Recently in biological nitrogen fixation Category

There's an interesting discussion of this issue at Biology Fortified blog. The post is by my first MS student, Andy McGuire, who also has interesting things to say on a blog from Washington State University, where he works.

Andy's post was apparently inspired partly by something I wrote last year, showing my estimates of how much nitrogen flows from synthetic fertilizer to conventional corn to chicken manure to organic farms.

I don't get too excited about whether organic farmers should be allowed to use manure from conventional farms, or whether we should "blame" them for the fertilizer used to grow the corn to feed the chickens. My point was simply that this dependence limits the potential for expanding organic agriculture. (So advocates of organic farming should be showering me with money for research on nitrogen-fixing legume crops and forages -- but that's another story.)

More interesting is this open-access paper, titled "To what extent does organic farming rely
on nutrient inflows from conventional farming?
" The paper has some actual numbers on nutrient fluxes to organic farms from conventional ones in France. While my earlier diagram assumed that an organic farm relied entirely on manure whose nitrogen originally came from synthetic fertilizer, the French study estimated that only 23% of nitrogen (though 73% of phosphorus) supply to organic farms came from conventional sources. But let's look at some details.

First, there are apparently some differences from US practices:

"European regulations recommend to use organically produced animal manure but allow the use of conventionally produced manure, provided that it is not the output of 'factory farming'."

I don't think we have that restriction in the US. Second, they found that:

"More than 80% of nutrient inflows through manures (82%, 85% and 81% for N, P and K, respectively)... came from conventional farming"

So 82% of N in manure came from conventional farms, but that was only 23% of estimated total N supply. The rest (almost 60% of total N inputs) was mostly estimated to come from biological nitrogen fixation:

"N fixation was estimated using the model proposed by Høgh-Jensen et al (2004)... based on the total N amount in leguminous crop biomass, multiplied by the ratio between the amount of symbiotically fixed N and the total N amount in the crop biomass."

That seems to be about all they said about biological nitrogen fixation -- I would have liked a little more detail. They also say that unsustainable "soil nutrient mining" (failure to replace nutrients exported in crops or otherwise lost from soil) was "not considered." A simple approach to that problem would have been to estimate nitrogen and phosphorus contents of products sold off-farm, and compared that to total inputs. If inputs exceed outputs, some of the difference may end up polluting groundwater, although some may be accumulating in soil, at least for a while. If outputs exceed inputs (including biological nitrogen fixation), over the long term, they will not be able to continue farming over the long term.

My conclusion is that organic farms that rely on manure from other farms (or on manure from their own animals, which are fed grain or hay from other farms) are dependent on conventional farms and therefore not a model that can be scaled up. That doesn't mean they are not making a positive contribution, if they are using manure that would otherwise end up polluting rivers.

If we can believe the nitrogen-fixation estimates in this paper, French farms are already getting a majority of their nitrogen from symbiotic rhizobia in the root nodules of their legume crops and forages. Maybe that could be scaled up. But if 75% of their phosphorus is coming from manure from conventional farms anyway, there may be little point in trying to replace the nitrogen in that manure with biological nitrogen fixation.

More reviews!


Janet Sprent, whose research I've admired for decades, reviewed my book in the Bulletin of the British Ecological Society. I couldn't find a web link to the review. She writes that "not all readers will agree with the arguments against these holy cows [perennial grain crops] but they deserve serious attention." Given our shared interest in nitrogen fixation, she was surprised by the lack of discussion of nitrogen-fixing cereals. But the book was already long enough to keep her "fully occupied on a 13 hour flight."

I probably could have lumped nitrogen-fixing cereals with C4 rice: both are big enough changes that we can't assume they have already been "tested and rejected by natural selection", but both may be "beyond anything humans today could design and implement from scratch." I may have to modify the latter statement for C4 rice after seeing what progress they've made at the International Rice Research Institute, next month, although copying other C4 plants isn't the same as designing a new photosynthetic system "from scratch." Making nitrogen-fixing cereals might be even more difficult, however, as I have discussed on my other blog.

Chris Smaje, a regular commenter here, reviewed my book for Permaculture Magazine (link to docx file here) and separately on this blog, Small Farm Future. Both reviews are examples of the kind of thoughtful discussion I hoped to generate with the book. He wrote:

"I suspect that it's ultimately impossible to create any kind of agriculture that can usefully be regarded as 'natural', but the further we depart from it the more we're flying blind..."
Similarly, I wrote (p. 74):
"the more we depart from nature, the more we enter unexplored territory, with possible unknown risks."
Still, the quantitative comparisons in Chapter 6 are consistent with my theoretical argument that it may be possible to improve on the overall organization of natural ecosystems. For example, crop rotation may be a good idea, even though natural ecosystems rarely have such dramatic changes in plant species from one year to the next. In contrast, Chapter 5 argues that making simple, tradeoff-free improvements in individual-plant traits like drought resistance will be much harder, even with biotechnology. This is because natural selection has tested individual traits competitively against alternatives, over millennia. Meanwhile, no natural process has consistently improved overall ecosystem organization on that time scale -- see previous post.