Recent theoretical and empirical studies demonstrate that intransitive competition is an important mechanism promoting species coexistence and biodiversity maintenance. Few studies, however, have examined how intransitive competition is affected by N enrichment and spatial scale in natural communities. Based on species abundance data from a decade-long field experiment with a control and six levels of N addition rate in semi-arid grassland, we examined the effects of fertilization treatment and spatial scale on competitive intransitivity by using a recently developed Markov chain model approach. Although the relative strength of competitive intransitivity was low (I < 0.3) it was common across all, common and rare species, and over different species pools, with significant competitive intransitivity being detected amongst 60–74 % of the plots in control and nutrient treatment communities. The overall species richness consistently increased with increasing competitive intransitivity among all species and common species; however, the positive effects of intransitivity among rare species on diversity among rare species occurred only at small to moderate spatial scales. For abundance-based species groups, a negative linear relationship between competitive intransitivity and N fertilization rate was common. For trait-based functional groups, the negative effect of high N inputs on competitive intransitivity was found in seven of eight functional groups. For all species groups, the degree of competitive intransitivity generally declined with increasing spatial scale. Our study presents strong evidence that a high degree of competitive intransitivity could promote species coexistence by reducing the negative effect of N fertilization on biodiversity in natural communities. Given that the global N deposition rate is projected to increase in this century, our study sheds new light on the effects and underlying mechanisms of increased N inputs on biodiversity maintenance in semi-arid grasslands.

最近的理论和经验研究表明，不及物竞争是促进物种共存和维持生物多样性的重要机制。然而，很少有研究研究过互斥竞争如何受自然群落中氮的富集和空间规模的影响。基于半干旱草原上长达十年的控制和六种氮素添加水平的田间试验的物种丰度数据，我们使用最近开发的马尔可夫链模型方法研究了施肥处理和空间规模对竞争不及物性的影响。尽管竞争不及格的相对强度很低（I<0.3）在所有常见和稀有物种以及不同物种库中都是常见的，在控制和养分处理社区的60-74％的地块中检测到明显的竞争性非传递性。随着所有物种和常见物种之间竞争性的不及物性的增加，总体物种丰富度不断提高；然而，稀有物种间不及物性对稀有物种多样性的积极影响只发生在小到中等的空间尺度上。对于以丰富度为基础的物种群，竞争不及物性与氮肥利用率之间存在负线性关系。对于基于特征的功能组，在八个功能组中的七个中发现了高氮输入对竞争不及物性的负面影响。对于所有物种组，随着空间规模的增加，竞争不及格的程度通常会下降。我们的研究提供了有力的证据，表明高度竞争性的不传递性可以通过减少氮肥对自然社区生物多样性的负面影响来促进物种共存。鉴于预计本世纪全球氮素沉积速率将增加，我们的研究为半干旱草原上氮素输入增加对生物多样性维持的影响和潜在机制提供了新的思路。