DNA is the genetic material of various organisms. Extracellular DNA adsorbed or bound on surface-active particles in soils has been shown to persist for long periods against nucleases degradation and still retain the ability to transform competent cells. This paper reviews some recent advances on the binding and transformation of extracellular DNA in soils, which is fundamental to understanding the nature of the soil, regulating biodiversity, and assessing the risk of releasing genetically engineered microorganisms (GEMs) as well as being belpful for development of the genetic evohtional theory of bacteria. Several inlluencing factors, such as soil pH, ionic strength, soil surface properties, and characteristics of the DNA polymer, are discussed. To date, the understanding of the type of molecular binding siLes and the conformation of adsorbed and bound DNA to soil particles is still in its infancy.

Adsorption of heavy metals and redox reactions of Cr(W) ions on Fe-Mn nodules from five soils of China were investigated by chemical analysis, equilibrium adsorption/redox, and X-ray photoelectron spectroscopy (XPS). Results show that Mn is mainly present as Mn3+ and Mn4+ forms in Fe-Mn nodules. The maximum adsorption amounts for different heavy metal ions follow the order Pb2+≈Cu2+>Zn2+>Co2+>Ni2+>Cd2+. The adsorption capacity for heavy metals by Fe-Mn nodules from calciaquert in Shandong province (N5-1) is the highest, while that from hapludalf in Shandong province (N6-1) is the lowest. About 44-100% of the heavy metals adsorbed on Fe-Mn nodules were dissolved in 0.1 mol/L hydroxylamine hydrochloride (HAHC). The maximum amounts of Cr(VI) production by Fe-Mn nodules follow the order of N1-1 (69 mmol/kg)> N4-1 (57 mmol/kg)> N2-1 (52 mmol/kg)> N5-1 (44 mmol/kg). Based on the content of MnO2 in Fe-Mn nodules dissolved in HAHC, the amount of Cr(VI) production by Mn oxides in N1-1, N2-1, N4-1, and N5-1 is 326, 624, 726, and 482 mmol/kg (MnO2), respectively. We propose that the amounts of Cr(VI) production through oxidation Cr(Ⅲ) by Mn oxides are related to the types of Mn oxides in Fe–Mn nodules.

Soil samples were collected at an altitude of 500, 1,060, 1,500, 1,950, 2,400 and 3,100m, respectively, from Shennongjia, a forest reserve in Hubei province (central China). Their corresponding pHs were 5.50, 4.91, 5.64, 5.28, 5.49 and 4.60. By using a plant trap method, a total of 25 soybean rhizobia were isolated from the soil above an altitude of 1,500 m and all identified to be Sinorhizobium fredii. Their genetic biodiversity was characterized by 16S-23S rDNA internally transcribed spacer (ITS) region polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and random amplification DNA (RAPD) analysis. All the tested strains produced a 2.1 kb 16S-23S rDNA ITS fragment. After digestion with three restriction endonucleases (HaeⅢ, MspI and CfoI), respectively, great variations in 16S-23S rDNA ITS PCR-RFLP patterns were observed. The tested strains could be differentiated into 11 ITS genotypes. The genotypes of rhizobia were not related to geographical location. Twelve primers were applied to RAPD analysis and a dendrogram was obtained, showing that all the strains (including reference strain S. fredii USDA205) were divided into two diverging groups. Moreover, each group could be further divided into two subgroups. Both RAPD and 16S-23S rDNA ITS PCR-RFLP analysis indicated that a high degree of genetic diversity existed among S. fredii strains isolated from Shennongjia virgin soils. Since Shennongjia is an unexploited forest region in central China and the gene centre of soybean is located in China, the symbiotic genes harboured by these strains may be of great importance and the rich diversity of these strains might contribute to the adaptation of soybean to an alpine environment.