Australian Journal of Soil Research

Transcription

1 CSIRO PUBLISHING Australian Journal of Soil Research Volume 38, 2000 CSIRO Australia 2000 A journal for the publication of original research in all branches of soil science All enquiries and manuscripts should be directed to Australian Journal of Soil Research CSIRO PUBLISHING PO Box 1139 (150 Oxford St) Collingwood Telephone: Vic Facsimile: Australia jenny.fegent@publish.csiro.au Published by CSIRO PUBLISHING for CSIRO Australia and the Australian Academy of Science

2 Aust. J. Soil Res., 2000, 38, Crop rotation effects on soil carbon and physical fertility of two Australian soils Nelly Blair AC and G. J. Crocker B A Agronomy and Soil Science, University of New England, Armidale, NSW 2350, Australia. B New South Wales Agriculture, Tamworth, NSW 2340, Australia. C Corresponding author; ndeane@metz.une.edu.au Abstract The effect of using different crop rotations, including legumes and fallows, on soil structural stability, unsaturated hydraulic conductivity, and the concentration of different carbon fractions was examined in a long-term rotation trial established in 1966 on a Black Earth (Pellic Vertisol) and a Red Clay (Chromic Vertisol) soil. There was a large decrease in the concentration of soil carbon fractions following cropping and cultivation on both soils. The inclusion of some legume rotation crops resulted in an increase in labile carbon concentrations compared with continuous wheat or a long fallow treatment. Aggregate stability to wetting under both immersion and tension wetting was reduced as a result of cropping and cultivation for both soil types. However, there was an improvement in aggregate stability with immersion wetting, on the Red Clay soil, for the lucerne (Medicago sativa), clover (Trifolium subterraneum), and continuous wheat (Triticum aestivium) treatments compared with the long fallow. Similar results were found for the Black Earth soil; however, on this soil the medic (Medicago scutella) rotation also showed an improvement in soil structure. On the Red Clay soil there was a decrease in hydraulic conductivity (K) with cropping, at all tensions measured. K for the Black Earth soil was higher at 30 and 40 mm tension on the cropped soil than on the uncropped reference soil, but at 10 mm tension the reference soil had a higher K value than all rotations except the lucerne. There was a significant correlation between labile carbon and all determinations of aggregate stability for the Red Clay soil. Farmers should be encouraged to eliminate long fallowing and to adopt no-till techniques combined with the return of residues from either the primary crop or rotation crops which have a slower breakdown rate, as this management is likely to have a better potential for increasing soil carbon content and improving soil structure. The investigation of ways to better increase the quantity and quality of soil organic matter and hence soil chemical and physical fertility is necessary if long-term sustainable agriculture is to be possible. Additional keywords: infiltration, soil structure, total organic carbon. Introduction The continuous cultivation and cropping of many of the worlds soils, which previously supported native vegetation, have generally resulted in a reduction in soil carbon content and consequent destabilisation of soil structure. Gretton and Salma (1996) reported that as much as 80 million hectares, or 16% of agricultural land, in Australia was affected by soil degradation in To have systems which are both sustainable and productive requires the development of agricultural practices that not only maintain but also increase soil organic matter levels. Soil is also increasingly becoming a potential carbon sink for lowering atmospheric concentrations of carbon dioxide (Bruce et al. 1998; Poulsen et al. 1998). It has been well documented that the cultivation of soils results in structural degradation and decreased soil organic matter (SOM). Oades (1993) stated that the repeated cultivation of soils, combined with limited SOM inputs, will eventually result in CSIRO /SR /00/010071

