Patterns of soil community structure differ by scale and ecosystem type along a large-scale precipitation gradient

Background/Question/Methods

Climate models predict increased variability in precipitation regimes, which will likely increase frequency/duration of drought. Reductions in soil moisture affect physical and chemical characteristics of the soil habitat and can influence soil organisms such as mites and nematodes. These organisms perform important roles in decomposition and nutrient cycling: changes to their community structuring have been shown to influence their contributions to these ecosystem processes. We investigated how soil moisture variation across regional scales might influence the community structure and population dynamics of mites and nematodes.

Research consisted of three types of precipitation regimes: 1) a regional scale precipitation gradient of wet to dry: two grassland (wet—Konza Prairie, KS—KNZ; arid—Shortgrass Steppe, CO—SGS) and two desert (hot—Jornada Basin, NM—JRN; cold—McMurdo Dry Valleys, Antarctica—MCM) long-term ecological research (LTER) sites across the western US and Antarctica; 2) landscape-scale soil moisture gradients of three plots along a topographic transect at each site; and 3) existing experiments at each site manipulating natural variation in moisture availability. For 2 and 3, soil samples were collected once in fall of 2009 and three times each in 2010 and 2011, with nematodes and mites extracted, counted and sorted to trophic groups. 

Results/Conclusions

Ordination analyses revealed that while nematode communities within two of the drier sites of the gradient (JRN and SGS) are similar, those of KNZ and MCM (the wettest and driest sites, respectively) are noticeably different. Regression analyses indicated patterns for nematode trophic groups across the entire gradient differ from those within individual sites; for example, across the entire gradient numbers of plant-feeding nematodes increase with available soil moisture, but within individual sites this pattern is only seen at JRN and decreases are observed at SGS and KNZ. In contrast, fungivorous nematodes increased with moisture across the entire gradient, while the only site displaying an increase was KNZ. Preliminary mite analyses suggest that mites also respond differently across different scales and sites.

Soil moisture effects on soil invertebrate community structure appear to be dependent upon scale and ecosystem type. Notably, while populations of mites and nematodes tend to increase as sites increase in annual precipitation, in many cases populations decrease within sites as soil moisture increases. Future work will examine how modeling techniques may be employed to forecast changes to soil communities as moisture regimes are altered across regional and local scales.