Enriching the Earth

Vaclav Smil presents data and poses serious questions for any attempt at imagining a world less reliant on fossil fuels, incl. agriculture, which is currently a major contributor to global warming and the climate disasters. If there is something missing in the book it is a more thorough review of the available data on nitrogen-fixing woody plants, specifically trees. He constantly compares the production of synthetic nitrogen in the form of ammonia to the potential productivity of green manures such as clover and other leguminous plants which are almost always annual nitrogen crops destroyed at the moment of harvest. Nitrogen-fixing trees do not die when they transfer nitrogen via the decomposition of leaves or roots. Importantly, trees can be integrated into other forms of agriculture, often with the result of enhancing the former crop. This can take many shapes like intercropping, alley cropping, silvopasture, woody agriculture and other tree-supported cropping systems. E.g. in Greece alone, around 1.6 million acres of olive trees are intercropped with annual crops and grapes. Farmers see many benefits to these kind of systems, and after the initial high investment (which is the primary reason this has not mainstreamed more), farmers often end up with a more efficient form of agriculture. What if the tree species was a nitrogen fixing species like Alder, Black Locust, Mesquite? (There are hundreds of species). Other than fixing nitrogen, trees could have other uses like shelter, food or fodder or eventually a timber crop. There is no doubt that nature cannot compete with the productivity of the Haber-Bosch process - but as long as we compare synthetic nitrogen to annual green manures cultivated alone, we might not get the proportions right. We need to compare synthetic fertilizer with the most up-to-date agroecological alternatives to know which forms of agricultures could reverse global warming and here we have to include tree-supported cropping systems.