3 72 N. Blair and G. J. Crocker major aggregate breakdown leaving the soil vulnerable to erosion and compaction. Many farming practices, such as fallows, burning, or removing crop residues, and conventional tillage practices remove organic matter from the system. Karlen and Cambardella (1996) suggested that changing to management such as reduced and no-till techniques, residue retention, the use of green manure crops and pasture leys, or the application of organic materials resulted in numerous beneficial effects on soil physical and chemical fertility. However, these processes may take as long as 100 years to reach the soil s maximum carbon storage (Bruce et al. 1998). SOM is made up of different fractions with turnover rates varying from hours to hundreds of years (Whitbread 1996; Chan 1997). Those fractions that are most sensitive to management practices are often the most important for both soil chemical and physical fertility. The measurement of these fractions can often give a more sensitive indication of short- and medium-term changes than is shown by the determination of total SOM. The measurement of the more labile carbon fractions by oxidation with KMnO 4, developed by Blair et al. (1995), has successfully been used by numerous researchers in SOM studies (Lefroy et al. 1993; Whitbread 1996; Bell et al. 1998; Blair et al. 1998; Whitbread et al. 1998). This method has been used in this study to investigate soil carbon dynamics and their relationship to soil structure. Whitbread et al. (1998) reported that declining SOM led to a decline in soil structure with lower infiltration and yield reductions. The introduction of legume crop rotations to increase the concentration of soil nitrogen and hence increase crop yields may potentially lead to increased SOM content and improved soil aggregate stability. This study examined the impact of legume rotations and fallows on the concentration of the different soil carbon fractions, aggregate stability, and unsaturated hydraulic conductivity on the two different soil types of this long-term experiment. Materials and methods In response to reduced grain yields and quality, a rotation trial was established in 1966 on adjacent Black Earth (Pellic Vertisol FAO, UNESCO classification) and Red Clay (Chromic Vertisol FAO, UNESCO classification) soils, 10 km south-east of Tamworth in north-western New South Wales, Australia. The mean daily temperature is 17.9 C and the average annual rainfall 671 mm (source: Australian Bureau of Meteorology). The Red Clay soil (0 100 mm depth) contained 320 g/kg of sand, 220 g/kg of silt, and 460 g/kg of clay, and the Black Earth soil (0 100 mm depth) contained 290 g/kg of sand, 170 g/kg of silt, and 540 g/kg of clay. Each trial comprised 6 rotation treatments arranged in a 6 6 Latin square. In Phase 1, , lucerne (Medicago sativa) was grown from 1966 until 1969 on the Black Earth, and until 1971 on the Red Clay, followed by wheat (Triticum aestivum) until In Phase 2, lucerne was grown on the 3 legume treatments until early The remaining 3 treatments were an annual legume [chickpea (Cicer arietinum)]/wheat rotation, a wheat/long fallow rotation, and continuous wheat. In late 1983 the 3 legume and the continuous cereal plots were sown to sorghum (Sorghum bicolor) and this crop was grown each year until The other 2 treatments were long fallow/sorghum and cowpea (Vigna unguiculata)/sorghum. Wheat was grown on all plots in Phase 3 commenced in 1988 with the sowing of the 3 legume plots to subterranean clover (Trifolium subterraneum), lucerne (Medicago sativa), or snail medic (Medicago scutellata). The remaining rotations were chickpea/wheat, long fallow/wheat, and continuous wheat. The legumes (subterranean clover, lucerne, and snail medic) were grown for 3 years followed by 3 years of wheat. The other treatments continued for the 6 years up to Wheat was grown on all treatments in 1994 and In 1996 the same legumes as were sown in 1988 were again planted. From 1981 to 1994, the plots were grazed with sheep following harvest, with sheep put on each experiment for 2 3 days. This was followed by cultivation and a 3-month cultivated fallow prior to sowing. This included one pass with a disk plough to a depth of 150 mm followed by 2 or more passes, depending on necessary weed control, with a scarifier to a depth of mm. From 1995 to 1997, the plots have been grazed and herbicides used for weed control with cultivation just prior to sowing using 3 passes with a scarifier to a depth of mm.

4 Crop rotations, soil carbon, and physical fertility 73 Measurement of unsaturated hydraulic conductivity In March 1997, just prior to ploughing for the next crop, in situ unsaturated hydraulic conductivity (K) was determined for the different rotations using tension infiltrometers designed by Perroux and White (1988). Quadruplicate measurements were made on 3 of the 6 replicates of each treatment for each soil type and also on a site adjacent to each experiment, which had been under pasture since at least the commencement of the trial in This was termed a reference sample. All rotations, except for the lucerne treatment, had no growing crop at the time of measurement. Sites were prepared by trimming all vegetation to ground level where necessary. A 5-mm-thick steel ring was then placed on the soil surface and filled with wet fine sand to act as contact material. The infiltrometer was placed on the levelled sand pad, and steady state infiltration was determined at 4 tensions (40, 30, 20, and 10 mm). By using simple capillarity theory it can be predicted that pores of 0.75, 1.0, 1.5, and 3.0 mm will drain at 40, 30, 20, and 10 mm tension, respectively. An automatic recording system using pressure transducers measured the rate of flow from the reservoir tube of the infiltrometer. This was logged to a laptop computer and the steady state flow for each tension (mm/min) was used to calculate K using the method of Ankeny et al. (1991). Following the determination of hydraulic conductivity the sites were allowed to drain for 24 h before soil samples were collected from mm depth (using a core of 80 mm diameter) from directly under the infiltrometers, for measurement of wet aggregate stability, dry aggregate size distribution, and concentrations of total and labile carbon. The sand used as the contact material was carefully removed from the soil surface prior to soil sampling. The 4 samples from within each plot were bulked and airdried. Large clods were gently broken by hand. Soil sample preparation The air-dry soil samples were broken down to pass through a 4-mm sieve. To achieve this, the soil was gently rolled on a board that had 4-mm-high ridges on the sides to maintain a gap between the board and the roller. This prevented total disruption of the sample and ensured that each soil sample received a similar energy input. Wet aggregate stability Wet sieving was undertaken by placing a 30-g soil sample on the top of a nest of 5 sieves of 2000, 1000, 500, 250, and 125 µm sizes with a diameter of 100 mm. A further 125-µm sieve was used for a lid. Soils were wet-sieved with both immersion and tension wetting. For immersion wetting, the air-dry soil was placed onto the 2000-µm sieve and immersed in distilled water for 30 s before sieving for 10 min through an amplitude of 17 mm at 34 cycles/min. For tension wetting, the soil was wetted on a sand pad at a tension of 40 mm before being immersed in distilled water and sieved as for immersion wetting. Following sieving, the sieves were drained and the soil dried at 40 C for 24 h prior to weighing. Mean weight diameter (MWD), percentage of aggregates >250 µm, and the percentage of aggregates <125 µm were calculated. Dry aggregate size distribution A dry sieving technique was used for measurement of the soils aggregate size distribution. A 30-g soil sample was placed on the largest mesh sieve of the set of sieves used for wet sieving. The soil was sieved by gently shaking by hand for less than 1 min to get as close as possible to the dry aggregate size distribution of the soil prior to wet sieving. The amount remaining on each sieve size was weighed and MWD, percentage of aggregates >250 µm, and percentage of aggregates <125 µm were calculated as above. Abbreviations For the aggregate stability determinations the following abbreviations are used: mean weight diameter, immersion wetting (MWDI), tension wetting (MWDT), dry aggregate size distribution (MWDD); percentage of aggregates >250 µm, immersion wetting (>250 I), tension wetting (>250 T), dry aggregate size distribution (>250 D); percentage of aggregates <125 µm, immersion wetting (<125 I), tension wetting (<125 T), dry aggregate size distribution (<125 D). Total and labile carbon and the derivation of the carbon management index Subsamples of the <4 mm soil were ground to <500 µm and total C (C T ) was determined by catalytic combustion on a Carlo Erba NA1500. Labile C (C L ) was determined by oxidation with 333 mm KMnO 4

