Abstract
Abstract. Deforestation and forest degradation in the tropics may substantially alter soil N-oxide emissions. It is particularly relevant to accurately quantify those changes to properly account for them in a REDD+ climate change mitigation scheme that provides financial incentives to reduce the emissions. With this study we provide updated land use (LU)-based emission rates (104 studies, 392 N2O and 111 NO case studies), we determine the trend and magnitude of flux changes with land-use change (LUC) using a meta-analysis approach (44 studies, 135 N2O and 37 NO cases) and evaluate biophysical drivers of N2O and NO emissions and emission changes for the tropics. The average N2O and NO emissions in intact upland tropical forest amounted to 2.0 ± 0.2 (n = 90) and 1.7 ± 0.5 (n = 36) kg N ha−1 yr−1, respectively. In agricultural soils annual N2O emissions were exponentially related to N fertilization rates and average water-filled pore space (WFPS) whereas in non-agricultural sites a Gaussian response to WFPS fit better with the observed NO and N2O emissions. The sum of soil N2O and NO fluxes and the ratio of N2O to NO increased exponentially and significantly with increasing nitrogen availability (expressed as NO3− / [NO3−+NH4+]) and WFPS, respectively; following the conceptual Hole-In-the-Pipe model. Nitrous and nitric oxide fluxes did not increase significantly overall as a result of LUC (Hedges's d of 0.11 ± 0.11 and 0.16 ± 0.19, respectively), however individual LUC trajectories or practices did. Nitrous oxide fluxes increased significantly after intact upland forest conversion to croplands (Hedges's d = 0.78 ± 0.24) and NO increased significantly following the conversion of low forest cover (secondary forest younger than 30 years, woodlands, shrublands) (Hedges's d of 0.44 ± 0.13). Forest conversion to fertilized systems significantly and highly raised both N2O and NO emission rates (Hedges's d of 1.03 ± 0.23 and 0.52 ± 0.09, respectively). Changes in nitrogen availability and WFPS were the main factors explaining changes in N2O emissions following LUC, therefore it is important that experimental designs monitor their spatio-temporal variation. Gaps in the literature on N oxide fluxes included geographical gaps (Africa, Oceania) and LU gaps (degraded forest, wetland (notably peat) forest, oil palm plantation and soy cultivation).
Highlights
Land use (LU) and land-use change (LUC) are important contributors to global greenhouse gas (GHG) emissions
We followed three main steps to assess how soil N2O and nitric oxide (NO) emissions were affected by LU and LUC in the tropics and subtropics: (i) compiling a database of all studies on soil N2O and NO fluxes, selecting those integrating seasonal variation in their experimental design and categorizing LU types; (ii) estimating average emission rate per LU category and exploring biophysical factors affecting them; and (iii) characterizing the magnitude of emission change due to LUC using a meta-analysis approach and evaluating how this change could be expressed as a function of the change in biophysical factors through a meta-analysis regression
We summarized the number of studies and assessed the representation of LU per continent categorizing them in five geographical areas: North-Central America, South America, Africa, South Asia and Oceania
Summary
Land use (LU) and land-use change (LUC) are important contributors to global greenhouse gas (GHG) emissions. The current contribution of LUC to total anthropogenic GHG emissions is estimated to be between 7 and 18 % (Houghton, 2003; Baumert et al, 2005; Baccini et al, 2012; Harris et al, 2012). This estimation heavily depends on biomass values and deforestation rates and is associated with high uncertainties, especially in the tropics (Houghton, 2005).
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