Abstract:

Red soil plays an important role in the sustainable development of agriculture and socio-economy in China. The red soil critical zone is the earth’s surface system which consists of water-soil-air-life-rock under the actions of natural and human activities in the red soil region. This paper summarizes the progresses on the acidification situation of red soil, the process and mechanism of proton (H⁺) production and consumption in the red soil critical zone, as well as the ecological and environmental effects. In the critical zone, carbon cycle is the main source of H⁺ in the natural soil acidification process. Atmospheric acid deposition (H⁺, nitrogen, sulfur) and the net uptake of base cations (K⁺, Na⁺, Ca²⁺, and Mg²⁺) by plants are the main sources of H⁺ in natural ecosystems. However, H⁺ from the nitrogen transformation process caused by chemical nitrogen fertilizer and base cations carried away by plant harvest are the main reasons for the intensification of red soil acidification in farmland ecosystem. The transformation of nitrogen in soil and the production process of H⁺ are complex. The source of nitrate ($\mathrm{NO}_3^{-}\mathrm{-N}$) in water can be quantified by using dual isotopes of nitrogen and oxygen, so as to quantify the contribution of different sources of nitrogen to H⁺ production in soil. Mineral weathering, cation exchange, iron and aluminum oxide buffering, special adsorption of sulfate and acid buffering of organic matter are important acid buffering mechanisms in the red soil critical zone. These processes are intertwined, so it is difficult to quantify individual buffering processes and the acidification rate of red soil. Based on the stoichiometric relationship between base cations and silicon released by mineral weathering, the proportion of H⁺ used for silicate weathering and base exchange in red soil regions with different weathering degrees can be distinguished, so as to better understand the difference of buffering pathways for H⁺ in red soil with different weathering degrees. Acidification will not only change the physical and chemical properties of the soil, activate heavy metal elements, cause aluminum toxicity, but also affect the growth of soil microorganisms and plants. $\mathrm{NO}_3^{-}\mathrm{-N}$ migration and deep accumulation caused by nitrogen transformation will bring potential risks to groundwater pollution. The process of H⁺ consumption can alleviate the negative effect caused by H⁺ production. The runoff water in the red soil region remained neutral, indicating that the soil consumed all input H⁺ and still had acid buffering capacity. In view of the above research status of H⁺ production and consumption in the red soil critical zone, this paper puts forward the prospects of future research and scientific issues that need to be further explored in the red soil critical zone.

Key words: acidification, acid buffering mechanism, mineral weathering, nitrogen transformation, element cycle