生態(tài)系統(tǒng)地下碳輸入與輸出過程是陸地生態(tài)系統(tǒng)碳分配和轉化的核心, 并直接影響著全球碳循環(huán)。了解水稻地下碳輸入及其周轉對于充分認識稻田生態(tài)系統(tǒng)碳循環(huán)和固定具有重要意義。之前有關作物地下碳輸入的評述往往都集中在旱地作物，對水稻根際沉積碳和地下碳的輸入尚未得到全面的總結。
為此，中國科學院亞熱帶農業(yè)生態(tài)研究所吳金水研究員團隊收集和整理利用碳同位素示蹤技術研究水稻光合碳分配和周轉的相關文獻資料并結合該團隊的前期工作，分析整合了水稻光合碳（通過根際沉積作用）的輸入量及其在水稻-土壤系統(tǒng)的分配特征，結果表明，水稻光合碳的土壤固持及其在水稻-土壤系統(tǒng)的分配與標記方法有關，在連續(xù)標記方法下，最終會有72%的光合碳被固定到水稻地上部，17.1%固定在根系，10.2%固定到土壤，還有1.3%固定在土壤微生物中；而在脈沖標記方法下，分別有79%、13.4%、5.5%和2.1%的光合碳被固定到水稻的莖葉、根系、土壤和土壤微生物（圖1）。
此外，該團隊還估算了水稻地下碳輸入量和根際沉積碳的數(shù)量，發(fā)現(xiàn)利用連續(xù)標記方法，水稻一個生長季地下碳輸入總量為1.58Mg ha-1，其中根際沉積碳占0.42Mg ha-1。而利用脈沖標記方法估算的碳輸入量則略低（總地下碳輸入，1.37Mg ha-1；根際沉積碳，0.35Mg ha-1）。
鑒于同位素示蹤技術費用高昂，而且需要專業(yè)的知識和特殊的分析設備，因此目前該技術的使用還不是非常普遍，因此該團隊還建立了一種快速和簡便估算水稻生態(tài)系統(tǒng)地下碳和根際沉積碳數(shù)量的方法?？紤]到水稻不同組織之間以及根系和根際沉積之間相對穩(wěn)定的比例關系，該團隊發(fā)現(xiàn)可以一個相對系數(shù)來估算水稻生態(tài)系統(tǒng)地下碳和根際沉積碳的數(shù)量，即用水稻的某個組織的生物量乘以該系數(shù)便可大致速算出水稻的地下碳和根際沉積碳的含量。發(fā)現(xiàn)，水稻總地下碳量的估算系數(shù)為：水稻莖葉生物量的0.24，水稻根系的0.79和水稻總生物量的0.17；而根際沉積碳數(shù)量的估算系數(shù)為：水稻莖葉生物量的0.06，水稻根系的0.28和水稻總生物量的0.04（圖2）
綜上，脈沖標記和連續(xù)標記法作為估算水稻地下碳輸入直接有效的方法，有助于研究根際來源碳的轉化和分配，并將其與土壤原有有機質區(qū)分開來。在標記后，水稻光合碳固定到莖葉、根系和土壤的比例迅速變化，但在1-2周后保持穩(wěn)定。連續(xù)標記法估算的水稻碳輸入量要比脈沖標記法高15%。用該方法測定水稻土中光合碳的輸入量為1370-1580 kg ha-1。但無論采用何種標記方法，水稻根際沉積碳約占總地下碳輸入量的26%（圖3）。
上述結果以Carbon input and allocation by rice into paddy soils: A review為題發(fā)表在土壤學Top期刊Soil Biology and Biochemistry上。該研究得到了國家重點研發(fā)計劃 、國家自然科學基金和研究所青年創(chuàng)新團隊的支持。
延伸閱讀：該團隊近年來發(fā)表的水稻光合碳土壤固持及其相關機制的文章
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7)Zhenke Zhu, Tida Ge, Shoulong Liu, Yajun Hu, Rongzhong Ye, Mouliang Xiao, Chengli Tong, Yakov Kuzyakov, Jinshui Wu. Rice rhizodeposits affect organic matter priming in paddy soil: The role of N fertilization and plant growth for enzyme activities, CO2 and CH4 emissions. Soil Biology and Biochemistry. 2018, 116: 369-377
8)Cornelius Talade Atere, Tida Ge, Zhenke Zhu, Chengli Tong, Davey L Jones, Olga Shibistova, Georg Guggenberger, Jinshui Wu. Rice rhizodeposition and carbon stabilisation in paddy soil are regulated via drying-rewetting cycles and nitrogen fertilization. Biology and Fertility of Soils. 2017, 53: 407-417
9)Zhenke Zhu, Tida Ge, Yajun Hu, Ping Zhou, Tingting Wang, Olga Shibistova, Georg Guggenberger, Yirong Su, Jinshui Wu. Fate of rice shoot and root residues, rhizodeposits, and microbial assimilated carbon in paddy soil-part 2: turnover and microbial utilization. Plant and Soil. 2017, 416: 243-257
10)Zhenke Zhu, Tida Ge, Mouliang Xiao, Hongzhao Yuan, Tingting Wang, Shoulong Liu, Cornelius Talade Atere, Jinshui Wu, Yakov Kuzyakov. Belowground carbon allocation and dynamics under rice cultivation depends on soil organic matter content. Plant and Soil. 2017, 410: 247-258
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14)Cornelius Talade Atere, Tida Ge, Zhenke Zhu, Liang Wei, Ping Zhou, Xinhua He, Yakov Kuzyakov, Jinshui Wu, Carbon allocation and fate in paddy soil depending on phosphorus fertilization and water management: results of 13C continuous labelling of rice, Canadian Journal of Soil Science, 2018, 98: 469-483
圖1 連續(xù)標記和脈沖標記方法下，光合碳在水稻-土壤系統(tǒng)中的分配
圖2 水稻地下碳和根際沉積碳與水稻不同組織生物量的相對比例（SB：莖葉生物量；RB：根系生物量；WB：總生物量）
圖3 連續(xù)標記和脈沖標記方法下水稻土壤光合碳輸入、分配和根際沉積的概述圖