Biochar application is widely promoted as a soil amendment to enhance soil fertility. However, significant knowledge gaps persist regarding its effect on herbicide efficacy and its impact on non-target organisms in tropical agroecosystems. This study aimed to evaluate, under laboratory and greenhouse conditions using a tropical Ultisol, how the addition of pyrolyzed carbonaceous materials influences (i) the efficacy and environmental behavior of the herbicides bromacil and diuron, and (ii) the response of soil fauna bioindicator species and microbial functional diversity. Pineapple stubble and coffee hulls, pyrolyzed at 300° (torrefied materials) or 600 °C (biochar), were used. These materials were applied at two rates (10 and 20 t ha− 1), both with and without the application of bromacil and diuron. Evaluations included herbicide phytotoxicity, plant growth, survival and reproduction of soil fauna (Collembola and Enchytraeids), soil avoidance/preference behavior, and microbial functional diversity. The addition of carbonaceous materials did not affect the phytotoxicity of the herbicides or initial plant growth in the absence of herbicides. The survival of non-target fauna species was also unaffected; conversely, their reproduction was consistently promoted. However, a contrasting ecotoxicological response in behavior was observed: while Enchytraeids showed a generalized avoidance of the amended soil, Collembola exhibited a marked preference. Soil microbial functional diversity remained largely unaltered. Pyrolyzed carbonaceous materials did not compromise the efficacy of the herbicides or the survival of non-target soil organisms. Nevertheless, alterations in faunal behavior suggest a selective ecological influence. Despite the observed benefits in promoting faunal reproduction, further field studies are required to better understand the long-term effects of these materials on the ecological dynamics of tropical agroecosystems. Addition of charred materials did not attenuate the efficiency of bromacil or diuron. Charred materials and herbicides showed no unintended toxic effects on soil invertebrates. Addition of charred materials promoted the reproduction of both collembolans and enchytraeids. Enchytraeids avoided charred material mixtures while collembolans showed a preference for them. Microbial functional diversity was not affected by the addition of herbicides or charred materials.

Springtails are abundant in forest litter and soils, where they play a crucial role in the transformation and decomposition of organic matter. However, most data on springtail communities originates from studies at the local and regional levels, while larger-scale studies are lacking. To address this gap, we applied standardized methods to investigate springtail communities in forests from a Europe-wide study, covering a pedo-climatic gradient across 25 sites in 17 countries. We evaluated which environmental factors influence springtail density and alpha- and beta-diversity. We sampled forest litter and underlying soil separately and applied both morphological identification and soil DNA metabarcoding. On average, we recorded approximately 2500 individuals m−2 for litter-dwelling and 10,500 individuals m−2 for soil-dwelling springtails, with the highest values recorded in the Arctic and Boreal regions and the lowest in the Mediterranean region. Species richness averaged 5.2 in the litter and 7.0 in the soil applying morphotaxonomy, whereas using metabarcoding indicated a higher richness of 10.2 in the soil. Using morphotaxonomy, the key factor influencing density and alpha-diversity of litter-dwelling springtails positively was litter dry mass, whereas mean annual temperature had a negative effect, and the key factors positively influencing the density of soil-dwelling springtail were soil N and clay content, whereas bulk soil density had a negative effect. When data from metabarcoding were used, similar key factors emerged. Our results demonstrate that metabarcoding represents a promising approach for future large-scale studies when simple and reliable methods are needed to rapidly assess shifts in springtail community profiles.

