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Nitrogen and grain production in Malawi

Snapp et al. have an interesting article in today’s issue of PNAS: Snapp SS, Blackie MJ, Gilbert RA, Bezner-Kerr R, Kanyama-Phiri GY (2010) Biodiversity can support a greener revolution in Africa (107, 20840-20845). They find that diverse crop rotations that include legumes (either in rotation or inter-cropping) can enhance the net production benefits of fertilizer – increasing production and fertilizer use efficiency using technologies preferred by Malawian producers. (BTW, Africa is much bigger than Malawi).

One test used to assess sustainability is a “Value Cost Ratio” that indicates returns. Snapp et al. found that returns were greater for the low fertilizer-input systems. Not surprisingly, they find that these systems respond better to a doubling of fertilizer prices relative to yields. This has not quite happened with US prices over the past 10 years, but fertilizer prices have more than doubled while grain yields have increased by 50%.

This article takes a step toward understanding nitrogen dynamics toward understanding how to increase production in nutrient-poor systems. However, because they didn’t include any information on N inputs from these different sources, it’s hard to know how efficient (and sustainable) they are. The metric they use “fertilizer efficiency” is faulty because it considers the costs of just one source of N inputs and fails to account for the most important aspect of nutrient sustainability – the supply of soil-mineralized N.

Two criticisms of the article. First, their focus on ecosystems services isn’t very comprehensive. VCR and fertilizer use efficiency are not ecosystem services. Second, this article is not about biodiversity. There are important differences in these crop rotations, but none of the systems foster preservation of native biodiversity.

I’m grateful to the authors for pointing me toward the 2000 article by Borlaug defending biotechnology in agriculture (Borlaug NE (2000) Ending World Hunger. The Promise of Biotechnology and the Threat of Antiscience Zealotry. 124, 487-490).


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