Fire-induced pine woodland to shrubland transitions in Southern Europe may promote shifts in soil fertility.

Since the mid of the last century, fire recurrence has increased in the Iberian Peninsula and in the overall Mediterranean basin due to changes in land use and climate. The warmer and drier climate projected for this region will further increase the risk of wildfire occurrence and recurrence. Although the impact of wildfires on soil nutrient content in this region has been extensively studied, still few works have assessed this impact on the basis of fire recurrence. This study assesses the changes in soil organic C and nutrient status of mineral soils in two Southern European areas, Várzea (Northern Portugal) and Valencia (Eastern Spain), affected by different levels of fire recurrence and where short fire intervals have promoted a transition from pine woodlands to shrublands. At the short-term (<1year), the amount of soil organic matter was higher in burned than in unburned soils while its quality (represented as labile to total organic matter) was actually lower. In any case, total and labile soil organic matter showed decreasing trends with increasing fire recurrence (one to four fires). At the long-term (>5years), a decline in overall soil fertility with fire recurrence was also observed, with a drop between pine woodlands (one fire) and shrublands (two and three fires), particularly in the soil microsites between shrubs. Our results suggest that the current trend of increasing fire recurrence in Southern Europe may result in losses or alterations of soil organic matter, particularly when fire promotes a transition from pine woodland to shrubland. The results also point to labile organic matter fractions in the intershrub spaces as potential early warning indicators for shifts in soil fertility in response to fire recurrence.

Divergent composition but similar function of soil food webs of individual plants: plant species and community effects.

Soils are extremely rich in biodiversity, and soil organisms play pivotal roles in supporting terrestrial life, but the role that individual plants and plant communities play in influencing the diversity and functioning of soil food webs remains highly debated. Plants, as primary producers and providers of resources to the soil food web, are of vital importance for the composition, structure, and functioning of soil communities. However, whether natural soil food webs that are completely open to immigration and emigration differ underneath individual plants remains unknown. In a biodiversity restoration experiment we first compared the soil nematode communities of 228 individual plants belonging to eight herbaceous species. We included grass, leguminous, and non-leguminous species. Each individual plant grew intermingled with other species, but all plant species had a different nematode community. Moreover, nematode communities were more similar when plant individuals were growing in the same as compared to different plant communities, and these effects were most apparent for the groups of bacterivorous, carnivorous, and omnivorous nematodes. Subsequently, we analyzed the composition, structure, and functioning of the complete soil food webs of 58 individual plants, belonging to two of the plant species, Lotus corniculatus (Fabaceae) and Plantago lanceolata (Plantaginaceae). We isolated and identified more than 150 taxa/groups of soil organisms. The soil community composition and structure of the entire food webs were influenced both by the species identity of the plant individual and the surrounding plant community. Unexpectedly, plant identity had the strongest effects on decomposing soil organisms, widely believed to be generalist feeders. In contrast, quantitative food web modeling showed that the composition of the plant community influenced nitrogen mineralization under individual plants, but that plant species identity did not affect nitrogen or carbon mineralization or food web stability. Hence, the composition and structure of entire soil food webs vary at the scale of individual plants and are strongly influenced by the species identity of the plant. However, the ecosystem functions these food webs provide are determined by the identity of the entire plant community.

Empirical and theoretical challenges in aboveground-belowground ecology.

A growing body of evidence shows that aboveground and belowground communities and processes are intrinsically linked, and that feedbacks between these subsystems have important implications for community structure and ecosystem functioning. Almost all studies on this topic have been carried out from an empirical perspective and in specific ecological settings or contexts. Belowground interactions operate at different spatial and temporal scales. Due to the relatively low mobility and high survival of organisms in the soil, plants have longer lasting legacy effects belowground than aboveground. Our current challenge is to understand how aboveground-belowground biotic interactions operate across spatial and temporal scales, and how they depend on, as well as influence, the abiotic environment. Because empirical capacities are too limited to explore all possible combinations of interactions and environmental settings, we explore where and how they can be supported by theoretical approaches to develop testable predictions and to generalise empirical results. We review four key areas where a combined aboveground-belowground approach offers perspectives for enhancing ecological understanding, namely succession, agro-ecosystems, biological invasions and global change impacts on ecosystems. In plant succession, differences in scales between aboveground and belowground biota, as well as between species interactions and ecosystem processes, have important implications for the rate and direction of community change. Aboveground as well as belowground interactions either enhance or reduce rates of plant species replacement. Moreover, the outcomes of the interactions depend on abiotic conditions and plant life history characteristics, which may vary with successional position. We exemplify where translation of the current conceptual succession models into more predictive models can help targeting empirical studies and generalising their results. Then, we discuss how understanding succession may help to enhance managing arable crops, grasslands and invasive plants, as well as provide insights into the effects of global change on community re-organisation and ecosystem processes.

