Data from: An African grassland responds similarly to long-term fertilization to the Park Grass experiment

We compared the results of a long-term (65 years) experiment in a South African grassland with the world’s longest-running ecological experiment, the Park Grass study at Rothamsted, U.K. The climate is warm and humid in South Africa and cool and temperate in England. The African grassland has been fertilized with two forms of nitrogen applied at four levels, phosphorus and lime in a crossed design in 96 plots. In 1951, about 84% of plant cover consisted of Themeda triandra, Tristachya leucothrix and Setaria nigrirostris. Currently, the dominant species are Panicum maximum, Setaria sphacelata and Eragrostis curvula, making up 71% of total biomass. As in the Park Grass experiment, we found a significant (additive) interaction effect on ANPP of nitrogen and phosphorus, and a (marginally significant) negative correlation between ANPP and species richness. Unlike the Park Grass experiment, there was no correlation between ANPP and species richness when pH was included as a covariate. There was also a significant negative effect of nitrogen amount and nitrogen form and a positive effect of lime on species richness and species diversity. Soil pH had an important effect on species richness. Liming was insufficient to balance the negative effects on species richness of nitrogen fertilization. There was a significant effect of pH on biomass of three abundant species. There were also significant effects of light on the biomass of four species, with only Panicum maximum having a negative response to light. In all of the abundant species, adding total species richness and ANPP to the model increased the amount of variance explained. The biomass of Eragrostis curvula and P. maximum were negatively correlated with species richness while three other abundant species increased with species richness, suggesting that competition and facilitation were active. Consistent with the results from the Park Grass and other long-term fertilization experiments of grasslands, we found a positive effect of soil pH and a negative effect of nitrogen amount on species richness, a more acutely negative effect on species richness of acidic ammonium sulphate fertilizer than limestone ammonium nitrate, a negative relationship between species richness and biomass, and a positive effect on species richness of lime interacting with nitrogen.

本研究将南非草原一项为期65年的长期生态学实验结果，与全球持续运行时间最长的生态学实验——英国洛桑实验站的公园草地实验（Park Grass Study）进行对比。南非地区气候暖湿，英国则为温凉的温带气候。该非洲草原实验共设置96个样地，采用交叉设计施加两种形态的氮肥（各设4个施用量梯度），同时配施磷肥与石灰。1951年，该区域约84%的植被盖度由黄背草（Themeda triandra）、Tristachya leucothrix与黑颖狗尾草（Setaria nigrirostris）构成。目前，群落优势种变为大黍（Panicum maximum）、Setaria sphacelata与弯叶画眉草（Eragrostis curvula），其总生物量占群落总生物量的71%。与公园草地实验一致，本研究发现氮肥与磷肥对地上净初级生产力（Aboveground Net Primary Production，ANPP）存在显著的（加性）交互效应，且ANPP与物种丰富度呈边际显著的负相关关系。但与公园草地实验不同的是，当以pH值作为协变量时，ANPP与物种丰富度之间未呈现显著相关关系。氮肥施用量与氮肥形态对物种丰富度与物种多样性存在显著负效应，而石灰施用则对其产生显著正效应。土壤pH值对物种丰富度具有重要调控作用。石灰施用不足以抵消氮肥施用对物种丰富度带来的负面效应。pH值对三种优势物种的生物量存在显著影响。光照对四种物种的生物量同样存在显著影响，其中仅大黍（Panicum maximum）对光照呈负响应。针对所有优势物种，在模型中纳入总物种丰富度与ANPP后，模型的方差解释量显著提升。弯叶画眉草（Eragrostis curvula）与大黍（Panicum maximum）的生物量与物种丰富度呈负相关，而其余三种优势物种的生物量则随物种丰富度升高而增加，这表明群落中竞争与促进作用同时存在。与公园草地实验及其他长期草地施肥实验的结果一致，本研究发现土壤pH值对物种丰富度呈正效应，氮肥施用量呈负效应；酸性硫酸铵肥料对物种丰富度的负向影响较硝酸铵石灰（Limestone Ammonium Nitrate，LAN）更为显著；物种丰富度与生物量呈负相关关系；而石灰与氮肥的交互作用则对物种丰富度产生正效应。