Monitoring and evaluating soil quality

Soil quality influences agricultural sustainability, environmental quality and, consequently, plant, animal and human health. Microorganisms are useful indicators of soil quality because they have key functions in the decomposition of organic matter, nutrient cycling and maintenance of soil structure. We summarize methods used for monitoring biomass, activity and diversity of soil organisms and show some results of the Dutch Soil Quality Network. In contaminated soils microbial community structure was changed, but diversity was not always reduced. In contrast, microbial biomass and activity were reduced markedly. In agricultural soils there were large differences between different categories of soil type and land use. Organic management resulted in an increased role of soil organisms, as indicated by higher numbers and activity. Replacement of mineral fertilizers by farmyard manure stimulated the bacterial branch of the soil food web. Reduced availability of mineral nutrients appeared to increase fungi, presumably mycorrhizas. Bacterial DNA profiles did not indicate low genetic diversity in agricultural soils, compared with some acid and contaminated soils. Organic farms did not show higher genetic diversity than intensively farmed areas. At extensive grassland farms and organic grassland farms nitrogen mineralization was about 50% higher than on intensively farmed areas. Also, microbial biomass and activity, and different groups of soil invertebrates, tended to be higher. Soil biodiversity cannot be monitored meaningfully with only a few simple tools. Extensive and long-term monitoring is probably the most realistic approach to obtain objective information on differences between, changes within, and human impact on, ecosystems. In most countries, microbial biomass, respiration and potential nitrogen (N) mineralization are regarded as part of a minimum data set. Adding the main functional groups of the soil food web brings us closer to understanding biodiversity, potentially enabling us to relate the structure of the soil community to functions.