Changes in the carbon and nitrogen isotopic composition of organic matter in soils of different thermal stability after free-air CO2 enrichment for three years

Abstract

The hypothesis that the biological availability of soil organic matter (SOM) pools is inversely proportional to their thermal stability was tested using the isotopic difference between the atmospheric CO2 (δ13C = −8.0‰) and 13C-enriched CO2 (δ13C = −47‰) fertilizers, as well as 15N-labeled fertilizers. The soil samples from spring wheat plots subjected to treatment with ambient (370 ppm) and elevated (540 ppm) CO2 concentrations for three years were analyzed by the thermogravimetric method. Based on the weight loss, five SOM pools were distinguished where the total C and N contents and isotopic compositions (δ13C and (δ15N) were determined. The contents of new C and N and their mean residence times in pools were calculated. The incorporation of 13C and 15N and their turnover rates did not depend on the thermal stability of the SOM pools, which disproved the hypothesis being tested.