Refining regional soil C stocks estimates in temperate highlands of Southern Brazil

Abstract Agricultural expansion on pristine woodlands and grasslands in Brazil usually causes major changes in soil organic C (SOC) stocks. Quantitative assessment of these stocks, their regional distribution and variability in rural landscapes are key scientific issues related to the Brazilian commitment to pursue climate-friendly agriculture. The aim of this research was to refine existing SOC stocks estimates in the Campos de Cima da Serra (temperate highlands) of Rio Grande do Sul state. Our approach included soil sampling campaigns to obtain primary SOC data, complemented by secondary data sources (published surveys and research projects). The field study sampled surface soil layers (to 30 cm depth) stratified by the predominant soil classes and agricultural land uses: primary grasslands with and without fire management, grasslands overseeded with legume forages, and fields fallowed after intensive annual horticultural crops or Pinus plantations (secondary grasslands). Large SOC stocks (0–30 cm depth) were measured in Ultisols (244.4 Mg C ha −1 ) and Inceptisols (191.1 Mg C ha −1 ) under grasslands without burning, whereas lower stocks were found in Inceptisols under agricultural crops. SOC stocks in this study were generally larger than those calculated using IPCC global reference SOC stocks, those estimated in SOC maps of Brazil − 70% higher in Inceptisols and 120% higher in Ultisols. Our estimates were 75% higher than those estimated for the study region in a state SOC map. Soils in this region store large C stocks, which had been generally underestimated in previous smaller scale inventory. Additionally, the SOC stock map obtained allows more detailed visualization of the spatial distribution of SOC than previously possible. This information can be used to define public policies for conservation agriculture and support the update and refinement of national inventories of SOC greenhouse gases.