Biogeochemical Cycling Of Nutrients Research Articles

Effects of pCO2 and iron on the elemental composition and cell geometry of the marine diatom Pseudo-nitzschia pseudodelicatissima (Bacillariophyceae)(1).

Partial pressure of CO2 (pCO2 ) and iron availability in seawater show corresponding changes due to biological and anthropogenic activities. The simultaneous change in these factors precludes an understanding of their independent effects on the ecophysiology of phytoplankton. In addition, there is a lack of data regarding the interactive effects of these factors on phytoplankton cellular stoichiometry, which is a key driving factor for the biogeochemical cycling of oceanic nutrients. Here, we investigated the effects of pCO2 and iron availability on the elemental composition (C, N, P, and Si) of the diatom Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle by dilute batch cultures under 4 pCO2 (~200, ~380, ~600, and ~800μatm) and five dissolved inorganic iron (Fe'; ~5, ~10, ~20, ~50, and ~100pmol·L(-1) ) conditions. Our experimental procedure successfully overcame the problems associated with simultaneous changes in pCO2 and Fe' by independently manipulating carbonate chemistry and iron speciation, which allowed us to evaluate the individual effects of pCO2 and iron availability. We found that the C:N ratio decreased significantly only with an increase in Fe', whereas the C:P ratio increased significantly only with an increase in pCO2 . Both Si:C and Si:N ratios decreased with increasing pCO2 and Fe'. Our results indicate that changes in pCO2 and iron availability could influence the biogeochemical cycling of nutrients in future oceans with high- CO2 levels, and, similarly, during the time course of phytoplankton blooms. Moreover, pCO2 and iron availability may also have affected oceanic nutrient biogeochemistry in the past, as these conditions have changed markedly over the Earth's history.

Hypoxia in the changing marine environment

The predicted future of the global marine environment, as a combined result of forcing due to climate change (e.g. warming and acidification) and other anthropogenic perturbation (e.g. eutrophication), presents a challenge to the sustainability of ecosystems from tropics to high latitudes. Among the various associated phenomena of ecosystem deterioration, hypoxia can cause serious problems in coastal areas as well as oxygen minimum zones in the open ocean (Diaz and Rosenberg 2008 Science 321 926–9, Stramma et al 2008 Science 320 655–8). The negative impacts of hypoxia include changes in populations of marine organisms, such as large-scale mortality and behavioral responses, as well as variations of species distributions, biodiversity, physiological stress, and other sub-lethal effects (e.g. growth and reproduction). Social and economic activities that are related to services provided by the marine ecosystems, such as tourism and fisheries, can be negatively affected by the aesthetic outcomes as well as perceived or real impacts on seafood quality (STAP 2011 (Washington, DC: Global Environment Facility) p 88). Moreover, low oxygen concentration in marine waters can have considerable feedbacks to other compartments of the Earth system, like the emission of greenhouse gases to the atmosphere, and can affect the global biogeochemical cycles of nutrients and trace elements. It is of critical importance to prediction and adaptation strategies that the key processes of hypoxia in marine environments be precisely determined and understood (cf Zhang et al 2010 Biogeosciences 7 1–24).

