Plant species and soil
We selected twelve herbaceous species that co-occur in natural old fields in southern China for setting up model plant communities, including three grasses (Dactyloctenium aegyptium ,Digitaria radicosa and Isachne repens ), eight non-legume forbs (Celosia argentea , Amaranthus viridis ,Achyranthes aspera , Emilia sonchifolia , Ageratum conyzoides , Eclipta prostrata , Ludwigia hyssopifolia andCapsella bursa-pastoris ), and a legume (Mimosa pudica ).Isachne repens and Achyranthes aspera are perennial species, Mimosa pudica can be either an annual or perennial and the rest are annual species. Seeds were collected from January to February 2019 in old fields in Fengkai County, China. Soil was collected in an old field in Fengkai County, China (23.51oN, 111.82oE). The climate is subtropical monsoon with a mean annual temperature of 19.6oC and a mean annual precipitation of 1532.8 mm. On March 22nd 2019, topsoil (15cm depth) was collected, sieved at 1cm mesh to remove large particles and organic debris, and homogenized thoroughly prior to the experiment.