灰漠土开垦后深层土壤中碳氮耦合效应与碳汇形成机制研究

Maintenance of soil organic carbon ( SOC ) is important not only for the sustainability of agroecosystems but also for the environmental effect. Deep soil layers have the greater potential for carbon sequestration than the topsoil, due to large volume of deep soil layers. In the arid north-west China, grey desert soil is the most important agricultural soil. Our previous study reveals that after more than ten years of cultivation, SOC content and storage significantly increased, in the deep layer of the grey desert soil. The rate of accumulation exceeded that of carbon loss in the topsoil. Accumulation of carbon in the deep layer can be assured as a result of leaching from the topsoil. Meanwhile, the low C:N ratio ( approximately 10 ) reveals that, part of dissolved organic carbon ( DOC ) and N were leached to the deep soil due to irrigation and fertilizer application, then they have been transformed with the help of soil microbes: small and unstable carbon molecules have become large and stable organic carbon molecules. In order to verify this process, We plan to conduct a serious experiments in a long-term fertilizer experiment started in 1990 and the oasis farmlands in the vicinity of the Fukang Station of Desert Ecology, Chinese Academy of Sciences, in the following three aspects: soil profile ( 0-3 m ), land use age ( 0-30 years ) and fertilization model, to determine the transfer characteristics of DOC and N and SOC dynamic process. And in particular we will focus on the response of deep soil microbe to extrinsic C and N and main ecological effects and mechanism in the C-N coupling process. We intent to explore the mechanism and controlling factor for carbon sequestration in the deep layer of cultivated grey desert soil.

土壤有机碳(SOC)的固持对于农业可持续性与环境效应具有双重意义。与表层土壤相比，深层土壤体积大，碳汇潜力更高。灰漠土是我国西北干旱区重要的农业土壤，我们的前期研究发现：开垦十余年后，其深层SOC含量与贮量有累积，补偿并超越了表层碳损失。碳累积推断为来自表层的可溶性有机碳(DOC)。开垦后深层较低的土壤碳氮比(10左右)则表明，灌溉与施肥导致部分DOC和氮淋溶至深层，经微生物作用发生转化：由易溶而不稳定的小分子化合物，转化为分子量更大、更稳定的土壤有机质。为验证这一过程，本项目拟依托中国科学院阜康荒漠生态站长期定位试验(1990至今)与其周边农田，从土壤剖面(0-3 m)、利用时间(0-30年)与施肥模式三个角度，通过相互联系的田间与室内实验，研究DOC、氮的迁移与SOC动态过程，重点聚焦于微生物在碳氮耦合过程中的作用、生态效应与机理，探讨灰漠土开垦过程中深层土壤碳汇的形成机制与控制因素。

灰漠土是西北干旱区典型农业土壤。由于灌溉洗盐与土壤质地的原因，其开垦必会影响深层土壤环境与有机碳（SOC）循环。微生物在土壤养分循环中作用关键。然而，灰漠土开垦过程中剖面SOC演化趋势、深层土壤微生物的响应以及对SOC 动力学产生怎样的影响，尚不清楚。本研究依托中国科学院阜康荒漠生态站长期定位实验与周边0-30年农田，研究灰漠土土开垦过程中可溶性有机碳（DOC）、氮的迁移与SOC演变特征，并聚焦于碳氮耦合作用中的微生物机理，探讨深层土壤碳汇形成机制与控制因素。主要结果如下：.1）.灰漠土在最初开垦的10年内，剖面SOC与养分含量（全氮、DOC、可溶性氮）在下层与深层土壤中明显下降，但开垦20年后在整个剖面（0-2 m）中均显著增加，开垦30年达到最高；氮素的响应最为敏感。暗示了开垦过程中初期的养分亏损与后期的积累均是从深层土壤开始。.2）.灰漠土开垦过程中，剖面土壤微生物生物量与其活性显著增加，其群落发生转换。荒漠以Cyanobacteria（占 25%）最丰富，而农田以Actinobacteria ( 26% ) 最丰富。一些极端抗逆与光合菌（Cyanobacteria，Deinococcus-Thermus、Firmicutes、Bacteroidetes、α-proteobacteria）显著降低或消失,但持R策略与持K策略及与氮循环相关的微生物群落(如γ-proteobacteria、Acidobacteria 与 Nitrospirae) 增加；农田中古菌、Xanthomonadales、Gp6 与厌氧菌的相对丰度也极显著增加。.3）.灰漠土开垦过程中剖面土壤C / N比的变化揭示了其深层存在碳氮耦合效应。 特定的微生物群落（如γ-proteobacteria、Nitrospirae、Crenarchaeota）与SOC、全氮、微生物量碳及大部分酶活性具有显著正相关关系。碳氮耦合培养过程中，细菌、真菌、古菌、氨氧化菌的丰度均显著增加；而真菌、古菌、氨氧化细菌及一些革兰氏阴性菌（如16:1w7c）的增加可能与农田深层土壤SOC积累的关系更为密切。.4）.灰漠土开垦为农田，灌溉方式引起了土壤水分与盐含量的剧烈改变，这是影响剖面微生物群落转换的最显著因子。同时，作物种植与施肥导致浅表层SOC与氮素间比例变化，特别是不同比例、不同量与质的可溶性碳与氮素随灌溉水向下