Has the anthropogenic production of reactive nitrogen impacted more remote ecosystems…and for how long?

There was report published yesterday from Holtgrieve, et al. in Science which I feel is highly relevant to the blog and is one of a steady number of papers being published in recent times on the issue of reactive nitrogen.

The report details the analysis of the stable nitrogen isotope ratios (¹⁵N:¹⁴N) found in dated sediment cores taken from 25 pristine northern hemisphere lakes.  Looking into the sediment record in this way enables scientists to infer the effect humans may have had on the nitrogen cycle in the past.  This is possible because changes in the N isotopic composition of the reactive nitrogen (Nr) found in the atmosphere can indicate historic variation in the contributions from human-derived sources.  The Nr produced through human activity (e.g. fossil fuel burning and the production and use of fertilizers) has depleted ratios when compared with catchment and preindustrial atmospheric N sources.

The analysis found that changes to the N isotope ratio and the increased presence of isotopically lighter nitrogen (often from anthropogenic sources) started during the late 19^th century, a period before artificial fertilizers, a major source of Nr, were widely used.  However, this period does coincide with a large global increase in fossil fuel burning as the world became more industrialised.  There is a second major shift in ratios noted across the range of sediment cores at around 1970, a time when the production of artificial nitrogen fertiliser was rapidly increasing.  The effects of these two anthropogenic influences were shown in the sediment record to be immediate and reached across all the core sites.  This challenges the perception of nitrogen deposition as a problem with impacts mainly limited to the area surrounding the source.