Covariation of soil nutrients drives occurrence of exotic and native plant species

Abstract

Nutrient enrichment with phosphorus (P) and nitrogen (N) threatens biodiversity globally, particularly because it drives invasion by exotic plant species. However, the effects of nutrients on plants can interact with other ecosystem processes such as competition and grazing, and these interactions can be scale‐dependent. Furthermore, P and N are often correlated, making it difficult to separate the effects of each. After implementing a herbivore‐exclusion experiment for 3 years in grassy woodlands, SE Australia, we assessed the relationship of native and exotic plant species with soil nutrients, grazing and spatial scale. Nitrate (NO3), available P and organic carbon (C) were partitioned into two principle components and further divided into plot‐ and site‐level variation. The correlated components of NO3, P and C (principle component 1) appeared to drive a dominant competitive relationship. Two exotic annual grasses likely out‐competed native and exotic species in areas high in NO3, P and C. In contrast, responses to principle component 2 (gradient from high NO3 to high P) indicated that native and exotic species generally benefitted when NO3 was high relative to P. Plant–nutrient relationships were further modified by grazing and spatial scale. Native and exotic species benefitted from grazing but did so under specific nutrient regimes. Summed occurrence of exotic species and native richness was reduced in the absence of grazing and high values of NO3 relative to P at site scales, but not at plot scales, suggesting competition is more important at a scale of hundreds of square metres while soil properties are most important at the plot scale (25 m2). Synthesis and applications. Dividing soil nutrients into uncorrelated principal components showed that niche space for native and exotic plant species is partitioned by positive and negative covariation of nitrate (NO3), phosphorus (P) and carbon (C), and interactions of these soil properties with grazing and spatial scale. Increasing NO3 relative to P could increase species richness, but requires further experimentation to define beneficial concentrations. Native plant richness can be enhanced with adequate levels of grazing by native herbivores and, in the long term, by reducing P from areas enriched with NO3 and P. These actions will help prevent likely competitive dominance by annual exotic grasses.

Measuring competitive impact: Joint‐species modelling of invaded plant communities

“…In our study, greater cover of three non-native species (Bromus diandrus, Acetosella vulgaris and especially the annual grass Avena fatua) was associated with strong declines in the cover of native species after accounting for differences in environmental responses. This outcome is consistent with previous studies that have measured the impact of non-native species in Australian temperate grasslands (Driscoll & Strong, 2017;Prober, Thiele, Lunt, & Koen, 2005) and in grasslands globally (Chang & Smith, 2014;Flores-Moreno et al, 2016;Harpole et al, 2016). Our results support the predictions outlined in the Introduction.…”

“…Total extractable nitrogen at sites along the fertility gradient ranged from 615 ppm to 2,420 ppm (Driscoll & Strong, ). Total soil carbon, nitrogen and phosphorus levels, as well as extractable nitrogen and phosphorus, all covaried strongly across the 10 sites (Appendix ), and we used total extractable nitrogen as a proxy for overall soil fertility.…”

Measuring competitive impact: joint-species modelling of invaded plant communities

“…In our study, greater cover of three non-native species (Bromus diandrus, Acetosella vulgaris and especially the annual grass Avena fatua) was associated with strong declines in the cover even after accounting differences in environmental responses. This outcome is consistent with previous studies on the impact of non-native species in Australian temperate grasslands (Driscoll, 2017;Driscoll & Strong, 2017;Prober, Thiele, Lunt, & Koen, 2005) and matching outcomes in grasslands globally (Chang & Smith, 2014;Flores-Moreno et al, 2016;Harpole et al, 2016).…”

“…Total extractable nitrogen at sites along the fertility gradient ranged from 615 ppm to 2420 ppm (Driscoll & Strong, 2017). Total soil carbon, nitrogen and phosphorus levels, as well as extractable nitrogen and phosphorus, all covaried strongly across the 10 sites (Appendix 1), and we used total extractable nitrogen as a proxy for overall soil fertility.…”

Plant-Dependent Soil Bacterial Responses Following Amendment With a Multispecies Microbial Biostimulant Compared to Rock Mineral and Chemical Fertilizers

“…Pasture plant species differ in their capacity to exploit limiting nutrients ( Hill et al, 2006 ) and this can influence their relative abundance within a sward ( Driscoll and Strong, 2017 ). Long-term application of fertilizers, including slow-release fertilizers, can influence soil microbial communities ( Zhao et al, 2014 ; Pan et al, 2016 ; Xun et al, 2016 ), and this in turn may influence the capacity of plants to access nutrients.…”