Torres-Rojas Dorisel, Hestrin Rachel, Solomon Dawit, Gillespie Adam W, Dynes James J, Regier Tom Z, Lehmann Johannes
Soil and Crop Sciences, Cornell University, Ithaca, NY 14853, USA.
CGIAR Research Program on Climate Change, Agriculture and Food Security (CCAFS), P.O. Box 5689, Addis Ababa, Ethiopia.
Geochim Cosmochim Acta. 2020 May 1;276:170-185. doi: 10.1016/j.gca.2020.02.034.
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- L.; Ryegrass- L.; and Willow- 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[bond, double bond]N in 6-membered rings (putative) increased threefold. Aromatic C[bond, double bond]N 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 (R = 0.44 < 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 C[bond, double bond]N 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, R = 0.15; < 0.27; for PyOM-700 °C, R = 0.22; < 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 (R = 0.56; 0.001; n = 100). Similarly, aromatic N in 6-membered heterocycles in PyOM remaining after toluene extraction reduced PyOM-C mineralization (R = 0.49; = 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.
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）的信息却有限。在本研究中，我们评估了将有机物质（OM）炭化为热解有机物质（PyOM）如何改变N分子结构并影响随后的C和N矿化作用。利用未炭化的OM、PyOM、PyOM甲苯提取物以及甲苯萃取后的PyOM的氮近边X射线吸收精细结构（NEXAFS）来预测PyOM-C和-N的矿化潜力。PyOM由三种不同的植物（例如玉米-L.；黑麦草-L.；柳树-L.）产生，每种植物在三个热解温度（350、500和700°C）下具有不同的初始N含量。通过在30°C下将未炭化的OM和PyOM在砂基质中培养256天来测量C和N的矿化作用。随着热解温度从350°C升高到700°C，六元环（假定）中的芳香族C[键，双键]N增加了两倍。六元氧化环中的芳香族C[键，双键]N增加了七倍，季铵芳香族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（R = 0.44 < 0.0001；n = 42）。与低OM-N含量相比，高OM-N含量时芳香族N杂环增加了55%，这表明较高浓度的N通过克服与C结构的电子和原子构型相关的能量屏障，有利于将N原子掺入芳香族结构中。PyOM中六元环（假定）中的芳香族C[键，双键]N增加十倍（占所有PyOM-N的比例）使C矿化作用降低了87%，而对于两个热解温度下的PyOM-C矿化作用，PyOM的总N含量和C:N比均无影响（对于PyOM-350°C，R = 0.15；< 0.27；对于PyOM-700°C，R = 0.22；< 0.21）。PyOM甲苯提取物中的氧化芳香族N与较高的C矿化作用相关，而六元杂环中的芳香族N与降低的C矿化作用相关（R = 0.56；0.001；n = 100）。同样，甲苯萃取后剩余的PyOM中六元杂环中的芳香族N降低了PyOM-C矿化作用（R = 0.49；= 0.0006；n = 100）。当N原子以氧化N官能团的形式位于C网络边缘时，或者当PyOM甲苯提取物中发现更多N且更易于微生物氧化时，PyOM-C矿化作用增加。这些结果证实了以下假设：火灾衍生的OM的C持久性受到其分子N结构的显著影响，并且所呈现的定量结构-活性关系可用于预测建模目的。
