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Biological Indicators for Monitoring Soil Quality under Different Land Use Systems

Introduction

Soil quality or health differs from air or water quality as the former is specified by its dynamic chemical, physical, biological and even ecological properties, whereas the latter are related to the concentration of specific contaminants with well-defined threshold limits. Moreover, when referring to air or water quality, there is hardly any well-defined ideal state due to the limitless number of environmental scenarios (Sojka and Upchurch 1999), unlike when soil quality is referred to. Since soil quality is central to agriculture, environment and other disciplines, authors have been defining soil quality from different perspectives from time to time (Pan- et al. 1992, Doran and Parkin 1994, Harris et al. 1996, Karlen et al. 1997). This becomes evident when one searches for the term ‘soil quality’ on internet search engine as more than 14,000 publications can be found since the 1940s, signifying the importance linked to this subject. This also reflects the tremendous effort of the soil scientists to objectify the subjective nature of soil quality, i.e., to quantify the quality of a soil (Sojka and Upchurch 1999). This is the reason the term continues to lack a universal definition and researchers define it according to specific situations or their interests. Perhaps, due to this, the “concept of quality” is much pertinent to use instead of focusing on soil quality as such and as Sojka and Upchurch (1999) affirm, anything that can be defined infinitely is actually indefinable. However, one must agree that maintenance of soil quality is critical for ensuring the sustainability of the environment and the biosphere despite the difficulty of defining soil quality in a universal framework (Smith et al. 1993, Arshad and Martin 2002). Rigorous studies on this particular aspect becomes more important as fertile agricultural soils are recently becoming more vulnerable to natural or environmental degradation than never before, often by erosion, leaching, or even without anthropogenic intervention (Popp et al. 2000).

However, from agricultural sustainability and soil health point of view, soil quality is defined as the capacity of a soil to function within its natural ecosystem boundaries to suppoit healthy plants and animals, maintain or enhance air or water quality and support hrunan health and habitation (Karlen et al. 1997) and is often described based on indices. A soil quality index may be defined as the minimum set of interrelated parameters that provides quantitative data on the capacity of a soil to cany out one or more functions. A soil quality indicator is a sensitive and measurable parameter that provides soil the capacity to cany out a given function (Acton and Padbury 1993). Studies on soil quality indices are complex in nature as diverse physical, chemical, microbiological and biochemical properties need to be integrated statistically to reach to such quality indices (Papendick and Pan 1992, Garcia et al. 1994, Halvorson et al. 1996).

Importance and characteristics of sensitive soil indicators in relation to soil quality and functions

Importance of soil indicators

Soil indicators are important parameters that explain the soil quality at a given time and which provide reference to measure trends and patterns of soil quality changes with cropping systems, management practices and land use systems. It also helps in assessing the soil quality parameters in relation to the precautionary levels so that pertinent actions may be taken before the deterioration of soil quality. The relationship between soil quality and soil functions are also described by the indicators (Bastida et al. 2008). Even though soil indicators are specific to land use systems, cropping systems and management practices, there are certain characteristics which are common for all situations. Thus, the characteristics of a good soil indicator, in general, may be ascribed as follows:

  • • It should be sensitive to land use under study.
  • • It should explain relationship with soil function.
  • • It should have ease and reliability of measurement.
  • • It should explain spatial and temporal patterns of soil quality variation.
  • • It should be sensitive to changes in soil management.
  • • It should be comparable during routine sampling and monitoring.
  • • It should involve simple skills for interpretation.

Sensitive indicators of soil quality in relation to soil functions

The important soil functions are as follows: biodiversity maintenance, food, feed, fibre and timber production, water and solute flow, filtering and buffering of different chemicals, nutrient cycling and providing structural support. There are sensitive indicators suited for specific soil functions (Table 1).

Table 1. Sensitive biological indicators (in bold letters) specific to soil functions.

Soil functions

Indicators

Production

OC, N, pH, EC, Al, Avl. water, weed species, soil strength

Biodiversity

OC, pH, bases, basal respiration, pH, EC, crusts, soil water tension

Water and solute flow

Tillage, aggregate stability, earthworms, termites, porosity, bulk density, soil structure

Filtering and buffering

Basal respiration, OC, texture, CEC, microbial biomass, cheimcal constituents’ concentration

Nutrient cycling

OC, N, CEC, basal respiration, microbial biomass, POM, PMN, enzyme activities (dehydrogenase, urease, phosphatase)

Structural support

Soil texture, structure, BD, landscape slope, aggregate stability

[ОС-organic carbon, EC-electrical conductivity, CEC-cation exchange capacity, N-nitrogen, POM-particulate organic matter, PMN-potentially nuneralizable nitrogen].

 
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