Abstract
Agricultural land is converted into semi-natural ecosystems in order to counteract the current loss of species-rich grasslands and heathlands. However, efforts to restore such plant communities on former agricultural land have shown variable success. Secondary succession is the process of species replacements which begins with some biological legacy following an initial disturbance (e.g. agricultural practices). Much is still unknown about how ecosystem development proceeds during secondary succession, especially about the role of interactions between plants and soil organisms. Soil organisms can play an important role in structuring plant communities. Directly, through beneficial or harmful effects on seedling establishment, plant growth and survival, and indirectly, through their effect on carbon and nutrient availability, which, in turn, influences plant growth and competition. Plants, in turn, provide the basic resources for soil organisms and this interdependence of plants and soil organisms can result in plant-soil feedbacks. Plants can influence the composition of their associated soil community, which, subsequently, can affect plant performance. So far, studies on plant community development following land abandonment have paid little attention to temporal changes in soil communities, the interrelationships between soil and plant community development and the potential use of soil organisms to restore species-rich grasslands and heathlands. The objectives of my thesis are 1) to analyse diversity patterns and changes in the composition of soil communities following land abandonment and how this relates to plant community development; 2) to study the role of plant-soil feedback in plant community development and successional replacements; and 3) to test the effects of soil community manipulations on restoration of species-rich grasslands.
Soil-dwelling nematodes are among the most numerous soil organisms in agricultural and grassland soils. They display high taxonomic richness, inhabit different trophic positions within soil food webs and are used as indicators for changes in ecosystem functioning. In a chronosequence consisting of former agricultural fields that differed in time since abandonment and spanning a range of 34 years, we analyzed soil nematode community composition and studied the relationship with plant community development. We showed that the plant and nematode communities do not necessarily develop in parallel towards the same reference system and that successful restoration of plant communities does not imply successful restoration of soil communities. Within the same chronosequence, three former agricultural sites (early, mid, late) as well as a reference heathland were selected and sampled in detail for soil mites and nematodes. From each site, samples were taken at different spatial locations, and at four different times throughout the growth season. We showed that diversity of the soil community differed between sites, but also depended on the spatial scale of the sampling regime and on the group of soil organisms considered. The data indicated that successional changes in the soil nematode community composition were mainly due to gradual shifts in dominance patterns in response to altered environmental conditions between sites, while successional changes in the soil mite community depended most on colonisation from local species pools. The data also revealed that the trophic structure of the soil food web initially changed following land abandonment. However, this was followed by a phase in which little change occurred, possibly due to retarded soil organic matter accumulation.
In microcosm experiments, we studied the role of plant-soil feedbacks in secondary plant community succession. In an experiment in which we constructed model systems of plants and soils associated to different successional stages, we showed that irrespective of abiotic soil conditions, negative plant-soil feedback effects enhance succession in early stages, while positive plant-soil feedback effects stabilise succession by enhancing the growth of slow-growing latesuccessional plant species. Positive plant-soil feedback effects were strongest in late-successional soil and these effects enhanced plant community evenness. In a second experiment, we studied for a range of early-successional plant species how they changed soil microbial communities and how these changes affected competitive interactions between early- and mid-successional plant species. We showed that microbe-mediated plant-soil feedback effects enhance replacement of early-successional plant species by mid-successional species. Interestingly, early-successional plant-soil feedback effects provided biotic legacies, which influenced dominance patterns of mid-successional plant species.
To test the potential of soil community manipulations as a management strategy in restoration of species-rich grasslands, two field experiments were set up. In one experiment, we tested the effect of carbon substrates (to induce microbial N-immobilisation) and top soil removal on plant and soil community development. To test the relative importance of soil nutrient status and dispersal limitation, the treatments were applied both with and without sowing later-successional plant species. The results show that seed addition is more important than soil fertility reduction measures for restoration of plant communities. Apparently, initial stages of secondary vegetation succession are determined by plant species arrival rather than by abiotic factors. In a second experiment, which was set up on an ex-arable site from which the top soil had been removed (the receptor site), we tested whether simultaneous introduction of later-successional plants and soil organisms can influence the direction of plant community development towards a later-successional ‘target’ system. Plants and soil organisms were collected from a mature, species-rich Cirsio-Molinietum fen meadow (the donor site) and introduced 1) by spreading hay and soil, independently or combined; or 2) by transplanting intact turfs. We could not demonstrate that introduction of later-successional soil organisms facilitate the establishment of later-successional plant species. Unfavorable soil conditions at the receptor site may have limited survival of later-successional soil organisms in the turfs and may have precluded successful establishment of the soil organisms at the receptor site.
