Effects of leafcutter ants on dissolved organic matter chemistry and soil CO2 concentration

Abstract

The leafcutter ant Atta cephalotes is a dominant herbivore, and their nest building activities may have an effect on dissolved organic and inorganic matter fluxes and soil CO2 concentrations. At a wet tropical forest in Costa Rica, I investigated the spatial and temporal heterogeneity of total dissolved carbon (TDC), dissolved inorganic nutrient concentrations, and soil CO2 concentrations at nine nest and six control plots at primary and secondary forests, alluvial and residual soils, and at 20 cm, 60 cm, and 100 cm depths. The concentrations of TDC, total dissolved nitrogen (TDN), dissolved C:N ratio, nitrate + nitrite (NOx), ammonium (NH4 +), and soluble reactive phosphorus (SRP) were measured monthly from April – July 2015. Water soluble carbon (WSC) and hot-water carbon (HWC) analyses were carried out on nest and control plots soils as a sensitive measure of ant mediated changes to the soil organic matter pool. Total dissolved carbon (TDC) concentrations ranged from 0.86 to 5.77 mg C L-1 at control plots, and 1.25 to 7.39 mg C L-1 at nest plots. Total dissolved nitrogen (TDN) concentrations ranged from 0.04 to 1.42 mg L-1 at control plots, and 0.04 to 2.71 mg L-1 at nest plots. Ammonium (NH4 +) concentrations ranged from 2.5 to 64.53 μg L-1 at control plots, and 2.5 to 55.52 μg L-1 at nest plots. Nitrite and Nitrate (NOx) concentrations ranged from 2.5 to 1368 μg L-1 at control plots, and 2.5 to 1912 μg L-1 at nest plots. Soluble reactive phosphorus (SRP) concentration ranged from 2.5 to 18.47 μg L-1 at control plots, and from 2.5 to 34.32 μg L-1 at nest plots. Linear mixed effect models were created and showed that TDC, TDN, dissolved C:N ratio, NOx, NH4 + and soil CO2 concentrations were significantly different with depth and time (P < 0.05). NH4 + was also significantly different across alluvial and residual soils (P < 0.05). Water soluble carbon was significantly different across alluvial and residual soils, and with depth (P < 0.05). Hot-water carbon was significantly different across primary and secondary forests, and with depth (P < 0.05). Soil CO2, total dissolved carbon and nutrient concentrations did not significantly differ between control and nest plots (P > 0.05). These results showed that concentrations of soil CO2, dissolved carbon and inorganic nutrients are spatially and temporally variable. Importantly, during one of the wettest periods on record the results showed that plot-to-plot spatial variation, and temporal variability associated with seasonal rainfall patterns are stronger drivers of soil CO2, TDC, and inorganic nutrient concentrations than the nests of A. cephalotes.