Rhizosphere pH and cation-anion balance determine the exudation of nitrification inhibitor 3-epi-brachialactone suggesting release via secondary transport.

Biological nitrification inhibition (BNI) of Brachiaria humidicola has been attributed to nitrification-inhibiting fusicoccanes, most prominently 3-epi-brachialactone. However, its release mechanism from B. humidicola roots remains elusive. Two hydroponic experiments were performed to investigate the role of rhizosphere pH and nutritional N form in regulating 3-epi-brachialactone release by B. humidicola and verify the underlying release pathway. Low rhizosphere pH and NH4 + nutrition promoted 3-epi-brachialactone exudation. However, the substitution of NH4 + by K+ revealed that the NH4 + effect was not founded in a direct physiological response to the N form but was related to the cation-anion balance during nutrient uptake. Release of 3-epi-brachialactone correlated with the transmembrane proton gradient ΔpH and NH4 + uptake (R2 = 0.92 for high ~6.8 and R2 = 0.84 for low ~4.2 trap solution pH). This corroborated the release of 3-epi-brachialactone through secondary transport, with the proton motive force (ΔP) defining transport rates across the plasma membrane. It was concluded that 3-epi-brachialactone release cannot be conceptualized as a regulated response to soil pH or NH4 + availability, but merely as the result of associated changes in ΔP.

Soil properties, seasonality and crop growth stage exert a stronger effect on rhizosphere prokaryotes than the fungal biocontrol agent Fusarium oxysporum f.sp. strigae.

Fusarium oxysporum f.sp. strigae (Fos) is an effective biocontrol agent (BCA) against the parasitic weed Striga hermonthica. It acts in the rhizosphere of several tropical cereals, where it may interfere with indigenous microbial populations. To test this impact, we assessed in a 2-season field experiment at two contrasting tropical agro-ecological sites the response of nitrifying and total indigenous prokaryotic communities in the rhizosphere of maize to the exposure of the Fos-BCA "Foxy-2". At early leaf development (EC30), flowering (EC60) and senescence (EC90) stage of maize, rhizosphere samples were obtained and subjected to community analysis of bacterial and archaeal amoA (ammonia monooxigenase) (AOB, AOA) and 16S rRNA genes. Abundance and community composition of all studied genes were predominantly influenced by soil type, crop growth stage and seasonality. No major effect of "Foxy-2" was found. Notably, total archaeal community relative to bacteria dominated in the clayey soil which was linked to its strong soil organic carbon (SOC) background. Compared to bacterial nitrifiers, domination of nitrifying archaea increased towards senescence stage which was explained by biochemical differences in organic resource availability between the crop growth stages. During the short rain season, the higher archaeal abundance was mainly driven by increased availability of organic substrates, i.e., extractable organic carbon. Our findings suggested that archaea had greater rhizosphere competence than "Foxy-2" in soils with higher clay and SOC contents. We verified that "Foxy-2" in maize rhizospheres is compatible with nitrifying prokaryotes under the given environments, in particular in clayey soils dominated by archaea.

Influence of coastal vegetation on the 2004 tsunami wave impact in west Aceh.

In a tsunami event human casualties and infrastructure damage are determined predominantly by seaquake intensity and offshore properties. On land, wave energy is attenuated by gravitation (elevation) and friction (land cover). Tree belts have been promoted as "bioshields" against wave impact. However, given the lack of quantitative evidence of their performance in such extreme events, tree belts have been criticized for creating a false sense of security. This study used 180 transects perpendicular to over 100 km on the west coast of Aceh, Indonesia to analyze the influence of coastal vegetation, particularly cultivated trees, on the impact of the 2004 tsunami. Satellite imagery; land cover maps; land use characteristics; stem diameter, height, and planting density; and a literature review were used to develop a land cover roughness coefficient accounting for the resistance offered by different land uses to the wave advance. Applying a spatial generalized linear mixed model, we found that while distance to coast was the dominant determinant of impact (casualties and infrastructure damage), the existing coastal vegetation in front of settlements also significantly reduced casualties by an average of 5%. In contrast, dense vegetation behind villages endangered human lives and increased structural damage. Debris carried by the backwash may have contributed to these dissimilar effects of land cover. For sustainable and effective coastal risk management, location of settlements is essential, while the protective potential of coastal vegetation, as determined by its spatial arrangement, should be regarded as an important livelihood provider rather than just as a bioshield.

