Eduardo Medina-Roldán

Grazing increases the temperature sensitivity of soil organic matter decomposition in a temperate grassland

We tested the effects of ungulate grazing and nutrient availability on the temperature sensitivity of soil respiration (CO2) and methane (CH4) emissions in semi-natural temperate grassland. To do this, soil taken from long term grazed and ungrazed grassland was incubated at four temperatures (4, 10, 15 and 20 C) with two levels of nutrient (NP) addition. The results showed that the variation in soil CO2 and CH4 emissions was explained by temperature and grazing, with grazing increasing the temperature sensitivity of CO2 and CH4 production by between 15 and 20 C. This response was constrained by nutrient availability for CO2, but not CH4. These findings suggest that grazing could potentially have important impacts on the temperature sensitivity of greenhouse gas emissions in nutrient limited grasslands.

Grazing-induced effects on soil properties modify plant competitive interactions in semi-natural mountain grasslands

Plant–soil feedbacks are widely recognized as playing a significant role in structuring plant communities through their effects on plant–plant interactions. However, the question of whether plant–soil feedbacks can be indirectly driven by other ecological agents, such as large herbivores, which are known to strongly modify plant community structure and soil properties, remains poorly explored. We tested in a glasshouse experiment how changes in soil properties resulting from long-term sheep grazing affect competitive interactions (intra- and inter-specific) of two graminoid species: Nardus stricta, which is typically abundant under high sheep grazing pressure in British mountain grasslands; and Eriophorum vaginatum, whose abundance is typically diminished under grazing. Both species were grown in monocultures and mixtures at different densities in soils taken from adjacent grazed and ungrazed mountain grassland in the Yorkshire Dales, northern England. Nardus stricta performed better (shoot and root biomass) when grown in grazing-conditioned soil, independent of whether or not it grew under inter-specific competition. Eriophorum vaginatum also grew better when planted in soil from the grazed site, but this occurred only when it did not experience inter-specific competition with N. stricta. This indicates that plant–soil feedback for E. vaginatum is dependent on the presence of an inter-specific competitor. A yield density model showed that indirect effects of grazing increased the intensity of intra-specific competition in both species in comparison with ungrazed-conditioned soil. However, indirect effects of grazing on the intensity of inter-specific competition were species-specific favouring N. stricta. We explain these asymmetric grazing-induced effects on competition on the basis of traits of the superior competitor and grazing effects on soil nutrients. Finally, we discuss the relevance of our findings for plant community dynamics in grazed, semi-natural grasslands.

Plant and soil responses to defoliation: a comparative study of grass species with contrasting life history strategies

The overall aim of this study was to test for inter-species variation in plant and soil responses to defoliation among a broad range of temperate grass species and life-history strategies. We used a microcosm experiment where a range of grass species differing in life history traits were subjected to different intensities of defoliation, and a range of aboveground and belowground plant and soil responses were measured. All plant attributes, including accumulated shoot biomass, root biomass and root length, showed a strong negative response to defoliation, although plant species exhibited subtle differences in the way that they responded to increased severity of defoliation. Defoliation also exerted a strong influence on soil properties, decreasing soil microbial carbon (C) and the soil microbial C:nitrogen (N) ratio, and increasing inorganic N availability and potential N mineralisation across all species. Despite the wide range in life history strategies, plant species did not differ in their influence on most of the soil variables, except for the rate of nitrate mineralisation, which was lowest under plant species that displayed the least relative detrimental responses to defoliation. Collectively, our results suggest that plant and soil responses to defoliation are reasonably consistent across a broad range of grass species, with only subtle inter-specific differences among species.

