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

Abstract

Effects of soil management on soil characteristics were investigated on the rhizosphere (RPP) and the nonrhizosphere (NRPP) soil 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 microaggregates 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 #m) were similarly predominant in the NRPP and RPP soils when the dispersible clay size fraction (<2 #m) represented more than 25% of the CC soil mass. The main increase of C content in RPP originated from the macroaggregates (> 2000 #m) 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.

Comparison ofnifHGene Pools in Soils and Soil Microenvironments with Contrasting Properties

“…5) revealed that most differences in genetic structure occurred between the coarse fractions (Ͼ250 m and 50 to 250 m) and the clay fraction (Ͻ2 m). Various studies, such as whole-cell counting (24,40) and biomass measurements (23), as well as specific bacterial enumerations (25) and determinations of the genetic structures (40) and activities (6,28,30) of bacterial subcommunities, have shown that soil microenvironments differ from each other. Ordering on PC1 grouped nifH gene pools located in microenvironments with similar granulometric characteristics: the two coarse fractions (Ͼ250 m and 50 to 250 m) were closely related, as were the two medium fractions (20 to 50 m and 2 to 20 m).…”

“…The cultivated LCSA-c soil sample was separated into five fractions, corresponding to various sizes of particles and aggregates, by a size fractionation procedure (25). The 250-to 2,000-m and 50-to 250-m fractions were coarse and fine sands, respectively, with associated macroaggregates.…”

Distribution of Microorganisms, Biomass ATP, and Enzyme Activities in Organic and Mineral Particles of a Long-Term Wastewater Irrigated Soil

“…In the latter respect, Trehen et al (1993) asked a similar question by formulating conceptual approaches in soil biology: ªWhere are the microbes located, and are there specific locations for the different microbial activities?º In our previous paper we demonstrated the feasibility of selected microbiological and biochemical soil parameters to indicate the effects of a longterm wastewater irrigation on a sandy Cambisol and Haplic Luvisol (Filip et al, 1999). Such an ecological approach to soil microbiology received recently an enhanced attention in studies published by Kabir et al (1994), Nacro et al (1996), Van Gestel et al (1996), Monreal and Kodama (1997), Chotte et al (1998) and Stemmer et al (1998). Yet, the ecologically based soil investigations require also the existing relationship between the biotic and abiotic soil constituents to be consequently elucidated (Filip, 1975;Munch 1995).…”

Prevalence of Autoplaque Phenomenon in Bacterial Communities from Spatially Heterogeneous Environment of an Abandoned Mine

“…The soil was fractionated using thestandard method outlined by Kabir et al, (1994). One hundred gram soil was taken in 250ml Erlenmeyer flask containing sterile distilled water and kept at 4C for 36h.…”