【作者】 陈懂懂；

【导师】 杜国祯；

【作者基本信息】 兰州大学 ， 生态学， 2010， 博士

【摘要】 青藏高原因海拔高和气候寒冷,被认为是气候变化的敏感区和脆弱区。随着全球气候变化的影响,土壤微生物量(SMB)、土壤中氮(N)的矿化将可能受到强烈影响,这些影响将可能使土壤中活性有机质、有效氮含量和植物生长速率改变,进而将对高寒草甸植被的生产力和碳汇功能产生一定的影响。尽管对多数陆地环境中的土壤养分、SMB和N矿化做过不少调查研究,但针对青藏高原的研究仍然较少。为了探讨青藏高原东北缘高寒草甸草地土壤养分和微生物量碳(Cmic)氮(Nmic)等指标的变化,我们研究了土地利用方式对土壤养分和SMB的影响,不同放牧强度对该区SMB和N矿化潜力的影响,以及土壤Cmic和Nmic在不同海拔梯度的分布状态,同时将室内培养下土壤N矿化潜力与野外原地培养净N矿化进行了比较。主要研究结果如下：1.对三类不同土地利用方式(自然草地、弃耕约10年的土地以及耕作至少50年以上的土地)下土壤养分及SMB的分析显示：与自然草地相比,耕作50年后的土壤有机碳(SOC)和全氮(TN)分别损失了45%和43%；而与耕作地相比,弃耕约10年后的SOC和TN分别升高了27%和23%。耕地中的SMB和微生物基础呼吸(MR)较低。弃耕地中的Cmic含量介于耕地和自然草地,而弃耕地中的Nmic含量和MR与自然草地的差异不显著。由此可见：在青藏高原东北缘的高寒草甸草地上,持续的耕作降低了土壤养分和SMB,提高了微生物代谢熵(qCO2)；而在该区域采取弃耕和对天然草地进行保护对土壤质量的恢复和维持是有效的措施。2.通过对青藏高原东北缘高寒草甸四类放牧强度(围栏封育/无放牧、轻度、适度和重度放牧)下的SOC、TN、Cmic和Nmic、矿化N的测量分析得出：SOC和TN对放牧的反应不敏感；而适度的放牧明显促进了微生物对碳氮元素的累积和固定。同时Cmic/SOC和Nmic/TN比值作为土壤有机碳/全氮向微生物碳/氮转化效率的指标,也是在适度放牧区最高。这证明在该研究地区,对草地进行合理的管理能促进土壤活性C库和N库的丰富度。3.作为土壤质量的检测指标之一,土壤N矿化潜力在围栏封育区最高,而在重度放牧区最低,这与SOC和TN的变化规律一致,但比后两者的变化更明显。进一步表明适当的围栏封育利于土壤质量的恢复和/或者保持,而重度放牧降低了土壤质量。4.通过对位于青藏高原东北缘高寒草甸5个海拔梯度(从低到高依次为3050m、3180m、3570m、3600m和3910m)的土壤理化特性、Cmic和Nmic、室内室外培养的净N矿化的研究得知：土壤Cmic在各海拔间差异不显著；而Nmic,微生物量碳、氮分别对土壤有机碳和全氮的贡献率(Cmic/SOC和Nmic/TN)在不同海拔间差异显著(p<0.05),且均与海拔梯度呈显著负相关(相关系数分别为-0.464、-0.806和-0.680),说明低海拔地区有较多的营养被微生物固定,潜在的有效营养源较高。5.Cmic和Nmic均随土壤pH值的降低而减少,是由于细菌适宜于中性或微碱性的环境(低海拔土壤pH值变化范围为6.7-7.1,土壤为中性),而高海拔土壤(pH变化范围为5.2-5.7)的酸性环境抑制了该研究地区以细菌为主导的微生物群落的活性,从而导致高海拔区微生物固定的C、N量较低。而Cmic和Nmic与土壤水分含量呈负相关(p>0.05),说明过多的水分导致土壤含氧量降低,从而对微生物活性产生抑制作用。6.室内培养的N矿化速率大于野外原位培养净N矿化率,说明在室内控制的适宜的温度和湿度(22℃和40-60%的田间持水量)条件下,矿化速率较高,进一步说明虽然高海拔气候环境恶劣,但其土壤潜在的可矿化、可利用的养分仍然较高。更多还原