Functional stability of microbial communities in contaminated soils near a zinc smelter (Budel, the Netherlands).

Environmental pollution causes adverse effects on many levels of ecosystem organization; it might affect the use efficiency of available resources which will make the system more sensitive to subsequent stress. Alternatively the development of community tolerance may make the system more resistant to additional stresses. In this study we investigate the functional stability, measured in the terms of resistance and resilience, of microbial populations inhabiting contaminated soils near a zinc smelter. With functional stability we mean that we look at processes rather than at population dynamics. We measure changes in respiration and bacterial growth rate in response to addition of stress (lead, salt) or disturbance (heat). We used soils that differ in the level of pollution with zinc and cadmium originating from an adjacent smelter. Our results showed, with regard to respiration, that the most polluted soils have the lowest stability to salt (stress) and heat (disturbance). This confirms the hypothesis that more stressed systems have less energy to cope with additional stress or disturbance. However, bacterial growth rates were affected in a different way than respiration. There was no difference between the soils in resistance and resilience to addition of lead. In case of salt treatment, the least polluted soils showed highest stability. In contrast, the least polluted soils were the least stable to increased temperature, which supports the hypothesis that more stressed soils are more stable to additional stress/disturbance due to properties they gained when exposed to the first stress (pollution by the smelter). Thus, the responses of microbial processes to stress, their nature and size, depend on the kinds of stress factors, especially whether a subsequent stress is similar to the first stress, in terms of the mechanism with which the organisms deal with the stress.

Effects of nematophagous fungi on numbers and death rates of bacterivorous nematodes in arable soil.

In a series of microcosm experiments with an arable, sandy loam soil amended with sugarbeet leaf, the short-term (8 weeks) dynamics of numbers of nematodes were measured in untreated soil and in gamma-irradiated soil inoculated with either a field population of soil microorganisms and nematodes or a mixed population of laboratory-propagated bacterivorous nematode species. Sugarbeet leaf stimulated an increase in bacterivorous Rhabditidae, Cephalobidae, and a lab-cultivated Panagrolaimus sp. Differences were observed between the growth rates of the nematode population in untreated and gamma-irradiated soils, which were caused by two nematophagous fungi, Arthrobotrys oligospora and Dactylaria sp. These fungi lowered the increase in nematode numbers due to the organic enrichment in the untreated soil. We estimated the annually produced bacterivous nematodes to consume 50 kg carbon and 10 kg nitrogen per ha, per year, in the upper, plowed 25 cm of arable soil.

Energetics, patterns of interaction strengths, and stability in real ecosystems.

Ecologists have long been studying stability in ecosystems by looking at the structuring and the strengths of trophic interactions in community food webs. In a series of real food webs from native and agricultural soils, the strengths of the interactions were found to be patterned in a way that is important to ecosystem stability. The patterning consisted of the simultaneous occurrence of strong "top down" effects at lower trophic levels and strong "bottom up" effects at higher trophic levels. As the patterning resulted directly from the energetic organization of the food webs, the results show that energetics and community structure govern ecosystem stability by imposing stabilizing patterns of interaction strengths.

Modelling food webs and nutrient cycling in agro-ecosystems.

Agricultural practices affect the spatial patterns and dynamics of the decomposition of soil organic matter and the availability of plant-limiting nutrients. The biological processes underlying these patterns and dynamics are the trophic interactions among the organisms in the soil community food web. Food web models simulate nutrient flow rates close to observed rates and clarify the role of the various groups of organisms in the cycling of nutrients. Several large interdisciplinary programs are currently focusing on these interactions, with a view to developing and managing sustainable forms of agriculture.

Influence of productivity on the stability of real and model ecosystems.

The lengths of food chains within ecosystems have been thought to be limited either by the productivity of the ecosystem or by the resilience of that ecosystem after perturbation. Models based on ecological energetics that follow the form of Lotka-Volterra equations and equations that include material (detritus) recycling show that productivity and resilience are inextricably interrelated. The models were initialized with data from 5-to 10-year studies of actual soil food webs. Estimates indicate that most ecological production worldwide is from ecosystems that are themselves sufficiently productive to recover from minor perturbations.