Soil profile distribution of total C content and natural abundance of 13C in two volcanic soils subjected to crop residue burning versus crop residue retention

Soil organic carbon (SOC) plays a key role in crop productivity and soil quality. Conservation agriculture has a positive effect on SOC accumulation in the surface soil horizons, but little information is available regarding the effect of the removal of crop residues by burning. This study aimed to assess the impact of different types of crop residue management practices on the total C distribution and natural abundance of ¹³C (‰, δ¹³C). Two volcanic soils, located in the Mediterranean temperate zone of Southern Chile, were studied: an Ultisol (Collipulli Series, CPL) and an Andisol (Santa Bárbara Series, SBA). Both soils had been cultivated under direct-drilling and a typical annual crop rotation system for a long period of time. Two different types of crop residue management practices were imposed in both soils: (i) crop residue burning (CPL-B; SBA-B) and (ii) crop residue retention over the soil (CPL-R; SBA-R), corresponding to treatments B and R, respectively. Soil profile distribution of the C content and natural abundance of ¹³C were analysed for bulk soils (down to 100 cm depth) and three particle-size fractions of the soils (down to 20 cm of soil depth): (a) ≤ 53 µm, (b) 53-212 µm and (c) ≥ 212 µm. It was found that the effect of crop residue management can be observed in the variations of C content and δ¹³C in the soil profile in both volcanic soils. Crop residue burning (B treatment) increased the C content in bulk soil and the particle-size fractions. On the other hand, soil organic matter of crop residue retention (R treatment) showed higher natural abundance of ¹³C (δ¹³C) compared with residue burning (B treatment) in the two volcanic soils. R treatment enriched the particle-size fractions (except ≥ 212 µm fraction of CPL soil) with ¹³C. Factors that could account for these findings are also discussed here.