Subsoiling increases aggregate-associated organic carbon, dry matter, and maize yield on the North China Plain

Background Soil degradation is one of the main problems in agricultural production and leads to decreases in soil quality and productivity. Improper farming practices speed this process and are therefore not conducive to food security. The North China Plain (NCP) is a key agricultural area that greatly influences food security in China. To explore the effects of different tillage measures on aggregate-associated organic carbon (AOC), the accumulation and transport of dry matter, and maize yield, and to identify the most suitable tillage method for use on the NCP, a field experiment was conducted at Shandong Agricultural University from 2016–2017 using plots that have been farmed using conservation tillage since 2002. Methods In this study, Zhengdan 958 summer maize was used as the test material and undisturbed soil and plant samples were obtained under four tillage methods—no-tillage (NT, tillage depth: 0 cm); rotary tillage (RT, tillage depth: 10 cm); conventional tillage (CT, tillage depth: 20 cm); subsoiling (SS, tillage depth: 40 cm)—which were used to determine the AOC and dry matter contents, as well as the yields of two summer maize growing seasons. Each sample was replicated three times and the AOC content was determined via potassium dichromate oxidation colorimetry. Potassium dichromate oxidized organic carbon in organic matter was employed to reduce hexadecent chromium into green trivalent chromium. Colorimetry was then used to determine the amount of reduced trivalent chromium and calculate the organic matter content. Results The resulting data were statistically analyzed and the results showed that, compared with CT, the AOC contents with NT and SS increased by 5.65% and 9.73%, respectively, while that with RT decreased by 0.12%. Conventional tillage resulted in the highest mean dry matter weight when the maize reached maturity, which was 19.19%, 9.83%, and 3.38% higher than those achieved using NT, RT, and SS, respectively. No significant difference was found between CT and SS treatments, both of which tended to increase the accumulation of dry matter as well as its contribution of assimilates to grain yield post-anthesis. Compared with CT, the mean yield increased at a rate of 0.18% with SS, while yields declined at rates of 17.17% and 11.15 with NT and RT, respectively. The yield with NT was the lowest, though the harvest indices with NT and SS were higher than those with RT and CT. Overall, SS increased the accumulation of dry matter and its contribution of assimilates to grain yields post-anthesis, as well as the AOC content and yields, making it the ideal tillage method for the NCP.