Fertilizing Fool's Gold

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Science  30 Oct 2009:
Vol. 326, Issue 5953, pp. 644
DOI: 10.1126/science.326_644b

High levels of nitrates released from agricultural fertilizers pose a serious threat to groundwater quality. Ameliorating this problem is not easy, but some help may be available from the natural soil bacterial communities that reduce nitrate into inert N2 gas. The rate of this denitrification step typically depends on the nature and concentration of the compounds that feed electrons into the process, providing the bacteria with energy. Most often bacteria use abundant organic matter, but in certain environments the iron(II) in minerals such as pyrite (FeS2) can also be oxidized. By monitoring the groundwater below a stretch of farmland in the Netherlands, Zhang et al. found that pyrite-coupled denitrification was the dominant nitrate-consuming reaction. Due at least in part to this reaction, nitrate levels in the groundwater were lower than levels measured 10 years ago; however, groundwater below a nearby unfertilized forest actually became enriched in nitrate over this same period. The transport of nitrate away from the farmland suggests that despite abundant pyrite remaining in the aquifer, complete denitrification occurs more slowly than nitrate's residence time. Complicating matters is the fact that levels of trace elements associated with pyrite (e.g., As, Zn, Ni) increased below the forest as well, signifying that these bacteria may be mobilizing heavy metals in the subsurface despite their benevolence in removing some of the nitrates.

Geochim. Cosmochim. Acta 73, 6716 (2009).

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