Early suggestions
Early reports suggest a linear relationship between increasing N input and increases in direct N2O emission in various agricultural systems (e.g., Bouwman, 1996; Dobbie et al., 1999). This relationship is adopted for current IPCC Tier I EF methodology (IPCC, 2006) which directly estimates direct N2O emission based on magnitude of N additions in agricultural soils.
Field observations
However, there is a growing body of evidence indicating a nonlinear, exponential response of direct N2O emission to N input (McSwiney and Robertson, 2005; Grant et al., 2006; Hellebrand et al., 2008; Zebarth et al., 2008; Cardenas et al., 2009; Jarecki et al., 2009; Kim et al., 2010). In addition, this nonlinear increase in direct N2O emission also seems to cause an increase in N2O EF with N additions, and therefore, N2O EF values are not constant but dependent on N input rates (Zheng et al., 2004; Grant et al., 2006; Halvorson et al., 2008; Hoogendoorn et al., 2008; Cardenas et al., 2009; Kim et al., 2010).
Through examining the acquired datasets, we found that nonlinear dependency of direct N2O emission on N input was more frequently observed than linear dependency. Among the compiled 26 datasets, only 4 datasets indicated a linear increase rather than a nonlinear increase in direct N2O emissions was better related to increases in N input. This meta-analysis suggests that both linear and nonlinear dependency of direct N2O emission on N input can occur and that the conditions for whether the N2O response to N inputs is linear or nonlinear are site specific. The majority of existing studies have applied only 2 or 3 different levels of N input to examine the response of N2O emission, and their results were not able to distinguish linear and nonlinear dependency (Hoben et al., 2011).
Modeling results
Irrespective of the range of N input, assessed experimental studies consistently indicated exponential increase of direct N2O emissions with increases in N input in these mineral, well-drained soils. This observed exponential relationship is being incorporated into process based models for direct N2O simulation. Results from mechanistic models such as Ecosys for maize cultivation in Ottawa, Ontario, Canada (Grant et al., 2006), APSIM model for sugarcane (Saccharum spp. L.) cultivation in Te Kowai, Mackay, Australia (Thorburn et al., 2010) and NZ-DNDC model for typical New Zealand grazed grassland (Saggar et al., 2007) also suggest exponential increases in direct N2O emission as a function of increases in N input. Collectively, these exponential patterns from both experimental and simulated data are contrary to the simplistic premise of linear increases in direct N2O emissions with increased N fertilizer input such as implemented in current IPCC Tier I EF methodologies (IPCC, 1997; IPCC, 2006).
Qin, S., Wang, Y., Hu, C., Oenema, O., Li, X., Zhang, Y., Dong, W., 2012. Yield-scaled N2O emissions in a winter wheat-summer corn double-cropping system. Atmos. Environ., DOI: 10.1016/j.atmosenv.2012.1002.1077.
ReplyDeleteAbstract
Emissions of nitrous oxide (N2O) from agricultural soils contribute to global warming and stratospheric ozone depletion. Applications of fertilizer nitrogen (N) increase N2O emission, but also increase agricultural production. Here, we report on the responses of crop yield, N2O emission and yield-scaled N2O emission (N2O emission per unit N uptake by grain and aboveground biomass) to different N fertilizer rates in a winter-wheat summer-corn double cropping system in the North China Plain. Soil N2O emission measurements were carried out for two years in a long-term field experiment, under semi-arid conditions with four flood irrigations events per year. Our results indicated that N2O emissions were linear functions and yield-scaled N2O emissions were cubic functions of N fertilizer application rate. Yield-scaled N2O emissions were lowest at application rates of 136 kg N ha−1 yr−1. Using a quadratic-plateau model, it was found that maximal crop yields were achieved at an application rate of 317 kg N ha−1 yr−1, which is 20% less than current practice. This level is suggested to be a compromise between achieving food security and mitigation N2O emissions.