Ecosystem multifunctionality of temperate grassland is impeded by long-term enclosure, especially in deep soil

Enclosure has been widely used as an effective and feasible management tool to restore grassland. The shift from positive to negative relationship between enclosure and ecosystem multifunctionality (EMF) along enclosure duration alters the outcome of enclosure effect on grassland restoration. Topsoil and subsoil exhibit distinct environmental and biological features influencing EMF, and that can lead to predictable variations in the enclosure effect on EMF. However, there is still a lack of understanding of the contrasting enclosure duration effects on EMF between topsoil and subsoil. Here, we conducted a research on between enclosure plots and paired grazed plots across 6 locations on the Mongolian Plateau to investigate the trends of biodiversity and EMFs along enclosure duration both in topsoil (0–10 cm) and subsoil (30–50 cm). Our results showed that relative higher EMFs in topsoil than in subsoil, and peak curves of EMFs along enclosure duration for both topsoil (21.5 years) and subsoil (17.5 years). However, compared with topsoil, we found a stronger quadratic enclosure effect in subsoil, which reduced the restoration effect of long-term enclosures on EMFs. Mean annual precipitation and soil water content were the main environmental drivers for EMFs in both soil layers, while contrasting biodiversity drivers were observed in different soil layers. Specifically, we found that plant diversity only had a negative effect in the subsoil. Moreover, fungal diversity had a stronger effect in topsoil, while bacterial diversity had a stronger effect in subsoil. Our research suggests that dynamic enclosure effects on soil multifunctionality differ across soil depths and these differences should be considered when evaluating grassland restoration models.