The influence of sucrose on soil nitrogen availability - A root exudate simulation using microdialysis

Root exudates are thought to promote nitrogen (N) availability via rhizosphere interactions, but empirical evidence is difficult to obtain given the scale and temporary nature of these processes. Microdialysis has potential to simulate root exudation patterns and quantify the effects on N availability simultaneously, but this has so far not been attempted. In a conceptual root exudation study, we used sucrose as a simple C source to investigate if microdialysis could detect the effects of continuous localised C supply on soil inorganic N fluxes. Through retrodialysis we released sucrose and simultaneously monitored diffusive soil N fluxes over one week, followed by a further seven days without sucrose. Based on current understanding of rhizosphere N dynamics, we hypothesised that N fluxes are inversely related to sucrose release, and upon ceasing release, N fluxes would increase. Using a 5 mM sucrose perfusate, C releases resulted in decreased N fluxes, but contrary to our hypothesis, N fluxes did not increase after ceasing sucrose release (c.f. control soil). Diffusive sucrose efflux from microdialysis probes increased in soils amended with N-rich litter suggesting that microbial activity and associated sucrose consumption altered sucrose concentration gradients. The fluxes of sucrose breakdown products glucose and fructose were greatest in litter treatments receiving sucrose, indicative of increased invertase activity in the presence of fresh organic matter. In the short term (days), sucrose release did not prompt an increase in inorganic N availability, possibly because of stimulated microbial growth and increased N demand under C-rich conditions. Our study confirms that microdialysis allows time-sensitive insight into the dynamic interactions of carbon and N in the rhizosphere.