Grazing triggers soil carbon loss by altering plant roots and their control on soil microbial community

摘要

1. Depending on grazing intensity, grasslands tend towards two contrasting systems that differ in terms of species diversity and soil carbon (C) storage. To date, effects of grazing on C cycling have mainly been studied in grasslands subject to constant grazing regimes, whereas little is known for grasslands experiencing a change in grazing intensity. Analysing the transition between C-storing and C-releasing grasslands under low- and high-grazing regimes, respectively, will help to identify key plant-soil interactions for C cycling. 2. The transition was studied in a mesocosm experiment with grassland monoliths submitted to a change in grazing after 14 years of constant high and low grazing. Plant-soil interactions were analysed by following the dynamics of plant and microbial communities, roots and soil organic matter fractions over 2 years. After disturbance change, mesocosms were continuously exposed to p#pdC-labelled CO, which allowed us to trace both the incorporation of new litter C produced by a modified plant community in soil and the fate of old unlabelled litter C. 3. Changing disturbance intensity led to a cascade of events. After shift to high disturbance, photosynthesis decreased followed by a decline in root biomass and a change in plant community structure 1.5 months later. Those changes led to a decrease of soil fungi, a proliferation of Gram(+) bacteria and accelerated decomposition of old particulate organic C (<6 months). At last, accelerated decomposition released plant available nitrogen and decreased soil C storage. Our results indicate that intensified grazing triggers proliferation of Gram(+) bacteria and subsequent faster decomposition by reducing roots adapted to low disturbance. 4. Synthesis. Plant communities exert control on microbial communities and decomposition through the activity of their living roots: slow-growing plants adapted to low disturbance reduce Gram(+) bacteria, decomposition of low and high quality litter, nitrogen availability and, thus, ingress of fast-growing plants. Our results indicate that grazing impacts on soil carbon storage by altering plant roots and their control on the soil microbial community and decomposition, and that these processes will foster decomposition and soil C loss in more productive and disturbed grassland systems.