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Soil Biodiversity and Nitrogen Cycling Under Agricultural (De-)Intensification Promotor: Prof

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Soil Biodiversity and Nitrogen Cycling Under Agricultural (De-)Intensification Promotor: Prof Soil biodiversity and nitrogen cycling under agricultural (de-)intensification Promotor: Prof. dr. L. Brussaard Hoogleraar Bodembiologie en Biologische Bodemkwaliteit, Wageningen Universiteit Co-promotoren: Dr. R.G.M. de Goede, Universitair docent, sectie Bodemkwaliteit, Wageningen Universiteit Dr. J. Bloem, Senior onderzoeker, Alterra Wageningen UR Dr. J.H. Faber, Senior onderzoeker, Alterra Wageningen UR Samenstelling Promotiecommissie: Prof. dr. ir. A.H.C. van Bruggen (Wageningen Universiteit) Prof. dr. ir. W.H. van der Putten (Nederlands Instituut voor Ecologie, Heteren) Dr. L.S. van Overbeek (Plant Research International, Wageningen Universiteit) Prof. dr. J. Filser (Universiteit Bremen, Duitsland) Dit onderzoek is uitgevoerd binnen de onderzoekschool Production Ecology and Resource Conservation Soil biodiversity and nitrogen cycling under agricultural (de-)intensification Maria B. Postma-Blaauw Proefschrift ter verkrijging van de graad van doctor op gezag van de rector magnificus van Wageningen Universiteit, Prof. dr. M.J. Kropff in het openbaar te verdedigen op dinsdag 15 april 2008 des namiddags te vier uur in de Aula CIP gegevens Koninklijke bibliotheek, Den Haag Postma-Blaauw, M.B. Soil biodiversity and nitrogen cycling under agricultural (de-)intensification PhD thesis Wageningen University, Wageningen, The Netherlands, 2008. - With summary in English and Dutch. ISBN 978-90-8504-890-9 Voor Jouke Abstract Soil biodiversity is often considered an essential element in sustaining ecosystem services in agricultural systems. Understanding functional biodiversity with respect to its impact on nitrogen mineralization and retention might, therefore, contribute to more sustainable production and prevention of environmental complications. In this thesis I determined how agricultural (de-)intensification affects soil biota abundances and diversity and I evaluated to what extent the soil biota abundances and community composition affect nitrogen cycling. The effects of agricultural (de-)intensification (including conversion of extensively managed grassland to arable land and vice versa , varying crop diversity and fertilization level) on soil biodiversity across taxonomic groups were studied in a long-term field experiment. Agricultural intensification was found to have strong negative effects on larger-sized soil biota and species- poor groups of soil animals such as earthworms. Furthermore, larger-sized soil biota were strongly negatively affected by short-term consequences of agricultural intensification such as soil disturbance, whereas smaller-sized soil biota were mostly negatively affected by long-term consequences, such as reduced organic matter content. Agricultural intensification affected soil biota abundances and taxonomic diversity, but not necessarily functional diversity. The relationship between soil biodiversity and nitrogen cycling was studied in two laboratory experiments. The difference in life-strategies between nematode species of the same trophic groups was found to be of importance for their communal effect on N mineralization. The effects of earthworms on nitrogen mineralization depended on the ecological traits of the species present, and hence, determined the outcome of their interactions. The results of the laboratory experiments indicate that the effect of soil biodiversity on ecosystem functioning is related to the diversity in ecological traits of the species present. The relationship between soil biodiversity and nitrogen cycling under agricultural (de-) intensification was further studied in the long-term field experiment mentioned before. Agricultural de-intensification generally resulted in increased N mineralization and decreased N leaching. Increased N mineralization in de-intensified systems was related to high abundances of earthworms and soil biota in the bacterial-based food chains. Furthermore, net N mineralization increased with a higher diversity of nematode life-strategy groups. Mineralized N had a lower risk of leaching during the cropping season than mineral N applied as synthetic fertilizer, but - year-round NO 3 leaching was not dependent on the source of mineral N. I conclude that in agricultural systems with reduced soil tillage and reduced external inputs, the abundances of certain soil biota trophic groups and diversity of life-strategy groups are promoted, which, under certain conditions, is associated with a higher nutrient use efficiency of the system. Contents Chapter 1 General introduction 1 Chapter 2 Within-trophic group interactions of bacterivorous nematode 19 species and their effects on the bacterial community and nitrogen mineralization