Biomonitoring of atmospheric nitrogen deposition with mosses

Mosses as indicator of effects of changes in nitrogen deposition

This research was prompted by the COVID-19 crisis and the associated lockdowns, with their sharp impact on human activities and also on the expected atmospheric emission of nitrogen, particularly that of oxidised nitrogen by traffic and industry. This unique situation offered the opportunity to trace the short-term effects of a sudden reduction in nitrogen deposition on the accumulation of nitrogen in ecosystems. A second important motivation was that it is difficult and costly to directly measure nitrogen deposition, and that its modelling involves numerous uncertainties. KWR thus studied whether pleurocarpus mosses could be suitable as a bioindicator for fine-scale monitoring of the effects of changes in nitrogen deposition in nature areas in the Netherlands.

Sampling moss at a dune location.

Sampling and analysis according to European protocols

Mosses were collected from a large number of locations in the Dutch coastal dunes, from Zeeland to the Wadden, and from a few locations in heath/woodland areas inland. For reference, mosses were also collected in several foreign coastal dunes that experience low nitrogen deposition levels. The sampling and analysis of the nitrogen concentration in the moss biomass was conducted according to European ICP Vegetation programme protocols, in which the concentrations of nitrogen, heavy metals and other problem substances in mosses are monitored every 5 years, so as to determine the large-scale spreading, and to acquire a picture of the spatio-temporal deposition pattern of these substances. The measurements of nitrogen concentrations in the moss biomass were compared with modelled nitrogen deposition and, in some of the locations, with measurements of ammonium concentrations in the air (MAN measurement locations). For 23 locations a comparison was made between 2020 and 2021, in order to determine the effects of the COVID-19 lockdowns on the nitrogen deposition.

One of the moss samples consisting of cypress-leaved plaitmoss.

Nitrogen deposition decrease as a result of COVID-19 lockdowns is measurable in mosses, but weather also has impact

The nitrogen concentration in the tops of the studied mosses, which indicate moss growth in the preceding year, was 7.5% lower in 2021 than in 2020. In the green parts of the mosses, which indicate moss growth in the preceding 2 to 3 years, this difference was 6%. The most significant decrease (22%) was measured in the inland heathland next to the A2 motorway. This could be the result of the lower traffic intensity during the COVID-19 lockdowns.

An analysis of the nitrogen isotopes was used to examine the shifts in the portions of reduced and oxidised nitrogen in the deposition. This analysis offers insight into the origin of the measured nitrogen, the nitrogen sources. Thus, traffic and industry are the main source of oxidised nitrogen, while reduced nitrogen originates for the most part in agriculture. The analysis showed that reduced nitrogen is predominant in the deposition. In 2021, the isotope ratio indicated that a relatively smaller proportion of reduced nitrogen was measured compared to 2020. It is difficult to ascribe these differences to the lockdowns, since meteorological differences (in drought and precipitation) between the measurement years can also have an impact. The nitrogen concentrations in the moss in the Dutch locations was high and in the range of nitrogen saturation. The nitrogen concentration in the moss showed a weak positive correlation with the ammonium concentration in the air.