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Soil organic matter as influenced by straw management practices and inclusion of grass and clover seed crops in cereal rotations.


In New Zealand, most arable crops have been grown as part of a mixed farming system consisting of 2-5 years of arable cropping followed by 2-5 years of grazed ryegrass/clover pasture (Haynes and Francis 1990). The length of the arable and pastoral phases varies with the relative profitability of the 2 types of farming. In recent years, the arable phase of the rotation has increased due to declining financial returns from livestock enterprises. This change in land use may result in a decline in soil organic matter because inputs of plant residues to arable soils are low compared with pastoral soils. In addition, cultivation accelerates organic C oxidation by improving aeration and by exposing aggregate-protected organic matter to microbial attack (Beare et al. 1994).

Burning of crop residues has traditionally been used in New Zealand as a way of clearing land to facilitate establishment of the next crop, and as a means of controlling pests and diseases. Because of concerns over the decline in organic matter and associated deterioration in soil structure under arable cropping (Francis et al. 1999), farmers are interested in incorporating post-harvest residues as a means of maintaining organic matter. Incorporating residues would also satisfy environmental groups who are concerned about atmospheric pollution from straw burning. Alternatively, surplus straw may be removed and used as livestock feed, animal bedding, fuel, and mulches for orchards and vineyards. However, the amount of straw currently used for these purposes is small in relation to annual production (Fraser and Francis 1996).

There is a dearth of information on the effects of straw management practices on soil organic matter under New Zealand conditions. Studies in North America, Europe, and Australia (summarised in Table 1) have given very inconclusive results. Straw retention (e.g. incorporation) has been shown to have little or no effect in many studies (Campbell et al. 1991; Nicholson et al. 1997; Soon 1998), whereas substantial gains in soil C have been observed in a number of long-term experiments (Thomsen 1993; Smith et al. 1997). Amongst the most positive results are those from long-term European studies where straw incorporation has been observed to increase organic C by between 5 and 50% (Powlson et al. 1987; Thomsen 1993; Smith et al. 1997). The wide range of responses in soil C to residue retention may be due to several factors including the duration of the study, amount of residue incorporated annually, the crop rotation followed, management history, initial organic matter content, and climatic factors. From a review of 8 studies in Western Europe (7-35 years duration), Smith et al. (1997) showed that when data from all sites were pooled, soil organic C increased with rate of straw incorporation according to the equation:

y = 0.11x + 0.19

where y is the annual increase in soil organic C (%) and x is the amount of straw incorporated (t/ha.year).

The labile soil organic matter fractions often show stronger responses to residue management practices than does total organic matter. For example, in a study in Denmark, Powlson et al. (1987) found that 18 years of straw incorporation increased soil organic C by only 5%, but mineralisable N increased by 40-50%. In an Australian cotton-growing soil, incorporation of cotton stubble doubled light fraction C (0-30 cm depth) compared with a stubble-burned treatment (Conteh et al. 1998). Changes in light fraction C accounted for a substantial proportion of the increase in total C in the stubble-incorporated soil. As well as providing an early indication of trends in total organic matter, the labile fractions are important in their own right because of their role in supplying nutrients such as N for plant growth and as a substrate for the soil microbial community (Janzen et al. 1992; Gregorich et al. 1994).

Overseas research on straw management has focused on long-term arable soils, frequently under cereal monoculture. The results may not be entirely applicable to New Zealand, where the period under arable cropping is relatively short, crops are rotated, and grass or clover seed crops are often included in the rotation. In this paper we report on effects of straw management practices and inclusion of grass/clover crops in the rotation on total and labile soil organic matter after 6 years.