Soil Water Content Dynamics Along a Range Condition Gradient in a Shortgrass Steppe

Abstract Evidence is accumulating on the importance of plant cover and plant species composition on the control of ecosystem processes. In this study we examined a gradient considering the proportional contribution of the key species Bouteloua gracilis H.B.K. Lag. to assess its influence on the average and dynamic changes in soil water content in the shortgrass steppe from Central Mexico. We chose 4 sites with the following proportions of the key species: < 25%, 25%–50%, 50%–75%, and > 75%, ascribing each proportion to the range condition categories poor, fair, good, and excellent, respectively. Soil water measurements were carried out during 14 months at the 4 sites. Our results showed that range condition had a significant effect on soil water content (P < 0.01). The excellent condition was overall 14.5% and 12.5% lower soil moisture content compared to the poor and good range conditions (P < 0.01), respectively. Our results indicated a negative correlation between the gradient of soil water content with the range condition classes. Soil water content dynamics also differed among range condition classes, with the excellent condition showing both faster water recharge and extraction patterns than the other 3 range condition categories. Differences in soil water content among the range condition classes appeared to be related to morphological and physiological traits associated with the dominant species cover observed at each site. These results offer insights into the importance of vegetation characteristics as potential indicators of thresholds in grazing ecosystem processes such as soil water dynamics.

