An estimate of potential threats levels to soil biodiversity in EU

Life within the soil is vital for maintaining life on Earth due to the numerous ecosystem services that it provides. However, there is evidence that pressures on the soil biota are increasing which may undermine some of these ecosystem services. Current levels of belowground biodiversity are relatively poorly known, and so no benchmark exists by which to measure possible future losses of biodiversity. Furthermore, the relative risk that each type of anthropogenic pressures places on the soil biota remains unclear. Potential threats to soil biodiversity were calculated through the use of a composite score produced from data collected from 20 international experts using the budget allocation methodology. This allowed relative weightings to be given to each of the identified pressures for which data were available in the European Soil Data Centre (ESDC). A total of seven different indicators were used for calculating the composite scores. These data were applied through a model using ArcGIS to produce a spatial analysis of composite pressures on soil biodiversity at the European scale. The model highlights the variation in the composite result of the potential threats to soil biodiversity. A sensitivity analysis demonstrated that the intensity of land exploitation, both in terms of agriculture and use intensity, as well as in terms of land‐use dynamics, were the main factors applying pressure on soil biodiversity. It is important to note that the model should not be viewed as an estimate of the current level of soil biodiversity in Europe, but as an estimate of pressures that are currently being exerted. The results obtained should be seen as a starting point for further investigation on this relatively unknown issue and demonstrate the utility of this type of model which may be applied to other regions and scales.     

Effects of the Biocontrol Agent Aspergillus flavipes on the Soil Microflora and Soil Enzymes in the Rooting Zone of Pepper Plants Infected with Phytophthora capsici

This study examined the effect of ASD strain (Aspergillus flavipes), isolated from continuous cropping soil for pepper and named by the sampling position, on soil microflora and soil enzymes in rooting zone soil of healthy and diseased (Phytophthora capsici) pepper plants. Results showed that the ASD strain could significantly reduce the number of bacteria and actinomycetes, with a significant increase in fungi in the rhizosphere soil of both healthy and diseased plants. With increasing colonization time of the ASD strain, the number of bacteria and actinomycetes decreased initially and then increased gradually, while the number of fungi was first increased significantly and later decreased slowly. The soil enzyme activities of urease, acid phosphatase, invertase and dehydrogenase were significantly increased by the ASD strain, while the activity of catalase was not significantly increased. As time from inoculation with the ASD strain increased, the activities of various enzymes were higher than controls. Maximum enzyme activities were found on the tenth day after ADS inoculation. The response of soil enzyme activities affected by the ASD strain was as follows: urease > dehydrogenase > invertase > acid phosphatase > catalase. These results suggest that the biocontrol of ASD strain could improve the micro ecology of rhizosphere soil.     

Leaf traits of two Mediterranean perennial tussock grass species in relation to soil nitrogen and phosphorus availability

Studying relationships of plant traits to ecosystem properties is an emerging approach aiming to understand plant's potential effect on ecosystem functioning. In the current study, we explored links between morphological and nutritional leaf traits of two Mediterranean perennial grass species Stipa tenacissima and Lygeum spartum, widely used to prevent desertification process by stabilizing sand dunes. We evaluated also relationships in terms of nitrogen (N) and phosphorus (P) availability between leaves of the investigated species and the corresponding soil. Our results showed that leaf P was very low in comparison with leaf N for the two investigated species. In fact, chlorophyll content, photosynthesis capacity and water conservation during photosynthesis are mainly linked to leaf nitrogen content. Our findings support previous studies showing that at the species levels, morphological and nutritional leaf traits were not related. On the other hand, significant relationships were obtained between soil N and leaf N for S. tenacissima (P = 0.011) and L. spartum (P = 0.033). However, leaf P was not significantly related to soil P availability for both species. We suggest that any decrease in soil N with the predicted increasing aridity may result in reduction in leaf N and thus in worst dysfunction of some biological processes levels.     

