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.
The field experiment was carried out on two different areas of a field (10ha) in the Experimental Farm “Hohenschulen”, an affiliate of Christian-Albrechts-University of Kiel in Northern Germany (54°18'N, 9°58'E). The soil is sandy loam in texture (sand 58 %, silt 29 % and clay 13 %) and classified as Luvisol with the following properties: bulk density 1.37gcm−3, pH 6.5, total organic C 1.5 %, total N 0.1 %, water holding capacity (WHC) 37 %.
The climate in this region is maritime, with the
Grain yield, N concentration and NUE
Grain yield was higher in 2012, and N fertilization significantly yielded 2 times higher grain or straw biomass than CK (Fig. 2). CAN resulted in very high grain yields in both years while U+UI+NI showed the highest grain yield across years among urea treatments, 8 % higher than U (Table 1). No significant differences in grain yield between fertilizer treatments were observed across years.
Compared to untreated U, neither single or combined use of inhibitors showed significant effects on straw
Nitrous oxide emissions
Averaged over all fertilizer types, N2O emissions were higher in 2012 than in 2013. Initial soil mineral N contents (0–0.3 m) were much higher in this year (on average 25 kg N/ha, Fig. S5) compared to 2013 (15 kg N/ha). In addition, higher temperature and much more frequent rainfall in 2012 were probably favorable for N2O production, and would have promoted denitrification which is the predominant source of soil N2O emission under central European conditions (Wrage et al., 2001). This
No difference between CAN and urea fertilizer on N2O emissions were observed under light soil and moderate rainfall conditions. As CAN fertilization resulted in higher crop yields than urea, yield scaled N2O emissions were lower for CAN. Nitrification inhibitor DCD/TZ addition to urea showed a beneficial effect on reducing soil N2O emission compared to both urea and CAN, but urea treated with NI alone showed a lower N recovery rate. Furthermore, combination of UI 2-NPT and NI on urea did not
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
We want to express our grateful thanks to Jun Yang, Gunda Schnack and the ﬁeld technicians’ team at Hohenschulen experimental farm for their technical support. This study was funded by the German fertilizer producer SKW Stickstoffwerke Piesteritz GmbH (SKWP). But SKWP has no influence and decision on the results and publication.
© 2023 Published by Elsevier B.V.