Enzymatic and microbial activities as influenced by tillage and fertilization in a semi-arid Mediterranean agroecosystem

<p>Soil enzymes respond rapidly to changes in soil managements, and therefore are used as early and sensitive indicators of alteration in soil properties induced by tillage and additions of fertilizers. The aim of this work was to compare the effects of different tillage (no, minimum, and conventional tillage), fertilization and soil depth (0-30, 30-60 and 60- 90 cm) on the microbial biomass, enzyme activity and their relationship with soil nutrients in a semiarid Mediterranean agro-ecosystem. Growing and total microbial biomass decreased with depth together with the activities of β-glucosidase and N-acetyl-β-glucosaminidase presumably because of the reduced carbon and oxygen content in the deeper layers of soils. The fertilization stimulated fast-growing microorganisms with low affinity of enzyme systems to substrate, enhanced the growing microbial biomass and facilitated the turnover rate of soil organics. Under no tillage, all enzymes showed higher potential activity in top layers of fertilized plots as compared with non-fertilized ones. The minimum tillage practice increased the growing microbial biomass, and stimulated N- and P-acquiring enzymes due to  increased nutrients limitation. Parameters of microbial growth and enzyme kinetics are suitable indicators of microbial activity in semiarid Mediterranean agroecosystems.</p>

Spatial Variability in Streambed Microbial Community Structure Across Two Watersheds

ABSTRACTThe spatial patterns of microbial communities are largely unknown compared to those of macro-fauna and flora. We investigated patterns of microbial community structure on streambed sediments from two watersheds across spatial scales spanning < 1m within a single stream to several hundred km between watersheds. Analyses of phospholipid fatty acids (PLFA) profiles indicated that the variations in microbial community structure were driven by increases in the relative abundance of microeukaryotic photoautotrophs and their contribution to total microbial biomass. Furthermore, streams within watersheds had similar microbial community structure, underscoring within-watershed controls of microbial communities. Moreover, bacterial community structure assayed as either polymerase chain reaction-denaturing gradient gelelectrophoresis (PCR-DGGE) fingerprints or PLFA profiles edited to remove microeukaryotes indicated a distinct watershed-level biogeography. No distinct stream order-level distributions were identified although DGGE analysis clearly indicated that there was greater variability in community structure among 1st-order streams compared to 2nd- and 3rd-order streams into which they flowed. Longitudinal gradients in microbial biomass and structure showed that the greatest variations were associated with 1st order streams within a watershed and 68% of the variation in total microbial biomass was explained by sediment C:N mass ratio, percent Carbon, sediment surface area, and percent water content. This study confirms a distinct microbial biogeography for headwater stream communities driven by environmental heterogeneity across distant watersheds and suggests that eukaryotic photoautotrophs play a key role in structuring sediment microbial communities.IMPORTANCEMicroorganisms in streams drive many biogeochemical reactions of global significance, including nutrient cycling and energy flow, yet the mechanisms responsible for the distribution and composition of streambed microbial communities are not well known. We sampled sediments from multiple streams in two watersheds; Neversink River (New York) and White Clay Creek (Pennsylvania) watersheds and measured microbial biomass, total microbial and bacterial community structures using phospholipid and molecular methods. Microbial and bacterial community structures displayed a distinct watershed-level biogeography. The smallest headwater streams within a watershed showed the greatest variation in microbial biomass, and C:N ratio, percent carbon, sediment surface area and percent water content explained 68% of the variations in microbial biomass. This study indicates a non-random distribution of microbial communities in streambeds, and that microeukaryotic photoautotrophs, environmental heterogeneity and geographical distance influence microbial composition and spatial distribution.

Methods for elucidating microbial biomass

<p>The microbial biomass of soil is defined as the part of the organic matter in the soil that constitutes living organisms smaller than the 5-10 mm^3.It is generally expressed in the milligrams of carbon per kilogram of soil or micrograms of carbons per gram of dry weight of soil. Typical biomass carbon ranges from 1 to 5% of soil organic matter. Biomass literally means “mass of living material” and can be expressed in units of weight (grams) or units of energy (calories or joules). Biomass is an important ecological parameter as it represents the quantity of energy being stored in a particular segment of biological community. </p> Measurement of biomass is used to determine standing crop of a population and transfer of energy between trophic levels within an ecosystem. A measure of the total microbial biomass in soil is often required when studying productivity or fertility of soils. Sometimes the biomass of specific parts of the microbiota is required-for example fungal biomass versus bacterial biomass.

Effect of forest harvesting and slash piling on microbial biomass and respiration in Newfoundland boreal forests

Clearcutting Newfoundland boreal forests significantly reduced organic layer fungal and total microbial biomass in clearcut areas with and without slash cover, compared with forested plots. However, aerobically incubated respiration rates were highest in organic layers from clearcut areas under slash, intermediate under forests, and lowest from clearcut areas without slash. Key words: Carbon, ergosterol, fumigation–extraction, fungal biomass, harvest slash, nitrogen