Chronic Atmospheric Reactive Nitrogen Deposition Suppresses Biological Nitrogen Fixation in Peatlands.

Biological nitrogen fixation (BNF) represents the natural pathway by which mosses meet their demands for bioavailable/reactive nitrogen (Nr) in peatlands. However, following intensification of nitrogen fertilizer and fossil fuel use, atmospheric Nr deposition has increased exposing peatlands to Nr loading often above the ecological threshold. As BNF is energy intensive, therefore, it is unclear whether BNF shuts down when Nr availability is no longer a rarity. We studied the response of BNF under a gradient of Nr deposition extending over decades in three peatlands in the U.K., and at a background deposition peatland in Sweden. Experimental nitrogen fertilization plots in the Swedish site were also evaluated for BNF activity. In situ BNF activity of peatlands receiving Nr deposition of 6, 17, and 27 kg N ha-1 yr-1 was not shut down but rather suppressed by 54, 69, and 74%, respectively, compared to the rates under background Nr deposition of ∼2 kg N ha-1 yr-1. These findings were corroborated by similar BNF suppression at the fertilization plots in Sweden. Therefore, contribution of BNF in peatlands exposed to chronic Nr deposition needs accounting when modeling peatland's nitrogen pools, given that nitrogen availability exerts a key control on the carbon capture of peatlands, globally.

Portable, low-cost, Raspberry Pi-based optical sensor (PiSENS): continuous monitoring of atmospheric nitrogen dioxide.

We have developed a sensing system that utilizes a low-cost computer (Raspberry Pi) and its imaging camera as an optical sensing core for the continuous detection of NO2 in the air (PiSENS-A). The sensor is based on colour development as a consequence of the interaction of the gas with an absorbing solution. The PiSENS-A is thoroughly calibrated over the hourly mean which is used as one of the key metrics in evaluating air quality. The calibration was performed in the range of 0 < [NO2] < 476 µg m-3 chosen to contain the threshold used to determine compliance to the UK's Air Quality Standard Regulations (2010) expressed as a maximum of 18 permitted exceedances of [NO2]hourly mean = 200 µg per m3 per year. Lab-based measurements were evaluated against UV-vis. The average precision expressed as a relative standard deviation was: RSD% = 2.8%, while the correlation of mock samples was excellent (Pearson's r = 1.000). Field-based measurements were evaluated against chemiluminescence-based instrument exhibiting a correlation coefficient of R2 = 0.993. The PiSENS-A was also deployed as an independent air quality analyser at the Keele University campus.

Chronic atmospheric reactive N deposition has breached the N sink capacity of a northern ombrotrophic peatbog increasing the gaseous and fluvial N losses.

Peatlands play an important role in modulating the climate, mainly through sequestration of carbon dioxide into peat carbon, which depends on the availability of reactive nitrogen (Nr) to mosses. Atmospheric Nr deposition in the UK has been above the critical load for functional and structural changes to peatland mosses, thus threatening to accelerate their succession by vascular plants and increasing the possibility of Nr export to downstream ecosystems. The N balance of peatlands has received comparatively little attention, mainly due to the difficulty in measuring gaseous N losses as well as the Nr inputs due to biological nitrogen fixation (BNF). In this study we have estimated the mean annual N balance of an ombrotrophic bog (Migneint, North Wales) by measuring in situ N2 + N2O gaseous fluxes and also BNF in peat and mosses. Fluvial N export was monitored through a continuous record of DON flux, while atmospheric N deposition was modelled on a 5 × 5 km grid. The mean annual N mass balance was slightly positive (0.7 ± 4.1 kg N ha-1 y-1) and varied interannually indicating the fragile status of this bog ecosystem that has reached N saturation and is prone to becoming a net N source. Gaseous N losses were a major N output term accounting for 70% of the N inputs, mainly in the form of the inert N2 gas, thus providing partial mitigation to the adverse effects of chronic Nr enrichment. BNF was suppressed by 69%, compared to rates in pristine bogs, but was still active, contributing ~2% of the N inputs. The long-term peat N storage rate (8.4 ± 0.8 kg N ha-1 y-1) cannot be met by the measured N mass balance, showing that the bog catchment is losing more N than it can store due its saturated status.