Microbial activity in the soil is an indicator of soil health. An analysis of data series from the cantonal soil-monitoring networks over a ten-year period confirms textbook knowledge and shows what we need to look out for.
Healthy soils are a scarce resource and of great environmental and economic value, as stipulated by the Swiss Federal Council in its Bodenstrategie Schweiz [‘Soil Strategy Switzerland’]. Among other things, for instance, the soil is the basis of food production, and acts as a filter for drinking-water production.
Consequently, we have been monitoring the soil for a fairly long time. In recent years – in addition to properties such as nutrient content or soil structure – we have been interested soil life, and microbial activity in particular. Microorganisms play an important role in breaking down organic matter, promoting nutrient cycling and forming humus, thus influencing soil fertility. The biological activity of a soil is therefore an integral indicator for its site-specific condition.
For over ten years now, microbial activity has been measured as part of cantonal soil monitoring in the cantons of Aargau, Grisons and Bern. The measurements have now been analysed. The aim of the study was to determine whether the values of the measured parameters have been subject to changes over time or to changes in soil use, and whether the methods used should be optimised. Microbial biomass, soil respiration and metabolic quotient – a measure of the energy efficiency of a community of microorganisms – were measured.
Human Impacts affect soil life
On the one hand, the investigation confirmed textbook knowledge: the greater human impacts, the lower the soil biological activity. Activity was lower in ploughed soils than in no-till soils, and lower in arable soils than in permanent grassland. Interestingly, microbial activity within the various types remained stable over the years.
By contrast, soil life changed dramatically on nature-conservation land whose humus-rich topsoil had been removed in order to create nutrient-poor sites for endangered plant species. After removal of the humus-containing topsoil layer, microbial activity fell drastically; thereafter, the measured values rose steadily, but even after 10 years were lower than the initial values before topsoil removal.
Parameters used measure soil life effectively
The results of the study show that the parameters used are suitable for long-term monitoring. For statistical reasons, it is important to measure sufficiently frequently for a sufficiently long period, ideally once a year for five years, or at least five times over a longer period. Furthermore, a minimum sampling depth of 10 cm should be respected for the long-term monitoring, since too shallow a sampling substantially increases the variability of the analysis results. Finally, it has also been shown that, given a reliable sampling strategy with sufficient and regularly spaced core samplings, one composite sample is sufficient. Laboratory reference samples must also be measured.
- The parameters microbial biomass and basal respiration can be used to record the initial state of soils and their changes over time.
- Permanent grassland sites and arable plots with controlled crop rotation did not exhibit any major changes in biological activity over time. By contrast, the measured values on nature-conservation land where the topsoil had recently been removed increased steadily over the investigation period.
- For statistically sound statements, at least five measuring points should be recorded over a fairly long time period.
- Sampling depths of less than 10 cm are unsuitable, owing to the wide variability.
- A composite sample is possible if the sampling strategy is robust. Laboratory reference samples must also be measured in order to guarantee the stability of the measurement over a long period of time.