Biochar amendments to agricultural soils: the potential for soil physicochemical improvement and carbon sequestration.
2017-03-02T04:34:02Z (GMT) by
Biochar is a carbon (C) rich, charcoal-like material produced by pyrolysing biomass in a low or oxygen-limited environment. Biochar may offer an innovative and sustainable solution to reduce water and fertiliser applications, providing a waste management option whilst also maintaining or increasing crop yields. As such, biochar has been proposed as a solution for both environmental and economic gains by promoting carbon sequestration and soil remediation whilst improving agro-ecosystem function, and hence productivity. This research assess the utility of biochar in agro-ecosystems with focus on improving soil fertility and crop productivity while achieving long-term stable C sequestration. A further objective was to evaluate the influence that pyrolysis temperature and feedstock have on the physicochemical properties of biochar. Grape marc (winery waste) pyrolysed at 350°C, 400°C, 450°C and 500°C was characterised using methods including ICPAES, ¹³C solid state NMR, SEM, XRD, SA, porosity and FT-IR, providing insight in this regard. While some variability in results was observed, biochars produced at higher temperatures possessed a higher C sequestration potential, whereas biochars produced at lower temperatures may contribute bio-available nutrient and mineralisable biomass when applied to soil. A randomised split-plot design field trial growing rockmelon (Cucumis melo var. Journey) was undertaken to evaluate the utility of biochar to improve water-use efficiency, reduce fertilizer requirements and act as an overall soil conditioner. These effects were investigated with biochar application rates 0 t ha¯¹, 0.25 t ha¯¹, 1 t ha¯¹, 20 t ha-1, interacted with two drip irrigation treatments (“full” ~ 4ML and “half” ~ 2 ML) and half-rates of commercial application. Biochar amendments to an irrigated, well-drained, loam soil did not improve crop yield, soil nutrient status or soil-water dynamics, while lower crop yield was associated with deficit irrigation. Biochar amendments of 20 t ha¯¹ increased water retention at permanent wilting point, but no other changes to the soil physical properties were observed after one field season. To evaluate the contribution of biochar to aggregate stabilization and the distribution of biochar within aggregate size fractions, sampling of a ¹³C-labelled biochar amended silt-loam soil after one field season was undertaken. Using a novel ¹³C isotopic three-part partitioning system, the direct contribution of biochar-C to soil aggregate size fractions were quantified. Biochar-C was greatest in the microaggregate fraction, suggesting it is preferentially bound within microaggregates. This demonstrates the potential for stable long-term C sequestration within the microaggregate structure, showing that biochar is extremely promising as a stable C sequestration tool. A 30-month incubation experiment evaluated the contribution of grape marc biochar to total C and N pools and changes to mineral NO₃¯-N and NH₄⁺-N. The maintained effect of elevated C and N with biochar application indicated medium-term recalcitrance of biochar C and N. Rates of decomposition of N in soil were decreased by biochar amendment. It is thus likely that biochar immobilized N and could reduce N₂O emissions in the medium-term. It is anticipated that the true effects of biochar will be realised at longer time scales. This thesis provides a multidimensional approach to understanding the interactions between the plant-soil environment and biochar additions to evaluate the true potential for biochar in agricultural systems as a soil conditioner and tool for C sequestration.