Three Decades of Irrigated Sugarcane Cropping Increased the Soil Carbon Stock and Substituted Most of the Original Carbon in the Brazilian Drylands

Soil carbon (C) dynamics are increasingly studied because of their role in theglobal C cycle and potential influence on atmospheric carbon dioxide con-centrations and climatic changes. The substitution of native forests by cropfields usually decreases soil C stocks. However, C stocks may increase inirrigated fields of semiarid areas because high biomass production and litterincorporation may compensate for the fast turnover of organic matter. Using13C isotopic techniques, we determined soil C dynamics in the 1 m super-ficial layer, in irrigated sugarcane fields established 10 and 30 years before.From 57% to 78% of the original C stock were lost after 10 years of cultivationand from 85% to 95% after 30 years. However, these high losses were morethan compensated by sugarcane biomass inputs, and the total soil C stockincreased from 94 to 169 Mg ha-1 between 10 and 30 years of cultivation.This last C stock is higher than any reported for soils in the Brazilian semiaridregion. Therefore, substituting native dry deciduous forests by irrigatedpermanent sugarcane in semiarid areas may increase soil C stocks, contraryto most rain-fed and irrigated cropping systems with crops that accumulateless biomass than sugarcane.