The invasive tree Piper aduncum alters soil microbiota and nutrient content in fallow land following small scale slash-and-burn farming in tropical lowland forest in Papua New Guinea

Piper aduncum, a tree species native to the Neotropics, has been introduced to other tropical regions and successfully invades secondary forest in fallow land after small scale slash-and-burn agriculture in Papua New Guinea (PNG). However, the effect of P. aduncum invasion on soil chemical properties as well as soil biota remains poorly understood. To fill this knowledge gap, we compared soil chemistry, soil microbiota and soil fauna between sites invaded by P. aduncum and sites with secondary native vegetation, where P. aduncum absent. Soils under P. aduncum had significantly lower P content at 0–5 cm depth, lower NO₃⁻ at 5–10 cm depth and lower N and C content at both depths compared to soil under secondary vegetation where P. aduncum absent. Sites invaded by P. aduncum also harbored lower microbial biomass measured by phospholipid fatty acid (PLFA) analysis, especially at 5–10 cm depth. The composition of microbial communities, based on PLFA and amplicon sequencing methods, also differed between soils invaded and uninvaded by P. aduncum, while soil macrofauna did not show any significant difference in the density of various faunal groups between invaded and uninvaded sites. Furthermore, we studied soil chemistry and foliar nitrogen in food gardens established after clearing of secondary vegetation in sites where P. aduncum had been experimentally excluded for five years. These gardens did not differ in soil chemistry compared to the control gardens where P. aduncum was not cleared. However, P. aduncum removal was associated with a higher N content in the leaves of sweet potatoes (Ipomoea batatas), but not bananas (Musa spp.). Our results suggest that P. aduncum has negative effect on soil microbiota and may slowdown nutrient turnover and availability as well as growth of plants on invaded soil. This finding requires further attention and may have practical implication for food gardening in tropical rainforests.