Browsing by Author "Doerr, Nicole"

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.

A long-term field experiment conducted in a Norway spruce forest at Solling, Central Germany, was used to verify and compare the response of lignin-decomposing fungal communities in soils receiving current and preindustrial atmospheric nitrogen (N) input for 14.5 years. Therefore, we investigated the decomposition of lignin compounds in relation to phenol oxidase activity and the diversity of basidiomycetes containing laccase genes in organic and mineral horizons. Lignin-derived CuO oxidation products and enzyme activity decreased with soil depth, while the degree of oxidative transformation of lignin increased. These patterns did not change with reduced atmospheric N input, likely reflecting a lasting saturation in available N. The laccase gene diversity decreased with soil depth in spring. In autumn, this pattern was only found in the control plot, receiving current N input. Principal component analysis confirmed the depth profile and distinguished a response of the fungal community to reduced N deposition for most organic layers in spring and a roof effect for the Oe layer in autumn. These responses of the fungal community did not translate into changes in enzyme activity and lignin content and decomposition, suggesting that transformation processes in soils are well buffered despite the rapid response of the microbial community to environmental factors.

A field-scale manipulation experiment conducted for 16 years in a Norway spruce forest at Solling, Central Germany, was used to follow the long-term response of total soil bacteria, nitrate reducers and denitrifiers under conditions of reduced N deposition. N was experimentally removed from throughfall by a roof construction ('clean rain plot'). We used substrate-induced respiration (SIR) to characterize the active fraction of soil microbial biomass and potential nitrate reduction to quantify the activity of nitrate reducers. The abundance of total bacteria, nitrate reducers and denitrifiers in different soil layers was analysed by quantitative PCR of 16S rRNA gene, nitrate reduction and denitrification genes. Reduced N deposition temporarily affected the active fraction of the total microbial community (SIR) as well as nitrate reductase activity. However, the size of the total, nitrate reducer and denitrifier communities did not respond to reduced N deposition. Soil depth and sampling date had a greater influence on the density and activity of soil microorganisms than reduced deposition. An increase in the nosZ/16S rRNA gene and nosZ/nirK ratios with soil depth suggests that the proportion of denitrifiers capable of reducing N2O into N-2 is larger in the mineral soil layer than in the organic layer.