Soil organic matter, the organic fraction of the soil, is a complex mixture of plant and animal products in various stages of decomposition, soil microbes, and substances produced by them. The importance of organic carbon to the physical, chemical, and biological aspects of soil quality is well recognized (eg, Stevenson, 1986; Johnston, 1986). In Australia, as in other parts of the world, soil degradation problems are often accompanied by soil organic carbon decline under exploitative farming practices (Greenland, 1981; Chartres et al., 1992; Gregorich et al., 1995)
Accumulating evidence suggests that certain fractions of soil organic matter are more important in maintaining soil quality and are, therefore, more sensitive indicators of the impact of management practices (Cambardella and Elliott, 1992; Duxbury and Nkambule, 1994; Chan, 1997). According to the hierarchical model of soil structure (Tisdall and Oades, 1982), different levels of soil structural organization are stabilized by different types of soil organic carbon. Macroaggregates (>250 μm) are stabilized mainly by transient forms of organic carbon such as root fragments, fungal hyphae, and polysaccharides, whereas microaggregates (<250 μm) are stabilized by more persistent forms of organic carbon such as humified organic carbon. Hence, macroaggregate stability, which determines the resistance of the soil to slaking, is expected to be influenced strongly by management practices. According to Oades (1984), stabilization of macroaggregate occurs most optimally under extensive root systems of perennial grasses.
Most conventional methods used in soil organic carbon determination have been developed to maximize oxidation and recovery of C (Walkley and Black, 1934; Heanes, 1984; Nelson and Sommers, 1982). However, total organic carbon measurements might not be sensitive indicators of changes in soil quality. Adoption of procedures that can extract the more labile fraction preferentially might be a more useful approach for the characterization of soil organic carbon resulting from different management practices. Using the amount of organic carbon oxidizible by potassium permanganate as a measure of soil organic carbon lability, Blair et al. (1995) demonstrated the decline of a more labile form of organic carbon under cropping but its accumulation under a legume pasture of lucerne.
In the higher rainfall areas of Australia, the pasture phase in the traditional crop/ley system becomes important in the maintenance of both soil chemical fertility and soil physical conditions by maintaining soil organic carbon levels (Greenland, 1971). A recent review has indicated that in the lower rainfall areas (<500 mm), soil organic carbon under cropping tends to decline even when no-tillage is practiced (Chan et al., 1998). Incorporation of a pasture phase of suitable duration in the cropping system may, therefore, be more important for maintaining soil organic carbon levels in the lower rainfall areas. However, it is likely that both quantity and quality of soil organic carbon sequestered under different pasture species are different, and these, in turn, can have different but important effects on soil quality, such as soil structural stability and chemical fertility. This knowledge is important for the selection of suitable pasture species, either singly or as mixes, to be incorporated into the cropping phase. Little information is available about the quantity and quality of soil organic carbon changes under different pasture leys.
Our objective was to determine the changes in the concentration and quality of the soil organic carbon of a degraded Oxic Paleustalf following establishment with three different pasture leys. The quality of soil organic carbon was assessed in terms of degree of oxidizability, which was also related to changes in soil structure and nitrogen availability.
MATERIALS AND METHODS
Site and Soil Description
Soil samples were collected from pastures of a soil amelioration experiment at a site located 30 km west of Nyngan (lat 31° 34′ S long 147° 12′ E) in western New South Wales. Annual average rainfall is 431 mm. Average daily maximum temperature is 25.7 °C, and average minimum temperature is 11.8 °C
The soil studied was an Oxic Paleustalf (Soil Survey Staff, 1994) or a red earth, according to the Great Soil Group. The surface (0-10 cm), was a clay loam with 32% clay, dominated by illite and kandite, and had a pH (in CaCl2) of 5.1. The area was originally an open woodland dominated by native pines (Callitris spp) and grasses (Danthonia spp and Stipa spp) but was cleared about 50 years ago and had been cropped primarily with wheat under a conventional system of repeated tillage, stubble burning, and fallowing. Wheat yield was low, averaging ∼1 t/ha. The site was chosen for its state of degradation caused by the previous cropping history. The surface soil was compact and exhibited hardsetting problems. Soil organic carbon of the 0-10-cm layer was 7.9 g/kg, typical of the degraded cropping soils of the area. In the adjacent uncleared area under native vegetation, soil organic carbon for the 0-10-cm layer was 21.4 g/kg.
