Mechanisms for increasing soil resistance to acidification by long-term manure application

Abstract Soil pH buffering capacity (pHBC) plays a crucial role in determining soil acidification rates and the amount of lime required to ameliorate soil acidity. However, it remains poorly understood how soil organic matter affects soil pHBC. Here, Alfisol and Ultisol samples from long-term fertilization experiments (control, chemical N, P, and K (NPK), manure only (M), 1/2 NPK plus 1/2 M (1/2NPKM), and NPK plus M (NPKM)) were used to investigate the effects and underlying mechanisms of manure application on the pHBC and soil resistance to acidification through simulated acidification experiments. The results indicated that the application of manure increased pHBC and the resistance of soils to acidification in both the Alfisol and Ultisol. The pHBC of the Alfisol in the M and NPKM treatments was increased by 81 and 60% compared with the control, respectively. Similarly, the pHBC of the Ultisol in the NPKM treatment was 66% higher than that of the control. The extent of protons consumption by the Alfisol followed the order: M > NPKM > 1/2NPKM > NPK ≈ control during stimulated acidification, which was consistent with their pHBC. These results suggested that manure application increased the resistance of the Alfisol to acidification by increasing soil pHBC. The protonation of organic anions from the dissociation of weakly acidic functional groups on soil organic matter to form neutral molecules was the main mechanism responsible for the increase in pHBC and soil resistance to acidification induced by manure application. During this process, some exchangebale base cations were released from negatively charged sites on organic matter into soil solution. This mechanism was confirmed by the experimental observations: the release of base cations from soils increased, while soil exchangeable base cations and effective CEC decreased as soil pH decreased. The results are of fundamental significance for understanding the role of organic matter in retarding soil acidification through long-term manure application.