Oxalic Acid and Succinic Acid Mediate the Weathering Process of Granite in the Cold-Temperate Forest Regions of Northeast China

Granite is an important soil-forming rock and is widely distributed in the cold-temperate forest area of northeast China, such as northern Greater Hinggan, where its biochemical weathering plays an important role in the formation of local soils. Due to the special latitude and elevation, lichen plays a crucial role in the biochemical weathering of granite here, of which the role of low molecular organic acids (short for organic acids) is of interest. We simulated the concentrations of oxalic acid (OA) and succinic acid (SA) in the local lichens of Northeast China, applied OA or SA (10, 20, 30, 40, and 50 mmol L^–1) to granite powders for 1 (10 min), 10, 20, 30, 40, 50, or 60 days, and studied the effects of organic acids on the bio-weathering process of granite, such as the dissolution regularity of various ions and the morphological changes of rock surfaces. OA and SA both induced the weathering and dissolving of granite, significantly promoted the release of Na⁺, K⁺, Al³⁺, Fe³⁺, Mg²⁺, Mn²⁺, Ca²⁺, and \({\text{SiO}}_{3}^{{2 - }}\) from granite powders. For the ion dissolution, the effect of OA was stronger than that of SA. Infrared analysis showed that both organic acids did not change the structures of their own groups, and the weathering mechanism was mainly the complexation of organic acids. The results of electron microscopy also presented the dissolution of granite powders by organic acids, and the effects were enhanced as the treatment time increased. After organic acid applications, the concentrations of dissolved ions from the granite powder varied with the type and concentration of organic acids, and treatment time. The concentration of each ion usually reached its maximum at 50 or 40 mmol L^–1 OA, and 50 mmol L^–1 SA (except for Fe³⁺ of some treatments). At day 1, the concentrations of \({\text{SiO}}_{3}^{{2 - }}\), Al³⁺, Fe³⁺, and Mn²⁺ treated with OA were higher than those of other times. Compared with SA, OA could dissolve more ions from granite powders, especially at day 1. However, the metal ions dissolved by OA were easy to complex with OA, so the concentrations of ions treated by SA were higher than those of OA, and the ions dissolved by OA would enter into the soils more frequently in OA–metal complexes.