Straw returning combined with controlled-release nitrogen fertilizer affected organic carbon storage and crop yield by changing humic acid composition and aggregate distribution

Soil organic carbon (SOC) sequestration effectively reduces global CO2 emissions while ensuring crop production and sustainable agricultural development. Evaluating the effects of straw returning combined with controlled-release nitrogen (N) fertilizer on soil organic carbon (SOC) sequestration is necessary. We conducted a long-term (2013–2018) insitu field trial with six treatments: straw combined with no N and two N fertilizer types [no N fertilizer (CKS), conventional N fertilizer (BBFS) and controlled-release N fertilizer (CRFS)] and their corresponding no N and two N fertilizer treatments (CK, BBF, and CRF), each in three replicates. Overall, straw returning combined with N fertilizer (BBFS and CRFS) improves carbon storage and crop yield by changing aggregate distribution, with the effect of CRFS being more substantial. We further found CRFS treatment significantly increased SOC storage in small macro-aggregates (0.25–2 mm) by 9.2% and 28.4%, respectively, compared to BBFS and CKS treatments, which is mainly due to CRFS treatment increasing the proportion of small macro-aggregates (0.25–2 mm) by 8.1% and 20.1%, respectively, in contrast with BBFS and CKS treatments. Compared to BBFS and CKS treatments, CRFS treatment increased the fluorescence intensity of hydrophobic compounds in humic acid (HA) by 45.0% and 107.6%, respectively. Similarly, in small macro-aggregates (0.2–2.5 mm), CRFS treatment significantly increased the fluorescence intensity of hydrophobic compounds in HA. Lastly, structural equation modeling showed that straw returning treatments promote aggregate formation and carbon sequestration by increasing the fluorescence intensity of hydrophobic compounds in HA, ultimately increasing crop yield. Therefore, CRFS treatment is theoretically an effective method to promote SOC storage and increase crop yield and is a viable practice for sustainable agriculture.