Dynamics of soil organic carbon and total nitrogen contents in short-rotation triploid Populus tomentosa plantations

The dynamics of soil organic carbon(C) and total nitrogen(N) contents were investigated in the triploid(vs.B304) and the control diploid(vs.1319) Populus tomentosa plantations from 2005 to 2008.The experiment was a split plot design with seven planting densities(2 m×2 m,2 m×3 m,2 m×3.5 m,2 m×4 m,2 m×5 m,3 m×3 m and 3 m×4 m).The following results were obtained:(1) Planting density,sampling year and their interactions had significant effects on soil organic C content.The organic C decreased in 2006 and then increased during the following years,and the maximum content was found in the 2 m×3 m plantation of triploid P.tomentosa in 2008.So this planting density was optional for the triploid clone to enhance the C sequestration.(2) Significant effects of sampling year and its interactions with planting density on soil total N were found during the 4-year growing period.In the 3 m×3 m plantations,the soil total N content decreased for the control diploid clone with the growth of the tree,and increased for the triploid clone in 2007 compared with the former years.The results suggested the planting density of 3 m×3 m was optional for the triploid clone to return the N element into plantation soil.(3) The dynamics of the ratio of soil organic C to total N was in accordance with the changes of the soil organic C.Both were lowest in 2006.(4) Significant positive correlations between soil organic C and total N were found in 2 m×3 m and 2 m×3.5 m growth densities in triploid P.tomentosa plantation in 2008.Significant negative correlation between soil organic C and total N was found in 2 m×5 m planting density for the control.