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PurposeThe environmental benefits of biochar application, ranging from improvements in crop yield to global change mitigation, have been extensively studied in the last decade. However, such benefits have not been profusely demonstrated under a Mediterranean climate and still less in combination with high pH soils. In our study, the short to medium effects of biochar application on a soil-plant system under Mediterranean conditions in an alkaline soil were assessed.Material and methodsBarley plants were grown in field mesocosms during three agronomical years at three biochar addition rates (0, 5, and 30 t ha−1). Related to soil, different physicochemical parameters were analyzed as well as microbial respiration, biomass, and functional diversity. In the plant domain, in vivo ecophysiology variables such as leaf transpiration rate, stomatal conductance, and photosynthesis rate were determined while photosynthetic pigment content and soluble protein concentrations were measured in the laboratory. Additionally, crop yield and nutrient composition were also analyzed. The soil-plant connection was investigated by the N content ratio in both fractions establishing the nitrogen efficiency in the system.Results and discussionThe highest rate of biochar amendment enhanced soil moisture and electrical conductivity combined with an increase of SO42−, Cl−, Mg2+, and K+, and decrease of NO3− and HPO4−. Notable variations regarding nutrition and moisture were induced in this Mediterranean alkaline soil after biochar addition although pH remained stable. Contrastingly, there were no major effects on microbial activity, but a lower abundance of the nosZ functional gene was found. Similarly, plant parameters were unaffected regarding chemical composition and ecophysiology although biochar induced a higher efficiency in the plant nitrogen uptake without increasing crop yield.ConclusionsBiochar addition at the highest rate (30 t ha−1) reduced soil-soluble nitrate although N uptake by the plant remained invariable, in turn coupled to no effects on crop productivity. Our study showed that, in a Mediterranean agroecosystem, a wood biochar produced by gasification was unable to increase crop yield, but enhanced soil water retention, decreased the need for N fertilization, and decreased soil-soluble nitrate concentrations, something that could help to mitigate the excessive nitrate levels associated with over-fertilization.

The soil of most Spanish vineyards is strongly eroded and carbon depleted and is very poor in biodiversity. Growing evidence of the negative impacts of soil degradation on climate change mitigation, water quality, and plant production is pushing a shift from intensive viticulture to more sustainable management strategies of the vineyards. Among them, minimum impact and regenerative viticulture are gaining ground. However, field data are still necessary to assess the real effect of these new farming schemes on soil carbon stocks and soil functional biodiversity. We compared soil quality at three vineyards managed under intensive, regenerative, and minimum impact strategies using physical, chemical, and biological indicators. Soil carbon stocks were 2.3 and 3.4 times greater in the regenerative and the minimal impact vineyards than in the intensive vineyard, respectively. Soil biota was particularly favored by regenerative viticulture, with 26.2 times more protists, 3.1 times more nematodes, and 29.4 more microarthropods in the regenerative than in the intensive vineyard. Our results indicate that the ecological intensification of agricultural practices is highly promising to restore degraded agricultural soils under Mediterranean conditions. We also propose cost-effective soil bioindicators sensitive to agricultural management for their possible inclusion in soil monitoring programs.

Tree-microbe interactions are essential for forest ecosystem functioning. Most plant–microbe research has focused on the rhizosphere, while composition of microbial communities in the phyllosphere remains underexplored. Here, we use 16S rRNA gene sequencing to explore differences between beech and Scots pine phyllospheric microbiomes at the European continental scale, map their functional profiles, and elucidate the role of host trees, forest features, and environmental factors such as climate and atmospheric deposition in phyllosphere microbiota assembly. We identified tree species and the associated foliar trait (specifically carbon:nitrogen ratio) as primary drivers of the bacterial communities. We characterized taxonomical and functional composition of epiphytic bacteria in the phyllosphere of beech and Scots pine across an environmental gradient from Fennoscandia to the Mediterranean area, with major changes in temperature and nitrogen deposition. We also showed that temperature and nitrogen deposition played a crucial role in affecting their assembly for both tree species. This study contributes to advancing our understanding on factors shaping phyllosphere microbial communities in beech and Scots pine at the European continental scale, highlighting the need of broad-scale comparative studies (covering a wide range of foliar traits and environmental conditions) to elucidate how phyllosphere microbiota mediates ecosystem responses to global change. Phyllosphere microbiota of beech and Scots pine at European continental scale is influenced by the host species and associated foliar traits, as well as by temperature and nitrogen deposition, according to 16S rRNA gene sequencing analyses on leaf epiphytic microbes.

