Regional differences in mineral weathering characteristics of zonal soils under intensive agriculture

Abstract

A good understanding of mineral evolution and soil properties in the context of agricultural activities is important for soil utilization and management. To investigate the effect various forms of intensive agriculture in different climate regions on the evolution of phyllosilicate minerals and iron (hydr)oxides, three zonal soils were studied under various long-term agriculture with different fertilizers and using fallow soil as reference state. The phyllosilicate composition and Fe species of the clay fractions (<2 μm) were determined, and the detailed mineralogical differences together with selected soil specific properties were analyzed. The results were used to unravel the most likely reasons for the distinctive mineral weathering differences caused by the different conditions. The variation of the extent of mineral weathering of the soils was mainly controlled by climate conditions. The relatively arid climate restrained the chemical weathering in Orthic Entisols. In subtropic humid area, intensive agriculture promoted the decilication of Udic Ferrisols, resulting in a slignt increase in kaolinite content by <5%. However, in the temperate humid area the phyllosilicate mineral assemblage of Udic Isohumosols was sensitive to tillage and fertilization practices. This was mainly reflected by the mutual transformation between non-expandable illite and expandable vermiculite or smectite depending on the variation of K pool. This conclusion was supported by the significant positive correlations between illite and available K (P = 0.005) and between illite and slow-available K (P = 0.047). Compared with phyllosilicate minerals, the iron species and crystallinity indicated by Feo/Fed ratio were more susceptible to intensive agriculture. This could be inferred from the one order of magnitude larger variation coefficients of the Feo/Fed ratio in the three soils under different agriculture/fertilization than the variation of the Fed/Fet ratio. The key factor affecting the Feo/Fed ratio was the OC/Fed molar ratio. Highest values of the Feo/Fed ratio and its variation coefficients in Udic Isohumosols also illustrated that iron (hydr)oxides were more vulnerable to agricultural activities in the temperate monsoon climate region. It can conclude that it is necessary to manage the agricultural utilization of arable soils in temperate humid areas with more by specifically regarding the K and OC content for a good soil quality.