Variations in soil properties and stoichiometric ratios with stand age under agarwood monoculture and polyculture on smallholder farms

Smallholder plantations have gained increasing attention in land management and conservation of the remaining forest resources locally. Plantation of agarwood (Aquilaria malaccensis), a critically endangered tree species, has become a promising source of income for smallholders because of its high economic value in parts of Asia. Agarwood is considered profitable with plantation cycles of 15 years or more. The objectives of the present study are to (i) quantify variations in soil physico-chemical properties, (ii) quantify variations in soil organic carbon (SOC) stocks, and (iii) evaluate variations in stoichiometric ratios with stand type, age and soil depth. Soil texture, bulk density, water holding capacity (WHC), soil pH, available nitrogen (N), phosphorus (P), potassium (K), exchangeable calcium (Ca), magnesium (Mg) and soil organic carbon (SOC) stocks in very labile, labile, less labile and non-labile pools were determined in different soil depths (0–100 cm) under <10, 10–20 and >20 years old monoculture and polyculture stands of agarwood on smallholder farms. Soil texture and bulk density did not significantly vary with stand type or age, but significantly higher WHC was found in >20 years old stands. Across stand ages and soil depths, available N and P concentrations were higher in monoculture, while available K concentrations were higher in polyculture. SOC content significantly varied with the stand type, age and soil depth and their interaction effects. The highest SOC content (28.9 g kg−1) was recorded in the 0–10 cm of >20 years old monoculture stands, while the lowest (4.2 g kg−1) in the 51–100 cm depth of 10–20 years old polyculture stands. SOC stocks significantly increased with increase in stand age in both monoculture and polyculture stands. The highest total SOC stock (40.7 Mg ha−1) was recorded in the 51–100 cm depth of >20 years old monoculture stands, while the lowest (13.4 Mg ha−1) was in the 31–50 cm depth of <10 years old polyculture stands. The SOC stocks in the recalcitrant (passive) pools, which are the main drivers of long-term soil carbon sinks, were 1.5-fold more in monoculture than polyculture stands.