Plots with the different pasture treatments were arranged in a completely randomized block design with four replicate blocks. Plot size was 2 m × 5 m. The three pasture types included (i) a perennial legume (lucerne, Medicago sativa cv. Trifecta), (ii) a perennial grass (Consol lovegrass, Eragrostis curvula), and (iii) annual legume (barrel medic, Medicago truncutulata cv sephi). The plots were sown in autumn/winter 1992 at a rate of 4 to 5 kg/ha (recommended sowing rate for the district). The plots were not grazed but were cut approximately four times per year to simulate a moderate level of grazing. Half of the vegetation cut was returned to the individual plots and the other half taken away for dry matter production measurements. For comparison purposes, a fallow treatment was also included which was kept free of vegetation throughout the investigation by the use of herbicides.
At the end of 4 years of pasture, a composite soil sample was collected from a 0-10-cm depth of each of the plots. Six months before the soil sampling, all the pastures had been killed by spraying with RoundupR, a common local practice, as part of the land preparation for subsequent wheat cropping. Five subsamples (10 × 10 × 10 cm) were collected at random within each plot and bulked to form a composite sample.
In the laboratory, the samples were air dried at 36 °C and mixed thoroughly. Subsamples were gently crushed to pass a 6.3-mm sieve. Further subsamples were ground to <0.5 mm.
Soil Organic Carbon Measurements
Total organic carbon (TOC) was determined by dry combustion using a LecoR Carbon Analyzer. About 0.5 g of finely ground soil (<0.5 mm) was weighed and burnt (Nelson and Sommers, 1982). As the soil pH was 5.1, no carbonate was present, and the carbon results obtained using this method were, therefore, equivalent to total organic carbon. All the measurements were duplicated.
Soil carbon fractions were determined by wet oxidation using three methods
Potassium Permanganate Solution (Cpm)
The potassium permanganate (KMnO4) procedure described by Blair et al. (1995) was used. Samples of soil (<0.5 mm) containing about 15 mg C were weighed into 30-mL plastic screw top centrifuge tubes and allowed to react with 25 mL of 333 mM KMnO4 for 1 h under tumbled shaking. The amount of carbon oxidized (Cpm) was calculated from the change in the concentration of KMnO4 when compared with the blank samples that contained no soil (Blair et al., 1995). All the measurements were triplicated.
Walkley-Black Method (CWB)
Oxidizible carbon concentration was also determined using the method of Walkley and Black (1934). One-half gram of ground (<0.5 mm) soil was placed in a 500 mmol/L Erlenmeyer flask to which 10 mL 0.167 M K2Cr2O7 was first added, followed by 20 mL concentrated sulphuric acid. After the reaction, the excess dichromate was determined by titrating against 1.0 M FeSO4. The amount of dichromate consumed by the soil was used to calculate the amount of oxidizible carbon based on the theoretical value of 1.0 mL 0.0167 M K2Cr2O7 oxidizes 3 mg C.
Modified Walkley-Black Method
The determination of oxidizable carbon was repeated using 5 and 10 mL of concentrated sulphuric acid instead of the 20 mL specified by Walkley and Black (1934). The resulting three acid-aqueous solution ratios of 0.5:1, 1:1, and 2:1 (which corresponded respectively to 12 N, 18 N, and 24 N of H2SO4) allowed comparison of oxidizable organic carbon extracted under increasing oxidizing conditions (Walkley, 1947). The amount of oxidizible organic carbon determined using 5, 10, and 20 mL of concentrated sulphuric acid when compared with total carbon concentration allowed separation of total organic carbon into four fractions of decreasing oxidizability:
Fraction 1 (12 N H2SO4)-organic carbon oxidizible under 12 N H2SO4,
Fraction 2 (18 N-12 N H2SO4)-the difference in oxidizible organic carbon extracted between 18 N and 12 N H2SO4;
Fraction 3 (24 N-18 N H2SO4)-the difference in oxidizible organic carbon extracted between 24 N and 18 N H2SO4. The 24 N H2SO4 is equivalent to the standard Walkley-Black method; and
Fraction 4 (TOC-24 N H2SO4)-residual organic carbon after reaction with 24 N H2SO4 when compared with the total carbon determined by the Leco combustion method.
