马尾松幼苗根系和光合碳供应对土壤性质变化和温室气体排放的驱动作用

森林植被受全球气候变化、森林经营活动及病虫害等多种干扰,导致林地光合碳供应水平及根系输入量发生变化。在此背景下,土壤性质及土壤温室气体排放的响应及其机理是预测森林碳汇功能变化及森林可持续经营的重要依据。以2年生马尾松盆栽苗为对象,通过单株/盆和3株/盆栽植密度控制根系输入量、通过环割和截干控制光合碳向地下的供应能力,模拟森林植被干扰导致的根系输入量及光合碳供应变化对土壤理化性质、微生物群落结构及温室气体排放的影响。结果表明,苗木根系非结构性碳水化合物(TNC)含量和氮含量比单株/盆低;3株/盆的土壤速效氮含量比单株/盆低,土壤革兰氏阳性菌、厌氧菌、放线菌及丛枝菌根真菌丰富度均比单株/盆显著增加,3株/盆的土壤二氧化碳(CO₂)排放速率较高,但土壤氧化亚氮(N₂O)排放速率差异不显著。无论是单株/盆还是3株/盆,环割和截干处理后,根系生物量、根系长度及表面积均比对照显著下降;根系TNC含量显著下降。土壤和根系氮含量都有增加趋势;土壤微生物生物量碳(SMBC)含量降低,而土壤微生物生物量氮(SMBN)则提高。环割和截干后,土壤中各种微生物组成丰富度均有下降趋势,土壤CO₂排放速率显著下降,土壤N₂O排放速率则显著提高。根系输入量及光合碳供应对土壤细菌和真菌含量均有显著影响,土壤细菌含量与根系生物量、SMBC和SMBN显著正相关;土壤真菌含量与土壤温度显著负相关,与根系生物量、SMBC和SMBN显著正相关。相关分析表明,土壤CO₂排放通量与土壤温度、土壤湿度及根系生物量显著正相关,与土壤硝态氮显著负相关;土壤N₂O排放通量与土壤温度和土壤湿度显著正相关。以上研究表明,根系输入量与地上光合碳供应共同作用,改变土壤理化性质及微生物环境,进而影响土壤温室气体排放。

Root input and photosynthetic carbon supply effect on soil properties and greenhouse gases release of Pinus massoniana seedling

Forest vegetation is affected by climate change, forest management activities, and diseases and pests, which can lead to changes in photosynthetic carbon supply, root input of forest land, composition of soil microbial community, and soil greenhouse gases emission. These changes and their underlying mechanisms are the basis for modelling change of forest ecological functions and for managing forest scientifically. Here, we studied level of root input of 2-year-old potted seedlings of Pinus massoniana under two densities, single-plant/pot and 3-plants/pot, and under two disturbance treatments, ring-barking and stem-severing to stop carbon transport to the root system. We aimed to simulate the effects of changes in root input and photosynthetic carbon supply on soil physical and chemical properties, microbial community structure and greenhouse gas emission under various conditions. Our results showed that the total nonstructural carbohydrate (TNC) and nitrogen in seedling roots were lower in 3-plants/pot treatment than in single-plant/pot treatment. The content of soil available nitrogen in 3-plants/basins was lower than that in single-plant/pot. In contrast, richness of soil Gram-positive bacteria, anaerobic bacteria, actinomycetes and arbuscular mycorrhizal fungi were higher in 3-plants/pot treatment than in single-plant/pot treatment. Further, the emission rate of soil carbon dioxide (CO₂) was higher in 3-plants/pot treatment than in single-plant/pot treatment. Planting density had no significant effects on emission rate of soil nitrous oxide (N₂O). Both the ring-barking and stem-severing treatments decreased the root biomass, root length, root surface and root TNC content in both single-plant/pot and 3-plants/pot treatments. Both the ring-barking and stem-severing treatments increased the content of nitrogen in soil and roots, the content of soil microbial biomass nitrogen (SMBN), and emission rate of soil N₂O, but decreased the content of soil microbial biomass carbon (SMBC), the richness of soil microbial and the emission rate of soil CO₂. Root biomass input and photosynthetic carbon supply have significant effect on soil bacterial and fungal contents. Soil bacterial contents were positively correlated with root biomass, SMBC and SMBN. Soil fungal content was negatively correlated with soil temperature, positively correlated with root biomass, SMBC and SMBN. Correlation analysis showed that soil CO₂ emission flux was positively correlated with soil temperature, soil humidity and root biomass, while negatively correlated with soil nitrate nitrogen; soil N₂O emission flux was positively correlated with soil temperature and soil humidity. The results suggest that root input and photosynthetic carbon supply work together to change soil physical and chemical properties and microbial environment, thus contribute to changes in the emissions rate of soil greenhouse gases.