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• Soil properties under monoculture and polyculture agarwood systems were studied. • BD, clay, sand and WHC varied significantly with the interaction effect of age and depth. • Exchangeable Ca and Mg significantly varied with the interaction effect of stand type and age. • The first two principal components accounted for 95.5% of the variation in soil physical and chemical properties. 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. [ABSTRACT FROM AUTHOR]

Indigenous agroforestry systems are important reservoirs of biodiversity, and ecosystem services with a potential contribution for conservation of biodiversity while sustaining the livelihood of the rural populace. Pineapple (Ananas comosus) agroforestry systems (PAFS) form an essential constituent of the rural landscape in the Indian Eastern Himalayas and other parts of Asia. The traditional PAFS management in southern Assam is unique in that it involves shifting cultivation transitioning from native forests to a PAFS. Scarcity of information on the functioning and services in terms of species diversity and carbon storage potential in the traditional PAFS has restricted the opportunities for consideration under the nature-based solutions of climate action including REDD+. Therefore, the present study assessed the tree diversity and ecosystem carbon storage in a chronosequence from swidden agriculture through different phases of PAFS establishment. The result demonstrated that basal area in the PAFS increased with age. The most dominant species in the native forests was Palaquium polyanthum , while agricultural land use and PAFS aged <5, 11–15 and > 15 years old were dominated by Gmelina arborea , Albizia procera , Areca catechu and Hevea brasiliensis , respectively. The highest value of Shannon-Wiener diversity index (H) was recorded in native forests (2.71), and lowest in 5–10 years old PAFS. The ecosystem carbon storage declined from 261.43 Mg ha−1 in native forests to 181.07 Mg ha−1 in <5-years old PAFS. In <5 years old PAFS, the ecosystem carbon storage was 30 % lower than the native forest; while at >15 years it was merely 5 % less than the native forests. The traditional PAFS maintains a steady ecosystem carbon stock while reducing land use related carbon emission and providing additional co-benefits to the communities. Therefore, the traditional PAFS constitute a good opportunity for REDD+. [Display omitted] • The Shannon-Wiener diversity index (H) was the highest in native forests (2.71), and the least in 5–10 years PAFS. • Forest clearing and burning prior to the establishment of PAFS caused a decline of 30 % of the ecosystem C. • The bush density of the pineapple increased with the increase in the PAFS age. • At >15 years of PAFS ecosystem carbon loss was merely 5 % compared to native forests. [ABSTRACT FROM AUTHOR]