Global change threatens biodiversity across ecosystems worldwide, yet soil biodiversity remains comparatively poorly understood due to the challenges of quantifying belowground life. As a result, its main drivers and spatial patterns remain unresolved across environmental gradients. Here, we analyzed soil biodiversity across 102 sites in the north-eastern Iberian Peninsula, spanning ecosystems from arid to boreal (alpine) biomes. Using multitaxon DNA metabarcoding, we assessed richness and Shannon diversity of bacteria, fungi, nematodes, and microarthropods and related these patterns to climatic variables, soil physicochemical properties, habitat and vegetation heterogeneity, and anthropogenic context. Diversity patterns showed coordinated variation among groups. Microbial diversity was strongly associated with soil pH-dependent nutrient relationships: soluble manganese (Mn) emerged as a key predictor of microbial diversity, showing positive effects under alkaline conditions, while calcium (Ca) covaried with pH and aluminium (Al) in ways consistent with constraints on bacterial communities, i.e. Ca alleviating Al toxicity in acidic soils, favouring bacterial diversity. Fungal richness also showed nutrient-related signals, negative with higher total nitrogen or nitrates with low pH, but positive with higher available phosphorus in alkaline conditions. In contrast, soil-fauna diversity was primarily linked to habitat structure and heterogeneity: richness increased with structural vegetation heterogeneity, amplified under high shrub cover for nematodes and varied with annual temperature and tree cover for microarthropods. Overall, combining soluble soil chemistry with reproducible habitat-heterogeneity indices in a compact regional design reveals how driver sets differ across trophic groups and improves the mechanistic interpretation of multi-taxon soil biodiversity patterns.

Soil fauna and microbial communities are key drivers of soil organic matter turnover and nutrient cycling, but we are still far from unraveling the mechanisms underlying the full complexity of their interactions. While soil fauna is generally hypothesized to release microbes from bottom-up resource limitations, they could also exert a strong top-down control by either direct feeding or by shifting the stoichiometric balance of the soil solution, thus constraining microbial growth. To try filling this knowledge gap we report novel data on a microcosm incubation experiment in which we controlled the presence of meso and macrofauna and measured the flows of carbon (C), nitrogen (N) and phosphorus (P) from litter to the different soil pools and tracked their effects on a comprehensive set of microbial functioning variables which included growth rates, stoichiometry, enzyme activity, substrate degrading capacity, and rates of N and P mineralization and consumption. Additionally, we evaluated changes in the microbial community composition through 16S, ITS and 18S DNA marker sequencing. Soil macrofauna boosted the release of C, N and P from the litter pool. This led to a strong increase in dissolved organic C and a moderate increase in free amino acids, ammonium and phosphate concentration, thus resulting in a sharp increment of the C:N and C:P ratios in the soil solution. Microbial C and growth were greater in the microcosms with meso and macrofauna, but their C-use efficiency did not change. Macrofauna presence boosted the microbial gross production and consumption of amino acids, ammonium and nitrate, but P mobilization and uptake rates remained equal across treatments. The activity of beta-glucosidase also increased with macrofauna while N and P mining enzyme activities did not change. Overall, soil macrofauna strongly up regulated microbial communities by releasing them from C limitation.

Phytomining is an environmentally friendly, cost-effective plant-based technique to recover valuable metals from mineralised or polluted soils. Nickel phytomining has garnered much attention due to the vast number of Ni hyperaccumulator species identified in ultramafic areas worldwide and their promising metal extraction yields. Over the last few decades, the need to optimise phytomining yields through agronomic and crop management practices has been emphasised. Hence, this study evaluated the effects of organic amendments on phytomining yields using the Ni hyperaccumulator species Odontarrhena serpyllifolia, which is endemic to the Bragança and Morais ultramafic massifs in Portugal. The primary objective was to determine if the organic amendments biochar, and a combination of biochar with industrial sludge, and biochar with pig slurry, were appropriate for improving Ni harvestable amounts (governed by plant biomass and Ni accumulation in shoots) with O. serpyllifolia in the Morais massif. Plant seeds and soils were collected from the Morais massif. Organic amendments were added to the collected soil, including 1.5 % and 3 % biochar (w/w), and the same biochar rates were tested in combination with 1 % dried industrial sludge or 1 % pelletized pig slurry. Plants were cultivated in a laboratory greenhouse under controlled conditions. Soil treatments containing both biochar and sludge yielded the most promising results, particularly when the highest biochar rate was applied. This can be attributed to the significant improvement in soil properties and nutrient levels. The application of 1.5 % and 3 % biochar alone had no significant effect. Pig slurry mixed with biochar had a pronounced negative impact on plants, which can be explained by its high salinity levels. This study confirms that certain types of organic waste can be valorised and incorporated into phytomining systems.

