Increasing Topsoil Depth Improves Yield and Nitrogen Fertilizer Use Efficiency in Maize

Topsoil degradation poses a significant threat to agricultural production worldwide. However, whether degraded topsoil is a net nitrogen source or sink depends on crop uptake and nutrient loss, and how it affects the sustainability of agricultural production remains unclear. To fill this gap in understanding, we conducted a three-year experiment with five topsoil depth treatments: 10 cm (D10), 20 cm (D20), 30 cm (D30), 40 cm (D40), and 50 cm (D50). Increasing topsoil depth significantly increased grain yield by a maximum of 49.4% (between D10 and D50). With increasing topsoil depth, the NFUE rises from 14.2% to 64.9% (between D10 and D50 treatments), while the G-NFUE climbs from 9.0% to 36.2% (between D10 and D40 treatments). Increasing topsoil depth reduced topsoil N depletion and the percentage of change in soil N stocks. In addition, N fertilizers applied during the season were generally enriched in soil at a depth of 30–40 cm. Therefore, increasing the depth of topsoil can effectively increase the source of nutrients absorbed by a crop by increasing access to additional resources stored in deeper soils, which ultimately increases maize grain yield and N fertilizer use efficiency. In this study, the threshold for maize to achieve high yields and efficiency was a topsoil depth of 30 cm. This study elucidated the differences in maize grain yield and resource utilization at different topsoil depths and established a link with soil N characteristics, and thus, it will provide a theoretical basis for the sustainable management of topsoil.