Introduction

Biodiversity controls soil functions. The expansion of the agricultural frontier and natural threats like prolonged drought periods impose risks to biodiversity and soil functions possibly causing the crossing of tipping points (TP). Therefore, a major challenge in modern research is to find suitable proxies for determining TP. In tropical soils, Phosphorus (P) acquisition of microorganisms is particularly prone to be affected by disturbances. P is hardly bioavailable as it is sequestered in mineral and organo-mineral complexes. In pristine and, therefore, functionally diverse forests, specialized microorganisms are able to mobilize sequestered P and relocate it to the living biomass, thus, keep it in the nutrient cycling. By loosing overall biodiversity under stress, it is highly likely that these organisms and thus the P cycle are impaired. Its effects could also cascade into Carbon and Nitrogen cycles.

Objectives

We propose that one of the main losses in functional diversity affecting tropical ecosystems is the loss of mobilization of P from hardly accessible organic sources. To test this, we selected the bacterial phoD functional gene as a proxy for potentially altered P acquisition functionality as phoD is directly affected by the availability of P.

Methods

We took soil samples in Madre de Dios, Peru, along gradients of above-ground biodiversity (from pristine and degraded forests to monoculture and pasture vegetation) with and without artificial rain exclusion and analysed the abundance and diversity of the phoD-bearing microbial communities at two sampling time points.

Results

Contrary to our expectations, we show that a reduced above-ground biodiversity and drought significantly increased the abundance and diversity as well as altered the overall community composition of the phoD-gene bearing communities. The results mirror altered P acquisition strategies and the communities’ necessity to adapt to stressful drought and disturbed conditions compared to pristine forests. We assume that the presence of the phoD gene poses a clear advantage of survival for microbes in tropical soils as it helps to overcome P-limiting conditions after disturbances.

Implications

Microbial marker genes can be used as an indicator to evaluate alterations in the functional biodiversity of soils after anthropogenic and natural stress. While we cannot conclude about the crossing of TP due to right now only two sampling time points, the results indicate a surprisingly flexible and vivid functional redundancy within the microbial P cycle especially on the disturbed sites and an uncertain fate of these ecosystems.