Impact of afforestation on soil nitrogen mineralization, and availability in a semi-arid sandy site in western Northeast China

Background/Question/Methods Assessment of the impacts of ecosystem restoration and vegetation change has been gaining much attention in the field of applied ecology in recent decades. Combating desertification in the Keerqin Sandy Lands is of national importance as a sustainable development issue in China. Revegetation projects in Zhanggutai (42°43' N, 122°22' E), the southeastern part of Keerqin Sandy Lands, were initiated in the mid 1950s. The earliest projects planted trees of Pinus sylvestris var. mongolica, P. tabulaeformis, P. densiflora and several Populus species. What were the outcomes of this enormous change in land-cover and land-use from grassland to forest plantations over the past 50 years? Addressing the patterns of soil ecological processes is beneficial for understanding altered ecosystems which is critical for ecological restoration. We compared soil accumulation of mineral nitrogen (N) using the ion exchange resin bags method and soil mineralization rates using PVC closed-top tube in situ incubation method under plantations of two pine species (Pinus sylvestris var. mongolica and P. densiflora) and one poplar species (Populus pseudo-simonii), and one grassland ecosystem in the Zhanggutai region of the southeastern Keerqin sandy lands, which is a pilot experimental site for afforestation and dune fixation in China.

Results/Conclusions The accumulation of mineral N during the growing season was significantly different due to stand types (P < 0.05), with the highest in Pinus densiflora (1671.30 μg·g^-1 dry resin), followed by P. sylvestris var. mongolica (626.40 μg·g^-1 dry resin) and Populus pseudo-simonii (592.65 μg·g^-1 dry resin), with the lowest accumulations in grassland (502.20 μg·g^-1 dry resin). The net mineralization rate during the growing season were also significantly different due to stand types, with the highest in Populus pseudo-simonii (42.90 kg ha^-1 yr^-1) plantation, followed by Pinus sylvestris var. mongolica (31.56 kg ha^-1 yr^-1) plantation, with the lowest in P. densiflora (22.80 kg ha^-1 yr^-1) plantation and grassland (20.10 kg ha^-1 yr^-1) (P < 0.05). This study indicates that afforestation of sandy grassland has important impacts on N cycling rate, which results in a shift in patterns of soil mineralization rates and N availability. In order to further improve the effects of sand fixation in Zhanggutai sandy land, proper thinning in Pinus sylvestris var. mongolica plantations and mixing P. densiflora with Populus pseudo-simonii would help reduce N limitation of trees.