Biological Soil Quality

Biological soil properties are critical indicators for soil quality. Soil biota play a major role in important soil processes, such as releasing nutrients via mineralisation of organic matter and creating and maintaining a good soil structure. Further, biological soil properties can quickly change in response to changes in soil- and crop management practices. We have performed a large-scale literature survey on the effects of good agricultural management practices on indicators for biological soil quality. The literature contains experimental data on earthworm and nematode abundance, microbial biomass carbon (MBC) and bacterial and fungal communities from more than 65 European long-term field experiments (LTEs). The effects of management practices on the selected biological soil indicators were analyzed. We evaluated whether climatic zone, soil texture and the number of consecutive years that a certain practice was applied influenced the effect of soil- and management practices. A sufficient amount of data was available for tillage and organic fertilization which allowed a thorough statistical analysis for those management practices in our study.

Figure 1: Relative (A) or absolute (B) increase/decrease of microbial biomass carbon (MBC) content (A) and earthworm number (B) when adopting potential best management practices (NT: no tillage; SNIT: shallow non-inversion tillage; DNIT: deep NIT; COMP: compost; FYM: farmyard manure; S: animal slurry) compared to a reference practice (i.e. ploughing and mineral fertilisation) (MBC: reference practice equals 1; earthworm number: reference practice equals 0)

Amongst the biological indicators, earthworm abundance and MBC content are frequently monitored in European LTEs. These indicators tend to respond well to a change in crop and soil management. Overall, farmyard manure and compost amendment emerged as the best management practices for increasing both earthworm abundance and MBC content. Apparently, soil biota benefit more from organic materials added to the soil, which serve as a food source, than they benefit from reduced soil disturbance (more stable habitat) (Figure 1). Further, the effect of a given management practice on soil biological quality was rarely influenced by climatic zone, soil texture or the number of consecutive years a practice was applied. Data on the nematode, bacterial and fungal community are still rather scarce. In this study, their response to management changes proved to be more difficult to interpret - we observed a shift in the community structure instead of a clear increase or decrease of a given species. However, because of the presence of nematodes in all trophical levels of the soil food web, nematode community indices could be likely candidates to become indicators of soil quality.

In general, it can be noted that soil biology, despite being known for its role in soil quality, is not often taken into account in soil research. To make progress in understanding and steering soil quality, we should step away from the current compartimentalisation into physical, chemical and biological disciplines. A multidisciplinary approach that takes chemical, physical and biological soil indicators into account will deliver more meaningful results and recommendations.

Effects of all investigated potential best management practices (BMPs) on biological indicators of soil quality

Table 1: Summary of relative response rate (RR) or absolute increase or decrease (DIFF) of biological indicators of soil quality when adopting potential BMPs. ++: very positive effect; +: positive effect; 0: neutral effect; -: negative effect; --: very negative effect; +/-  depends on the conditions (all compared to the baseline treatment (indicated in grey)). PPNEM: plant-parasitic nematodes; FUNGNEM: Fungivorous nematodes; BACNEM: bacterivorous nematodes; BACPLFA: bacterial phospholipid fatty acids; FUNGPLFA: fungal phospholipid fatty acids.