Most leguminous plants require a neutral or slightly acidic soil for growth, especially when they depend on symbiotic N2 fixation.

Bordeleau L M, Prevost D. Nodulation and nitrogen fixation in extreme environments. Plant Soil. 1994;161:115–124.

Brockwell J, Pilka A, Holliday R A. Soil pH is a major determinant of the numbers of naturally-occurring Rhizobium meliloti in non-cultivated soils of New South Wales. Aust J Exp Agric. 1991;31:211–219.

It has been recently reported (207, 309) that pasture and grain legumes acidify soil to a greater extent and that the legume species differ in their capacity to produce acids.

McLay C D A, Barton L, Tang C. Acidification potential of ten grain legume species grown in nutrient solution. Aust J Agric Res. 1997;48:1025–1032.

Tang C, Barton L, Raphael C. Pasture legume species differ in their capacity to acidify soil. Aust J Agric Res. 1998;49:53–58.

The number of nodules and nitrogen fixation got enhanced under acidic conditions, but nitrogenase activity was drastically decreased at pH 7.0.

Ferreira, T.C., Aguilar, J.V., Souza, L.A. et al. pH effects on nodulation and biological nitrogen fixation in Calopogonium mucunoides . Braz. J. Bot 2016; 39: 1015–1020.

One reason could be that leguminous plants that fix atmospheric N2 require more nutrient cations than anions to grow. This can cause exclusive root H3O+ ion efflux into the rhizosphere that eventually contributes to declining soil pH. Experts could explain more, I guess!

Greetings! I am no expert in legumes physiology but I can name several reasons for pH shift in rhizosphere of terrestrial plants. As was mentioned above by Shamim Hasan, a plant in order to maintain electroneutrality of the soil solution needs to excrete something in exchange for sorbed nutrients like Na+, K+, Ca2+, Mg2+. In the vast majority of cases this «something» is H+, which eventually leads to rhizosphere acidification.

The second reason is exudation of organic acids by roots or rhizospheric microorganisms. Exudates increase nutrient availability, regulate plant-microorganisms relations and have many other functions. Almost every organism in rhizosphere exudates something and it is often acid.

There is possibility to aluminium ion accumulation in rhizosphere due to clay mineral weathering intensification and its migration to root surface by soil water flow. It is well known that Al3+ hydrolysis shifts pH to more acidic values.

Also we should consider how soil pH was measured. Sample preparation and the choice of method have a huge impact on the result you will get.

Generally the rhizosphere environment has a lower pH, lower oxygen and higher carbon dioxide concentrations. However, exudates from the roots to maintain the metabolic equilibrium can make the soil in the rhizosphere more acid or alkaline, depending on nutrients roots are taking from the soil.