Carbon Turnover and Carbon‐13 Natural Abundance under Land Use Change in Semiarid Savanna Soils of La Pampa, Argentina

Half‐lives of 50 to 100 yr have been reported for native soil C in temperate Mollisols taken under cultivation, whereas some C fractions have shown mean residence times of hundreds of years. In the Tropics, C turnover is much more rapid. This lower C stability may be attributable to climate or to a mineral suite with low C stabilization potential typical of many tropical soils. We present data on the mineralization and accretion pattern of organic C under land use conversion in illitic loessial loams of the semiarid, warm temperate Province of La Pampa, Argentina. These soils should have a low C stabilization potential, and the semiarid temperate climate should provide for relatively slow turnover compared with tropical conditions. Using 13 C‐natural abundance and soil fractionation, we determined soil C derived from C3 and C4 plants under land use changes between native Calden savannah, pastures, and arable fields, specifically: (i) the conversion of C3 natural savannah to agriculture that includes (C4) maize in the rotation, (ii) the conversion of a 40‐yr‐old C4 pasture into arable land under C3 crops, and (iii) the reclamation of highly degraded C3 cultivated land with C4 pastures. Although none of the land cover changes represented 100% conversions between plants of C4 and C3 metabolism, C turnover could still be determined. All sites provided evidence for rapid C losses with C half‐lives just above 10 yr and no evidence for long‐term stabilized C in any soil fractions. Soil under long‐term pasture or the natural vegetation of the region, with initial C contents between 24 and 33 mg C g −1 bulk soil, lost 33 to 57% of this original bulk soil C within 12 to 18 yr of continuous cultivation. On degraded soils under restoration with C4 pasture, C accretion was also rapid but leveled off well below the original C levels. Theses results provide evidence for potentially irreversible soil degradation and corroborate local practical experience that these temperate, coarse‐textured, illitic soils are highly susceptible to degradation and should be managed carefully.