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

Abstract

Abstract. Amending soils with biochar, a pyrolyzed organic material, is an emerging practice to potentially increase plant available water. However, it is not clear (1) to what extent biochar amendments increase soil water storage relative to non-amended soils and (2) whether plants grown in biochar amended soils access different pools of water compared to those grown in non-amended soils. To investigate these questions, we set up an upland rice field experiment in a tropical seasonally dry region in Costa Rica, with plots treated with two different biochar amendments and control plots, from where we collected hydrometric and isotopic data (δ18O and δ2H from rain, soil, groundwater and rice plants). Our results show that the soil water retention curves for biochar treated soils shifted, indicating that rice plants had 2 % to 7 % more water available throughout the growing season relative to the control plots. In addition, we observed a within treatment variability in the soil water retention curves which was in the same order of magnitude as one would expect from responses due to differences in biochar application rates or due to differences in biochar typologies. The stable water isotope composition of plant water showed that the rice plants across all plots preferentially utilized the more variable soil water from the top 20 cm of the soil instead of using the deeper and less variable sources of water. Our results indicated that rice plants in biochar amended soils could access larger stores of water more consistently and thus could withstand dry spells of seven extra days relative to rice grown in non-treated soils. Though supplemental irrigation was required to facilitate plant growth during extended dry periods. Therefore, biochar amendments can complement, but not necessarily replace, other water management strategies.

Drone-Based Hyperspectral and Thermal Imagery for Quantifying Upland Rice Growth and Water Use Efficiency After Biochar Application

“…The shallower the measurement location, the stronger the influence from atmospheric forcing and consequently the larger the fluctuations of the measurements, explaining the higher variability of the in situ measurements compared to the remote sensing estimates. Besides, the groundwater level was reported 0.8 m below the surface during the dry period [25] and the capillary rise in clay loam soil of the site might dampen the fluctuations of root zone soil moisture. The mismatch of in situ sensor-measured and UAV-derived  makes the two sets of data not readily comparable.…”

“…Harvest took place on 21 November 2018 and indicated the end of the experiment. See [25] for details of biochar experimental design. Figure 1.…”

“…where and are the dry and wet edges of the ΔT-fc triangle, estimated following [42]; and [% in m 3 /m 3 ] is the soil moisture content to be estimated. and are soil moisture content at field capacity and wilting point respectively, estimated from soil water retention curve at soil matric potential of -0.05 and -1.5 MPa respectively (see [25] for details of water retention curve estimation using in situ measurements). The spatially explicit soil matric potential (, [MPa]) was then estimated from the UAV-derived soil moisture content using the Van Genuchten model [43]:…”

“…where Θ = − − , = 1 − 1 , and , , , and n are the water retention curve parameters reported in [25].…”

Comments to hess-2020-404

“…Based on these findings, the authors hypothesized that amending biochar into the top 10-30 cm of the soil, could increase soil water storage and rice plants be able to use water from different soil water pools in comparison to plants grown in non-amended soils (L113-125) across the different time periods studied. First, I should say that after the work of Brooks et al (2020), countless studies across regions (including tropical wet environments; see Goldsmith et al 2012;Muñoz-Villers et al 2018) and revisions have showed that plants use evaporatively fractionated soil water (Sprenger et al, 2016;Sprenger and Allen 2020). The general finding is that plant water is isotopically similar to bulk soil water but not to low suctionâȂŘlysimeter water, implying that roots are generally located in less conductive (mobile) pores where water tends to travel more slowly and can reside for longer times.…”