A big‐microsite framework for soil carbon modeling

Soil carbon cycling processes potentially play a large role in biotic feedbacks to climate change, but little agreement exists at present on what the core of numerical soil C cycling models should look like. In contrast, most canopy models of photosynthesis and leaf gas exchange share a common ‘Farquhaur‐model’ core structure. Here, we explore why a similar core model structure for heterotrophic soil respiration remains elusive and how a pathway to that goal might be envisioned. The spatial and temporal variation in soil microsite conditions greatly complicates modeling efforts, but we believe it is possible to develop a tractable number of parameterizable equations that are organized into a coherent, modular, numerical model structure. First, we show parallels in insights gleaned from linking Arrhenius and Michaelis–Menten kinetics for both photosynthesis and soil respiration. Additional equations and layers of complexity are then added to simulate substrate supply. For soils, model modules that simulate carbon stabilization processes will be key to estimating the fraction of soil C that is accessible to enzymes. Potential modules for dynamic photosynthate input, wetting‐event inputs, freeze–thaw impacts on substrate diffusion, aggregate turnover, soluble‐C sorption, gas transport, methane respiration, and microbial dynamics are described for conceptually and numerically linking our understanding of fast‐response processes of soil gas exchange with longer‐term dynamics of soil carbon and nitrogen stocks.     

Interannual climate variability mediates changes in carbon and nitrogen pools caused by annual grass invasion in a semiarid shrubland

Exotic plant invasions alter ecosystem properties and threaten ecosystem functions globally. Interannual climate variability (ICV) influences both plant community composition (PCC) and soil properties, and interactions between ICV and PCC may influence nitrogen (N) and carbon (C) pools. We asked how ICV and non-native annual grass invasion covary to influence soil and plant N and C in a semiarid shrubland undergoing widespread ecosystem transformation due to invasions and altered fire regimes. We sampled four progressive stages of annual grass invasion at 20 sites across a large (25,000 km²) landscape for plant community composition, plant tissue N and C, and soil total N and C in 2013 and 2016, which followed 2 years of dry and wet conditions, respectively. Multivariate analyses and ANOVAs showed that in invasion stages where native shrub and perennial grass and forb communities were replaced by annual grass-dominated communities, the ecosystem lost more soil N and C in wet years. Path analysis showed that high water availability led to higher herbaceous cover in all invasion stages. In stages with native shrubs and perennial grasses, higher perennial grass cover was associated with increased soil C and N, while in annual-dominated stages, higher annual grass cover was associated with losses of soil C and N. Also, soil total C and C:N ratios were more homogeneous in annual-dominated invasion stages as indicated by within-site standard deviations. Loss of native shrubs and perennial grasses and forbs coupled with annual grass invasion may lead to long-term declines in soil N and C and hamper restoration efforts. Restoration strategies that use innovative techniques and novel species to address increasing temperatures and ICV and emphasize maintaining plant community structure—shrubs, grasses, and forbs—will allow sagebrush ecosystems to maintain C sequestration, soil fertility, and soil heterogeneity.     

Sustained effects of atmospheric [CO2] and nitrogen availability on forest soil CO2 efflux

Soil CO₂ efflux (F_soil) is the largest source of carbon from forests and reflects primary productivity as well as how carbon is allocated within forest ecosystems. Through early stages of stand development, both elevated [CO₂] and availability of soil nitrogen (N; sum of mineralization, deposition, and fixation) have been shown to increase gross primary productivity, but the long‐term effects of these factors on F_soil are less clear. Expanding on previous studies at the Duke Free‐Air CO₂ Enrichment (FACE) site, we quantified the effects of elevated [CO₂] and N fertilization on F_soil using daily measurements from automated chambers over 10 years. Consistent with previous results, compared to ambient unfertilized plots, annual F_soil increased under elevated [CO₂] (ca. 17%) and decreased with N (ca. 21%). N fertilization under elevated [CO₂] reduced F_soil to values similar to untreated plots. Over the study period, base respiration rates increased with leaf productivity, but declined after productivity saturated. Despite treatment‐induced differences in aboveground biomass, soil temperature and water content were similar among treatments. Interannually, low soil water content decreased annual F_soil from potential values – estimated based on temperature alone assuming nonlimiting soil water content – by ca. 0.7% per 1.0% reduction in relative extractable water. This effect was only slightly ameliorated by elevated [CO₂]. Variability in soil N availability among plots accounted for the spatial variability in F_soil, showing a decrease of ca. 114 g C m^?2 yr^?1 per 1 g m^?2 increase in soil N availability, with consistently higher F_soil in elevated [CO₂] plots ca. 127 g C per 100 ppm [CO₂] over the +200 ppm enrichment. Altogether, reflecting increased belowground carbon partitioning in response to greater plant nutritional needs, the effects of elevated [CO₂] and N fertilization on F_soil in this stand are sustained beyond the early stages of stand development and through stabilization of annual foliage production.     