Materials and methods


The experiment was initiated in November 1992 on a Wakanui silt loam [Immature Pallic Soil (Hewitt 1993) or Orthic Tenosol (Isbell 1996)] at Lincoln in the South Island of New Zealand (43[degrees]38'S, 172[degrees]30'E). The sand, silt, and clay contents of the soil were 60, 670, and 270 g/kg, respectively, and soil cation exchange capacity (at pH 7.0) was 15 cmolc/kg. Initial organic C contents of the 0-7.5 and 7.5-15 cm soil layers were 30 and 32 g/kg, respectively. Soil pH at the commencement of the experiment was 5.3-5.4 in the 0-15 cm layer and 5.6 in the 15-30 cm depth. The site is located on the Canterbury Plains where nearly 60% of New Zealand's arable crops are grown. Mean annual rainfall and temperature are 680 mm and 11.3[degrees]C, respectively. A summary of climatic information for each year of the experiment is given in Table 2. During the 4 years prior to the experiment the site had been under ryegrass/white clover pasture.

Three straw management treatments were evaluated: (1) straw incorporated by ploughing [straw had previously been mulched (~6-cm lengths)]; (2) straw burned; and (3) straw baled and removed (stubble was cut at a height of ~6 cm). Plots (100 [m.sup.2]) were cropped to wheat (Triticum aestivum L.) in 1992-93 and 1993-94, to barley (Hordeum vulgate L.) in 1994-95 and 1995-96, and to oats (Avena sativa L.) in 1996-97 and 1997-98. Each of the cereal crops was grown for 2 successive years only in order to minimise disease severity. A fourth treatment involved growing cereals in alternate years only. In this treatment, the cereals,were undersown with either grass or clover. After harvest of the cereals, the undersown crop was allowed to grow through the cereal straw and was grazed by sheep during the winter/early spring before being harvested for seed in summer (hereafter this will be referred to as the undersown treatment). In 1992-93, the undersown treatment was seeded to wheat, which was undersown with perennial ryegrass (Lolium perenne L.). White clover (Trifolium repens L.) was the undersown crop during the remainder of the experiment. It was sown under barley in 1994-95 and under oats in 1996-97. Treatments were laid out in a randomised block with 3 replicates.

All plots were ploughed (nominal depth of 15 cm) in the autumn of each year, except for undersown plots, which were cultivated in alternate years only (i.e. after harvest of cereal crops). Secondary cultivation took place in spring, immediately prior to drilling the cereal crops. The secondary cultivation usually consisted of 2 passes of a spring-tined cultivator with an attached rotary crumbier, followed by harrowing and rolling. Crops were usually sown in September/October and harvested in late summer (usually February). Soil fertility status was monitored regularly using routine tests for available P, K, and S. Each year, nutrient additions for cereal production were the same across all treatments. Because of high soil fertility levels following 4 years of pasture, the 1992-93 wheat crop required little fertiliser [160 kg/ha of Cropmaster 15 (15% N; 10% P; 10% K; 8% S)]. Wheat grown in 1993-94 was treated with 500 kg/ha of Cropmaster 20 (20% N; 10% P; 12.5% S) at sowing. The barley crops were treated with Cropmaster 20 at sowing (230 kg/ha in 1994-95 and 217 kg/ha in 1995-96) followed by side dressings of urea at tillering (200 kg/ha in 1994-95 and 110 kg/ha in 1995-96). The oat crops were fertilised with diammonium phosphate (18% N; 20% P; 1% S) at sowing (222 kg/ha in 1996-97 and 170 kg/ha in 1997-98). Ammonium sulfate (100 kg N/ha) was applied to the grass seed crop in 1993-94. The clover seed crops did not receive fertiliser in either 1995-96 or 1997-98.

To maintain soil pH at a level suitable for clover (>pH 6), slaked lime [Ca[(OH).sub.2]] was applied to the whole site at a rate of 2.8 t/ha in the spring of 1994 and ground limestone (5 t/ha) was applied to all plots in spring 1996. Soil pH (0-15 cm) measured at the end of the experiment ranged from 6.1 to 6.5 (mean 6.3). During the growing season, the cereal crops were regularly monitored, and where needed, fungicides were applied at recommended rates. Weeds were also monitored and were controlled with appropriate herbicides, applied at recommended rates. In summer, plots were irrigated when evapotranspiration amounted to 40 mm, calculated by the Penman (1948) equation as described by French and Legg (1979). Forty mm of water was applied at each irrigation event.

