Plant-Soil Food Webs Create Solar Powered Nitrogen Factories
Recently, I was invited into a discussion on the potential for “solar powered nitrogen factories”. The dialog dealt with using legumes and grasses as cover crops to increase soil fertility. This is of course, a great strategy which farmers are adopting at large scales. As the cover crops grow, they release sugars that feed soil microbes, nitrogen rich amino acids, and a long list of other good biomolecules into the soil. Often, interplanting carefully selected cover crops with the main crop results can not only eliminate the need for chemical fertilizers and increase yields of the main crop, but also supplement the grower with a second “crop,” the cover crop, which might be used to feed livestock directly, or to make silage. As I listen to the success growers are having with cover crops, I cannot help but wonder how we ever bought into the myth that feeding plants only Nitrogen (ammonium sulfates, ammonium nitrates, etc.) or even N, P, and K, was a good idea. We’ve understood nitrogen fixation for decades.
Throughout my career, I’ve heard many discussions about the potential to engineer grain crops, like wheat, to fix their own nitrogen. In the 80’s, we spoke as though nitrogen fixing grasses were just around the corner. Later, it seemed the process was more complex than initially imagined. I shudder to think of the millions that have been invested in related research since the 1980’s. But in the era of the microbiome, scientists and growers are awakening to the reality that the biggest barrier to self-fertilizing plants may be the now-dogmatic belief that crops grow better when we add manufactured forms of nitrogen. We have sorely underestimated a number of factors that work into the nitrogen availability formula. We have failed to reflect adequately on the number of species that fix nitrogen, the impact our agrochemicals and tillage practices have on nitrogen fixation dynamics, and our own inability to accurately measure nitrogen fluxes in a dynamic living system. We have also been blind to the reality that far too often, the nitrogen we add is what is making our crops prone to disease and weed infestations. No wonder we are now looking for microbial alternatives.
In terms of seeking microbes that serve as solar powered nitrogen factories, it is important to consider cyanobacteria along with rhizobia. These single celled algae include many strains that can fix nitrogen. Because they are photosynthetic, they will be most abundant on soil and plant surfaces where the sunlight can reach them. Like free living chloroplasts, the contributions of cyanobacteria to soil carbon and nitrogen cycling are too often ignored in agriculture, probably because conventional horizontal tillage and fertilization practices render soil microbes incapable of doing their job.
In a healthy soil, when nitrogen levels are low, both cyanobacteria and rhizobia increase (assuming other nutrient levels are adequate), and the system is supplied with converted atmospheric nitrogen. However, when you add nitrogen fertilizers, these “factories” shut down, and when you plow the soil, you bury the cyanobacteria, rendering them incapable of growing and fixing nitrogen.
Another problem created by adding nitrogen, especially from simple sources like ammonium or nitrate salts, is that doing so feeds soil microbes that “denitrify” the system. As the number of denitrifying bacteria increase, your nitrogen gets released back into the atmosphere where it has no value at all for the plant. The grower is quite literally throwing his or her money away.
Because nitrogen in its various forms is both critical and potentially toxic to all living cells, nature regulates it carefully, and has devised numerous mechanisms to insure that plants get enough, but not too much. When we work with these systems, rather than override them, it is very possible to grow crops without added N. Remember, the air around us is more than 78% N! Build the plant and soil microbiology, and N fertilizer becomes obsolete.
We did not always believe N fertilizer was essential. It’s use became widespread after WWII, largely because the industrial giants who made millions off weapons development (TNT, etc) needed new markets that would endure in times of peace. Large investments were made in 1) plant genetics to develop hybrids that could tolerate large amounts of nitrogen 2) demonstration plantings around the globe designed to illustrate orders of magnitude difference when N is added.
Today, we know that the only way these differences would have ever been observed would have been on if field trials were made on nutrient depleted, microbial imbalanced soils. (Read papers by Norman Borlaug, father of the green revolution). Universities followed the money, and began teaching the use of nitrogen as the “right” way to grow crops. This allowed them access to enormous streams of grant money, because the imbalances created by adding N created new problems to solve.
Hiding the truth, that a healthy soil can feed itself, served not to benefit the farmer, (how many farms have we lost since WWII) but to ensure jobs for academia, government, and agrochemical companies. Today’s successful farmers are awakening, not only to the value of soil microbes and soil health, but also to the fallacies of institutions that serve big money under the disguise of public service.