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In the second half of the 20th century, intensification of agricultural management led to a reduction in the extent of species-rich grasslands throughout Europe, through the increased use of inorganic fertilizers and biocides and the conversion of land to arable cropping. As a response, specific mitigation efforts have been introduced in the European Union (EU) through agri-environment schemes that aim to promote both the extensification of the remaining grassland and creation of species-rich grassland on ex-arable land (Anonymous 1998). Although widely practised, the outcome of such schemes is highly variable and remains difficult to predict (Buckingham et al. 1999; Kleijn & Sutherland 2003). This has been attributed to a number of factors, such as the lack of seed propagules (Hutchings & Booth 1996; Bekker et al. 1997), herbivory (Kleijn 2003) and a paucity of trophic linkages (Van der Heijden 2004).
One of the most critical factors determining the success of creation of species-rich grasslands on ex-arable land is the high nutrient availability resulting from previous land use (Marrs 1993). In habitat-creation projects on fertile soils, a rapid establishment of annual and fast-growing perennial early seral plants often characterizes the initial phase of vegetation development (Hansson & Fogelfors 1998; Baer et al. 2004). These continue to dominate as long as the nutrient availability remains high, and thus impede the establishment of late-seral plant species, even when such species are introduced by sowing (Kindscher & Tieszen 1998). Negative effects of high nitrogen (N) availability on the restoration and maintenance of species-rich grassland have been shown in experiments where N availability was increased by fertilization (Tilman 1993; Hansson & Fogelfors 1998). These results suggest that a key prerequisite for succession towards more diverse vegetation on ex-arable land is a reduction in plant-available N (Marrs 1993; Tilman 1993).
Different methods have been proposed to reduce the nutrient availability in ex-arable soils undergoing restoration management (Marrs 1993). These include topsoil removal, maximizing offtake and increasing storage in organic and inorganic nutrient pools. Addition of carbon (C) to the soil has recently been put forward as a means to reduce plant-available nutrients and alter competitive interactions among plant species (Morgan 1994). It has been hypothesized that C addition induces soil microbial activity, which would be paralleled by increased immobilization of inorganic N (Johnson & Edwards 1979; Schmidt, Michelsen & Jonasson 1997; Paschke, McLendon & Redente 2000; Blumenthal, Jordan & Russelle 2003). Several studies have found that C addition leads to decreased rates of net N mineralization (Johnson & Edwards 1979; Averett et al. 2004; Gilliam et al. 2005) and nitrification (Gilliam et al. 2005) and reduced concentrations of ammonium (; Hopkins 1998) and nitrate (; Schmidt, Michelsen & Jonasson 1997; Török et al. 2000; Blumenthal, Jordan & Russelle 2003) in the soil. As plant growth is thought to be primarily limited by the availability of inorganic N in the soil (Tilman 1985; Schimel & Bennett 2004), a reduction in the amount of plant-available N in response to C addition should result in reduced plant growth (Blumenthal, Jordan & Russelle 2003). While this method has been tested repeatedly to reduce the competitive ability of alien invasive plants (Reever Morghan & Seastedt 1999; Alpert & Maron 2000; Blumenthal, Jordan & Russelle 2003; Corbin & d’Antonio 2004; Perry, Galatowitsch & Rosen 2004) it has rarely been considered as a tool to alter the community composition of native vegetation (Michelsen et al. 1999; Török et al. 2000; Eschen, Müller-Schärer & Schaffner 2006). C addition has also been found to reduce above-ground vegetation biomass (Michelsen et al. 1999; Alpert & Maron 2000; Blumenthal, Jordan & Russelle 2003). In nutrient-rich environments, such as ex-arable soils, is expected to be the dominant pool of plant-available N (Schimel & Bennett 2004); it might therefore be expected that a reduction in through C addition will affect the growth rate of plants on ex-arable land.
The type of C source added to the soil is likely to influence the effect of C addition on the soil environment and plant growth as a result of the rate at which the source is available to micro-organisms. A readily available C source such as sugar (sucrose) may stimulate microbial activity within hours (Dalenberg & Jager 1981) while other sources, consisting of structurally more complex molecules, take longer to degrade (Magill & Aber 2000), especially when applied as coarse structures with small surface to volume ratios. The decay of sawdust is slower than that of sugar (Török et al. 2000) but presumably faster than that of wood chips. The relatively short decay rate of sawdust makes it a potential substitute for the expensive sugar, while the addition of small pieces of wood may have a slower but longer-lasting effect on the soil environment and vegetation.
