FUTURE WORK
Including regional variations in soil invertebrate effects on biogeochemistry to conceptual and predicting models. The temperate bias of studies examining the role of invertebrates in decomposition means that much of the raw data for existing biogeochemical models are from non-tropical zones, which may result in an underestimation of soil invertebrate effects on carbon flux and nutrient turnover. Moreover, invertebrate biodiversity has been under tremendous pressure, with knock-on effects on soil health, plant growth and climate regulation. The tropics represent 36% of earth’s landmass and support the majority of aboveground plant biomass (Crowther et al. 2019). The regional differences in invertebrate functions we have reported suggests that the loss of invertebrate biodiversity could be particularly catastrophic in the Tropics.
More empirical studies needed in Africa and Australia. As with many meta-analyses in ecology, only a small number of studies have occurred in Australia and Africa, where large areas of tropical dry forests and rainforests are located (Pan et al. 2013). We have repeatedly stressed the relative importance of the termites in driving tropical decomposition. Termites are diverse in both Australia and Africa. Africa, indeed, is the evolutionary cradle of termites and hosts the highest number of termite species (Buitenwerf et al. 2011; Bignell 2019). The lack of observations in these areas suggests that we may have underestimated tropical invertebrate effects on forest litter decomposition. We urge, extensive data collection in the tropics, especially in Africa, as essential for future assessments of global carbon cycling and nutrient turnover.
Detailed information on litter traits and environmental variables are needed. We suggest that detailed measurements of leaf litter traits need to be included in further studies of litter decomposition. We advocate for the establishment of a global dataset of leaf litter traits. No such comprehensive global dataset for leaf litter traits exists. Contrasts with living leaves (for which global data bases do exist (Kattgeet al. 2020)) will be informative. In addition, information on plant communities, plot-based microclimates and soil properties should accompany future studies since they are important factors driving the ecological functions of soil invertebrate. Collecting such data at a local scales is essential (Bradford et al. 2014).
Protocol used to exclude soil invertebrates. To date, most studies have used physical methods (i.e. litterbags and mesocosms) to exclude soil invertebrates in field experiments, accounting for 85% of observations in our meta-data. Sixteen percent of observations in our meta-data showed negative effects of invertebrates on litter mass loss. These negative faunal effects on decomposition have not been well tested. Possible explanations would be non-target effects of chemicals (e.g. naphthalene) on both invertebrates and microbes (Lan et al.2020). Within the observations of negative faunal effects in our meta-data, 62% of observations were from the use of naphthalene to exclude invertebrates. The inclusion of fungivores such as Collembola and Nematoda may affect microbial decomposition negatively (Johnsonet al. 2005) in cold or dry regions where invertebrate activity is constrained by temperature and water availability (Wall et al.2008). Our analysis shows an insignificant relationship between exclusion method and invertebrate effects on forest litter decomposition. The potential non-target effects of chemicals such as naphthalene on soil organisms and environments (Lan et al. 2020), however, leads us to advocate use of physical exclusion methods (Lanet al. 2019), particularly in highly stochastic tropical rainforests and in regions with high precipitation.