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Background and aims Soil aggregation is a crucial aspect of ecosystem functioning in terrestrial ecosystems. Arbuscular mycorrhizal fungi (AMF) play a key role in soil aggregate formation and stabilization. Here we quantitatively analyzed the importance of experimental settings as well as biotic and abiotic factors for the effectiveness of AMF to stabilize soil macroaggregates. Methods We gathered 35 studies on AMF and soil aggregation and tested 13 predictor variables for their relevance with a boosted regression tree analysis and performed a meta-analysis, fitting individual random effects models for each variable. Results and conclusions The overall mean effect of inoculation with AMF on soil aggregation was positive and predictor variable means were all in the range of beneficial effects. Pot studies and studies with sterilized sandy soil, near neutral soil pH, a pot size smaller than 2.5 kg and a duration between 2.2 and 5 months were more likely to result in stronger effects of AMF on soil aggregation than experiments in the field, with non-sterilized or fine textured soil or an acidic pH. This is the first study to quantitatively show that the effect of AMF inoculation on soil aggregation is positive and context dependent. Our findings can help to improve the use of this important ecosystem process, e.g. for inoculum application in restoration sites.

Background In the Mediterranean climate, plants have evolved under conditions of low soil-water and nutrient availabilities and have acquired a series of adaptive traits that, in turn exert strong feedback on soil fertility, structure, and protection. As a result, plant-soil systems constitute complex interactive webs where these adaptive traits allow plants to maximize the use of scarce resources. Scope It is necessary to review the current bibliography to highlight the most know characteristic mechanisms underlying Mediterranean plant-soil feed-backs and identify the processes that merit further research in order to reach an understanding of the plant-soil feedbacks and its capacity to cope with future global change scenarios. In this review, we characterize the functional and structural plant-soil relationships and feedbacks in Mediterranean regions. We thereafter discuss the effects of global change drivers on these complex interactions between plants and soil. Conclusions The large plant diversity that characterizes Mediterranean ecosystems is associated to the success of coexisting species in avoiding competition for soil resources by differential exploitation in space (soil layers) and time (year and daily). Among plant and soil traits, high foliar nutrient re-translocation and large contents of recalcitrant compounds reduce nutrient cycling. Meanwhile increased allocation of resources to roots and soil enzymes help to protect against soil erosion and to improve soil fertility and capacity to retain water. The long-term evolutionary adaptation to drought of Mediterranean plants allows them to cope with moderate increases of drought without significant losses of production and survival in some species. However, other species have proved to be more sensitive decreasing their growth and increasing their mortality under moderate rising of drought. All these increases contribute to species composition shifts. Moreover, in more xeric sites, the desertification resulting from synergic interactions among some related process such as drought increases, torrential rainfall increases and human driven disturbances is an increasing concern. A research priority now is to discern the effects of long-term increases in atmospheric CO₂ concentrations, warming, and drought on soil fertility and water availability and on the structure of soil communities (e.g., shifts from bacteria to fungi) and on patching vegetation and root-water uplift (from soil to plant and from soil deep layers to soil superficial layers) roles in desertification.

Background and aims Soil depth is a critical attribute of any soil, and determines rooting, moisture and nutrient storage, mineral reserve, anchorage, and a range of conditions that affect plant growth. We reviewed papers from four primary soil science journals and extracted how deep the soils were studied in those papers. Methods Soil depth was obtained over a 30-years period (1989–2019) from papers in: European Journal of Soil Science , Geoderma, Plant and Soil , and Soil Biology and Biochemistry . In total, 1146 papers were reviewed, and 37% (420 papers) included information on how deep the soil was studied. Results The number of papers that included soil depth increased from 31% in 1989 to 47% in 2019. The average soil depth studied was 27 cm, but it was 53 cm between 1989 and 1999, and 24 cm between 2004 and 2019. Most of the studies were from Europe, and 41% of the papers contained soil classification. Research that focused on soil mineralogy and technology tended to study soils to a greater depth (average 74 cm), whereas the depth in soil biology research was on average 18 cm. Over 80% of the soils were sampled by fixed depth and not by soil horizon. Conclusions Soil depth is lacking from about half of the papers in these four journals. The depth of the soil studied has halved in the past 30 years. Soil processes, soil properties, and microbial communities are depth-dependent, and for a more complete understanding, soils should be studied to a greater depth.