Impacts of Land Use Changes on Soil Properties and Processes

Land use is one of the main drivers of many processes of environmental change, as it influences basic resources within the landscape, including the soil resources. Poor soil management can rapidly deteriorate vast amounts of land, which frequently becomes a major threat to rural subsistence in many developing and developed countries. Conversely, impact of land use changes on soil can occur so unnoticed that land managers hardly contemplate initiating ameliorative measures. Knowledge and understanding of soil properties and processes ensures remediation or reclamation of disturbed or damaged soils. This special issue brought together an international group of scientists presenting results from field trials and data harvesting carried out in a range of different soils and environments, from Poland, Italy, Spain, and USA to China, Indonesia, Venezuela, Brazil, and Argentina, together with laboratory experiments, reviews, and modeling with advanced mathematical tools. The strength of such issue was derived from a mutual interest in the mechanisms that regulate the impact of land use and changes in land use on soil properties and processes and also in the development and use of the most advanced methods and procedures for assessing them. Drawing on the latest research and opinion, first this issue contains one state-of-the-art review and two research articles highlighting the usefulness and efficiency of the approach adopted here in a general context. W. Zhou et al. reviewed the effect of paddy upland rotation on soil properties. W. Shangguan et al. addressed the soil pedodiversity of China, mapping the distribution and extent of different soil taxa; this allowed identification of nearly 90 endangered soils, also suggesting that at least two dozens of soils have already gone extinct due to inadequate land use. J. Rejman et al. addressed the role of land use change in soil losses and relief modification in Loess areas of Poland. Various authors reported laboratory experiments aiming to clarify the role of external inputs (amendments, irrigation, etc.) in selected soil properties. More specifically, A. D. Karathanasis et al. used several amendment materials together with extracts from crop biomass to accelerate fragmentation of fragipans and, therefore, to increase the water holding capacity of these soils. L. Chu et al. assessed the potential of microsprinkler irrigation as a method to alleviate soil salinization, allowing crop growth. M. Garcia-Albacete et al. conducted leaching experiments to analyse phosphorus mobility in both soil-compost ad soil-digestate systems, showing that phosphorus losses were higher for the former than for the latter; in addition this study provided evidence of the importance of wastes wettability for assessing P sorption mechanisms and risk of leaching losses. M. Garcia-Sanchez et al. using a batch experiment showed that both organic amendments and a sulfur compound added to two different Hg contaminated soils (luvisol and chernozem) were able to reduce Hg mobile fractions and increased availability of macro- and micronutrient. The role of land use in soil organic matter and nitrogen dynamics has been illustrated by case studies carried out in contrasting soil and climatic conditions. A. F. Gonzalez-Pedraza and N. Dezzeo studied soil nitrogen seasonality in the Western Llanos of Venezuela. S. Dori et al. addressed soil carbon dynamics in different regions of Europe and discussed the efficiency of management practices that control the potential of sequestration of soil organic carbon. A. Liang et al. analysed the mechanisms of soil organic carbon turnover on agricultural systems using a combination of isotopic tracer and physical fractionation under no-tillage and mouldboard ploughing; results showed that shot term impact of the studied tillage treatments varied in the different fractions analysed. D. A. McGranahan et al. analysed the reliability of carbon sequestration estimations associated with the effect of unexplained variability and due to interactions of vegetation, land use management, and soil properties with belowground ecosystem function; subsequently, even if rangeland soils are important carbon pools, it is unlikely that rangeland plant communities can be effectively categorized by their carbon sequestration potential. Land use impacts on greenhouse gasses have been a major topic of this special issue. Z. S. Zhang et al. conducted a field trial to evaluate the effect of mulching from residues of a previous crop on paddy fields under no-tillage; this management system was found to significantly increase CO2 and N2O emissions, while decreasing CH4 emissions. S. F. Smith and K. R. Brye reported results from a field trial on a silt loan soil under soybean, showing that the impact of irrigation on seasonal CO2 emissions differed between years, whereas no-tillage management reduced seasonal CO2 emissions; the tillage effect on total CO2 emissions was not dependent on the irrigation scheme used. L. N. L. K. Choo and O. H. Ahmed used a lysimeter experiment to analyse both CO2 emissions and dissolved organic carbon leaching in a drained peat land cropped to pineapple under tropical conditions. Y. Lu and H. Xu performed an incubation experiment to test the effects of soil temperature, flooding, and organic matter addition on N2O emissions in a wetland soil. Two manuscripts addressed the role of land use in soil organism. G. M. Siqueira et al. used the classical pitfall trap method to study the interactions between the soil arthropod community and land use and management of an entisol under semiarid climate in Brazil; the arthropod abundance under native forest was much lower than under native biomes with tropical climate. Agricultural land use strongly decreases the abundance of Formicidae compared to natural biome. E. E. Kuramae et al. studied the role of several land uses in the structure and composition of microbial communities in Netherlands using DNA analysis; the functional gene diversity found in different soils did not group the sites accordingly to land management, and the main factors driving differences in functional genes between land uses or management systems were carbon : nitrogen ratio, phosphatase activity, and total nitrogen. New experimental and conceptual methods are needed to assess the effects of land use changes on soil properties and processes. J. L. M. P. de Lima et al. mapped soil surface macropores using infrared thermography; this technique is expected to provide a better understanding of the complex relationships between soil pores and soil physical and hydraulic properties. C. Moreno et al. developed an image analysis method to estimate the soil cover by different types of mulching materials during degradation in the field; particular attention was paid to thresholding methods in image treatment; proportion of areas lacking mulch have been automatically assessed. The applications of fractals and multifractals in soil and earth sciences are increasing, since many soil properties and processes have been shown to depend on complex interactions that could be assessed by fractal models. Also there is an increasing availability of data sets allowing computation and modelling using these mathematical tools. J. de Castro et al. described fractal analysis of Laplacian pyramidal and applied this method to segmentation of soil micromorphology; the algorithm used produced more reliable results than the commonly employed OTSU algorithm. Geostatistics was used to evaluate the spatial variability of several soil properties as related to land use at various sampling scales. G. M. Siqueira et al. used soil apparent electrical conductivity for devising soil sampling schemes in an agricultural field that in a further step were analyzed by geostatistical techniques; as a result, a first manuscript was devoted to estimated spatial patterns of soil compaction and a second one provided insight into the spatial variability of selected general soil properties. L. A. Morales et al. studied the spatial distribution of ammonium-nitrogen, phosphorus, and potassium in a paddy field at Argentina during three different vegetative periods of the rice crop. X. Tan et al. analyzed the spatial variability of sixteen soil properties, including several soil enzymes, focusing on soil quality assessment; it was concluded that the spatial patterns of soil quality were better reflected using an integrated index based on soil enzyme activities. P. L. Aguado et al. used multifractal analysis to characterize a landscape, based on a high resolution digital elevation model; it was shown that the use of the multifractal approach with mean absolute gradient data is a useful tool for analyzing topographical features. We believe that the present special issue reflects recent advances on the effects of land use over a range of soil properties and processes, complemented with insightful case studies using advanced mathematical techniques and new experimental methods for assessing soil surface characteristics.