Biometric variability of inflorescence and flower traits among ex situ accessions of the neotropical oilseed palm Acrocomia Mart.

The oilseed palm genus Acrocomia is suitable for sustainable oil production in South America. The high phenotypic diversity of wild populations poses a challenge for the delimitation of the genus. Comparing the inflorescence architecture, a first-order panicle, and staminate and pistillate flower traits could be a valuable tool in resolving the taxonomic disarray. Thus, this study aims to characterize the differences in the inflorescence architecture and floral structures of three common and economically significant Acrocomia species: A. aculeata, A. totai, and A. intumescens. Biometric traits of the inflorescence architecture and floral structures of various Acrocomia accessions in an ex situ germplasm collection in Brazil were assessed. The unweighted pair group method with arithmetic mean (UPGMA) cluster analysis based on the Gower distance was used to measure dissimilarities between the individual plants of the accessions. To our best knowledge, this study provides the first evidence of the presence of second-order rachillae in the genus Acrocomia. Evaluated traits showed a high level of variation within and between accessions, emphasizing the phenotypic diversity of the genus. The accessions of A. totai were distinguishable from those of the other two species by their inflorescence architecture and flower traits. The dissimilarities between A. aculeata and A. intumescens were not sufficient to differentiate both. In conclusion, the quantitative assessment of inflorescence and floral traits is a valuable tool for taxonomic resolution of the genus.

Leaf growth, gas exchange and assimilation performance of cowpea varieties in response to Bradyrhizobium inoculation.

Supplying nitrogen to crops through selecting high N fixing legumes and effective inoculant is one of the key strategies to improve crop productivity. However, studies related to the effect of Bradyrhizobial inoculation on leaf growth, its functioning in relation to photosynthesis, and transpiration efficiency (WUE) of cowpea [Vigna unguiculata (L.) Walp] varieties in the tropics were inadequate. A two-year field experiment was conducted at three sites to evaluate the effect of inoculation on leaf growth, gas exchanges and photosynthetic efficiency of cowpea varieties. The study treatments were composed of four varieties, Keti (IT99K-1122), TVU, Black eye bean, and White wonderer trailing and three levels of inoculation (non-inoculated or inoculated with Bradyrhizobium strains CP-24 or CP-37). Gas exchange was measured on live plants at 67-77 days after sowing, between 8:00 to 11:00 a.m. and 14:00 to 16:00 p.m. Leaf growth parameters (leaf number and leaf area) were measured by destructive sampling, and the yield data was determined by harvesting plants in the three central rows at physiological maturity. Variety TVU performed best in terms of leaf number, photosynthesis rate, and WUE. Whereas, Black eye bean revealed superior performances for leaf area, leaf area index, and stomatal conductance compared with the rest two varieties. The effect of inoculation was significant with 14.0, 23.8, 13.7, and 11.0% advantage in leaf area, leaf area index, net photosynthesis, and WUE, respectively. Moreover, the performance of cowpea of the 2018 cropping season showed a relative advantage over 2019 in terms of leaf number, leaf area, leaf area index, net photosynthesis, and stomatal conductance. Therefore, inoculating cowpea varieties with effective Bradyrhizobium strain can be a viable alternative to enhance growth, gas exchange, photosynthetic efficiency, and grain yield.

Missing association between nutrient concentrations in leaves and edible parts of food crops - A neglected food security issue.

Crop nutrient deficiencies are determined based on leaf nutrient composition, and rarely on food composition. Consequently, it remains unclear whether leaf nutrients are useable to form conclusions on quality of produced foods. This study aimed to investigate the relationships between plant macro- (Mg, P, S, K, Ca) and micronutrient (Fe, Zn, Mn, Cu) concentrations of leaves and edible parts of three East African staple crops: Zea mays, Manihot esculenta, and Musa acuminata. Low phloem mobile nutrients Ca, Mn, Fe, Zn, and Cu showed the largest differences in correlations between leaves and edible parts. Perennial crops showed lower correlations between nutrient concentrations of leaves and edible parts than annuals. Leaves may provide information on plant health, however do not provide enough information to gauge both yields and food quality, particularly regarding micronutrients. Therefore, agricultural and nutritional scientists should harmonize methods to develop sustainable management options for increased food and nutrition security.

