Facilitation of Urtica dioica colonisation by Lupinus arboreus on a nutrient-poor mining spoil

Facilitation is an important process during succession. Legumes often play a significant role as facilitators, particularly in primary succession, enriching the soil with nitrogen (N). The leguminous shrub Lupinus arboreus (Sims) can fix significant N on acidic, nutrient-poor soils. An apparent association between L. arboreus and Urtica dioica (L), which requires high concentrations of soil N and phosphorus (P), suggested that L. arboreus might facilitate colonisation by Urtica of nutrient-poor soils by increasing both soil N and P. I measured significantly higher concentrations of extractable soil P and higher values of soil pH beneath L. arboreus canopy, compared with areas between bushes, occupied by herbaceous vegetation. Litter inputs beneath L. arboreus were more than two and a half times higher in terms of mass of material and P and three times higher in terms of N, than in areas between bushes. This high litter input accounted for the higher soil P concentration and higher pH. It did not lead to higher soil organic matter content however, probably because high nutrient concentration in L. arboreus litter leads to rapid decay. A glasshouse trial showed that Urtica grew poorly on soil collected from areas between bushes of L. arboreus without the addition of supplementary N and P fertiliser, indicting co-limitation, by both N and P. Growth of Urtica on soil from beneath L. arboreus was more than four times higher than on soil from between L. arboreus. Amendment of the latter soil with N significantly increased growth of Urtica, but amendment with P did not, indicating that Urtica growth on this soil was not P limited, even when amended with N. Facilitation of colonisation of this site by Urtica therefore can be attributed to increased soil N and P, derived from litter of L. arboreus. However, in the field Urtica was only found beneath dead and senescent L. arboreus, suggesting a period of inhibition caused by shading, before senescence of L. arboreus allows light penetration to the nutrient-rich soil below.