Distribution of mesozooplankton biomass in the global ocean

Abstract. Mesozooplankton are cosmopolitan within the sunlit layers of the global ocean. They are important in the pelagic food web, having a significant feedback to primary production through their consumption of phytoplankton and microzooplankton. In many regions of the global ocean, they are also the primary contributors to vertical particle flux in the oceans. Through both they affect the biogeochemical cycling of carbon and other nutrients in the oceans. Little, however, is known about their global distribution and biomass. While global maps of mesozooplankton biomass do exist in the literature, they are usually in the form of hand-drawn maps for which the original data associated with these maps are not readily available. The dataset presented in this synthesis has been in development since the late 1990s, is an integral part of the Coastal and Oceanic Plankton Ecology, Production, and Observation Database (COPEPOD), and is now also part of a wider community effort to provide a global picture of carbon biomass data for key plankton functional types, in particular to support the development of marine ecosystem models. A total of 153 163 biomass values were collected, from a variety of sources, for mesozooplankton. Of those 2% were originally recorded as dry mass, 26% as wet mass, 5% as settled volume, and 68% as displacement volume. Using a variety of non-linear biomass conversions from the literature, the data have been converted from their original units to carbon biomass. Depth-integrated values were then used to calculate an estimate of mesozooplankton global biomass. Global epipelagic mesozooplankton biomass, to a depth of 200 m, had a mean of 5.9 μg C L−1, median of 2.7 μg C L−1 and a standard deviation of 10.6 μg C L−1. The global annual average estimate of mesozooplankton in the top 200 m, based on the median value, was 0.19 Pg C. Biomass was highest in the Northern Hemisphere, and there were slight decreases from polar oceans (40–90°) to more temperate regions (15–40°) in both hemispheres. Values in the tropics (15° N–15° S) were intermediate between those at the northern and southern temperate latitudes. Datasets available at doi:10.1594/PANGAEA.785501.

A mechanistic‐based framework to understand how dissolved organic carbon is processed in a large fluvial lake

Lay AbstractDissolved organic carbon (DOC) is a fundamental component of the biogeochemical cycling of nutrients in aquatic ecosystems and is the main carbon source supporting bacterial production. The efficiency at which heterotrophic (nonphotosynthetic) bacteria convert this substrate into biomass depends mainly on the quality of DOC in the water column. DOC is constantly processed through various physical, chemical, and biological mechanisms that operate simultaneously and alter its quality. It is paramount to understand how these different processes interact to drive the fate of DOC in aquatic ecosystems. Based on field data collected in a large fluvial lake, we developed and validated a mechanistic model that provides a framework to understand the relative contribution of the main processes involved in both labile (DOCL) and semilabile (DOCSL) DOC pool kinetics. The model revealed that during the downstream flow, each category of DOC pool was processed differently by bacteria: DOCL was preferentially used for biomass production, whereas DOCSL completed bacterial carbon demand. Our results also suggest that a decrease in DOCL abundance will further determine the intake of DOCSL.

Altered growth and enzyme expression profile of ZnO nanoparticles exposed non-target environmentally beneficial bacteria

The extensive production and usage of nanoparticles with ultimate disposal in the environment leads to unintentional exposure of non-target environmentally beneficial bacteria thereby posing a serious threat to the native soil inhabitants. Soil microflora is an important link in the biogeochemical cycling of nutrients, affecting ecosystem functioning and productivity. This study evaluates the effect of one of the widely used nanoparticles, zinc oxide on two predominant soil bacteria, Gram-positive Bacillus subtilis and Gram-negative Pseudomonas aeruginosa with respect to their biocatalytic activities. Growth profiles of these bacteria in the presence of zinc oxide nanoparticles (ZnONPs) at a concentration of 20 ppm exhibited a prolonged lag phase in B. subtilis, whereas no significant effect was observed in the case of P. aeruginosa even at 200 ppm. Interestingly, the enzymatic profile of both the organisms was affected at non-lethal ZnONPs concentrations. The most pronounced effect was on the enzymes associated with amylolytic activity, denitrification and urea degradation wherein total inhibition of activity was noted in B. subtilis. The enzyme activities were lowered in the case of P. aeruginosa. The results presented here reiterate a critical need for exposure assessment and risk characterization of nanomaterial disposal on soil microflora while formalizing waste management strategies.

Trace metal chemistry in the pristine freshwater Lake Hauroko, Fiordland, New Zealand

The south-western region of the South Island of New Zealand is remote from human activities and is thought to be minimally affected by anthropogenic contaminants, making it a good candidate for understanding the geochemistry of pristine systems, and for providing baseline concentrations for trace elements and other chemical parameters. We have determined the inorganic and physical characteristics of a remote mountain lake (Lake Hauroko) in this region. The lake exhibits oligotrophic character and major ion concentrations and total alkalinity are very constant throughout summer and winter as well as with depth. Because of the low ionic strength, trace metal concentrations were measured directly on acidified samples by Quadrupole ICP-MS. Concentrations are low throughout, averaging (in ngl−1) for Mn 102.9, Fe 4570, Co 5.31, Ni 98.6, Cu 247.8, Zn 140.7, Cd 1.23 and Pb 24.85. We have compared these results with other lakes in New Zealand, with freshwater systems in Europe and North America as well as with the temperate stream model of Meybeck (1987). These comparisons confirm the very pristine character of this lake, making it an ideal natural laboratory for the study of natural biogeochemical cycles of trace element and nutrients at temperate latitudes.