Impact of forest cover and conservation agriculture on sediment export: A case study in a montane reserve, south-western China.

Reforestation and agricultural conservation have long been recognized as important in reducing on-site soil loss and off-site sediment export. Quantitative assessment of their effectiveness is critical, and assists cost-benefit analysis and decision-making in land management and landscape planning. We applied a paired watershed approach to monitor 1-year sediment export in two watersheds with forest-dominated (reference) and mosaic (target) land use in the Naban River Watershed National Natural Reserve (NRWNNR) in Xishuangbanna, south-western China. Analysis of land-use change in the target watersheds showed decreasing total forest cover (FC) (from 57% to 47%), but increasing FC in steep areas (from 54% to 59%) from 2007 to 2012. A distributed hydrological model (Land-Use Change Impact Assessment, LUCIA) was well calibrated and validated through field data from the two watersheds. Scenarios were created representing different FCs (from 31% to 83%) and agricultural management (as-usual and conservation). Simulation results quantified the relation between FC and sediment export as a logarithmic or logit model, indicating at least one turning point of FC, beyond which further forest reduction should significantly increase sediment export. This point was identified in the range between 57% and 61% of the target watershed under as-usual management; it was shifted to 47%-53% by conservation agriculture. Compared with the reference (with 83% FC), conservation agriculture was able to almost fully compensate for increased sediment export by forest reduction to 57% in 2007. However, when forest was reduced further to 47% in 2012, sediment export increased significantly. We concluded that total FC was as important as FC in montane watershed management in steep areas; and crop type conversion, such as rubber to maize in this study, and on-site agriculture management affect more to sediment export than agricultural expansion. We recommend conservation agriculture as an efficient tool for reducing sediment export on a watershed scale.

Soil and farm management effects on yield and nutrient concentrations of food crops in East Africa.

Crops that grow on soils with higher fertility often have higher yields and higher tissue nutrient concentrations. Whether this is the case for all crops, and which soil and management factors, or combinations mostly affect yields and food nutrient concentrations however, is poorly understood. Here, the main aim was to evaluate effects of soil and management factors on crop yields and food nutrient concentrations in (i) grain, fruit and tuber crops, and (ii) between high and low soil fertility areas. Total elemental concentrations of Mg, P, S, K, Ca, Fe, Zn, Mn and Cu were measured using a portable X-Ray Fluorescence Spectrometer (pXRF) in maize grain (Zea mays; Teso South, Kenya: n = 31; Kapchorwa, Uganda n = 30), cassava tuber (Manihot esculenta; Teso South: n = 27), and matooke fruit (Musa acuminata; Kapchorwa, n = 54). Soil properties measured were eCEC, total N and C, pH, texture, and total elemental content. Farm management variables (fertilisation, distance to household, and crop diversity) were collected. Canonical Correspondence Analyses (CCA) with permutation rank tests identified driving factors of alterations in nutrient concentrations. Maize grain had higher correlations with soil factors (CCA > 80%), than cassava tuber (76%) or matooke fruit (39%). In contrast, corresponding correlations to management factors were much lower (8-39%). The main soil properties affecting food nutrients were organic matter and texture. Surprisingly, pH did not play an important role. A positive association of crop diversity with nutrient concentration and yield in lower fertility areas was observed. Considering, food nutrient composition, apart from yield, as response variables in agronomic trials (e.g. fertilisation or soil improvement strategies), would contribute towards discounting the notion that crops growing on fertile soils always produce healthy and high quality foods.

Brachialactone isomers and derivatives of Brachiaria humidicola reveal contrasting nitrification inhibiting activity.

