The relative impact of invasive Australian acacias, fire and season on the soil chemical status of a sand plain lowland fynbos community

Measurements of soil pH, resistance, cation exchange capacity and six macro- and five micro-elemental concentrations were made over a 12-month period in disturbed (burnt and Acacia saligna-infested) and undisturbed habitats in a sand plain lowland fynbos community. Two null hypotheses were tested, namely that there were no differences in soil chemical properties between (i) disturbed and undisturbed habitats and (ii) different seasons. A Friedman test was used to identify significant differences in soil chemical properties between different (i) habitats against seasons and (ii) seasons against habitats. With respect to acacia infestation, the first null hypothesis was rejected (P < 0.01) for all measured soil chemical properties, apart from Na, available P, Fe and Cu. With respect to disturbance by fire, this null hypothesis was rejected (P < 0.01) only for soil pH, Ca, available P and Mn. The second null hypothesis was rejected (P < 0.05) for soil pH, K, Na, available P, Fe and Zn. The impact of acacia infestation on soil chemical status was considerably greater than that of fire or season. The approximate two-fold increase in soil elemental concentrations evident in acacia-infested fynbos may detrimentally affect the survival of indigenous species adapted to a nutrient-impoverished environment. Grond-pH, weerstand, katioon-uitruilvermoë en die konsentrasies van ses makro- en vyf mikro-elemente is gemeet oor ’n tydperk van 12 maande in habitatte wat versteurd (deur brand en infestasie van Acacia saligna) en wat onversteur is, in ’n sandvlakte laagland-fynbosgemeenskap. Twee nul-hipoteses is getoets, naamlik dat daar geen verskille in grond chemiese eienskappe is tussen (i) versteurde en onversteurde habitatte en (ii) tussen verskillende seisoene. Die Friedman-toets is gebruik om verskille in grond chemiese eienskappe te identifiseer tussen verskillende (i) habitatte teenoor seisoene en (ii) seisoene teenoor habitatte. Met betrekking tot infestasie deur acacia, is die eerste nul-hipotese verwerp (P < 0.01) ten opsigte van alle grond-chemiese eienskappe behalwe vir Na, beskikbare P, Fe en Cu. Met betrekking tot versteuring deur vuur, was hierdie nul-hipotese verwerp (P < 0.01) net vir grond-pH, Ca, beskikbare P en Mn. Die tweede nul-hipotese is verwerp (P < 0.05) ten opsigte van grond-pH, K, Na, beskikbare P, Fe en Zn. Infestasie deur acacia het dus ’n groter effek op grond-chemiese status uitgeoefen as óf vuur óf seisoen. Die ongeveer tweevoudige toename in grondelement-konsentrasies in fynbos wat deur acacia ingedring is mag ’n negatiewe uitwerking hê op die oorlewing van inheemse spesies wat aangepas is teenoor ’n omgewing met ’n gebrek aan voedingstowwe.