Nutrient enrichment
Nutrient enrichment did not have an overall significant effect on soil
fauna communities. However, the effects on soil fauna did vary depending
on the nutrient-based environmental stressors. Given that all nutrient
stressors were considered to be press stressors, this once again
highlights that the use of just a single trait does not capture the
complexity of the stressors, especially when it is unclear how long each
nutrient type remained in the soil before degrading. Organic-based
amendments and fertilisers increased soil biodiversity, and several
mechanisms may explain this pattern. It may be due to the increased
carbon in the soil . found that inorganic nitrogen fertilisers
simplified nematode communities but organic fertilisers (for example,
crop residue) increased carbon in the soil and had positive impacts on
the nematode community. However, it may also be because the
organic-based fertilizers provide the nutrient resources at a slower
rate , as well as create micro-habitats for a variety of soil organisms
. The fact that we did not find a significant decline in biodiversity
with synthetic fertilizers was somewhat surprising, although this result
has been found in another meta-analyses .
, in their meta-analysis, found that nutrient addition had positive
effects on soil biodiversity at low doses, and effects became negative
at higher concentrations. Thus, if concentration amount was not taken
into consideration, the overall mean effect size was neutral.
Additionally, found that synthetic fertilisers had a significant
negative effect on soil fauna but were no longer significant when the
duration of the application was increased (>5 years).
Similar results were found in , where long-term application of synthetic
fertilizers did not negatively impact earthworms. Both hypothesised that
any effect on soil biodiversity was indirect, as the increased nitrogen
in the soil benefited the crops, and thus in the longer term the
increased plant biomass in the soil resulted in increased carbon,
ultimately benefiting the soil community . Without looking into the
concentration and application rates, or the temporal aspects of the
treatment, all of which are hard to standardise across such a wide
variety of studies, we would not be able to test these mechanisms, but
these may explain the lack of a significant decline with synthetic
fertilizers.