Biological Nitrification Inhibition (BNI) of Brachiaria humidicola has been mainly attributed to the root-exuded fusicoccane-type diterpene brachialactone. We hypothesized, however, that according to the high diversity of fusicoccanes described for plants and microorganisms, BNI of B. humidicola is caused by an assemblage of bioactive fusicoccanes. B. humidicola root exudates were collected hydroponically and compounds isolated by semi-preparative HPLC. Chemical structures were revealed by spectroscopic techniques, including HRMS as well as 1D and 2D NMR. Nitrification inhibiting (NI) potential of isolated compounds was evaluated by a Nitrosomonas europaea based bioassay. Besides the previously described brachialactone (1), root exudates contained 3-epi-brachialactone (2), the C3-epimer of 1 (m/z 334), as well as 16-hydroxy-3-epi-brachialactone (3) with an additional hydroxyl group at C16 (m/z 350) and 3,18-epoxy-9-hydroxy-4,7-seco-brachialactone (4), which is a ring opened brachialactone derivative with a 3,18 epoxide ring and a hydroxyl group at C9 (m/z 332). The 3-epi-brachialactone (2) showed highest NI activity (ED50 ~ 20 µg mL-1, ED80 ~ 40 µg mL-1), followed by compound 4 with intermediate (ED50 ~ 40 µg mL-1), brachialactone (1) with low and compound 3 without activity. In coherence with previous reports on fusicoccanes, stereochemistry at C3 was of high relevance for the biological activity (NI potential) of brachialactones.

Do we need more drought for better nutrition? The effect of precipitation on nutrient concentration in East African food crops.

Soil, inputs, and environmental factors such as weather control plant nutrient availability and nutrient content in food. Drought periods affect nutrient bioavailability. Nutrient transport within the plant and allocation of nutrients within organs of the plant is water dependent and therefore drought susceptible. This study compared Kapchorwa, Uganda and Teso South, Kenya that experienced drought during the second season in 2016. The main research questions were: (i) do droughts have an impact on the nutrient composition of food; (ii) is there a difference in nutrient concentrations in food based on their xylem or phloem mobility? Maize (Zea mays) grain (n = 62) and matooke (Musa acuminata) fruit samples (n = 90) in Kapchorwa, and maize grain (n = 61) and cassava (Manihot esculenta) tuber (n = 64) in Teso South were collected during a normal season (March-July) and drought season (October-December) in 2016. Crop samples were analysed using a pXRF for P, K, Ca, Mg, S, Fe, Mn, Cu, and Zn. The Standardized Precipitation Index (SPI) was calculated using TAMSAT database to compare drought intensities. The drought in Kapchorwa (SPI: -1.14 to -0.32) was severer and began 2 months prior to Teso South (SPI: 0.09 to 0.55). Nutrient concentration in Kapchorwa decreased significantly from normal to drought in both crops. In contrast, during the moderate drought in Teso South, nutrient concentrations increased significantly. Lacking nutrient phloem mobility is suggested to play a vital role in mobilisation of micronutrients (Fe, Mn, and Cu) as shown by their decreased concentration under severe drought in the yield. Total nutrients assimilated in crop samples were significantly higher in the normal than the drought for almost all samples. Micronutrients and yields during drought were strongly affected, leading to a double-burden for consumers through affected quantity and quality. Future research considerations should particularly include the focus on potential nutrient increases during mild drought.

Nitrate reductase activity in leaves as a plant physiological indicator of in vivo biological nitrification inhibition by Brachiaria humidicola.

The tropical forage grass Brachiaria humidicola (Bh) controls soil microbial nitrification via biological nitrification inhibition (BNI). The aim of our study was to verify if nitrate reductase activity (NRA) in Bh roots or leaves reflects in vivo performance of BNI in soils. NRA was measured in roots and leaves of contrasting accessions and apomictic hybrids of Bh grown under controlled greenhouse and natural field conditions. Nitrate (NO3-) contents were measured in soil solution and in Bh stem sap to validate NRA data. Potential soil nitrification rates (NRs) and leaf δ15N values were used to verify in vivo BNI by the NRA assay in the field study. NRA was detected in Bh leaves rather than roots, regardless of NO3- availability. NRA correlated with NO3- contents in soils and stem sap of contrasting Bh genotypes substantiating its reflectance of in vivo BNI performance. Additionally, leaf NRA data from the field study significantly correlated with simultaneously collected NRs and leaf δ15N data. The leaf NRA assay facilitated a rapid screening of contrasting Bh genotypes for their differences in in vivo performance of BNI under field and greenhouse conditions, but inconsistency of the BNI potential by Bh germplasm was observed. Among Bh genotypes tested, leaf NRA was closely linked with nitrification activity, and consequently with actual BNI performance. It was concluded that NRA in leaves of Bh can serve as an indicator of in vivo BNI activity when complemented with established BNI methodologies (δ15N, NRs) under greenhouse and field conditions.

