P.C. de Ruiter

SPECIES RICHNESS-PRODUCTIVITY PATTERNS DIFFER BETWEEN N-, P-, AND K-LIMITED WETLANDS

We evaluated whether the kind of nutrient limitation (N, P, or K) may affect species richness–productivity patterns and subsequently may explain variation in species richness and in richness of threatened species. We present a data set from previous studies in wetlands in Poland, Belgium, and The Netherlands and examine species richness–productivity patterns for vascular plants in all 150 sites together as well as for N-, P-, and K-limited sites separately. The kind of nutrient limitation was assessed by N:P, N:K, and K:P ratios in the vegetation. Critical values for these ratios were derived from a literature review of fertilization experiments. The kind of nutrient limitation influenced species richness–productivity patterns in our 150 sites through large differences in productivity. P (co)-limitation occurred only at low productivity, K (co)-limitation up to intermediate productivity, and N limitation along the entire productivity gradient. There was a decreasing trend in species richness with increasing productivity for K (co)-limited sites, whereas for both the N-limited sites and P (co)-limited sites a sort of “filled hump-shaped curve” was observed. The species richness–productivity relationship for threatened species was restricted to a much narrower productivity range than that for all species. Richness of threatened species was higher in P (co)-limited sites than in N-limited sites, suggesting that increased P availabilities in wetlands may be particularly important in causing disappearance of threatened species in western Europe. The role of nutrient limitation in species richness–productivity relationships not only reveals mechanisms that may explain variation in species richness and occurrence of threatened species, but it also may be important for nature management practice.

Simulation of nitrogen mineralization in the belowground food webs of two winter wheat fields.

Food webs in conventional (high-input) and integrated (reduced-input) farming systems were simulated to estimate the contribution of soil microbes and soil fauna to nitrogen mineralization during the growing season. Microbes accounted for approximately 95% of the biomass and 70% of total nitrogen mineralization in both management practices. Among the soil fauna, amoebae and bacterivorous nematodes were the most important contributors to nitrogen mineralization. The contribution of nematodes showed more temporal and spatial variability than the contribution of amoebae. The model calculated nitrogen mineralization rates close to the observed rates for both fields and depth layers. In the integrated plot there were relatively high rates of mineralization in the 0-10 cm layer compared with the 10-25 cm layer, whereas in the conventional plot no differences were observed between depth layers (...)

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.

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.

Calculation of nitrogen mineralization in soil food webs

In agricultural practices in which the use of inorganic fertilizer is being reduced in favour of the use of organic manure, the availability of nitrogen (N) in soil for plant growth depends increasingly on N mineralization. In simulation models, N mineralization is frequently described in relation to the decomposition of organic matter, making a distinction in the quality of the chemical components available as substrate for soil microbes. A different way to model N mineralization is to derive N mineralization from the trophic interactions among the groups of organisms constituting the soil food web. In the present study a food web model was applied to a set of food webs from different sites and from different arable farming systems. The results showed that the model could simulate N mineralization rates close to the rates obtained from in situ measurements, from nitrogen budget analyses, or from a decomposition based model. The outcome of the model suggested that the contribution of the various groups of organisms to N mineralization varied strongly among the different sites and farming systems.

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.

N, P, AND K BUDGETS ALONG NUTRIENT AVAILABILITY AND PRODUCTIVITY GRADIENTS IN WETLANDS

Nutrient enrichment in Western Europe is an important cause of wetland deterioration and the concomitant loss of biodiversity. We quantified nitrogen, phosphorus, and potassium budgets along biomass gradients in wet meadows and fens (44 field sites) to evaluate the importance of various nutrient flows (atmospheric deposition, flooding, groundwater flow, leaching, soil turnover rates) for availability of the growth-limiting nutrient(s). From the nutrient budgets, we assessed N, P, and K availabilities for plants and compared them with N, P, and K in aboveground biomass. Also, potential long-term effects of annual hay harvesting on nutrient limitation were assessed. Comparing N, P, and K availabilities with N, P, and K amounts in the vegetation revealed that (1) the assessed availabilities could explain amounts and variation of nutrients in the vegetation along the biomass gradients, and (2) N was likely the major limiting nutrient along the gradients and P and K could (co)limit growth in some of the sites. Increasing N availabilities along the biomass gradients were caused by increasing N turnover rates in the soil. The contribution of atmospheric N deposition (43 kg N·ha 21 ·yr 21 at all sites) to N availability varied from ;63-76% in low-productivity meadows and fens to 24-42% in highly productive meadows and fens. P and K availabilities along the biomass gradients were primarily influenced by soil processes, as indicated by soil extractable nutrient pools. Flooding could explain 20-30% of K in aboveground higher plants but was less important for P or N availabilities. Nutrient input and output by groundwater flow were more or less negligible for nutrient availability. At low-productivity sites, N output by hay harvesting just accounted for N input from atmospheric deposition, whereas there was net output of P and K. At highly productive sites, there was net output of all three nutrients. Compared to total N, P, and K pools in the top soil, net K output (1-20% of soil K pool) was at many sites much larger than that of P (generally 0.5-3%) or N (0-3%). Hay harvesting particularly seems to create K limitation. Our results indicate that conservation or restoration of low productivity wetlands in Western Europe requires (1) stable site conditions controlling low N, P, and K turnover rates in the soil, and (2) in case of N limitation, annual removal of biomass by harvesting hay, or another management measure to counterbalance the N input from atmospheric deposition. Key words: atmospheric deposition; eutrophication; flooding; groundwater; mineralization; na- ture management; nitrogen; nutrient cycling; phosphorus; potassium; soil nutrient turnover; wetlands.

