IV. Conceptual representation of the relationship between N input and direct N2O emission

On the basis of the observed responses of direct N2O emissions on N inputs and the postulated hypothetical mechanisms for explaining the relationship as discussed above, we preliminary proposed a conceptual representation to capture the understanding gained about this relationship.

This hypothetical representation separates the response of direct N2O emission on N input into three sequential phases using the optimal N uptakes of both vegetation and soil microbes as boundaries.

Phase I: Linear increase
As N input initially increases, the provided N is consumed by plants and microbes and N2O emission primarily controlled by plant vs. microbial competition for the available N. Therefore, in the phase I, direct N2O emission would show slight linear increase.

Phase II: Exponential increase
As N additions exceed beyond optimal N plant uptake rates, phase II would exhibit exponential increases of direct N2O emission since soil N2O production highly increases with the excessive N supply.

Phase III: Steady state
Finally, as N additions rates continue to progressively increase beyond the capacity of soil microbes to uptake and utilize N (phase III), increase rate of N2O production would slow down and finally reach steady-state; under this stage, soil C availability would presumably control N2O production and emission. Laboratory incubation (Koops et al., 1997) and field (Rochette et al., 2010) studies can support the existence of this postulated steady-state phase as they both observed relatively high rates of N2O release and overall no response to multiple levels of N additions when assessing organic soils (e.g., total C: 374.4 g C kg-1 soil, total N: 21.4 g N kg-1 soil; Rochette et al., 2010).

Sigmoidal variation
This proposed hypothetical conceptualization of direct N2O emission is supported by the sigmoidal variation.