Nitrous oxide fluxes and production pathways
Agricultural soils are an important source of the greenhouse gas nitrous oxide (N2O), which is mainly produced by microbial Nitrogen (N) transformations and is often enhanced when available N exceeds crop demand (Oenema et al. 2005). Different pathways, e.g., denitrification, nitrification, and nitrifier denitrification, among others, are involved in the production of N2O under a range of soil conditions (Wrage-Mönnig et al. 2018). Among environmental factors, N availability and soil water content are major drivers of N2O emissions (Butterbach-Bahl et al. 2013; Chen et al. 2014). However, there are also interactions with other elements, e.g., carbon (C) and phosphorus (P) (He and Dijkstra 2015; O'Neill et al. 2020).  Like N, P is a critical nutrient for plant and microbial growth. Phosphorus is a primary constituent of biomolecules such as nucleic acids (DNA and RNA), phospholipids, and adenosine triphosphate (ATP), which regulates many biological processes including energy transfer reactions and activation of enzymes (Nannipieri and Eldor 2009; Rouached et al. 2010). Availability of P is essential for the metabolisms of C and amino acids in microbes, which play an important role in regulating N cycling and availability (Elanchezhian et al. 2015). Besides, N transformations in soil are multi-enzyme processes carried out by diverse microbial species. The effects of P on various enzyme activities and composition of the microbial community differ, leading to P sensitivity of N mineralization, nitrification, and denitrification (Olander and Vitousek 2000). Moreover, P requirements of plants and microbes and the relationship with N are altered by increased N inputs (Fan et al. 2018). Changes in the stoichiometric N:P ratios and increasing P limitation could affect N availability and transformations in terrestrial systems (Peñuelas et al. 2013; He and Dijkstra 2015).   In P limited environments, P fertilization may alleviate P limitation of N cycling microorganisms and thus, stimulate nitrification and denitrification resulting in increased N2O emissions (Wang et al. 2014; He and Dijkstra 2015). Furthermore, increased microbial biomass may stimulate microbial respiration leading to the formation of anoxic conditions and increased N2O emissions from denitrification (Mori et al. 2013; Wang et al. 2014). Recent studies indicated increasing or decreasing effects of P fertilization on N2O emissions, depending on amounts and combinations of nutrient addition (P, N, C), plant growth, vegetation type, and ecosystem. For instance, alleviation of P limitation resulted in the reduction of N2O emission in cropland (Baral et al. 2014), grassland (O'Neill et al. 2020; Gebremichael et al. 2022), and wetland soils (Sundareshwar et al. 2003), but increased N2O production in P-limited grassland soils (He and Dijkstra 2015; Mehnaz and Dijkstra 2016; Mehnaz et al. 2019b), and in paddy soils irrespective of soil P status (Shah et al. 2022). Under C limited conditions, no significant effect of soil P content on N2O losses was found, while with C addition, cumulative N2O was significantly higher from low-P than high-P soils, confirming a strong interaction among nutrients (O'Neill et al. 2020).  Thus, the effect of P depends on the circumstances and possibly the pathways of N2O production. So far, no robust information is available for P effects and P history on sources of N2O emissions. Therefore, we used isotopic approaches to improve knowledge on N2O sources and N transformations after addition of P fertilizer to soils with different histories of P-addition in a mesocosm experiment. We hypothesized that 1) P addition would decrease inorganic N in soil and N2O emissions, 2) P addition would have no effect on the relative importance of nitrification or denitrification for N conversion or as sources of N2O and 3) P addition effects would depend on the history of P fertilization.