UNDERSTANDING THE NITROGEN CYCLE
by Todd Dalotto
A core concept of sustainable cannabis agriculture is viewing the farm as an ecosystem within a broader ecosystem. Crop management is much more than just providing for the needs of one plant species–it’s managing soils to optimize the habitat of mutualistic soil organisms, managing irrigation to favor aerobic microorganisms, and managing nutrition to maximize the roles of soil microbes and mycorrhizae to break-down organic molecules and atmospheric gasses into plant-utilizable nutrients.
The cannabis industries are centered around growing plants, so it’s vital to understand how plants play a central role in the continuum of atmospheric, pedospheric, lithospeheric, hydrospheric, and biospheric interactions. The number of these dynamic interactions is seemingly infinite, so we will focus our attention on the Nitrogen Cycle to understand how management of soil, plants, microbes, and inputs affects nutrient efficiency.
THE NITROGEN CYCLE
1. Nitrogen (N) is considered a plant macronutrient because N atoms are part of lipid, protein, and nucleic acid (DNA & RNA) molecules that are abundant in every plant cell. N is very plant mobile, so if there is a deficiency in the soil, older plant leaves will gladly give up their N to provide for the nutritional needs of new growth shoots–just as good parents give up their life force for the good of our children. This is why N-deficiency symptoms show as chlorosis (yellowing) of older leaves.
2. When plants die or drop leaves, the tissue becomes organic matter (biomass), consisting of mostly organic N, to be consumed and transformed ultimately to plant- utilizable forms of N. Decomposing plant roots are also a significant source of organic N and carbon in the soil.
3. Nitrate (NO3-) is taken-up and utilized by plants more efficiently than any other molecular form of N, which is why inorganic (mineral) fertilizers are an effective means of rapid plant growth, although at a cost of reducing beneficial soil microbes and increased risk of nutrient burn. Some mineral fertilizers are certified-organic because they are synthesized from plants, however they don’t contain any organic N. If the fertilizer solution is clear, it is not chemically organic. N in organic matter/fertilizers are broken-down from organic N by soil microbes and made available plants by the next two processes:
4. Mineralization: Soil microbes decompose organic matter/fertilizer and transform organic N into ammonium (NH4+), and then into Nitrate (NO3-).
5. Nitrification: Aerobic soil bacteria and archaea oxidize ammonia (NH3) and ammonium (NH4+) into nitrite (NO2-), and then oxidizes nitrite into nitrate (NO3-)
6. When wood chips and incompletely-decomposed organic matter are visible in soil or media, a high carbon (C) to nitrogen ratio exists and will cause immobilization, which transforms plant-available N into organic N (the opposite of mineralization) because soil microbes are consuming N in order to decompose carbon-rich matter. When there is a high C:N, you may see N-deficiency symptoms, even with otherwise sufficient N-fertilizer is added.
7. Soil organisms not only consume and release N as described above, but also release organic N when their own bodies die, to be decomposed by living soil organisms.
8. Good irrigation and soil management results in a healthy balance of water:air in soil pore space, which promotes soil microbes to consume atmospheric N2 gas, break the N-N triple bond (which is too strong for plant cells to break), and form nitrate. Water-logged soils cause microbes to release N2 gas, among other undesirable effects, including favoring pathogenic microbes over beneficials.
9. Atmospheric N2 gas is also transformed into nitrate by rhizobium, which are N-Fixing bacteria that have a mutualistic relationship with plant roots (particularly with legumes). Plant roots form nodules, where rhizobium lives and enjoys the carbohydrates fed by the roots. In exchange, the bacteria transform N2 into nitrate and release it at the root nodule. N-fixation is one of many benefits of cover-cropping with legumes (bean and pea family).
10. Clay particles and aggregates are mostly negatively-charged, so clay plays a particularly useful role in the cation-exchange of positively-charged ammonium (NH4+). Ammonium is held by clay colloids tightly-enough to not leach easily, but loosely-enough to move through the soil towards areas of lower NH4+ concentration (diffusion) in the rhizosphere, then transformed into nitrate by microbes (nitrification), and taken-up by plants. When over-fertilization occurs, ammonium returns to the lower-concentrated clay colloid (diffusion), and leaching occurs when cation exchange sites overflow, causing water pollution.
11. Animals (humans included) are an important part of the Nitrogen Continuum. Just like microbes, we consume plants and other animals, break-down organic N in the lipids, proteins, and nucleic acids of our food, and transform it (with the help of our intestinal microbes) into organic N in every cell in our bodies, as well as in our feces. When animals die and return to the soil, our organic N becomes the next meal for soil microbes, which break it down eventually into plant-utilizable nitrate.
12. Although not a significant source of nitrogen for plants, it is notable that lightning produces gaseous nitric acid (HNO3).
13. Plants can’t utilize the gaseous nitric acid or dinitrogen from the atmosphere directly, but they can utilize gaseous ammonia (NH3), which is taken-in through stomata (gas- regulating leaf pores) from the atmosphere. Gaseous ammonia is also released in small quantities by plants.
14. Fertilizer manufacturers recapture gaseous ammonia from the atmosphere to be used in the manufacture of inorganic nitrogen fertilizers, which can be thought of as a sustainable practice.
15. Methane gas (CH4, a fossil fuel) is used to produce ammonia, which is the precursor to numerous other inorganic fertilizers and pesticides, such as urea, ammonium nitrate, and anhydrous ammonia.
17. Over-fertilization combined with poor soil & irrigation management leads to soil erosion and eutrophication of surface water, which leads to toxic algal blooms and other pollution in oceans, lakes and rivers.
18. The combustion of fossil fuels produces toxic gasses such as nitrous oxide (N2O) and nitric oxide (NO).
19. Denitrification is a microbial process that transforms nitrate into gaseous dinitrogen, which is not a harmful product. However, it may be seen as an unsustainable process because it results in the loss of plant-utilizable nitrate and is partially the result of over watering and over-fertilization.
The entire cannabis industry flows downstream from farms, so applying this understanding of the good, the bad and the ugly of the Nitrogen Cycle, and how plants and soil microbes play a central role, can help purify the headwaters of our industry.