Fine root dynamics in relation to nutrients in oligotrophic Bornean rain forest soils

The fine roots of plants are key structures enabling soil resource acquisition, yet our understanding of their dynamics and the factors governing them is still underdeveloped, especially in tropical forests. We evaluated whether Bornean tree communities on soils with contrasting resource availability display different soil resource uptake strategies, based on their fine root properties and dynamics, and related responses of fine roots to the availability of multiple nutrients. Using root cores and ingrowth cores, we quantified variation in community-level fine root properties (biomass, length, and area) and their growth rates, biomass turnover rate, and specific root length (SRL) between clay and sandy loam soils, on which tree community composition differs dramatically. We found that standing fine root biomass and biomass, length, and area growth were higher in sandy loam, the soil type that is better-drained, coarser-textured, and less fertile for most nutrients. In clay SRL was significantly greater, and turnover tended to be faster, than in sandy loam. Across both soils, greater supplies of K+, NH4 (+), and PO4 (3-) were associated with greater standing biomass and growth rates of fine roots, suggesting foraging for these nutrients. Our data support the hypothesis that the sandy loam tree community achieves fine root absorptive area through faster growth and greater investment on a mass basis, whereas trees on clay achieve a similar standing absorptive area through slower growth of less-dense fine root tissues. Furthermore, our results suggest colimitation by multiple nutrients, which may enhance tree species coexistence through increased dimensionality of soil-resource niches.