Soil Structural Stability
Water stable aggregation of the soil under the different pasture treatments was determined by a wet sieving procedure. About 20 g of the air-dried soil (6.3-2 mm fraction) was weighed and wet sieved for 10 min (38 mm stroke length and 30 strokes min−1) using sieves of 2-mm and 250-μm apertures in a 2-L cylindrical container. After the wet sieving, the container was inverted 10 times, and the <50-μm fraction was determined using a pipette sampling technique. Percentages of water stable aggregation >2 mm, 2-0.25 mm, 0.25-0.05 mm, and <0.05 mm were calculated after correction for sand content.
Mineralizable nitrogen (MN) was determined by the anaerobic incubation method following Keeney (1982).
The results were analyzed using one way analysis of variance. The treatment means were compared using least significant differences (P = 0.05). Relationships between different carbon concentrations and soil quality parameters, namely mineralizable nitrogen and water stable aggregation, were compared by correlation analysis using the means of the different treatments (n = 4).
RESULTS AND DISCUSSIONS
Soil Organic Carbon Fractions
Significantly higher total organic carbon concentrations were found in the soils that had been under pasture compared with those under fallow (Table 1). The soil under lucerne had the highest total organic carbon concentration (9.88 g/kg), which was 25.6% higher than that under fallow. Total organic carbon concentrations were comparable in the Consol lovegrass and medic soils; both were lower than that of the lucerne soil but higher than that of the fallow soils. Dry matter production over the period was 7.1, 3.9 and 1.8 t/ha, respectively, from the lucerne, lovegrass, and medic pastures. Thus, the increases in total soil organic carbon under the different pasture treatments followed the same trend as the dry matter production.
The amounts of oxidizable carbon extracted from the different soils by the standard Walkley-Black Method (CWB) and the potassium permanganate method (Cpm) were significantly higher in the two perennial pastures than in either the annual pasture or the fallow treatment (Table 1). Although the soil under lucerne had a significantly higher level of total carbon than that under Consol lovegrass, the amounts of oxidizable carbon extracted from the two soils by both the Walkley-Black and permanganate methods were not significantly different, indicating presence of less oxidizible forms of carbon in the lucerne soil. The Walkley-Black Method extracted a much higher proportion of total organic carbon (averaging 78%) than the potassium permanganate method (averaging 17%).
The fractions of organic carbon extracted under a gradient in oxidizing conditions were significantly different among the different treatments (Table 2). However, the majority (78-92%) of the differences, when compared with the fallow soil, were found in the two most easily oxidizible fractions, Fraction 1 and Fraction 2. These two fractions together extracted more than half of the total organic carbon content (65%). Amounts of carbon in fraction 1 were significantly higher under lucerne and lovegrass soils compared with the fallow soil or the annual medic. A total of 53, 53, and 32% of the changes in total organic carbon under lovegrass, lucerne, and medic, respectively, compared with the fallow soil, occurred in the fraction oxidized by 12 N H2SO4 (Fraction 1). The difference in carbon oxidized between 18 N and 12 N H2SO4 (Fraction 2) was comparable under all pasture species (2.59 g/kg) and was significantly higher than the fallow soil. With the exception of a significantly higher carbon concentration in fraction 4 in the case of lucerne when compared with the others (Table 2), little difference was detected in the other two fractions.
Differences in Soil Structural Stability and Nitrogen Status
Figure 1 presents the water stable aggregate distribution of the different soils. Four years of pasture improved the structural stability of the soil significantly, as indicated by the higher percentage of >2-mm water stable aggregates in the pasture soils when compared with fallow (Fig. 1). The stability of the >2-mm aggregates of the lucerne soil and Consol lovegrass soil was similar, and both were higher than that of the medic soil (Fig. 1). There was no significant difference in the <50-μm fraction.
