Opposing Effects of Plant-Community Assembly Maintain Constant Litter Decomposition over Grasslands Aged from 1 to 25 Years

International audience; Litter decomposition is central to ecosystem functioning and depends, under constant abiotic conditions, on litter quality and decomposer activity. During the assembly of a plant community following disturbance, litter quality is expected to decrease, due to an increasing proportion of resource-conservative species, whereas decomposer activity is expected to increase, due to the establishment of decomposer populations and their response to local vegetation (“home-field advantage”, HFA). To date, the combined effect of these processes remains poorly understood. We studied 27 seminatural grasslands in western France, ranging from 1 to 25 years since last cultivation. We measured the functional composition of plant communities using litter traits (specific leaf area, leaf dry matter content, C/N ratio, phenolics), characterized the entire community of decomposers (macrofauna, mesofauna, microbes) and performed reciprocal litter transplants to quantify HFA. We found that, overall, decomposition was rapid, and HFA was not evident. While there was substantial among-grassland variation in decomposition and HFA, neither changed with grassland age. Litter quality and decomposer efficiency also remained, overall, unchanged. However, grassland age determined all measured litter traits and caused soil microbial C/N ratio to decline. Although these changes impacted decomposition individually, together they canceled out each other, resulting in constant decomposition across the chronosequence. Our results suggest that processes driving decomposition differ during grassland succession and suggest that HFA may be lower in communities with high litter quality. Moreover, simultaneous assembly processes have opposing and therefore stabilizing effects on decomposition, possibly explaining the outstanding resilience of primary production in temperate grassland ecosystems.