Methods to evaluate pesticide damage to the biomass of the soil microflora

Respiratory methods to estimate the amount of C in the soil microbial biomass and the relative contributions of prokaryotes and eukaryotes in the biomass were used to evaluate the influence of pesticides on the soil microflora. Experiments were conducted with 5 and 50 micrograms per gram of three fungicides, captan, thiram and verdesan. At 5 micrograms per gram they caused significant decreases (40%) in the biomass; the organomercury fungicide verdesan also caused a shift from fungal to bacterial dominance. Within 8 days, biomass in captan- and thiram-amended soils had recovered to that of controls. Although the fungal to bacterial balance was restored in verdesan-amended soils, biomass recovery was not complete. At 50 micrograms per gram the fungicides caused long-term decreases in the biomass and altered the relative proportions of the bacterial and fungal populations. Verdesan had the greatest effect on soil microbial biomass and competition.

Assessment of physical and hydrological properties of biological soil crusts using x-ray microtomography and modelling

Biological soil crusts (BSCs) are formed by aggregates of soil particles and communities of microbial organisms and are common in all drylands. The role of BSCs on infiltration remains uncertain due to the lack of data on their role in affecting soil physical properties such as porosity and structure. Quantitative assessment of these properties is primarily hindered by the fragile nature of the crusts. Here we show how the use of a combination of non-destructive imaging X-ray microtomography (XMT) and Lattice Boltzmann method (LBM) enables quantification of key soil physical parameters and the modeling of water flow through BSCs samples from Kalahari Sands, Botswana. We quantify porosity and flow changes as a result of mechanical disturbance of such a fragile cyanobacteria-dominated crust. Results show significant variations in porosity between different types of crusts and how they affect the flow and that disturbance of a cyanobacteria-dominated crust results in the breakdown of larger pore spaces and reduces flow rates through the surface layer. We conclude that the XMT–LBM approach is well suited for study of fragile surface crust samples where physical and hydraulic properties cannot be easily quantified using conventional methods.