Biogeochemical Cycles

We have previously brought up elements of below-ground nutrient cycling like (i) legumes & nodulation, (ii) rhizophagy, (iii) and an introduction to the soil food web.  We are now going to "dig" deeper into these concepts to help us understand nutrient cycling and its relation to food production and water health.

What are Soil Aggregates?

NUTRIENT CYCLING 

There are several plant-available nutrients that are important to the overall health of a soil. The three major nutrients of importance are N, P, and K. Nutrients occur naturally in soils in mineral form, through biological inputs, atmospheric deposition, and the application of fertilizers. Nutrients are lost from systems through processes like runoff, water solubility, plant uptake and leaching. When managing a soil for plant production, it is important to understand nutrient cycles, availability, and potential for loss.

Nitrification 

• Nitrification is the oxidation of reduced forms of N, typically ammonium (NH4 + ), via nitrite (NO2 - ), to nitrate (NO3 - ). The process begins with autotrophic and chemolithotrophic bacteria oxidizing ammonium generated by organic matter mineralization and fertilizer addition, producing nitrate and acid, leading to soil acidification. The process of nitrification determines the relative amounts of different inorganic N sources available for plant and crop growth and is responsible for significant loss of added N fertilizer. 

Denitrification 

• Denitrification involves the reduction of nitrate to nitrite, nitric oxide, nitrous oxide and nitrogen gas. This process is catalyzed by bacteria, archaea and fungi, and is essential for returning N to the atmosphere. 

Nitrogen Fixation 

• Nitrogen fixation is the microbial conversion of nitrogen gas (N2) to ammonium (NH4 + ). Various bacteria in plant roots perform this process. Recently, soil microbiologists have reclassified all of the microorganisms that fix nitrogen. The following list provides the new classification of these microbes and the plants that they infect. 

Mineralization 

• Mineralization is the conversion of organic nitrogen to inorganic ammonium (NH4 + ). 

Immobilization 

• Immobilization is the assimilation of ammonium (NH4 + ) and nitrate (NO3 - ) into tissue 

Volatilization 

• Volatilization is the chemical conversion of ammonium (NH4 +) ions to ammonia gas (NH3) in high pH soils and is accelerated by drying of the soil.

Compared to other macronutrients, like S and Ca, the concentration of phosphorus in the soil solution is very low, generally ranging from 0.001 mg/L in infertile soil to 1 mg/L in heavily fertilized soils. Plant roots absorb P dissolved in soil solution, mainly as phosphate ions (HPO4 2- and H2PO4 - ), determined by the soil pH. In strongly acid soils (pH 4-5.5), H2PO4 - dominates, while alkaline soils contain mostly HPO4 2-. Phosphorus is lost from the soil by plant removal, erosion of P-carrying soil particles, P dissolved in surface runoff water and leaching to groundwater. Additions of P to the soil from the atmosphere are small but may balance losses in undisturbed ecosystems. For optimal crop production, input from fertilizer to exceed the removal in crop harvest may be required. Phosphorus held in organic forms can be mineralized and immobilized by the same general processes that release N and C from SOM: immobilization and mineralization. Net immobilization of soluble P is likely to occur if residues added to the soil have a C/P ratio that is greater than 300:1, while net mineralization is likely if the ratio is below 200:1. These processes are influenced by the same factors that control general decomposition of SOM: temperature, moisture, and tillage.   

In contrast to phosphorus, potassium is found in comparatively high levels in most mineral soils, yet the quantity of K held in an easily exchangeable condition is often very small. However, over time K can be released to exchangeable and dissolved forms in the soil solution and can be quickly taken up by plants. Potassium is readily lost by leaching, which can be reduced by increasing the cation exchange capacity (CEC) of the soil.CEC is the attraction of positively charged potassium ions to the negatively charged cation exchange sites on clay and organic matter. Liming an acidic soil to raise pH can reduce the leaching losses as well. Plants take up very large amounts of K so biomass removal is another source of K loss from soils. This situation is increased by luxury consumption, or the tendency of plants to take up soluble potassium in excess if sufficiently large quantities are present. Forms of potassium in soils, as shown in the graphic above, include K in primary mineral structure (unavailable to plant uptake), nonexchangeable K in secondary minerals & compounds (slowly available), and exchangeable K+ on soil colloids and K2O soluble in water (readily available, only 1-2% of total soil potassium).

THE CARBON CYCLE

The carbon cycle illustrates the role of soil in the global C cycle. There is more C stored in soil than in the atmosphere and above-ground biomass combined! Soil C is in the form of organic compounds originally created through photosynthesis, in which plants convert atmospheric CO2 into plant matter, and enter the soil system as plants and animals die and decompose. Soil organisms consume the organic matter extracting energy and nutrients and releasing water, heat, and CO2 back into the atmosphere. If organic matter is added to the soil at a faster rate than organisms convert it to CO2, C will gradually be removed from the atmosphere and sequestered in the soil. Cultivation aerates the soil, triggering increased biological activity, and therefore rapid decomposition, loss of SOM and the release of CO2 into the atmosphere. Most soil C losses occur in the first several years after cultivation begins, and currently farmers and scientists are interested in reversing that effect by increasing C stored in the soils through management. (Source: NRCS East National Technology Support Center)

NUTRIENT AVAILABILITY 

In addition to nutrient cycling, the actual availability of those nutrients to the plants is critical to consider when assessing soil health. Certain nutrients are only available under specific soil conditions. Soil pH is especially important when it comes to nutrient availability. Consider the following figure: The width of the bar indicates the relative range of availability for each nutrient at various pH levels. Thicker bar widths reflect increased availability, and thinner sections of the bar reflect conditions at which nutrient availability is decreased. 

Soil Organisms 

Soil organisms usually comprise much less than 0.5 % of the dry soil mass. Practically nothing can be done to permanently alter the population of a soil. Normally, the soil contains a vast array and abundance of organisms. Consequently, most direct additions of organisms (inoculation) to this community will be ineffective. Usually populations will be altered more by changing the crop growing in the soil and how it is grown than by adding other organisms.  The average biomass distribution in an acre-furrow-slice of soil is approximately as presented in Table 1.

A number of conditions affect populations in soils. The optimum temperature range for decay organisms is between 70-100 °F (about 20-40 °C). Soil temperatures outside this range will retard the activity of most soil organisms. Excessive water in soil reduces the numbers and kinds of living organisms due to poor aeration. However, at low moisture levels soil organisms thrive better than do higher plants. Numbers of fungi, bacteria and actinomycetes vary with soil pH. If pH of the soil is <6.0, the fungi become the dominant soil microorganisms. The supply of nutrients, organic material for energy, and free oxygen gas affect microbial numbers. Fortunately, optimum soil conditions for most plants and for most soil organisms are similar. 

Tullgren Funnel (AKA Berlese Funnel)

https://mississippientomologicalmuseum.org.msstate.edu/collecting.preparation.methods/Berlesefunnel.htm