Increase of soil fertilization produces an increase of N exported to the hydrosphere. The amount of nitrate that reaches the aquifers is controlled by processes affecting N-species within the soils. The most relevant processes are nitrification, denitrification, assimilation, mineralization, and immobilization. This work studies the fate of N compounds in soil after manure application in a lysimeter study. To this end the isotopic composition of N and O of dissolved nitrate (δ 15 N-NO 3 - and δ 18 O-NO 3 - ) was studied coupled with the evolution of N-compounds retained and leached from the soil. Results showed an increase in the δ 15 N-NO 3 - of the leached nitrate towards values similar to the δ 15 N-NH 4 + from the applied manure. The highest δ 15 N-NO 3 - values were measured after 100 days of manure application, and thereafter, values decreased progressively towards the initial δ 15 N-NO 3 - of the soil before manure application.

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76 kg N ha −1  yr −1 . We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling. Canopy nitrification contributes up to 80% of the nitrate reaching the soils via throughfall in European forests, according to analyses of nitrogen deposition and oxygen isotopes in nitrate at ten forested sites.

The CO2 emission rates have been continuously incremented during the last decades. To mitigate it, a method to store carbon in terrestrial ecosystems is the addition of biochar to soil. After its application to soil, biochar suffers an ageing process, able to deteriorate its functional properties as soil improver. However, at present, it is not clear how to evaluate biochar ageing. The main aim of this study is to evaluate biochar ageing by determination of temporal changes on (a) soil respiration after biochar addition and (b) the relationship between CO2 adsorption capacity and wettability of biochar as measurable parameters indicating biochar ageing. Results show that 1 month after biochar addition, soil respiration decreased when poplar and pine biochars were applied to bare soils, in the absence of vegetation. One year after biochar addition, this reduction on soil respiration disappeared, evidencing biochar ageing due to decrements on its CO2 adsorption capacity. Compared with fresh biochar, decreased CO2 adsorption capacity of biochar corresponded with enhanced biochar wettability for both biochar types. Its means that poplar and pine biochars, while initially hydrophobic, became hydrophilic after 1 year of its application to soil. It is concluded that changes of biochar CO2 adsorption capacity in time go along with improved wettability as mutually opposed processes. Globally, pine biochar tends to adsorb a higher quantity of CO2 than poplar biochar. The absence of CO2 adsorption of soil without biochar demonstrates the remarkable capacity of both biochars to adsorb carbon dioxide and promote carbon storage in soils.