Toxicity of Functionalized Single-Walled Carbon Nanotubes on Soil Microbial Communities: Implications for Nutrient Cycling in Soil

Culture-dependent and -independent methods were employed to determine the impact of carboxyl-functionalized single-walled carbon nanotubes (SWNTs) on fungal and bacterial soil microbial communities. Soil samples were exposed to 0 (control), 250, and 500 μg of SWNTs per gram of soil. Aliquots of soil were sampled for up to 14 days for culture-dependent analyses, namely, plate count agar and bacterial community level physiological profiles, and culture-independent analyses, namely, quantitative real-time polymerase chain reaction (qPCR), mutliplex-terminal restriction fragment length polymorphism (M-TRFLP), and clone libraries. Results from culture-independent and -dependent methods show that the bacterial soil community is transiently affected by the presence of SWNTs. The major impact of SWNTs on bacterial community was observed after 3 days of exposure, but the bacterial community completely recovered after 14 days. However, no recovery of the fungal community was observed for the duration of the experiment. Physiological and DNA microbial community analyses suggest that fungi and bacteria involved in carbon and phosphorus biogeochemical cycles can be adversely affected by the presence of SWNTs. This study suggests that high concentrations of SWNTs can have widely varying effects on microbial communities and biogeochemical cycling of nutrients in soils.

Consequences of increased temperature and acidification on bacterioplankton community composition during a mesocosm spring bloom in the Baltic Sea

Despite the paramount importance of bacteria for biogeochemical cycling of carbon and nutrients, little is known about the potential effects of climate change on these key organisms. The consequences of the projected climate change on bacterioplankton community dynamics were investigated in a Baltic Sea spring phytoplankton bloom mesocosm experiment by increasing temperature with 3°C and decreasing pH by approximately 0.4 units via CO₂ addition in a factorial design. Temperature was the major driver of differences in community composition during the experiment, as shown by denaturing gradient gel electrophoresis (DGGE) of amplified 16S rRNA gene fragments. Several bacterial phylotypes belonging to Betaproteobacteria were predominant at 3°C but were replaced by members of the Bacteriodetes in the 6°C mesocosms. Acidification alone had a limited impact on phylogenetic composition, but when combined with increased temperature, resulted in the proliferation of specific microbial phylotypes. Our results suggest that although temperature is an important driver in structuring bacterioplankton composition, evaluation of the combined effects of temperature and acidification is necessary to fully understand consequences of climate change for marine bacterioplankton, their implications for future spring bloom dynamics, and their role in ecosystem functioning.

Suspended sediment dynamics in the Amazon River of Peru

The erosion and transport of sediments allow us to understand many activities of significance, such as crust evolution, climate change, uplift rates, continental processes, the biogeochemical cycling of pollutants and nutrients. The Amazon basin of Peru has contrasting physiographic and climatic characteristics between the Andean piedmont and the plains and between the north and south of the basin which is why there are 8 gauging stations located along the principal rivers of the Andean piedmont (Marañón, Huallaga, Ucayali) and the plain (Marañón, Tigre, Napo, Ucayali and Amazon rivers). Since 2003, the ORE-Hybam (IRD-SENAMHI-UNALM) observatory has performed out regular measurements at strategic points of the Amazon basin to understand and model the systems, behavior and long-term dynamics. On the Andean piedmont, the suspended yields are governed by a simple model with a relationship between the river discharge and the sediment concentration. In the plain, the dilution effect of the concentrations can create hysteresis in this relationship on a monthly basis. The Amazon basin of Peru has a sediment yield of 541 *106 t year−1, 70% comes from the southern basin.

