I am afraid it is not very reasonable to prepare such maps, for this reason they don't exists. The N availability depends on many factors, which are strongly correlated. In natural soil (without N fertilization) the main N source is soil organic matter. The decomposition of soil organic matter produces available N, but the rate of this process depends on soil humidity, temperature, pH etc. One soil may have different N-availability depending on the season, month, etc.  Not always soils rich in organic matter are rich in available N - high content of organic matter is frequently associated with excessive humidity.

Probably one of the main constrains in the tropics is high acidity (and Al toxicity), drought, P deficiency, and so on.

Perhaps, the amount of natural biomass could be an indicator of N and water (!) availability. In this case, also the maps of vegetation indices (e. g. NDVI) might be useful. They may be derived from satellite data (Landsat, Sentinel 2, MODIS). However, it must be remembered, that the greatest biomass may be produced in places with good supply in N and water at the same time. It is possible to find excessive N supply both in too wet and too dry environments.

Soil N fertility is the status of a soil with respect to its ability to provide plants with N in the form, amount and rate needed for optimum growth. The N fertility of a soil primarily depends on: (1) total N content in the soil, (2) N transformations in the soil, and (3) various factors that influence N absorption by plant roots. Total N concentration in the plough layer of soils thus gives a reasonably good indication of  plant N availability. For assessing N availability to rice in tropical Asia, Kawaguchi and Kyuma (1977) measured total N concentration in the plow layer of 410 rice soils sampled from 9 countries, viz. Bangladesh, Cambodia, India, Indonesia, Malaysia, Myanmar, Philippines, Sri Lanka and Thailand. A kind of map of rice nitrogen availability in tropical Asia was thus obtained from this work (Kundu and Ladha, 1999). Using similar idea/concept, it will be possible to prepare a global map of plant nitrogen availability.

According to the documentation (http://www.fao.org/fileadmin/user_upload/gaez/docs/GAEZ_Model_Documentation.pdf), the SQ1 dataset (nutrient availability) was compiled from soil texture, soil organic carbon, soil pH, total exchangeable bases. Michal and Dilip already mentioned that available N is highly variable and that most of soils N is bound to organic matter, which is part of the SQ1 dataset (soil organic carbon). So I guess the topsoil C from the HWSD (T_OC) represents total N to some degree as well, but also could be of some use as a proxy for N availability (keeping in mind the bottleneck of humus decomposition).

My computer doesn't allow to handle large datasets, so the org. C map I extracted from the HWSD for the Manaus region should suffice as an example. This region is indicated as "very severely constrained" in the SQ1 dataset.

Yes, Dr. Thomas. I agree with your opinion. The top soil C in general and organic C in particular could also be a proxy for N availability to plants. Large data sets (on organic C) available with FAO may be used to prepare a global level mapof indicative plant N availability.

look at the enormous spatial variabiliy of N availybility within a single field, then you will see that a global mape on this issue makes not much sense!

I agree with Dr Ewald Schung and Dr Michał Stępień. I don't think the map could be of much use even if prepared

Dear all, the map would be used to make global predictions, so it does not necessarily need to reproduce N availability at a very fine scale, just make sense globally based on what we know from field studies (boreal areas low in N availability, most tropical areas high, US rather low,…). So far, based on all I have read, C:N ratio seems to be the best single indicator of N availability. pH is more important for P.

If I could have access to a map of N mineralization based on field measurements and scaled up to the globe that would probably be the best indicator available

It is possible. Soil nitrogen is a matter related to soil organic matter content. As nitrogen and carbon got a definite relationships in soil and one can not be increased with out increasing the other and vice-versa, and mainly dependent on climate (temperature, rainfall etc), vegetation etc and in cultivable soil, management practices.

As Dr. Kundu referred for rice soils, this can be done for other geographical situations for other group of crops.

I agree with most of the above. By the way FAO did prepare a soil N map for top- and sub-soil on the basis of the FAO/UNESCO Soil Map of the World. See for instance under chemical soil properties in the FAO Soils Portal http://www.fao.org/soils-portal/soil-survey/soil-properties/chemical-properties/it/. The full set (for what it is worth) can be downloaded from GeoNetwork. http://www.fao.org/geonetwork/srv/en/main.home

Nitrogen is the most critical element obtained by plants from the soil and when deficient is a bottleneck in plant growth. Plants can use the nitrogen as either the cation ammonium, NH4+, or the anion nitrate, NO3-. Nitrogen is seldom missing in the soil, but is often in the form of raw organic material which cannot be used directly.  Nitrogen is also available in gas forms in the soil, however these quantities are very small and difficult to detect such as :nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide (NO2) , ammonia (NH3) and molecular nitrogen (N2) present in the air space of the soil.

Considering the enormous spatial variability of available nitrogen even on small scales, a global map on plnat N availability makes not much sens, except you think on a map wher the economical availability of the nutrient is addressed!

Yes, Dr. Thomas. I agree with your opinion