III. Mechanistic hypotheses to explain observed dependency of N2O emissions on N input

Several hypotheses can be postulated to account for the observed linear and nonlinear exponential and hyperbola model response of direct N2O emissions to N input .


Linear response
Nitrogen provided is consumed by plants and microbes, and N2O emissions are primarily controlled by plant vs microbial competition for the available N. Therefore, as N input increases direct N2O emissions would increase linearly.

 
Exponential response

1. Substrate for additional N2O production

This response can be primarily associated with both excessive N supply beyond plant demands (e.g., > 100 kg N ha−1; Bouwman et al., 2002) and soil microbial mediation. This soil N surplus would concomitantly lead to lower plant N uptake efficiency (Liang and MacKenzie, 1994; Hong et al., 2007), and therefore, the resulting soil residual N would likely served as substrate for additional N2O production (Chantigny et al., 1998; McSwiney and Robertson, 2005; Grant et al., 2006; Zebarth et al., 2008). This initial hypothesis can be in part supported by data from several experimental studies (Zebarth et al., 2008; Abdalla et al., 2010; Velthof et al., 2010).

2. Inhibit reduction of N2O

Excessive soil N due to levels of N additions beyond plant and microbial uptake capacities could also hypothetically promote additional soil N2O production as it is known that an increased NO3− accumulation can inhibit biochemical reduction of N2O to dinitrogen (N2) also resulting in wider N2O:N2 ratios (Firestone et al., 1979; Mosier et al., 1982; Weier et al., 1993).

3. Priming effects

Exogenous N additions to soils can cause priming effects by stimulating microbial mobilization of native N boned within pre-existing soil organic matter (Jenkinson et al., 1985; Kuzyakov et al., 2000). This enhanced soil native N mobilization and accessibility can likely result in increased direct N2O emissions derived from the soil N pool as found in a lysimeter study after addition of animal urine in a ryegrass (Lolium perenne L.) - white clover (Trifolium repens L.) pasture (Di and Cameron, 2008).


Hyperbola response

As N input initially increases, direct N2O emissions would increase. However, as N additions continue to increase progressively beyond the capacity of soil microbes to take up and utilize N, the rate of N2O production would slow. Combining the initial increase in direct N2O emissions and the later slowing down of the increase as a response to N input increase, the direct N2O emissions vs N input would fit the hyperbola model.

1 comment:

  1. 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.

    Abstract
    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.

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