Effects of no-tillage and straw returning on soil aggregates composition and organic carbon content in black soil areas of Northeast China

Abstract: Soil aggregates and organic carbon content are important soil quality evaluation indices. They play important roles in guiding agriculture and ensuring food production safety. In order to solve the problem of deterioration of soil structural properties and decrease of organic carbon content caused by irrational farming in the black soil area of Northeast China, an experiment was carried out in Xiangyang experimental base of Northeast Agricultural University in Harbin since 2015. To study effects of no tillage and straw returning on soil aggregate stability, soil organic carbon content and the soil organic carbon content of the aggregates in 0-10 and 10-20 cm soil layers in the Northeast thin black soil area, four treatments including straw returning with no-tillage (NTS), no-tillage(NT), straw returning with conventional tillage(CTS) and conventional tilliage(CT) were designed. Soil samples were collected in 2018 and 2019. The impact of conservation tillage on the physical and chemical properties of soil was analyzed. The results showed that the percentage content of water-stable aggregates larger than 5 mm and Mean Weight Diameter (MWD) of NTS treatment were significantly higher than those in the other three treatments of 0-10 and 10-20 cm soil layers in 2018 and 2019. Compared with 2018, the percentage content of water-stable aggregates larger than 5 mm of NTS and NT treatments increased to varying degrees in 2019. In 2018, the MWD values of 0-10 cm soil layer ordered from high to low were NTS, NT, CTS and CT. In 2019, the NTS treatment was significantly higher than NT treatment (P<0.05), and no significant difference was found between CTS treatment and CT treatment. In 2018 and 2019, the MWD values of 10-20 cm soil layer ordered from high to low were NTS, NT, CT and CTS, and the difference was significant between treatments (P<0.05). The organic carbon content of soil and organic carbon content in aggregates decreased with the increase of soil depth generally. In 2018, the soil organic carbon content of NTS and NT treatments in 0-10 and 10-20 cm soil layers were significantly higher than those in CTS and CT treatments (P < 0.05). In 2019, no significant difference in 0-10 cm soil layer was found between CT treatment and CTS treatment. In 10-20 cm soil layer of 2019, the contents of soil organic carbon of NTS and NT treatments were not significantly different, and CTS treatment was significantly higher than CT treatment (P < 0.05). In 2018 and 2019, the peak value of organic carbon content of water stable aggregates in 0-10 and 10-20 cm soil layers appeared at 1-2 mm, the organic carbon content of water-stable aggregates smaller than 0.25 mm was the lowest. In 0-10 and 10-20 cm soil layers of 2018 and 2019, the organic carbon contribution rates of water-stable aggregates of >5, 2-5 and 1-2 mm of NTS and NT treatments were higher than those in CTS and CT treatments generally, and those of water stable aggregate smaller than 0.25 mm was smaller than those of CTS and CT treatments. The research showed that no tillage and straw returning were beneficial to stability of soil aggregates and accumulation of organic carbon in different aggregates at sloping farmland of thin black soil. The treatments of no-tillage combined with straw returning had the best effect compared with treatments of no-tillage, conventional tillage and conventional tillage combined with straw returning.