Biotic and abiotic controls of nitrogen and phosphorus cycling in Central European forests

Schwarz, Martin (2015). Biotic and abiotic controls of nitrogen and phosphorus cycling in Central European forests. (Dissertation, Universität Bern, Philosophisch-naturwissenschaftliche Fakultät)

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The functioning and services of Central European forests are threatened by global change and a loss
of biodiversity. Nutrient cycling as a key forest function is affected by biotic drivers (e.g., dominant
tree species, understory plants, soil organisms) that interact with abiotic conditions (e.g., climate, soil
properties). In contrast to grassland ecosystems, evidence for the relationship of nutrient cycles and
biodiversity in forests is scarce because the structural complexity of forests limits experimental control
of driving factors. Alternatively, observational studies along gradients in abiotic conditions and biotic
properties may elucidate the role of biodiversity for forest nutrient cycles.

This thesis aims to improve the understanding of the functional importance of biodiversity for
nutrient cycles in forests by analyzing water-bound fluxes of nitrogen (N) and phosphorus (P) along
gradients in biodiversity in three regions of Germany. The tested hypotheses included: (1) temperate
forest canopies retain atmospheric N and retention increases with increasing plant diversity, (2) N release
from organic layers increases with resource availability and population size of decomposers but N
leaching decreases along a gradient in plant diversity, (3) P leaching from forest canopies increases with
improved P supply from recalcitrant P fractions by a more diverse ectomycorrhizal fungal community.

In the canopies of 27 forest stands from three regions, 16 % to 51 % of atmospheric N inputs were
retained. Regional differences in N retention likely resulted from different in N availability in the soil.
Canopy N retention was greater in coniferous than in beech forests, but this was not the case on loessderived soils. Nitrogen retention increased with increasing tree and shrub diversity which suggested
complementary aboveground N uptake. The strength of the diversity effect on canopy N uptake differed
among regions and between coniferous and deciduous forests. The N processing in the canopy directly
coupled back to N leaching from organic layers in beech forests because throughfall-derived N flushed
almost completely through the mull-type organic layers at the 12 studied beech sites. The N release
from organic layers increased with stand basal area but was rather low (< 10 % of annual aboveground
litterfall) because of a potentially high microbial N immobilization and intensive incorporation of litter
into the mineral soil by bioturbation. Soil fauna biomass stimulated N mineralization through trophic
interactions with primary producers and soil microorganisms. Both gross and net leaching from organic
layers decreased with increasing plant diversity. Especially the diversity but not the cover of herbs
increased N uptake. In contrast to N, P was leached from the canopy. Throughfall-derived P was also
flushed quickly through the mull-type organic layers and leached P was predominantly immobilized in
non directly plant-available P fractions in the mineral soil. Concentrations of plant-available phosphate
in mineral soil solution were low and P leaching from the canopy increased with increasing concentrations
of the moderately labile P fraction in soil and increasing ectomycorrhiza diversity while leaf C:P
ratios decreased. This suggested that tree P supply benefited from complementary mining of diverse
mycorrhizal communities for recalcitrant P. Canopy P leaching increased in years with pronounced
spring drought which could lead to a deterioration of P supply by an increasing frequency of drought
events.
This thesis showed that N and P cycling in Central European forests is controlled by a complex
interplay of abiotic site conditions with biological processes mediated by various groups of organisms,
and that diverse plant communities contribute to tightening the N cycle in Central European forests and
that diverse mycorrhizal communities improve the limited P availability. Maintaining forest biodiversity
seems essential to ensure forest services in the light of environmental change.

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