AIMS: We estimate organic carbon (C): total nitrogen (N): total phosphorus (P) ratios in soils under Australia’s major native vegetation groups. METHODS: We use digital datasets for climate, soils, and vegetation created for the National Land and Water Resources Audit in 2001. Analysis-of-variance is used to investigate differences in nutrient ratios between ecosystems. Linear discriminant analysis and logistic regression are used to investigate the relative importance of climatic variables and soil nutrients in vegetation patterns. RESULTS: We find that the N:P and C:P ratios have a greater range of values than the C:N ratio, although major vegetation groups tend to show similar trends across all three ratios. Some apparently homeostatic groupings emerge: those with very low, low, medium, or high N:P and C:P. Tussock grasslands have very low soil N, N:P, and C:P, probably due to frequent burning. Eucalypt woodlands have low soil N:P and C:P ratios, although their total P level varies. Rainforests and Melaleuca forests have medium soil N:P and C:P ratios, although their total P level is different. Heathlands, tall open eucalypt forests, and shrublands occur on soils with low levels of total P, and high N:P and C:P ratios that reflect foliar nutrient ratios and recalcitrant litter. CONCLUSIONS: Certain plant communities have typical soil nutrient stoichiometries but there is no single Redfield-like ratio. Vegetation patterns largely reflect soil moisture but for several plant communities, eucalypt communities in particular, soil N and P (or N:P) also play a significant role. Soil N:P and the presence of Proteaceae appear indicative of nutrient constraints in ecosystems.

Accurate and detailed spatial soil information is essential for environmental modelling, risk assessment and decision making. The use of Remote Sensing data as secondary sources of information in digital soil mapping has been found to be cost effective and less time consuming compared to traditional soil mapping approaches. But the potentials of Remote Sensing data in improving knowledge of local scale soil information in West Africa have not been fully explored. This study investigated the use of high spatial resolution satellite data (RapidEye and Landsat), terrain/climatic data and laboratory analysed soil samples to map the spatial distribution of six soil properties-sand, silt, clay, cation exchange capacity (CEC), soil organic carbon (SOC) and nitrogen-in a 580 km2 agricultural watershed in south-western Burkina Faso. Four statistical prediction models-multiple linear regression (MLR), random forest regression (RFR), support vector machine (SVM), stochastic gradient boosting (SGB)-were tested and compared. Internal validation was conducted by cross validation while the predictions were validated against an independent set of soil samples considering the modelling area and an extrapolation area. Model performance statistics revealed that the machine learning techniques performed marginally better than the MLR, with the RFR providing in most cases the highest accuracy. The inability of MLR to handle non-linear relationships between dependent and independent variables was found to be a limitation in accurately predicting soil properties at unsampled locations. Satellite data acquired during ploughing or early crop development stages (e.g. May, June) were found to be the most important spectral predictors while elevation, temperature and precipitation came up as prominent terrain/climatic variables in predicting soil properties. The results further showed that shortwave infrared and near infrared channels of Landsat8 as well as soil specific indices of redness, coloration and saturation were prominent predictors in digital soil mapping. Considering the increased availability of freely available Remote Sensing data (e.g. Landsat, SRTM, Sentinels), soil information at local and regional scales in data poor regions such as West Africa can be improved with relatively little financial and human resources.

Background and Aims Crop residues are important for the redistribution of alkalinity within soils. A net increase in pH following residue addition to soil is typically reported. However, effects are inconsistent in the field due to confounding soil processes and agronomic practises. Methods A column experiment investigated the effects of canola, chickpea and wheat residues, differing in alkalinity content and C:N ratio, on soil pH changes in a Podosol (Podzol; initial pH 4.5) and Tenosol (Cambisol; initial pH 6.2) under field conditions. Results Residues (10 g dry matter kg⁻¹ soil; 0-10 cm) increased soil pH, and temporal changes in alkalinity depended on the residue and soil type. Alkalinity was generated via abiotic association reactions between H⁺ and added organic matter and via ammonification and decarboxylation processes during decomposition. Alkalinity from canola and chickpea residues moved down the soil profile (10-30 cm) and was attributed to nitrate immobilisation and organic anion decomposition by soil microbes. Conclusions The application of residues to acid and moderately acid soils increased the pH of both topsoil and subsoils, which persisted over 26 months. Maximal increase of pH observed at 3 months was correlated with the concentration of excess cations in the residues.

