Micron-scale spatial patterns of manganese oxidation states and acid phosphatase in soils under annual row crops and a native plant community

Manganese (Mn) plays a critical role in regulating litter decomposition, C protection, and nutrient cycling. However, the contribution of Mn to these processes can be modulated by land use and management. We investigated spatial patterns in Mn oxidation states and their associations with soil pores and extracellular enzyme activity in soils under two contrasting long-term (34 years) land uses: conventional row crop agriculture and native succession vegetation. Intact soil cores (5 cm Ø 2.5 cm height) were collected from replicated plots of (i) chisel-plowed maize-soybean-wheat rotation and (ii) early successional plant community abandoned from agriculture in 1989. Manganese distribution and speciation were obtained using a combination of synchrotron X-ray fluorescence microscopy and X-ray absorption near-edge structure spectroscopy. Soil pores were characterized using X-ray computed micro-tomography. Mapping of acid phosphatase activity was conducted via time-lapse zymography. Mn(II) and Mn(III) hotspots (i.e. localized regions of high Mn abundance) were more prevalent in the native succession soil than in conventional agriculture, while the reverse was true for Mn(IV) hotspots. Acid phosphatase activity was 8% higher in native succession and was especially high within its Mn(II) hotspots. Mn(II) and Mn(III) hotspots in the soil under native succession were particularly abundant in proximity (