Agronomy 2022, 12, 2374. https://doi.org/10.3390/agronomy12102374 www.mdpi.com/journal/agronomy Article Effects of 10 Years of the Return of Corn Straw on Soil Aggregates and the Distribution of Organic Carbon in a Mollisol Jiawei Gan 1,2,† , Chen Qiu 1,2,† , Xiaozeng Han 1 , David Kwaw‐Mensah 3 , Xu Chen 1 , Jun Yan 1 , Xinchun Lu 1 and Wenxiu Zou 1, * 1 Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences, Harbin 150081, China 2 University of Chinese Academy of Sciences, Beijing 150049, China 3 Department of Agronomy, Iowa State University, Ames, IA 50011, USA * Correspondence: zouwenxiu@iga.ac.cn; Tel.: +04‐51‐866‐02940 † These authors contributed equally to this work. Abstract: The return of straw is a widely used agricultural practice for increasing the soil organic carbon (SOC) content and improving soil structure in Mollisols, owing to the decline caused by continuous high‐intensity tillage. We conducted a field experiment where corn straw was continu‐ ously returned for 10 years to investigate effects of the straw on the size distribution and stability of soil aggregates and on SOC density fractions. The treatments were no straw return (CK) and four rates of straw return: 6000 kg hm −2 (S1), 9000 kg hm −2 (S2), 12 000 kg hm −2 (S3), and 15 000 kg hm −2 (S4). SOC contents after straw return for bulk soil, a free light fraction (F‐LF), an occluded light fraction (O‐LF), and a heavy fraction (HF) were significantly higher by 27.0, 644.3, 720.0, and 69.2%, respectively, in S4 than CK. The contents of F‐LF, O‐LF, and HF in aggregates >2.00 mm were sig‐ nificantly higher by 194.2, 162.1, and 35.8%, respectively, in S4 than CK. Structural equation model‐ ing indicated that SOC contents and aggregates >0.25 mm were directly correlated with the amount of straw returned. We conclude that returning 15 000 kg m −2 of straw would be an effective agro‐ nomic practice to restore Mollisol fertility. Keywords: straw retention; soil aggregate stability; soil organic carbon; density fraction of soil organic carbon; black soil 1. Introduction Northeastern China is the Chinese granary, with an average corn production of 41.0% and corn commercialization of >80.0%, playing an important role in and greatly contributing to food security in China [1]. The fertility of the Mollisol soil, however, has been decreasing after years of high‐intensity planting and cultivation. Xu et al. [2] re‐ ported that Mollisol regions around the world are losing soil organic carbon (SOC). For example, the SOC contents in northeastern China decreased by 46.0% after 150 years of cultivation, and the SOC contents in North America decreased by >50% during the last 100 years, which led to yield reductions of >16.5% by 2020 [2]. The loss of SOC contributes to global warming and decreases crop growth and yield [2,3], which affects food security. SOC is key to soil fertility and plays an important role in regulating the physical and chemical properties of soil [4]. The accumulation of SOC is therefore very important for sustainable crop production [5] and increasing the sequestration of SOC is critical for food security and decreasing the loss of carbon (C). The return of straw in agroecosystems is considered an essential and effective man‐ agement practice for increasing SOC storage and improving soil quality and health [6]. An estimated 3.8 billion tons of crop residues are produced globally each year [7] and Citation: Gan, J.; Qiu, C.; Han, X.; Kwaw‐Mensah, D.; Chen, X.; Yan, J.; Lu, X.; Zou, W. Effects of 10 Years of the Return of Corn Straw on Soil Aggregates and the Distribution of Organic Carbon in a Mollison. Agronomy 2022, 12, 2374. https://doi.org/10.3390/ agronomy12102374 Academic Editor: Di Wu Received: 20 July 2022 Accepted: 28 September 2022 Published: 30 September 2022 Publisher’s Note: MDPI stays neu‐ tral with regard to jurisdictional claims in published maps and institu‐ tional affiliations. Copyright: © 2022 by the authors. Li‐ censee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and con‐ ditions of the Creative Commons At‐ tribution (CC BY) license (https://cre‐ ativecommons.org/licenses/by/4.0/).