Straw decomposition

A litter bag technique (Cookson et al. 1998) was used to measure the rate of straw decomposition. Litter bags (20 by 20 cm) were prepared from fibreglass-nylon material (4 mm mesh) and filled with 25 g of wheat residue (equivalent to 6.25 t/ha). The wheat residue, which contained 5.9 g N/kg (C :N ratio ~70) and 165 g/kg of lignin, was collected from an area adjacent to the trial site in autumn 1993. Decomposition was measured by burying litter bags on 4 separate occasions, i.e. 2 April 1993, 15 July 1994, 30 October 1995, and 9 October 1996 (hereafter referred to as the 1993, 1994, 1995, and 1996 measurements, respectively). The same residues were used in all decomposition measurements and burial depth was 15 cm. In 1993 the litter bags were buried in a fallow area adjacent to the trial site. In 1994 measurements were made in plots of the straw-incorporated treatment (2 bags in each of the 3 replicate plots), and in 1995 and 1996, bags were buried in plots of the burned and straw-incorporated treatments (2 bags in each of 3 replicate plots). The litterbags were recovered at intervals of about 2 months during the first 6 months and less frequently thereafter. The longest burial period was 12 months except for the 1993 measurements, when the final recovery was 19 months after burial. After recovery of the litter bags from the soil, remaining straw was oven-dried at 60[degrees]C and ground (<0.5 mm). Subsamples were ashed in a muffle furnace (550[degrees]C for 5 h) to adjust residue recovery to an ash-free dry weight basis.

Soil sampling and analysis

After harvest of the 1997-98 crop, soil samples were taken from the 0-7.5 and 7.5-15 cm depths. Five cores (5.5 cm diameter) per depth increment were taken in each plot and bulked to give a composite sample. Additional cores were collected to measure bulk density. Samples to be used for C and N analyses were divided in two. One part was air-dried and the remainder was stored in field moist condition in a cold room (~0[degrees]C). The air-dry sample (passed through a 2-mm sieve and visible plant debris removed) was used to measure total C and N (Leco C/N analyser), light fraction organic matter, and mineralisable N. Light fraction was determined using a procedure similar to that described by Janzen et al. (1992). Soil (20 g) was suspended in 40 mL of NaI solution (specific gravity 1.7) and shaken overnight on an end-over-end shaker. The soil was allowed to settle for 48 h, after which the suspended material (light fraction) was transferred by suction to a Millipore filtration unit (0.45 gm). After washing with distilled water, the LF was dried at 70[degrees]C. The LF was then finely ground and analysed for C and N using a Carlo Erba C/N analyser (Carlo Erba, Milan, Italy). Nitrogen mineralisation potential was measured by anaerobic incubation at 40[degrees]C for 7 days (Keeney and Bremner 1966).

To measure C mineralisation (soil respiration), field-moist soil equivalent to 50 g of oven-dry soil was transferred to a biometer flask, wetted to field capacity with distilled water, and incubated at 20[degrees]C for 30 days. Evolved C[O.sub.2] was trapped in 0.1 M NaOH contained in the side arm of the biometer flasks and determined by titration with 0.05 M HCl (Biederbeck et al. 1994). Microbial biomass was determined on field moist soil by chloroform fumigation followed by extraction of microbial C and N in 0.5 M [K.sub.2]S[O.sub.4]. Carbon was measured by dichromate digestion (Voroney et al. 1989), and organic N by the persulfate oxidation method of Cabrera and Beare (1993). Microbial biomass C and N were estimated using an extraction efficiency factor of 0.38 for C (Vance et al. 1987) and 0.45 for N (Jenkinson 1988).