Low 15N Natural Abundance in Shoot Tissue of Brachiaria humidicola Is an Indicator of Reduced N Losses Due to Biological Nitrification Inhibition (BNI).

The tropical forage grass Brachiaria humidicola (Bh) suppresses the activity of soil nitrifiers through biological nitrification inhibition (BNI). As a result, nitrate ( NO 3 - ) formation and leaching are reduced which is also expected to tighten the soil nitrogen (N) cycle. However, the beneficial relationship between reduced NO 3 - losses and enhanced N uptake due to BNI has not been experimentally demonstrated yet. Nitrification discriminates against the 15N isotope and leads to 15N depleted NO 3 - , but 15N enriched NH 4 + in soils. Leaching of 15N depleted NO 3 - enriches the residual N pool in the soil with 15N. We hypothesized that altered nitrification and NO 3 - leaching due to diverging BNI magnitudes in contrasting Bh genotypes influence soil 15N natural abundance (δ15N), which in turn is reflected in distinct δ15N in Bh shoot biomass. Consequently, high BNI was expected to be reflected in low plant δ15N of Bh. It was our objective to investigate under controlled conditions the link between shoot value of δ15N in several Bh genotypes and leached NO 3 - amounts and shoot N uptake. Additionally, plant 15N and N% was monitored among a wide range of Bh genotypes with contrasting BNI potentials in field plots for 3 years. We measured leaf δ15N of young leaves (regrown after cutback) of Bh and combined it with nitrification rates (NRs) of incubated soil to test whether there is a direct relationship between plant δ15N and BNI. Increased leached NO 3 - was positively correlated with higher δ15N in Bh, whereas the correlation between shoot N uptake and shoot δ15N was inverse. Field cultivation of a wide range of Bh genotypes over 3 years decreased NRs in incubated soil, while shoot δ15N declined and shoot N% increased over time. Leaf δ15N of Bh genotypes correlated positively with NRs of incubated soil. It was concluded that decreasing plant δ15N of Bh genotypes over time reflects the long-term effect of BNI as linked to lower NO 3 - formation and reduced NO 3 - leaching. Accordingly, a low δ15N in Bh shoot tissue verified its potential as indicator of high BNI activity of Bh genotypes.

Variable responses of the depth of tree nitrogen uptake to pruning and competition.

Trees in cropped fields may improve nitrogen (N) use efficiency by intercepting leached N, but crop yield will be reduced if the trees compete strongly with crops for N. Ideal trees for intercropping will take up N from deeper soil layers not accessed by the crop species. Spatiotemporal aspects of tree nitrogen capture niches were investigated within a hedgerow intercropping system by placing 15N at three depths and monitoring 15N uptake by trees pruned either 25 or 4 days before application of 15N. Trees with contrasting rooting patterns (Gliricidia sepium L. and Peltophorum dasyrrachis (Miq.) Kurz) were grown in mixed hedgerows and intercropped with maize (Zea mays L.). Neither species showed significant N uptake during the 5-14 days after pruning, even though some shoot regrowth occurred during this time. Mean topsoil (0-5 cm depth) root length density of G. sepium was 520% greater than that of P. dasyrrachis, whereas total root length (0-100 cm depth) of G. sepium was only 450% greater. On average, G. sepium recovered 15 times as much 15N as P. dasyrrachis, following application of 15N at 5 cm depth, but the two species recovered a similar amount following application at 80 cm depth, suggesting that P. dasyrrachis had better niche complementarity with shallow rooting crops. However, both species showed strong plasticity in vertical N uptake pattern in response to competition from establishing maize plants. The species differed in their response: N uptake activity by G. sepium shifted down the soil profile in response to increasing competition from a growing maize crop (uptake from 80 cm depth changed from 4 to 9% of uptake from 5 cm depth), whereas N uptake by P. dasyrrachis became relatively shallow (uptake from 80 cm depth changed from 305 to 25% of uptake from 5 cm depth). Niche avoidance and increased competitiveness within the topsoil represent alternative responses to competition. The response displayed may be related to soil fertility in the species' natural habitats.

Biocontrol agent Fusarium oxysporum f.sp. strigae has no adverse effect on indigenous total fungal communities and specific AMF taxa in contrasting maize rhizospheres.

