Relationships Between Carbon Fractions and Soil Nutrients in Organic Cassava Cultivation in the Sandy Soil of Northeastern Thailand

This research investigated the interaction between the labile and stable fractions of soil organic carbon (SOC) during the cultivation of organic cassava in sandy soil in northeastern Thailand over a period of five years. We collected surface soil samples (0–30 cm) from this sandy region, utilizing a combination of cow and chicken manure along with dried distilled grains (DDGs) from cassava fermentation for ethanol production, to monitor and compare the effects of continuous mixed organic fertilization on SOC, carbon fractions, soil pH, and nitrogen and phosphorus levels throughout a five-year period of varying land use ages (LUA) to the pre-fertilization state. This study proposed that the use of a combination of organic fertilizers could increase soil organic carbon levels. This study indicates that the continuous application of organic fertilizers over five years does not lead to a significant increase in soil carbon; however, it may result in temporary alterations in different organic carbon fractions. The study showed that the mixed organic fertilization could the increase carbon fractions. Labile carbon (LBC) fraction was at its lowest before fertilization, peaking at LUA 3 and increasing by 5.44–25.50% after organic fertilizer addition. The first year revealed high non labile carbon (NLBC) levels, exceeding 60%, in comparison to the pre-fertilizer period. In the second year, NLBC levels declined to LUA 5, a change that was not statistically significant. After pre-fertilization, the concentration of recalcitrant carbon (REC) did not significantly decrease. Nitrate (NO3−-N) concentrations exhibited no significant fluctuation pre-and post-fertilization. Furthermore, the Bray II-extractable phosphorus (P(B)) decreased (i.e., LUA 1 and 4). The pH levels dropped after the addition of organic fertilizer, particularly in the second year. We found strong positive links between SOC and carbon fractions such as NLBC (r = 0.54 ***) and POXC (r = 0.49 ***). However, neither LBC nor less labile carbon fraction (LLBC) showed any significant correlations with SOC. The negative correlations were observed between ammonium (NH4+-N) and NO3−-N with labile carbon types, such as LBC, LLBC, and POXC, while positive correlations were noted with stable carbon fractions, such as NLBC, and REC. From the application of this organic fertilizer, there are various amounts of organic carbon which cause the following effects: The inclusion of LBC from mixed organic fertilization seems to enhance SOC decomposition rather than accumulation. NLCB may persist in sandy soil for a longer duration than LBC, resulting in the retention of SOC in sandy soil. Our results suggested the implementation of a systematic soil testing strategy to monitor temporal variations in carbon fractions and nutrient levels. Using the right amounts of both LBC and NLBC would improve soil health and help store carbon through organic fertilizers.