Long-term impacts of infrequent biosolids applications on chemical and microbial properties of a semi-arid rangeland soil

A plot study was conducted to quantify long-term (>12 years) impacts of a single biosolids application, and short-term impacts (<2 years) of a repeated application, on semi-arid rangeland soil chemical and microbial parameters. In 2003 and 2004, plots which had received 0, 2.5, 5, 10, 21, or 30 Mg biosolids ha⁻¹ once in 1991 (long-term plots), or again in 2002 (short-term plots), were sampled and analyzed for soil chemical parameters, microbial biovolumes, C and N mineralization activities, Biolog EcoPlate substrate utilization potential, and plant productivity and tissue quality. Repeated applications temporarily exacerbated differences in soil chemical properties among treatments, but after 2 years, soil chemistry trends were similar between short-term and long-term plots. Soils which received a repeated application of 21 or 30 Mg biosolids ha⁻¹ had greater bacterial biovolumes and C and N mineralization activities. In long-term plots, mineralization activities were stimulated only at the highest rate. Biosolids-amended soil communities also utilized Biolog substrates more quickly compared to communities from control plots. Plant biomass increased, whereas plant diversity and plant C/N ratio decreased with increasing application rate for both short- and long-term plots. Infrequent biosolids application had positive ecosystem effects in terms of site management objectives, with relatively low extractable metal levels in soil and greater plant biomass and tissue quality despite reduced species richness.     

Reciprocal transfer effects on denitrifying community composition and activity at forest and meadow sites in the Cascade Mountains of Oregon

In order to examine the effects of disturbance, vegetation type, and microclimate on denitrification and denitrifier community composition, experimental plots were established at the H. J. Andrews Experimental Forest in the Cascade Mountains of Oregon. Soil cores were reciprocally transplanted between meadow and forest and samples were collected after 1 and 2 years. Denitrifying enzyme activity (DEA) was measured using the acetylene block assay and terminal restriction length polymorphism profiles were generated with nosZ primers that target the gene coding for nitrous oxide reductase. Nitrate concentrations, C mineralization, and water content were measured to gain additional insights into soil properties controlling DEA. Meadow soils were significantly higher in DEA than forest soils, and the highest DEA levels were observed in cores transferred from the meadow into the forest. Nitrate concentrations were also different between forest and meadow soils, but did not correlate to DEA. DEA was higher in open versus closed cores, suggesting an association between denitrification and the rhizosphere. Denitrifier communities of undisturbed forest and meadow soils shifted through a 4-year period but remained distinct from each other. Similarly, denitrifier communities clustered by vegetation type of origin regardless of manipulation, suggesting that the overall denitrifier communities are well buffered against environmental changes.     

Increased N availability in grassland soils modifies their microbial communities and decreases the abundance of arbuscular mycorrhizal fungi

Two complementary studies were performed to examine (1) the effect of 18 years of nitrogen (N) fertilization, and (2) the effects of N fertilization during one growing season on soil microbial community composition and soil resource availability in a grassland ecosystem. N was added at three different rates: 0, 5.44, and 27.2 g N m⁻² y⁻¹. In both studies, Schizachyrium scoparium was the dominant plant species before N treatments were applied. Soil microbial communities from each experiment were characterized using fatty acid methyl ester (FAME) analysis. Discriminant analysis of the FAMEs separated the three N fertilizer treatments in both experiments, indicating shifts in the composition of the microbial communities. In general, plots that received N fertilizer at low or high application rates for 18 years showed increased proportions of bacterial FAMEs and decreased fungal FAMEs. In particular, control plots contained a significantly higher proportion of fungal FAMEs C18:1(cis9) and C18:2(cis9,12) and of the arbuscular mycorrhizal fungal (AMF) FAME, C16:1(cis11), than both of the N addition treatment plots. A significant negative effect of N fertilization on the AMF FAME, C16:1(cis11), was measured in the short-term experiment. Our results indicate that high rates of anthropogenic N deposition can lead to significant changes in the composition of soil microbial communities over short periods and can even disrupt the relationship between AMF and plants.     

Dynamics of microbial biomass and nitrogen supply during primary succession on blastfurnace slag dumps in dry tropics