Plant litter chemistry alters the content and composition of organic carbon associated with soil mineral and aggregate fractions in invaded ecosystems

Through the input of disproportionate quantities of chemically distinct litter, invasive plants may potentially influence the fate of organic matter associated with soil mineral and aggregate fractions in some of the ecosystems they invade. Although context dependent, these native ecosystems subjected to prolonged invasion by exotic plants may be instrumental in distinguishing the role of plant–microbe–mineral interactions from the broader edaphic and climatic influences on the formation of soil organic matter (SOM). We hypothesized that the soils subjected to prolonged invasion by an exotic plant that input recalcitrant litter (Japanese knotweed, Polygonum cuspidatum) would have a greater proportion of plant‐derived carbon (C) in the aggregate fractions, as compared with that in adjacent soil inhabited by native vegetation that input labile litter, whereas the soils under an invader that input labile litter (kudzu, Pueraria lobata) would have a greater proportion of microbial‐derived C in the silt‐clay fraction, as compared with that in adjacent soils that receive recalcitrant litter. At the knotweed site, the higher C content in soils under P. cuspidatum, compared with noninvaded soils inhabited by grasses and forbs, was limited to the macroaggregate fraction, which was abundant in plant biomarkers. The noninvaded soils at this site had a higher abundance of lignins in mineral and microaggregate fractions and suberin in the macroaggregate fraction, partly because of the greater root density of the native species, which might have had an overriding influence on the chemistry of the above‐ground litter input. At the kudzu site, soils under P. lobata had lower C content across all size fractions at a 0–5 cm soil depth despite receiving similar amounts of Pinus litter. Contrary to our prediction, the noninvaded soils receiving recalcitrant Pinus litter had a similar abundance of plant biomarkers across both mineral and aggregate fractions, potentially because of the higher surface area of soil minerals at this site. The plant biomarkers were lower in the aggregate fractions of the P. lobata‐invaded soils, compared with noninvaded pine stands, potentially suggesting a microbial co‐metabolism of pine‐derived compounds. These results highlight the complex interactions among litter chemistry, soil biota, and minerals in mediating soil C storage in unmanaged ecosystems; these interactions are particularly important under global changes that may alter plant species composition and hence the quantity and chemistry of litter inputs in terrestrial ecosystems.     

Disentangling effects of air and soil temperature on C allocation in cold environments: A 14C pulse‐labelling study with two plant species

Carbon cycling responses of ecosystems to global warming will likely be stronger in cold ecosystems where many processes are temperature‐limited. Predicting these effects is difficult because air and soil temperatures will not change in concert, and will affect above and belowground processes differently. We disentangled above and belowground temperature effects on plant C allocation and deposition of plant C in soils by independently manipulating air and soil temperatures in microcosms planted with either Leucanthemopsis alpina or Pinus mugo seedlings. Daily average temperatures of 4 or 9°C were applied to shoots and independently to roots, and plants pulse‐labelled with ¹⁴CO₂. We traced soil CO₂ and ¹⁴CO₂ evolution for 4 days, after which microcosms were destructively harvested and ¹⁴C quantified in plant and soil fractions. In microcosms with L. alpina, net ¹⁴C uptake was higher at 9°C than at 4°C soil temperature, and this difference was independent of air temperature. In warmer soils, more C was allocated to roots at greater soil depth, with no effect of air temperature. In P. mugo microcosms, assimilate partitioning to roots increased with air temperature, but only when soils were at 9°C. Higher soil temperatures also increased the mean soil depth at which ¹⁴C was allocated. Our findings highlight the dependence of C uptake, use, and partitioning on both air and soil temperature, with the latter being relatively more important. The strong temperature‐sensitivity of C assimilate use in the roots and rhizosphere supports the hypothesis that cold limitation on C uptake is primarily mediated by reduced sink strength in the roots. We conclude that variations in soil rather than air temperature are going to drive plant responses to warming in cold environments, with potentially large changes in C cycling due to enhanced transfer of plant‐derived C to soils.     