The dilemma of controlling cultural eutrophication of lakes

The management of eutrophication has been impeded by reliance on short-term experimental additions of nutrients to bottles and mesocosms. These measures of proximate nutrient limitation fail to account for the gradual changes in biogeochemical nutrient cycles and nutrient fluxes from sediments, and succession of communities that are important components of whole-ecosystem responses. Erroneous assumptions about ecosystem processes and lack of accounting for hysteresis during lake recovery have further confused management of eutrophication. I conclude that long-term, whole-ecosystem experiments and case histories of lake recovery provide the only reliable evidence for policies to reduce eutrophication. The only method that has had proven success in reducing the eutrophication of lakes is reducing input of phosphorus. There are no case histories or long-term ecosystem-scale experiments to support recent claims that to reduce eutrophication of lakes, nitrogen must be controlled instead of or in addition to phosphorus. Before expensive policies to reduce nitrogen input are implemented, they require ecosystem-scale verification. The recent claim that the ‘phosphorus paradigm’ for recovering lakes from eutrophication has been ‘eroded’ has no basis. Instead, the case for phosphorus control has been strengthened by numerous case histories and large-scale experiments spanning several decades.

Dissolved organic matter and nutrient dynamics of a coastal freshwater forested wetland in Winyah Bay, South Carolina

Seasonally flooded, freshwater cypress-tupelo wetlands, dominated by baldcypress (Taxodium distictum), water tupelo (Nyssaaquatica), and swamp tupelo (Nyssa sylvatica var. biflora) are commonly found in coastal regions of the southeastern United States. These wetlands are threatened due to climate change, sea level rise, and coastal urban development. Understanding the natural biogeochemical cycles of nutrients in these forested wetlands as ecosystems services such as carbon sequestration and nitrogen processing can provide important benchmarks to guide conservation plans and restoration goals. In this study, surface water and soil pore water samples were collected weekly from a cypress-tupelo wetland near Winyah Bay, South Carolina and analyzed for dissolved organic carbon (DOC), dissolved organic nitrogen (DON), inorganic nitrogen, and phosphate during its flooding period between October 2010 and May 2011. DOC was further characterized by specific ultra-violet absorbance at 254 nm, spectral slope ratio (SR) (ratio of two spectral slopes between 275–295 nm and 350–400 nm), E2/E3 ratio (ratio between A254 and A365), and fluorescence excitation-emission matrix. In addition, litterfall was collected on a monthly basis for a year while the biomass of the detritus layer (i.e., decomposed duff lying on the wetland floor) was determined before and after the flooding period. Results of the field study showed that concentrations of DOC, DON, NH4+–N, and (NO2− + NO3−)–N in the surface water were generally higher during the fall, or peak litterfall season (October to December), than in the spring season (March to May). Highest concentrations of 54.8, 1.48, 0.270, and 0.0205 mg L−1, for DOC, DON, NH4+–N, and (NO2− + NO3−)–N respectively, in surface waters were recorded during October. Lower SUVA, but higher SR and E2/E3 ratios of DOC, were observed at the end of the flooding season comparing to the initial flooding, suggesting the wetland system converts high aromatic and large DOC molecules into smaller and hydrophilic fractions possibly through photochemical oxidation. A similar trend was observed in soil pore water, but the pore water generally had greater and relatively stable concentrations of dissolved nutrients than surface water. No obvious temporal trend in phosphate concentration and total nitrogen to total phosphorus ratio (N:P) were found. Results of the laboratory extraction and mass balance calculation suggested fresh litter was a major source of DOC whereas decomposed duff was the source of dissolved nitrogen in surface water. In summary, the biogeochemistry of this isolated cypress-tupelo wetland is not only driven by the vegetation within the wetland system but also by hydrology and weather conditions such as groundwater table position, precipitation, and temperature.