5 74 N. Blair and G. J. Crocker according to the method of Blair et al. (1995). Carbon oxidised by 33 mm KMnO 4 [the more easily oxidisable C (C 33 )] (Lefroy et al. 1993) was also determined. The non-labile carbon (C NL ) equals C T C L. The carbon management index (CMI) derived by Blair et al. (1995) was calculated for each treatment for each soil using the reference values for the calculation. This is calculated as follows. Firstly, a carbon pool index (CPI) is calculated: CPI = Then a lability index (LI) is calculated: sample total C (mg/g) reference sample total C (mg/g) lability of C in sample soil LI = lability of C in reference soil C where: Lability of C(L) = C The carbon management index (CMI) can then be calculated as follows: CMI = CPI * LI * 100 Statistical analysis Analysis of variance was determined using the NEVA program developed by Professor E. J. Burr, Department of Computing Science, University of New England, Armidale, NSW, Australia. Data transformation was carried out if required. Treatment means were compared using Duncan s Multiple Range Test (DMRT) at P = Regression equations were determined using the Excel program. Reference soils were not included in any statistical analysis due to lack of replication. Results Soil carbon The impact of cropping and cultivation on soil carbon concentration can be seen by the large decrease in the concentration of all carbon fractions in all rotations when compared with the reference soils (Table 1). The largest decline was in C L for the Black Earth where cropping decreased C L by an average of 71% across all rotations. Red Clay The growing of clover and lucerne rotations increased C L by 41% and 32%, respectively, when compared with the long fallow (Table 1). The clover rotation also had 25% and 28% more C L than the grain legume and continuous wheat treatments (Table 1). These differences in C L are reflected in the different CMI values for the different rotations (Table 1). There was no significant difference between rotations for C 33, C T, and C NL (Table 1). Black Earth The growing of lucerne, clover, and medic rotations increased C 33 by 21%, 22%, and 23%, respectively, and C L by 33%, 32%, and 40%, respectively, when compared with the long fallow treatment (Table 1). These rotations also had a significantly higher CMI than did the long fallow (Table 1). The medic rotation also had a significantly greater concentration of C L than did the grain legume rotation (Table 1). C T and C NL were significantly higher in the medic rotation than either the grain legume or long fallow rotations. L NL C sample T = C reference

6 Crop rotations, soil carbon, and physical fertility 75 Table 1. Total C (C T ) and carbon fractions [C oxidised by 33 mm KMnO 4 (C 33 ); C oxidised by 333 mm KMnO 4 (C L ); non-labile C (C NL =C T C L )] (all mg/g), and carbon management index (CMI) for the different rotations and the reference soil for the Red Clay and Black Earth soils Values in the same row, for the different rotations and soils, followed by the same letter are not significantly different according to DMRT (at P = 0.05) Reference Long Lucerne Clover Grain Medic Continuous fallow legume wheat Red Clay C C L c 3.11ab 3.33a 2.67bc 2.85abc 2.61bc C T C NL CMI 44c 58ab 64a 50bc 54abc 50bc Black Earth C b 1.30a 1.31a 1.19ab 1.32a 1.20ab C L c 2.60ab 2.58ab 2.19bc 2.75a 2.42abc C T b 16.00ab 16.00ab 14.97b 16.90a 15.37ab C NL b 13.40ab 13.42ab 12.77b 14.15a 12.95ab CMI 21b 29a 29a 24ab 30a 27ab Aggregate stability There were some 350% more aggregates <125 µm in the mean of the rotations than the reference soil for the Black Earth with immersion wetting (Table 3). Much smaller differences were found for tension wetting for the Red Clay soil (Table 2). There was very little difference between rotations in the dry aggregate size distribution for both soils except for the percentage <125 µm for the Red Clay soil, which was 37% higher than for the mean of the rotations (Tables 2 and 3). Red Clay MWDI and >250 I were 72% and 35% higher in the lucerne, 78% and 37% higher in the clover, and 78% and 32% higher in the continuous wheat rotations than in the long fallow (Table 2). The long fallowing treatment increased the <125 I by 65% and 57% when compared with the lucerne and clover rotations (Table 2). The clover rotation had a significantly greater MWDT than the long fallow but there were no significant differences for >250 T, <125 T, MWDD, >250 D, and <125 D between the different rotations (Table 2). Black Earth The lucerne, clover, medic, and continuous wheat treatments increased MWDI by 46%, 38%, 40%, and 37%, respectively, and >250 I by 265%, 231%, 299%, and 201%, respectively, compared with the long fallow (Table 3). Long fallowing resulted in a larger <125 I than all other treatments (Table 3). The clover rotation had a significantly higher MWDT than the long fallow but there were no significant differences between the different rotations for >250 T, <125 T, MWDD, >250 D, and <125 D (Table 3).

