Spatio-temporal Variability Of Soil Microbial Biomass Carbon And Nitrogen In Five Temperate Forests

Soil microbe is the most important decomposer in forest ecosystems and sensitive indicator to environmental changes. It plays a crucial role in forest carbon and nitrogen cycles. However, few studies on soil microbial biomass carbon (Cmic) and nitrogen (Nmic) have been conducted in the temperate forests of northeastern China. In this study, we used a fumigation-extraction method (FE) to examine temporal and spatial dynamics of Cmic and Nmic along soil profiles in representative temperate forests in this region. The experimental design included five forest types, three 20 m x 30 m plots in each forest type. The forest types were Dahurian larch (Larix gmelinii) plantation, Korean pine (Pinus Koraiensis) plantation, Mongolian oak (Quercus mongolica) forest, aspen-birch forest (dominated by Populus davidiana and Betula platyphylla), and hardwood forest (dominated by Fraxinus mandshurica, Juglans mandshurica and Phellodendron amurense). In each plot, we randomly took soil samples by two soil layers (0-10 cm and 10-20 cm) from five spots every month from March of 2008 to October of 2009, and almost every week from March to May of 2009 during the soil thawing. The samples in each plot were mixed as one sample for Cmic and Nmic determination. All soil samples were refrigerated then analyzed within one week from sampling.The Cmic and Nmic were significantly different (P<0.01) among the forest types. The Cmic in the Dahurian larch, Korean pine, Mongolian oak, aspen-birch, and hardwood forests varied from 278-937,218-1101,313-1082,510-1406, and 440-2265 mg kg-1, respectively, while the Nmic varied from 18-72,18-108,24-123,43-183, and 40-239 mg kg-1, respectively. The Cmic and Nmic in broadleaved forests were higher than coniferous forests. Both Cmic and Nmic decreased with the soil depth increasing for all forests:by the soil layer of 0-10 cm, the Cmic varied from 537-1471 mg kg-1 and the Nmic varied from 51-162 mg kg-1; by the soil layer of 10-20 cm, the Cmic varied from 335-716 mg kg-1 and the Nmic varied from 33-69mg kg-1.Overall, the Cmic and Nmic showed similar seasonal patterns for all forests during 2008 and 2009, in which they keeped a high level in winter, decreased with a substantial decline at the beginning during the soil thawing, and had 1-2 in-between peaks during the growing season, then increased in late autumn. However, the values and occurring times of the peaks varied with forest types and soil layers. The seasonal variations in the Cmic and Nmic were greater at the 0-10 cm soil layer than those at 10-20 cm. The variations during the growing season in 2009 was greater than that in 2008.The Cmic and Nmic were significantly positively correlated with the leaf litterfall, soil organic carbon content and total soil nitrogen content. The Cmic was positively correlated with soil water content, but negatively with the environmental temperatures. The Nmic was positively correlated with soil water content expect Dahurian larch plantation, but no significant connection with the environmental temperatures. We concluded that the variations in litterfall amount and composition, soil physical and chemical properties among the forest types mainly contributed to the differences among the forest types. At the same time, the seasonal changes of environment factors and different soil microbial conmunity maked up the spatio-temporal patterns of the soil microbial biomass in the five forests.