Microbial activity plays a central role in nitrogen (N) cycling, with effects on forest productivity. Although N biotransformations, such as nitrification, are known to occur in the soil, here we investigate whether nitrifiers are present in tree canopies and actively process atmospheric N. This study was conducted in a Mediterranean holm oak (Quercus ilex L.) forest in Spain during the transition from hot dry summer to cool wet winter. We quantified NH4+—N and NO3-—N fluxes for rainfall (RF) and throughfall (TF) and used δ15N, δ18O and Δ17O to elucidate sources of NO3-. Finally, we characterized microbial communities and abundance of nitrifiers on foliage, RF and TF water through metabarcoding and quantitative polymerase chain reaction respectively. NO3—N fluxes at the site were larger in TF than RF, suggesting a contribution from dry deposition, as also supported by δ15N and δ18O. However, Δ17O indicated that about 20% of NO3- in TF derived from canopies nitrification in August, after a severe drought, with a lower proportion in September (≈8%). This seasonal partitioning between biologically and atmospherically derived NO3- coincided with a decreasing trend of the abundance of archaeal nitrifiers. Tree canopies and TF had more diverse microbial communities than RF. Yet, RF showed higher variability in microbial composition, likely associated with the origin of air masses. Synthesis. Atmospheric N deposition is significantly altered after passing through tree canopies. While nitrification has been proposed as one of the mechanisms responsible for these changes, very few studies directly investigate its occurrence. Here, we showed that nitrification by epiphytic leaf microbes contributed to increasing NO3 in TF and that nitrifiers' activity was reduced going from the dry and hot summer to the cool winter. Overall, these results highlight the power of coupling microbial community analysis, functional gene amplification and stable isotope approaches to examine ecosystem‐scale processes. Resumen La actividad microbiana desempeña un papel central en el ciclo del nitrógeno (N) y tiene efectos en la productividad forestal. Aunque son bien conocidas las biotransformaciones de N (como la nitrificación) en el suelo, en este trabajo investigamos la presencia de nitrificadores en las copas de los árboles y exploramos si éstos procesan activamente el N atmosférico. Este estudio se realizó en un encinar (Quercus ilex L.) mediterráneo durante la transición del verano seco y cálido al invierno fresco y húmedo. Se cuantificaron los flujos de NH4+—N y NO3‐— N en la lluvia (RF) y la trascolación (TF) y se utilizaron los isótopos δ15N, δ18O y Δ17O para dilucidar el origen del NO3‐. Asimismo, se caracterizaron las comunidades microbianas y la abundancia de nitrificadores en el follaje, RF y TF por medio de la metabarcodificación y la reacción cuantitativa en cadena de la polimerasa, respectivamente. Los flujos de NO3‐— N fueron mayores en TF que en RF, sugiriendo una contribución de la deposición seca, lo que corroboraron los resultados de δ15N y δ18O. Sin embargo, los resultados de Δ17O indicaron que aproximadamente el 20% de NO3‐— N en TF derivó de nitrificación en las copas en agosto, tras una severa sequía, seguido de una proporción menor en septiembre (≈ 8%). Esta división estacional entre NO3 derivado de la actividad biológica y de la deposición atmosférica coincidió con una tendencia decreciente en la abundancia de nitrificadores arqueales. Las copas de los árboles y la trascolación contuvieron comunidades microbianas más diversas que la lluvia. Sin embargo, ésta mostró una mayor variabilidad en su composición microbiana, probablemente asociada a variaciones en el origen de las masas de aire. Síntesis. La deposición atmosférica de N se ve significativamente modificada después de pasar por las copas de los árboles. Si bien se ha propuesto la nitrificación como uno de los mecanismos responsables de estos cambios, muy pocos estudios lo han investigado directamente. Aquí, mostramos que la nitrificación microbiana foliar contribuyó a aumentar el NO3 en TF y que la actividad de los nitrificadores se redujo al pasar del verano seco y caluroso al frio invierno. En general, estos resultados resaltan la potencia de combinar análisis de la comunidad microbiana, de amplificación funcional de genes y de isótopos estables para examinar los procesos a escala de ecosistema. Our study investigated whether nitrifying microbes were present in holm oak tree canopies and actively processed atmospheric nitrogen (panel a). No difference were found between RF and TF for N fluxes in the form of NH4, whereas higher N fluxes in the form of NO3 were observed in TF versus RF. By using the Δ17O tracer we found that about 20% of NO3- in TF derived from canopies nitrification (fBio) in August, after a severe drought, with a lower proportion in September, while atmospheric deposition (fAtm) was the dominant source of NO3- for October–December (panel b). This temporal partitioning between biologically and atmospherically derived NO3- coincided with a decreasing trend of the abundance of archaeal nitrifiers (as shown by the Archaeal amoA copies per ng microbial DNA; panel c). These results suggest the important, but overlooked, role of foliar microbes in N cycling by contributing to the biological transformation of atmospheric N.

