Vegetation fires are known to have broad geochemical effects on carbon (C) cycles in the Earth system, yet limited information is available for nitrogen (N). In this study, we evaluated how charring organic matter (OM) to pyrogenic OM (PyOM) altered the N molecular structure and affected subsequent C and N mineralization. Nitrogen near-edge X-ray absorption fine structure (NEXAFS) of uncharred OM, PyOM, PyOM toluene extract, and PyOM after toluene extraction were used to predict PyOM-C and -N mineralization potentials. PyOM was produced from three different plants (e.g. Maize-Zea mays L.; Ryegrass-Lollium perenne L.; and Willow-Salix viminalix L.) each with varying initial N contents at three pyrolysis temperatures (350, 500 and 700 °C). Mineralization of C and N was measured from incubations of uncharred OM and PyOM in a sand matrix for 256 days at 30 °C. As pyrolysis temperature increased from 350 to 700 °C, aromatic C 000000000000 000000000000 000000000000 111111111111 000000000000 111111111111 000000000000 000000000000 000000000000 N in 6-membered rings (putative) increased threefold. Aromatic CN in 6-membered oxygenated ring increased sevenfold, and quaternary aromatic N doubled. Initial uncharred OM-N content was positively correlated with the proportion of heterocyclic aromatic N in PyOM (R2 = 0.44; P < 0.0001; n = 42). A 55% increase of aromatic N heterocycles at high OM-N content, when compared to low OM-N content, suggests that higher concentrations of N favor the incorporation of N atoms into aromatic structures by overcoming the energy barrier associated with the electronic and atomic configuration of the C structure. A ten-fold increase of aromatic CN in 6-membered rings (putative) in PyOM (as proportion of all PyOM-N) decreased C mineralization by 87%, whereas total N contents and C:N ratios of PyOM had no effects on C mineralization of PyOM-C for both pyrolysis temperatures (for PyOM-350 °C, R2 = 0.15; P < 0.27; for PyOM-700 °C, R2 = 0.22; P < 0.21). Oxidized aromatic N in PyOM toluene extracts correlated with higher C mineralization, whereas aromatic N in 6-membered heterocycles correlated with reduced C mineralization (R2 = 0.56; P = 0.001; n = 100). Similarly, aromatic N in 6-membered heterocycles in PyOM remaining after toluene extraction reduced PyOM-C mineralization (R2 = 0.49; P = 0.0006; n = 100). PyOM-C mineralization increased when N atoms were located at the edge of the C network in the form of oxidized N functionalities or when more N was found in PyOM toluene extracts and was more accessible to microbial oxidation. These results confirm the hypothesis that C persistence of fire-derived OM is significantly affected by its molecular N structure and the presented quantitative structure-activity relationship can be utilized for predictive modeling purposes.

中文翻译：

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火源有机质中的氮形态及其转化

众所周知，植被火灾对地球系统中的碳 (C) 循环具有广泛的地球化学影响，但关于氮 (N) 的可用信息有限。在这项研究中，我们评估了将有机物 (OM) 炭化为热解 OM (PyOM) 如何改变 N 分子结构并影响随后的 C 和 N 矿化。未烧焦的 OM、PyOM、PyOM 甲苯提取物和甲苯提取后的 PyOM 的氮近边 X 射线吸收精细结构 (NEXAFS) 用于预测 PyOM-C 和 -N 矿化潜力。PyOM 由三种不同的植物（例如 Maize-Zea mays L.；Ryegrass-Lollium perenne L.；和 Willow-Salix viminalix L.）生产，每种植物在三个热解温度（350、500 和 700 °C）下都具有不同的初始 N 含量. C 和 N 的矿化是通过在 30 °C 下在沙基质中培养 256 天的未烧焦 OM 和 PyOM 来测量的。由于热解温度增加350至700℃，芳族C 000000000000 000000000000 000000000000 111111111111 000000000000 111111111111 000000000000 000000000000 000000000000 N 6元环（推定的）增加了三倍。6 元氧化环中的芳香族 CN 增加了 7 倍，季芳香族 N 增加了一倍。初始未炭化的 OM-N 含量与 PyOM 中杂环芳香族 N 的比例呈正相关（R2 = 0.44；P < 0.0001；n = 42）。与低 OM-N 含量相比，高 OM-N 含量的芳香族 N 杂环增加 55%，表明较高浓度的 N 有利于通过克服与 C 结构的电子和原子构型相关的能垒，将 N 原子结合到芳族结构中。PyOM 中 6 元环（推定）中芳香族 CN 的 10 倍增加（占所有 PyOM-N 的比例）使 C 矿化减少了 87%，而 PyOM 的总 N 含量和 C:N 比对 C 没有影响两种热解温度下 PyOM-C 的矿化（对于 PyOM-350 °C，R2 = 0.15；P < 0.27；对于 PyOM-700 °C，R2 = 0.22；P < 0.21）。PyOM 甲苯提取物中的氧化芳香 N 与较高的 C 矿化相关，而 6 元杂环中的芳香 N 与减少的 C 矿化相关（R2 = 0.56；P = 0.001；n = 100）。相似地，甲苯萃取后残留的 PyOM 中 6 元杂环中的芳香 N 减少了 PyOM-C 矿化（R2 = 0.49；P = 0.0006；n = 100）。当 N 原子以氧化的 N 官能团的形式位于 C 网络的边缘时，或者当在 PyOM 甲苯提取物中发现更多的 N 并且更容易被微生物氧化时，PyOM-C 矿化增加。这些结果证实了火源 OM 的 C 持久性受其分子 N 结构显着影响的假设，并且所提出的定量构效关系可用于预测建模目的。当 N 原子以氧化的 N 官能团的形式位于 C 网络的边缘时，或者当在 PyOM 甲苯提取物中发现更多的 N 并且更容易被微生物氧化时，PyOM-C 矿化增加。这些结果证实了火源 OM 的 C 持久性受其分子 N 结构显着影响的假设，并且所提出的定量构效关系可用于预测建模目的。当 N 原子以氧化的 N 官能团的形式位于 C 网络的边缘时，或者当在 PyOM 甲苯提取物中发现更多的 N 并且更容易被微生物氧化时，PyOM-C 矿化增加。这些结果证实了火源 OM 的 C 持久性受其分子 N 结构显着影响的假设，并且所提出的定量构效关系可用于预测建模目的。