Organic Nitrogen in Precipitation: Real Problem or Sampling Artefact?

Abstract

Published observations of organic nitrogen (N) compounds in precipitation go back almost a century. Several different methods have been used to measure both the total and ionic concentrations of N. There is therefore some uncertainty as to whether reported "organic N" is real, or simply the result of uncertainties in chemical analyses or inadequate sampling methods. We found that the materials from which the collector was made (polypropylene, steel, or glass) had no significant effect on the composition of dissolved organic N (DON). The use of a biocide was found to be very important during sampling and storage of samples before analysis. We set up a network of seven collectors across the U.K., from the Cairngorms to Dorset, all operating to the same protocol, and including a biocide. Samples were analysed centrally, using proven methods. Over 6 months, organic N contributed about 20% to the total N in U.K. precipitation, but with a large variation across the country. This means that current estimates of wet deposited N to the U.K., which are based only on the ammonium and nitrate concentrations, are too small. Organic N is not an artefact, but a real problem that needs to be addressed.

Experimental field estimation of organic nitrogen formation in tree canopies

“…Rain and TF samples were preserved in the field using thymol (2-isopropyl-5-methylphenol), to prevent microbial degradation during and after collection (Ayers et al, 1998, Cape et al, 2001b, Hadi and Cape, 1995, and were collected approximately weekly, depending on rainfall amounts. Rain was sampled (in triplicate) using 20 cm diameter polyethylene funnels mounted 1.5 m above ground, draining to black polyethylene bottles.…”

“…However, the sampling of wet-only or bulk precipitation for inorganic N, and the range of sampling methods for TF and SF, also introduce quantitative uncertainties into the values of inorganic N reported, whether through the contribution of dry-deposited material on the collectors (González Benítez et al, 2009), or losses of material through biological or chemical activity in the sample in the field prior to collection and preservation by cooling, freezing or addition of biocide (Cape et al, 2001b, Michalzik et al, 1997. Consequently, the estimation of 'organic N' as the difference between 'total N' and the sum of the inorganic ions (ammonium and nitrate) is inevitably less precise than for many other constituents of wet deposition.…”

Atmospheric nitrogen deposition in south-east Scotland: Quantification of the organic nitrogen fraction in wet, dry and bulk deposition

“…One of the main limitations in DON determination in water samples is that it is not possible to quantify directly (Cape et al, 2001, Jones and Willett, 2006, Vandenbruwane et al, 2007, Zhang et al, 2008. DON concentration is calculated by the subtraction of several independently measured concentrations, which leads to an important analytical uncertainty Westerhoff, 2005, Vandenbruwane et al, 2007).…”

Acid atmospheric deposition in a forested mountain catchment

“…The monthlong storage of rain/fog water in gauges also introduces the risk of error, namely for nitrogen that can be altered by bacterial activity. Nevertheless, Cape et al (2001) reported negligible changes of NO3 contents in longer sampled rainwater in cold mountain environments, finding that significant errors might occur only in high summer months (July, August), but not exceeding 20%.…”

“…Although bacterial activity may alter the chemical composition of water after collection (especially nitrogen -Golterman 1969), in our case the relatively cold mountain climate (subarctic region), relatively high concentrations of nitrogen, and low values of pH and dissolved organic carbon likely limited bacterial activity (Cape et al 2001). The potential growth of algae was reduced by using dark sampling bottles and keeping samples in the dark during transport.…”