Anthropogenic biodiversity decline threatens the functioning of ecosystems and the many benefits they provide to humanity1. As well as causing species losses in directly affected locations, human influence might also reduce biodiversity in relatively unmodified vegetation if far-reaching anthropogenic effects trigger local extinctions and hinder recolonization. Here we show that local plant diversity is globally negatively related to the level of anthropogenic activity in the surrounding region. Impoverishment of natural vegetation was evident only when we considered community completeness: the proportion of all suitable species in the region that are present at a site. To estimate community completeness, we compared the number of recorded species with the dark diversity-ecologically suitable species that are absent from a site but present in the surrounding region2. In the sampled regions with a minimal human footprint index, an average of 35% of suitable plant species were present locally, compared with less than 20% in highly affected regions. Besides having the potential to uncover overlooked threats to biodiversity, dark diversity also provides guidance for nature conservation. Species in the dark diversity remain regionally present, and their local populations might be restored through measures that improve connectivity between natural vegetation fragments and reduce threats to population persistence.

Microplastics (MPs) are a growing concern as they are increasingly detected in various environmental compartments. An important pathway for MPs to enter terrestrial ecosystems is the application of organic amendments for soil improvement. This study aims to investigate the size, morphology, and composition of MPs in different types of treated organic waste commonly used in agricultural soils and soil restoration. A total of seven organic amendments, including sewage sludge, horse manure, two composts and two digestates from agri-food industries and selectively collected municipal organic waste, respectively, and a biostabilized product from non-selectively collected municipal organic waste, were analyzed. The samples underwent pretreatment steps including advanced Fenton oxidation, alkaline and enzymatic digestion, and density separation, followed by spectroscopic confirmation. The results revealed high plastic concentrations ranging from 6774 to 551,696 items/kg of dry weight, depending on its origin and treatment. Sewage sludge samples exhibited the highest MPs concentrations, while horse manure showed the lowest. The most prevalent MPs morphologies were fragments, followed by fibers and films. Among the particles, the most prevalent polymers observed were PE, PP, PET, PVC, and PS, whereas PES constituted the primary composition of the fibers. The present study underscores the substantial presence of MPs in organic waste amendments and when they are applied to soil can be ingested by terrestrial organisms, entering the food chain and posing risks to human health.

Environmental changes and their effects are among the most pressing topics of today's ecological research. Shrublands, although widespread across the globe, remain understudied in this respect. We conducted a global meta-analysis of 81 shrubland sites subjected to experimental warming, shifts in precipitation (e.g. increased precipitation and drought), and nitrogen addition to quantify seven types of vegetation responses, including density and cover, species diversity, shrub proportion, and ecosystem functions. Our results indicated that the magnitude of responses varied depending on the vegetation metrics and treatment conditions. Specifically, aboveground biomass (AGB) was most sensitive to warming, increased precipitation, and nitrogen addition, while density was most responsive to drought treatment. Short-term treatments (1-5 yr) generally elicited stronger responses than long-term ones (> 5 yr), particularly under drought. High sensitivity to changes in climate and nitrogen addition was observed at extremely arid sites (aridity index < 0.2), and water availability strongly mediated sensitivity variation. Surprisingly, many vegetation metrics revealed no association between sensitivity variability and site water availability. Our research offers a global perspective on shrubland vegetation responses to environmental changes, highlighting the importance of water availability in sustaining shrubland biodiversity and functioning under future conditions.

