Natural Organic Compounds in Soil Solution: Potential Role as Soil Quality Indicators

This review focuses on the chemical nature of that fraction of already dissolved organic matter into soil solution, or extracted by mild extractants, which is truly readily available for microbial activity and, consequently, more sensitive than total soil organic matter to changes in management and/or environmental conditions. In particular, we deal with low molecular weight compounds such as monosaccharides, amino sugars and amino acids. Soil sampling strategy and extraction procedure, prior to analyses, are crucial to make comparable results among laboratories. Although soil management and climatic conditions may cause large variability, extractable organic C and N may indicate the amount of substrates available to microbial biomass. Hot water extractable carbohydrates are among the most sensitive indicators of various factors such as tillage, cropping system, soil inundation frequency, seasonality and wildfires. Moreover, hot water extracts mainly microbial carbohydrates, i.e. those more involved to bind soil aggregates. The turnover rate of free amino acids in soils is very high, ranging from 1 h to about 30 hours. This explains the usually very low amino acid-N extractable from soils. The pHvariable charge of amino acids favours their adsorption on soil colloids. Amino sugars concentration in soils (mainly glucosamine, galactosamine, mannosamine and muramic acid) is more useful indicators of microbial necromass than microbial living biomass since they tend to stabilize and accumulate in soil. Moreover, their use as specific biomarkers is questionable due the widespread presence among microorganisms and plants.