Oleg V. Menyailo

Stable isotope discrimination during soil denitrification: Production and consumption of nitrous oxide

Measuring the stable isotope composition of nitrous oxide ( N(2)O) evolved from soil could improve our understanding of the relative contributions of the main microbial processes ( nitrification and denitrification) responsible for N(2)O formation in soil. However, interpretation of the isotopic data in N(2)O is complicated by the lack of knowledge of fractionation parameters by different microbial processes responsible for N(2)O production and consumption. Here we report isotopic enrichment for both nitrogen and oxygen isotopes in two stages of denitrification, N(2)O production and N(2)O reduction. We found that during both N(2)O production and reduction, enrichments were higher for oxygen than nitrogen. For both elements, enrichments were larger for N(2)O production stage than for N(2)O reduction. During gross N(2)O production, the ratio of delta(18)O- to-delta(15)N differed between soils, ranging from 1.6 to 2.7. By contrast, during N(2)O reduction, we observed a constant ratio of delta(18)O- to-delta(15)N with a value near 2.5. If general, this ratio could be used to estimate the proportion of N(2)O being reduced in the soil before escaping into the atmosphere. Because N(2)O- reductase enriches N(2)O in both isotopes, the global reduction of N(2)O consumption by soil may contribute to the globally observed isotopic depletion of atmospheric N(2)O.

The effect of single tree species on soil microbial activities related to C and N cycling in the Siberian artificial afforestation experiment

The effects of grassland conversion to forest vegetation and of individual tree species on microbial activity in Siberia are largely unstudied. Here, we examined the effects of the six most commonly dominant tree species in Siberian forests (Scots pine, spruce, Arolla pine, larch, aspen and birch) on soil C and N mineralization, N2O-reduction and N2O production during denitrification 30 years after planting. We also documented the effect of grassland conversion to different tree species on microbial activities at different soil depths and their relationships to soil chemical properties. The effects of tree species and grassland conversion were more pronounced on N than on C transformations. Tree species and grassland conversion did significantly alter substrate-induced respiration (SIR) and basal respiration, but the differences were not as large as those observed for N transformations. Variances in SIR and basal respiration within species were markedly lower than those in N transformations. Net N mineralization, net nitrification, and denitrification potential were highest under Arolla pine and larch, intermediate under deciduous aspen and birch, and lowest beneath spruce and Scots pine. Tree species caused similar effects on denitrification potential, net N mineralization, and net nitrification, but effects on N2O reduction rate were idiosyncratic, indicating a decoupling of N2O production and reduction. We predict that deciduous species should produce more N2O in the field than conifers, and that Siberian forests will produce more N2O if global climate change alters tree species composition. Basal respiration and SIR showed inverse responses to tree species: when basal respiration increased in response to a given tree species, SIR declined. SIR may have been controlled by NH4+ availability and related therefore to N mineralization, which was negatively affected by grassland conversion. Basal respiration appeared to be less limited by NH4+ and controlled mostly by readily available organic C (DOC), which was higher in concentration under forests than in grassland and therefore basal respiration was higher in forested soils. We conclude that in the Siberian artificial afforestation experiment, soil C mineralization was not limited by N.

Tree species mediated soil chemical changes in a Siberian artificial afforestation experiment

Natural and human-induced changes in the composition of boreal forests will likely alter soil properties, but predicting these effects requires a better understanding of how individual forest species alter soils. We show that 30 years of experimental afforestation in Siberia caused species-specific changes in soil chemical properties, including pH, DOC, DON, Na+, NH4+, total C, C/N, Mn2+, and SO42-. Some of these properties –- pH, total C, C/N, DOC, DON, Na+ –- also differed by soil depth, but we found no strong evidence for species-dependent effects on vertical differentiation of soil properties (i.e., no species × depth interaction). A number of soil properties –- NO3−, N, Al3+, Ca2+, Fe3+, K+, Mg2+ and Cl− –- responded to neither species nor depth. The six studied species may be clustered into three groups based on their effects on the soil properties. Scots pine and spruce had the lowest pH, highest C/N ratio and intermediate C content in soil. The other two coniferous species, Arolla pine and larch, had the highest soil C contents, highest pH values, and intermediate C/N ratios. Finally, the two deciduous hardwood species, aspen and birch, had the lowest C/N ratio, intermediate pH values, and lowest C content. These tree-mediated soil chemical changes are important for their likely effects on soil microbiological activities, including C and N mineralization and the production and consumption of greenhouse gases.

Interactive effects of tree species and soil moisture on methane consumption

Abstract Methane consumption by temperate forest soils is a major sink for this important greenhouse gas, but little is known about how tree species influence CH 4 uptake by soils. Here, we show that six common tree species in Siberian boreal and temperate forests significantly affect potential CH 4 consumption in laboratory microcosms. Overall, soils under hardwood species (aspen and birch) consumed CH 4 at higher rates than soils under coniferous species and grassland. While NH 4 + addition often reduces CH 4 uptake, we found no effect of NH 4 + addition, possibly because of the relatively high ratio of CH 4 -to-NH 4 + in our incubations. The effects of soil moisture strongly depended on plant species. An increase in soil moisture enhanced CH 4 consumption in soils under spruce but had the opposite effect under Scots pine and larch. Under other species, soil moisture did not affect CH 4 consumption. These results could be explained by specific responses of different groups of CH 4 -oxidizing bacteria to elevated moisture.

