Linking nematode and microathropod bioindicators to ecosystem function in forest soils

Background/Question/Methods

Interpretation of biological indicators of soil quality depends on their ability to reflect ecological processes such as nutrient mineralization and decomposition.  Soil in pairs of relatively undisturbed and disturbed pine forests in each of three land resource regions of North Carolina (i.e., coastal plain, piedmont, and mountains) was monitored 18 times over two years. Abundance of nematodes in 35 families, collembolans in 8 families, and 16 microarthropod taxa were enumerated.   Microarthropods were identified separately in litter and soil substrates.  Rates (mg lost per day) of cellulose and lignin decomposition were estimated based on weight loss of standardized museum board (100% cellulose) and balsa wood (19.9% insoluble lignin, 0.4% soluble lignin,  77.1% cellulose) substrates enclosed in litter baskets. Nitrate and ammonium concentrations in soil were quantified using colorimetric reactions.  Principal components analysis was performed on 59 invertebrate taxa to indentify the taxa explaining the greatest proportion of variation. Soil microclimate and chemical properties (pH, electrical conductivity, and organic matter) were included as covariables.  Canonical correlation was performed to determine how well soil invertebrates predicted ecological processes. 

Results/Conclusions

Increases in nitrogen resulted in greater rates of museum board decomposition and reduced rates of balsa wood decomposition. The linkage between nitrogen and substrate-specific decomposition rates supports conclusions from previous studies that use extra-cellular soil enzymes to characterize the functional role(s) of microbes. Taxa explaining the greatest proportion of variance in the invertebrate community included six nematode families (Cephalobidae, Dorylaimidae, Longidoridae, Pratylenchidae, Tylenchidae, and Tylencholaimellidae), two collembolan families (Entomobryidae and Hypogastruridae), and three other taxa (Acari , Hymenoptera, and Chilopoda). Microarthropods consistently explained at least 5% percent of decomposition. Collembolans and many of the mites are responsible for litter comminution while most Hymenoptera identified were Formicidae which have been observed to impact decomposition and often categorized as ecosystem engineers. The association of nematode abundance and availability of nitrate and ammonium was inconsistent. Protozoa may have overshadowed any association of nematodes and mineral nitrogen. Although not quantified in this study, protozoa are abundant in moist forest soils. These results support decomposition alone as an indicator of nitrogen, and the interaction of decomposition and soil microarthropods as useful bioindicators for monitoring ecosystem services in forest soil. However, nematodes alone were not able to reflect N mineralization. Therefore, protozoa may be a more useful indicator of that process.