Applying the denitrification method to 15N and 18O analysis of nitrate in natural groundwater samples

<p>This study aims to develop a simplified denitrification method for the δ<sup>15</sup>N and δ<sup>18</sup>O analysis of nitrate (NO<sub>3</sub><sup>–</sup>) in natural groundwater samples following Zhu et al. (Sci Total Environ. 2018; 633: 1370–1378) and Sigman et al. (Anal Chem. 2001; 73: 4145–4153). With the help of Pseudomonas aureofaciens bacteria, the simplified method induced denitrification of the sample and completely converted the NO<sub>3</sub><sup>– </sup>into measurable N<sub>2</sub>O while avoiding sample fractionation. In contrast to the classic denitrification method (Sigman et al., 2001), which is based on anaerobic cultivation, the bacteria are cultivated aerobically in the simplified method (Zhu et al., 2014). In this study, aerobic cultivation was performed in a nitrate-free medium. Unlike the other two methods, aerobic cultivation was performed without the addition or removal of nitrate in the liquid medium. This eliminates the need for another preparation step, saving time. There was no contamination with external NO<sub>3</sub><sup>–</sup>. After further optimising the influencing factors, the method yielded high accuracy and precision (standard deviations were generally ≤ 0.7‰ for δ<sup>18</sup>O and ≤ 0.3‰ for δ<sup>15</sup>N), confirming the suitability of this procedure. Finally, the potential applicability of the method was demonstrated by measuring the isotopic composition of NO<sub>3</sub><sup>–</sup> in natural groundwater samples.</p>

Grain Yield and Resource Use Efficiencies of Upland and Lowland Rice Cultivars under Aerobic Cultivation

Aerobic rice has the potential to replace transplanted flooded rice, as rice cultivation is seriously threatened by environmental and social factors. Although the recently released upland rice cultivars have higher drought tolerance, low yield potential of these cultivars makes them less feasible for high-yielding rice planting regions under aerobic cultivation. In this study, typical lowland rice cultivars (Huanghuazhan and Yangliangyou6) were evaluated for grain yield and resource use efficiencies under aerobic cultivation as compared with upland rice cultivars (Lvhan1 and Hanyou3). Averaged across different years, lowland rice cultivars recorded 26.9%, 14.6%, and 26.6% higher grain yield, water productivity, and nitrogen use efficiency for grain production (NUEg), respectively, as compared with upland cultivars. The higher grain yield of lowland rice cultivars under aerobic cultivation was mainly attributed to the higher aboveground biomass and the spikelet number per panicle, along with a higher harvest index and panicle number per unit area with respect to upland rice cultivars. During the entire growing season in aerobic cultivation, rainfall accounted for 60% to 85% of the total water use, which indicates that lowland rice cultivars could make better use of the rainfall because of a longer growth duration and a higher growth potential. In summary, this study suggests that with appropriate irrigation, lowland rice cultivars could grow well and furnish higher yield than the current upland rice cultivars under aerobic cultivation.