In conclusion, microcosm experiments show that there is strong potential for interdependence in rate and direction of plant and soil community development in secondary grassland succession following land abandonment. Feedback between plants and soil organisms can strongly affect plant competitive interactions and successional replacements at small spatial and temporal scales. Through ecological legacies, such feedback effects may affect long-term plant community composition patterns. However, at the field scale level, plant and soil communities may develop independent of each other to a great extent, and other factors, especially the order of plant species arrival, may overrule the effects of soil organisms in plant community succession. We know little about the required conditions for successful establishment of later-successional soil organisms and, so far, we could not demonstrate that introduction of soil organisms is an effective measure to improve grassland restoration.
Soil-dwelling nematodes are among the most numerous soil organisms in agricultural and grassland soils. They display high taxonomic richness, inhabit different trophic positions within soil food webs and are used as indicators for changes in ecosystem functioning. In a chronosequence consisting of former agricultural fields that differed in time since abandonment and spanning a range of 34 years, we analyzed soil nematode community composition and studied the relationship with plant community development. We showed that the plant and nematode communities do not necessarily develop in parallel towards the same reference system and that successful restoration of plant communities does not imply successful restoration of soil communities. Within the same chronosequence, three former agricultural sites (early, mid, late) as well as a reference heathland were selected and sampled in detail for soil mites and nematodes. From each site, samples were taken at different spatial locations, and at four different times throughout the growth season. We showed that diversity of the soil community differed between sites, but also depended on the spatial scale of the sampling regime and on the group of soil organisms considered. The data indicated that successional changes in the soil nematode community composition were mainly due to gradual shifts in dominance patterns in response to altered environmental conditions between sites, while successional changes in the soil mite community depended most on colonisation from local species pools. The data also revealed that the trophic structure of the soil food web initially changed following land abandonment. However, this was followed by a phase in which little change occurred, possibly due to retarded soil organic matter accumulation.
In microcosm experiments, we studied the role of plant-soil feedbacks in secondary plant community succession. In an experiment in which we constructed model systems of plants and soils associated to different successional stages, we showed that irrespective of abiotic soil conditions, negative plant-soil feedback effects enhance succession in early stages, while positive plant-soil feedback effects stabilise succession by enhancing the growth of slow-growing latesuccessional plant species. Positive plant-soil feedback effects were strongest in late-successional soil and these effects enhanced plant community evenness. In a second experiment, we studied for a range of early-successional plant species how they changed soil microbial communities and how these changes affected competitive interactions between early- and mid-successional plant species. We showed that microbe-mediated plant-soil feedback effects enhance replacement of early-successional plant species by mid-successional species. Interestingly, early-successional plant-soil feedback effects provided biotic legacies, which influenced dominance patterns of mid-successional plant species.
To test the potential of soil community manipulations as a management strategy in restoration of species-rich grasslands, two field experiments were set up. In one experiment, we tested the effect of carbon substrates (to induce microbial N-immobilisation) and top soil removal on plant and soil community development. To test the relative importance of soil nutrient status and dispersal limitation, the treatments were applied both with and without sowing later-successional plant species. The results show that seed addition is more important than soil fertility reduction measures for restoration of plant communities. Apparently, initial stages of secondary vegetation succession are determined by plant species arrival rather than by abiotic factors. In a second experiment, which was set up on an ex-arable site from which the top soil had been removed (the receptor site), we tested whether simultaneous introduction of later-successional plants and soil organisms can influence the direction of plant community development towards a later-successional ‘target’ system. Plants and soil organisms were collected from a mature, species-rich Cirsio-Molinietum fen meadow (the donor site) and introduced 1) by spreading hay and soil, independently or combined; or 2) by transplanting intact turfs. We could not demonstrate that introduction of later-successional soil organisms facilitate the establishment of later-successional plant species. Unfavorable soil conditions at the receptor site may have limited survival of later-successional soil organisms in the turfs and may have precluded successful establishment of the soil organisms at the receptor site.
In conclusion, microcosm experiments show that there is strong potential for interdependence in rate and direction of plant and soil community development in secondary grassland succession following land abandonment. Feedback between plants and soil organisms can strongly affect plant competitive interactions and successional replacements at small spatial and temporal scales. Through ecological legacies, such feedback effects may affect long-term plant community composition patterns. However, at the field scale level, plant and soil communities may develop independent of each other to a great extent, and other factors, especially the order of plant species arrival, may overrule the effects of soil organisms in plant community succession. We know little about the required conditions for successful establishment of later-successional soil organisms and, so far, we could not demonstrate that introduction of soil organisms is an effective measure to improve grassland restoration.
| Original language | English |
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| Qualification | Doctor (dr.) |
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| Award date | 25 May 2007 |
| Place of Publication | Wageningen |
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| Publication status | Published - 25 May 2007 |