7 76 N. Blair and G. J. Crocker Table 2. Mean weight diameter (MWD), percentage of aggregates >250 µm, and percentage of aggregates <125 µm for immersion and tension wetting, and for dry aggregate size distribution for the different rotations and the reference soil for the Red Clay soil Values in the same row, for the different rotations, followed by the same letter are not significantly different according to DMRT (at P = 0.05) Reference Long Lucerne Clover Grain Medic Continuous fallow legume wheat Immersion wetting MWD (mm) b 0.810a 0.839a 0.631ab 0.631ab 0.838a >250 (%) b 62.2a 63.5a 55.5ab 55.8ab 61.0a <125 (%) a 20.8b 21.8b 27.5ab 26.0ab 27.4ab Tension wetting MWD (mm) b 1.282ab 1.543a 1.236ab 1.279ab 1.287ab >250 (%) a 79.0a 82.9a 77.2a 72.7a 76.6a <125 (%) a 12.7a 11.1a 13.7a 20.7a 17.2a Dry aggregate size distribution MWD (mm) >250 (%) <125 (%) Table 3. Mean weight diameter (MWD), percentage of aggregates >250 µm, and percentage of aggregates <125 µm for immersion and tension wetting, and for dry aggregate size distribution for the different rotations and the reference soil for the Black Earth soil Values in the same row, for the different rotations, followed by the same letter are not significantly different according to DMRT (at P = 0.05) Reference Long Lucerne Clover Grain Medic Continuous fallow legume wheat Immersion wetting MWD (mm) b 0.603a 0.570a 0.529ab 0.581a 0.567a >250 (%) b 40.1a 36.4a 31.7ab 43.9a 33.1a <125 (%) a 33.2b 39.4b 39.1b 32.2b 37.6b Tension wetting MWD (mm) b 0.716ab 0.887a 0.711ab 0.665ab 0.716ab >250 (%) a 61.9a 67.7a 68.0a 65.6a 65.3a <125 (%) a 22.9a 23.4a 21.6a 22.9a 22.4a Dry aggregate size distribution MWD (mm) >250 (%) <125 (%) Relationships between carbon fractions and wet aggregate stability There was a significant positive correlation between MWDI or >250 I and C L for the Red Clay soil but not for the Black Earth (Table 4). There was also a significant negative correlation between C L, C 33, and C T and <125 I for the Red Clay (Table 4). This was not found for the Black Earth. There was no correlation between C 33 or C T and MWDI or

8 Crop rotations, soil carbon, and physical fertility 77 Table 4. Linear relationships between measures of aggregate stability measured by immersion wetting [mean weight diameter (MWDI), percentage of aggregates <250 µm (<250 I), percentage of aggregates <125 µm (<125 I)] and soil C fractions [C oxidised by 33 mm KMnO 4 (C 33 ), C oxidised by 333 mm KMnO 4 (C L ), non-labile C (C NL = C T C L )], in the Red Clay and Black Earth soils Y Red Clay Black Earth bx r 2 bx r 2 MWDI 0.32C L 0.56 * 0.14C L 0.27n.s. 0.05C T 0.33n.s C T 0.09n.s. 0.02C n.s. 0.44C n.s. <250 I 14.52C L 0.59 ** 23.16C L 0.34n.s. 2.48C T 0.35n.s. 5.24C T 0.15n.s. 0.86C n.s C n.s. <125 I 11.07C L 0.63 ** 20.08C L 0.25n.s. 2.22C T 0.51 * 3.93C T 0.08n.s C * 63.58C n.s * P < 0.05; ** P < 0.01; n.s., not significant. Table 5. Hydraulic conductivity (K) values (mm/h) for the different rotations and the reference soil for the Red Clay soil Values in the same row, for the different rotations, followed by the same letter are not significantly different according to DMRT (at P = 0.05) Tension Reference Long Lucerne Clover Grain Medic Continuous fallow legume wheat 40 mm mm 77 23ab 22ab 14b 33a 31a 29a 20 mm ab 54ab 33b 68ab 80a 73a 10 mm b 200a 79c 167ab 150ab 184a >250 I for either soil. There was always a stronger correlation between C L and the factors of wet aggregate stability with immersion wetting than for C 33 or C T for the Red Clay. There were no significant relationships between any carbon fractions and any factors of wet aggregate stability with tension wetting for either soil type. Unsaturated hydraulic conductivity Surface infiltration is dependent on several factors including soil surface conditions, the number and continuity of the pores, and aggregation and stability. The rooting characteristics of the plants growing in the soil can also influence infiltration. On the Red Clay soil, cropping and cultivation have resulted in a decrease in infiltration at all tensions (Table 5). However, this is not the case for the Black Earth where K at 40 and 30 mm tension was 107% and 70% higher, respectively, in the cropped soils than in the reference (Table 6). At 20 mm tension there was little difference between the cropped soil and the reference soil, whereas at 10 mm, K was higher for the reference soil than for all rotations except for the lucerne (Table 6). Red Clay At 10 mm tension the lucerne and continuous wheat had a significantly higher K value than did both the long fallow and clover rotations, whereas K in both the grain legume

