Soil organic carbon stock for carbon credit in smallholder farms

Abstract Soil carbon (C) credits provide monetary incentives to farmers who are motivated to improve soil organic carbon (SOC) stock through agricultural practices such as no‐tillage system, cover cropping, and complex crop rotation as a mitigation and adaptation strategy for climate change. Several major challenges associated with measuring SOC stock for C credit protocol include a high cost and variability. Thus, this study was aimed at characterizing the variability of bulk density (BD), SOC concentration, and stock at an on‐farm operation site under small and aggregated farms to suggest feasible soil sampling methods for SOC stock. In addition, 80% of the farmland is managed by smallholders who work on less than 1–10 ha and provide up to a large proportion of the food supply in Asia and sub‐Saharan Africa. Therefore, evaluating SOC stock, including variability under small and aggregated farms, is critically important. The objective of this study was to characterize the variability of soil parameters, such as BD, SOC concentration, and SOC stock, at an on‐farm operation site to suggest feasible soil sampling methods for assessing the SOC stock. The results show that the SOC stock variability (coefficient of variance; ranged from 6.9% to 21.2%) strongly affected BD and SOC concentration. The number of required samples could be minimized with acceptable uncertainty levels, and it was estimated that six samples are needed to achieve an error of 20% (110 ± 22 Mg ha −1 ).