We analyzed the relationship between organic C and N in the upper soil and the quantity (mass) and quality (N, soluble phenolic, and lignin concentrations) of plant aboveground litter and upper fine roots at contrasting microsites (plant patch and bare soil) in 15 study sites across an aridity gradient in Patagonia. At each site, we estimated the total, grass, and shrub cover and randomly selected 10 plant patches of modal size (height and crown diameter) and species composition. We extracted an upper soil core (0–10 cm depth) and collected the aboveground litter underneath each plant patch canopy and at the contiguous bare soil. We separated the fine roots (< 2 mm in diameter) from the soil and assessed the biomass and the concentrations of organic C and N in soil and the lignin, soluble phenolic, and N in shrub and grass components of aboveground litter and in fine roots. Total plant and grass cover decreased with increasing aridity. Total litter mass did not vary across the aridity gradient, but the proportion of shrub components in litter increased and litter quality decreased with increasing aridity. The mass of fine roots was positively correlated to soil organic C and N and decreased with increasing aridity. All these trends were consistent at plant patch and bare soil microsites. We conclude that the decrease in soil organic C and N across the aridity gradient was the outcome of different above- and belowground controls resulting from the replacement of grasses by shrubs. Accordingly, the main controls of soil organic C and N could be the quality of aboveground litter and the quantity of the organic matter input to soil through fine-root turnover from belowground.