Miscanthus sacchriflorus exhibits sustainable yields and ameliorates soil properties but potassium stocks without any input over a 12‐year period in China

The perennial C₄ Miscanthus spp. is used in China for bio‐fuel production and its ecological functions. However, questions arise as to its economic and environmental sustainability in abandoned farmland where the costs should be very low. Little is known about its yield performance and effects on soil properties when it was harvested annually without any inputs in China. To address these questions, an experiment was implemented for 12 years on annually harvested Miscanthus sacchariflorus planted in 2006 and managed without fertilization, irrigation, or any other inputs. We determined biomass yields each year, biomass allocation, and soil properties before and after its cultivation. Biomass yields of M. sacchariflorus reached a peak value (29.67 t/ha) 3 years after cultivation and was maintained at a stable level (averaged 22.22 t/ha) during 2012–2017. Its root shoot ratio increased due to more biomass allocated below‐ground with time. Long‐term cultivation of M. sacchariflorus increased organic carbon contents, pH (for the absence of fertilization), microbial carbon, nitrogen and phosphorus contents, and soil carbon nitrogen ratios (0–100 cm). Soil bulk density was decreased significantly (p < .05) independent of soil depths. Annual harvest did not reduce total nitrogen and phosphorus, available nitrogen, and potassium, but total the potassium content of soil (0–100 cm). Cultivation of M. sacchariflorus increased available phosphorus contents in 40–100 cm soil and reduced that value in 20–40 cm soil. Biological nitrogen fixation provided ~218.74 kg ha^?1 year^?1 (1 m depth) nitrogen for the system offsetting nitrogen export by biomass harvest and stabilizing nitrogen levels of soil. In conclusion, M. sacchriflorus exhibited sustainable biomass yields and ameliorated soil properties but the decrease of total potassium contents after 12 years’ cultivation without any input. These conclusions could provide important information timely for the government and encourage farmers to promote large‐scale utilization of M. sacchriflorus on the abandoned farmland in China.     

Host specificity of two pollinating seed‐consuming fly species is not related to soil moisture of host plant in the high Himalayas

Studying the drivers of host specificity can contribute to our understanding of the origin and evolution of obligate pollination mutualisms. The preference–performance hypothesis predicts that host plant choice of female insects is related mainly to the performance of their offspring. Soil moisture is thought to be particularly important for the survival of larvae and pupae that inhabit soil. In the high Himalayas, Rheum nobile and R. alexandrae differ in their distribution in terms of soil moisture; that is, R. nobile typically occurs in scree with well‐drained soils, R. alexandrae in wetlands. The two plant species are pollinated by their respective mutualistic seed‐consuming flies, Bradysia sp1. and Bradysia sp2. We investigated whether soil moisture is important for regulating host specificity by comparing pupation and adult emergence of the two fly species using field and laboratory experiments. Laboratory experiments revealed soil moisture did have significant effects on larval and pupal performances in both fly species, but the two fly species had similar optimal soil moisture requirements for pupation and adult emergence. Moreover, a field reciprocal transfer experiment showed that there was no significant difference in adult emergence for both fly species between their native and non‐native habitats. Nevertheless, Bradysia sp1., associated with R. nobile, was more tolerant to drought stress, while Bradysia sp2., associated with R. alexandrae, was more tolerant to flooding stress. These results indicate that soil moisture is unlikely to play a determining role in regulating host specificity of the two fly species. However, their pupation and adult emergence in response to extremely wet or dry soils are habitat‐specific.     

Decomposition and nutrient release of grass and tree fine roots along an environmental gradient in southern Patagonia

