Nitrogen deposition effects on soil organic matter

Tracing the fate of 13C and 15N labeled litter under increased N deposition. Foto: Frank Hagedorn.

One of the unknowns in soil organic matter cycling is the impact of the currently increased N deposition to forest ecosystems. By adding ¹³C and ¹⁵N double-labelled litter (leafs, wood) and tracing the fate of C and N in soils under normal and high N inputs, we aim at elucidating the mechanism how N deposition affects soil organic matter dynamics.  

Rationale

Soils contain the largest fraction of the terrestrial carbon pools. Their role as sinks for atmospheric CO₂, however, is strongly debated. One of the unknowns is the impact of the currently increased N deposition to forest ecosystems. Nitrogen is the limiting nutrient for many processes in the plant and soil system, and as the cycling of C and N are tightly coupled, it is very likely that increased N deposition affects also the cycling of soil organic matter (SOM). Modelling studies suggest that increased N inputs stimulate the turnover of C due to decreasing C/N ratios. In contrast, many field studies have shown that high N inputs suppress CO₂ effluxes from soils. Recent radiocarbon studies and ¹³C tracer experiments have indicated that N deposition retards particularly the decomposition of older and stable SOM-pools and thus, increased N inputs might lead to a sequestration of C in the long-term. The underlying mechanisms of this preservation effect, however, are not well known.

Aims

In our Swiss National Science Foundation project we want to improve our understanding of N deposition effects on the cycling of SOM:

1. How does increased N deposition affect CO₂ effluxes and DOC leaching?
2. Which SOM-pools (labile litter-derived or stabile SOM) respond most sensitive to the increased N inputs?
3. Does high N inputs in fact retard lignin degradation as hypothesized by some authors?
4. How does N deposition changes the microbial communities, e.g. by favouring fast growing microbes specialised on fresh substrate and suppressing slow-growing microbes relying mainly on SOM?

Methods

We are adding double-labelled (¹³C and ¹⁵N) beech litter, leafs and wood-chips to forest soil and to trace the fate of the added C and N through the soil system under ambient and experimentally increased N inputs (+50 kg N ha-1y-1). The labelled litter is available in exceptionally large amounts, 7 kg of leaves and 15 kg of wood, from previous CO₂ enrichment and ¹⁵N tracer experiments. This provides the unique opportunity to conduct a field study with labelled, but still natural, forest litter under undisturbed conditions. We will follow the fate of the added C and N in the litter itself, in mineral soils and their microbial communities, in leached DOC, and in respired CO₂.

Partners

• Institute of Plant Sciences, ETH Zürich (Nadine Rühr, Werner Eugster, Nina Buchmann)
• Agroscope Reckenholz (J. Furer)