Inter-cropped perennial legumes in commercial eucalyptus spp. plantations enhance soil quality

Area planted with Eucalyptus spp. in Uruguay has largely increased in the last fi fteen years, mainly through afforestation of grasslands. Such changes in land use are known to affect soil properties, including increased soil acidity and exchangeable aluminum, decreased organic carbon content and water retention capacity, as well as changes in soil enzyme activities. The low mineralization rate of tree litter due to its high C:N ratio is suggested to be in the base of changes in soil properties. Our assumption was that the 2-year period between Eucalyptus grandis plantation and tree canopy closure allows the commercial forest to benefi t from the inclusion of an inter-cropped perennial legume. The aim of this research was to test if inter-cropping an adapted (native) legume can supply nitrogen from the atmosphere to the system, contributing to soil conservation through enhanced soil microbial activity and nutrient cycling, while improving tree growth. A fi eld experiment was carried out in a commercial forest area to assess the effects of perennial Desmanthus species (virgatus, illinoensis and bicornutus) intercropped between tree rows on soil microbial biomass and soil enzyme activities (soil quality indicators); legume shoot dry matter and total nitrogen content; and tree diameter at breast height. Enhanced soil microbial biomass, soil respiration and hydrogenase activities were associated to legume inclusion. Both D. illinoensis and D. virgatus contributed comparable amounts of nitrogen to the system when inoculated with effective rhizobia. Tree diameter 12 months after planting was 6.5 cm in average when Desmanthus spp. were intercropped and 5.6 cm when trees grew without the legume. The results indicate a direct and positive effect of intercropped legumes in Eucalyptus plantations on soil quality and early tree growth. An increase in soil microbial activities may lead to enhanced nutrient biogeochemical cycles, increased soil fertility and improved agroecosystem sustainability.

PHYTOPLANKTON SELENIUM REQUIREMENTS: THE CASE FOR SPECIES ISOLATED FROM TEMPERATE AND POLAR REGIONS OF THE SOUTHERN HEMISPHERE(1).

A series of laboratory culture experiments was used to investigate the effect of selenium (Se, 0-10 nM) on the growth, cellular volume, photophysiology, and pigments of two temperate and four polar oceanic phytoplankton species [coccolithophore Emiliania huxleyi (Lohmann) W. W. Hay et H. P. Mohler, cyanobacterium Synechococcus sp., prymnesiophyte Phaeocystis sp., and three diatoms-Fragilariopsis cylindrus (Grunow) Kriegar, Chaetoceros sp., and Thalassiosira antarctica G. Karst.]. Only Synechoccocus sp. and Phaeocystis sp. did not show any requirement for Se. Under Se-deficient conditions, the growth rate of E. huxleyi was decreased by 1.6-fold, whereas cellular volume was increased by 1.9-fold. Se limitation also decreased chl a (2.5-fold), maximum relative electron transport rate (1.9-fold), and saturating light intensity (2.8-fold), suggesting that Se plays a role in photosynthesis or high-light acclimation. Pigment analysis for Antarctic taxa provided an interesting counterpoint to the physiology of E. huxleyi. For all Se-dependent Antarctic diatoms, Se limitation decreased growth rate and chl a content, whereas cellular volume was not affected. Pigment analysis revealed that other pigments were affected under Se deficiency. Photoprotective pigments increased by 1.4-fold, while diadinoxanthin:diatoxanthin ratios decreased by 1.5- to 4.9-fold under Se limitation, supporting a role for Se in photoprotection. Our results demonstrate an Se growth requirement for polar diatoms and indicate that Se could play a role in the biogeochemical cycles of other nutrients, such as silicic acid in the Southern Ocean. Se measurements made during the austral summer in the Southern Ocean and Se biological requirement were used to discuss possible Se limitation in phytoplankton from contrasting oceanographic regions.

The Disappearing Cryosphere: Impacts and Ecosystem Responses to Rapid Cryosphere Loss

The cryosphere—the portion of the Earth's surface where water is in solid form for at least one month of the year—has been shrinking in response to climate warming. The extents of sea ice, snow, and glaciers, for example, have been decreasing. In response, the ecosystems within the cryosphere and those that depend on the cryosphere have been changing. We identify two principal aspects of ecosystem-level responses to cryosphere loss: (1) trophodynamic alterations resulting from the loss of habitat and species loss or replacement and (2) changes in the rates and mechanisms of biogeochemical storage and cycling of carbon and nutrients, caused by changes in physical forcings or ecological community functioning. These changes affect biota in positive or negative ways, depending on how they interact with the cryosphere. The important outcome, however, is the change and the response the human social system (infrastructure, food, water, recreation) will have to that change.