Soils under pasture also had significantly higher levels of total nitrogen and mineralizable nitrogen than soils under fallow (Table 3). The levels of mineralizable nitrogen of the pasture soils were from 2.0 (medic) to 2.9 (lucerne) times that of the fallow.
Correlation analysis indicated that the total organic carbon level (TOC) was not significantly related to mineralizable nitrogen (MN) and macroaggregate stability (>2 mm) (P = 0.05). Conversely, the levels of oxidizable carbon extracted by the Walkley-Black (CWB) and potassium permanganate methods (Cpm) were both significantly related to macroaggregate stability although not to MN (P = 0.05) (Table 4A).
Of the individual fractions obtained using the modified Walkley-Black method, the more oxidizible fractions, namely fraction 1 and fraction 2, are more highly correlated with MN and macroaggregate stability than fraction 3 and fraction 4 (Table 4B). However, only the correlation between fraction 2 and macroaggregate stability was significant (P = 0.05). By combining fraction 1 and fraction 2, oxidizible carbon levels were significantly related to both MN and macroaggregate stability (P = 0.05) (Table 4B).
Effectiveness of Different Pastures
Significant increases in soil organic carbon concentrations were detected at the end of 4 years of pasture on a degraded red earth. Lucerne (perennial legume) was more effective than medic (annual legume pasture) for increasing soil organic carbon concentration because of the higher dry matter production. Most of the increases were in the more oxidizible forms (Tables 1 and 2). Nevertheless, the highest increase in organic C concentration (26%, lucerne) at the end of 4 years was equivalent to only 15% of the total loss in organic carbon, as indicated by the differences between the undisturbed soil (21.4 g/kg) and the initially degraded cropped soil (7.9 g/kg). Sequestration of soil organic carbon is expected to be slow under the semiarid environment due to low productivity and high decomposition rate as a result of higher temperature.
Oxidizible Organic Carbon Fractions as Indicators of Soil Quality
Historically, much of the research efforts to improve organic carbon determination procedures by wet digestion were focussed on achieving complete digestion and, therefore, recovery of total organic carbon (Walkley, 1947; Tinsley, 1950; Heanes, 1984). Accurate measurement of total organic carbon is important for soil carbon sequestration studies. However, the results of our studies supported earlier findings of Loginow et al. (1987) and Blair et al. (1995) regarding the existence of soil organic carbon fractions with different degrees of oxidizability. Blair et al. (1995), using a mild oxidation of potassium permanganate, identified a carbon fraction that declined more rapidly under cropping and increased more rapidly under lucerne pasture than total organic carbon. These authors suggested monitoring this fraction as an indicator of sustainability of agricultural systems. In the present investigation, we also obtained a proportionally greater increase in Cpm (+49%) compared with TOC (+26%) after 4 years of lucerne (Table 1). This oxidizible C fraction is, therefore, a more sensitive indicator of changes in soil organic carbon resulting from different management practices than is total organic C.
Our results indicated further that the differences in soil quality detectable after the pasture leys, namely increases in mineralizable nitrogen and water stable aggregation, were significantly correlated with the more oxidizible organic carbon fractions obtained using a modified Walkley-Black method (namely Fractions 1 and 2) (Table 4b). Our results supported previous findings that macroaggregate stability and nitrogen availability are dependent on labile forms of carbon (Tisdall and Oades, 1982; Janzen, 1987) comprised largely of decaying young organic matter, fungal hyphae, polysaccharides, and other microbial products, the so-called light fraction (Janzen, 1987). It is interesting to note that the remaining fractions (Fraction 3 and Fraction 4), which accounted for 35% of the total organic carbon (Table 2), fall within the range of 30 to 40% assigned to the "passive pool" of soil organic carbon used in the Century Model (Parton et al., 1992). According to the latter, the carbon in the passive pool is inert, with a turnover time of 2000 years. Further research is needed to verify these findings over a range of soil types and agroecosystems.
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