Impact of Stand Age on Soil C, N and P Dynamics in a 40 year Chronosequence of Alder - Cardamom Agroforestry Stands of the Sikkim Himalaya

The impact of age (5,10,15,20,30,40years) on chemical characteristics of mineral soil under anage sequence falder- ardamomagro forestry stands was studied in the Eastern Himalayas.These asonal variation in soil organic carbon(OC),soil organic matter (SOM),total nitrogen (TN), forms of phosphorus (total P, organic P, in organic P,availableP, fractionated forms of P),mineral nitrogen,potential mineralization and nitrification was measured in the chrono sequence across three replicate sites each having six representative stand ages. We hypothesized that nutrient stocks would belower in younger agroforestry stands, would eventually increase with stand age due to the influence of alder but then decline as the stands mature further. The expected pattern of increasing soil nutrient stocks with stand age did occur with a peak at 15–20 years ; nutrient stocks then substantially declined in 30- to 40-year-old stands. A significant seasonality, which coincided with card a mom flowering and fruiting , was observed in soil nutrient contents and transformation rates.The 15–20-year-old stand shade the highest nutrient pools and potential transformation rates, where as the youngest and oldest stand shad the lowest nutrient pools. Soil acidity increased with stand age.Soil was negatively related to stand age and SOM in the linear regression. Nutrient dynamics varied with age depending on the successional stage, which limited soil nutrient availability for plant up take after the 20-year point. The performance of both alder and card mom was reduced after this age likely due to limited soil nutrient availability and nutrient dynamics as a result of recurrent biomass removal – part of the traditional management practice.