Drought intensity alters nitrogen cycling at the tree and soil interface in Scots pine mesocosms

More frequent and intense episodes of drought are expected to affect terrestrial nitrogen (N) cycling by altering N transformation rates, the functioning of soil microorganisms, and plant N uptake. However, there is limited empirical evidence of how progressive water loss affects N cycling processes at the plant-soil interface. In this study, we addressed this challenge by employing ¹⁵N tracing techniques, and metagenomic analyses of microbial genes involved in N cycling. Our goal was to assess how different levels of soil water availability influence the fate of N derived from decomposing needle litter within a Scots pine saplings and forest soil mesocosm platform. We observed that with increasing water limitation, the release of N from the decomposing needle litter into the soil declined rapidly. However, moderate levels of water limitation barely affected the microbial metagenome associated with N cycling processes and the uptake of N by the saplings. Comparatively, severe levels of water limitation clearly impaired plant N uptake, and increased the prevalence of microbial N cycling genes potentially involved in mechanisms that protect against water stress, as opposed to genes associated with the uptake and release of N during mineralization and nitrification processes. An increased allocation of N to fine roots was further observed under reduced levels of soil moisture, to support the physiology of the saplings and potentially enhance drought resilience. Our study overall indicates that when soil water becomes largely unavailable, the cycling of N at the plant-soil interface is slowed down, and microbial and plant tolerance mechanisms may prevail over N uptake and microbial decomposition processes.