To fertilize or acidify? Nitrogen plays both sides in soils

The human industrial and agricultural sectors contribute to air pollution by releasing nitrogen oxides (sometimes denoted NOx) into the atmosphere. And just like ocean acidification occurs when carbon dioxide dissolves into the ocean, soil acidification can occur when nitrogen oxides dissolve into soils.

But we also know that nitrogen is a major component of fertilizers, which add nutrients to soils.  So how do these two processes–soil acidification and eutrophication, or the increase of soil nutrients–interact?  Do the acidifying and fertilizing actions cancel each other out?

This quagmire was the topic of a recent paper in Functional Ecology. The authors, led by Carly Stevens of The Open University in the UK, investigated the decrease in plant biodiversity related to nitrogen deposition that had been observed in UK grasslands with highly acidic soils. Although this phenomenon is well known, its mechanisms remained a mystery.

In eutrophic communities, plants and animals have more nutrients available to them, which leads to rapid proliferation and, consequently, inter-species competition. The researchers compared a measurement of relative competitiveness among plant species over a nitrogen gradient, and found no apparent relationship between competition and levels of nitrogen.  On the other hand, areas with high nitrogen levels had more plants that were tolerant of lower pH, or more acidic conditions. 

These data led the team to conclude that soil acidification has a stronger effect on these grasslands than eutrophication. They write:

The results suggest that soil acidification as opposed to eutrophication and consequent competition between species is contributing to shifts in species composition and diversity linked to N deposition in calcifuge grasslands. Soil acidification may be leading to reduced nutrient availability preventing the effects of N addition from being apparent.

These results highlight the fact that high levels of naturally occurring elements can play very different roles in different ecosystems.  Whereas in more normal soils high nitrogen levels could lead to eutrophication and competition, in these soils that competition was not a factor in species interactions. Just goes to show that even ecosystems have their own personalities.

Stevens, C., Thompson, K., Grime, J., Long, C., & Gowing, D. (2009). Contribution of acidification and eutrophication to declines in species richness of calcifuge grasslands along a gradient of atmospheric nitrogen deposition Functional Ecology DOI: 10.1111/j.1365-2435.2009.01663.x