Human activities have greatly increased the reactive nitrogen in the biosphere, thus profoundly altering global nitrogen cycling. The large increase in nitrogen deposition over the past few decades has led to eutrophication in natural ecosystems, with negative effects on forest health and biodiversity. Recent studies, however, have reported oligotrophication in forest ecosystems, constraining their capacity as carbon sinks. Here we demonstrate the widespread biological transformation of atmospheric reactive nitrogen in the canopies of European forests by combining nitrogen deposition quantification with measurements of the stable isotopes in nitrate and molecular analyses across ten forests through August–October 2016. We estimate that up to 80% of the nitrate reaching the soil via throughfall was derived from canopy nitrification, equivalent to a flux of up to 5.76¿kg¿N¿ha-1¿yr-1. We also document the presence of autotrophic nitrifiers on foliar surfaces throughout European forests. Canopy nitrification thus consumes deposited ammonium and increases nitrate inputs to the soil. The results of this study highlight widespread canopy nitrification in European forests and its important contribution to forest nitrogen cycling.

Aims We investigated the effects of volatile organic compounds (VOCs) emitted by pine litter, specifically terpenes, on soil microbial biomass carbon and nitrogen and heterotrophic soil respiration under different microclimatic scenarios of water availability and temperature. Methods Soil in glass jars (0.6 L headspace) was exposed to pine needle litter, avoiding any physical contact between soils and litter. Treatments were subjected to two moisture levels, control and drought (20 % and 10 % gravimetric soil water content respectively) and to different temperatures (temperature response curve from 5 °C to 45 °C). Results In control soils, exposure to litter was associated with a significant decrease in microbial biomass carbon and ninhydrin extractable organic nitrogen, and with a significant increase in heterotrophic respiration (up to 46 %) under optimum temperature (25 °C). Drought, on the other hand, restricted the effects of litter exposure on heterotrophic respiration but exposure to litter was associated with a significant increase in microbial biomass nitrogen. We did not detect significant overall microbial consumption of terpenes in this study. Conclusions These results suggest either that other VOCs not measured in the study were being consumed and/or that VOCs emissions were triggering strong changes in the composition and functioning of soil microbial communities. More studies under field conditions are needed to assess the magnitude of litter VOCs effects on carbon and nitrogen cycles.

This study reports the relationship between the diversity and functioning of fungal and bacterial soil communities with vegetation in Mediterranean woodland that experienced severe die-off after a drought episode. Terminal restriction fragment length polymorfism (TRFLP) was used to describe microbial community structure and diversity five years after the episode in different habitats (Juniperus woodland, shrubland, grassland), when the vegetation had not yet recovered. Vegetation diversity was positively related to TRF bacterial richness under unaffected canopies and was higher in diverse grassland. Fungal TRF richness correlated with vegetation type, being greater in Juniperus woodland. Microbial respiration increased in grassland, whereas microbial biomass, estimated from soil substrate-induced respiration (SIR), decreased with bacterial diversity. Die-off increased bacterial richness and changed bacterial composition, particularly in Juniperus woodland, where herbaceous species increased, while fungal diversity was reduced in Juniperus woodland. Die-off increased microbial respiration rates. The impact on vegetation from extreme weather episodes spread to microbial communities by modifying vegetation composition and litter quantity and quality, particularly as a result of the increase in herbaceous species. Our results suggest that climate-induced die-off triggers significant cascade effects on soil microbial communities, which may in turn further influence ecosystem C dynamics. Drought episodes, likely related to climate change, may produce large impacts on vegetation resulting in extensive die-off, which in turn modifies soil microbial communities and their contribution to ecosystem functioning.