The restoration of Nardus grasslands is often hampered by high bioavailability of soil phosphorus and disturbed soil communities. In order to better understand these bottlenecks, we studied Nardus grassland species grown together in communities with fast-growing species in 50-liter pots along a gradient of bioavailable phosphorus with or without inoculated soil biota. These mesocosms allowed the plants to freely interact, including competition for light and nutrients.

We investigated changes in the plant community composition along the phosphorus gradient using Threshold Indicator Taxa Analysis (TITAN). We found a negative threshold of 11.5 mg P_Olsen kg⁻¹ with six significant indicator plant species. Above the threshold, a small increase in phosphorus resulted in a disproportionally large drop in biomass for the indicator species, including four typical Nardus grassland species. The decline in these ‘oligotrophic indicator species’ was also linked to increasing plant community biomass, so we suggest the oligotrophic indicator species to be outcompeted for light by fast-growing plant species. We did not find an effect of the soil biota treatment on the biomass of the oligotrophic indicator species, but did observe a positive effect of inoculation with soil biota on the total biomass of the plant community.

Interestingly, the threshold for the plant communities in the mesocosm experiment was comparable to the upper bioavailable phosphorus concentrations in remnant Nardus grasslands in northern Belgium. For the restoration of Nardus grasslands, such phosphorus-poor soil conditions appear to be essential, because the plant species that typically occur in these grasslands are able to handle nutrient limitation, but not light limitation.

Nardus草原的恢复常常受到土壤磷的高生物利用度和受干扰的土壤群落的阻碍。为了更好地理解这些瓶颈，我们研究了Nardus草地物种，这些物种与迅速生长的物种一起在50升盆中沿着生物可利用的磷（有或没有接种土壤生物量）的梯度一起生长。这些中观世界使植物能够自由相互作用，包括争夺光和营养。

我们使用阈值指标分类法分析（TITAN）调查了沿磷梯度的植物群落组成的变化。我们发现负阈值为11.5 mg P _Olsen kg ^-1，具有六个重要的指示植物种类。高于阈值时，磷的少量增加导致指示物物种（包括四个典型的纳尔达斯）的生物量下降幅度过大草原物种。这些“营养不足指示物种类”的减少也与植物群落生物量的增加有关，因此我们建议通过快速增长的植物种类来取代营养不足的指示物种类。我们没有发现土壤生物区系对贫营养指示物物种生物量的影响，但是确实观察到接种土壤生物区系对植物群落总生物量的积极影响。

有趣的是，中观实验中植物群落的阈值与比利时北部残留的纳尔杜斯草原中较高的生物利用磷浓度相当。对于Nardus草原的恢复来说，这种缺磷的土壤条件似乎是必不可少的，因为通常在这些草原中出现的植物能够处理养分限制，但不能限制光照。