Objectives The combined application of lime, Chinese milk vetch and rice straw have been widely used in paddy fields in southern China, the effect and mechanism of their synergistic improvement on soil remains unclear. This study explores the long-term impacts of the synergistic utilization of lime, Chinese milk vetch and rice straw on the composition of soil aggregates and the distribution of carbon and nitrogen in paddy fields, aiming to provide a theoretical basis for their efficient utilization.

Methods Alluvial loamy paddy soil (AL) and reddish yellow clayed paddy soil (RY), which were widely distributed in the southern paddy fields of China, were selected for a six-year micro-plot positioning experiment. Three treatments were set up: chemical fertilizer (F); Chinese milk vetch + rice straw + chemical fertilizer (GRF); Chinese milk vetch + rice straw + lime + chemical fertilizer (GRFL). After the harvest of late rice, soil samples of the 0−20 cm layer were collected. The soil aggregates were divided into four grades by the wet sieving method. The quantity, proportion, as well as the contents of organic carbon and nitrogen of each grade of aggregates were measured. The mean weight diameter (MWD), geometric mean diameter (GMD) and stability (R_0.25) of soil aggregates were calculated. The yields of early rice, late rice and annual rice were investigated, and the correlations between the organic carbon and nitrogen reserves of different particle-size aggregates and the annual rice yield were analyzed.

Results Alluvial loamy paddy field, compared with F, the annual rice yield of GRF and GRFL was significantly increased by 17.29% and 20.88%, respectively; the content of aggregates with a particle size of 0.25−2 mm of GRF treatment was significantly increased by 13.53%, and R_0.25 was significantly increased by 6.65%. Compared with GRF, GRFL increased the MWD and GMD by 10.45% and 12.00%, respectively, enhanced the organic carbon and total nitrogen contents as well as the total reserve of organic carbon in the aggregates of >2 mm, 0.053−0.25 mm and <0.053 mm. The rice yield was positively correlated with organic carbon reserves in soil aggregates of >2 mm and 0.25−2 mm. Reddish yellow clayed paddy field, compared with F, GRF and GRFL significantly increased annual rice yield by 13.99% and 14.98%, respectively; GRF increased the content of aggregates of 0.25−2 mm and the R_0.25 by 41.41% and 5.25%, and enhanced the contents and total storage of carbon and nitrogen in the soil aggregates of >2 mm and 0.25−2 mm. Compared with GRF, GRFL decreased the MWD and GMD by 1.73% and 8.66%, R_0.25 by 3.17%, the storage of organic carbon and nitrogen by 4.70% and 4.74%, respectively. The rice yield was positively correlated with the organic carbon storage in the soil aggregates of >2 mm and 0.25−2 mm.

Conclusions The combined utilization of Chinese milk vetch and rice straw can significantly increase the rice yield, promote the formation of soil macro-aggregates and improve the soil structure. Adding lime on the basis of combined utilization can further improve the stability of alluvial loamy paddy soil aggregates and then increase the storage of organic carbon and nitrogen, but it reduces the stability of soil aggregates and the storage of carbon and nitrogen in reddish yellow clayed paddy soil.