Statistical analyses

A standard 1-way analysis of variance was used to evaluate treatment effects on soil organic matter components, with separate analyses conducted for each sampling depth. Relationships between measured attributes were evaluated by standard correlation analysis.

Results and discussion

Carbon inputs in straw

Straw management practices had no significant effect on yield of straw (Table 3) or grain except in 1994-95 when the straw-incorporated treatment yielded less than the burned and removed treatments. These results are in keeping with other studies showing that straw burning or removal generally does not adversely affect crop yields (Biederbeck et al. 1980; Campbell et al. 1991). Straw yields shown in Table 3 can be considered typical of those achieved on the Canterbury Plains. Grain yields of wheat, barley, and oats average about 5.8, 5.3, and 4.8 t/ha, respectively (J. M. de Ruiter, pers. comm.). Assuming harvest indices of 0.5 (wheat), 0.45 (barley), and 0.4 (oats), straw yields would average about 6, 6.5, and 7 t/ha, respectively. In the incorporated treatment, the total amount of straw returned to the soil in the 5 growing seasons prior to sampling (straw from the 1997-98 season was not incorporated when the soil samples were taken) was about 25 t/ha. Assuming a straw C content of 45%, total C input in straw amounted to about 11.2 t/ha.

Straw decomposition

Information on the rate of straw decomposition is essential in order to evaluate the effect of straw incorporation on soil C. In all 4 years in which measurements were made, straw decomposition was rapid in the first 2 months of burial with 31-39% of the original mass being lost. Although, decomposition slowed thereafter, only 21-39% of the original mass remained after 10 months of burial. Decomposition rates measured in this study appear to be consistent with the model of Douglas and Rickman (1992), which was developed to describe crop residue decomposition in the USA (Fig. 1). That model estimates straw decomposition based on temperature (degree-days), soil moisture, and initial mineral N content of the straw from the following equation:

In([R.sub.S]/[I.sub.S]) = [k.sub.fN][f.sub.w] CDD

where In([R.sub.S]/[I.sub.S])is the natural logarithm of the ratio of straw remaining to the initial amount of straw, k is the decomposition constant, [f.sub.N] is a coefficient that is dependent on the N content of the residue, CDD is cumulative degree-days from the date of residue burial, and [f.sub.W] is a coefficient that ranges from 0.2 to 1 depending on soil moisture availability. For residue buffed in fallow soil,[f.sub.W] is assigned a value of 1 and for residue buried in cropped soil the value of [f.sub.w] is 0.8 (Douglas and Rickman 1992). Cumulative degree-days are calculated by summing, for each day, the mean daily temperature above a base temperature of 0[degrees]C.


The Douglas-Rickman model predicted decomposition well in 1993, 1994, and 1995 (Fig. 1). Although the model over-predicted decomposition in 1996, the overall agreement for the 4 years was satisfactory, with 86% of variability in straw recovery being explained by the model. The reason why decomposition was less than predicted in 1996 was not identified. There was no consistent difference in decomposition between straw buffed in the straw-incorporated v. straw-burned plots (results not shown). Previous work (Cookson et al. 1998) suggested that a history of straw incorporation may alter the composition and activity of the soil microbial community and result in more rapid decomposition than in soil where straw had previously been burned or removed. The results in Fig. 1 show that decomposition of buried residue can be estimated from cumulative degree-days (CDD). The initial rapid decomposition which occurs in the first 1000 CDD after burial is dependent on straw quality (N content) (Douglas and Rickman 1992), but within the range of N levels found in cereal straws, the effect is relatively small. For example, 1000 CDD after burial of high N residue (8 g N/kg, C:N ~50), the amount remaining was estimated at 53%. The corresponding value for low N (4 g N/kg, C:N ~100) residue was 65%. The average annual CDD for our trial site over the past 20 years was 4182 (range 3789-4522). Model calculations and field measurements are in agreement that, typically, only about 25% of residue would remain un-decomposed 1 year after incorporation.


