Species effects on root exudates and rhizosphere N mining
Plants invest carbon into root exudates to acquire nitrogen derived from enhanced extracellular enzyme activities of rhizosphere microbes, i.e. rhizosphere priming effects. The magnitude of rhizosphere priming effects for mining N and their controlling factors are less known. We conducted an in situexperiment on mature trees with different life forms (deciduous vs. evergreen) and mycorrhizal types (ECM vs. AM) to test species effects on root exudate rates and rhizosphere N mining. Based on the assumption that the magnitude of root exudates controls levels of rhizosphere N mining processes, we predicted: i) between deciduous and evergreen species, deciduous species have higher root exudate rates due to their greater photosynthesis capacity and nitrogen demand. ii) AM trees will invest more carbon into root exudates than ECM trees in order to acquire the same amount of nitrogen because the benefit from root-associated free microbes in response to unit root exudate might be less than the benefit from ECM fungi. To test these hypotheses, we measured intact fine-root exudate rates, rhizosphere microbial activity and aboveground physiological traits of four tree species from two genera ( also two mycorrhizal types).
Root exudate rate was ca. 50% higher in the rhizosphere of deciduous species than of evergreen species, which mirrored a greater nitrogen demand in the deciduous species. Root exudate rates also closely related to leaf nitrogen contents which represented distinct photosynthesis capacities. The variation of rhizosphere NAG and LAP activities among species was explained by root exudate rates. However, PPO activity, which indicates decomposition of recalcitrant N, was significantly different between mycorrhizal types. Although enzyme activities were closely related to root exudate rates within a single species and these relationships had similar regression slopes among different species, the magnitude of exudate rates could not fully predict the level of rhizosphere N priming in different species. The uncertainties were likely related to mycorrhizal fungi, and also to the variability in the composition of both root-associated free microbe communities and root exudates.