A.J. Schouten

Monitoring and evaluating soil quality

This book provides a selection of microbiological methods that are already applied in regional or national soil quality monitoring programs. It is split into two parts: part one gives an overview of approaches to monitoring, evaluating and managing soil quality. Part two provides a selection of methods, which are described in sufficient detail to use the book as a practical handbook in the laboratory. The methods are described in chapters on soil microbial biomass and numbers, soil microbial activity, soil microbial diversity and community composition, and plant-microbe interactions and soil quality

Dynamics and stratification of functional groups of nematodes in the organic layer of a Scots pine forest in relation to temperature and moisture

Abstract Nematode trophic groups were studied in litterbags in a Pinus silvestris forest using sampling periods of 8 weeks during 2.5 years. Temperature, moisture relationships and annual periodicity of nematodes were analyzed in the litter (L), fragmentation (F) and humus (H) horizons. Litterbags containing L, F and H material were placed in stacks and buried in the organic layer. Undisturbed core samples were used to compare the nematode abundance under normal field conditions with that in the litterbags. Time dependence of population growth and colonization was also studied in separate litterbags that were replaced every 8 weeks. During the first 4 to 6 months of the experiment, nematodes in the litterbag stacks grew rapidly to circa 5×106m–2. After that period, abundance gradually decreased to about 2.5×106m–2. These abundances were similar to those found in undisturbed cores. Nematode abundance during the first year was most pronounced in the top (L) litterbags; subsequently densities were more or less the same in the three organic horizons, reflecting the gradual change of L to F material. On average, during 2.5 years, bacterial feeding nematodes were the dominant group in the organic horizons (73%), with 17% hyphal feeders and 9% plant feeders. There were dissimilarities between layers and in the course of time. The number of hyphal feeding nematodes differed significantly between layers. In the first 2 to 4 months, hyphal feeding nematodes equalled the bacterial feeders in the L layer. Later bacterial feeders became dominant. The highest number of plant feeding nematodes was found in the F litterbags. Significant effects of temperature and moisture were mainly found on bacterial feeding nematodes. Regression coefficients for trophic group abundances and moisture were generally positive. Temperature was negatively correlated with the three functional groups in the L horizon only. Bacterial and hyphal feeding nematodes showed a significant decrease with time in the L layer, reflecting diminishing substrate quality (and food availability) during decomposition. A significant annual periodicity could be demonstrated for bacterial feeders in L litterbags and plant feeding nematodes in the H material.

Estimating the effect on soil organisms of exceeding no-observed effect concentrations (NOECs) of persistent toxicants

In estimating the effects of toxic substances on ecosystems we generally lack information on the sensitivity (expressed as a no-observed effect concentration, NOEC) of individual species in the field, and have to rely on information from laboratory test species, expressed as a frequency distribution of NOECs. In this case we can express toxic stress as the fraction of organisms that is exposed above its NOEC: the potentially affected fraction (PAF). This paper describes a model of the soil food web and the effect of toxic stress by persistent pollutants. The model predicts that in the absence of competition, individual species disappear from the foodweb at toxic concentrations 3–5 times their NOEC. With competition present, species affected by toxic stress are replaced by less sensitive ones. This has a twofold effect: species disappear from the foodweb at a lower concentration because loss of competitiveness occurs well before absolute extinction, but the replacement of disappearing species implies that the effect on total biomass and diversity becomes only noticeable at a PAF level near 100%. Model predictions are in good agreement with observations on nematode communities in experimental fields contaminated with copper and zinc. The model serves to illustrate why overall measures of ecosystem functioning (total biomass, production, diversity) are affected by toxic stress only at high levels of pollution, which is particularly true for systems with a high diversity. This apparent robustness masks a considerable genetic ‘erosion’, i.e. the disappearance of sensitive species or genotypes. The PAF is a good indicator of the latter effect.