Summary and concluding remarks

Land-use change always affects the cycling of N in soil and the exchange of N2O with the atmosphere. Emissions tend to increase for a number of years following the conversion of tropical forest to grassland, or the conversion of temperate grassland to cropland. In the longer term, shrinking pools of soil organic matter and turnover rates of N may reverse the situation. Smaller emissions after a land-use change have been observed, often as a consequence of reduced ecosystem productivity, such as in degraded pastures on previously forested land. Where productivity, and with it the turnover of N, are maintained, such as in agroforestry systems, emissions of N2O often tend to continue at an elevated level. Where N pools are large, such as in organic soils, emissions increase dramatically following drainage and cultivation. For a long time they continue to remain substantially larger than before. Even abandoning cultivation may not reduce emissions for decades. Reduced soil tillage, as a more recent development within mainly arable crop production systems, may either reduce or enhance N2O emissions, the outcome depending on soil type and aeration status.

In summarizing all these findings, we can say that any particular type of land-use or management change does not always lead to the same result in terms of N2O emissions and that the result may change with the time horizon. Also, it strongly depends on the specific soil conditions, in particular on the balance between factors limiting and factors promoting N2O emission, and how this balance is shifted by the land-use or management change. Motivations for changing land use or farming practices are diverse, involving decisions by settlers, farmers, regulators, institutions and others, often with contrasting interests. As a consequence, arguments are plentiful. One of them should be about the effect on parameters affecting N2O emissions, especially where land-use or management change is promoted as a way of mitigating climate change by increasing CO2 sequestration.

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