Comparative Analysis of Eukaryotic Marine Microbial Assemblages from 18S rRNA Gene and Gene Transcript Clone Libraries by Using Different Methods of Extraction

Eukaryotic marine microbes play pivotal roles in biogeochemical nutrient cycling and ecosystem function, but studies that focus on the protistan biogeography and genetic diversity lag-behind studies of other microbes. 18S rRNA PCR amplification and clone library sequencing are commonly used to assess diversity that is culture independent. However, molecular methods are not without potential biases and artifacts. In this study, we compare the community composition of clone libraries generated from the same water sample collected at the San Pedro Ocean Time Series (SPOTs) station in the northwest Pacific Ocean. Community composition was assessed using different cell lysis methods (chemical and mechanical) and the extraction of different nucleic acids (DNA and RNA reverse transcribed to cDNA) to build Sanger ABI clone libraries. We describe specific biases for ecologically important phylogenetic groups resulting from differences in nucleic acid extraction methods that will inform future designs of eukaryotic diversity studies, regardless of the target sequencing platform planned.

How do persistent organic pollutants be coupled with biogeochemical cycles of carbon and nutrients in terrestrial ecosystems under global climate change?

Purpose Global climate change (GCC), especially global warming, has affected the material cycling (e.g., carbon, nutrients, and organic chemicals) and the energy flows of terrestrial ecosystems. Persistent organic pollutants (POPs) were regarded as anthropogenic organic carbon (OC) source, and be coupled with the natural carbon (C) and nutrient biogeochemical cycling in ecosystems. The objective of this work was to review the current literature and explore potential coupling processes and mechanisms between POPs and biogeochemical cycles of C and nutrients in terrestrial ecosystems induced by global warming.

Chapter 2 - Elemental Economy: Microbial Strategies for Optimizing Growth in the Face of Nutrient Limitation

Microorganisms play a dominant role in the biogeochemical cycling of nutrients. They are rightly praised for their facility for fixing both carbon and nitrogen into organic matter, and microbial driven processes have tangibly altered the chemical composition of the biosphere and its surrounding atmosphere. Despite their prodigious capacity for molecular transformations, microorganisms are powerless in the face of the immutability of the elements. Limitations for specific elements, either fleeting or persisting over eons, have left an indelible trace on microbial genomes, physiology, and their very atomic composition. We here review the impact of elemental limitation on microbes, with a focus on selected genetic model systems and representative microbes from the ocean ecosystem. Evolutionary adaptations that enhance growth in the face of persistent or recurrent elemental limitations are evident from genome and proteome analyses. These range from the extreme (such as dispensing with a requirement for a hard to obtain element) to the extremely subtle (changes in protein amino acid sequences that slightly, but significantly, reduce cellular carbon, nitrogen, or sulfur demand). One near-universal adaptation is the development of sophisticated acclimation programs by which cells adjust their chemical composition in response to a changing environment. When specific elements become limiting, acclimation typically begins with an increased commitment to acquisition and a concomitant mobilization of stored resources. If elemental limitation persists, the cell implements austerity measures including elemental sparing and elemental recycling. Insights into these fundamental cellular properties have emerged from studies at many different levels, including ecology, biological oceanography, biogeochemistry, molecular genetics, genomics, and microbial physiology. Here, we present a synthesis of these diverse studies and attempt to discern some overarching themes.