We extrapolated the relationship between straw decomposition and CDD shown in Fig. 1 to estimate the amount of incorporated straw remaining in the soil at the time of sampling (April 1998). The proportion of incorporated straw remaining in the soil was estimated to be 0.5% of 1992-93 straw, 1.3% of 1993-94 straw, 3.5% of 1994-95 straw, 9.9% of 1995-96 straw, and 27.5% of 1996-97 straw. Of the 11.2 t/ha of straw-C incorporated, the amount retained in the soil was estimated to be 1.1 t C/ha. This estimate is based on rates of wheat straw decomposition. Decomposition rates of barley and oat straw may be slightly faster than that of wheat straw (Fraser and Francis 1996). Because the estimate of straw-C retention involved extrapolation of the measured decomposition v. time curve, the value of 1 t C/ha should be regarded as a rough approximation of C retention. From a Canadian experiment in which decomposition of [sup.14]C-labelled wheat straw, incorporated into a soil with texture similar to that at our site, was followed for 10 years, Voroney et al. (1989) showed that the amount of labelled C remaining in the soil (y) was described by the equation:

y = 80 [e.sup-1.5t] + 20 [e.sup.-0.077t]

where t is decomposition time in years. Applying this equation to our experiment provided an estimate of straw-C retention of 2.4 t/ha. Given that climatic conditions in Canterbury were more favourable for straw decomposition than those at the Canadian site [80 frost-free days annually; 1110 degree-days above 5[degrees]C (Voroney et al. 1989)], our estimate of straw-C retention of 1 t/ha does not appear unrealistic.

Soil organic matter

There was no significant difference (P > 0.05) in concentrations of total soil organic C or N between any of the treatments in the 0-7.5 or 7.5-15 cm layers (Table 4). As soil within the upper 15 cm of plots of the 3 straw management treatments was extensively mixed by tillage operations, there was relatively little difference in levels of total or labile organic matter between the 0-7.5 and 7.5-15 cm layers. Bulk density was not affected (P > 0.05) by straw management practices in either soil layer (mean bulk density was 1.14 Mg/[m.sup.3] at the 0-7.5 cm depth and 1.21 Mg/[m.sup.3] at the 7.5-15 cm depth). The total mass of C (or N) in the top 15 cm, expressed in tonnes per hectare, was not significantly (P > 0.05) affected by straw management (total C masses for the burned, incorporated and removed treatments were 54.6, 54.5, and 55.9 t/ha, respectively).

Light fraction organic matter and microbial biomass C and N were higher in soil from the undersown treatment compared with soil under continuous cereals (Table 4). The light fraction, which is composed mainly of partially decomposed plant residues, is highly labile and can change rapidly in response to management (Janzen et al. 1992; Biederbeck et al. 1994). Straw management had no effect on LF-N or LF-C. This contrasts with other studies showing a positive relationship between crop residue inputs and LF (Biederbeck et al. 1994; Conteh et al. 1998). Conteh et al. (1998), for example, showed that after just 3 years of cotton stubble incorporation, LF-C was about double that found in soil where stubble was burned. However, the amount of LF was much lower than in our soil, and the absolute increase in LF was not very large (LF-C increased from ~1 mg/kg soil where stubble was burned to ~2 mg/kg soil where stubble was incorporated). Between 3 and 5% of total C and 1.5-3% of total N in the top 15 cm layer of our soil was in the LF. These values are within the ranges reported by Janzen et al. (1992), i.e. 2-17% of soil C and 1-12% of soil N. The C:N ratio of LF from soil collected in the undersown treatment was lower than that from soil under continuous cereals, reflecting the influence of high N, legume-derived litter. Our LF C:N ratios (19-26) are somewhat wider than those reported from other studies on arable soils [e.g. 15-20 (Biederbeck et al. 1994), 12-21 (Curtin and Wen 1999)], possibly due to the presence of charcoal fragments, which were noted by microscopic examination. Charcoal in LF may be a result of previous crop residue burning (Elliot et al. 1991), but the straw management treatments imposed in this study did not affect the C:N ratio of LF.

