Forest canopy nitrogen uptake can supply entire foliar demand

Plant canopies intercept, process, and potentially assimilate atmospheric nitrogen (N) additions, but the forest-scale effects of canopy processes on N cycling and plant nutrition are not clear. Substantial method artefacts and scaling issues exist in previous experimental studies which measure relevant N fluxes either at i) natural abundance, ii) via a 15N tracer, or iii) by incorporation of additional 15N into measured biomass, meaning these processes are often overlooked or discounted.

Here, working in a mature Sitka spruce plantation and under ambient conditions, we used all three of these methods independently at stand, tree, and branch scale to assess the effects of canopy interception on capture, assimilation, and incorporation of inorganic N deposition by canopy biomass.

We consistently found that above 70 % of N deposition was unaccounted for (i.e. assimilated, processed beyond detectability, volatilised or retained) when passing through the canopy, independent of the assessment method. Using short-term 15N tracers we found that this unaccounted N was retained in canopy tissues. Apparent uptake from monitoring of ecohydrological fluxes was sustained and consistent, suggesting that the tree canopies were a sink for N deposition. Seasonal variation in NO3- recovery also suggested biological activities influenced fluxes and transformations of this ion within the canopy.
The total amount of this canopy N uptake was greater than the mean annual N in litterfall, implying that this flux was supporting the turnover of the canopy. When we used 15N to directly assess uptake into branch biomass we also did not find evidence for immediate relocation to other parts of the tree.

Our findings suggest that at our N-limited site, canopy uptake of N deposition is likely to be as important as root uptake of N but may be limited to supplying N to the canopy rather than the whole tree. Further research in this area is crucial to better understand the interactions of future changes in N deposition on primary production and carbon storage in forests.