Using quantitative differentiation method of proton sources to study the factors causing rice rhizosphere acidification

Objectives This paper is to quantitatively analyze the proton contribution and differentiate the effects of acidification processes induced mainly by the inorganic nitrogen uptake, other mineral nutrient uptake, and organic acid secretion by plant roots, which provides a scientific basis for the better nutrient regulation to control soil acidification in croplands.

Methods Using rice variety, Shenliangyou 3206 as experimental material, three series of hydroponic experiments were carried out separately. They were experiments of four NH₄⁺/NO₃⁻ ratios (1:3, 2:2, 3:1, 4:0) with total N concentration of 3 mmol/L, three inorganic P levels (0, 0.08, 0.32 mmol/L), and three Cd concentrations (0, 2.5, 5 μmol/L), all the experiments were under two nutrient solution pH 4.5 and 6.5. After 48 hours of treatment, the mineral nutrient and H⁺ content of the nutrient solution were quantitatively determined, and the difference with the initial values were used for the calculation of indexes.

Results 1) The nutrient solution pH in all the treatments, except for NH₄⁺/NO₃⁻ of 1:3 at pH 4.5, decreased after 48 h cultivation, indicating net proton release from the rice roots. With the increase of NH₄⁺/NO₃⁻ ratios, this acidification effect was enhanced. The proton was mainly originated from uptake of NH₄⁺-N, which accounted for more than 70% of total proton net release in treatments with NH₄⁺/NO₃⁻ ratio of 3:1 and 4:0. 2) P deficiency promoted the relative uptake of cations and then increased net proton release of rice root, while Cd stress inhibited the inorganic N uptake and net proton release. 3) The rice root exudates showed acid buffering capacity mainly by associating protons at low solution pH, and this capacity was enhanced with the decrease of pH or the increase of P deficiency. The results implied that the root exudates contributed to soil acidification directly in the neutral or alkaline soils.

Conclusions Based on the co-relation between nutrient uptake and induced proton release or consumption by rice roots, proton sources could be divided into inorganic N uptake, cation uptake, and root organic acid secretion. The proton sources caused by NH₄⁺/NO₃⁻ ratio, P deficiency and Cd stress were significantly reflected in their acid-inducing effects, indicating that the quantitative proton differentiation method can accurately evaluate the rhizosphere acidification effect induced by environmental factors.