Impact of Nitrogen Fertilization on Soil Organic Matter in Forest Soils – INFOSOM

In the last decades the effect of elevated nitrogen (N) inputs on the forest carbon (C) cycle has been debated controversially and its impact on soil C storage is highly uncertain. From boreal forests we know that increased N inputs may largely increase forest canopy growth and litter production. At the same time C transport into roots may be impeded and microbial communities altered, with uncertain consequences for SOM (soil organic matter) accumulation. The aim of this project is to reveal, if and how enhanced N inputs into temperate forests change SOM pools and their chemical characteristics as well as depth distribution. Which are the microbial processes involved and how is long-term soil C sequestration affected? Hypotheses: We hypothesize that (i) increased N inputs increase the accumulation of SOM in temperate forest soils, (ii) the additionally stabilized SOM shows specific molecular characteristics, e.g. lower humification, (iii) The depth distribution of stabilized SOM is altered and SOM is enhanced predominantly in the topsoil layers, (iv) the microbial community changes with increasing N inputs in temperate forests sites, decomposing activity of microorganisms decreases. Approach: We have selected two unique long-term N fertilization experiments in Klosterhede (Denmark) and Alptal (Switzerland) which have received artificial N additions for up to 20 years to simulate elevated N deposition. We will combine a comprehensive set of chemical and microbiological methods for detailed soil analyses: (1) Soil organic matter quantity, quality and molecular characteristics will be addressed by spectroscopic analysis (mid-infrared-; solid state 13C-NMR spectroscopy) of bulk soil. Soil extracts will be obtained to analyse the labile SOM pool. Spectroscopic analyses and pyrolysis GC-MS will enable evaluations of the isolated soil extracts. The contribution of microbial and plant inputs to soil fractions will estimated. Isotope ratio mass spectrometry (IRMS) will reveal information about d13C enrichment and hence about the degree of SOM degradation with increasing soil depth. (2) Microbial pool sizes will be assessed by microbial biomass C and N and ergosterol determination, microbial activity by enzyme and respiration analyses. Metaproteomics will give us a deeper insight into the origin of soil proteins/enzymes and changes in microbial community structure and function. Innovation: The outcome of this investigation will provide answers to the unresolved question if long-term additions of N increase C sequestration in temperate forest soils. The novelty of the investigation lies in the combination of cutting-edge techniques for SOM as well microbial characterization, involving most advanced expertise from several international renown laboratories and field experiments. The strong methodological focus will bring forward future directives for investigations on soil C sequestration.

Today, temperate forest take up more carbon (C) from the atmosphere than they release through respiration. Part of this C-sink function can be attributed to the atmospheric input of reactive nitrogen (Nr) which fuels tree growth. However, most of the C in temperate forests is stored in soils as soil organic matter (SOM). Within this project, we investigated the effect of long-term experimental addition of Nr on amount and composition of SOM in two experimental sites in Europe. In contrast to most similar studies, Nr addition did not increase the amount of soil C at the experimental sites. Rather, Nr resulted in a shift in soil C storage from mineral (less C) to organic soil horizons (more C). The total amount of soil C storage down to 30 cm was not affected by Nr. In addition, the impact of Nr on tree growth, soil pH and exchangeable cations strongly depended on soil type. Furthermore, the addition of Nr altered the chemical composition of SOM (more lignin, more plant protein, less lipids) as well as function and composition of soil microbial communities. These results suggest that Nr slows the decomposition of plant-derived SOM but enhances the decomposition microbial-derived SOM. In summary, long-term Nr addition modified microbially-mediated decomposition and chemical SOM composition but did not affect the total amount of C in temperate forest soils.