We studied the effects of Fusarium oxysporum f.sp. strigae (Fos), a soil-borne biocontrol agent (BCA) against Striga hermonthica, on total fungal and arbuscular mycorrhizal fungal (AMF) taxa in rhizospheres of maize in both clayey and sandy soil. Effects of Fos-BCA 'Foxy-2' were evaluated against (1) S. hermonthica presence, and (2) organic fertilization with Tithonia diversifolia residues at 14, 28 and 42 d after 'Foxy-2' inoculation, via DNA-based quantitative PCR and TRFLP fingerprinting. In both soils, 'Foxy-2' occasionally promoted total fungal abundance, while the community composition was mainly altered by T. diversifolia and S. hermonthica. Notably, 'Foxy-2' stimulated AMF Gigaspora margarita abundance, while G. margarita was suppressed by S. hermonthica. Total fungal and AMF abundance were promoted by T. diversifolia residues. In conclusion, 'Foxy-2' resulted in no adverse effects on indigenous rhizosphere fungal communities substantiating its environmental safety as BCA against S. hermonthica.

Proliferation of the biocontrol agent Fusarium oxysporum f. sp. strigae and its impact on indigenous rhizosphere fungal communities in maize under different agro-ecologies.

Our objectives were to (1) monitor the proliferation of the biocontrol agent (BCA) Fusarium oxysporum f. sp. strigae strain "Foxy-2", an effective soil-borne BCA against the parasitic weed Striga hermonthica, in the rhizosphere of maize under different agro-ecologies, and (2) investigate its impact on indigenous rhizosphere fungal community abundance and composition. Field experiments were conducted in Busia and Homa Bay districts in western Kenya during two cropping seasons to account for effects of soil type, climate, growth stage and seasonality. Maize seeds were coated with or without "Foxy-2" and soils were artificially infested with S. hermonthica seeds. One treatment with nitrogen rich organic residues (Tithonia diversifolia) was established to compensate hypothesized resource competition between "Foxy-2" and the indigenous fungal community. Rhizosphere soil samples collected at three growth stages (i.e., EC30, EC60, EC90) of maize were subjected to abundance measurement of "Foxy-2" and total indigenous fungi using quantitative polymerase chain reaction (qPCR) analysis. Terminal restriction fragment length polymorphism (TRFLP) analysis was used to assess potential alterations in the fungal community composition in response to "Foxy-2" presence. "Foxy-2" proliferated stronger in the soils with a sandy clay texture (Busia) than in those with a loamy sand texture (Homa Bay) and revealed slightly higher abundance in the second season. "Foxy-2" had, however, only a transient suppressive effect on total indigenous fungal abundance which ceased in the second season and was further markedly compensated after addition of T. diversifolia residues. Likewise, community structure of the indigenous fungal community was mainly altered by maize growth stages, but not by "Foxy-2". In conclusion, no adverse effects of "Foxy-2" inoculation on indigenous fungal rhizosphere communities were observed corroborating the safety of this BCA under the given agro-ecologies.

Soil functional responses to excess nitrogen inputs at global scale.

There is little evidence that nitrogen (N) cycling in the highly weathered, low-phosphorus (P), acidic soils found in Southern Hemisphere continents will differ greatly from that in North America and Europe. Evidence from the 'south' shows: the similarity in forms and temporal patterns in losses of N from different land uses; that the C:N ratios of the forest floor/litter layer from different continents are strongly predictive of a range of processes on a global scale; that generalizations based on Northern Hemisphere experience of the impact of N additions to 'P-limited' ecosystems are likely to fail for southern ecosystems where anatomical and physiological adaptation of native plants to low-P soils makes questionable the concept of 'P-limitation'; that the greatest threats in the 'south' are probably changes in land use that may greatly increase N inputs and turnover; that localized increases in N inputs produce similar effects to those seen in the 'north'.

Quantification of total element concentrations in soils using total X-ray fluorescence spectroscopy (TXRF).

