Indirect nitrous oxide emissions from oilseed rape cropping systems by NH3 volatilization and nitrate leaching as affected by nitrogen source, N rate and site conditions
Introduction
Generally, 15.3 % of the global N2O emissions and 41.8 % of anthropogenic-induced N2O emissions come from agricultural soils (Denman et al., 2007). For quantification purposes, direct and indirect N2O emissions have to be considered. Direct N2O emissions can be measured directly at the place of origin, whereas indirect N2O emissions result from processes like ammonia volatilization (NH3) or nitrate leaching (N leaching) (IPCC, 2006). Assessment of indirect emissions is still uncertain. Mosier et al. (1998) estimated indirect N2O emissions in a same amount as direct N2O emissions. They also defined the following N input pathways triggering indirect emissions: volatilization and subsequent atmospheric deposition of ammonia, nitric oxide and nitrogen dioxide, formation of N2O in the atmosphere from ammonia and also N leaching and runoff as well as food processing and human consumption of crops followed by municipal sewage treatment.
The mitigation of both direct and indirect N2O emissions is still challenging. The emission level depends on numerous interacting factors, such as site (temperature and soil moisture) and management factors, e.g. fertilizer type (mineral vs. organic), time of fertilization and fertilizer rate as well as tillage practices (Wells et al., 2018; Di and Cameron, 2002; Thomas et al., 2005). The use of organic fertilizers like digestates (DIG) to oilseed rape (OSR), for instance, may be a mitigation option, because GHG emissions from organic fertilizer production have not to be considered in the current GHG calculation of the fertilized crop (RED, 2009). On the other hand NH3 volatilization and only partial availability of the organic component of DIG N result in higher amounts of applied N to achieve similar yields (Herrmann et al., 2013; Svoboda et al., 2013). This study focuses on the major sources of indirect N2O emissions from OSR cultivation, ammonia volatilization and nitrate leaching.
OSR is characterized by a low N-efficiency (Sieling and Kage, 2010; Rathke et al., 2006; Sieling et al., 1998), mainly due to its comparable low N harvest index leading to low N utilization efficiency. OSR plant residues have higher N concentrations resulting in narrower C/N ratios than cereal straw (Garnier et al., 2003) and are less effective in immobilizing mineralized soil N (Nicolardot et al., 2001). Due to its comparably early harvest date and especially in soils with a high N mineralization capacity, soil mineral nitrogen (SMN) in autumn after OSR or canola harvest usually increase to higher levels compared to cereals (Sieling et al., 1999; Ryan and Kirkegaard, 2006; Engström and Lindén, 2012). Within crop rotations of northern Europe, winter wheat is normally grown after OSR, having a small N uptake before winter (<30 kg N ha−1) increasing the leaching potential over winter (Lickfett, 1993; Sieling et al., 1999).
To calculate the GHG balance of OSR cultivation used for biodiesel production, IPCC guidelines (2006) suggest several global and uncertain emission factors (EF) relating to direct and indirect soil N2O emissions. Referring to N losses by leaching, 30 % of the total N applied by fertilization and crop residues is assumed to be globally leached under humid conditions, whereas 0.75 % of the leached N is assumed to be emitted as N2O afterwards. But the authors of the IPCC guidelines mentioned uncertainties in emission factor estimation because of natural variability and a lack of measurements. Furthermore, these emission factors were neither made for site specific analyses nor for specific crops, but in absence of better methods they were still used to calculate GHG emissions. To improve GHG balancing of biofuels, further research is needed to take the variability of environmental preconditions and differing management practices for calculations of N2O emissions into account.
