Past, present and future of soil quality indices: A biological perspective

Abstract The concept of soil quality gives rise to more controversy than that of water or air quality. However, despite the difficulty in providing a definition, the maintenance of soil quality is critical for ensuring the sustainability of the environment and the biosphere. Literature exhibit a great number of soil quality indices for both agro-ecosystems and natural or contaminated soils. The objective of this work was to make a review of some of the soil quality indices established up to date as well as of the parameters that make up them, and to offer a reflection on the lack of consensus concerning the use of these indices. We will focus in those indices including biological parameters. The most straightforward index used in the literature is the metabolic quotient ( q CO 2 ) (respiration to microbial biomass ratio), widely used to evaluate ecosystem development, disturbance or system maturity. However, q CO 2 and other indices integrating only two parameters provide insufficient information about soil quality or degradation. For this, lately there has been a wide development of multiparametric indices that clearly establish differences between management systems, soil contamination or density and type of vegetation. These indices integrate different parameters, among which the most important are the biological and chemical ones, such as pH, organic matter, microbial biomass C, respiration or enzyme activities. The major part of multiparametric indices has been established on the basis of either, expert opinion (subjective), or using mathematical–statistics methods (objective). Molecular indicators have not yet been used for soil quality indices establishment. However, the development of genomic, transcriptomic or proteomic methodologies could have importance in the evaluation of soil quality, not only in a diversity sense but also in a functional way. These methods can provide information about what is the role of specific microorganisms and their enzymes in key processes related to soil functionality. Despite of the great diversity of indices, they have never been used on larger scales, nor even in similar climatological or agronomic conditions. The lack of applicability of soil quality indices resides on: i) poor standardization of some methodologies; ii) some methods are out of reach in some parts of the world; iii) spatial scale problems (soil heterogeneity); iv) poor definition of soil natural conditions (climate and vegetation); and v) poor definition of soil function to be tested for soil quality.