Lysimeter-based full fertilizer 15N balances corroborate direct dinitrogen emission measurements using the 15N gas flow method

The $^{15}$N gas flux ($^{15}$NGF) method allows for direct in situ quantification of dinitrogen (N$_2$) emissions from soils, but a successful cross-comparison with another method is missing. The objectives of this study were to quantify N$_2$ emissions of a wheat rotation using the $^{15}$NGF method, to compare these N$_2$ emissions with those obtained from a lysimeter-based $^{15}$N fertilizer mass balance approach, and to contextualize N$_2$ emissions with $^{15}$N enrichment of N$_2$ in soil air. For four sampling periods, fertilizer-derived N$_2$ losses ($^{15}$NGF method) were similar to unaccounted fertilizer N fates as obtained from the $^{15}$N mass balance approach. Total N$_2$ emissions ($^{15}$NGF method) amounted to 21 ± 3 kg N ha− 1, with 13 ± 2 kg N ha− 1 (7.5% of applied fertilizer N) originating from fertilizer. In comparison, the $^{15}$N mass balance approach overall indicated fertilizer-derived N$_2$ emissions of 11%, equivalent to 18 ± 13 kg N ha− 1. Nitrous oxide (N$_2$O) emissions were small (0.15 ± 0.01 kg N ha− 1 or 0.1% of fertilizer N), resulting in a large mean N$_2$:(N$_2$O + N$_2$) ratio of 0.94 ± 0.06. Due to the applied drip fertigation, ammonia emissions accounted for < 1% of fertilizer-N, while N leaching was negligible. The temporal variability of N$_2$ emissions was well explained by the δ$^{15}$N$_2$ in soil air down to 50 cm depth. We conclude the $^{15}$NGF method provides realistic estimates of field N$_2$ emissions and should be more widely used to better understand soil N$_2$ losses. Moreover, combining soil air δ$^{15}$N$_2$ measurements with diffusion modeling might be an alternative approach for constraining soil N$_2$ emissions

Coupling Langmuir with Michaelis-Menten-A practical alternative to estimate Se content in rice?

Selenium plays an important, but vastly neglected role in human nutrition with a narrow gap between dietary deficiency and toxicity. For a potential biofortification of food with Se, as well as for toxicity-risk assessment in sites contaminated by Se, modelling of local and global Se cycling is essential. As bioavailability of Se for rice plants depends on the speciation of Se and the resulting interactions with mineral surfaces as well as the interaction with Se uptake mechanisms in plants, resulting plant Se content is complex to model. Unfortunately, simple experimental models to estimate Se uptake into plants from substrates have been lacking. Therefore, a mass balance of Se transfer between lithosphere (represented by kaolinite), hydrosphere (represented by a controlled nutrient solution), and biosphere (represented by rice plants) has been established. In a controlled, closed, lab-scale system, rice plants were grown hydroponically in nutrient solution supplemented with 0-10 000 μg L-1 Se of either selenate or selenite. Furthermore, in a series of batch experiments, adsorption and desorption were studied for selenate and selenite in competition with each of the major nutrient oxy-anions, nitrate, sulfate and phosphate. In a third step, the hydroponical plants experiments were coupled with sorption experiments to study synergy effects. These data were used to develop a mass balance fitting model of Se uptake and partitioning. Adsorption was well-described by Langmuir isotherms, despite competing anions, however, a certain percentage of Se always remained bio-unavailable to the plant. Uptake of selenate or selenite by transporters into the rice plant was fitted with the non-time differentiated Michaelis-Menten equation. Subsequent sequestration of Se to the shoot was better described using a substrate-inhibited variation of the Michaelis-Menten equation. These fitted parameters were then integrated into a mass balance model of Se transfer