MANAGEMENT PRACTICE ON THE CONTENT OF ORGANIC SUBSTANCES, MICROBAL BIOMASS ACTIVITY AND JOINT STABILITY IN THE PROFILES. The loss of soil organic matter under sugarcane resulted in a simultaneous decrease in soil microbial biomass C, microbial quotient, basal respiration, aggregate stability, aryl sulfatase and acid phosphatase activity. A decrease in sugar cane yield is often accompanied by a decrease in soil organic matter content (Masilacaet al., 1986).
However, very little is known about the effects of sugarcane monoculture on soil organic matter content and quality, as well as soil biological activity (Haynes et al. earth.
ROLE OF SOIL ORGANIC MATTER IN SOIL QUALITY
Light fraction is a transient pool of organic matter (Gregorich and Janzen, 1996) and consists mainly of plant residues in various stages of decomposition in the soil (Stevenson, 1994). Humus is the relatively biologically resistant, mainly dark brown to black, fraction of soil organic matter (Tate, 1987). Aeration, water-holding capacity, and permeability all improve with high levels of soil organic matter (Stevenson, 1994).
The C:N ratio of the fraction of mineralizable organic matter indicates the composition of the active fraction of soil organic matter (Gregorichet al., 1994). Long-term pasture management generally results in the accumulation of high soil organic matter (Haynes and Williams, 1993).
BIOLOGICAL INDICATORS OF SOIL QUALITY
Microbial biomass is the small (1 to 4%) living component of soil organic matter (Jenkinson and Ladd, 1981), excluding macroinvertebrates and plant roots (Sparling, 1997). Carbon and nitrogen measurements in microbial biomass are useful in determining the response of total soil microbial biomass to changes in agricultural management (Roper and Gupta, 1995). The living and dynamic nature of the microbial biomass makes it more responsive to changes in soil management than the total organic matter content (Powlson et al., 1987).
At very low soil moisture contents, however, the microbial biomass activities are also limited (Roper and Gupta, 1995). Clay provides more refuges for protection for the microbial biomass and organic matter than sand (Hassink and Whitmore, 1997). Plant biomass input can be increased by certain crop rotations (specifically those that include pastures), thus increasing the size of the microbial biomass (Collinset al., 1992).
Rotations that incorporate large amounts of organic matter into the soil generally result in higher soil microbial biomass (Blairetai., 1998). Carbon input is usually considered the limiting factor on the extent of soil microbial biomass (Anderson and Domsch, 1985), although inorganic phosphorus concentrations can sometimes be limiting due to P deficiencies (Srivastava, 1992). Microbial biomass is therefore typically high under pastures (e.g. 1200 Ilg C g-l) and serves as a large labile pool of nutrients (Haynes and Williams, 1993).
The dynamics of organic matter is partly reflected by the ratio of microbial biomass C to total organic C, i.e. the microbial quotient (Carter, 1991). For example, a greater increase in microbial biomass carbon compared to total organic carbon will increase the microbial quotient (Sparling, 1997). Although enzymes are not classified as soil biota, they are the products of the microbial biomass.
SOU- AGGREGATION AND STRUCTURAL ORGANISATION
Flocculation is also increased by high concentrations of electrolyte in soil solution, minimal soil disturbance and high organic matter (Rimmer and Greenland, 1976). This is believed to be due to the exposure of organic matter once physically protected by macroaggregate structure (Gregorichet al., 1989). The greater the clay content of the soil, the greater the soil organic matter content and the more organic carbon required to reach a certain level of aggregate stability (Douglas and Goss, 1982; Hayneset al., 1991).
Microbial biomass is directly related to soil organic C content in long-term experiments, but is more indicative of short-term changes in soil organic matter dynamics due to its rapid turnover rate (Jenkinson and Ladd, 1981). The microbial biomass is a store of labile organic matter and is involved in organic matter cycling. It is the living component of organic matter and is dynamic and highly responsive to changes in carbon availability.
It thus serves as a more suitable indicator of carbon availability than total organic matter content. EFFECT OF LONG-TERM SUGAR CANE PRODUCTION ON THE STATUS OF ORGANIC SUBSTANCES IN SOIL AND RELATED PROPERTIES OF TWO CONTRASTING SOILS. Several authors have suggested that the most serious factor associated with soil degradation under sugarcane is the loss of soil organic matter (Wood, 1985; Haynes and Hamilton, 1999).
A loss of soil organic matter can have adverse effects on soil physical, chemical and biological properties. In addition, OBC methodology is used to try to quantify the contribution of sugarcane residues to the soil organic matter content. The second was sieved to collect aggregates (2 - 4 mm diameter) which were air dried for aggregate stability analysis. The third sub-sample was air-dried at room temperature, sieved «2 mm) and ground «0.5 mm) for analysis of organic C.
