More efficient aboveground nitrogen use in more diverse Central European forest canopies

Abstract

We hypothesized that biodiversity improves ecosystem functioning and services such as nutrient cycling because of increased complementarity. We examined N canopy budgets of 27 Central European forests of varying dominant tree species, stand density, and tree * Corresponding author: M. and shrub species diversity (Shannon index) in three study regions by quantifying bulk and fine particulate dry deposition and dissolved below canopy N fluxes. Average regional canopy N retention ranged from 16% to 51%, because of differences in the N status of the ecosystems. Canopy N budgets of coniferous forests differed from deciduous forest which we attribute to differences in biogeochemical N cycling, tree functional traits and canopy surface area. The canopy budgets of N were related to the Shannon index which explained 14% of the variance of the canopy budgets of N, suggesting complementary aboveground N use of trees and diverse understorey vegetation. The relationship between plant diversity and canopy N retention varied among regional site conditions and forest types. Our results suggest that the traditional view of belowground complementarity of nutrient uptake by roots in diverse plant communities can be transferred to foliar uptake in forest canopies.

Drivers of nitrogen leaching from organic layers in Central European beech forests

“…The organic layers consist of Oi and Oe horizons, except on one plot (AEW8) where a thin discontinuous Oa horizon was detected additionally. Total N deposition in the growing season 2010 was 17.1 ± standard deviation (SD) 4.3 kg ha −1 in the Schwäbische Alb and 13 ± SD 2.5 kg ha −1 in Hainich-Dün (Schwarz et al 2014).…”

“…We did not include stem ow uxes because we believe that stem ow is of limited importance for the budget of the organic layers because it is spatially concentrated around trees where it causes erosion of the litter layer and thus in ltrates directly into the mineral soil at the base of trunks (Levia and Frost 2003). Furthermore, stem ow deposition of TDN in the growing season 2010 was only 0.27 ± SD 0.15 kg ha −1 in the Schwäbische Alb and 0.71 ± SD 0.28 kg ha −1 in Hainich-Dün (Schwarz et al 2014). …”

“…Plants in uence N cycling by controlling external N inputs to soils like deposition from the atmosphere via canopy properties (Schwarz et al 2014) and losses like N leaching and N uptake (Knops et al 2002). About 50 to 60 % of plant-assimilated N in deciduous forests is annually returned to the soil via litterfall ).…”

“…While this implies a relationship with functional diversity, no study yet tested the in uence of plant diversity in the canopy and the understory on N uxes in organic layers. This is surprising given the positive relationship of plant diversity to N use e ciency in grasslands (Tilman et al 1996;Kahmen et al 2006) and to primary productivity and canopy N uptake in forests (Morin et al 2011;Brassard et al 2013;Schwarz et al 2014). …”

“…The study forests were exposed to a total N deposition from the atmosphere of about 15 kg ha −1 year −1 (Schwarz et al 2014). We hypothesized that (1) gross N uxes with litter leachate are dominated by throughfall-derived N while the contribution of organic layer mineralization is small, (2) the small contributions of organic layer mineralization to gross N uxes in litter leachate are positively related with stand basal area (as a proxy for litterfall), (3) vascular plant diversity reduces dissolved N uxes in organic layers, and (4) the soil fauna increases dissolved N uxes in litter leachate by accelerating soil organic matter decomposition.…”