Seasonal Fluctuations of Microbial Biomass Phosphorus and Nutrient Composition within a Vernal Pond System

Abstract:

Vernal ponds experience hydrologic changes throughout the year that cause heterogeneity in soil redox conditions which alter nutrient bioavailability in the soil. During flooded conditions, iron minerals tend to be in a reduced soluble form, unable to sorb nutrients like Phosphate (PO43-). As vernal ponds dry out, oxygen diffuses through the soil causing iron oxides to precipitate. Iron oxide minerals can sorb PO43-, limiting its bioavailability. Seasonal changes within temperate vernal pond soils can affect PO43- availability depending on precipitation, temperature and organic matter quality. Generally, as frozen soils thaw in spring, PO43- bioavailability increases due to microbial cell lysis in which nutrients are expelled from cells. Therefore, I predict that there will be higher concentrations of PO43- in microbial biomass during the autumn and spring seasons since PO43- is more bioavailable due to the lack of sorption from iron minerals. To assess changes in PO43- concentrations in microbial biomass as a function of seasonal variations, we obtained soil samples from lowland soils within a vernal pond and upland soils adjacent to them in Northeast Ohio during the four seasonal fluctuations. Upland soils were moist in spring and dried throughout autumn, while lowland soils were submerged in spring, and dried by late summer. We measured microbial biomass P using traditional chloroform fumigation followed by a salt extraction. Preliminary results show that upland soils have a higher concentration of PO43- averaging at 21.08+/-17.91 µP/g dry soil compared to lowland soils which averaged out to 1.51 +/-1.33 µP/g dry soil. Further investigation of microbial biomass P will determine how significant seasonal events like thawing contribute to PO43- bioavailability in vernal pond soils.