Soil microinvertebrates as bioindicators in soils of intercropped loblolly-switchgrass systems managed for biofuels
Efficient management of nutrients within intercropped systems requires integrating the multiple effects of management and species composition on the rhizosphere processes that drive nutrient availability. Soil microinvertebrate assemblages may be particularly responsive indicators for management practices due to their close association with decomposition and soil nutrient cycles. However, identification of appropriate bioindicator taxa has been limited by a lack of information on taxonomy, distribution, and functional role. Indicator Species Analysis (ISA) is a non-parametric, objective method that may potentially overcome these limitations. ISA assesses the indicator potential of different taxa without regard to their ecological role or expected management response. Previous studies have demonstrated success in using ISA for unsupervised classification in intercropped systems. This study evaluated the potential for using soil microinvertebrates as bioindicators of soil quality in intercropped switchgrass and loblolly pine stands. Study plots were established and maintained by Weyerhaeuser Company and Catchlight Energy LLC (a joint venture between Chevron and Weyerhaeuser) on land owned and managed by Weyerhaeuser Company. Samples were collected from replicate plots within switchgrass, pine, and intercropped pine-switchgrass stands from 2010-2012 (6 samples x 3 treatments x 3 blocks x 3 years). Microinvertebrates were extracted using a modified Berlese-funnel system and classified to morphotaxa.
A total of 6,800 specimens were collected during this study, representing 79 taxonomic groups. Microinvertebrate communities in switchgrass and pine stands differed significantly from one another in terms of taxa richness and diversity (Shannon’s H) but not from the intercropped treatments. However, Mantel tests did provide evidence for emerging treatment effects between switchgrass and intercropped stands during the first years following establishment. Differences in community composition as a function of treatment (as measured by MRPP) were statistically significant for both the complete dataset (2010-2012) as well as for all three years considered individually. Three-way ISA (pine, switchgrass, and intercropped) identified 18 morphotaxa with statistically significant Indicator Values (p < 0.10) that were greater than 25 (8 switchgrass, 9 pine, and 1 intercropped). Restricting ordination to significant indicator morphotaxa reduced the dimensionality of the community data matrix by 76% while increasing the percentage of total variability explained by the first two PCA axes. Although care needs to be taken to account for temporal and spatial variability, ISA has the potential to reduce the taxonomic expertise and costs associated with identification of soil microarthropods and may make routine microarthropod monitoring more feasible.