9 78 N. Blair and G. J. Crocker Table 6. Hydraulic conductivity (K) values (mm/h) for the different rotations and the reference soil for the Black Earth soil Values in the same row, for the different rotations, followed by the same letter are not significantly different according to DMRT (at P = 0.05) Tension Reference Long Lucerne Clover Grain Medic Continuous fallow legume wheat 40 mm 22 59a 23b 44a 51a 50a 47a 30 mm 43 87a 45b 72ab 80a 78ab 78ab 20 mm mm c 497a 244bc 261bc 245bc 280b and medic rotations was greater than that in the clover rotation (Table 5). The medic and continuous wheat had significantly larger K than did the clover rotation at 20 and 30 mm tension, and the grain legume rotation also had a higher K than the clover at 30 mm tension (Table 5). There were no significant differences between rotations at 40 mm tension. Black Earth At 40 mm tension, the lucerne rotation had a significantly lower K value than all other rotations (Table 6). The long fallow and grain legume rotations had significantly greater K than the lucerne at 30 mm tension. At 10 mm tension, K in the lucerne rotation was significantly higher than in all other rotations, and continuous wheat had a significantly larger K value than the long fallow rotation (Table 6). There was no significant difference between rotations at 20 mm tension (Table 6). Discussion Soil carbon Crocker and Holford (1995) recorded a decline in total soil organic carbon for the fallow treatment on both soil types since 1966 in this experiment. Holford (1990) and Holford et al. (1998) found no effect of legume rotations on total soil organic carbon in their studies at this site since the commencement of the experiment in In the present studies a large decrease in concentration of each carbon fraction as a result of cropping was recorded and differences in C T were recorded between rotations on the Black Earth soil (Table 1). Tisdall and Oades (1982) and Lefroy et al. (1993) also found that cultivation resulted in a decline in the organic matter content of soils. Syers and Craswell (1995) suggested that such a decline was due to increases in decomposition rate by shattering of macro-aggregates, mixing of surface soil, and increases in the intensity and number of wetting and drying cycles. Cropping and cultivation has resulted in a decline, compared with the reference soil, in the concentration of the different carbon fractions (meaned over all rotations) in the Red Clay soil (44 46%); however, the Black Earth soil had a much greater decline in C L (71%) than in C 33 (56%), C T (56 %), and C NL (52%) (Table 1). Blair et al. (1995) showed that cropping generally results in a greater decrease in C L than C T, and Whitbread (1996) found that C L also increased faster than did C T as the result of changing management from residue removal and/or continuous cropping to residue retention and/or legume rotations.