Perhaps the best indicators of labile organic matter are C and N mineralisation, since these indices provide a direct measure of organic matter turnover (Biederbeck et al. 1994). The results in Figs 2 and 3 show that the undersown soil had significantly more mineralisable N and C in the 0-7.5 and 7.5-15 cm layers than the other treatments. Among the straw management treatments, there was a tendency for mineralisable C and N to be higher where straw was incorporated, although the effect was not significant (P > 0.05). There was a good relationship between C mineralisation and LF-C (Fig. 4), supporting suggestions (Janzen et al. 1992; Gregorich and Ellert 1993) that LF is a biologically active component of soil organic matter. There was a significant negative correlation (r = -0.71***) between C mineralisation and the C:N ratio of LF. This indicates that LF material with elevated C:N ratios tended to be less susceptible to microbial decomposition. Light fraction N was a slightly better predictor of C mineralisation than was LF-C (r = 0.97*** for LF-N v. 0.89*** for LF-C).


The results show that, after 6 years, straw management had little effect on soil C or N fractions. The soil C data are consistent with our estimate that only about 1 t C/ha of incorporated straw C would remain in the soil at the time of sampling. The trial site had about 55 t C/ha in the upper 15 cm layer and an increase in soil C of 1 t C/ha would not be detectable against background variability. The incorporated straw returned about 150 kg N/ha to the soil during the experiment. Although this amount of N would not have a measurable effect on total soil N, the labile organic N pools (mineralisable N, LF-N, microbial biomass N) might be expected to show an increase in response to incorporation of straw-N, as each of these pools contained <100 kg N/ha. Lack of an effect of straw incorporation on labile organic N may reflect a rapid turnover of this form of N, which would preclude its accumulation when straw-N is added in increments over a period of years.

The number of thermal units received at a site has a major influence on the decomposition of incorporated straw C (Douglas and Rickman 1992). The Canterbury Plains accumulate a relatively large number of degree-days compared with overseas locations where straw incorporation has been found to increase soil organic matter. For example, Rothamsted in southern United Kingdom averages about 3400 CDD per year (based on weather data for the 1960-1990 period) compared with 4200 CDD per year at our site. Studies at Rothamsted indicate a gain in soil C of 3 t/ha as a result of incorporation of straw at a rate of 9 t/ha.year for 7 years (Smith et al. 1997). Thus, even under cooler conditions and with large returns of straw compared with our study, the increase in soil C was relatively small. It seems reasonable to postulate that greatest increases in soil C will occur where straw is added in large quantities over a long period and cool conditions slow decomposition. Under the cool conditions prevailing in Uppsala, Sweden (~2600 CDD annually, estimated from 1995-99 data; P Chiverton, pers. comm.), soil C increased by 24 t/ha when straw was incorporated at a rate of 8.7 t/ha.year for 35 years (Smith et al. 1997).

Including seed crops in the rotation can have beneficial effects on labile organic matter fractions, although total organic matter was not increased within the time span of this experiment. In a study at an adjacent site, Francis et al. (1999) observed that growing ryegrass or clover for 3 years did not increase total soil C compared with barley-producing plots, but the merits of grass and clover were clearly evident when the plots were re-sampled 3 years later. An important factor contributing to increases in labile organic matter are large inputs of organic matter from the seed crops (including roots, above-ground plant debris, and dung from grazing sheep). Seed crops, particularly grass crops, have large root masses compared to cereals (Francis et al. 1999; Williams et al. 2000), and there is evidence that roots may be more important in contributing to soil C storage than are inputs of above-ground crop residues (Balesdent and Balabane 1996). Although clover has a more sparse root system than grass (Francis et al. 1999), legume-derived organic matter with a low C:N ratio may enhance the retention of C and N in soil (Drinkwater et al. 1998; Gregorich et al. 2001). Another important factor is that the undersown treatment was cultivated every second year only, whereas the straw management plots were cultivated annually. Local comparisons of direct drilled versus annually cultivated ryegrass plots have shown that after 6 years cultivated soil may have as much as 5 t/ha less C than direct-drilled soil (Francis et al. 1999). Increases in labile organic matter due to the inclusion of seed crops in cereal rotations may have beneficial effects (e.g. improved N supply) on the succeeding cereal crop. Some evidence of this can be seen in the 1996-97 data (Table 3), which show that the yield of oat straw following a clover crop was 8.2 t/ha, whereas straw yield in systems growing cereals continuously averaged 5.6 t/ha. Larger earthworm populations found in the undersown treatment compared with continuous cereal plots have been partly attributed to the greater abundance of food provided by labile organic matter and clover roots (Fraser and Piercy 1998).


