Targeting yield and reducing nitrous oxide emission by use of single and double inhibitor treated urea during winter wheat season in Northern Germany (2023)

Nitrous oxide (N₂O) is the most powerful long-lived greenhouse gas (GHG) with ∼268 times higher global warming potential (GWP) than carbon dioxide (CO₂) on a 100-year perspective (IPCC, 2019). Beside global warming, nitrous oxide also has a severe effect in depleting stratospheric ozone (Ravishankara et al., 2009). Agriculture is the most important source of anthropogenic emission of N₂O, mostly by N fertilizer application (Davidson, 2009). Nevertheless, sufficient synthetic N fertilizer should be used to meet the global food demand with expected population increase over time (Tilman et al., 2011). Thus, decreasing the N₂O emission with adapted N application rates and timings, and the use of enhanced efficiency fertilizes with urease and nitrification inhibitors, have great potential for improving future agricultural management practices.

Since N fertilizer derived N₂O is mostly produced by microbial driven nitrification and denitrification processes, reducing the corresponding substrate concentration or retarding the related microbial activity in the soils is a reliable path to mitigate soil N₂O emission (Wrage et al., 2001). Using nitrification inhibitors (NI) to slow down the transformation of ammonium to nitrate has been considered as an efficient way to reduce N₂O emission or N leaching in many studies (Di et al., 2016; Meng et al., 2021). Nevertheless, single use of NI may increase the ammonium availability in soil and subsequent ammonia emissions, especially in calcareous soil with high pH, which was substantiated in a review study by Lam et al. (2017). Fan et al. (2018) reported that use of a nitrification inhibitor decreased soil N₂O emissions by 1.8–61.0 %, but promoted NH₃ volatilization by 3.2∼44.6 % in vegetable production systems in China. Thus, the combined use of NI with urease inhibitor (UI) has been promoted to concomitantly control the emission of ammonia and N₂O (Weiske et al., 2001, Tao et al., 2018).

There are three commonly used NI, i.e. dicyandiamide (DCD), 3,4-dimethylpyrazole phosphate (DMPP) and Nitrapyrin. Generally, DMPP showed the strongest effect (per unit weight of compound) on nitrification inhibition since it has a lower mineralization rate and can remain longer in soil than DCD (Zhou et al., 2020). Moreover, the efficacy of NI seems also to depend on crop type, field management, soil and climatic conditions (Behnke et al., 2018, Essich et al., 2020, Ottaiano et al., 2020). Thus, in addition to laboratory testing, the efficacy and performance of a new developed inhibitor should be assessed under field conditions,

A new nitrification inhibitor, DCD/TZ (mixture of dicyandiamide and 1H-1,2,4-triazole) was introduced in Germany in the early 2000s. The positive effect of DCD/TZ alone on retarding N₂O emission after urea application were found in both lab incubation and field studies (Ni et al., 2018, Hu et al., 2020). Like NI, urease inhibitor (UI) is another nitrogen fertilizer synergist which slows down the speed of transformation of urea to ammonium. UI is beneficial for decreasing nitrogen loss by ammonia volatilization. But the knowledge on the effect of UIs on N₂O emission is scarce.

The UI N-(2-Nitrophenyl) phosphoric triamide, shortly as 2-NPT, was also introduced in early 2000s, and its efficacy to reduce NH₃ emission has been assessed (Ni et al., 2014, Schraml et al., 2016, Adhikari et al., 2020), but its efficiency on decreasing N₂O emission has not yet been tested. It was reported that additional 2-NPT increased the N₂O emission compared with DCD/TZ addition alone (Hu et al., 2020). The decrease of efficiency of NI by UI addition implies the key role of climate and soil conditions on N₂O emissions.

In addition to the effect of inhibitors on N₂O losses, the specific N form (e.g. urea vs. calcium ammonium nitrate) can play a role. Cowan et al. (2019) showed that under the conditions of Ireland (clayey soils, high rainfall) N₂O emissions from the nitrate-based fertilizer CAN could be considerably higher than from urea, due to faster and stronger denitrification. Overall, there is a strong need to evaluate the N₂O emission behavior of fertilizer N forms and the influence of inhibitors on this behavior under various agroecological conditions. This is in particular of importance for regionalized N₂O emission assessment which can support optimized allocation of N₂O emission reduction measures and policies (Mathivanan et al., 2021).

Another important aspect of N form and use of inhibitors is their agronomic effect on crop yield and nutrient efficiency with relevant repercussions for fertilizer economy and additional environmental benefits by allowing the reduction of application rates and of concomitant environmental risks of excess N supply including reduction of indirect N₂O emissions. On a global scale, recent meta studies found general agronomic benefits (Li et al., 2018, Kanter and Searchinger, 2018) combined with emission reduction. But effects also depent on inhibitor active ingredients and agronomic environment, with smaller effects e.g. in grain crops. For a full assessment of a potential benefit of an inhibitor active ingredient both agronomic outcomes and emissions need to be accounted for and can be combined in the variable yield scaled emissions.

In this study, a field experiment was conducted to evaluate the effect of urea and CAN and the efficacy of 2-NPT, DCD/TZ and their combination on N₂O emission reduction, crop yield and nitrogen use efficiency in winter wheat in northern Germany. The main objective was to quantify the response of crop yield and N₂O emission to fertilizer type and different inhibitor treatments of urea. We hypothesized that (1) CAN has no higher relative N₂O emission than urea under typical central European climate and arable soil conditions, (2) UI or NI addition alone have positive effect on reducing N₂O emission, but combined use of UI and NI has no additive effect as the NI dominates the urea turnover process, (3) single use of UI or NI can increase yield due to reduced N losses, and a combined use of UI and NI has a synergistic effect.