Structure of and carbon flux through soil food webs of temperate grassland as affected by land use management

In Europe grasslands are among the most important agricultural systems. They are subject to continuous management intensification, which is potentially threatening ecosystem services. Since soil comprises the largest terrestrial carbon (C) pool, processes affecting C sequestration are receiving increasing attention. Exploring the structure and functioning of food webs is crucial for improving the understanding of C fluxes within the soil system and how they react to anthropogenically induced land use changes. Despite growing attention, soil processes such as the flux of C from the aboveground to belowground food webs are still poorly understood. I examined how agricultural management influences soil communities (including microorganisms and invertebrates), in particularly how it affects the flux of C through the soil food web. The study was performed in the framework of the ‘GRASSMANN’ (Grassland Management) experiment, manipulating agricultural management practices including fertilizer addition (-NPK and +NPK), sward cutting frequency (one and three cuts per year) and sward composition by applying herbicide targeting either dicotyledons or monocotyledons in a temperate grassland. In the study described in Chapter 2, the abundances of soil microorganisms and soil animals were recorded and evaluated. Microbial biomass (Cmic) decreased in treatments cut once with fertilizer addition. However, in treatments cut thrice, Cmic did not decline with fertilizer addition, which suggests that cutting frequency lessens the detrimental effect of fertilizer input. Fungal but not bacterial PLFAs (phospholipid fatty acids) varied with sward composition, indicating fungi to respond more sensitively to changes in grassland management and sward composition as compared to bacteria. Abundances of dipterans and beetles increased with fertilizer addition but decreased with higher cutting frequency. Epigeic earthworms benefited from reduced proportion of monocotyledons. Overall, however, the results indicate that the majority of soil invertebrate taxa are rather insensitive to variations in grassland management practices. Consequently, the results indicate that microbial parameters are more sensitive to intensification of grassland management practices than soil animals, particularly in the short-term after changes of management practices. In order to identify root exudate associated trophic chains and their dynamics under different management regimes an in situ 13C labelling experiment was performed by applying 13C labelled glucose mimicking root exudates to each of the treatments except ‘cutting frequency’, aiming to trace the flux of C into microorganisms (Chapter 3) and soil animals (Chapter 4). As documented in the second study (Chapter 3), focusing on soil microorganisms, the fungal-to-bacterial atom percent excess ratio of PLFAs was at a maximum early in the experiment, indicating that in relative terms fungi are initially more efficient in capturing low molecular weight C compounds entering the soil in pulses than bacteria, whilst in later stages of the experiment the opposite was true. These results challenge the established idea of a ‘fast bacterial’ and a ‘slow fungal’ channel. Furthermore, the incorporation of glucose C into fungi was most pronounced at the first two sampling dates, two and six weeks after glucose addition, and declined thereafter, whereas in bacteria it remained at a similar level for one year, suggesting that microbial residues are processed predominantly by bacteria, but not by fungi. In fertilized swards the fungal-to-bacterial 13C ratio increased, while bacterial 13C incorporation remained at the same level suggesting that NPK fertilizer addition increases the efficiency of resource capture by fungi. Sward composition generally had little effect on microbial community composition and glucose C incorporation. Chapter 4 focuses on the incorporation of glucose C into soil animals, differentiating species into those feeding as primary decomposers on litter or living as herbivores by feeding on roots (not incorporating glucose C), from those functioning as secondary decomposers feeding on microorganisms (incorporating glucose C). Each of the studied soil animal species incorporated glucose C indicating that the majority of grassland soil animal species relies on microorganisms as food resources, which themselves rely on root exudates. However, incorporation of glucose C into soil animal species varied markedly with species identity, suggesting detritivorous microarthropods to complement each other in channelling microbial C through soil food webs. Fertilizer addition markedly reduced the concentration of glucose C in most soil animal species as well as the absolute transfer of glucose C into oribatid mites as major secondary decomposers. This suggests that fertilizer addition shifts the basis of the decomposer food web towards utilisation of unlabelled resources, presumably roots, i.e. towards a herbivore system, thereby lessening the link between microorganisms and microbial grazers and hampering the propagation of microbial C to higher trophic levels. The results of this thesis advance the understanding of soil food web structure and their role in managed grassland for cycling of C. They document the value of innovative techniques, especially stable isotope analysis using glucose as 13C tracer, in advancing our understanding the functioning of soil food webs. The combination of stable isotope analysis, fatty acid analysis and compound specific 13C analyses provide a powerful tool for studying the structure and the flux of C through the bacterial and fungal channel of grassland. This thesis documented that grassland management markedly alters the structure of and carbon flux through soil food webs with the effects varying between management regimes and soil organisms. Overall, the results suggest that herbivore species benefit from higher biomass and nutritional status of plants due to fertilizer addition with some detritivorous species shifting to a more herbivorous diet. In contrast, microorganism and secondary decomposer relying on rhizodeposits are detrimentally affected. This suggests that grassland management practices and in particular fertilizer addition alter the interrelationship between the above- and belowground food web which needs closer consideration if we are to conserve the functionality of European grassland systems.