Elucidating relationship between nitrous oxide emission and functional soil microbes from tropical lowland rice soil exposed to elevated CO2: A path modelling approach

Abstract

Understanding the effect of elevated atmospheric CO2 on nitrous oxide (N2O) emission and plant-microbe interactions in rice soils is of great significance for predicting the long-term response of rice ecosystems to elevated CO2 concentration. Elevated atmospheric CO2 concentration has the potential to augment rice production and alter the soil nitrogen (N) dynamics. Because N2O emission is controlled by microbial activity, we reasoned that changes in soil N dynamics and activity of soil microbes may affect N2O flux from rice soil. In order to examine the influence of elevated CO2, different N doses and varying moisture regimes on N2O emission from rice soil, we designed a field experiment with 3 CO2 levels [C1 = ambient CO2 (400 ± 10 μmol mol−1), C2 = elevated CO2 (550 ± 20 μmol mol−1), C3 = elevated CO2 (700 ± 20 μmol mol−1)], 3 N doses [No = No nitrogen, N1 = 100 kg ha−1, N2 = 150 kg ha−1] and 2 moisture regimes (M1 = well watered, M2 = water deficit stress upto −60 kPa). In this study, path analyses using partial least square path modelling (PLS‐PM) approach was used to distinguish the direct and indirect factors influencing N2O emission under ambient and elevated CO2 conditions. Significant increase was found for labile N fractions such as microbial biomass-N (32 %; p ≤ 0.03) whereas significant decrease was found for NH4+-N (40 %; p ≤ 0.0005) and NO3--N (32 %; p ≤ 0.001) concentration under elevated over ambient CO2. Rhizospheric denitrifier population was increased (39 %; p ≤ 0.001) whereas, nitrifier population decreased (41 %; p ≤ 0.001) under elevated as compared to ambient CO2. Exposure of elevated CO2 decreased (20 %; p ≤ 0.001) N mineralization whereas it increased N2O emission (43 %; p ≤ 0.001). Elevated CO2 level increased activities of β- glucosidase, urease and fluorescein diacetate showing significant effects on soil N dynamics. Path modelling (PLS-PM) approach indicated a direct effect of nitrifiers and denitrifiers population on N2O emission and the effect was more pronounced under elevated CO2 as compared to ambient CO2 levels. The developed predictive models for N2O emission clarifies that the effect of varying CO2 levels is more pronounced at rice vegetative stage than reproductive stage.