The goal of this study was to evaluate biochar’s resistance to microbial decomposition and its impact on native soil organic matter (SOM) decomposition. Conducted in a vineyard with a sandy loam Mediterranean soil with neutral pH and low organic carbon content, the experiment involved the application of 6.5 g biochar kg−1 derived from pine (PB) and corn cob (ZB). The monitoring period spanned two years, with soil samples collected at short- and medium-term timepoints (2 and 26 months post-application) and incubated in the lab for an additional 250 days. Soil respiration, the CO2-C isotopic signature, and dissolved organic carbon (DOChw) were assessed to identify potential priming effects (PE) and evaluate their persistence over two years. In the short term, biochar-induced priming effects were feedstock and pyrolysis temperature dependent, exhibiting negative priming in high-temperature wood biochar and positive priming in low-temperature grass biochar. The mechanism behind short-term positive priming was attributed to the higher labile organic carbon (OC) content in ZB compared to PB. In the medium term, initial strong priming effects shifted to slightly negative priming effects in both biochars, indicating the depletion of labile carbon fractions and the emergence of physical protection processes that mitigated priming.

Grassland management aims to ensure sufficient yield, forage quality and biodiversity. Robust knowledge supports sustainable management practices. In South Tyrol (NE Italy), we studied the effect of organic fertilisation on soil acid and alkaline phosphatase activity (ACP, ALP, or both APase), phosphorus (P) availability and forage yield in mountain permanent meadows. Three factors were included in the experimental design, which was arranged as a split-plot design: the initial vegetation class (C1 = moderately species-poor or C2 = moderately species-rich), being the main plot, randomised within three study areas, as well as the manure type (slurry, farmyard manure, and a combination of farmyard manure and manure effluent) and the nitrogen (N) fertilisation input (0, 55.5 and 111 kg N ha(-1) yr(-1)), both randomised within the vegetation class. Soil samples were collected from the top 10 cm before the last cut in summer 2022. Results showed that the combined use of farmyard manure and manure effluent decreased ACP activity with increasing N input, whilst ALP activity remained unaffected. These novel findings show that organic N input does not imply an increase in APase activity. Moreover, the C2 meadow class showed higher ACP activity than C1, possibly due to higher species diversity, a lower mowing frequency and the legacy effect of more extensive management prior to the start of the trial. Both ACP and ALP activity responded to pH (negatively for ACP activity and positively for ALP activity) and both were negatively affected by soil moisture, highlighting their sensitivity to changes in the soil conditions. ALP activity was positively influenced by total organic carbon (TOC) and by the Shannon diversity index of the plant communities, possibly due to its link with the soil microbial community. Soil available P increased with pH, TOC, soil moisture, K2O content, and N organic input from farmyard manure, which provided the highest P input. The forage yield of the last growth cycle was positively affected by organic N input but negatively affected by TOC, whilst the activity of both APase had no effect on it. The annual yield increased with the N input and was higher in the C1 meadow class than in the C2. On the whole, the results suggest that organic fertilisation, rather than APase activity, was the main driver of forage yield.

Agricultural and livestock production cover more than a third of the Earth's land surface and are crucial to food supply. Soil extracellular enzymes play an important role in the transformation of elements and compounds in soil, particularly acid (ACP) and alkaline (ALP) phosphatases (both, APases). These enzymes have a vital role in releasing phosphorus (P) from organic matter. However, the effect of climate variables and agro-ecosystem management on APase activity in croplands remains unclear, as does its eventual relationship with agricultural productivity. Therefore, we compiled a global database of APase activity in croplands (between 1977 and 2022) and we analysed 5876 observations across 474 papers to study climate variables, crop family, and management effects on ACP and ALP activity, and their relationship with yield. ACP activity is reduced by higher temperatures (p<0.001) and lower rainfall (p=0.002). There was an interaction effect of temperature and precipitation on ALP activity (p=0.046), with the negative effect of temperature being stronger with high precipitation, and low precipitation showing low ALP activity levels at any temperature. The crop family greatly influenced APase activity (p<0.001). Management practices affected ACP and ALP activity differently; ACP activity was positively influenced by organic fertilization combined with, crop rotation or irrigation by an average of 15.6 % and 30.7 %, respectively. ALP activity was mainly positively influenced by the interaction of two different factors: organic or inorganic-organic fertilization and reduced or zero tillage. Further understanding of soil enzyme mechanisms would aid global food security and yield. As ACP activity doubles from 100.0 to 200.0 mg pNP kg(-1)h(-1), the crop yield increases by more than two-fold, an outcome not demonstrated in croplands until now. These results could enhance yield potential through the promotion of APase activity, and the consideration of climate variables and agro-ecosystem management, which could ultimately improve cost-benefit ratios for sustainable crop growth.

