Investigating the impacts of biochar on water fluxes in tropical agriculture using stable isotopes

Abstract

25 Keywords: biochar, stable isotopes of water, soil and plant water, soil water retention curves, 26 plant water uptake 27 Abstract 28 Amending soils with biochar, a pyrolyzed organic material, is an emerging practice to potentially 29 increase plant available water. However, it is not clear (1) to what extent biochar amendments increase 30 soil water storage relative to non-amended soils and (2) whether plants grown in biochar amended soils 31 access different pools of water compared to those grown in non-amended soils. To investigate these 32 questions, we set up an upland rice field experiment in a tropical seasonally dry region in Costa Rica, 33 with plots treated with two different biochar amendments and control plots, from where we collected 34 hydrometric and isotopic data (δ 18 O and δ 2 H from rain, soil, groundwater and rice plants). Our results 35 show that the soil water retention curves for biochar treated soils shifted, indicating that rice plants had 36 2 % to 7 % more water available throughout the growing season relative to the control plots. In addition, 37 we observed a within treatment variability in the soil water retention curves which was in the same 38 order of magnitude as one would expect from responses due to differences in biochar application rates 39 or due to differences in biochar typologies. The stable water isotope composition of plant water showed 40 that the rice plants across all plots preferentially utilized the more variable soil water from the top 20 41 cm of the soil instead of using the deeper and less variable sources of water. Our results indicated that 42 rice plants in biochar amended soils could access larger stores of water more consistently and thus could 43 withstand dry spells of seven extra days relative to rice grown in non-treated soils. Though supplemental 44 irrigation was required to facilitate plant growth during extended dry periods. Therefore, biochar 45 amendments can complement, but not necessarily replace, other water management strategies. 46