In mine-waste affected terrestrial ecosystems, land-cover change is often the result of heavy metal/metalloid toxicity. The alteration or loss of aboveground and belowground plant inputs degrades soil biogeochemical properties, causing erodible bare substrates devoid of vegetation and nutrients. While it is increasingly recognized that aboveground net primary productivity (ANPP) of many terrestrial ecosystems is co-limited by both nitrogen (N) and phosphorus (P), N-limitation of ANPP still predominates at cool, high latitudes on younger soil types. Accordingly, restoration efforts of smelter-impacted landscapes in southwest Montana, USA use an organic N-based fertilizer to reestablish native plant communities and improve soil biogeochemical processes. However, this approach does not take into account the unique residual geochemical and ecological factors that can influence P and other nutrient availability in these metal(loid)-contaminated ecosystems. We used a factorial nutrient addition study to assess changes in plant cover (m2 plot) and ANPP (0.2025 m2 clip plot) following the addition of N, P, and potassium plus micronutrients (Kμ), alone and in combination, to answer questions about how the interactions of metal(loid)s and nutrient additions are influencing plant community composition and ANPP on highly-degraded smelter-affected soil.
Results/Conclusions
The response of the plant community differed among treatments, with graminoid cover increasing significantly compared to all other lifeforms measured (forb/tree). The NP combined treatment resulted in the highest cover of the non-native species Agrostis stolonifera, whereas native grasses responded best to the NPKμ combined treatment. However, grass cover treatment-level differences between NP and NPKμ were not significant, and the response of Agrostis stolonifera and native grass cover for both treatments was higher than the sum of the individual fertilizer treatments (N+P and N+P+Kμ), suggesting grass species at this site are synergistically co-limited by multiple nutrients. The treatment effects on overall ANPP (sum of graminoid+forb+tree) are less pronounced, but some treatment-level differences have emerged. The NP treatment combination produced the greatest increase in ANPP, but the nutrient limitation effect appears to be additive (sum of N+P alone = NP combined) rather than synergistic. The discrepancy between cover and ANPP may be due to heterogeneity between the clip and cover plots, or it may be that these two indicators are responding differently to nutrient additions. This study provides additional evidence that multiple-nutrient constraints should be considered in ecosystem-scale restoration of metal(loid)-contaminated sites, even in cool, high latitude terrestrial ecosystems.