Soil Organic Carbon and Humus Characteristics: Response and Evolution to Long-Term Direct/Carbonized Straw Return to Field

While numerous studies have examined the effects of direct and carbonized straw return on soil fertility, most focus on short-term impacts. Long-term research is needed to understand how these practices affect soil fertility and organic carbon transformation, providing guidance for the use of straw and biochar in agriculture. This study examined the long-term effects of corn straw (CS), straw biochar (BIO), and biochar-based fertilizer (BF) applied over 5, 10, and 15 years on soil aggregates, organic carbon, and humus composition in a peanut field microplot experiment. Using pig manure compost (PMC) as a control, we assessed soil water-stable aggregate distribution, carbon content in soil and aggregates, and organic carbon’s infrared spectral characteristics. The goal was to understand how different straw utilization methods impact soil carbon retention and humus composition for sustainable agricultural practices. The results showed that the straw biochar (BIO) had a significantly better long-term effect on increasing the proportion and stability of large soil aggregates compared to direct straw return (CS) and biochar-based fertilizer (BF). After ten years of continuous fertilization, the organic carbon content in soils treated with BIO was higher than that in soils treated with CS. After fifteen years of continuous fertilization, the organic carbon content in soils treated with BF was similar to that of the CS treatment. Additionally, humus carbon primarily accumulated in the 2–0.25 mm aggregates (SMA), and the BIO treatment significantly improved the quality of soil humus. FTIR spectra indicated that the long-term application of BIO and BF increased the proportion of aromatic carbon in the soil, enhancing the stability of soil organic carbon. Long-term application of carbonized straw returns improved soil organic carbon, aggregate stability, and humus quality, contributing to carbon sequestration. Fertilization duration directly and indirectly regulated humus formation through its impact on organic carbon and aggregates, while organic materials influenced humus indirectly.