The loss of above- and belowground biodiversity in degraded grasslands drives the decline of ecosystem multifunctionality

It is well established that grassland degradation negatively affects ecosystem functioning. Many studies have explored the changes of individual functions involved with grassland degradation, while fewer considered an ecosystem multifunctionality (EMF) summarizing the multiple simultaneous changes in the response of ecosystem functions to grassland degradation. Further, how changes in multiple ecosystem functions with grassland degradation in relation to above- and belowground biodiversity has been rarely explored. Here we assessed the effects of grassland degradation on nutrient cycling, biological productivity and overall EMF in a Tibetan Plateau alpine grassland. We quantified both the direct effect of grassland degradation and indirect effects mediated by changes in plant richness, soil biodiversity and abiotic factors, on EMF. We found that 1) there were negative effects of grassland degradation on EMF, plant richness and soil biodiversity; 2) EMF was positively related to plant richness, soil biodiversity and soil moisture, but not with soil pH; 3) grassland degradation had a strong negative direct effect on EMF and the positive interaction between aboveground and belowground biodiversity enhanced the negative effects on EMF, especially, through plant richness. Our study reveals how grassland degradation affects EMF directly and indirectly through biotic and abiotic pathways and highlights the importance of both above- and belowground biodiversity for maintaining multiple ecosystem functions. Protecting belowground diversity is as important as preserving aboveground diversity for maintaining ecosystem functioning and the restoration of degraded grassland ecosystem.