Soil trophic effects: Do nitrogen cycling rates respond to nematode abundance

Background/Question/Methods

Soil organisms play a key role in decomposition and nutrient cycling processes. Bactivorous nematodes might indirectly affect nitrogen cycling rates by consuming mineralizing and nitrifying bacteria. We collected nematodes during June 2011 from two sites located at the extremes of an elevation gradient at the Hubbard Brook Experimental Forest in New Hampshire’s White Mountains. Previous studies have found that high elevation sites experience shallower soil frost and higher net nitrogen cycling rates than low elevation sites. This study sought to determine (1) whether nematode abundance differs between the high and low elevation sites and (2) if nematodes indirectly influence mineralization and nitrification rates. Five replicate soil samples (0-5 cm) were collected from high (HF) and low (LF) soil freezing sites. Soil moisture, organic matter content, and pH of all samples were measured. To address question 1, nematodes were extracted from the soil using the Baermann Funnel method and counted. Abundances for both sites were recorded. To address question 2, nematodes were extracted from or added to HF or LF soils. These soils were incubated for 20 days in mesocosms at 20°C and mineralization and nitrification rates were determined at the end of the incubation period.

Results/Conclusions

Soil moisture and organic matter did not differ between the sites, while soil pH was higher at HF. Nematode abundance was significantly higher at LF than at HF. For the no nematodes treatment the mineralization rate was significantly greater in LF soil than in HF soil. There were no significant differences in mineralization rates comparing the nematode to the no nematode treatment. The difference in nematode abundance between HF and LF may be due to physical factors such as pH or winter soil frost depth. The similarity in mineralization and nitrification rates between nematode treatments does not support the idea that nematodes indirectly influence nitrogen cycling rates. However, the trend of higher mineralization rates at the LF site suggests that there is a site difference in the microbial community mineralization ability. This trend supports the previous results of higher nitrogen cycling rates of high elevations. Further studies should investigate potential interactions between site differences in nematodes abundance and microbial mineralization.