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Afterward, the chemical structural changes and reaction mechanism of KL during the modification process were surveyed in depth using diverse analytical techniques. The results revealed that the phenolization treatment markedly raised the active site number in KL from 5.79 to 25.5 mmol\/g, which led to a significant increase in the chemical reactivity of KL. Meanwhile, the amine group was successfully grafted onto the best phenolized kraft lignin (PKL) after the Mannich reaction. Furthermore, the effects of amination reagent, reactant mass ratio, temperature and time on the nitrogen content of APKL were systematically examined to optimize the reaction conditions for amination. Using FTIR, molecular weight and elemental analyses, the optimal amination conditions were determined as a reaction temperature of 75 \u00b0C, reaction time of 3 h and PKL6\/arginine\/formaldehyde mass ratio of 3:21:28. Under these parameters, APKL10 with a higher nitrogen content of 19.2% and lower C\/N ratio of 2.46 was acquired. In addition, TG and SEM results revealed that the obtained APKL10 possessed a flake-like structure and outstanding thermal stability, which was beneficial for its subsequent application as a slow-release soil fertilizer. More importantly, the soil column leaching test confirmed that the as-prepared APKL10 had excellent nitrogen slow-release properties in the soil. As a result, this kraft lignin derivative generated by phenol treatment followed by amination-based modification could serve as an efficient nitrogen fertilizer, providing a long-term nitrogen source for plant growth in soil.<\/jats:p>","DOI":"10.3390\/polym18111281","type":"journal-article","created":{"date-parts":[[2026,5,25]],"date-time":"2026-05-25T00:30:33Z","timestamp":1779669033000},"page":"1281","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Combining Phenolization Treatment with the Mannich Reaction for Modification of Kraft Lignin to Produce Highly Efficient Lignin-Based Nitrogen Fertilizer"],"prefix":"10.3390","volume":"18","author":[{"ORCID":"https:\/\/orcid.org\/0009-0009-9244-8067","authenticated-orcid":false,"given":"Xinkai","family":"Mo","sequence":"first","affiliation":[{"name":"State Key Laboratory of Green Papermaking and Resource Recycling, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-4877-7180","authenticated-orcid":false,"given":"Yingchao","family":"Wang","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Green Papermaking and Resource Recycling, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0328-9906","authenticated-orcid":false,"given":"Zhongjian","family":"Tian","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Green Papermaking and Resource Recycling, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China"},{"name":"Shandong Huatai Paper Co., Ltd., Dongying 257335, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Xingxiang","family":"Ji","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Green Papermaking and Resource Recycling, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-3642-3246","authenticated-orcid":false,"given":"Fengshan","family":"Zhang","sequence":"additional","affiliation":[{"name":"Shandong Huatai Paper Co., Ltd., Dongying 257335, China"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Jingpeng","family":"Zhou","sequence":"additional","affiliation":[{"name":"Shandong Huatai Paper Co., Ltd., Dongying 257335, China"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2026,5,23]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"544","DOI":"10.1016\/j.ijbiomac.2019.01.076","article-title":"Amination of biorefinery technical lignin by Mannich reaction for preparing highly efficient nitrogen fertilizer","volume":"127","author":"Jiao","year":"2019","journal-title":"Int. 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