10 Crop rotations, soil carbon, and physical fertility 79 Legumes used in rotation with arable crops can result in increases in concentration of soil carbon and the increase will vary depending on the type of crop and the biomass inputs into the soil. Karlen and Cambardella (1996) reported that often the increase in concentration of soil carbon as a result of legume rotations is the result of higher yields in the following crop from the nitrogen added to the system by the legumes. On the Red Clay soil only the inclusion of lucerne or clover in the rotation increased C L relative to the long fallow treatment (Table 1), indicating a greater concentration of more active carbon from these residues. Rotations had no significant effect on C T. This may have been caused not only by the return of clover and lucerne residues through grazing and root growth, but also from root growth and residue return from the higher yielding wheat crops which have followed legume rotations in previous years (Holford and Crocker 1997). The reduction in C L for the long fallow treatment is most likely the result of less organic input and more tillage to control weeds (Holford 1981; Haynes and Beare 1996). Holford (1990) and Holford et al. (1998) also found no accumulation of organic carbon following the legume growing periods in phase 1 ( ) and Phase 2 ( ) at this site. In the experiment of Whitbread (1996), the growing of a lucerne rotation on a Red Earth soil resulted in a 55% increase in C L and only a 19% increase in C T when compared with the mean results of chickpea, medic, and fallow rotations. The significant increase in C L following the growing of lucerne and clover compared with the long fallow treatment, on the Red Clay soil (Table 1), resulted from the input of high quality residues from roots, dung return, and trampling from the grazing sheep. It may be expected that medic should also have increased C L ; however, Whitbread (1996) also found no increase in C L following a medic rotation, compared with chickpea or fallow rotations on a Red Earth soil at Warialda, NSW. The greater decrease in C L compared with C T for the Black Earth soil is similar to the survey findings of Whitbread et al. (1998), who found greater decreases in C L than C T for 6 of the 7 soils studied across north-western NSW. Blair and Daniel (1996) also found a greater decrease in C L than C T for a range of soils from the cotton-growing areas of eastern Australia. The changes in concentration of labile carbon fractions (C 33 and C L ) measured in the Black Earth soil were similar to the Red Clay soil except that medic did not increase C L on the Red Clay soil. The medic crop on the Black Earth soil had a higher yield than the clover in 1996 (G. Crocker, pers. comm.) and may have resulted in greater organic inputs than on the Red Clay. This may also be the reason for the increase in C T on the medic rotation compared with the grain legume/wheat rotation and long fallow. Changes in CMI The higher CMI values in the legume rotations compared with the long fallow treatments for both the Red Clay and Black Earth soils show how the return of legume residues is resulting in better carbon dynamics in the system. The growing of a lucerne rotation on a Red Earth at Warialda also increased the CMI compared with a chickpea, medic, or fallow rotation (Whitbread 1996). These increases in CMI values following the legume rotations will likely increase the soil chemical and physical fertility, thus increasing the sustainability of the systems. Aggregate stability The decline in wet aggregate stability as a result of cropping and cultivation for both soils (Tables 2 and 3) is similar to the findings of Tisdall and Oades (1980), Dormaar (1983), Cambardella and Elliot (1993), Bridge and Bell (1994), and Whitbread et al. (1998).

11 80 N. Blair and G. J. Crocker Cultivation also breaks up roots and fungal hyphae which also positively influence the stability of soil aggregates (Tisdall 1991). The large increase in the percentage of aggregates <125 µm may cause problems with surface sealing. Loch (1994) contended that the percentage of particles of this size had the greatest influence on surface sealing of dry-land cropping soils when measured followed by wetting with simulated rainfall. Tisdall and Oades (1982) reported that the aggregates most susceptible to disintegration from agricultural practices were those >250 µm, which, when they break down, can lead to pore blockages that reduce infiltration, which can lead to an increased erosion risk. The lack of any significant difference in MWDD, >250 D, and <125 D between treatments on both soils for the dry aggregate size distribution shows that each soil commenced with a similar particle size distribution before undergoing wet sieving (Table 2 and 3). The Red Clay reference soil appeared to be more friable and easier to break down than the corresponding cropped soils, which resulted in a 36% increase in the percentage of aggregates <125 µm (meaned over all rotations) following dry sieving (Table 2). The decrease in the percentage of these aggregates, in the cropped soil compared with the reference soil, may have been due to a soil crust present on the cropped soil at the time of sampling. The lack of cover on all treatments, except for the lucerne, and the upright habit of lucerne plants, leaves much of the soil exposed to raindrop impact, particularly following heavy summer storms, leading to soil slaking and crust formation (Harte 1984). The improvement in wet aggregate stability (Table 2) with immersion wetting for the Red Clay soil in the clover, lucerne, and continuous wheat treatments when compared with that of the long fallow reflects a general increase in C L for these treatments (Table 1). Although C L for the continuous wheat was not significantly higher than for the long fallow, the binding of soil particles by roots also influences aggregate stability and the fibrous root system of the wheat could increase the stability of the aggregates in this treatment (Tisdall 1991). Lower organic inputs resulting in a reduction in C 33, C L, and C T concentration have likely resulted in a reduction in aggregate stability for the long fallow treatment on the Black Earth soil (Table 3). The greater structural breakdown of the long fallow for both of the soils could result in an increased erosion risk from decreased infiltration and lack of cover. The only significant difference in aggregate stability with tension wetting, for both of the soils, was for mean weight diameter between the clover and long fallow rotations (Tables 2 and 3). For both soils, tension wetting has resulted in better aggregate stability than immersion wetting, indicating a degree of slaking. Relationships between carbon fractions and wet aggregate stability For the Red Clay soil there is a significant relationship between C L and all aggregate stability measurements with immersion wetting, with the strongest relationship with the percentage of aggregates <125 µm. This indicates the importance of this labile carbon fraction in maintaining aggregate stability. Blair and Daniel (1996) also reported a relationship between mean weight diameter and C L for a range of soils (with <49% clay) from the cotton-growing areas of eastern Australia. As was found in the present experiment, this relationship was stronger than with C T. Many researchers have found a stronger relationship between aggregate stability and the more labile fractions of organic carbon. Hot water extractable carbohydrate has been found to be more closely correlated to aggregate stability than organic carbon (Haynes and Swift 1990; Ball et al. 1996). Conteh et al. (1999) showed that the carbon oxidised by 333 mm KMnO 4 had a high correlation with total and labile polysaccharides, indicating that these were some of the

