Monitoring plant physiological characteristics to evaluate mine site revegetation: A case study from the wet-dry tropics of northern Australia

Biologically driven markers or monitors were used to evaluate plant and ecosystem health of uranium-mining affected sites. Plant water, nitrogen (N) and phosphorus (P) status were used to measure physiological characteristics of tree and shrub species at sites perturbed by mining activities (waste rock dumps: WRD 1, WRD 2; mine wastewater irrigated woodland) and of species at undisturbed woodland (tropical savanna). Plant water status was evaluated by measuring leaf relative water content (RWC) and carbon isotope discrimination (delta(13)C). Leaf RWC varied significantly (P < 0.0001) between wet and dry season in species at the woodland sites with higher RWC in the wet season compared to the dry season. No seasonal differences were observed in RWC in species at the WRDs. Leaf delta(13)C was similar in species at woodland sites and WRD 2 (-28.8 to -28.1 parts per thousand) but was significantly (P < 0.05) lower in species at WRD 1 (-27.6 parts per thousand). This suggests that species at WRD 1 had a lower water availability and/or lower water use compared to species at all other sites. WRD substrate had an up to 4-orders of magnitude greater availability of inorganic phosphate (Pi) compared to woodland soil as determined using in situ ion exchange resin. Pi concentrations in xylem sap of species at WRDs were 2- to 3-fold higher compared to species at woodland sites. Plant nitrate reductase (NR) activity was low in most species at woodland and WRD 1. In contrast, Eucalyptus and Acacia species had high NR activities of up to 300-700 pkat g(-1) fw at WRD 2 indicating that these species had greater nitrate use than species at all other sites. Nitrate availability in the top five cm of the profile, as determined using in situ ion exchange resins, increased at all sites in the wet season, but no significant differences were observed between sites using this method. However, traditional soil analysis revealed that WRD substrate had a 2-times higher nitrate content (0 to 1000 mm depth) compared to woodland soil. Thus, it is likely that plants at WRD2 accessed nitrate from deeper parts of the profile. Proline, an indicator of plant stress, was found in appreciable quantities in leaves of herbaceous species but not in woody species. Soil and leaf delta(15)N were measured to investigate N-cycling and the contribution of diazotrophic N-2 fixation to plant N nutrition. Soil delta(15)N values were highest and most variable at WRD 2 (6.2 parts per thousand) compared to all other sites (irrigated woodland 3.1 parts per thousand, undisturbed woodland 2.5 parts per thousand, WRD 1 0.9 parts per thousand). This may indicate that N-turnover and nitrification was greatest at WRD 2 leading to greater N-15 enrichment of soil N. At all sites, Acacia species were nodulated and putatively fixing N-2. With the exception of WRD 2 where leaf delta(15)N of Acacia species averaged 0.9 parts per thousand, Acacia species had N-15 depleted values characteristic of species that receive N derived from N-2 fixation (-0.8 to -0.6 parts per thousand). Eucalyptus species at the woodland also had N-15 depleted values (average -0.4 parts per thousand) but N-15 enriched values (0.3 to 1.8 parts per thousand) at the three mining affected sites. The results show that for the plants studied foliar delta(15)N could not be used as an unequivocal measure of plant N sources. The results suggest that biomonitoring of plant and ecosystem health has potential in evaluating performance of mine site revegetation