Gains in soil C as a result of straw incorporation are likely to be relatively small in the New Zealand mixed farming system, where the length of the arable phase of the rotation is <6 years. Under the relatively warm conditions of the Canterbury Plains, about three-quarters of incorporated straw decomposes within a year of burial, and relatively little of the straw-C is stored in soil organic matter. The Douglas-Rickman model appears to be a useful tool for predicting the rate of incorporated straw decomposition under New Zealand conditions. An important advantage of the model is that it enables estimates of straw decomposition to be made using information that is readily available (air temperature data and straw placement information).

Including seed crops in cereal rotations may increase the labile pools of organic matter and this may have beneficial effects (e.g. improved nutrient supply) on the succeeding cereal crop. However, growing a seed crop every second year only may not be enough to increase total organic matter content.
Table 1. Effects of crop residue management on soil organic C,
as reported in the literature

Reference Length of Location Crop/rotation
 study (years)

Nicholson et al. 8-10 UK Cereals-oilseed
 (1997) rape
Nicholson et al. 8-10 UK Cereals-sugarbeet
Conteh et al. 3 New South Cotton
 (1998) Wales
Powlson et al. 18 Denmark Barley
Powlson et al. 18 Denmark Barley
Soon (1998) 10 Alberta, Barley
Rasmussen et al. 45 Oregon, USA Fallow-wheat
Campbell et al. 30 Saskatchewan, Fallow-wheat
 (1991) Canada
Barber (1979) 11 Indiana, USA Corn/maize
Singh et al. 25 Norway Cereal rotation
Smith et al. 7 Rothamsted, UK Winter wheat
Thomsen (1993) 10 Denmark Spring barley
Smith et al. 22 Germany Cereals

Reference Residue treatment Soil organic C

Nicholson et al. Incorporated (plough) 1.24% n.s.
 (1997) Incorporated (tine) 1.26%
 Burned 1.21%
Nicholson et al. Incorporated (plough) 0.98% n.s.
 (1997) Burned 1.01%
Conteh et al. Incorporated 1.11% *
 (1998) Burned 1.01%
Powlson et al. Incorporated 61.9 t/ha n.s.
 (1987) Burned 59.3 t//ha
Powlson et al. Incorporated (plough) 30.2 t/ha n.s.
 (1987) Burned 28.7 t/ha
Soon (1998) Removed 2.07%
 Incorporated (plough) 1.94% n.s.
 Incorporated (disc) 2.08% n.s.
Rasmussen et al. Incorporated 1.06%
 (1980) Burned (autumn) 0.99%
 Burned (spring) 1.05%
Campbell et al. Incorporated 38 t/ha n.s.
 (1991) Removed 38 t/ha
Barber (1979) Removed 2.75%
 Returned 3.25% *
Singh et al. Returned 3.12% *
 (1997) Burned 3.02%
Smith et al. Incorporated (18 t/ha.year) 79 t/ha n.d.
 (1997) Incorporated (9 t/ha.year) 75 t//ha
 Incorporated (4.5 t/ha.year) 76 t/ha
 Removed 72 t/ha
Thomsen (1993) Incorporated (12 t/ha.year) 1.74 % n.d.
 Burned 1.34%
Smith et al. Incorporated (6.5 t/ha.year) 60 t/ha n.d.
 (1997) Removed 50 t/ha

* P < 0.05. n.s., not significant; n.d., not determined.

