Nitrogen and phosphorus fertilizer regime affect jarrah forest restoration after bauxite mining in Western Australia

Abstract

Question Removal of vegetation and disturbance of the soil profile during mining reduce nutrient pools available for plants during mine site restoration. Thus, fertilizer is needed to replace the loss of nutrients available to support vegetation establishment. Yet the application of fertilizer can promote the establishment of competitively dominant species to the exclusion of others, particularly in low‐fertility ecosystems. Here, we test effects of fertilizer application rates on the establishment of jarrah forest. Location Five restored mine pits in the jarrah forest that grows on low‐fertility soils in southwest Western Australia. Methods Nitrogen (0 and 20 kg elemental N·ha−1) and phosphorus (0, 10, 20 and 40 kg elemental P·ha−1) were applied factorially to 20 m × 20 m field plots at five sites restored after bauxite mining with seeds and seedlings of jarrah forest species. Results Two and a half years after the experiment was established, there were no effects of N application rates on species richness, percentage vegetation cover or community composition of the restored jarrah forest. In contrast, P application had a number of effects on the restored jarrah forest. Species richness was highest at intermediate P application rates (10–20 kg·ha−1) and lowest at the two extreme treatments (0 and 40 kg·ha−1). Higher rates of P application resulted in a higher seedling density (2.5 vs 5.0 m−2 for 0 and 40 kg P·ha−1, respectively) and increased plant cover. Fertilizer P also significantly affected community composition, favouring ephemerals and weeds at high application rates and re‐sprouting species at lower application rates. Community composition was more similar to the restoration target (unmined jarrah forest) at 0 kg·P·ha−1 (similarity value of 36%) than at higher P application rates. Conclusions The data suggest that, in terms of maximizing species richness and percentage cover, a P application rate of between 10 kg·P·ha−1and 20 kg·P·ha−1is optimal for jarrah forest restoration after bauxite mining. Additionally, the data suggest no benefit of N application at a rate of 20 kg·N·ha−1.

Phosphorus fertilisation and large legume species affect jarrah forest restoration after bauxite mining

“…We found that species richness, density and cover of weeds and the density of native ephemerals increased with increasing P. This result is consistent with other studies that have reported higher rates of fertiliser to increase species richness, cover and density of exotic annuals (e.g. Prober and Wiehl, 2012;Daws et al, 2013) and native ephemeral species (Daws et al, 2013). However, for these groups, cover and density increased to a lesser extent at the highest P-application rate when large legumes were seeded, i.e.…”

“…the premise that additional legumes may increase tree growth due to higher N-availability was not supported. Although the sampling regime for soil nutrients was low (6 sample locations plot À1 in 2006), these results are consistent with other studies that have also shown low availability of N in restored jarrah forest soils (Daws et al, 2013;Standish et al, 2008). While large legumes had no effect on tree growth in our study, the competitive environment resulting from high legume densities can in fact reduce tree growth in both restored sites and forestry plantations (Turvey et al, 1983;Koch, 1987;Forrester et al, 2011).…”

“…Shane et al, 2004) and for post-mining restoration in South Africa, Holmes (2001) found that adding 26 kg P ha À1 increased mortality of Proteaceae. In contrast, we found no negative effect of P on density of re-sprouters or Proteaceae suggesting that P-toxicity is unlikely to be a major cause of reduced growth at elevated P. Similarly, Daws et al (2013) found no effect of applied-P on density of Proteaceae in jarrah forest restoration. An absence of P-toxicity may result from the strong P immobilising properties of jarrah forest soils that reduce P-availability to plants (Bolan et al, 1983;Handreck, 1997).…”

“…This suggests that at >20 kg ha À1 P there were limitations to growth caused by the availability of other soil resource(s). In restored sites 5 years of age, water availability is unlikely to significantly constrain plant growth (Croton and Reed, 2007), but soil-N may be limiting: soil NO 3 À is rapidly lost by leaching due to the gravelly nature of jarrah forest soils (Banning et al, 2008;Daws et al, 2013). In contrast, the N-fixing leguminous species did not exhibit growth limitation at >20 kg P ha À1 .…”

“…For example, in fynbos restoration after a simulated mining disturbance, Holmes (2001) found that adding 26 kg P ha À1 increased overall plant density and cover, although this effect was mainly limited to exotic weeds, and increased the mortality of Proteaceae. Similarly, in jarrah forest restored after mining, Daws et al (2013) found that weed species and native ephemerals benefited from fertiliser P. However, the growth and density of Proteaceae was unaffected by P, although the species richness of native species of the restored sites was reduced at P-application rates greater than 20 kg ha À1 . However, the combined effects of P-application and a restoration strategy that includes a significant understorey legume component are unclear.…”