In any given year, for any given site, the net benefit of NT on annual GHG emissions will probably depend on time elapsed since adoption of NT. This may be especially true for CO2. Rates of SOC accrual under NT are usually highest soon after adoption, and then gradually diminish as the soil organic matter approaches a new steady state, perhaps after several decades (West and Post, 2002; Alvarez, 2005). Thus, NT systems can be initially a significant sink for CO2, but the magnitude of that sink wanes with time.
N2O emissions, too, will probably change with time elapsed since the adoption of NT, though the nature of this response is not yet well defined. If supplemental nitrogen is required to offset nitrogen immobilization in the first years after adoption, emissions might initially increase. If NT results in persistent changes to soil conditions such as bulk density or moisture, the effect on N2O emissions, if any, would likely also persist. Finally, N2O fluxes might change in response to evolving nitrogen mineralization patterns - enhanced immobilization in the early stages, and increasing mineralization later as the system matures with higher organic matter.
CH4 uptake by soil may also change with time from inception of NT, increasing gradually over years or decades. Since CH4 oxidation plays only a small role in net GHG emissions, this effect will likely be small. Finally, CO2 emissions from fossil energy use may change with time, particularly if dependence on fertilizer and herbicide evolves as the NT system matures.
This brief overview demonstrates that any effect of NT on net GHG emissions is not fixed; it will almost certainly vary with time, perhaps in a complex, somewhat unpredictable pattern. This complexity may be further enhanced by the lingering influence of history - at any point in time, the GHG emissions from a given site depend not only on the practices currently imposed on the ecosystem, but also on how the land was managed in previous years (or decades).
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