12 Crop rotations, soil carbon, and physical fertility 81 materials removed by oxidation. The negative correlation found between all carbon fractions and the percentage of aggregates <125 µm indicates the importance of carbon fractions in reducing the amount of these smaller aggregates. This can have positive benefits in reducing surface sealing so increasing water infiltration and reducing the risk of erosion (Loch 1994). The lack of any correlation between the different carbon fractions and aggregate stability with tension wetting may be the result of the elimination of the effect of rapid wetting. As the soil wets more slowly and air is able to escape during the wetting process, the importance of organic binding agents in holding the aggregates together is reduced. The lack of any relationship between aggregate stability and any carbon fractions for the Black Earth soil (Table 4) is similar to the findings of Blair and Daniel (1996), who found no relationship between mean weight diameter and C L or C T for soils of >49% clay. Whitbread et al. (1998) also reported significant correlations between percentage of aggregates >250 µm and C L and C T for Grey Clay and Red Earth soils but not for Black Earth soils with higher clay contents. This contrasts with the findings of Bell et al. (1998) who showed a significant relationship between C 33, C L, and C T and the percentage of aggregates <125 µm, following wetting with simulated rainfall, for a range of Red Ferrosol soils with average clay content of 63%. However, these soils also had much higher C 33, C L, and C T, means of 2.14, 6.46, and 25.4 mg/g, respectively, than the soils used in the present study and in that of Blair and Daniel (1996) with means of 0.91, 1.83, and mg/g, respectively (N. Blair and H. Daniel, unpubl. data). Those soils would also have different clay minerals to those used in this study and those of Whitbread et al. (1998) and Blair and Daniel (1996). Oades (1993) reported that biological factors were not as important in clay-rich soils whose structure is dominated by the behaviour of the clay particles, which may be affected by the clay minerals, whereas Haynes and Beare (1996) suggested that soils with higher clay contents require a higher organic matter content for stabilising soil structure. When determining C 33 and C L, Bell et al. (1998) used soil ground to <125 µm compared with <500 µm used in this study. This may affect the results obtained for these fractions due to the disruption of some micro-aggregates, which increases the accessibility of some carbon, which would not be oxidised if the soil was only ground to <500 µm. Edwards and Bremner (1967) and Elliot (1986) showed that the grinding of soil generally increased C mineralisation because previously inaccessible organic matter was released for attack by microbes. Unsaturated hydraulic conductivity The pore sizes that are most susceptible to collapse from agricultural practices are those greater than 0.75 mm, which represents 40 mm tension (Murphy et al. 1993). Coughlan et al. (1991) stated that the values for K determined using tension infiltrometers of the type used here refer to the immediate surface soil only and not to the whole profile. The lowering of K at all tensions as a result of cropping and cultivation for the Red Clay soil (Table 5) is similar to the results of Whitbread (1996), who showed that cultivation of a Red Earth soil at Warialda, in north-western NSW, reduced K at all tensions except 40 mm. In a survey of 7 soils in north-western NSW, Whitbread et al. (1998) reported that 6 of the 7 soils surveyed had reduced infiltration following cropping compared with a reference soil. Chan and Mead (1989) observed that 4 years of conventional cultivation reduced macropore density and continuity compared with a pasture soil. The low K value for the clover treatment on the Red Clay soil was unexpected. However, it may be the result of grazing by sheep foraging for clover seed

13 82 N. Blair and G. J. Crocker buried in the soil and hence scuffing the soil surface and reducing infiltration. This grazing occurred just prior to the measurements being made and this low K value may be only a temporary reduction. The higher infiltration on the lucerne compared with the long fallow treatment is most likely the result of a combination of some surface cover, deep rooting plants, and more water-stable aggregates in this treatment. When the K value for the clover treatment, which appears to be an anomaly, is left out, there is a significant relationship between aggregate stability with immersion wetting and K, with the strongest relationship being with the percentage of aggregates <125 µm (r 2 = 0.57). There is also a similar relationship between C L and K (10 mm) when the clover treatment is removed (r 2 = 0.57). As C L is correlated to percentage of aggregates <125 µm, this is not surprising and shows the importance of maintaining the more labile carbon factors for reducing surface sealing and increasing surface infiltration. The relationships are not strong, as other factors such as surface cover and the rooting characteristics of plants also affect infiltration. At 40 and 30 mm tension on the Black Earth soil, the reference and lucerne treatments had lower infiltration than all other treatments (Table 6). This indicates that lack of surface cover on all except these soils may have caused the larger pores to collapse, resulting in an increased number of smaller pores. At 10 mm tension the lucerne rotation on the Black Earth soil had a higher K than the pasture soil, which may be attributed to the strong root growth and deep rooting characteristics of lucerne, resulting in many large continuous pores on this treatment (Cresswell and Kirkegaard 1995). Pasture growing on the Black Earth reference soil was mainly summer-growing annuals, which would be expected to be shallow rooted, and unlikely to produce large continuous pores as would the lucerne. Although the reference soil was much more stable to wetting than was the lucerne treatment, the abundance of large pores would be expected to overcome the lack of stability and allow for faster water entry. The strong rooting characteristics of the lucerne (Cresswell and Kirkegaard 1995), combined with more surface cover, has allowed for a significant increase in K at 10 mm for this treatment compared with all others. Conclusion This study has shown the impact of cropping on reducing soil carbon concentration and aggregate stability. This is particularly so when practices such as long fallowing are used. Although the use of legume rotations has increased aggregate stability and the concentration of soil carbon, particularly the more labile fractions, the rapid breakdown of the plant materials returned to the soil, and the continued practice of cultivation, mean that the increases are small when compared with the values for the reference soils. Farmers should be encouraged to eliminate long fallowing and to adopt no-till techniques combined with the return of residues from either the primary crop or rotation crops which have a slower breakdown rate, as this management is likely to have a better potential for increasing soil carbon concentration and improving soil structure. This general principal of synchronising crop residue breakdown rate with the growth rate of the following crop has been explored in many agricultural systems. The results presented here show the significant impact that the type of rotation crop can have on soil physical fertility. The investigation of ways to better increase the quantity and quality of SOM and hence soil chemical and physical fertility is necessary if long-term sustainable agriculture is to be possible.

