Objectives The influence of biochar and N fertilizer on soil water-stable aggregates, stability, organic carbon distribution and yields of winter wheat and summer maize in rotation system were studied.

Methods A six-years’ experiment was conducted. Four levels of biochar amendments (0, 1000, 5000, 10000 kg/hm², as B0, B1, B2, B3) and 2 levels of N fertilizer application (480 kg/hm², and 240 kg/hm², as NT, NH) were set in this experiment with 8 treatments (B0NT, B1NT, B2NT, B3NT, B0NH, B1NH, B2NH, B3NH). All samples (0–10 cm and 10–20 cm) were separated into five aggregate-size classes ( > 2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm, < 0.25 mm) by TTF-100 soil aggregate analyzer. The mean weight diameter (MWD), geometric mean diameter (GMD) and > 0.25 mm aggregate content (R_0.25) were used to indicate the stability of water-stable aggregates. The soil organic carbon (SOC) of aggregates and crop total yield were determined.

Results Compared with B0NT and B0NH, the treatments with biochar significantly increased the content of SOC water-soil aggregates (P < 0.05) of > 2 mm and 1–2 mm by 3.5%–180.3% and 9.4%–98.9%, respectively. The values of MWD, GMD and R_0.25 in B3NT and B3NH treatments were the highest, which increased by 12.5%–112.5%, 25.0%–65.7%, 20.0%–65.0%, respectively. At the same time, the contents of SOC in aggregates were significantly increased in biochar, and the macro-aggregates ( > 0.25 mm) concentrated more organic carbon than micro-aggregates ( < 0.25 mm). Compared with B0NT and B0NH, the content of SOC in soil aggregates > 2 mm, 1–2 mm, 0.5–1 mm, 0.25–0.5 mm were increased by 6.3%–30.5%, 0.2%–28.2%, 0.2%–41.6%, 4.6%–39.1% respectively. Meanwhile, the soil aggregate stability and contributing rates of aggregates in SOC were lower in 10–20 cm than those in 0–10 cm, especially in NH treatments, while the higher application rate of biochar (10000 kg/hm²) could improve soil aggregate stability and change SOC distribution. In 10–20 cm soil layer, compared with B0NT, the values of MWD, GMD and R_0.25 in B0NH was significantly reduced by 79.2%, 25.7%, 30.0% respectively, but those of B3NH had no difference with B3NT treatment. As for the contributing rates of aggregates in Lou soil SOC, the contributing rates of < 0.25 mm aggregates in SOC in B0NH treatment was significantly increased by 17.4% compared with B0NT, while B3NH treatment had no difference with the counterpart of B3NT. In addition, the crop total yields in different biochar treatments were increased, which were higher in B2NT, B3NT, B3NH treatments compared to the B0NT with the increment of 27.0%, 23.6% and 27.9% respectively, and there was no significant difference among the three treatments. The results of correlation analysis showed that there was a significant positive correlation between the GMD of water-stable macro-aggregate, the contents of SOC and crop total yield.

Conclusions Application of biochar and N fertilizer significantly improved the content of soil water-stable macro-aggregates and stability, which was conducive to increase SOC content and crop total yield. The optimal amounts for biochar and N fertilizer according to the comprehensive results should be 10000 kg/hm² and 240 kg/hm², respectively.