增氮对青藏高原东缘典型高寒草甸土壤有机碳组成的影响

土壤有机碳动态是陆地生态系统碳平衡研究的关键环节,有关青藏高原高寒草甸土壤有机碳组成对大气氮沉降增加的响应研究至今尚未开展。基于中国科学院海北生态站的大气氮沉降模拟控制实验平台,于2010年5月、7月和9月中旬分别测定不同施氮处理下0—10cm、10—20cm、20—30cm土壤中粗颗粒态有机碳(CPOC)、细颗粒态有机碳(FPOC)和矿质结合有机碳(MOC)含量,研究不同施氮类型(NH4Cl,(NH4)2SO4和KNO3)和施氮水平(0、10、20、40 kgN.hm-.2a-1)对土壤POC和MOC含量以及POC/MOC比值的影响。结果表明:青藏高原高寒草甸土壤POC积聚在土壤表层,占总土壤有机碳(SOC)含量的64%以上,稳定性较差。施氮水平显著改变了土壤CPOC、FPOC和MOC含量,而施氮类型的影响不显著。不同月份土壤POC和MOC含量对增氮的响应不同,反映了SOC组分对增氮响应的时间异质性。在生长季中期,施氮倾向于增加表层土壤POC含量,而在生长季初期和末期恰好相反。土壤MOC对增氮的响应不敏感。另外,施氮显著降低生长季初期表层土壤POC/MOC比例,SOC稳定性增加。表明,青藏高原高寒草甸土壤有机碳活性组分较高,未来大气氮沉降增加短期内即可降低活性有机碳含量,相应地改变了其组成和稳定性。

Effects of N addition on soil organic carbon components in an alpine meadow on the eastern Qinghai-Tibetan Plateau

Increasing atmospheric nitrogen(N) deposition caused by human activities significantly changes carbon cycles and carbon budgets in terrestrial ecosystems.Compared with plant carbon pools,soil pools are more complex in their components and they respond in a variety of ways to N addition.Thus,contrasting conclusions have been reached as to the consequences of N addition for carbon storage in N-limited forest and grassland ecosystems including promotion,no change and inhibition.Alpine meadows are a N-limited grassland ecosystem on the Qinghai-Tibetan Plateau,where plants and soil microorganisms have adapted to the environment of low available N.N addition might be expected to affect inputs and outputs of soil organic carbon(SOC) via changing returns of plant residues and soil CO2 release.However,a related study of this ecosystem has not so far been carried out.To assess the effects of atmospheric N deposition on SOC dynamics and the stability of an alpine meadow ecosystem on the Qinghai-Tibetan Plateau,a multi-form,low-level N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station in 2007.Three N fertilizers,NH4Cl,(NH4)2SO4,and KNO3,were added at four rates: control(0 kg N · hm-2 · a-1),low N(10 kg N · hm-2 · a-1),medium N(20 kg N · hm-2 · a-1),and high N(40 kg N · hm-2 · a-1).Each N treatment had three replicates.Each plot had an area of 9 m2(3 m × 3 m) and a 2 m isolation band was established between adjacent plots.During the 2010 growing season,soil samples were collected to 30cm depth at 10cm intervals in mid-May,July and September.The contents of three size SOC fractions,coarse particulate organic carbon(CPOC,>250μm),fine particulate organic carbon(Fine POC,53—250μm) and mineral associated organic carbon(MOC,<53μm) as well as POC/MOC ratios were measured to examine the dynamics,shifts and stability of SOC caused by N addition.Soil POC in the alpine meadow mainly accumulated in the top 10cm and accounted for more than 64% of the total SOC content,reflecting the lability and poor stability of the soil organic matter.Three-year N addition significantly changed the contents of soil CPOC,FPOC and MOC,and there were significant differences between various N levels,rather than N forms.Both soil POC and MOC responded in contrasting ways to N addition in the early,end and peak of the growing season,suggesting that temporal variability in the dynamics of SOC components responded to N addition.N addition tended to increase soil CPOC and FPOC contents in the peak of the growing season,while significantly reducing them in the early and end of the growing season.However,soil MOC content responded insensitively to N addition.N addition also significantly lowered the topsoil POC/MOC ratio in the early growing season,suggesting an increase in the stability of SOC.These results suggest that increasing atmospheric nitrogen deposition in the future may cause significant short-term changes in soil organic carbon composition and stability in the alpine meadow due to its lability.