Soil Carbon and Nitrogen in a Mountain Forest in Northern China: 40 Years After Reforestation.

China has the largest area of “man-made” forest in the world and it continues to convert more than 2 million hectares of cropland and wasteland into forest each year. As the area of planted land continues to increase, issues related to the quality and health of these plantations and their ecological impact give rise to intensified attention. This paper presents a case study on soil carbon and nitrogen as affected by community type and management in a mid-altitude mountain forest ecosystem in northern China, which was largely reforested more than 40 years ago. Four dominant communities, including two reforested [Chinese-pine (Pinus tablaeformis) and Chinese pine- purpleblow maple (Acer truncatum) mixture] and two secondary [Manchurian linden (Tilia mandshurica) and mixed hardwoods, mainly Ailanthus altissima, Syringa reticulata (Blume), Prunus siberica (Linn), and Ulmus spp.], were compared in two orthogonal contrasts: (1) Chinese pine monoculture vs. Chinese pine-maple mixture, and (2) the two replanted vs. the two secondary communities. Soil samples were taken from three depths (0-10, 10-20 and 20-40 cm) at three random locations in each stand. Soil C and N stocks, C fractionation by density, and dissolved organic C (DOC) were determined. The Chinese pine-maple mixture had higher C and N stocks, lower light-fraction C and DOC than Chinese pine; the two secondary communities had higher C and N stocks, lower light-fraction C and DOC than the two replanted pine-dominated communities. Results suggested 40 yr after the restoration in this northern ecosystem, while mixing broad-leafed species with pine monoculture is beneficial, fostering secondary regeneration may still be a more effective management in enriching and stabilizing C and N in the soil.