The effects of feral swine (Sus scrofa) bioturbation on belowground processes in Florida subtropical prairie

Background/Question/Methods

In high abundance, invasive ecosystem engineers introduce broad scale disturbances that negatively affect community structure and ecosystem function.  The feral swine (Sus scrofa) is a prolific ecosystem engineer that alters soil properties and plant community attributes, primarily via destructive rooting habits.  Here, we examined soil properties and belowground net primary productivity after a rooting event in a grassland experimental plot that had been fenced to exclude cattle and feral swine for ten years.  We expected increased soil nutrient content and soil respiration in rooted versus unrooted patches due to altered root and microbial activities.  We expected greater rhizome biomass in rooted areas due to the dominance of Lachnanthes caroliniana, a species with large rhizomes, but greater fine root biomass in unrooted areas due to the dominance of bunch grasses.  Soils were analyzed for pH, bulk density, soil moisture, and inorganic nitrogen at the end of the wet season.  In addition, in-situ soil respiration measurements were conducted at the end of the wet season and two times during the dry season.  To quantify belowground productivity, 24 root in-growth cores (8-cm diameter x 60-cm long) were installed in rooted and unrooted areas in early 2014 and removed a year later.  

Results/Conclusions

We found that rooted patches contained greater ammonium, lower pH, and greater soil moisture.  Soil respiration was marginally greater in rooted than in unrooted patches, possibly due to increased soil moisture, soil nutrient content and rhizome biomass.  Soil nutrients and soil respiration measurements presented here represent dry-down conditions and we expect that the effects of rooting may be amplified when soil moisture is higher during the wet season.

After one year of growth, 85% of total belowground productivity occurred in the top 20 cm of the soil. Fine root and rhizome biomass ranged from 37 to 326 g/m²yr and 0 to 274 g/m²yr respectively, and rhizomes were limited to the top 20 cm of soil.  In the top 10 cm of soil, rooted areas contained 1.4-fold more rhizome biomass (66±77 g/m²yr; entirely Lachnanthes caroliniana) than unrooted areas (28±55 g/m²yr); otherwise, there were no significant differences in fine root production between the rooted plots (116±75 g/m²yr) and unrooted plots (119±75 g/m²yr).  This suggests that despite an increase of rhizomes, there was not an associated loss of fine root production. Our results demonstrate that bioturbation by ecosystem engineers alters multiple belowground processes.