This paper reports the role of microbial biomass in the establishment of N pools in the substratum during primary succession (till 40-year age) in Blastfurnace Slag Dumps, an anthropogenically created land form in the tropics. Initially in the depressions in the slag dumps fine soil particles (silt+clay) accumulate, retaining moisture therein, and providing microsites for the accumulation of microbial biomass. In all sites microbial biomass showed distinct seasonality, with summer-peak and rainy season-low standing crops. During the summer season microbial biomass C ranged from 18.6 μg g⁻¹ in the 1-year old site to ca. 235 μg g⁻¹ in the 40-year old site; correspondingly, microbial biomass N ranged from 1.22 to 40 μg g⁻¹. On sites 2.5-years of age and younger, the microbial biomass N content accounted for more than 50% of the organic N in the soil, whereas the proportion of microbial biomass N was ca. 7% of organic N in 40-year old site. The strong correlation between microbial biomass and total N in soil indicated a significant role of microbes in the build-up of nitrogen during the initial stages of succession in the slag dumps. Though the organic N pool in the soil was low (594 mg kg⁻¹) even after 40 years of succession, the available N (NH₄-N and NO₃-N) contents in the soil were generally high through the entire age series (ca. 16-32 μg g⁻¹) during the rainy season (which supports active growth of the herbaceous community). The high mineral-N status on the slag dump was related with high N-mineralization rates, particularly in the young sites (20.6 and 13.9 μg g⁻¹ month⁻¹ at 1 and 2.5-year age). We suggest that along with the abiotic factors having strong effect on ecosystem functioning, the microbial biomass, an important biotic factor, shows considerable influence on soil nutrient build-up during early stages of primary succession on the slag dumps. The microbial biomass dynamics initiates biotic control in developing slag dumps ecosystem through its effect on nitrogen pools and availability.     

Soil microbial community characteristics along an elevation gradient in the Laguna Mountains of Southern California

We sampled soil at four sites in the Laguna Mountains in the western Sonoran Desert to test the effects of site and sample location (between or beneath plants) on fatty acid methyl ester (FAME) and carbon substrate ulilization (Biolog) profiles. The four sites differed in elevation, soil type, plant community composition, and plant percent cover. Soil pH decreased and plant density increased with elevation. Fertile islands, defined as areas beneath plants with greater soil resources than bare areas, are present at all sites, but are most pronounced at lower elevations. Consistent with this pattern, fertile islands had the greatest influence on FAME and Biolog profiles at lower elevations. Based on the use of FAME biomarker and principal components analyses, we found that soil microbial communities between plants at the lowest elevation had proportionally more Gram-negative bacteria than all other soils. At the higher elevation sites there were few differences in FAME profiles of soils sampled between vs. beneath plants. Differences in FAME profiles under plants among the four sites were small, suggesting that the plant influence per se is more important than plant type in controlling FAME profiles. Since microbial biomass carbon was correlated with FAME number (r=0.85,P<0.0001) and with FAME named (r=0.88,P<0.0001) and total areas (r=0.84,P<0.0001), we standardized the FAME data to ensure that differences in FAME profiles among samples were not the result of differences in microbial biomass. Differences in microbial substrate utilization profiles among sampling locations were greatest between samples taken under vs. between plants at the two lower elevation sites. Microbial substrate utilization profiles, therefore, also seem to be influenced more by the presence of plants than by specific plant type.     

Changes in soil microbial biomass and aggregate stability following burning and soil rehabilitation

A greenhouse experiment was conducted in order to determine the influence of organic amendment, as compared with inorganic fertilization, and vegetation cover (Lolium perennne L.) on microbial biomass and aggregate stability in burnt soils. The study was performed with soil samples from the ash layer of three pine forests differing in their physicochemical and chemical properties and affected by high-intensity wildfires about 3 and 36 months before the sampling. Two different doses of both poultry manure and NPK fertilizer were used. Similar results were observed independent of the soil; however, the fertilization effect was most pronounced in the soil collected 36 months after the wildfire. Significant increases in microbial biomass C were detected following poultry manure addition, particularly at high dose, while no changes or slight increases were found as a consequence of inorganic fertilization. A significant but varied response of microorganisms to plant growing was also observed. Revegetation and fertilization also modified the aggregate stability values, but different effects were detected depending on the fertilizer used (organic or inorganic) and the dose of application. A high positive and significant relationship between soil microbial biomass and aggregate stability was observed. The results clearly indicate that organic amendment combined with the implantation of a vegetation cover can improve the reestablishment of both microbial biomass and soil structure in burnt pine forests.     

Carbon stocks, soil respiration and microbial biomass in fire-prone tropical grassland, woodland and forest ecosystems

