Reply on CC1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Total denitrification, the natural process capable of removing reactive N from ecosystems through conversion to N₂, is one of the most poorly constrained processes in terrestrial N cycling. In situ quantification of total denitrification could help identify mitigation options for N pollution. This study provides proof-of-concept for a novel natural abundance isotope based model for depth differentiated in situ quantification of total denitrification; it does so by examining the use-case of the impact of wheat (Triticum aestivum) varieties with different root biomass on total denitrification. We set up a mesocosm experiment in 1.5 m tall lysimeters with four wheat varieties, each replicated three times. Temporal data for soil moisture, nitrous oxide (N₂O) concentrations in the soil pore space, site preference (SP) and &delta;¹⁸O values of soil pore space N₂O were collected at soil depths of 7.5, 30, 60, 90 and 120 cm over a five month growing period and used as input variables in the new model. Here, we define total denitrification as gross N₂O consumption, with N₂O produced either through nitrification or denitrification. The model, further referred to as &lsquo;Process Rate Estimator&rsquo; or PRE, constrains temporally explicit gross N₂O production and consumption rates at each depth increment based on a combination of diffusion and isotope mixing and fractionation models. Estimated production and consumption of N₂O, integrated over the five month experiment, ranged from 3.9 to 170.3 kg N ha^-1, with a trend for greater N₂O production from denitrification compared to nitrification. N₂O concentrations where greatest at 60 and 90 cm depth, while N₂O production and consumption peaked at 7.5 and 30 cm depth, illustrating the important role of N₂O dynamics along the soil profile in understanding ecosystem N budgets. Both N₂O production and consumption were greater in varieties that had previously been characterized to have greater root biomass. We demonstrate that PRE is able to constrain nitrification and denitrification leading to gross daily N₂O production, and gross reduction to N₂ across the depth profile, based on the temporal change in concentrations, &delta;¹⁸O and SP of N₂O. We conclude that our results support the potential of PRE to estimate total denitrification in situ, which could form the basis for developing promising strategies to reduce N pollution.