Decomposition of fine roots is a fundamental ecosystem process that relates to carbon (C) and nutrient cycling in terrestrial ecosystems. However, this important ecosystem process has been hardly studied in Patagonian ecosystems. The aim of this work was to study root decomposition and nutrient release from fine roots of grasses and trees (Nothofagus antarctica) across a range of Patagonian ecosystems that included steppe, primary forest and silvopastoral forests. After 2.2 years of decomposition in the field all roots retained 70–90% of their original mass, and decomposition rates were 0.09 and 0.15 year^?1 for grass roots in steppe and primary forest, respectively. For N. antarctica roots, no significant differences were found in rates of decay between primary and silvopastoral forests (k = 0.07 year^?1). Possibly low temperatures of these southern sites restricted decomposition by microorganisms. Nutrient release differed between sites and root types. Across all ecosystem categories, nitrogen (N) retention in decomposing biomass followed the order: tree roots > roots of forest grasses > roots of steppe grasses. Phosphorus (P) was retained in grass roots in forest plots but was released during decomposition of tree and steppe grass roots. Calcium (Ca) dynamics also was different between root types, since trees showed retention during the initial phase, whereas grass roots showed a slow and consistent Ca release during decomposition. Potassium (K) was the only nutrient that was rapidly released from both grass and tree roots in both grasslands and woodlands. We found that silvopastoral use of N. antarctica forests does not affect grass or tree root decomposition and/or nutrient release, since no significant differences were found for any nutrient according to ecosystem type. Information about tree and grass root decomposition found in this work could be useful to understand C and nutrient cycling in these southern ecosystems, which are characterized by extreme climatic conditions.     

Epidemics of Rhizoctonia Root Rot in Association with Biological and Physicochemical Properties of Field Soil in Bean Crops

During two growing seasons (2008 and 2009), the associations of Rhizoctonia root rot (RRR) with a number of soil properties were determined at different growth stages in 122 commercial bean fields in Zanjan, Iran. Mean RRR incidence at a level of 4–25% sand content was lower than that at 45–65% level. Damage by fly puparia had no significant effect on RRR incidence and occurrence. A greater RRR incidence was detected in field soils treated with fungicides compared with non‐treated soils. A lower RRR incidence was associated with the highest level of soil organic matter (1.2–1.8) compared with the lowest level, 0.4–0.8. The highest RRR incidence corresponded with no rhizobial nodulation compared with highly nodulated bean roots. RRR incidence was negatively correlated with soil silt and organic matter content at R6–7 and R9 growth stages. RRR‐affected fields were recognized with a greater soil pH (V3) and sand content (R9), and a lower silt (R9) and organic matter content (R6–7 and R9) in comparison with RRR‐free fields. Loadings and linear regressions between RRR incidence and principal component scores indicated that the most effective soil characteristic linked to the disease was silt at V3, sand at R6–7 and organic matter at R9 stage. This new epidemiological information extends our knowledge of the bean–RRR–soil interaction on a regional basis.     

Climate change is predicted to alter the current pest status of Globodera pallida and G. rostochiensis in the United Kingdom

The potato cyst nematodes Globodera pallida and G. rostochiensis are economically important plant pathogens causing losses to UK potato harvests estimated at £50 m/ year. Implications of climate change on their future pest status have not been fully considered. Here, we report growth of female G. pallida and G. rostochiensis over the range 15 to 25°C. Females per plant and their fecundity declined progressively with temperatures above 17.5°C for G. pallida, whilst females per plant were optimal between 17.5 and 22.5°C for G. rostochiensis. Relative reproductive success with temperature was confirmed on two potato cultivars infected with either species at 15, 22.5 and 25°C. The reduced reproductive success of G. pallida at 22.5°C relative to 15°C was also recorded for a further seven host cultivars studied. The differences in optimal temperatures for reproductive success may relate to known differences in the altitude of their regions of origin in the Andes. Exposure of G. pallida to a diurnal temperature stress for one week during female growth significantly suppressed subsequent growth for one week at 17.5°C but had no effect on G. rostochiensis. However, after two weeks of recovery, female size was not significantly different from that for the control treatment. Future soil temperatures were simulated for medium‐ and high‐emission scenarios and combined with nematode growth data to project future implications of climate change for the two species. Increased soil temperatures associated with climate change may reduce the pest status of G. pallida but benefit G. rostochiensis especially in the southern United Kingdom. We conclude that plant breeders may be able to exploit the thermal limits of G. pallida by developing potato cultivars able to grow under future warm summer conditions. Existing widely deployed resistance to G. rostochiensis is an important characteristic to retain for new potato cultivars.     