Chapter 3 Earthworm species composition affects the soil bacterial 43 community and net nitrogen mineralization Chapter 4 Agricultural (de-)intensification differentially affects 65 abundances, functional diversity and community structure in soil micro-, meso- and macrofauna Chapter 5 Agricultural (de-)intensification differentially affects 107 taxonomic diversity of predatory mites, enchytraeids, nematodes and bacteria Chapter 6 Nitrogen cycling and soil biodiversity after (de-)intensification 141 of agroecosystem management Chapter 7 General discussion 173 Summary 183 Samenvatting 187 Dankwoord 193 Curriculum vitae 195 Education certificate 197 Chapter 1 General Introduction Introduction Introduction The richness of the species on earth has astounded both scientists and lay people as long as man exists. Although hidden to the eye, a large part of this diversity is found belowground, which led Usher et al. (1979) to call the soil the poor man's tropical rainforest. While soil biodiversity is worthwhile studying for its own sake, ecologists have been intrigued by the question whether biodiversity is somehow related to ecosytem functioning. This question has become ever more urgent, as biodiversity has drastically declined in recent decades, due to human impacts such as deforestation, pollution and agricultural intensification. Soil biota are known to be of great importance for life-support functions such as soil formation, decomposition of organic matter, disease suppresiveness and nutrient availability for plant growth (Brussaard et al., 1997; De Ruiter et al., 1994; Faber et al., 2006). In conventional agricultural systems these services are partly bypassed, through the use of synthetic fertilizers and pesticides and mechanical soil tillage. Although these practices have enhanced crop production, negative consequences are the loss of nutrients and pesticides from the agroecosystem, leading to pollution of the environment. Furthermore, soil biota abundance and diversity may be reduced under intensive agricultural management, potentially leading to impaired ecosystem functioning in the longer term (Swift et al., 2004). Growing concern about the long-term negative consequences of conventional agricultural practices gave rise to political and social incentives towards long-term sustainable agriculture. Scientific research has been directed towards finding solutions that minimize external inputs and losses from the system, while maximizing the role of soil biota for agroecosystem functioning. In this context, the scientific Stimulation Programme Biodiversity of NWO, the Netherlands Organization for Scientific Research was started. This program has funded studies that investigate the drivers for above-ground and below-ground biodiversity and the relationships between biodiversity and ecosystem functioning. As part of this program, this thesis describes my research on the relationships between biodiversity and nitrogen cycling in agricultural soil, whereas a related project was focused on the relationship between microbiological diversity and disease suppressiveness (Garbeva et al., 2003; Garbeva et al., 2004; Garbeva et al., 2006). Nitrogen cycling is an important process in agroecosystems, as nitrogen supply is needed for optimal crop production, whereas loss of nitrogen leads to eutrophication of the environment. Whereas conventional agricultural systems rely on artificial fertilizers, addition of organic matter in less intensive agricultural practices stimulates nitrogen mineralization by the soil biota and promotes soil biodiversity (Bloem et al., 1997). Many questions remain, however, on the role of different soil biota groups in nitrogen mineralization and the consequences for nitrogen leaching in different agroecosystems. In this thesis, I investigate the effects of agricultural 3 Chapter 1 (de-)intensification on soil biota abundances and diversity, and evaluate the role of the soil biota in nitrogen cycling in (de-)intensified agroecosystems. A better understanding of the role of soil biodiversity is needed to facilitate more sustainable agriculture by enhancing ecosystem services and reducing the use of mineral fertilizers and losses to the environment. Biodiversity and ecosystem functioning The role of biodiversity in ecosystem functioning has been the subject of ongoing debate in ecological studies. Several hypotheses have been formulated for the relationship between biodiversity and ecosystem functioning (Brussaard et al., 2004). The rivet hypothesis poses that each species makes a significant contribution to ecosystem functioning , thus leading to enhanced ecosystem functioning with increased species diversity. In contrast, the redundancy hypothesis poses that species are redundant with respect to ecosystem functioning: as long as all functional groups are represented, ecosystem functioning is not affected by species
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