Total X-ray fluorescence spectroscopy (TXRF) determines concentrations of major and trace elements in multiple media. We developed and tested a method for the use of TXRF for direct quantification of total element concentrations in soils using an S2 PICOFOX™ spectrometer (Bruker AXS Microanalysis GmbH, Germany). We selected 15 contrasting soil samples from across sub-Saharan Africa for element analysis to calibrate the instrument against concentrations determined using the inductively coupled plasma-mass spectroscopy (ICP-MS) standard method. A consistent underestimation of element concentrations using TXRF compared to ICP-MS reference analysis occurred, indicating that spectrometer recalibration was required. Single-element recalibration improved the TXRF spectrometer's sensitivity curve. Subsequent analysis revealed that TXRF determined total element concentrations of Al, K, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, and Ga accurately (model efficacy/slope close to 1:1 line, and R(2)>0.80) over a wide range of soil samples. Other elements that could be estimated with an acceptable precision (R(2)>0.60) compared with ICP-MS although generally somewhat under- or overestimated were P, Ca, As, Rb, Sr, Y, Pr, Ta and Pb. Even after recalibration, compared to ICP-MS the TXRF spectrometer produced underestimations for elements Na, Mg, Ba, Ce, Hf, La, Nd, W and Sm and overestimations for elements Bi, Tl and Zr. We validated the degree of accuracy of the TXRF analytical method after recalibration using an independent set of 20 soil samples. We also tested the accuracy of the analysis using 2 multi-element standards as well as the method repeatability on replicate samples. The resulting total element concentration repeatability for all elements analyzed were within 10% coefficient of variability after the instrument recalibration except for Cd and Tl. Our findings demonstrate that TXRF could be used as a rapid screening tool for total element concentrations in soils assuming that sufficient calibration measures are followed.

Technical considerations for the use of 15N-DNA stable-isotope probing for functional microbial activity in soils.

Stable-isotope DNA probing is a culture-independent technique that may provide a link between function and phylogeny of active microorganisms. The technique has been used in association with 13C substrates while here we evaluate feasibility and limitations of 15N-DNA stable-isotope probing (SIP) using labelled and unlabelled pure microbial cultures or soil extracts. Our results showed that (15)N-DNA probing is feasible for cultures as well as soil samples. Limitations of 15N-DNA-SIP are (a) the need for relatively large quantities of DNA to visualise bands (although molecular resolution is much higher) and (b) 15N-DNA enrichment needed to ideally be >50 at%; however, this requirement can be lowered to approx. 40 atom% 15N with pure cultures using a modified CsCl centrifugation method (140K g for 69 h). These advances in 15N-DNA-SIP methodology open new opportunities to trace active microbial populations utilising specific N substrates in situ.

Discrimination against 13C during degradation of simple and complex substrates by two white rot fungi.

Changes in isotopic 13C signatures of CO2-C evolved during decomposition of a sugar (glucose), a fatty acid (palmitic acid), a protein (albumin), a structural biopolymer (lignin) and bulk plant tissue (aerial shoots from Lolium perenne) were monitored over a period of 76 days. All materials were sterilized and inoculated with either of two different species of white rot fungi, Phanerochaete chrysosporium or Coriolus versicolor, and incubated in sealed bottles at 28 degrees C. The CO2 concentration in the jars was periodically determined using an infrared gas analyzer and its isotopic (13C) signature was assessed using a trace gas (ANCA TGII) module coupled to an isotope ratio mass spectrometer (IRMS, Europa 20-20). L. perenne material inoculated with C. versicolor showed the highest C mineralization activity with approximately 70% of total C evolved as CO2 after 76 days of incubation, followed by glucose. Substrates inoculated with C. versicolor generally decomposed faster than when degraded by P. chrysosporium, except for lignin, where no significant differences between the two fungi types were found and CO2-C released was less than 2% of the initial C. Considerable 13C isotopic fractionation during the degradation of plant tissue and of pure biochemical compounds was revealed as well as progressive shifts in cumulative CO2-13C isotopic signatures over time. During the first stages of decomposition, the CO2-C released was usually depleted in 13C as compared with the initial solid substrate, but with ongoing decomposition the CO2-C evolved became progressively more enriched in 13C. P. chrysosporium usually showed a slightly higher 13C fractionation than C. versicolor during the first decomposition phase. At posterior decomposition stages isotopic discrimination was often stronger by C. versicolor. These findings on isotopic 13C discrimination during microbial degradation both of simple biochemical compounds and of complex vegetal tissue confirmed not only the existence of significant 13C isotopic fractionation during plant residue decomposition, but also the existence of non-random isotopic distribution within substrates. They also demonstrated the ability of microorganisms to selectively discriminate against 13C even when degrading an isolated simple substrate.