The increase in microbial N immobilization through C addition may be a temporary phenomenon. Little is known about how the soil microbial community and the pool of plant-available N in C-amended soil responds to a cessation of C addition, and how these effects will translate to a shift in vegetation composition. Once C addition is stopped, the inorganic N concentration in the soil may rise again as a result of an increased release from decaying microbial biomass (Török et al. 2000). The rate of release of stored N may affect the extent to which C addition will influence soil N concentration and the vegetation composition after the period of C addition. The timing and rate of such a release may depend on the group of micro-organisms involved in the storage of nutrients. The addition of C as sugar may elicit a quick response in terms of bacterial biomass, while addition of C as wood chips may particularly stimulate the activity of decomposer fungi because of their ability to exploit larger structures. The release from bacterial biomass is likely to be faster than from groups with slower turnover rates, such as fungi. As we expect wood chips to stimulate decomposing fungi in particular, the release of N in soils where wood chips were added may occur later and at a lower rate than in soils where sugar was added.
The general effect of C addition on the vegetation is a reduction in plant growth, resulting in reduced cover and competition for light (Blumenthal, Jordan & Russelle 2003 and references therein). Yet to make C addition a useful tool to manage vegetation composition in habitat-creation projects, it should affect the growth of plant species in a species-specific way. In a greenhouse experiment testing the effect of C addition on the growth of a large number of early seral and late-seral plant species in the absence of competition, Eschen, Müller-Schärer & Schaffner (2006) found that C addition reduced the growth of all plant species but that the level of biomass reduction varied significantly among the plant species. Both functional group and life form explained significant amounts of the variation among individual plant species. For example, biomass reduction in legumes was lower than that in other forbs and grasses, and the shoot:root ratios of grasses were significantly reduced while those of legumes and other forbs were not. Moreover, the growth of annual plants was affected more by C addition than that of perennial plant species. These findings are in line with the results of a study by Averett et al. (2004) and suggest that C addition may indeed be a promising tool for manipulating vegetation composition on N-rich ex-arable land.
In the present study we aimed to assess the effect of different forms of C addition on vegetation composition at ex-arable sites. We examined the effect of C addition on the soil nutrient availability, soil microbial community and vegetation composition at two recently abandoned fields in Switzerland and two 6-year-old restoration sites in the UK. We monitored changes in these factors during a 2-year period of C addition and the year following cessation of the treatments. We tested the hypotheses that: (i) C addition leads to a reduction in plant-available N in the soil; (ii) this reduction coincides with an increase in microbial biomass; (iii) the changes in the soil environment lead to changes in plant above-ground biomass and changes in vegetation composition; (iv) the changes in vegetation cover facilitate the establishment of sown late-seral plant species; (v) the magnitude of the effect during the period of C addition depends on the form of the C source applied; and (vi) the effects will persist for at least 1 year beyond the period of C addition.
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AM fungi take up C from the soil before they associate with a host-plant (Bago et al. 1999), which may explain the correlation between the change in concentration and change in AM fungal biomass in the weeks after the first C addition at the Swiss sites. An increase in C availability after C addition, as in this study, may therefore increase the rate of consumption and activity of AM fungi for a short period of time. AM fungal growth is no longer affected by soil C once associations have been formed with a host-plant, which may explain why we did not find a consistent effect of C addition on AM fungal abundance. The significant treatment effect on AM fungal biomass at the Swiss sites after the C additions were stopped was because of differences on one sampling date only.
In general, the pattern of N availability at the UK sites was similar to that at the Swiss sites (Fig. 1); however, there were no treatment effects on during the first weeks of C addition, which may explain why no relationship between changes in N availability and changes in microbial biomass was found at the UK sites. The slower response of the soil at the UK sites may be a result of the extended period of set-aside of the fields prior to the start of the experiment, which may have resulted in the establishment of below-ground and above-ground communities that make the system more resilient to changes in resource availability.
The magnitude of the reduction in in the present study depended on the C source. The sugar/sawdust mixture elicited faster and stronger changes in the concentration, and the vegetation parameters we measured, than the wood chip/sawdust mixture (Figs 1 and 3). The faster response in sugar/sawdust plots than in wood chip/sawdust plots is likely to be because of the sugar, which is more easily available to micro-organisms than sawdust and can thus elicit a quicker reduction in concentration, than structurally more complex C sources (Török et al. 2000). Similarly, Johnson & Edwards (1979) found that adding sucrose increased net N immobilization more rapidly than adding less-labile C sources such as root and litter leachates. The release of C from wood chips may have been slower because of the coarser structure of wood chips compared with sawdust. In addition, the wood chip/sawdust plots received less C than the sugar/sawdust plots, because no wood chips were added in 2003. The rate of N immobilization by the soil microbial community has been found to increase logarithmically in response to C addition (Gilliam et al. 2005). Similarly, plant growth responses have been shown to depend on the amount of C added to the soil (Blumenthal, Jordan & Russelle 2003; Eschen, Müller-Schärer & Schaffner 2006). In our study, the difference in magnitude of the responses between the two C treatments was already apparent in 2002, indicating that the results are best explained by the different availability of the added C.
