Invasive<i> Acacia mangium</i> Leaf Litter Modifies Soil Chemical Properties of A Bornean Tropical Heath Forest: A Soil Incubation Study

This study investigated the effects of <i>Acacia mangium</i> Willd. leaf litter on soil chemical properties of a tropical heath forest in Borneo using a controlled soil incubation experiment. The litter of exotic <i>A. mangium</i> and selected native heath forest species (<i>Buchanania arborescens</i> Blume., <i>Calophyllum inophyllum</i> L., <i>Dillenia suffruticosa</i> Griff. and <i>Ploiarium alternifolium</i> Vahl.) were incubated with heath forest soils collected under natural conditions and nine different treatments of heath forest soils (soils without leaf litter, soils treated with single species leaf litter, and soils treated with native leaf litter with and without <i>A. mangium</i> leaf litter). We quantified mass litter loss (%), and soil concentrations of exchangeable nitrogen ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:mrow> </mml:math> and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:mrow> </mml:math> ) and cations (K <i>⁺</i> , Ca²⁺ and Mg²⁺), available phosphorus (P), total organic carbon (TOC) and organic matter (OM), and total acidity with exchangeable concentrations of Al³⁺ and H⁺ in each treatment after a 9-month incubation period. Mass litter loss (%) varied significantly between species, with <i>A. mangium</i> leaf litter only showing higher mass loss than <i>D. suffruticosa</i> litter, but lower than <i>C. inophyllum</i> litter. The effects of incubation with single-species native leaf litter were variable and species-specific, but incubation with <i>A. mangium</i> litter increased soil pH and exchangeable <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NO</mml:mtext></mml:mrow> <mml:mn>3</mml:mn> <mml:mo>-</mml:mo></mml:msubsup> </mml:mrow> </mml:mrow> </mml:math> and K⁺ concentrations and decreased exchangeable Al³⁺ concentrations. Soils incubated with a combination of <i>C. inophyllum</i> and <i>A. mangium</i> leaf litters, as opposed to those incubated with <i>C. inophyllum</i> alone, exhibited decreased pH, lower total organic carbon (TOC), and reduced exchangeable concentrations of potassium (K⁺) and magnesium (Mg²⁺). Additionally, there was an increase in organic matter (OM) content, total acidity, and exchangeable concentrations of ammonium ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mrow> <mml:msubsup><mml:mrow><mml:mtext>NH</mml:mtext></mml:mrow> <mml:mn>4</mml:mn> <mml:mo>+</mml:mo></mml:msubsup> </mml:mrow> </mml:mrow> </mml:math> ) and hydrogen (H⁺). Our results provide preliminary evidence that <i>C. inophyllum</i> may be a promising native plant species for use in enrichment planting of degraded or disturbed tropical heath forests with co-occurring invasive <i>A. mangium</i>.