Background and aims Tundra soils, which usually contain low concentrations of soil nutrients and have a low pH, store a large proportion of the global soil carbon (C) pool. The importance of soil nitrogen (N) availability for microbial activity in the tundra has received a great deal of attention; however, although soil pH is known to exert a considerable impact on microbial activities across ecosystems, the importance of soil pH in the tundra has not been experimentally investigated. Methods We tested a hypothesis that low nutrient availability and pH may limit microbial biomass and microbial capacity for organic matter degradation in acidic tundra heaths by analyzing potential extracellular enzyme activities and microbial biomass after 6 years of factorial treatments of fertilization and liming. Results Increasing nutrients enhanced the potential activity of β-glucosidase (synthesized for cellulose degradation). Increasing soil pH, in contrast, reduced the potential activity of β-glucosidase. The soil phospholipid fatty acid concentrations (PLFAs; indicative of the amount of microbial biomass) increased in response to fertilization but were not influenced by liming. Conclusions Our results show that soil nutrient availability and pH together control extracellular enzyme activities but with largely differing or even opposing effects. When nutrient limitation was alleviated by fertilization, microbial biomass and enzymatic capacity for cellulose decomposition increased, which likely facilitates greater decomposition of soil organic matter. Increased soil pH, in contrast, reduced enzymatic capacity for cellulose decomposition, which could be related with the bioavailability of organic substrates.

Aims Little is known about how plant leaf litter decomposing on the soil surface is affecting microbial communities in the underlying soil. Here we examined the effects of decomposing leaf litter of different initial chemistry on biomass, stoichiometry, community structure and activity of microorganisms in the soil underneath the decaying litter layer.Methods Leaf litter from six different neotropical tree species with contrasted quality decomposed on top of a common tropical soil in a laboratory microcosm experiment over 98 days. At the end of the experiment we determined microbial biomass C, N, and P, microbial community structure (PLFA), and community level physiological profiles (CLPP) from the top soil.Results Despite growing in a common soil substrate, soil microorganisms were strongly affected by litter species, especially by the soluble litter fraction. While litters with low soluble C content did not affect the soil microbial community, litters with high soluble C content led to an increase of microbial biomass and to a structural shift to relatively more Gram-negative bacteria. Changing community structure resulted in changes of catabolic capacity of microorganisms to metabolize a range of different C substrates. The large differences in leachate N and P among litter species, in contrast, had no effect on soil microbial parameters.Conclusions Our data suggest that plant litter decomposing on the soil surface exhibit a strong and predictable leachate C-control over microbial community biomass, structure and function in the underlying soil.

Biochar is produced as a by-product of the low temperature pyrolysis of biomass during bioenergy extraction and its incorporation into soil is of global interest as a potential carbon sequestration tool. Biochar influences soil nitrogen transformations and its capacity to take up ammonia is well recognized. Anthropogenic emissions of ammonia need to be mitigated due to negative environmental impacts and economic losses. Here we use an isotope of nitrogen to show that ammonia-N adsorbed by biochar is stable in ambient air, but readily bioavailable when placed in the soil. When biochars, containing adsorbed 15N labelled ammonia, were incorporated into soil the 15N recovery by roots averaged 6.8% but ranged from 26.1% to 10.9% in leaf tissue due to differing biochar properties with plant 15N recovery greater when acidic biochars were used to capture ammonia. Recovery of 15N as total soil nitrogen (organic+inorganic) ranged from 45% to 29% of 15N applied. We provide a proof of concept for a synergistic mitigation option where anthropogenic ammonia emissions could be captured using biochar, and made bioavailable in soils, thus leading to nitrogen capture by crops, while simultaneously sequestering carbon in soils.

Aims Organic amendments to arable soil build soil organic matter (SOM), which can increase crop yields. However, organic amendments can influence crop yields independently of SOM by providing nutrients directly to plants. The relative importance of native organic matter versus organic amendments is not well quantified. We experimentally manipulated both organic amendments and native SOM concentrations to quantify their relative importance to crop yields. Methods We created OM concentration gradients by (1) diluting an organic-rich A-horizon with a mineral base and (2) amending compost to the same mineral base, generating OM concentrations for both treatments of approximately 2, 4 and 8%. We grew buckwheat and measured plant productivity and a range of soil fertility variables. Results Higher concentrations of OM, whether native or amended, were associated with higher soil water holding capacity and nutrients, and improved soil structure. Consequently, increases in both native and amended OM were associated with strong positive but saturating impacts on productivity, though amendment effects were greater. Conclusions Our results suggest that native SOM can support productivity levels comparable to those observed with organic amendments. Although our quantitative findings will likely vary for different soils and amendments, our results lend support to the idea that SOM stocks directly increase productivity.