Dear Roghayeh Vahedi,

Long-term effects of biochar on nutrients occur through complex physiochemical reactions with soil particles (Spokas et al., 2011). One such reaction that affects

P is biochar-induced increases in soil alkalinity (liming). In acidic soils phosphorus can be adsorbed onto iron oxides, which makes it unavailable to plants. Liming agents

reduce the concentration of iron and aluminum in the soil solution and the previously bound phosphorus then becomes available (Cui et al., 2011).

References:

Spokas KA, Cantrell KB, Novak JM et al. (2011) Biochar: synthesis of its agronomic

impact beyond carbon sequestration. Journal of Environmental Quality, 41, 973–989.

Cui H-J, Wang MK, Fu M-K, Ci E (2011) Enhancing phosphorus availability in phosphorus-fertilized

zones by reducing phosphate adsorbed on ferrihydrite using

rice straw-derived biochar. Journal Soils Sediments, 11, 1135–1141

For more details, please use the follow link which contains a review article which covers this topic:

http://www.fao.org/fsnforum/sites/default/files/resources/biederman%20review%20biochar%20yield%20and%20nutrient%20cycling%202013.pdf

Hoping this will be helpful,

Rafik

 Ms.Vahedi,good question but a bit difficult to answer. Dr Antoun has provided good reply.Dr. Karaman gave good comments and references especially the last one.To increase the bio-availability of iron or other micronutrients, one has to increase the microbial activity in soil.When biochar is amended to soil, how it affects the microbial activity?Because of large surface area it can provide anchorage to microbes. Biochar can also can hold water and provide congenial moisture regime to microbes.Can the biochar provide food/carbon source/energy to microbes to accelerate their activity?Low temperature produced biochars may contain some labile carbon.Also, it is said that because of their surface charges and surface area the biochar can adsorb or retain dissolved organic carbon.But these two sources of carbon may a limited stock.So the microorganisms have to attack the soil organic carbon or the non-degrading portion of biochar. More possibility is there for decomposing a portion or pool of soil organic matter.It appears that the benefit of biochar on microbial activity and mobilization of insoluble iron  through siderophore  or organic acid/proton production may for a short term and limited extent In long run the microorganisms have to depend on soil organic matter or other organic materials added to soil.I welcome the suggestions of microbiologists.The following reference may be consulted.

The biochar dilemma by A. Mukherjee and R.Lal Soil Research ,2014,52,217-230

http://dx.doi.org/10.1071/SR13359