Impact of conservation tillage on the distribution of soil nutrients with depth

Conservation tillage, defined as no-till with/without straw stubble covering, reduced tillage, or minimum tillage in this study, is widely used to mitigate the negative effects of intensive tillage. Achieving sustainable agricultural development and eco-environmental protection requires a deeper understanding of the effect of conservation tillage on soil chemical properties (e.g., organic matter (OM), nutrient elements, metal cations, and pH); we investigated these properties in this study using a global meta-analysis based on the data collected from 203 studies published worldwide since 1983. The results showed that compared with conventional tillage without straw stubble covering (CT), conventional tillage with straw stubble covering (CTS), no-till with straw stubble covering (NTS), no-till without straw stubble covering (NT), reduced tillage without straw stubble covering (RT), and minimum tillage without straw stubble covering (MT) significantly increased the topsoil (0–15 cm) organic matter (OM), organic carbon (OC), total carbon (TC), total nutrients (N, P, and K), available nutrients (N, P, and K), calcium ions (Ca²⁺), magnesium ions (Mg²⁺), and cation exchange capacity (CEC), respectively. However, the effects of conservation tillage on sodium ion (Na⁺) and pH are insignificant or inconsistent. In particular, the existing research data could not conclude whether conservation tillage can significantly increase Na⁺, or if pH was affected by conservation tillage practices, and both significant increases and decreases were observed. This meta-analysis also suggested that CTS, NTS, and NT significantly (p < 0.05) increased the subsoil (15–40 cm) OC, total nutrients (N, P, K), AN, and AP, respectively, compared to CT. Finally, compared with NT, NTS significantly increased the topsoil OM, OC, TC, TN, TP, TK, AN, and AP by 14%, 5%, 37%, 8%, 8%, 3%, 11%, and 16%, respectively. These findings indicate that conservation tillage can effectively improve the soil chemical properties, and straw stubble covering can greatly enhance the content of OM, OC, nutrient elements, and other soil chemical properties.