RESULTS
The relationships between the different measurements of organic matter and size and activity of the microbial biomass from both sites are presented in Table 3.1. Microbial biomass C was linearly related to organic C, and the regression equation and line of best fit are shown in Fig 3.8. Basal respiration, arylsulfatase and acid phosphatase activity were best related to microbial biomass C by quadratic relationships.
CII 600
DISCUSSION
The loss of soil organic matter occurred in both the Glenrosa and Hutton soils when the natural vegetation was converted to sugarcane monoculture. In fact, 10hnston (1986) found that soil texture is one of the most important factors determining the equilibrium of soil organic matter status. The lower clay content of the Glenrosa soil meant that it had a lower potential to store organic matter during cultivation than the Hutton soil.
Thus, surprisingly, even though organic matter content was significantly lower in the Glenrosa soil under sugarcane, aggregate stability was similar to that in the Hutton soil. The main mechanism involved in physical protection of the microbial biomass is thought to be the ability of clay to retain soil organic matter and therefore more substrate C is available to support a greater microbial biomass. The decrease in arylsulfatase and acid phosphatase activities (Figures 3.6 and 3.7 respectively) with long-term sugarcane cultivation was to be expected due to the significant decreases in soil organic matter content and in the size and activity of the microbial biomass (Gupta and Germida, 1988).
Thus, the Hutton site, with higher organic matter and clay contents, showed significantly higher arylsulfatase and acid phosphatase activities than the Glenrosa site. Long-term sugarcane production causes a noticeable decrease in the soil organic matter content and related soil microbial and physical properties in the surface 10 cm. The loss of soil organic matter is often considered the most serious factor associated with agriculturally induced soil degradation (Gregorichet a!., 1994; Paustianet a!., 1997).
Maintaining and improving soil organic matter content is widely accepted as an important objective of every country. There is therefore little information about the effects of common agricultural practices on soil organic matter content and associated soil properties. In the corn sector, the trend from conventional to zero tillage is aimed at conserving soil water and soil organic matter.
MATERIALS AND J\1ETHODS
Basal respiration in the top 10 cm of soil profiles followed the very clear order of kikuyu. At the 5 cm surface the trend followed was kikuyu > native grassland ~ annual ryegrass = maize ~ sugar cane, while between 30 and 40 cm the trend was maize = sugar cane ~ native grassland grass = annual ryegrass ~ kikuyu. The metabolic coefficient (Fig 4.2b) for sugarcane was higher than the other treatments at 15 cm high.
Differences in organic C and microbial biomass C between land uses were generally greatest in the top 10 cm. Bulk density values in the 0–10 cm layer were lowest for kikuyu pastures and native grassland and greatest under annual pasture and sugarcane. However, treatment effects were still most evident in the top 5 cm, where the trend was kikuyu pasture ~ native grassland > maize (ZT) ~ annual ryegrass pasture > maize (CT).
At the 5 cm surface, both values were significantly higher for maize (ZT) than maize (CT). Metabolic coefficient for maize (CT) was significantly higher than the other treatments in the top 20 cm, while values for kikuyu pastures were lower at all depths. At the 10 cm surface, a clear ranking in bulk density values was followed (Table 4.2), kikuyu grassland.
Values for organic C and microbial biomass C in the top 40 cm were adjusted to a volume basis and both followed the order of kikou pasture=native grassland> ryegrass pasture> maize (ZT)=maize (CT). Both organic C and microbial biomass C in the surface 10 cm were significantly higher for the corn (ZT) treatment compared to corn (CT). a) b). Substantial Grassland Kikuyu P.ltuur AImual Ryegrnss Maize(CT) Maize(ZT) . Fig 4.4 Effects of long-term agricultural management practices on a) microbial quotient and b) metabolic quotient in the soil profile to a depth of 40 cm at Cedara Research Farm (site 2).
DISCUSSION
This results mainly from the turnover of the large, branched grass root system and associated microbial biomass, and from the cycling of organic matter via manure deposition (Haynes and Knight, 1989). The organic matter content in the soil is often closely related to the aggregate stability of a soil (Haynes and Beare, 1996). The results of this trial showed that a permanently improved pasture increased soil organic matter content compared to native grassland.
In contrast, conventional arable farming of maize and sugar cane resulted in a significant decrease in organic matter content. The turnover of soil organic matter in long-term field experiments, as indicated by the natural abundance of carbon-13. Effect of crop rotation and rotation phase on soil organic matter characteristics in a dark brown Chernozemian soil.
Soil organic matter transport and carbon storage from maize residues estimated from natural 13C abundance. Changes in microbial biomass and organic matter levels during the first year of modified tillage practices and stubble on red soil. Differential effects of crop rotation, crop residues and nitrogen fertilizer on microbial biomass and organic matter in an Australian Alfisol.
The effects of fallow and fallow crops on the quantity and quality of soil organic matter in sugarcane soils. Measurement of microbial biomass provides an early indication of changes in the total organic matter in the soil as a result of straw incorporation. The ratio of microbial biomass carbon to soil organic carbon as a sensitive indicator of changes in soil organic matter.
