Automatic Estimation of Soil Biochar Quantity via Hyperspectral Imaging

Introduction

Food security and climate change are two key global issues for the 21^st century. With growing world population and rising living standard, global demand for agricultural products will rise 70% by 2050 (Food and Agriculture Organization, 2009), while agricultural productivity is facing emerging plateau and high exposure to climate change (Keating and Carberry, 2010). However, many conventional farming practices are based on high resource inputs (e.g. fertilizer, irrigation and fuel), and tending to generate high greenhouse gas (GHG) emissions and exacerbate soil degradation, thereby unlikely to sustain the rate of productivity gain (Robertson, 2010). Thus, there is an imperative need for farming approaches that can efficiently use constrained resources (e.g. land and water) and effectively mitigate greenhouse gas emissions.

Soil amendment with biochar, a carbon (C)-rich product of burning biomass in the absence of oxygen (pyrolysis), is recognized globally as an emerging approach to improve soil fertility and increase soil C stock (Woolf et al., 2010). Biochar has unique properties to improve soil chemo-physical and biological properties for crop growth (Chan and Xu, 2009, Bai et al., 2015). The porous physical structure of biochar can improve soil bulk density and aeration (Alburquerque et al., 2014; Mukherjee and Zimmerman, 2014). The large surface area also creates a great sorption capacity to retain soil moisture and nutrients and improve soil cation exchange capacity (CEC) (Chan and Xu, 2009; Liu et al., 2012; Novak et al., 2012). The alkaline nature of many biochar makes such materials especially suitable for improving acidic soil (Novak et al., 2009). Biochar made from specific feedstocks (e.g. manure) have high nutrient content and promotes plant growth (Hass et al., 2012; Lentz and Ippolito, 2012; Uzoma et al., 2011). These positive effects of biochar on crop yield are especially significant in degraded soils (Spokas et al., 2012, Xu et al., 2015). Many types of biochar also has high proportion of recalcitrant C with hundreds to thousands of years of durability, making it a potentially effective soil C sink to mitigate climate change (Cheng et al., 2008; Kuzayakov et al., 2009). Agronomic benefits of biochar and the potential of biochar for soil carbon sequestration have been widely demonstrated in many on-ground trials over the world (Atkinson et al., 2010; Spokas et al., 2012).