Table 2. Rainfall (mm) and mean temperature ([degrees]C)
in spring (September-November), summer (December-February),
autumn (March-May), and winter (June-August) during the
six years of the trial

LTM, long-term mean

Year Spring Summer

 Rain Temp. Rain Temp.

1992-93 169 10.3 135 14.4
1993-94 195 10.5 190 15.4
1994-95 98 10.6 82 16.0
1995-96 153 10.7 64 16.7
1996-97 86 11.9 171 15.3
1997-98 81 11.4 73 17.5
LTM 151 11.2 163 16.4

Year Autumn Winter

 Rain Temp. Rain Temp.

1992-93 191 10.7 65 6.6
1993-94 146 11.4 177 6.3
1994-95 89 12.7 225 6.0
1995-96 159 11.5 215 6.1
1996-97 155 11.8 157 6.5
1997-98 97 13.3 n.a. n.a.
LTM 161 11.9 205 6.5

n.a., not applicable.

Table 3. Cereal straw yields (t/ha) for the four experimental
treatments during the four years of the trial

Values in parentheses are yields of straw for seed crops, i.e.
grass in 1993-94 and white clover in 1995-96 and 1997-98

Treatment Growing Season

 1992-93 1993-94 1994-95

Incorporated 3.6 (A) 5.8 3.3
Burned 3.6 5.8 4.7
Removed 3.6 5.7 4.3
Undersown 3.6 n.a. (4.0) 4.5

Treatment Growing Season

 1995-96 1996-97 1997-98

Incorporated 6.3 5.9 5.7
Burned 6.3 5.7 6.3
Removed 6.0 5.3 5.4
Undersown n.a. (7.9) 8.2 n.a. (11.1)

n.a., not applicable.

(A) Low yields in the 1992-93 season were due to
late sowing of the crop.

Table 4. Effect of straw management practices and inclusion of
seed crops in the rotation on total organic C and N, light
fraction C and N, and microbial biomass C and N

LF, light fraction; MB, microbial biomass

Treatment Total C Total N LF-C LF-N LF C:N MB-C MB-N
 (g/kg) (mg/kg) (mg/kg)

 0-7.5 cm soil layer

Incorporated 31.5 2.67 1060 44 24 414 29
Burned 31.3 2.67 1114 43 26 377 26
Removed 31.3 2.62 1006 39 26 380 27
Undersown 33.4 2.82 1707 88 19 510 39
Significance n.s. n.s. * ** * *** **
l.s.d. (P = 0.05) -- -- 445 22 4 44 7

 7.5-15 cm layer

Incorporated 31.6 2.69 968 44 24 310 22
Burned 30.9 2.62 980 43 26 348 24
Removed 31.2 2.64 948 39 26 342 26
Undersown 32.1 2.77 1160 88 19 405 39
Significance n.s. n.s. * ** * * **
l.s.d. (P = 0.05) -- -- 132 22 4 60 8

* P < 0.05; ** P < 0.01; *** P < 0.001; n.s., not significant.


We thank Richard Gillespie for his conscientious management of the trial site, and Fran McCallum, Jacqueline Piercy, and Charles Wright for technical assistance. This work was carried out as part of Foundation for Research, Science and Technology contracts C02302, C02613 and C02X0024.


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Manuscript received 20 November 2001, accepted 28 August 2002

D. Curtin (A,B) and P. M. Fraser (A)

(A) New Zealand Institute for Crop and Food Research, Private Bag 4704, Christchurch, New Zealand.

(B) Corresponding author; email:
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Author:Curtin, D.; Fraser, P.M.
Publication:Australian Journal of Soil Research
Geographic Code:8NEWZ
Date:Jan 1, 2003
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