Limited carbon inputs from plants into soils in arid ecosystems: a study of changes in the d13C in the soil-root interface

Abstract

Background and aims. The tracing of C assimilation and the subsequent partitioning among plant organs has been a central focus of studies utilising Free Air CO2 Enrichment (FACE) facilities. The approach makes use of the fossil origin of this carbon, which is depleted in 13C. However, there is little data for desert environments. The Nevada Desert FACE Facility (NDFF), located in the Mojave Desert, has been one of the main facilities for the study of C dynamics in arid ecosystems and how they respond to rising atmospheric CO2 concentrations. In this experiment, we studied the incorporation of fixed CO2 during the previous two years (detectable by its lower 13C) in the soil fraction surrounding roots. Methods. The soil was collected monthly in direct vicinity to the roots during a complete growth season, at two depths (5 and 15 cm). Soil samples were dried and fractionated by size (> 50 μm and < 50 μm) by wet sieving, and both size fractions were then analysed for the 13C of their organic matter and their carbonates. Results. In the coarse fraction (> 50 μm), 13C values ranged between -1 and -2¿ for carbonates and between -23 and -25¿ for soil organic matter. These values did not significantly change throughout the experiment and were not affected by depth (5 or 15 cm). In contrast, 13C values for both organic and inorganic carbon in the fine fraction (< 50 μm) were much more variable than in the coarse fraction (> 50 μm). The 13C values for organic C ranged mostly between -20¿ and -27¿, and were roughly maintained throughout the sampling period. For inorganic C, the 13C values were mostly between 0¿ and -15¿, and tended to become less negative during the course of the sampling period. Overall the effect of [CO2] on 13C values of either organic or inorganic carbon was not significant for any experimental condition (plant species, depth, fraction). Conclusion. Little or no signs of recently fixed CO2 (13C-depleted) were detected in the soils close to the roots, in the coarse fraction (> 50 μm), the fine fraction (< 50 μm), the organic matter, or in carbonates. This indicates a slow C turnover 45 in the studied soils, which can result from a highly conservative use of photoassimilates by plants, including a very low release of organic matter into the soil in the form of dead roots or root exudates, and from a conservative use of available C reserves.