Fate of ammonium N-15 in a Norway spruce forest under long-term reduction in atmospheric N deposition

In the last decades, in particular forest ecosystems became increasingly N saturated due to elevated atmospheric N deposition, resulting from anthropogenic N emission. This led to serious consequences for the environment such as N leaching to the groundwater. Recent efforts to reduce N emissions raise the question if, and over what timescale, ecosystems recover to previous conditions. In order to study the effects on N distribution and N transformation processes under the lowered N deposition treatment, we investigated the fate of deposited NH4 (+)-N-15 in soil of a N-saturated Norway spruce forest (current N deposition: 34 kg ha(-1) year(-1); critical N load: 14 kg ha(-1) year(-1)), where N deposition has been reduced to 11.5 kg ha(-1) year(-1) since 14.5 years. We traced the deposited N-15 in needle litter, bulk soil, and amino acids, microbial biomass and inorganic N in soil. Under reduced N deposition, 123 +/- A 23% of the deposited N was retained in bulk soil, while this was only 72 +/- A 15% under ambient deposition. We presume that with reduced deposition the amount of deposited N was small enough to become completely immobilized in plant and soil and no leaching losses occurred. Trees receiving reduced N deposition showed a decline in N content as well as in N-15 incorporation into needle litter, indicating reduced N plant uptake. In contrast, the distribution of N-15 within the soil over active microbial biomass, microbial residues and inorganic N was not affected by the reduced N deposition. We conclude that the reduction in N deposition impacted only plant uptake and drainage losses, while microbial N transformation processes were not influenced. We assume changes in the biological N turnover to start with the onset of the decomposition of the new, N-depleted litter.