Stable carbon isotope ratios in soil and vegetation shift with cultivation practices (Northern Laos)

Stable carbon isotopes (delta C-13) can serve as a natural tracer of how plants contribute to soil organic pools (SUM). The present study investigates whether a better knowledge of the vegetation could help to account more accurately for changes in SOM. Secondly it aims to identify environmental drivers for changes in the delta C-13 of SOM. The study site was a small catchment where the initial forest had been cleared for agriculture in the 1960's. The delta C-13 was determined in the surface vegetation and in SOM across 59 survey plots comprising fields and fallow land, 22 environmental and land use variables were also determined in the plots. Carbon isotope ratios in the C-3 pathway species (n = 209) ranged from -37.7 to -26.1 parts per thousand, in the C-4 species (n = 29) from -15.2 to -10.3 parts per thousand, in SUM from -28.1 to -22.6 parts per thousand. Cultivation of C-4 crops did not affect the delta C-13 of SOM suggesting that maize and Jobs' tears residues supplied little C-4 material to SOM. Plots covered by a mix of C-3 and C-4 weeds had significant higher delta C-13 values in SUM than plots with only C-3 plants, suggesting that C-4 weeds more than C-4 crops contributed to SOM. Accounting only for the perennials in the plot population gave strongest associations between SUM and vegetation. Although C-4 annual crops and C-4 annual weeds often cover the soil extensively during cultivation years, their biomass contribution to SUM is therefore much less than perennial C-4 plants occupying the site for longer periods. Soil delta C-13 increased significantly with short fallow periods in between cultivation years which can be explained by our finding that very short fallow periods were associated with the invasion of C-4 weeds. Competition with C-4 weeds in turn pushed the farmers to cultivate hardier crops like maize and Jobs tears, replacing rain fed rice