Shifts in leaf nitrogen to phosphorus ratio of Lolium rigidum grown in highly alkaline bauxite-processing residue sand with differing age of rehabilitation and amendments

Abstract Stoichiometric ratios of leaf nitrogen (N) to phosphorus (P) have been used as a tool to characterize ecological problems and ecosystem productivity. This study used leaf N:P stoichiometry to assess nutrient limitation and productivity in alkaline bauxite-processing residue sand (BRS) vegetated with Lolium rigidum. Fresh BRS [amended with either greenwaste compost (BRSgc) or biochar (BRSbc)], and aged BRS [e.g. 5, 7, and 15 years under established rehabilitation (5, 7, and 15YrBRS)] were grown with L. rigidum for 3 months in a controlled environment. Leaf N:P ratios were positively correlated with the pH of growth media [freshly amended BRS (r = 0.66, P 5YrBRS (20.9) > 7YrBRS (19.0) > 15YrBRS (17.2)]. This suggests that when rehabilitated BRS becomes older, the leaf N:P ratio will shift towards the ratio of the reference sample (i.e. L. rigidum with adequate levels of N and P), demonstrating an improved plant N and P uptake in aged BRS-plant systems. Results highlight that leaf N:P ratios can effectively reflect N and P limitations to productivity in vegetated alkaline BRS, which can be a critical ecological indicator in such environment. These results however, have to be verified with other species (e.g. native shrubs and trees) used for plant establishment of alkaline BRS before using this indicator for assessing ecological rehabilitation performance