Nanoplastics and antibiotics are among the most abundant chemical pollutants of soils, but their interplay with global warming remains poorly understood. The springtail Folsomia candida (Class Collembola) is a standard model for ecotoxicological assays with potential as a bioindicator of xenobiotics. Little is known, however, about their gut microbiome and how it might respond to warming and these pollutants. We exposed populations of F. candida to nanoplastics and antibiotic under two temperatures. The antibiotic treatment consisted of colistin addition, and the nanoplastic treatment consisted of polystyrene particles (50 mg kg−1 and 0.1 g kg−1 of dry soil, respectively). Both treatments were incubated at 20 and 22 °C for two months, and the bacterial gut microbiomes of springtails were then sequenced. Exposure to nanoplastics at 20 °C decreased the abundance of the dominant bacterial phyla and families, and decreased the evenness of the gut microbiome. At 22 °C, however, the abundances and evenness of the dominant families increased. Surprisingly, Gramnegative bacteria targeted by colistin were not globally affected. And at genus-level, the endosymbiont Wolbachia controlled the compositional shifts under nanoplastic addition, potentially driving the gut microbiome. Our results also indicated that warming was a major driver modulating the impacts of the antibiotic and nanoplastics. We illustrate how the gut microbiomes of springtails are sensitive communities responsive to xenobiotics and provide evidence of the need to combine multiple factors of global change operating simultaneously if we are to understand the responses of communities of soil arthropods and their microbiomes.

Phosphorus (P) is a vital macronutrient crucial for crop productivity. Plants absorb P salts, mainly orthophosphate, from the soil, yet the primary P source resides in organic materials. Acid and alkaline phosphatases (the predominant forms of soil phosphomonoesterases (APases)) are crucial for alleviating P deficiency in plants and play a vital role in releasing P from organic materials via hydrolysis. Our aim was to summarize the direction of the relationship between a variety of influential factors on acid and alkaline phosphatase activity in agricultural lands and identify gaps in knowledge. Our findings indicate a strong linkage between both APases and soil pH, positively influenced by clay content, organic matter, microbial biomass carbon, and nitrogen. Adopting healthy soil practices like balanced organic fertilizer usage, optimal soil water levels, reduced tillage, crop rotation, and using beneficial plant microbes help boost both APase activity. However, the connection between APases and crop productivity remains uncertain due to insufficient research in this area. We identified gaps in knowledge in relation to meso-macrofauna, alongside essential plant nutrients such as potassium, nutrient ratios, and the synergistic effects of various factors on APase response. Understanding the rapid, efficient assimilation of P through APases in the plant-soil and/or plant-microbiota ecosystem it can be crucial for crop productivity and yields.

&lt;p&gt;The Catalonia Plot System for Terrestrial Biodiversity Monitoring (SISEBIO) is a long-term project aiming to monitor above- and below-ground biodiversity changes due to global change in permanent experimental sites. The project aims: a) to catalogue the existing biodiversity using metabarcoding, b) to describe the environmental drivers explaining such, and c) to identify habitats acting as biodiversity hotspots.&lt;/p&gt; &lt;p&gt;For this purpose, 109 permanent plots were set up in natural areas to covering all the main habitats and climates of Catalonia (NE Spain). Plant, microbial (bacteria and fungi), protists, and microarthropods richness and diversity were assessed between 2018-2021, together with a variety of environmental drivers (soil physicochemical properties, habitat structure, climate, and topography). While plant diversity was assessed through traditional morphological identification, that of soil organisms was assessed by metabarcoding and using operational taxonomic units (OTUs) for this purpose.&lt;/p&gt; &lt;p&gt;Concerning soil biodiversity, 42077 unique OTUs were identified, with around 40% of them only found in once. The highest biodiversity values corresponded to sites located in the Pyrenees, and the environmental factors driving biodiversity were clearly different depending on the taxa studied. However, we failed to find habitat-specific hotspots except for microarthropods, with higher richness values in conifer forests when compared to deciduous forests, shrublands and grasslands.&lt;/p&gt; &lt;p&gt;The existence of biological interactions and historical factors may hinder the emergence of strong environmental trends to describe soil biodiversity patterns. Our results might may guide stakeholders with the implementation of management policies in the most vulnerable habitats to protect their biodiversity, but are also of interest for modelling the impact of global change on soil biodiversity and their ecosystem services.&lt;/p&gt;

