We have just finished a review on N mineralisation potential of soils world wide. The rule of thumb as suggested by Paul Hepperly seems a good estimator. We find similar values in the range of 1-3% of total nitrogen to be potentially released from tropical and humid subtropical soils, whereas temperate soils potentially release 10-15%. The actual release per year is then a function of soil moisture and temperature mainly, land use and related C input altering the release significantly.

As a rule of thumb 1 to 3 % of the total nitrogen is mineralized on a yearly basis. The ability of nitrogen to be metabolized is dependent on several factors such as temperature and moisture conditions and the availability of organic carbon carbon to fuel the microbes which break down the organic nitrogen ammonifying and nitrifying the organic source which is many faciltated by Nitrosomas and Nitrobacter bacteria. Increasing the total Carbon and Nitrogen and the temperature are going to facilitate optimum relaease of organic nitrogen and its conversion into nitrate. Highest release of mineralized nitrogen occurs at the longest day length where the factors we take about are all optimized. Under an aerobic environment bacteria will denitrify nitrates and nitrites converting them to gasesous nitrous oxide which is loss to the atmosphere as a greenhouse gas. Bacteria that denitriufy use the oxidized nitrogen for a source of oxygen in anaerobic metaboilism. Incubation soil with covering of water and oil will create a denitrying anaerobic environment. The difference of N in the original soil and the determination after incubation under the controlled anaerobic conditions gives the potentially mineralized nitrogen value. The 0.18% total nitrogen will be nitrogen limited for crops such as maize. When maize is 15 cm high soil can be measured for nitrate content above 25 ppm Nitrate response to additional nitrogen will not be dramatic if the soil test is below 25 and especially below 20 ppm the application of nitrogen by side dressing is recommendied the maize crop. In tropical grasses the bacteria in the rhizosphere can fix sizable amounts of nitrogen all these factors make the prediction of how much nitrogen might be mineralized quite messy. 

When the soil is cultivated for the first time  there will be a relatively large release of N and loss of C. 

In tropical rice growing soils of Asia,3 to 4% of total N is mineralized annually. This information is based on our own research in Philippines and India.

Nitrogen mineralisation mainly depends upon the conditions that are good for microbial activity, such as warm temperature (25-35 degree Celsius) and moist soil. The rate at which N becomes available is determined by the complexity and stability of the organic matter and by microbial activity. In arid soils of India, we find that the rate of N mineralisation does not exceed 3% annually under field condition.

 My experience is different than Dr. Hepperly and Dr. Kundu. I found N mineralization varied from 2-3% of total N in temperate soil (Kader et al, 2010. Soil organic matter fractionation as a tool for predicting nitrogen mineralization in silty arable soils, Soil Use and Management 26(4):494 - 507) while it varied from 3-15% of total N in sub-tropical paddy soil under aerobic condition and 4.5-28%f total N under anaerobic condition(Kader et al, 2013. Nitrogen mineralization in sub-tropical paddy soils in relation to soil mineralogy, management, pH, carbon, nitrogen and iron contents. European Journal of Soil Science 64(1):47-57). However, both the study were conducted under controlled environment at lab condition.

Please check the resources related to your topic

https://www.frontiersin.org/articles/10.3389/fpls.2016.01038/full

We have just finished a review on N mineralisation potential of soils world wide. The rule of thumb as suggested by Paul Hepperly seems a good estimator. We find similar values in the range of 1-3% of total nitrogen to be potentially released from tropical and humid subtropical soils, whereas temperate soils potentially release 10-15%. The actual release per year is then a function of soil moisture and temperature mainly, land use and related C input altering the release significantly.

Thanks dear peers for the information.

Nitrogen availability is related to amount of Carbon and Nitrogen in soil and the biological activity in the soil all of which are inter related. When a legume is planted the soil reserve of both Carbon, Nitrogen and the biological activity are all increased. At a 1% or 3% mineralization will lead to 100 to 300 kg/ha/yr of Nitrogen when the reserve was at the same 10,000 kg/ha level. At a 1% mineralization the cereal crop would be compromised for yield potential while at 3% and the same SON there would sufficient SON to optimize the yield of crops such as maize, rice and wheat our dietary staple cereals. When soil organic matter is increased both the gross amount of soil nitrogen and the conversion rate are improved. This gives support to the critical roles of legumes in the cropping system as Soil Organic Carbon, Soil Organic Nitrogen and biological activity or conversion through the increased soil respiration all increase in tandem. The ability to supply optimized nitrogen can be addressed directly by nitrogen additions but also indirectly by using legumes to increase soil nitrogen, soil organic carbon and soil biological activity capacity.

Tassadduq Rasool.