A thorough understanding of the role of microbes in C cycling in relation to fire is important for estimation of C emissions and for development of guidelines for sustainable management of dry ecosystems. We investigated the seasonal changes and spatial distribution of soil total, dissolved organic C (DOC) and microbial biomass C during 18 months, quantified the soil CO₂ emission in the beginning of the rainy season, and related these variables to the fire frequency in important dry vegetation types grassland, woodland and dry forest in Ethiopia. The soil C isotope ratios (δ¹³C) reflected the 15-fold decrease in the grass biomass along the vegetation gradient and the 12-fold increase in woody biomass in the opposite direction. Changes in δ¹³C down the soil profiles also suggested that in two of the grass-dominated sites woody plants were more frequent in the past. The soil C stock ranged from being 2.5 (dry forest) to 48 times (grassland) higher than the C stock in the aboveground plant biomass. The influence of fire in frequently burnt wooded grassland was evident as an unchanged or increasing total C content down the soil profile. DOC and microbial biomass measured with the fumigation-extraction method (C_mic) reflected the vertical distribution of soil organic matter (SOM). However, although SOM was stable throughout the year, seasonal fluctuations in C_mic and substrate-induced respiration (SIR) were large. In woodland and woodland-wooded grassland C_mic and SIR increased in the dry season, and gradually decreased during the following rainy season, confirming previous suggestions that microbes may play an important role in nutrient retention in the dry season. However, in dry forest and two wooded grasslands C_mic and SIR was stable throughout the rainy season, or even increased in this period, which could lead to enhanced competition with plants for nutrients. Both the range and the seasonal changes in soil microbial biomass C in dry tropical ecosystems may be wider than previously assumed. Neither SIR nor C_mic were good predictors of in situ soil respiration. The soil respiration was relatively high in infrequently burnt forest and woodland, while frequently burnt grasslands had lower rates, presumably because most C is released through dry season burning and not through decomposition in fire-prone systems. Shifts in the relative importance of the two pathways for C release from organic matter may have strong implications for C and nutrient cycling in seasonally dry tropical ecosystems.     

Organic matter status and the size, activity and metabolic diversity of the soil microbial community in the row and inter-row of sugarcane under burning and trash retention

The concentrations of organic C, labile organic fractions and the size and activity of the microbial community were measured to a depth of 30 cm below the plant row and at distances of 30 and 60 cm into the inter-row area under sugarcane under pre-harvest burning or green cane harvesting with retention of a crop residue (trash) mulch. Total root mass was similar under burning and trashing but under trashing there was a redistribution of roots towards the surface 0-10 cm in the inter-row space as roots proliferated beneath the trash mulch. Soil organic C content decreased in response to both increasing distance from the plant row (to a depth of 20 cm) and burning rather than trashing (to a depth of 10 cm). Declines in K₂SO₄-extractable C, light fraction C, microbial biomass C, basal respiration and aggregate stability in response to distance and burning were much more marked than those for organic C and occurred to a depth of 30 cm. Bulk density was greater under burnt than trashed sugarcane and was greater in the inter-row than row, particularly under burning. Heterotrophic functional diversity (measured by analysis of catabolic response profiles to 36 substrates) was also investigated. Principal component analysis of response profiles demonstrated that soils below the row and those under trashing at 30 cm out from this row were separated from the other soils on PC1 and the sample from the inter-row centre (60 cm out) under burning was separated from the others on PC2. Catabolic evenness was least for the latter soil. It was concluded that soil in the inter-row of burnt sugarcane receives few inputs of organic matter and that conversion to green cane harvesting with retention of a trash mulch greatly improves the organic matter, microbial and physical status of the inter-row soil.     

Microbial dynamics in an anaerobic soil slurry amended with glucose, and their dependence on geochemical processes

Under anoxic conditions, microbial activities interact closely with geochemical reactions and consequently affect soils, underlying aquifers, and the atmosphere. Recent studies have noted the relationships between microbial biodiversity and environmental conditions, but the dynamics of numerous coexisting microbial groups in connection with soil biogeochemical processes has never been investigated. In this work, we investigated the dynamics of anaerobic microbial populations using a new method combining PCR-SSCP and epifluorescent direct counts, and analysed these results in the light of biogeochemical changes. Batch incubations were performed over an 8-day period on a calcic cambisol (WRB) incubated anaerobically, either without amendment (treatment C) or after adding glucose (treatment +G). In treatment +G, the predominant microbial processes included (i) NO₃⁻ and NO₂⁻ reduction during the first 12 and 24 h of incubation respectively, (ii) fermentations during the first 6 days with non-symbiotic N₂ fixation between days 1 and 6, enabling bacterial growth during this period, and probably (iii) reduction of FeIII by H₂ oxidation throughout the incubation period. In treatment C, microbiological and geochemical measurements revealed no prominent microbial activities, and the PCR-SSCP method led to complex bacterial density profiles without prominent peaks. In treatment +G, 78 microbial groups were distinguished; these were divided into seven sets (A to G) according to their dynamics. Bacteria belonging to sets A, E and F grew during the period of intense fermentation and were probably able to fix N₂, as is the case with Clostridium butyricum (set A). Bacteria belonging to sets B, D, and G were probably able to reduce FeIII to FeII with concomitant oxidation of H₂ into H₃O⁺, but unable to fix N₂. Two microbial groups in these sets were closely related to Clostridium favosporum (set B) and the genus Bacillus (set B). Bacteria belonging to class C were probably only able to reduce N oxide(s). Lastly, we obtained two similar estimates of the gross increase in microbial biomass by taking into account either (i) the sum of gross increases for the 78 microbial groups, or (ii) the energy yield of catabolic reactions minus the energy requirement for N₂ fixation.