|dc.description.abstracteng||The availability of mineral nutrients (i.e. N, P) in unfertilized soils is often not adequate to meet plant demand, which in turn creates a necessity to fulfil the supply requirements from nutrient release from soil organic matter (SOM). SOM has a high chemical complexity, and it varies in terms of temporal and spatial availability. It contains essential nutrients in heterogeneous com-position and distribution (Lal, 2009; Murphy et al., 2015; Schmidt et al., 2011). Root exudates are a key component of the mechanisms for acquisition and mobilization of nutrients in the soil. Their influences can be classified as both direct, i.e. the compounds released react directly by degrading soil material, or indirect, which requires intermediate steps of biological or physical nature, which in turn trigger effects. Direct effects are triggered, for example, by organic acids that raise P availability by complex alteration (Jones & Darrah, 1994; Jones, 1998; Bais et al., 2006; Eldhuset et al., 2007). Indirect effects are mostly priming effects or the modification of soil chemistry like acidification. Priming effects describe a short-term change in SOM decom-position rates as a response to an input of labile carbon (C) from living roots (Kuzyakov et al., 2000). The `nutrient-mining' theory presumes that microbial growth is promoted by the input of exogenous C and, as a consequence, a limitation of other nutrients such as N, raising the micro-bial decomposition of SOM and the mobilization of limited nutrients (Blagodatskaya & Kuzya-kov, 2008; Wang et al., 2015).
Effects in the soil caused by the inflow of root exudates have been object of a multitude of stud-ies, in contrast, the causative factors determining the amount of C exuded were poorly exam-ined. Aiming to investigate influences of soil nutrient distribution, availability and climatic con-ditions on root exudation, three projects have been conducted, each using different gradual ap-proaches. In a first project, investigations were carried out along a depth gradient in a soil near Nienburg (Weser), deriving from glacial sediments. Root exudate quantity, root morphology and architecture, soil chemistry and nutrient availability, different soil carbon classes and micro-bial biomass C were investigated. As part of a second project, beech forests were chosen along a nutrient availability gradient, containing different soil types, that originated from different bed-rock material but were characterized by comparable climate. Measurements of the amount of released C by exudation and annual C fluxes, root morphology and biomass, parameters of soil chemistry and nutrient availability as well as fungal biomass and exoenzyme activity were de-termined. Both projects were associated to a larger research project, `SUBSOM' dealing with organic matter storage and turnover in subsoils.
The third project addressed an elevational gradient located in northern Hesse, Germany, with beech forests ranging from 300 to 800 m a.s.l., that had comparable soils deriving from bedrock material with similar properties but was characterized by distinct climatic differences. Climatic factors, particularly short- and longer-term temperature regimes, were documented, as well as root exudation rates, annual C fluxes, root morphology, fine root biomasses, soil chemistry and nutrient availability.
Root exudation decreased strongly with increasing depth; strong decreases in fine root biomass also reduced the annual flux per soil volume from topsoil to deep subsoil. Total N and P con-tents declined strongly with depth, accompanied by SOC and microbial biomass. Root exuda-tion was related to EOC and N in topsoil but decreased to a minimal basal rate in subsoil. This may be viewed as a strategy of optimizing use efficiency, limiting carbon loss by exudation in soil regions where positive effects by priming are unlikely, but increased where SOM can be mined by microorganisms. Along the nutrient availability gradient, a negative relationship of exudation, org. C : N ratios and pH were found. In addition, increasing acidity led to decreasing proportions of fungal biomass and activity of enzymes that degrade poorly bioavailable carbon, while root exudation increased. This suggested, that in acidic, nitrogen-poor soils, root exuda-tion increases while fungal abundance and activity decrease, and a greater proportion of assimi-lated carbon is invested by the plant into the surrounding rhizosphere.
Distinct relationships between root morphology and exudation rates were observed in both pro-jects. With depth, the relative surface area and length of the roots became smaller, while tissue density and average diameter increased. Similar observations were made along the nutrient gra-dient, increasing relative length was associated with an exponentially raised amount of exuded C. Decreasing fibrosity reduces the functionally metabolically active areas of fine roots, result-ing in reduced exudation. This appears to be a mechanism to restrict to basal exudation in re-gions where no benefit would be met. The elevation gradient provided results that indicate a highly significant relationship between the amount of exuded C and short-term temperatures. We could not find relations between fine root morphology and climate, while the long-term cli-mate was related to the proportion of soil organic material and the mass of the organic layer as well as accumulation of org. layer FRB. The correlation between the short-term temperature regime and root exudation can be interpreted as a consequence of increased assimilate influx into the root and energetically accelerated higher outflow following the concentration gradient to the soil, as well as a consequence of increased priming of microorganisms in order to mobilize mainly N by exuding labile C according to demand.
Priming of soil microorganisms appears to be the most important function of root exudates. The presence of nutrients in microbially degradable form seems to determine the surplus of carbon released for direct purposes and the basal efflux.
In summary, the interplay between environmental conditions and fine roots is plastic at several levels. Abundance and morphological expression of fine roots occur in relation to local soil conditions, both soil chemistry and the pattern in which nutrients occur. Exudation appears to be a factor capable of compensating short-term nutrient needs and can be adjusted depending on the given morphology.||de