Predicting soil nitrogen availability

Nitrogen fertilizer is often the single-most expensive input into a farming system. Lack of nitrogen limits crop yield, so getting enough to meet the high plant demand is important. However, applying excess nitrogen leads to loss to the environment, either as runoff, leaching, or gaseous emission.

Measuring the organic nitrogen naturally available in soil can help farmers decide how much additional fertilizers to apply in a growing season. This research is focusing on soil-test biological activity to predict soil nitrogen availability. Optimizing nitrogen fertilizer application rate can reduce nitrous oxide (N2O) emissions and reduce the overall carbon footprint of the farming system. The carbon cost of nitrogen fertilizer manufacture is about 1.2 pounds of carbon per pound of nitrogen. Additionally, N2O emission is about 300 times more potent than carbon dioxide (CO2).

Conservation agricultural systems can significantly increase the accumulation of biologically active soil and organic nitrogen, reducing the need for additional nitrogen fertilizer input.

These conservation systems are more adapted to drought and excessive heat conditions, increasing the overall resiliency of farming.

What type of research is being conducted?

On-farm trials are being conducted with farmers in the region to assess how their management has influenced soil biological activity and the supply of nitrogen with this condition. Trials have included strips of different nitrogen rates using the equipment of farmers and small plots with hand-applied nitrogen at different rates to obtain yield response curves. Field trials have been supplemented with laboratory incubations and greenhouse growth studies to support concepts.

Matching yield responses with soil-test biological activity and soil nitrogen supply has been the goal to obtain field calibration of the concept.

What research results have been found so far?

Laboratory studies supported the role of soil-test biological activity in predicting nitrogen availability

Soil nitrogen mineralization can be predicted with soil-test biological activity
Soil texture does not alter the relationship between soil-test biological activity and nitrogen mineralization
A large quantity of mineralizable nitrogen in surface soils is possible with conservation management
Soil-test biological activity is a rapid and reliable indicator of soil nitrogen availability

Source: Franzluebbers, A.J., Pershing, M.R., Crozier, C., Osmond, D., & Schroeder-Moreno, M. (2018). Soil-test biological activity with the flush of CO2: I. C and N characteristics of soils in corn production. Soil Science Society of America Journal, 82, 685-695.

Greenhouse growth studies validated that nitrogen was a limiting factor to plant growth; soil-test biological activity identified the limitation

Grass growth in the greenhouse was dependent on soil nitrogen mineralization
Soil-test biological activity was a valuable indicator of nitrogen mineralization
Biological activity, residual inorganic nitrogen, and total nitrogen were most important in predicting growth

Source: Franzluebbers, A.J., & Pershing, M.R. (2018). Soil-test biological activity with the flush of CO2: II. Greenhouse growth bioassay from soils in corn production. Soil Science Society of America Journal, 82, 696-707.

Corn yield response to nitrogen fertilizer was predicted by soil-test biological activity

Relative corn yield was associated with residual inorganic and mineralizable nitrogen
Soil-test biological activity was a good surrogate for predicting nitrogen availability
Economically optimum nitrogen fertilizer requirement could be adjusted with soil testing

Source: Franzluebbers, A.J. (2018). Soil-test biological activity with the flush of CO2: III. Corn yield responses to applied nitrogen. Soil Science Society of America Journal, 82, 708-721.

Soil-test biological activity is a sensitive indicator of soil biological health

Soil-test biological activity was an effective short-term indicator of cumulative carbon mineralization and basal soil respiration across a diversity of soil textures, soil organic matter contents, soil types, and geographies
A standard methodology was provided in detail
The flush of CO2 following rewetting of dried soil should be considered a rapid, robust, and reliable method to indicate soil biological activity, and should be a major component of any soil health assessment

Source: Franzluebbers, A.J. (2018). Short-term C mineralization (aka the flush of CO2) as an indicator of soil biological health. CAB Reviews, 13, 017.

Soil-test biological activity was validated as a predictor of corn yield response to nitrogen fertilizer on 111 more fields

Soil nitrogen mineralization is a biological process predictable from carbon mineralization
Economically optimum nitrogen rate is a function of soil-test biological activity
Cost-to-value thresholds must be a part of any fertilizer recommendation system
Soil-test biological activity is a key metric of soil health condition
Farmers who improve soil health can reduce nitrogen fertilizer inputs

Source: Franzluebbers, A.J. (2020). Soil-test biological activity with the flush of CO2: V. Validation of nitrogen prediction for corn production. Agronomy Journal, 112, 2188-2204.

Nitrogen fertilizer recommendations using soil-test biological activity resulted in better economic return than standard approaches

Soil-test biological activity was an effective indicator of soil nitrogen mineralization
Soil-test biological activity can predict fertilizer nitrogen need for corn in North Carolina and surrounding states
A soil-based system led to equal or greater economic returns than other systems
Economic benefit was greatest for fields managed with conservation agricultural approaches

Source: Franzluebbers, A.J., & Shoemaker, R. (2020). Soil-test biological activity with the flush of CO2: VI. Economics of optimized nitrogen inputs for corn. Agronomy Journal, 112, 2848-2865.

The need for nitrogen fertilizer on fall-stockpiled tall fescue was predicted by soil-test biological activity

Soil biological activity is a reliable indicator of soil nitrogen availability
Nitrogen fertilization of fall stockpiled tall fescue should be adjusted based on soil testing
The flush of CO2 is a robust indicator of soil biological activity
A new paradigm of soil testing based on soil biological activity is possible

Source: Franzluebbers, A.J., Pehim-Limbu, S., & Poore, M.H. (2018). Soil-test biological activity with the flush of CO2: IV. Fall-stockpiled tall fescue yield response to applied nitrogen. Agronomy Journal, 110, 2033-2049.

Soil-test biological activity was validated as a predictor of tall fescue yield response to nitrogen fertilizer on 37 more fields

Soil-test biological activity was validate as a good indicator of soil nitrogen availability
Nitrogen fertilizer on stockpiled tall fescue could be based on soil-test biological activity
Cost-to-value threshold should be considered for nitrogen fertilizer recommendations
Nitrogen was recycled in pastures through mineralization of organic matter
Stockpiled forage mass did not respond to phosphorus fertilizer application at any location

What remains to be done?

Additional field testing is underway for cotton production in North Carolina and other Cotton Belt states
Field data from wheat production need to be summarized
Commercialized soil testing based on soil-test biological activity is needed
Revised nitrogen fertilizer recommendations need to be adopted

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