Background and Objective

Despite comprehensive investigations in the field of C and N cycling, there is still little knowledge about direct implications of alterations in N deposition on ecosystem productivity and GHG exchange, which can at least partly be attributed to the lack of comprehensive reactive N measurements with high temporal resolution. Thus, the junior research group NITROSPHERE will contribute to ecosystem-atmosphere research by

1. establishing an improved methodology (the TRANC) for continuous monitoring of Nr exchange and by
2. synthesizing already existing datasets with a special emphasis on the  impact of Nr exchange/deposition on gross ecosystem productivity and on other GHG fluxes of affected ecosystems.

Outcomes of the project are also expected to (i) further develop research infrastructures such as ICOS through instrumental and analytical improvements, (ii) increase the potential of global data assimilation networks such as FLUXNET, (iii) providing a better data basis for (re-)formulating Nr emission factors and climate protection regulations, and (iv) enhance the understanding of C and N cycling in both natural and managed ecosystems.

Approach

The group NITROSPHERE will consist of 3 researchers, i.e. two PhD students beside the group leader, and a technician to support all field- and lab-related work. The two PhD students will be assigned to one working package (WP) each and will mainly be responsible for the subtasks and milestones within the respective WP. The focus of WP I lies on Nr measurements and methodology improvement of the recently developed TRANC system. Measurements at peatland, agricultural, and forest sites are planned. WP II deals with data synthesis and Nr modeling to elaborate the relationship between Nr and GHG exchange. All subtasks of the project are expected to be finalized within the first 4 years; a fifth year is intended for further result application such as a regionalization of the obtained relationships.

Results

Our results show that with the newly developed measurement technology the absorption capacity of peatland ecosystems for atmospheric nitrogen can be considerably lower than previously assumed (approx. 35% lower than with conventional methods). For the first time, TRANC and NH₃-QCL could were coupled and measured simultaneously as part of NITROSPHERE. QCL and a further developed thermal converter showed good agreement in the measured NH₃ concentrations during a comparison campaign. Emission factors of 3.6% could be measured a few days after fertilization on a corn field. This value is well below the 8.2% given by the European Environment Agency (2016). Global radiation and atmospheric concentration play an important role in explaining N-fluxes. For example, 41% of the variability in the exchange behavior of reactive nitrogen could be explained by both drivers. An analysis of the high-frequency losses of ΣN_r and NH₃ showed that established methods, which are optimized for CO₂ and H₂O, do not appear to be suitable for reactive gases and, according to the current state of the art, empirical methods should be used to estimate the attenuation.
In the area of ​​modeling, significant progress has been made at various levels of the NH₃ exchange determination: In Schrader & Brümmer (2013, 2014), a current review of the literature on NH₃ deposition rates was presented. The core results confirm the basic assumption that deposition is particularly high in ecosystems with a high receptor surface (e.g. coniferous forests), whereas there is a high variability in the deposition rate, especially in actively managed ecosystems (e.g. arable land). On the one hand, these updated figures can be used directly for plausibility control and for the conservative estimation of NH₃ dry deposition. On the other hand, they also underline the need for local and process-based modeling and the inadequate resolution of constant factors for scientific applications. Schrader et al. (2016) show gaps in knowledge and physically implausible processes in the modeling of the non-stomatal NH₃ exchange that are currently being actively worked on. Schrader et al. (submitted) discuss and demonstrate a procedure for the more physiologically correct calculation of the stomatal exchange of NH₃ based on the photosynthetic activity of the vegetation. This approach allows mechanistically correct, location-based parameterization of NH₃ exchange models and is an important step in the modular structure of NH₃ deposition monitoring on existing infrastructures. It also opens up the possibility of a better estimate of the NH₃ deposition under future climatic conditions. With the help of the correction method by Schrader et al. (2018) it is possible to calculate valid NH₃ deposition rates even with slow-response, low-cost sensors. In the past, significant systematic errors were often ignored and never quantified precisely and rigorously reproduced.