组成和结构驱动的水田和旱地土壤可溶性有机碳差异形成机理

Organic carbon content in paddy soils is generally higher than that in upland soils within a geomorphic unit in southern China. The dissolved organic carbon (DOC) concentration, however, is lower in paddy soils. Based on the previous results, we hypothesize the discrepancy of internal chemical composition and structure of the DOC controls the main fates of DOC (mineralization to CO2 and absorption by soil mineral), and thereby leads to differences of DOC content between the two agricultural soils under contrasting management regimens. In the present project, field observation will be carried out to analyze the characteristics and discrepancies of composition and structure of the DOC between the paddy soils and upland soils by using 13C nuclear magnetic resonance (13C-NMR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) technologies. 13C isotope tracing incubation experiment will be conducted to clarify the fates and the change of composition and structure of DOC during the decomposition process. Isothermal adsorption-desorption and its kinetics processes of DOC and the relationship between these processes and the internal composition and structure of DOC will be monitored. The purpose is to clarify the discrepancies of decomposition and adsorption-desorption processes of DOC in paddy soils and upland soils which result from the different composition and structure of DOC. This project will reveal the key physical and chemical mechanisms of different DOC content between paddy soils and upland soils, and also offer the theoretical basis of biogeochemical carbon cycle in agricultural ecosystems.

南方同一地貌单元内水田土壤有机碳含量普遍高于旱地，但可溶性有机碳（DOC）含量却以旱地高于水田。基于前期研究结果，假设DOC本身化学组成和结构的差异引起DOC主要去向（矿化为CO2和被土壤吸附）不同，进而造成水田和旱地土壤DOC含量差异。基于此，本项目拟通过原位取样分析结合模拟培养试验，运用13C同位素示踪、热裂解气相质谱（Py-GC/MS）和液相/固相核磁共振（13C-NMR）等定量与定性相结合的技术，深入分析南方典型样区水田和旱地土壤DOC化学组成、分子结构特征，明确DOC分解转化过程中各种去向的分配及其与组成结构的变化关系，探讨DOC组成结构对DOC在土壤中等温吸附-解吸和吸附动力学过程的影响，从而阐明由DOC组成和结构引起的DOC在土壤中分解转化和吸附-解吸过程的差异，系统揭示水田和旱地土壤DOC含量差异的形成机理，为完善农田生态系统碳循环理论提供依据。

可溶性有机碳（DOC）作为土壤有机碳的重要组成部分，化学性质极其活跃，其去向对于土壤质量及碳平衡具有重要意义。前期研究结果表明，组成结构可能是引起DOC去向差异的关键因子，这更可能是造成旱地和水田有机碳差异的一个重要原因。本项目通过田间采样、13C标记有机物培养试验和吸附-解吸试验，从DOC组成结构角度展开系列研究。结果表明：①土壤DOC含量受土壤类型、取样时间影响。易变化DOC组分（有机酸、脂类）受环境因素影响大（P<0.05），而稳定DOC组分（芳香化合物）受地貌类型影响较大（P<0.05）。②培养期内（30d），乙酸在旱地和水田中矿化比例最高，在旱地中葡萄糖的矿化量显著高于草酸，水田中趋势相反。旱地中三种低分子有机酸的矿化均低于水田，这说明化合物组成结构和土壤条件共同决定有机物矿化速率。草酸和葡萄糖在水田中的激发效应低于旱地。因此，水田中低分子有机物的矿化和激发效应较低，导致其碳累积高。激发效应与真菌丰度正相关，水田中真菌丰度比旱地低，淹水抑制真菌活性可能是水田中有机碳周转慢的一个重要因素。③DOC的累积矿化受土壤类型影响，但其利用与转化受土壤类型与DOC性质共同影响。微生物更易利用水溶性碳组分合成自身，死亡后这部分碳组分以氨基糖的形式固持在土壤中。④DOC吸附和解吸主要受土壤矿物类型影响。蒙脱石对土壤DOC吸附能力较强，其次为针铁矿和混合矿物，高岭石吸附DOC能力最小。旱地中DOC被高岭石和混合矿物吸附后溶液中芳香性组分比例变化不显著，而被蒙脱石和针铁矿吸附后芳香性组分所占比例显著增加。水田DOC被土壤矿物吸附后芳香性组分所占比例显著减小，且吸附能力为高岭石<混合矿物<针铁矿<蒙脱石。水田DOC被土壤吸附后溶液中的芳香性组分比例显著减少。该研究从来源及组成结构差异角度研究了DOC在土壤中的周转及吸附过程，结果可为丰富亚热带地区农田土壤有机碳循环理论及指导农田管理提供参考。

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