Changes in soil carbon and nutrient pools along a chronosequence of poplar plantations in the Columbia Plateau, Oregon, USA

Establishment of short-rotation woody crop (SRWC) plantations for meeting the demand of wood and bioenergy production necessitates reclamation of agricultural lands and desert soils, such as those in the southern Columbia Plateau of Oregon, USA. The effects of plantation management on soil carbon (C) storage and nutrient concentration were evaluated, using a chronosequence of poplar (Populus spp.) stands on soils of eolian origin (Xeric Torripsamments). Stands of ages 1, 3, 4, 7, 9, and 10 years (n = 3 per stand age), as well as adjacent agricultural and desert lands, were compared based on soil C, inorganic C (SIC), total nitrogen (N), and nutrient concentrations within the 0- to 50-cm soil depth. The 7- through 10-year-old stands that were in a first-rotation cycle were irrigated and fertilized. The 1- through 4-year-old stands in a second-rotation cycle received a mulch application treatment in addition to the irrigation and fertilization treatments. At age 11 years, the projected plantation C (147.5 Mg ha−1) accumulated almost entirely in the aboveground biomass (62.2%), forest floor (24.3%), and roots (11.7%). There were no significant increases in the mineral soil C and N pools with stand age, despite the presence of increasing trends within the surface layer. The accumulation of the mineral soil C pool (∼1.8%), from the first- (23.5 ± 1.7 Mg C ha−1) to the second-rotation stands (26.3 ± 3.5 Mg C ha−1), was partially offset by a loss of SIC due to irrigation. The SIC pool had a decreasing trend, which was related to dissolution of calcite along the soil profile, from the first- (16.7 ± 3.4 Mg C ha−1) to the second-rotation stands (8.4 ± 5.0 Mg C ha−1). Soil pH (r > 0.6) and exchangeable acidity (r = −0.5) patterns were dependent upon the concentration of exchangeable Ca2+. Soil Mg2+ and K+ concentrations were correlated with soil C concentration in the surface layer (r = 0.5). In coarse-textured soils, a decadal time scale was insufficient to measure significant changes in the mineral soil C pool. Carbon benefits may be gained, however, in aboveground (tree and forest floor) and belowground (roots) biomass accumulations. SRWC plantations are an effective land-use option to restore degraded lands of arid regions.