Leaf traits in Chilean matorral: sclerophylly within,among, and beyond matorral,and its environmental determinants

Studies of leaf traits often focus on tradeoffs between growth and resource conservation, but little is known about variation in the mechanical traits that influence resource conservation. This study investigates how leaf mechanical traits vary across matorral vegetation in central Chile, how they correlate with environmental factors, and how these trends compare at a broader geographic scale. Leaf toughness, strength, stiffness, and associated traits were measured in five matorral types in central Chile, and relationships with soil N and P and climate variables were assessed. Trends with soil and climate were then analyzed across shrubland and woodland in Chile, Western Australia, and New Caledonia. Chilean species varied in leaf mechanics and associated traits, both within and among matorral types, with more species in sclerophyll matorral having strong, tough, and stiff leaves than in arid and littoral matorral. Overall, leaves with high leaf dry mass per area were stiffer, tougher, stronger, thicker, denser, with more fiber, lignin, phenolics and fiber per unit protein and less protein: tannin activity and N and P per mass, forming a broad sclerophylly syndrome. Mechanical traits of matorral species were not correlated with soil N or P, or predictably with climate variables, except flexural stiffness (EI_W) which correlated positively with annual reference evapotranspiration (ET₀). However, soil P made strong independent contributions to variation in leaf mechanics across shrublands and woodlands of Chile, Western Australia, and New Caledonia, either separately (strength) or together with ET₀ (toughness) explaining 46–90% of variation. Hence ET₀ was predictive of EI_W in Chilean matorral, whereas soil P was highly predictive of variation in leaf strength, and combined with ET₀ was highly predictive of toughness, at a broader geographic scale. The biological basis of these relationships, however, may be complex.     

Responses of the soil fungal communities to the co‐invasion of two invasive species with different cover classes

• Soil fungal communities play an important role in the successful invasion of non‐native species. It is common for two or more invasive plant species to co‐occur in invaded ecosystems.
• This study aimed to determine the effects of co‐invasion of two invasive species (Erigeron annuus and Solidago canadensis) with different cover classes on soil fungal communities using high‐throughput sequencing.
• Invasion of E. annuus and/or S. canadensis had positive effects on the sequence number, operational taxonomic unit (OTU) richness, Shannon diversity, abundance‐based cover estimator (ACE index) and Chao1 index of soil fungal communities, but negative effects on the Simpson index. Thus, invasion of E. annuus and/or S. canadensis could increase diversity and richness of soil fungal communities but decrease dominance of some members of these communities, in part to facilitate plant further invasion, because high soil microbial diversity could increase soil functions and plant nutrient acquisition. Some soil fungal species grow well, whereas others tend to extinction after non‐native plant invasion with increasing invasion degree and presumably time. The sequence number, OTU richness, Shannon diversity, ACE index and Chao1 index of soil fungal communities were higher under co‐invasion of E. annuus and S. canadensis than under independent invasion of either individual species.
• The co‐invasion of the two invasive species had a positive synergistic effect on diversity and abundance of soil fungal communities, partly to build a soil microenvironment to enhance competitiveness of the invaders. The changed diversity and community under co‐invasion could modify resource availability and niche differentiation within the soil fungal communities, mediated by differences in leaf litter quality and quantity, which can support different fungal/microbial species in the soil.

     

Why does oriental arborvitae grow better when mixed with black locust: Insight on nutrient cycling?

To identify why tree growth differs by afforestation type is a matter of prime concern in forestry. A study was conducted to determine why oriental arborvitae (Platycladus orientalis) grows better in the presence of black locust (Robinia pseudoacacia) than in monoculture. Different types of stands (i.e., monocultures and mixture of black locust and oriental arborvitae, and native grassland as a control) were selected in the Loess Plateau, China. The height and diameter at breast height of each tree species were measured, and soil, shoot, and root samples were sampled. The arbuscular mycorrhizal (AM) attributes, shoot and root nutrient status, height and diameter of black locust were not influenced by the presence of oriental arborvitae. For oriental arborvitae, however, growing in mixture increased height and diameter and reduced shoot Mn, Ca, and Mg contents, AM fungal spore density, and colonization rate. Major changes in soil properties also occurred, primarily in soil water, NO₃‐N, and available K levels and in soil enzyme activity. The increase in soil water, N, and K availability in the presence of black locust stimulated oriental arborvitae growth, and black locust in the mixed stand seems to suppress the development of AM symbiosis in oriental arborvitae roots, especially the production of AM fungal spores and vesicles, through improving soil water and N levels, thus freeing up carbon to fuel plant growth. Overall, the presence of black locust favored oriental arborvitae growth directly by improving soil water and fertility and indirectly by repressing AM symbiosis in oriental arborvitae roots.