Dynamics of microorganisms, microbivores and nitrogen mineralisation in winter wheat fields under conventional and integrated management☆

Abstract To reduce environmental problems, integrated farming has been proposed, which may involve a considerable reduction of fertiliser-N input. A reduced fertiliser-N input must be compensated for by a higher N mineralisation from organic matter. To reduce losses and to facilitate optimal use of the N mineralised for crop growth, knowledge of the effects of management on soil orgaisms and on their role in N cycling is needed. Therefore, biomass and activity of bacteria, biomasses of fungi, bacterivorous amoebae, flagellates and nematodes, and in situ N mineralisation were monitored during a full year in a winter wheat field under conventional management (CONV) and integrated management (INT). Fungal biomass was about 100-fold lower than bacterial biomass. The average bacterial biomass was not significantly higher in INT than in CONV, whereas amoebae and nematodes were 64% and 22% higher, respectively. Average N mineralisation was 30% higher in INT. The differences are attributed to the approximately 30% higher organic matter content of INT. Bacterial biomass and frequency of dividing-divided cells (FDDC) were relatively low in December and January, probably owing to temperatures just above 0°C. At about 5°C in February and March, relatively high FDDC values and a doubling of bacteria occurred. During summer, FDDC values were relatively low and bacterial numbers were stable, probably owing to nutrient limitation. After harvest and skim ploughing, rapid increases in FDDC and bacteria were found. In the non-fumigated INT field, protozoan peaks coincided with the bacterial peak, whereas in CONV bacterivorous fauna were drastically reduced by soil fumigation. Nevertheless, the bacterial peaks were similar in CONV and INT, indicating that bacteria were not controlled by bacterivores. Nitrogen mineralisation was relatively low in winter. The increased bacterial growth in February and March, and in September appeared to enhance immobilisation rather than mineralisation of N. During the growing season from April to the end of August, bacterial growth was relatively low and N mineralisation was relatively high. This probably resulted from bacterivore feeding and from substrate- or nutrient-limited bacteria with a low growth efficiency. Considerable mineralisation rates after harvest confirmed the need for measures to stimulate immobilisation during periods without crop uptake.

Contribution of earthworms to carbon and nitrogen cycling in agro-ecosystems.

Earthworms contribute to N mineralisation directly, through consumption, digestion, respiration/ excretion and indirectly, by influencing population dynamics of other soil biota through predation or through affecting their environmental conditions. A comparison between a high-input conventional farming system with a reduced-input integrated system showed that earthworms had not colonized the conventional fields. Two methods were used to estimate the contribution of earthworms to N mineralisation in the integrated field. In the first method, the direct contribution of earthworms to N mineralisation is derived from feeding rates based on energy conversion efficiencies, life-history parameters and C:N ratios. This method also allowed the estimation of the indirect contribution, by assuming that the feeding of earthworms on their prey stimulates the growth rates of their prey. The second method calculated the amounts of mineral N derived from two sources: dead biomass and excretion products. Application of these two methods on several agro-ecosystems showed that earthworms may make a considerable contribution to N mineralisation. A sensitivity analysis, however, showed that the outcome of both methods depends on parameter estimates that are frequently uncertain. This implies that more detailed investigations on these parameters are required before the role of earthworms in energy and nutrient cycling can be established more reliably.

Population dynamics in the belowground food webs in two different agricultural systems

Abstract The biomass of 17 different groups of organisms was established every 6 weeks during 1 year in two arable fields cropped to winter wheat; one field was under conventional management (CONV) and the other under integrated management (INT). Bacteria showed the highest average biomass, followed by earthworms (INT only) and amoebae. Most of the groups of organisms had higher biomasses in INT than in CONV. The difference was statistically significant for protozoans, bacterivorous, fungivorous, and phytophagous nematodes and earthworms. Predatory Collembola, cryptostigmatic and bacterivorous mites, and enchytraeids showed a smaller biomass in INT than in CONV. The annual biomass production for each group was estimated using simulation model calculations. Bacteria showed the highest production followed by amoebae and earthworms (INT only). Most of the groups showed a higher biomass production in INT than in CONV. Exceptions were predatory and nematophagous mites, predatory and omnivorous Collembola, and enchytraeids. The total annual production was approximately 32 kg C ha−1 cm−1 depth in CONV and approximately 57 kg C in INT. The population dynamics were analysed by hierarchical cluster analysis. Four different clusters were found in CONV and INT. Bacteria, fungi, protozoans, bacterivorous nematodes and predatory mites showed the same trend in population dynamics in CONV and INT. All other groups showed different population dynamics in CONV and INT. This observation and the composition of these clusters suggested different conditions in CONV and INT.