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Soil Science:Volume 166(1)January 2001pp 61-67

OXIDIZIBLE ORGANIC CARBON FRACTIONS AND SOIL QUALITY CHANGES IN AN OXIC PALEUSTALF UNDER DIFFERENT PASTURE LEYS

[Technical Articles]

Chan, K. Y.1; Bowman, A.2; Oates, A.1

1Wagga Wagga Agricultural Institute, NSW Agriculture, PMB, WaggaWagga, NSW 2650, Australia. Dr. Chan is corresponding author. E-mail: u

2Agricultural Research Centre, NSW Agriculture, Trangie, NSW 2823, Australia.

Received Jan. 26, 2000; accepted Aug 22, 2000.

Abstract

To compare the effectiveness of different pasture species in restoring soil quality, changes in concentration and quality of soil organic carbon (C) were measured in the surface 10 cm of an OxicPaleustalf (red earth) in the semiarid area of New South Wales, Australia, at the end of 4 years under lucerne (Medicago sativa cv. Trifecta), Consol lovegrass (Eragrostiscurvula), and barrel medic (Medicagotruncutulatacvsephi). Before the investigation, the soil had been degraded by 50 years of cropping. Soil samples were analyzed for water stable aggregation, mineralizable N, and C by three procedures: Total carbon (C) by dry combustion, oxidizible C by potassium permanganate, and oxidizible C by potassium dichromate/sulphuric acid with varying concentrations of acid.

Higher dry matter production caused lucerne to be was more effective than barrel medic in increasing soil organic carbon concentration. Compared with fallow plots, total soil organic carbon concentration increased by 16, 26, and 11%, respectively, in the Consol lovegrass, lucerne, and barrel medic treatments. Nevertheless, even in the case of lucerne, the 26% increase in organic carbon in the 0-10-cm layer at the end of 4 years (7.87 vs. 9.88 g/kg) represented only 15% of the total loss in organic carbon after 50 years of cropping.

Most (78-92%) of the organic carbon increases under the various pastures were of the more labile forms, as indicated by their removal under much milder oxidizing conditions than those recommended in the standard methods for organic carbon determination. Significant improvements in structural stability and nitrogen availability were detected in the perennial pasture soils. Our results suggested that the amount of organic carbon oxidizible by a modified Walkley-Black method, which involves using only half the amount of sulphuric acid, is a more sensitive indicator of the improvement in soil quality parameters under investigation, namely increases in mineralizable nitrogen and water stable aggregation. Further research is needed to verify these findings over a range of soil types and agroecosystems.

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 OxicPaleustalf 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.