NO3-N dynamics within the soil profile is affected by several transformation and transport processes as well as soil properties, weather conditions and agricultural management (Vos, 2001; Silva et al., 2005; Engström et al., 2011; Bednorz et al., 2016). The amount of N leaching strongly depends on the soil water storage capacity of the soil profile (Abdirashid et al., 2004). The difference between SMN in autumn and spring is not an exact method to estimate leaching losses, but it can be used as a first proxy for N leaching (Sieling et al., 1997). Installing ceramic suction cups at a defined depth to collect leachate is an established method (Svoboda et al., 2013), but time-consuming and expensive. The use of dynamic models to calculate nitrogen mineralization and leaching can overcome this problem. Several models are available with varying complexity in aspects like plant N uptake, N mineralization/immobilization, soil water dynamics and consideration of management events (Molina-Herrera et al., 2016; Henke et al., 2008; Eckersten et al., 1995; Hansen et al., 1991; Berghuijs van Dijkm et al., 1985). For the present study, a newly tailored plant-soil-atmosphere model is introduced using components from existing models allowing to calculate crop-specific N uptake dynamics of OSR and winter wheat within a crop rotation and to describe dynamics of soil water, mineralization and N balance within heterogenic soil profiles and under time-specific and site-specific weather conditions. In comparison to the study of Henke et al. (2008), the inclusion of two N sensitive crop growth modules for OSR (Weymann, 2015) and winter wheat (Ratjen and Kage, 2015, 2016) are main improvements, now enabling extrapolations e.g. running scenario simulations.
Using this model, the main aim of our study was to quantify indirect nitrous oxide emissions from oilseed rape cropping at different sites in Germany by (i) measuring NH3 volatilization from organic fertilization, (ii) quantifying N leaching levels using a site-specifically fitted model and (iii) comparing them with the emission factors following IPCC calculation rules. Data from field experiments at five sites in three consecutive years were analyzed. Furthermore, we used the model to (iv) investigate functional relationships between N fertilizer amount and N leaching for different sites and two fertilizer types.
Section snippets
Experimental sites and field trials
As part of a larger project field trials were established in autumn 2012 at five different sites covering main climatic conditions in Germany (Ruser et al., 2017). Geographic and long-term climatic characteristics of these sites are shown in Table 1. At each site, the crop rotation consisted of winter oilseed rape (OSR; cv ‘Visby’), winter wheat (cv ‘Julius’) and winter barley (cv ‘Tenor’ in Berge, cv ‘Souleyka’ at all other sites). Each crop was grown each year in four replicates within a
NH3 volatilization
Depending on local differences in digestate properties, three-year-means of applied total N ranged from 238 to 345 kg N ha−1 between sites and also from year to year at one site (Table 4). Measurements resulted in NH3 emissions ranging from 14 to 76 kg NH3-N ha−1, with a mean of 38.7 kg NH3-N ha−1 (Fig. 1) with strong annual and site-specific variations. Table 4 shows the relative NH3-N losses depending on total applied N with the corresponding figures of air temperature and wind speed at the
Discussion
The aim of this study was to quantify site-specific indirect nitrous oxide emissions from OSR cropping systems in Germany, resulting from NH3 emissions after fertilizer application and nitrate leaching. Therefore, NH3 volatilization from organic fertilization was measured and N leaching was estimated using site-specific input values, e.g. weather data, soil characteristics (see Appendix A). The model consists of two crop growth models and soil water and soil nitrogen components. Parameters for
Conclusion
The present study showed that NH3 emissions after DIG application differed from site to site, but the general volatilization losses were clearly below those calculated with the default emission factor (20 % of applied fertilizer N). The Plant-Soil-Atmosphere (PSAM) model is a useful tool to estimate site-specific N leaching, since it was able to successfully reproduce year-specific observed dynamics of important components of the N balance other than N loss via leaching across different sites.
CRediT authorship contribution statement
Thomas Räbiger: Conceptualization, Methodology, Investigation, Validation, Writing - original draft. Monique Andres: Investigation, Validation. Hannes Hegewald: Investigation, Validation. Katharina Kesenheimer: Investigation, Validation. Sarah Köbke: Investigation, Validation. Teresa Suarez Quinones: Investigation. Ulf Böttcher: Methodology, Software, Data curation. Henning Kage: Conceptualization, Methodology, Supervision, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare no conflicts of interests.
Acknowledgements
This work was funded by the German Federal Ministry of Food and Agriculture and managed by the Agency for Renewable Resources under grants 22403212, 22403312, 22403412, 22403512, 22403712, 22403812, and 22403912. We also thank the Union for the Promotion of Oil and Protein Plants, Germany for their financial support and all technical stuff und students for their assiduous work in the field and the labs.
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