The changes in saprophytic fungi were paralleled by an increase in the fungal:bacterial ratio in wood chip/sawdust plots in both countries. The fact that the wood chip/sawdust treatment induced a biomass increase of saprophytic fungi at the UK sites and at one of the two Swiss sites provides evidence that the wood chips were available to fungi as a C source. The shift in the fungal:bacterial ratio in the soil of wood chip/sawdust plots lasted beyond the period of wood chip addition (Fig. 3), which is likely to be a consequence of the slow degradation of the wood chips; wood chips were still detectable on the soil surface at the end of the study. The present results suggest that it may be possible to manipulate below-ground microbial succession on ex-arable fields differently by adding either slowly or rapidly decomposing C sources to the soil.
In a greenhouse study, native European grassland species, when grown without competition, showed species-specific reductions in growth in response to C addition (Eschen, Müller-Schärer & Schaffner 2006). The above-ground biomass of annual plant species and grasses was significantly more reduced than that of legumes and perennial forbs. In the present field study, the changes in cover of the different plant functional groups during the period of C addition support the findings of the greenhouse study and show that different growth rates after C addition can have a significant impact on the composition of the vegetation. Similarly, in a study on ex-arable land invaded by exotic species, Averett et al. (2004) found that the reduction in above-ground biomass after C addition was less pronounced for prairie forbs compared with prairie grasses and exotic species. These and other studies documenting a change in the vegetation composition after C addition (Michelsen et al. 1999; Blumenthal, Jordan & Russelle 2003; Perry, Galatowitsch & Rosen 2004) demonstrate that C addition is a promising tool for manipulation of vegetation composition in disturbed habitats.
The Swiss and the UK sites differed in several aspects at the start of the experiment, such as time since abandonment and management (Table 1). Therefore the different responses of the plant functional groups to C addition at the UK and Swiss sites cannot be compared directly. One potential explanation for the observed differences is that the age of the vegetation at the start of the treatment application varied between the sites. The seedlings colonizing the Swiss sites were most probably free or almost free of mycorrhizae during the first days and weeks after germination, and were therefore likely to be more susceptible to changes in soil inorganic nutrient content than the established plants at the UK sites, the majority of which belonged to mycorrhizal plant species. Mycorrhizae are known to act as a support system to plant growth because they facilitate nutrient uptake by the plants (Schimel & Bennett 2004). We therefore hypothesize that C addition has the strongest impact on the vegetation composition on ex-arable land when applied immediately after taking the land out of cultivation.
The higher level of bare ground on C-amended plots at the Swiss sites was paralleled by a reduction in the cover of unsown, spontaneously occurring plant species. It appears that the creation of open gaps enabled better establishment of sown species during the period of C addition at the Swiss site in Courchapoix, which was characterized by a high abundance of unsown grasses in the control plots (Fig. 4). In the UK, the abundance of bryophytes, a group absent at the Swiss sites, was reduced, creating more bare patches on those sites. Tilman (1993) suggested that increased light penetration promotes the rate of establishment by late-seral species in grasslands. The promotion of open patches at the beginning of grassland development, as observed in this experiment, shows the potential of C addition to create conditions that are desirable for the creation of species-rich grasslands. Moreover, the increase in abundance of the sown species at the Swiss site in Courchapoix, where the abundance of unsown grasses was high compared with the level found at the other Swiss site at Movelier, indicates that C addition can make the outcome of restoration programmes more predictable.
The results from the Swiss sites show that C addition can create gaps in the vegetation and may thus increase the possibility for late-seral species to establish over an extended period of time. Whether the persistence of open space is a desirable outcome or not depends on the identity of the colonizing species. At the Swiss sites the bare patches that were present when C addition was stopped in summer 2003 were colonized by annual forbs in 2004. Thus, although in 2004 the cover of legumes remained significantly higher and the cover of grasses lower on the sugar/sawdust-amended plots than on the control plots, the vegetation composition on the C-amended plots had already started to approach that of the control plots. We suggest that C addition should be continued until the gaps in the vegetation have been more or less fully colonized by desirable plant species. This may be attained by either adding smaller amounts of C, which would reduce the level of growth reduction, or applying C over a longer period than in this experiment.
In summary, our study provides evidence that C addition is a useful tool that could be used to reduce N availability in ex-arable soils, thereby increasing the likelihood of successful creation of species-rich grassland. The rapid response and the low cost of C addition make it a useful alternative to other methods for reducing soil nutrient availability (Marrs 1993), such as topsoil removal and hay cutting, which are more expensive or have slower effects on nutrient availability. We therefore support the recommendation for management of ex-arable fields on fertile soils made by Hansson & Fogelfors (1998), that the initial dominance of early seral species can be reduced and diversity of the vegetation increased by adding C to the soil. C addition could be combined with other management tactics, such as sowing a species-rich seed mixture or cutting to increase species richness.