【Abstract】 Although the researches about the soil nutrients, the immobilization of C and N in microbes and N mineralization have been investigated in many terrestrial environments, the similar research on the Tibetan Plateau is scarce. To better understand the dynamics of the soil nutrients and microbial biomass on the Tibetan plateau, we analyzed the effect of land-use management (natural grassland, abandoned old-field and crop field) on soil physiochemical properties and microbial biomass (SMB); investigated the effects of fencing and grazing on the microbial biomass carbon (Cmic) and nitrogen (Nmic), and mineralized N in soils at four sites with different grazing intensity:no-grazing (fencing), light-, moderate-and heavy-grazing of an alpine meadow on the northeastern Tibetan plateau. At the same time, we analyzed the distribution of Cmic and Nmic from sites at 5 different altitudes (3050, 3180,3570,3600,3910 m), the relationships among them and soil properties, and compared the net N mineralization rate both from indoor-and in-situ incubation. The results showed:1. The losses of SOC and TN were about 45% and 43%, respectively, due to cultivation after more than 50 years comparing with natural grassland. Because of the abandonment of cultivation for about a decade, SOC and TN were increased by 27% and 23%, respectively, in comparison with the crop field. Microbial carbon (ranging from 357.5 to 761.6 mg kg-1 soil) in the old-field was intermediate between the crop field and grassland. Microbial nitrogen (ranging from 29.9 to 106.7 mg kg-1 soil) and respiration (ranging from 60.4 to 96.4 mg CO2-Cg-1Cmic d-1) were not significantly lower in the old-field than those in the grassland. Thus it could be concluded that cultivation decreased the soil organic matter and SMB, while the adoption of abandonment has achieved some targets of grassland restoration in the alpine region of Gansu Province on the northeastern Tibetan plateau.2. The effect of grazing on soil nutrients was not statistically significant, while moderate grazing markedly stimulated the accumulation and immobilization of C and N in microbes. As a good measure of the efficiency of soil nutrients conversion into microbial C and N, Cmic/SOC and Nmic/TN were higher in the moderate-grazing site, which demonstrated that proper management of grassland promoted enrichment of the available soil C and N pool in the study area.3. As a measure of soil quality, N mineralization potential was higher in the fencing site and lower in the heavy-grazing site, showing the similar trend with SOC and TN, which demonstrated that the fencing is beneficial to the restoration and/or maintenance of soil quality, while the heavy grazing reduces soil quality.4. There was no significant difference for Cmic among different altitudes; but significant differences for Nmic, Cmic/SOC and Nmic/TN were detected among different altitudes, and the latter three had a significantly negative correlation with elevation, which demonstrated that the microbes fixed more and effective potential nutrition in the low altitude.5. There was a significantly positive correlation between soil pH and microbial biomass (p<0.01):The bacteria adapt to the neutral or slightly alkaline environment (pH in the low-altitude ranges from 6.7 to 7.1, the neutral soil), while the acidic soil environment of high-elevation (pH ranges from 5.2 to 5.7) maybe suppress the activity of bacteria-dominated microbial communities, which caused less C and N fixed by microbes in high-altitude. There was a negative correlation between soil moisture and microbial biomass (p>0.05), which was because too much water led to reduced oxygen, thus inhibited microbial activity in the soil.6. The net N mineralization from the indoor incubation was larger than that from the in-situ incubation, which indicated that there would be higher N mineralization rates under the conditions with appropriate temperature and moisture.更多还原