Nitrogen vs Carbon (written about 2007)

Prepared by Jim Burnett (deceased)

(Founder of Texas Earth, Idalou, Texas - https://www.texasearth.com/)

For decades, the conventional thinking has been that applications of synthetic nitrogen fenilizers help enhance soil carbon levels by stimulating soil microbes to feed on organic matter from crop residues.  New research indicates that, in fact, the opposite may be true.

A group of scientists at the University of lllinois says that research from the Morrow plots, the oldest  research plots ln the country, is indicating a decline in soil carbon from the use of synthetic nitrogen fertilization. The way it works is that the plant residues are left behind in crop production, and various tilling and residue management methods, make use of that residue as a means of adding organic matter to the soil. What we have understood about synthetic nitrogen is that the soil microorganisms are stimulated by it, as an additional food source, which causes these microbes to increase their activities in breaking down plant residues and adding to the humus/carbon content of the soil. What is being discovered now, however, is alarming. Soil microbes degrade plant residues and reduce their carbon content and nutritional content into plant available forms and long-term fully degraded carbon, which is the backbone for forming soil humus. The other result of this microbial activity is that when microbes feed on carbon and plant residues, and as their bodies die, they release the carbon as CO2. Volatilization of this rapid cycling carbon from plants may be the larger part of what happens in this process, so as high inputs of nitrogen stimulates the microbes to feed, eventually the organic matter disappears before it can become humified, The pathway of humification is interrupted by removal in volatitization of carbon before it reaches the form of humus. "Over time, this enhanced microbial appetite outweighs the benefits of more crop residues", says Tom Philpott of Grist.org. 

The last aspect of this carbon loss is explained by taking a look at carbon based organic acids in the soil: humin, hurnic acids and fulvic acids. Due to the acceleration of microbial oxidation of humin by nitogen stimulation, these acids are reduced from the carbon to rich humin, to less carbon-rich humic acids, and  finally to fulvic acids, which have very little carbon content. This process is natural and healthy, but when the soils lashed with excessive amounts of nitrogen, the process is accelerated dramatically, allowing less time for new carbon inputs from residues and proper humidification of organic matter to occur. The net effect of this is that soil carbon levels decline, making it even more difficult for soils to store nitrogen. As the ability of the soil to store nitrogen decline, the more nitrogen inputs are needed.  Soil tilth, water holding capacity and nutrient retention also suffered causing compaction and leaching of nutrients. Thus, the vicious cycle is born.

New information from three University of lllinois professors Richard Mulvaney, Saeed Khan and Tim Ellsworth have been raising some eyebrows with new data test that testifies to this. ln two recent papers, "The Myth of Nitrogen Fertilization for Soil Carbon Sequestration" (Khan et al 2007) and "Syr6[hetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Prodttction" (R.1. Mulvaney et al 2009) The researchers show that the net effect of synthetic nitrogen use is to reduce or soil carbon levels. The proposed mechanism is stimulation of the soil microbes by nitrogen fertilizer fertilizers, causing the macros to consume excessive amounts of organic matter. To make matters worse, inputs of synthetic nitrogen create tough competition from niffogen fixing bacteria populations like Rhizobium and Azospirillum. Adding synthetic nitrogen is highly stimulatory to microbes that feed on it. These microbes then outcompete the nitrogen-fixing bacteria, making nitrogen from the atmosphere even less accessible to the crop.

 How important are the nitrogen fixing bacteria? Rccording to the A&L Agronomy Handbook, "lt is estimated that 35,0001bc of nitrogen are available over a single acre of land. ln return for the supply of food and minerals they get from the plant, these nitrogen fixing bacteria supply the plant with part of its nitrogen needs, generally not more than 50 to75% of it", What we do know is that nitrogen inputs are important in crop and turf production, however its suggested that nitrogen inputs be managed much, much more carefully, Also, recommended is the use of humates to restore soil carbon in its most active form. Applications of humates will not only increase soil carbon levels, but will improve water retention, drainage, soil tilth and nutrient retention. Humates also provide a healthy substrate for beneficial soil microbes. Research has shown that soils with less than 3% organic matter can lose 15% to 4O% of nitrogen in a growing season. Anchoring nitrogen in the soil is part of the important job of sole carbon/humus, as nitrogen binds readily with these carbon-based acids such as humic and fulvic, This   reduces the need for heavy, extensive inputs of nitrogen fertilizers. Moreover, the simulation of symbiotic and free-living and nitrogen-fixing bacteria by humic substances adds that availabitity of nitrogen from the atmosphere, supplying in many cases up to 75% of crops needs.  By applying humates to soils where synthetic nitrogen and other fertilizers are used, we can restore soil carbon levels, balance the ratio of carbon to nitrogen and break the vicious cycle we have created in most conventional production soils. Humates are a direct input of soil organic matter, providing the most important aspect of a healthy productive soil.

R.L. Mulvaney, T.R. Ellsworth and C.W. Boast, 2007, "The Myth of Nitrogen Fertilization for Soil Carbon Sequestration" published in J Environ Qual 35:1821-1832 (2007). R.L. Mulvaney*, S.A. Khan and T.R. Ellsworth, 2009, "synthetic Nitrogen Fertilizers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production" published in J Environ Oual38:2295-2374 (2009)" Tom Phitpott, 2010, "New Research: Synthetic Nitrogen DestroysSoil Carbon, Undermines Soil liealth".