Abstract
There has been much debate about the uncertainties associated with the estimation of direct and indirect agricultural nitrous oxide (N2O) emissions in developing countries and in particular from tropical regions. In this study, we report an up-to-date review of the information published in peer-review journals on direct N2O emissions from agricultural systems in tropical and sub-tropical regions. We statistically analyze net-N2O-N emissions to estimate tropic-specific annual N2O emission factors (N2O-EFs) using a Generalized Additive Mixed Model (GAMM) which allowed the effects of multiple covariates to be modelled as linear or smooth non-linear continuous functions. Overall the mean N2O-EF was 1.2% for the tropics and sub-tropics, thus within the uncertainty range of IPCC-EF. On a regional basis, mean N2O-EFs were 1.4% for Africa, 1.1%, for Asia, 0.9% for Australia and 1.3% for Central & South America. Our annual N2O-EFs, estimated for a range of fertiliser rates using the available data, do not support recent studies hypothesising non-linear increase N2O-EFs as a function of applied N. Our findings highlight that in reporting annual N2O emissions and estimating N2O-EFs, particular attention should be paid in modelling the effect of study length on response of N2O.
Highlights
In the past century, to meet the increasing demand for food from a growing global population, agriculture has undergone a global “green revolution” that introduced high yielding cereal varieties together with improved agronomy including use of mineral fertilizer
The underrepresentation of tropical and sub-tropical agricultural soils in global N2O emissions studies represents a major limitation in the development of accurate N2O-emission factors (EF) for these regions
To our knowledge this is the first study reporting a review of agricultural N2O-EFs measured exclusively across the tropical and sub-tropical regions
Summary
To meet the increasing demand for food from a growing global population, agriculture has undergone a global “green revolution” that introduced high yielding cereal varieties together with improved agronomy including use of mineral fertilizer. Since net-N2O-N emissions may depend on multiple factors in an additive, non-linear or interactive manner we applied a Generalized Additive Mixed Model (GAMM) to assess the effects of multiple potential explanatory variables on N2O-EF, including continuous functions of N application rate and experiment duration. This modelling approach provides a statistical exploration of published data that can be used to guide further studies aimed at improving our knowledge of the factors influencing the relationship between N2O-EF and N input in the tropics
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