Effects of straw and biochar returning on soil aggregates distribution and organic carbon content in brown soil

Objectives The influence characteristics of straw returning and biochar application on different sizes of soil aggregate contents is closely related with their efficiencies in soil fertility improvement. The study aimed to clarify their differences and find suitable ways for the improvement of brown soil fertility.

Methods A field microplot experiment was conducted for six years under continuous single corn production in brown soil of Shenyang City, Liaoning Province. The experiment had six treatments, including: no fertilizer control (CK), chemical fertilizer (NPK), biochar application alone (B), biochar with chemical fertilizer application (BNPK), straw application alone (S) and straw with chemical fertilizer application (SNPK). After harvest each year, soil samples of 0–20 cm and 20–40 cm layers were collected, and soil aggregates were classified and the contents were determined using the method of Yoder.

Results Compared with NPK treatment, BNPK and SNPK treatments significantly increased the percentages of aggregate in size of > 1 mm, 1–0.5 mm and 0.25–0.5 mm in both 0–20 cm and 20–40 cm soil layers, decreased the percentage of aggregates of 0.25–0.053 mm, the increase effect of SNPK treatment was significantly higher than that of BNPK. Compared with NPK treatment, both BNPK and SNPK treatments increased MWD, GMD and R_0.25 significantly, the increase effects in MWD and GMD of SNPK were significantly higher than those of BNPK, those in R_0.25 of BNPK and SNPK treatments were not significantly different. The organic C contents decreased with the decrescent aggregate sizes, > 1 mm soil aggregate had the highest organic C content. Compared with CK, the five treatments increased the soil organic carbon contents in all sizes of aggregates. The BNPK treatment had the most significant impact on the organic carbon content of 0.25–0.053 mm aggregate in 0–20 cm soil layer, which was increased by 44.57%.

Conclusions Long-term straw and biochar application could change the distribution of soil aggregates, promote the formation of macro-aggregate, improve soil structure, improve the content of soil organic carbon and the stability of soil aggregates and crop yield. The influence of straw retuning is better than biochar application in increasing soil aggregates stability, while biochar application performs better in increasing the organic carbon contents in the aggregates, especially in 0.25–0.053 mm size.