Southwest monsoon season over India exhibits spatial variation in the relative humidity (RH) levels. We took advantage of rice (Oryza sativa L.), cultivated in varied RH condition during the southwest monsoon season, to study the relationship of oxygen and carbon isotopic composition in the bulk grain organic matter (δ18OOM, δ13COM) with the hydroclimatic parameter of RH. Seasonal harvests of the years 2010 through 2014, sampled from 23 sites located in different climatic zones over India, were used for this experiment. Several fields and different genotypes (n = 105) were sampled at each site to arrive at representative values of δ18OOM and δ13COM. Based on regression analyses, the δ18OOM variation across sites showed significant dependence on the δ18O of source water (δ18OSW) used by the crops, with the humid region registering the strongest correlation (r2 = 0.95, p < 0.0001). After normalizing δ18OOM values with respect to δ18OSW and expressing the deviation as Δ18OOM, we obtained a significant relationship between the growing‐season average RH and Δ18OOM (r2 = 0.90, p < 0.0001). This is represented by a linear response function: Δ18OOM = (− 0.45 ± 0.03) × RH + (66.4 ± 2.7). The relationship established in this study enables the reconstruction of RH level from the isotopic measurement of rice grain OM in modern‐day and palaeo samples. Further, the relationship of the carbon isotope discrimination in rice grain OM (Δ13COM) with RH gradient demonstrates the role of the evaporative demand in governing the dual‐isotope variability.
Journal of Geophysical Research: Biogeosciences – Wiley
Published: Jan 1, 2018
Keywords: ; ;
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