Hypoxia and nitrogen processing in the Baltic Sea water column

Steep redoxclines form between oxic surface water and the stagnant, sulfidic hypolimnion in the eutrophied, brackish water Baltic Sea. Nitrification, denitrification, and anammox were measured at and below the redoxcline to quantify the role of water‐column nitrogen processes in the overall magnitude of nitrogen removal in the Baltic Sea. Rates of nitrification were very high (up to 85 nmol N L−1 d−1) at the oxic‐anoxic interface, but, surprisingly, nitrification was separated from the processes reducing nitrate to N2 (up to 810 nmol N L−1 d−1) by tens of meters in depth. N2 production was dominated by chemolithotrophic denitrification, with anammox playing only a negligible role, and limited to the water layers in which nitrite or nitrate coexisted with sulfide. The alternating oxygen concentrations in the basin induce irregular bursts of nitrogen removal. Nitrification takes place after mixing of ammonium‐rich deep water with oxic water, and denitrification uses the formed nitrite and nitrate once anoxic conditions re‐establish. Although removal rates can be high, conditions allowing such rates are likely short‐lived. While the sedimentary denitrification rates at the shallower, oxic areas are lower, they are more constant in time, highlighting the need to avoid hypoxia, which would prevent sedimentary denitrification.

Inter-cropped perennial legumes in commercial eucalyptus spp. plantations enhance soil quality

Area planted with Eucalyptus spp. in Uruguay has largely increased in the last fi fteen years, mainly through afforestation of grasslands. Such changes in land use are known to affect soil properties, including increased soil acidity and exchangeable aluminum, decreased organic carbon content and water retention capacity, as well as changes in soil enzyme activities. The low mineralization rate of tree litter due to its high C:N ratio is suggested to be in the base of changes in soil properties. Our assumption was that the 2-year period between Eucalyptus grandis plantation and tree canopy closure allows the commercial forest to benefi t from the inclusion of an inter-cropped perennial legume. The aim of this research was to test if inter-cropping an adapted (native) legume can supply nitrogen from the atmosphere to the system, contributing to soil conservation through enhanced soil microbial activity and nutrient cycling, while improving tree growth. A fi eld experiment was carried out in a commercial forest area to assess the effects of perennial Desmanthus species ( virgatus , illinoensis and bicornutus ) intercropped between tree rows on soil microbial biomass and soil enzyme activities (soil quality indicators); legume shoot dry matter and total nitrogen content; and tree diameter at breast height. Enhanced soil microbial biomass, soil respiration and hydrogenase activities were associated to legume inclusion. Both D. illinoensis and D. virgatus contributed comparable amounts of nitrogen to the system when inoculated with effective rhizobia. Tree diameter 12 months after planting was 6.5 cm in average when Desmanthus spp. were intercropped and 5.6 cm when trees grew without the legume. The results indicate a direct and positive effect of intercropped legumes in Eucalyptus plantations on soil quality and early tree growth. An increase in soil microbial activities may lead to enhanced nutrient biogeochemical cycles, increased soil fertility and improved agroecosystem sustainability.

Aeolian nutrient fluxes following wildfire in sagebrush steppe: implications for soil carbon storage

Abstract. Pulses of aeolian transport following fire can profoundly affect the biogeochemical cycling of nutrients in semi-arid and arid ecosystems. Our objective was to determine horizontal nutrient fluxes occurring in the saltation zone during an episodic pulse of aeolian transport that occurred following a wildfire in a semi-arid sagebrush steppe ecosystem in southern Idaho, USA. We also examined how temporal trends in nutrient fluxes were affected by changes in particle sizes of eroded mass as well as nutrient concentrations associated with different particle size classes. In the burned area, total carbon (C) and nitrogen (N) fluxes were as high as 235 g C m−1 d−1 and 19 g N m−1 d−1 during the first few months following fire, whereas C and N fluxes were negligible in an adjacent unburned area throughout the study. Temporal variation in C and N fluxes following fire was largely attributable to the redistribution of saltation-sized particles. Total N and organic C concentrations in the soil surface were significantly lower in the burned relative to the unburned area one year after fire. Our results show how an episodic pulse of aeolian transport following fire can affect the spatial distribution of soil C and N, which, in turn, can have important implications for soil C storage. These findings demonstrate how an ecological disturbance can exacerbate a geomorphic process and highlight the need for further research to better understand the role aeolian transport plays in the biogeochemical cycling of C and N in recently burned landscapes.