1:45 PM - 2:00 PM
[MIS19-13] Probing the Future: Response of Soil Organic Carbon and Nitrogen dynamics to elevated CO2 in a paddy field
Keywords:Soil, C and N cycling, Free Air CO2 Experiment (FACE), Density fractionation, 13C, Climate change
Both soil C and N contents progressively decreased over the four year period due presumably to a shift in management (i.e. removal of rice straw form the field since 2010). It is worth mentioning that this decrease in soil organic matter was 25% more pronounced in the control plot compared to the plot exposed to eCO2, suggesting that eCO2 significantly enhanced organic matter input to the soil. The decrease in N contents was not as pronounced relative to C, leading to an increase in C:N ratios. The higher C:N ratios of soil and plant observed are likely to promote N immobilization by soil microbial community, which might lowered N availability for rice.
Further soil analysis using stable isotopes measurements revealed that, after four years, significant proportions of original soil C were replaced with recent C at different degrees among the three soil density fractions. The mean residence time of C was on average 6.5, 120 and 56 years in free light fraction, occluded light fraction and heavy fraction, respectively. About 60% of soil organic matter was distributed in the heavy fraction and the relatively rapid turnover of C in this fraction was not expected. The heavy fraction contains the organic matter bound with mineral particles, which is more stable into the soil. We found that, after four years, about 7% of the C in the heavy fraction was originating from C assimilated by rice plants since 2010.
The relatively rapid incorporation of newly added carbon into the heavy fraction and the higher soil C contents measured under eCO2 suggest that the soil may be acting as a C sink under the open-field eCO2 conditions. As it corresponds to a negative feedback for the rise in atmospheric CO2 concentration, further research investigating this process are necessary.