14 Crop rotations, soil carbon, and physical fertility 83 Acknowledgments The consultation on the carbon analysis provided by Graeme Blair is gratefully acknowledged. The technical advice for the carbon analysis provided by Leanne Lisle and Judi Kenny was greatly appreciated. The Australian Centre for International Agricultural Research (ACIAR) Project 9448 provided funding for the carbon analysis. References Ankeny MD, Ahmed M, Kaspar TC, Horton R (1991) Simple field method for determining unsaturated hydraulic conductivity. Soil Science Society of America Journal 55, Ball BC, Cheshire MV, Robertson EAG, Hunter, EA (1996) Carbohydrate composition in relation to structural stability, compaction and plasticity of two soils in a long-term experiment. Soil Tillage and Research 39, Bell MJ, Moody PW, Connolly RD, Bridge BJ (1998) The role of active carbon fractions of soil organic matter in physical and chemical fertility of Ferrosols. Australian Journal of Soil Research 36, Blair GJ, Chapman L, Whitbread AM, Ball-Coelho B, Larsen P, Tiessen H (1998) Soil carbon changes resulting from trash management at two locations in Queensland, Australia, and in North-East Brazil. Australian Journal of Soil Research 36, Blair GJ, Lefroy RDB, Lisle L (1995) Soil carbon fractions, based on their degree of oxidation, and the development of a carbon management index for agricultural systems. Australian Journal of Agricultural Research 46, Blair N, Daniel H (1996) The impact of cropping on soil aggregate stability and carbon content. In ASSSI and NZSSS national soils conference Oral Papers. pp Bridge BJ, Bell MJ (1994) Effect of cropping on the physical fertility of krasnozems. Australian Journal of Soil Research 32, Bruce JP, Frome M, Haites E, Janzen H, Lal R, Paustian K (1998) Carbon sequestration in soil. In Proceedings of the carbon sequestration in soils workshop. Calgary, Alberta, Canada May pp (Soil and Water Conservation Society). Cambardella CA, Elliot ET (1993) Carbon and nitrogen distribution in aggregates from cultivated and native grassland soils. Soil Science Society of America Journal 57, Chan KY (1997) Consequences of changes in particulate organic carbon in vertisols under pasture and cropping. Soil Science Society of America Journal 61, Chan KY, Mead JA (1989) Water movement and macroporosity of an Australian Alfisol under different tillage and pasture conditions. Canadian Journal of Soil Science 14, Conteh A, Blair GJ, Lefroy RDB, Whitbread AM (1999) Labile organic carbon determined by permanganate oxidation and its relationships to other measurements of soil organic carbon. Humic Substances in the Environment Journal 1, Coughlan KJ, McGarry D, Loch RJ, Bridge B, Smith GD (1991) The measurement of soil structure some practical initiatives. Australian Journal of Soil Science 29, Cresswell HP, Kirkegaard JA (1995) Subsoil amelioration by plant roots the process and the evidence. Australian Journal of Soil Research 33, Crocker GJ, Holford ICR (1995) The Tamworth legume/cereal rotation. Evaluation of soil organic matter models using existing long term datasets. In NATO advanced sciences institute series 1. (Eds D Powlson, P Smith, J Smith) Vol. 38, pp (Springer-Verlag: Berlin) Dormaar JF (1983) Chemical properties of soil and water-stable aggregates after sixty-seven years of cropping to spring wheat. Plant and Soil 75, Edwards AP, Bremner JM (1967) Micro-aggregates in soils. Journal of Soil Science 18, Elliot ET (1986) Aggregate structure and carbon, nitrogen, and phosphorus in native and cultivated soils. Soil Science Society of America Journal 50, Gretton P, Salma U (1996) Land degradation and the Australian agricultural industry. Staff Information Paper, Report for the Industry Commission. Harte AJ (1984) Effect of tillage on the stability of three red soils of the northern wheat belt. Journal of Soil Conservation 40, Haynes RJ, Beare MH (1996) Aggregation and organic matter storage in meso-thermal, humid soils. In Structure and organic matter storage in agricultural soils. (Eds MR Carter, BA Stewart) pp (Advances in Soil Science, CRC Press Inc.: Boca Raton, FL)

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