Soil microbial activities in tree-based cropping systems and natural forests of the Central Amazon, Brazil

Little information is available about the factors controlling soil C and N transformations in natural tropical forests and tree-based cropping systems. The aim of this work was to study the effects of single trees on soil microbiological activities from plantations of timber and non-timber species as well as species of primary and secondary forests in the Central Amazon. Soil samples were taken in the primary forest under Oenocarpus bacaba and Eschweilera spp., in secondary regrowth with Vismia spp., under two non-timber tree species (Bixa orellana L. and Theobroma grandiflorum Willd.), and two species planted for wood production (Carapa guianensis Aubl. and Ceiba pentandra). In these soils, net N mineralization, net nitrification, denitrification potential, basal and substrate-induced respiration rates were studied under standardized soil moisture and temperature conditions. Individual tree species more strongly affected N transformations, particularly net nitrification, than C respiration. Our results suggest that soil C respiration can be affected by tree species if inorganic N becomes a limiting factor. We found a strong correlation among almost all microbiological processes suggesting close inter-relationship between C and N transformations in the studied soils. Correlation analysis between soil chemical properties and microbiological activities suggest that such strong inter-relationships are likely due to competition between the denitrifying and C-mineralizing communities for NO3−, which might be an important N source for the microbial population in the studied soils.

Nitrogen stable isotope composition of leaves and roots of plants growing in a forest and a meadow

In controlled N-nutrition experiments, differences in δ 15N composition of leaves and roots are regularly found. In this paper we report results from a survey of nitrogen stable isotope signatures of leaves and roots of 16 plant species growing under natural conditions in a meadow and a forest understorey, which differed in nitrate and ammonium availability. Significant differences between leaf and root were observed. The range of Δ 15N [leaf-root] values was m 0.97 to +0.86, small compared to published values from controlled N-nutrition experiments, but almost as large as the range of leaf δ 15N values (m 1.04 to +1.08). Forbs showed the largest differences between leaves and roots and showed a significant difference with respect to habitat. Grasses and legumes did not show significant differences in Δ 15N [leaf-root] between the two habitats. Care must be taken when using leaf δ 15N values as representative for whole-plant 15N composition in these two habitats.

Tree species of the central amazon and soil moisture alter stable isotope composition of nitrogen and oxygen in nitrous oxide evolved from soil

The use of stable isotopes of N and O in N2O has been proposed as a way to better constrain the global budget of atmospheric N2O and to better understand the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N2O formation in soil. This study compared the isotopic composition of N2O emitted from soils under different tree species in the Brazilian Amazon. We also compared the effect of tree species with that of soil moisture, as we expected the latter to be the main factor regulating the proportion of nitrifier- and denitrifier-derived N2O and, consequently, isotopic signatures of N2O. Tree species significantly affected δ 15N in nitrous oxide. However, there was no evidence that the observed variation in δ 15N in N2O was determined by varying proportions of nitrifier- vs. denitrifier-derived N2O. We submit that the large variation in δ 15N-N2O is the result of competition between denitrifying and immobilizing microorganisms for NO 3 m . In addition to altering δ 15N-N2O, tree species affected net rates of N2O emission from soil in laboratory incubations. These results suggest that tree species contribute to the large isotopic variation in N2O observed in a range tropical forest soils. We found that soil water affects both 15N and 18O in N2O, with wetter soils leading to more depleted N2O in both 15N and 18O. This is likely caused by a shift in biological processes for 15N and possible direct exchange of 18O between H2O and N2O.

The effect of afforestation on mineralization of soil organic matter

The effect of afforestation on the activity of microbiological mineralization of soil organic matter has been studied in Siberia. The results show that this effect concerns mainly net nitrogen mineralization and net nitrification, while carbon mineralization (CO2 formation) does not depend on the type of ecosystem. It is proposed to use the rates of net nitrogen mineralization and nitrification as the most sensitive indicators of changes in an ecosystem.

Biological sources of soil CO 2 under Larix sibirica and Pinus sylvestris

Mycorrhizal ingrowth collars were used to study the effect of tree species on the seasonal dynamics of carbon dioxide flux from three major sources of soil respiration: (1) plant roots, (2) mycorrhizal hyphae, and (3) microorganisms. Distinct seasonality in carbon transport to mycorrhizae was revealed, with its highest values being observed during the second half of the growing season. The annual amount of C transferred through mycorrhizae did not differ between the two tree species, and the contribution of mycorrhizae to soil surface CO2 emission was about 20%.

The Influence of Tree Species on the Biomass of Denitrifying Bacteria in Gray Forest Soils

The biomass of two groups of microorganisms was studied in gray forest soils under six tree species (spruce, Scotch pine, Arolla pine, larch, birch, and aspen) and in the soil of a layland (a clearing in the forest) using kinetic methods. The biomass was the highest in the soil of the layland. The lowest (19.4 μg C/g of soil) biomass of heterotrophic microorganisms was found in the soil under the birch trees, and the highest one (41.7 and 32.0 μg C/g), under the pine and spruce ones. The biomass of denitrifying microorganisms was lower by thirty times than that of the heterotrophic ones. In the soils under the pine and spruce trees (8.4 and 9.2 μg C/g, respectively), the biomass of the denitrifying microorganisms was the lowest; under the birch and larch trees, it was the highest (16.7 and 13.7 μg C/g).