Long‐Term Integration of Cover Crops in Olive Orchards Restores Soil Nitrogen Fertility on Transitional Drylands

ABSTRACT Transitional drylands are anthropogenically fragile and climate‐susceptible land surfaces situated across sub‐humid, arid and semi‐arid ecosystems worldwide. Understanding land use dynamics is essential for food security and soil conservation, particularly in ecologically vulnerable ecosystems such as transitional drylands. Maintaining optimal soil nitrogen (N) fertility is a major challenge in intensive land use scenarios on these drylands. At the same time, understanding how soil N and associated soil functions respond to different land‐use changes is crucial for developing sustainable nitrogen management strategies. This study investigates the changes in soil N fertility and associated key ecosystem functions in topsoil (0–15 cm) and subsoil (15–30 cm) by comparing three land‐use systems along 5‐, 10‐ and 18‐year chronosequences: (i) groundnut (native land use), (ii) olive without cover crop (olive − CC) and (iii) olive with cover crop (olive + CC). Our results showed that long‐term integration of olive + CC, particularly after 18 years, led to the most noticeable decrease in soil pH and bulk density (BD). In contrast, we observed significantly higher water extractable organic carbon (WEOC), pentose (plant derived‐C), hexose (microbe derived‐C) and soil enzyme activities (i.e., dehydrogenase, urease and protease) in olive + CC compared to either groundnut or olive − CC. For total N stocks, the olive − CC showed a consistent decline (≈30% in topsoil; 27% in subsoil from 5 to 18 years), indicating progressive N depletion without organic inputs. In contrast, olive + CC restored soil N fertility with the highest N stocks after 18 years, including total N (131.58 kg N ha −1 in topsoil; 222.10 kg N ha −1 in subsoil), greater soil NO 3 ‐N and NH 4 + ‐N stocks at all depths, and a significant increase in organic N and microbial biomass nitrogen (MBN), especially in subsoil after 18 years. Regardless of soil depth, the mineral N stock under olive + CC increased steadily over 18 years, whereas changes under olive − CC were not significant. Furthermore, the regressions highlight the importance of both the quantity and quality of WEOC, which decisively influence soil N multifunctionality, especially through the additive effects of olive + CC. Overall, our study provides empirical evidence that long‐term diversification of olive orchards with cover crops is a practical, time‐dependent strategy to restore and improve soil nitrogen fertility in transitional drylands.