Impact of crop production and eucalyptus forestry on a Ferralsol under native grassland in southern Brazil

Abstract

The extreme south of Brazil is marked by a moist subtropical climate, where land use has changed widely from traditional grazing to intensive agriculture and plantation forestry, and in most cases the effects on soil properties are poorly known. Here, we assessed how conversion from Pampa native grasslands biome to annual crops and eucalyptus stands changed physical, chemical and mineralogical properties of a Dystric Rhodic Ferralsol (kaolinitic) after 30 yrs. (1988–2018). The soil was analyzed by chemical extractions, X-ray diffraction, X-ray fluorescence, diffuse reflectance, magnetic susceptibility, and petrographic microscopy. Soil organic carbon concentrations in the top 5 cm under eucalyptus was more than twice as high as under native grasslands, with no significant differences beneath that layer, or under annual crops. Basic cations and available P increased under croplands as an effect of liming and fertilization, but unexpectedly also under non-fertilized eucalyptus, which was probably due to biogeochemical cycling from deeper layers. Significant decreases in magnetic susceptibility, total Fe and Al (determined by X-ray fluorescence), and dithionite-extractable Fe in the soils under eucalyptus and cropland could be ascribed to lower clay contents compared to those of the native grasslands. Such changes were also evidenced by X-ray diffraction through the decrease in peak areas of hematite and kaolinite, the latter probably decreasing its crystallinity whereas an interstratified 1:1–2:1 phase was more stable. Soil microstructure was massive, i.e. dense and with few fissure pores under native grassland, but under croplands and especially eucalyptus was much looser and contained more excrements and faunal features. Our data helps understand how land use change in this poorly studied biome affects differently key soil properties involved in agriculture and ecosystems, especially SOC and chemical composition.