Soil organic carbon and nitrogen pools in a chronosequence of poplar (Populus deltoides) plantations in alluvial soils of Punjab, India

Poplar based agroforestry system has the potential to make soil a net sink for carbon thereby attenuating CO2 load in the atmosphere and improving soil fertility and productivity. The sequestered soil carbon varies with land use system, age of tree plantations and other edaphic factors. We selected four sites with different duration of use as poplar plantation (6, 12, 18 and 24 years representing 1, 2, 3 and 4 cutting cycles) to study the temporal effects of poplar based agroforestry system on dynamics of carbon and nitrogen pools in comparison to rice-wheat system. Soil samples were collected up to 45 cm depth at an interval of 15 cm each from farmers' fields in Punjab, India and analyzed for carbon and nitrogen pools. The soil organic carbon (SOC) content increased from 0.434 in 1 cutting cycle to 0.902 % in 4 cutting cycles in the surface soil layer. The SOC, total organic carbon and total carbon (TC) stock increased significantly from one to four cutting cycles of poplar. Among all treatments highest SOC and TC contents were observed under rice-wheat sequence. Poplar plantation increased the total C content more effectively because higher duration of plantation resulted in accumulation of more recalcitrant carbon forms. The recalcitrant carbon stock increased from 3.14 Mg ha(-1) in 1 cutting cycle to 6.41 Mg ha(-1) in 4 cutting cycles in the surface layer. Ammonical, nitrate and total nitrogen also increased in the four cutting cycles of poplar. Redundancy analysis indicated positive correlation between TC and total nitrogen suggesting that a composite of soil indicators could better distinguish the influence of poplar cutting cycles on organic carbon and nitrogen pools. The long term adoption of poplar based agroforestry system suggests high carbon sequestration efficiency not only quantitatively but also qualitatively as indicated by the buildup of stable pools of carbon.