Distribution of soil fractions and location of soil bacteria in a vertisol under cultivation and perennial grass

Effects of soil management on soil characteristics were investigated on the rhizosphere (RPP) and the non-rhizosphere (NRPP) siol of a re-grass vertisol under #Digitaria decumbens and in the soil under continuous cultivation (CC). A low energy technique allowed to separate eight size and density fractions, including macro- and micro-aggregates while preserving soil bacteria. Organic C and N, microbial biomass C and the number of total bacteria (AODC) and of #Azospirillum brasilense and their distribution were determined in soil fractions isolated from the CC, NRPP and RPP soils. Soil macroaggregates (>2000 micrometers) were similarly predomminant in the NRPP and RPP soils when the dispersible clay size fraction (2000 micrometers) and from the root fraction, not from the finer separates. The proportion of organic C as microbial biomass C revealed the low turnover of microbial C in the PP situations, especially in the clay size fraction of the NRPP soil. A common shift of AODC toward the finer separates from planted soils (CC and RPP) revealed the influence of living plants on the distribution of soil bacteria. The relative abundance of #A. brasilense showed the presence of the active roots of #Digitaria in the macroaggregates and their contact with the dispersible clay size fraction of the rhizosphere soil. (Résumé d'auteur

80-річчя академіка НАН України В.І. Старостенка

Qualitative and quantitative changes in soil and fertilizer-derived organic N fractions were assessed during a cropping season in an intertropical Alfisol, under maize and pasture, fertilized with15 N-urea. Before the sowing, after fertilizing and after the harvest, the organic N of top soil samples was fractionated by a two-step acid hydrolysis under reflux (H1 = 1 M HCl for 3 h; H2 = 3 M HCl for 3 h). The total hydrolysable N (HN) from H1 decreased significantly during the cropping season in both maize and pasture soils. Contrastingly, the content of HN from H2 and that of non-hydrolysable N did not vary significantly during the cropping season. The easily hydrolysable fractions, especially amino acid N, amino sugar N and amide N, were the most active N pools and the major source of N potentially available for plants. The urea-derived N that remained in the soil was mainly in organic forms at both 7 and 108 d after fertilizing (70–82% and 93–98%, respectively), higher figures being found in pasture than in maize soil. The total amount of urea-derived HN decreased significantly during the crop period in both maize and pasture soils. This decrease was largely due to the decline in HN from H1. The amount of non-hydrolysable urea-derived N was significantly higher in pasture than in maize soil and it decreases in the former and increases in the latter, during the cropping season. During the crop period, the decrease of urea-derived organic N was 4.6 to 9.1 times higher than that of native organic N. Shortly after fertilizing, the proportion of urea-derived N in the easily hydrolysable (H1) organic fractions was higher than that of soil N, whereas the reverse was true for the slowly hydrolysable (H2) or insoluble fractions. These differences were less marked, but still significant, at the end of cropping. The easily hydrolysable organic N fractions were more sensitive than total N to the impact of land use intensification and are, therefore, a more useful index for early detection of soil biological degradation.Peer reviewe

Survival and spread of Shiga toxin-producing Escherichia coli in alpine pasture grasslands

International audienceAims: To determine the fate of Shiga toxin-producing Escherichia coli (STEC) strains defecated onto alpine grassland soils. Methods and Results: During the summers of 2005 and 2006, the field survival of STEC was monitored in cowpats and underlying soils in four different alpine pasture units. A most probable number (MPN)-PCR stx assay was used to enumerate STEC populations. STEC levels ranged between 3Æ9 and 5Æ4 log10 CFU g)1 in fresh cowpats and slowly decreased until their complete decay (inactivation rates k < 0Æ04 day)1). PFGE typing of STEC strains isolated from faecal and soil samples assessed the persistence of various clonal types for at least 2 months in cowpats and their vertical dispersal down through the soil at a depth up to at least 20 cm. STEC cells counts in soil were always below 2 log10 CFU g)1, regardless of the pasture unit investigated. The soil became rapidly free of detectable STEC once the cowpat had decomposed. The eight STEC strains isolated during this study belonged to six distinct serotypes and tested positive for the gene(s) stx2, including the stx2g and stx2 NV206 variants. Conclusions: STEC were able to persist in cowpats and disseminate down through the soil but were unable to establish. Significance and impact of the Study: This study provides useful information concerning the ecology of STEC in alpine pasture grasslands and may have implications for land and cattle management