Emissions intensity and carbon stocks of a tropical Ultisol after amendment with Tithonia green manure, urea and biochar

Bernard Fungo,Johannes Lehmann,Karsten Kalbitz,Moses Tenywa,Margaret Thionģo,Henry Neufeldt

doi:10.1016/j.fcr.2017.05.013

Abstract

Biochar has been shown to reduce soil emissions of CO2, CH4 and N2O in short-term incubation and greenhouse experiments. Such controlled experiments failed to represent variable field conditions, and rarely included crop growth feedback. The objective of this study was to assess the effect of biochar, in comparison to green manure and mineral nitrogen, on greenhouse gas Emissions Intensity (EI=emissions in CO2 equivalents per ton of grain yield) in a low-fertility tropical Ultisol. Using a field trial in western Kenya, biochar (0 and 2.5tha−1; made from Eucalyptus wood) was integrated with urea (0 and 120kgNha−1) and green manure (Tithonia diversifolia; 0, 2.5 and 5tha−1) in a factorial design for four consecutive seasons from October 2012 to August 2014. Compared to the control, biochar increased soil CO2 emissions (9–33%), reduced soil CH4 uptake (7–59%) and reduced soil N2O emissions (1–42%) in each season, with no seasonal differences. N2O emissions increased following amendment with T. diversifolia (6%) and urea (13%) compared to the control. Generally, N2O emissions decreased where only biochar was applied. The greatest decrease in N2O (42%) occurred where all three amendments were applied compared to when they were added separately. EI in response to any of the amendments was lower than the control, ranging from 9 to 65% (33.0±3.2=mean±SE). The amendments increased SOC stocks by 0.1–1.2tha−1year−1 (mean±SE of 0.8±0.09tha−1year−1). The results suggest decreased net EI with biochar in low fertility soils mainly through greater net primary productivity (89% of the decrease).

Highlights

The search for climate-smart agricultural production technologies is directing research to identify innovations that address multiple benefits such as crop productivity, carbon sequestration and mitigation of soilatmosphere greenhouse gas (GHG) emissions
The objective of this study was to assess the effect of biochar, in comparison to green manure and mineral nitrogen, on greenhouse gas Emissions Intensity (EI = emissions in CO2 equivalents per ton of grain yield) in a low-fertility tropical Ultisol
Biochar effects were not observable for daily CO2 measurements, but its effect was observed for CH4 and N2O, where it generally reduced the CH4 sink capacity of the soil and reduced N2O emission

Summary

Introduction

Addition of biochar (pyrogenic organic matter) to agricultural soils as a management strategy has reportedly increased crop yields in several studies but has shown variable effects on GHG fluxes (Knoblauch et al, 2011; Cayuela et al, 2014). The amount of fertilizer N additions is considered proportional to the N2O emissions (Manzoni and Porporato, 2009; Mori and Hojito, 2011). It is not clear, if simultaneous addition of biochar with either fertilizer N or legume mulch or a combination if the two may result in GHG emission reductions. Uncertainty exists whether any emission reductions would persist over several cropping seasons

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Conclusion