Inter-Specific Competition, but Not Different Soil Microbial Communities, Affects N Chemical Forms Uptake by Competing Graminoids of Upland Grasslands

Evidence that plants differ in their ability to take up both organic (ON) and inorganic (IN) forms of nitrogen (N) has increased ecologists’ interest on resource-based plant competition. However, whether plant uptake of IN and ON responds to differences in soil microbial community composition and/or functioning has not yet been explored, despite soil microbes playing a key role in N cycling. Here, we report results from a competition experiment testing the hypothesis that soil microbial communities differing in metabolic activity as a result of long-term differences to grazing exposure could modify N uptake of Eriophorum vaginatum L. and Nardus stricta L. These graminoids co-occur on nutrient-poor, mountain grasslands where E. vaginatum decreases and N. stricta increases in response to long-term grazing. We inoculated sterilised soil with soil microbial communities from continuously grazed and ungrazed grasslands and planted soils with both E. vaginatum and N. stricta, and then tracked uptake of isotopically labelled NH4 + (IN) and glycine (ON) into plant tissues. The metabolically different microbial communities had no effect on N uptake by either of the graminoids, which might suggest functional equivalence of soil microbes in their impacts on plant N uptake. Consistent with its dominance in soils with greater concentrations of ON relative to IN in the soluble N pool, Eriophorum vaginatum took up more glycine than N. stricta. Nardus stricta reduced the glycine proportion taken up by E. vaginatum, thus increasing niche overlap in N usage between these species. Local abundances of these species in mountain grasslands are principally controlled by grazing and soil moisture, although our results suggest that changes in the relative availability of ON to IN can also play a role. Our results also suggest that coexistence of these species in mountain grasslands is likely based on non-equilibrium mechanisms such as disturbance and/or soil heterogeneity.

Next-generation sequencing showing potential leachate influence on bacterial communities around a landfill in China

The impact of contaminated leachate on groundwater from landfills is well known, but the specific effects on bacterial consortia are less well-studied. Bacterial communities in a landfill and an urban site located in Suzhou, China, were studied using Illumina high-throughput sequencing. A total of 153 944 good-quality reads were produced and sequences assigned to 6388 operational taxonomic units. Bacterial consortia consisted of up to 16 phyla, including Proteobacteria (31.9%-94.9% at landfill, 25.1%-43.3% at urban sites), Actinobacteria (0%-28.7% at landfill, 9.9%-34.3% at urban sites), Bacteroidetes (1.4%-25.6% at landfill, 5.6%-7.8% at urban sites), Chloroflexi (0.4%-26.5% at urban sites only), and unclassified bacteria. Pseudomonas was the dominant (67%-93%) genus in landfill leachate. Arsenic concentrations in landfill raw leachate (RL) (1.11 × 103 μg/L) and fresh leachate (FL2) (1.78 × 103 μg/L) and mercury concentrations in RL (10.9 μg/L) and FL2 (7.37 μg/L) exceeded Chinese State Environmental Protection Administration standards for leachate in landfills. The Shannon diversity index and Chao1 richness estimate showed RL and FL2 lacked richness and diversity when compared with other samples. This is consistent with stresses imposed by elevated arsenic and mercury and has implications for ecological site remediation by bioremediation or natural attenuation.

Biomineralisation performance of bacteria isolated from a landfill in China

We report an investigation of microbially induced carbonate precipitation by seven indigenous bacteria isolated from a landfill in China. Bacterial strains were cultured in a medium supplemented with 25 mmol/L calcium chloride and 333 mmol/L urea. The experiments were carried out at 30 °C for 7 days with agitation by a shaking table at 130 r/min. Scanning electron microscopic and X-ray diffraction analyses showed variations in calcium carbonate polymorphs and mineral composition induced by all bacterial strains. The amount of carbonate precipitation was quantified by titration. The amount of carbonate precipitated in the medium varied among isolates, with the lowest being Bacillus aerius rawirorabr15 (LC092833) precipitating around 1.5 times more carbonate per unit volume than the abiotic (blank) solution. Pseudomonas nitroreducens szh_asesj15 (LC090854) was found to be the most efficient, precipitating 3.2 times more carbonate than the abiotic solution. Our results indicate that bacterial carbonate precipitation occurred through ureolysis and suggest that variations in carbonate crystal polymorphs and rates of precipitation were driven by strain-specific differences in urease expression and response to the alkaline environment. These results and the method applied provide benchmarking and screening data for assessing the bioremediation potential of indigenous bacteria for containment of contaminants in landfills.