Metabolic substances (ergosterol andgiomalin)
Several compounds are found in soil, e.g., sterols, antibiotics, protein, enzymes, etc. as a result of microbe mediated catabolic and anabolic processes in soil. The two most important of them with respect to soil health monitoring include ergosterol and giomalin.
It is predominantly considered as the principal sterol secreted endogenously from fungi, actinomycetes, and some of the microalgae. The concentration of ergosterol is a critical indicator of fungal proliferation on organic compounds and its mineralization efficiency (Battilani et al. 1996). It was demonstrated that ergosterol content was not affected by heavy metals concentrations (Cu 80 ppm, Zn 50 ppm or Cd 10 ppm) and fungicides (Thiram 3 ppm or pentachlorophenol
- 1.5 ppm) even at concentrations that reduce the metabolic activity by 18% to 53% (pollutant stressed cultures), while Zineb, a fungicide at a concentration of (25 ppm), reduced the ergosterol content significantly. Similarly, a significant correlation was observed between fungal hyphae and ergosterol concentration in pastures and arable soils. It was also demonstrated through electron microscopy that a beneficial role of fungi in thixotropy (a physical process involving the orientation of the claymicelles) also exists in soil (Barajas et al. 2002, Molope et al. 1987). Puglisi et al. (2003) worked out the concentrations of cholesterol, sitosterol, and ergosterol in some agricultural soils (hazelnut irrigated with contaminated water and intensive horticuhure) and reported that crop rotation does not affect the concentration of these sterols, but ergosterol hastened the metabolic activity in soils with industrial contamination. In the Pacific region, Joergensen and Castillo (2001) found positive correlations between qC02 and ergosterol to biomass C. They opined that the low soil microbial availability was attributed to the low soil organic matter and phosphorus leading to poor soil fertility and health in Nicaragua. The reduced fungal population caused P deficiency to crops probably due to the inability of plant roots to access immobile nutr ients for plant growth.
- 5.3.2 Giomalin
It is an important fungal component which is hydrophobic and proteinaceous in nature (Wright and Upadhyay 1996). Giomalin as glomalin-related soil protein (GRSP) is reported to improve soil stability by avoiding w'ater mediated deflocirlation (Wright et al. 2008, Wright and Upadhyay 1998). A good correlation between giomalin concentration and the amount of water stable aggregates (WSA) was observed. Glomalin-related soil protein (GRSP) being gelatinous in nature seals most soil pores, thereby hindering penetration of w'ater in soil aggregates (Wright and Upadhyaya 1998, Rillig 2004, Hamer et al. 2004, Rillig 2004). GRSP is often used as a biochemical marker in soil due to its stability even with negative management effects (Rosier et al. 2006) on mycorrhizal fungi, viz. tillage, and inclusion of fallow' into crop rotation. Bedini et al. (2009) used isolates of Glomus mosseae and Glomus intraradices for inoculating Medicago saliva plants in a microcosm experiment and reported enhanced soil aggregate stability as mean w'eight diameter (MWD) of macro aggregates of 1-2 mm diameter, in inoculated soils compared to non-inoculated ones. They also reported a strong positive correlation between GRSP concentration and soil aggregate stability with mycorrhizal root volume and a w'eak correlation with total root volume.
Microbial biomass carbon (MBC)
Microorganisms are w'idely recognized as sensitive soil quality indicators. They play a crucial role in nutrient cycling and energy flow' (Li and Chen 2004) and are sensitive to intercropping, organic matter addition, and management practices (64) affecting soil structure stabilization (Suman and Gaur 2006). Microbial biomass carbon is a meager but active fraction of soil organic matter and is of immense interest in soil fertility due to its sensitivity to management practices than the bulk organic matter (Janzen 1987). It acts as a pool of plant nutrients and is vital in governing the nutrient availability to plants. Several authors (Azmal et al. 1996, Sridevi et al. 2003) have reported a significant enhancement in microbial biomass following incorporation of crop residues. Post residue incorporation, microbial biomass-C (MBC) enhanced and reached maximum value of MBC (as per the studies of Azmal et al. 1996, Sridevi et al. 2003). Ocio and Brooks (1990) repoxted that addition of straw, as compared to control, improved the microbial biomass by 87.5% in a sandy loam soil and by about 50% in a clay soil. Malik et al. (1998) observed a large increase in microbial biomass dining the early stages of rice crop on wheat straw and green manure application in a rice-wheat cropping system. This also resulted in synchronization of N release from soil and its uptake by plants. Patra et al. (1992) opined that biomass C content varied with type of crop residues as they found higher MBC in wheat straw amended soil compared to cowpea; however, for microbial biomass N (MBN), it was vice-versa. Azmal et al. (1997) showed an immediate increase in microbial biomass C and N on rice straw incorporation under aerobic incubations in a clay loam soil, which reached maxima after 7 days of each application (2 g C kg"1 soil as lice straw at an interval of 1.5 months), and decreased thereafter. The maximum biomass formation reached its optima after the second application, signifying the limited capacity of soil to incoiporate biomass. In rice-lentil (Lens esculenta) crop rotation under dry land conditions, microbial C was maximum (408-420 pg g"1) with the application of wheat straw (101 ha"1) and fertilizer, followed by only straw (360-392 pg g"1) and only feitilizer treatments (238-246 pg g"1) (Singh and Singh 1991). Straw incoiporation coupled with feitilizer treatment showed 77% more microbial biomass C accumulation over control. The rapid initial increase of microbial activity may probably be attributed to the faster catabolism of simple C compounds contained in crop residues.