Soil colloids-bound plasmid DNA: Effect on transformation of E. coli and resistance to DNase I degradation

The adsorption and binding of plasmid p34S DNA on four different colloidal fractions from a Brown soil and clay minerals in the presence of various Ca²⁺ concentrations, the ability of bound DNA to transform competent cells of CaCl₂-treated Escherichia coli, and the resistance of bound DNA to degradation by DNase I were studied. DNA adsorption on soil colloids and clay minerals was promoted in the presence of Ca²⁺. Kaolinite exhibited the highest adsorption affinity for DNA among the examined soil colloids and clay minerals. In comparison with organo-mineral complexes (organic clays) and fine clays (<0.2 μm), DNA was tightly adsorbed by H₂O₂-treated clays (inorganic clays) and coarse clays (0.2-2 μm). The transformation efficiency of bound DNA increased with increasing concentrations of Ca²⁺ at which soil colloid or clay mineral-DNA complexes were formed. DNA bound by kaolinite showed the lowest transformation efficiency, and especially no transformants were observed with kaolinite-DNA complex prepared at 5-100 mM Ca²⁺. Compared to organic clays and fine clays, DNA bound on inorganic clays and coarse clays showed a lower capacity to transform E. coli at different Ca²⁺ concentrations. The presence of soil colloids and minerals provided protection to DNA against degradation by DNase I. Montmorillonite, organic clays and fine clays showed stronger protective effects for DNA than inorganic clays and coarse clays. The protection mechanisms as well as the differences in transforming efficiency of plasmid DNA molecules bound on various soil colloidal particles are discussed. The information obtained in this study is of fundamental significance for the understanding of the horizontal dissemination of recombinant DNA and the fate of extracellular DNA in soil environments.     

The effect of mixing organic biological waste materials and high-N crop residues on the short-time N2O emission from horticultural soil in model experiments

Manipulating the N release from high-N crop residues by simultaneous mixing of these residues with organic biological waste (OBW) materials seems to be a possible method to reduce NO₃^– leaching. The aim of this study was to examine whether the incorporation of OBW materials together with a high-N crop residue (celery) had also an effect on N₂O emission from horticultural soil under short-term and optimised laboratory conditions. A sandy loam soil and celery residues were mixed with different OBW materials and brought into PVC tubes at 80% water-filled pore space and 15°C. Every 2.5 h, a gas sample was taken and analysed by gas chromatography for its N₂O concentration. The soil amended with only celery residues had a cumulative N₂O emission of 9.6 mg N kg^–1 soil in 50 h. When the celery residues were mixed with an OBW material, the N₂O emission was each time lower than the emission from the celery-only treatment (between 3.8 and 5.9 mg N kg^–1 soil during maximum 77 h), except with paper sludge (17.2 mg N kg^–1 soil in 100 h). The higher N₂O emission from the paper sludge treatment was probably due to its unusually low C:N ratio. Straw, green waste compost 1 (GWC1) and 2 (GWC2), saw dust, and tannic acid reduced the N₂O emission of the celery treatment by 40 to 60%. Although the N₂O reduction potential can be expected to be lower and with differing dynamics under field conditions, this study indicates that apart from reducing NO₃^– leaching, OBW application may at the same time reduce N₂O emissions after incorporation of high-N crop residues.