Nematodes are numerous in soils and play a crucial role in soil food-webs. DNA metabarcoding offers a timeeffective alternative to morphology-based assessments of nematode diversity. However, it is unclear how different DNA extraction methods prior to metabarcoding could affect community analysis. We used soils with woody vegetation from a European latitudinal gradient (29 sites, 39 to 79 degrees N, similar to 4500 km, covering six biomes) to systematically evaluate the effect of two sources of nematode DNA either directly extracted from soils vs. extracted from nematodes previously isolated from soils hypothesizing that the DNA source material may produce different diversities, community structures and abundances of feeding types. Nematode-sample DNA exhibited a higher richness, while no difference in Shannon diversity was found between the approaches. The DNA sources also created significantly different community structures, with greater differences observed across soil-extracted DNA than nematode-sample DNA. The most overrepresented species in nematode-sample DNA were Heterocephalobus elongatus, Eucephalobus striatus and Hexatylus sp., whereas Phasmarhabditis sp. and Eumonhystera filiformis were overrepresented in soil-extracted DNA. Read abundances of feeding types signifi-cantly differed between the DNA sources and across sites, with a significant effect of biome on both ecto-and endoparasitic herbivores in soil-extracted DNA and for ectoparasitic herbivores only in nematode-sample DNA. Collectively, our data suggest that choice of the DNA source material may lead to different patterns of nematode community composition across space and environmental conditions. Improving the sensitivity of the soil -extracted DNA method by developing protocols using larger amounts of soil and designing nematode-specific primers will make this approach an efficient screening tool to analyse nematode diversity and community structure complementing the labour-intensive isolation of intact nematodes from soils (nematode-sample DNA).

Biochar, a carbon-rich solid produced from biomass pyrolysis, has attracted growing interest as a fertiliser ingredient due to its ability to non-permanently retain nutrients. A greenhouse pot experiment was set up to compare three commercial organo-mineral fertiliser formulations (NPK, NP and K) with the corresponding formulations containing a slow-pyrolysis wood biochar (NPK+B, NP+B and K+B) (6 replications each). Nutrient leaching as well as crop growth and nutrient uptake was monitored using barley as model species. Nutrient leaching was slowed down in the NPK+B compared to the NPK fertiliser. The most responsive ions were nitrate and potassium, whose leaching during the two first weeks was reduced by 28% and 22%, respectively, while this trend reversed from the third week on. One plausible explanation would be a microbial nutrient immobilisation mediated by the concurrent NPK and biochar habitat provision. NPK+B significantly enhanced barley straw biomass (23.43% increase respect to NPK), whereas all the biochar-based fertilisers showed increases in nutrient content and export (involving potassium, sulphur, calcium and manganese), possibly indicating that biochar acted as a nutrient source. These results provide some evidence of the potential use of the studied biochar in biochar-based fertilisers to meet nutrient availability with plant demands.

