; ZT, zero till; RT,
reduced till

                               C concentration
                                   0-0.1 m
Treatment       Rotation       (mg [g.sup.-1])

S1                W-RT             lO.Oabc
S2              Pe/W/B-RT           9.4ad
S3              V/W/B-RT            10.6c
S4              C/W/Pe-RT           9.3d
S5              C/W/Pe-TT          10.1 be
S6           L/L/L/C/W/Pc-RT        9.9ab
S7              CAV/Pe-ZT          9.9abed
S8              F/W/Ch-RT           8.2e

             C stock (A)      Estimated C
              0-0.3 m Mg       inputs (B)
Treatment    [ha.sup.-1])   (Mg [ha.sup.-1])

S1             25.7abc            24.1
S2              24.7ab            31.5
S3              26.1c
S4              24.6a             29.1
S5              25.8bc            52.6
S6              26.1c
S7             25.6abc            49.3
S8              23.0d             11.9

(A) Approximate depth for equivalent soil mass
of 3400 Mg [ha.sup-1].

(B) Estimated cumulative root + stubble inputs.
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Author:Robertson, Fiona; Armstrong, Roger; Partington, Debra; Perris, Roger; Oliver, Ivanah; Aumann, Colin;
Publication:Soil Research
Article Type:Report
Geographic Code:8AUST
Date:Sep 1, 2015
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