Accessible, affordable, fine‐scale estimates of soil carbon for sustainable management in sub‐Saharan Africa

Abstract

Soil C status is acritical component of greenhouse gas mitigation efforts, and supports food security through affecting crop growth and management. In much of the world, laboratory-based measures of soil C are expensive and logistically challenging, whereas map-based predictions generated at the continental scale (e.g., African Soil Information Service [AfSIS]; www.soilgrids.org) may be unreliable at the management-relevant, policy-relevant, field, and regional scales. We test whether an USD 350, open source, field portable reflectometer can provide site-specific estimates of soil C status, and also predict whether a crop will respond to fertilizer across 1,155 sites in central and southern Malawi based on an established threshold of 9.4 g C kg −1 soil. When compared with soil C measured by combustion, the scanner calibrated with covariates of field-estimable texture class and slope class provides unbiased (0.42 ± 0.44 g C kg −1 soil; p = .06), precise (R 2 = .57), and actionable (area under the receiver operating characteristic curve [AUC] = 0.88) data at the field scale, including at unmeasured locations (relative prediction error 19-23%) and at the village scale. The reflectometer outperformed predictions from the continental-scale AfSIS database, which were neither precise (R 2 = .044) nor actionable (AUC = 0.63) at the field scale, and underestimated soil C in the region by 2.5 ± 0.5 g C kg −1 soil (p < .001). The reflectometer is an accessible tool to monitor soil C in sub-Saharan Africa to improve field-level management, and guide regional policies that support food security while combating climate change.

Ideas and perspectives: Enhancing research and monitoring of carbon pools and land-to-atmosphere greenhouse gases exchange in developing countries

“…While soil bulk density can be measured with locally available instruments (Grossman and Reinsch, 2002), alternative approaches with respect to the standard can be adopted to produce reliable SOC content data (Table 1, Gessesse and Khamzina, 2018;Apesteguia et al, 2018;Jha et al, 2014). A low-cost, field-portable reflectometer (a hardware cost of USD 350) provided precise and accurate soil C estimates in central and southern Malawi (Ewing et al, 2021).…”

Advancing the science and practice of ecological nutrient management for smallholder farmers

“…Newly developed handheld sensors and digital tools that measure soil C org levels are one example; feedback on soil C org accrual is valuable to farmers who practice ENM and for implementing payments for C sequestration (Tieszen et al, 2004). For instance, in Malawi, inexpensive handheld sensors enabled extension educators to visit with farmers and provide reliable information on real-time soil C org status and crop N response (Ewing et al, 2021).…”

Soils, Science and Community ActioN (SoilSCAN): a citizen science tool to empower community-led land management change in East Africa

“…Through the associated app, a rapid assessment of the following soil characteristics is obtained: pH, soil texture class, TOC (g kg −1 ), total nitrogen (TN; g kg −1 ), TP (g kg −1 ), total potassium (TPo; mmol + kg −1 ), and CEC (mmol + kg −1 ). This hand-held soil scanner can enable farmers to become citizen scientists empowered to collect data to establish research data bases that support critical landscape decisions (Ewing et al 2021). This offers a step change from existing piecemeal, often commercially driven data gathering by external actors (figure 1).…”