Limits of agricultural greenhouse gas calculators to predict soil N2O and CH4 fluxes in tropical agriculture.

Meryl Richards,George Nyamadzawo,Emma Keller,Jonathan Hillier,Ngonidzashe Chirinda,Andreas De Neergaard,Quynh Duong Vu,Jørgen E Olesen,Daniella Malin,Eva Wollenberg,Myles Oelofse,Ruth Metzel,Proyuth Ly,Todd S Rosenstock

doi:10.1038/srep26279

Abstract

Demand for tools to rapidly assess greenhouse gas impacts from policy and technological change in the agricultural sector has catalyzed the development of ‘GHG calculators’— simple accounting approaches that use a mix of emission factors and empirical models to calculate GHG emissions with minimal input data. GHG calculators, however, rely on models calibrated from measurements conducted overwhelmingly under temperate, developed country conditions. Here we show that GHG calculators may poorly estimate emissions in tropical developing countries by comparing calculator predictions against measurements from Africa, Asia, and Latin America. Estimates based on GHG calculators were greater than measurements in 70% of the cases, exceeding twice the measured flux nearly half the time. For 41% of the comparisons, calculators incorrectly predicted whether emissions would increase or decrease with a change in management. These results raise concerns about applying GHG calculators to tropical farming systems and emphasize the need to broaden the scope of the underlying data.

Highlights

Background soilN20Multivariable empirical modelc Not includedN20 from fertilizers N2O from crop CH4 from riceBiomass C stock changes residue management cultivation Soil C stock changes within a land use categorySingle emission factoraMultiple emission factorsa,dMultiple emission factorsa,eAllometric equationsa
Our analysis shows that Cool Farm Tool (CFT) consistently overestimated net greenhouse gas (GHG) emissions from the agricultural systems represented in this sample (Fig. 1), while EX-ACT over-estimated and under-estimated emissions in near-equal proportions
N inputs in the treatments included in our analysis ranged from 0 to 250 kg N/ha, with a mean of 97 N kg/ha, and given the evidence on non-linearity of N2O emissions to input, extrapolation is not trivial. Attempts to improve both the exponential model used in CFT and the IPCC emission factor used in EX-ACT, both of which are based on the same data set, have so far been limited by a lack of empirical studies in tropical climates[12,16]

Summary

Introduction

Background soilN20Multivariable empirical modelc Not includedN20 from fertilizers N2O from crop CH4 from riceBiomass C stock changes residue management cultivation Soil C stock changes within a land use categorySingle emission factoraMultiple emission factorsa,dMultiple emission factorsa,eAllometric equationsa. We compiled GHG emissions data from field experiments in tropical smallholder systems and compared them with predicted emissions from two of the most commonly used GHG calculators, EX-ACT10 and Cool Farm Tool (CFT)[5].

Results

Conclusion