The freezing winter temperatures in Northeast China hinder the breakdown speed of straw, making it challenging to implement widespread straw return in rice fields, meaning that crop remnants are not efficiently utilised. This study involved a pot experiment conducted on rice plants that were treated with varying quantities of biochar: CK (no biochar); BC1 (5 t ha⁻¹); BC2 (10 t ha⁻¹); and BC3 (20 t ha⁻¹). An investigation was conducted to examine the impact of biochar on the soil organic carbon (SOC), humus (HS) composition, humic acid (HA) structure, and rice yield of paddy fields. The findings demonstrated that the use of biochar led to a substantial rise in SOC and HA-C concentrations in the soil layer between 20 and 40 cm. Additionally, biochar’s application enhanced soil humification. Notably, the treatment with BC3 (20 t ha⁻¹) had the most pronounced impact. The O/C ratio in the HA and the relative strength of the peaks at 1620 cm⁻¹ on the infrared spectra showed a more pronounced response to the BC3 treatment compared to the other biochar treatments. However, the application of the BC1 treatment at a rate of 5 t ha⁻¹ and the BC2 treatment at a rate of 10 t ha⁻¹ had a minimal impact on the fluorescence intensity of humic acid (HA). The application of the BC3 treatment increased the aromatic nature of the humic acid (HA) in paddy soil, leading to the formation of an intricate and enduring HA structure. Furthermore, the use of the BC3 treatment resulted in a notable enhancement in the quantity of spikes, spike weight, and number of grains per spike. Additionally, it positively impacted the accumulation of dry matter in the spike, leading to a substantial 13.7% increase in the rice yield. Applying biochar at a rate of 20 t ha⁻¹ is a sensible and effective approach to enhance the soil organic carbon (SOC) content, enhance the stability of the humic acid (HA)’s structure, and raise the rice yield in the rice-growing area of Northeast China. This study’s findings will establish a theoretical foundation for utilising soil biochar in the rice fields located in Northeast China.

The continuous input, slow decomposition, and retention of straw can impede tillage and crop growth, and the decomposition process of the straw in soil is affected by its composition and the method of application. Experimental Station of Jilin Agricultural University, Changchun City, Jilin Province. The soil type was Argiudolls, the experimental field area was 30 m², and the maize was planted continuously for 12 years without returning the straw to the field. There were four treatments: (1) control (CK), 10 g of straw was placed on the surface of a nylon mesh bag, and the nylon mesh bag was separated from the soil by polyethylene film without contacting with the soil; (2) straw mulching (CM), a nylon mesh bag with 10 g of straw was placed on the soil surface; (3) straw burying (CD), a nylon mesh bag with 10 g of straw was buried at a depth of 40 cm from the soil surface; (4) straw burying (CE), a nylon mesh bag with 40 cm of soil depth. Fifteen micro-zones were set up with a micro-zone area of 1 m² and each micro-zone consisted of four treatments randomly grouped in three replicates per treatment. The on-site nylon bag burying trial started on 20 April 2021 and ended on 15 April 2022. Nylon bag and soil samples were collected on days 0, 30, 90, 180, 270, and 360. SOC, TN, straw component decomposition, and water droplet-soil contact angle were determined. Our results showed that the CE treatment increased soil organic carbon (SOC) and total nitrogen (TN) content compared to the CM and CD treatments. Compared with CK, straw decomposition rates increased by 13.3%, 30.8%, and 22.3% in the CM, CD, and CE treatments, respectively. Lignin decomposition rates increased by 7.8%, 27.3%, and 16.2%; cellulose decomposition rates increased by 14.6%, 35.4%, and 27.3%; and hemicellulose decomposition rates increased by 17.2%, 31.7% and 23.7% in the CD treatment, respectively. Compared with CK, the contact angle of droplets in the CD treatment was statistically significantly increased by 91.5% when the droplets remained on the soil surface for 5 s. The rate of decrease of droplet contact angle with time was statistically significantly decreased by 11.8%, and the penetration rate of droplets on the soil surface was slower. Overall, the CD treatment promoted straw decomposition and increased SWR compared to the CM and CE treatments. which are important attributes to enhance soil quality and improve soil structural stability.