Influence of arbuscular mycorrhizal fungi on soil structure and aggregate stability of a vertisol

The influence of arbuscular mycorrhizal (AM) fungi on aggregate stability of a semi-arid Indian vertisol was studied in a pot experiment in which Sorghum bicolor (L.) was grown as test plant for 10 weeks. Pasteurized soil inoculated with AM fungi was studied with pasteurized and unpasteurized soils as references. A part of the soil in each pot was placed in nylon mesh bags to separate effects of roots and hyphae. The sorghum plants were planted outside the mesh bags which permitted AM hyphae to enter while excluding roots. Aggregate stability of the soil was determined by wet-sieving and turbidimetric measurements. Development of the AM fungi was quantified as colonized root length and external hyphal length. Soil exposed to growth of roots and hyphae (outside mesh bags) showed aggregates with larger geometric mean diameter (GMD) in pasteurized soil inoculated with AM fungi than in pasteurized uninoculated soil. There was no significant difference in GMD of the inoculated, pasteurized soil and the unpasteurized soil. No significant effects of inoculation or plant growth were found in pasteurized soil exposed to hyphal growth only (inside the mesh bags). However, the unpasteurized soil had significantly higher GMD than the pasteurized soil, irrespective of plants and inoculum. Turbidimetric measurements of soil exposed to roots and hyphae (outside mesh bags) showed the highest aggregate stability for the inoculated pasteurized soil. These results demonstrate that AM fungi contribute to the stabilization of soil aggregates in a vertisol, and that the effect is significant after only one growing season. The effect was associated with both AM hyphae and the stimulation of root growth by AM fungi. The contribution from plant roots and AM hyphae to aggregate stability of different size fractions is discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Influence of rainfall,temperature and somes oil physico-chemical variables on seasonal population structure and vertical distribution of earthworms in two semi-arid tropical grassland soils

Two semi-arid tropical grasslands, one of an alfisol and the other on vertisol were sampled for earthworm populations during July 1985 to July 1987. The soil of the alfisol grassland was slightly acidic while that of the vertisol was slightly alkaline. The alfisol grassland has a lower range of soil moisture, soil temperature, electrical conductivity, organic carbon, available nitrogen and potassium, and a higher range of phosphorus compared to the vertisol. The alfisol grassland was inhabited byOctochaetona phillotti (Michaelsen) with rare occurrence ofBarogaster annandalei (Stephenson) whereas the vertisol grassland was inhabited only byB. annandalei. The total population ofO. phillotti ranged between 16 and 96/m² while its juvenile population ranged between 3.2 and 25.6/m², pre-adult population between 5.3 and 53.6/m² and adult population between 0 and 28.8/m². The total population ofB. annandalei ranged between 3.2 and 58.3/m² and its juvenile population between 0 and 26.6/m², pre-adult population 0 and 10.6/m² and adult population between 0 and 22.4/m². The earthworms migrated to deeper layers during winter and summer,O. phillotti reaching a maximum depth of 40 cm andB. annandalei reaching a depth of 45 cm, and seen in quiescent stage. Their seasonal population structure was significantly influenced by the seasonal patterns in rainfall, soil temperature, pH, electrical conductivity and phosphorus. These variables along with organic carbon, available nitrogen and potassium together accounted for 32–63% and 79–91% respectively, of the variation in seasonal population fluctuation of differentO. phillotti andB. annandalei age groups. The physical factors of the soil were collectively more effective in causing the seasonal variation in their population size than the chemical factors.Visiting scientist (under the Rockefeller Foundation Environmental Research Fellowship in International Agriculture) in Resource Management Programme of ICRISAT, Andhra Pradesh, India     

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 nonrhizosphere (NRPP) soil of a re-grass vertisol underDigitaria 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 ofAzospirillum 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) respresented 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 ofA. brasilense showed the presence of the active roots ofDigitaria in the macroaggregates and their contact with the dispersible clay size fraction of the rhizosphere soil.