Soil invertebrates (i.e., soil fauna) are important drivers of many key processes in soils including soil aggregate formation, water retention, and soil organic matter transformation. Many soil fauna groups directly or indirectly participate in litter consumption. However, the quantity of litter consumed by major faunal groups across biomes remains unknown. To estimate this quantity, we reviewed > 1000 observations from 70 studies that determined the biomass of soil fauna across various biomes and 200 observations from 44 studies on litter consumption by soil fauna. To compare litter consumption with annual litterfall, we analyzed 692 observations from 24 litterfall studies and 183 observations from 28 litter stock studies. The biomass of faunal groups was highest in temperate grasslands and then decreased in the following order: boreal forest > temperate forest > tropical grassland > tundra > tropical forest > Mediterranean ecosystems > desert and semidesert. Tropical grasslands, desert biomes, and Mediterranean ecosystems were dominated by termites. Temperate grasslands were dominated by omnivores, while temperate forests were dominated by earthworms. On average, estimated litter consumption (relative to total litter input) ranged from a low of 14.9% in deserts to a high of 100.4% in temperate grassland. Litter consumption by soil fauna was greater in grasslands than in forests. This is the first study to estimate the effect of different soil fauna groups on litter consumption and related processes at global scale.

Biochar applications can have important implications for many of the soil functions upon which agroecosystems rely, particularly regarding organic carbon storage. This study evaluated the impacts of adding a highly aromatic gasification biochar at different rates (0, 12 and 50 t ha-1) to a barley crop on the provision of crucial soil functions (carbon sequestration, water content, greenhouse gas emissions, nutrient cycling, soil food web functioning, and food production). After natural ageing in the field for six years, a wide range of soil properties representative of the studied soil functions were measured and integrated into a soil quality index. Results showed that C sequestration increased with biochar rate (23 and 68% higher than in the control for the 12 and 50 t biochar ha-1 treatments, respectively). Water content was enhanced at the 50 t ha-1 treatment depending on the sampling date. Despite biochar additions neither abating nor increasing CO2 equivalent emissions (carbon dioxide plus nitrous oxide and methane), the system shifted from being a methane sink (-0.017 ± 0.01 mg CH4-C m-2 h-1 at the 12 t ha-1 treatment), to a net source (0.025 ± 0.02 mg CH4-C m-2 h-1 at the 50 t ha-1 treatment). In addition, biochar ageing provoked a loss of nitrate mitigation potential, and indeed ammonium production was stimulated at the 50 t ha-1 rate. The 50 t ha-1 treatment also adversely affected nematode and collembolan functional diversity. Lastly, biochar did not affect barley yield. The results of the soil quality index indicated that no biochar treatment provided more benefits to our agricultural soil, and, although the 50 t ha-1 treatment increased C sequestration, this was potentially offset by its harmful effects on soil faunal communities. Therefore, application of this biochar at high rates should be avoided to prevent risks to soil biological communities.

Biochar is a carbon rich product obtained from pyrolysis of biomass. The use of biochar as soil amendment has been boosted in the last years due to its possible influence on fertility, including its potential ability to lower mineral nitrogen losses, but specially for its potential to reduce greenhouse gases and to increase carbon sequestration in soil. However, the studies on the effects of biochar on nitrogen forms in soil are heterogeneous and contradictory. The present work aims to clarify this point by applying 6 different biochars (with different origin and production process) on 6 different soils (of different properties). The amendment corresponded to an agronomic addition rate of 30 Mg ha(-1), together with the addition of urea at a 100 kg N per ha rate. Then those mixtures were incubated for one year at a 60% of the WHC. The samples were analyzed for nitrogen forms (Kjeldahl-N, ammonium-N, nitrate-N, nitrite-N, and microbial-N) at different incubation times (1 week, 1 month, 4 months and 1 year after the addition). The results showed that the effects of different biochars on the soil nitrogen forms were variegated, mainly attributable to soil properties, and to a lesser extent to the particular biochar used. Overall, the Kjeldahl-N (KN) decreased after the incubation time, and only the mixtures with N-rich biochars achieved slightly higher KN compared to controls. Also, biochars tended to induce a decrease in NH4+-N, and, especially, in NO3--N. The biochars causing highest shifts on N inorganic forms were those produced from agronomic sources (olive and corn wastes) and the one from pine wood materials subjected to high pyrolysis temperature conditions.