Temporal Dynamics of Soil Carbon Stocks and Mineralization Rates in Coffea arabica Agroforestry Systems

This study assesses how different Coffea arabica cropping systems affect soil carbon dynamics in northern Thailand, at a 13-ha highland research station (850–900 m a.s.l., ~20% slope, 1,355 mm mean annual rainfall). Four land uses were compared — natural forest (NF) as reference, coffee monoculture (CA), coffee intercropped with forest trees (CF), and coffee intercropped with persimmon (CP) — with triplicate sampling at three depths (0–20, 20–40, 40–60 cm) across the rainy, cold and summer seasons of 2022–2023. Measurements covered physicochemical properties (bulk density, texture, pH, EC, total C and N, P, K, Ca, Mg), C and N stocks, and microbial activity: microbial respiration (MR) via 50-day incubation, microbial biomass carbon (MBC) and dissolved organic carbon (DOC) via chloroform fumigation–extraction, analysed with ANOVA/LSD, t-tests, linear regression and Pearson correlations. Relative to natural forest, conversion to coffee with forest trees (CF) raised soil carbon by 12.10% over 0–60 cm (p = 0.004), whereas coffee with persimmon (CP) lowered it by 11.89% (p = 0.039) and monoculture by a non-significant 9.06%. MBC was higher in CF and CP, comparable to natural forest, while CA showed significantly lower MBC (p < 0.05) and the highest DOC concentration in the rainy season. MBC and MR correlated positively, but high MBC did not necessarily translate into greater microbial activity. Seasonal variation significantly influenced all measured parameters. The authors conclude that both agroforestry coffee systems mitigate deforestation impacts and sustain soil fertility better than monoculture, with coffee–forest tree intercropping the optimal choice for carbon stock enhancement — underscoring that species selection determines whether agroforestry actually gains carbon.