Species of fast bulk-soil nutrient cycling have lower rhizosphere effects: A nutrient spectrum of rhizosphere effects

Tree roots not only acquire readily-usable soil nutrients but also affect
microbial decomposition and manipulate nutrient availability in their
surrounding soils, i.e., rhizosphere effects (REs). Thus, REs challenge
the basic understanding how plants adapt to environment and co-exist with
other species. Yet, how REs vary among species in response to
species-specific bulk soil nutrient cycling is not well-known. Here we
studied how plant-controlled microbial decomposition activities in
rhizosphere soils respond to those in their corresponding bulk soils and
whether these relations depend on species-specific nutrient cycling in the
bulk soils. We targeted 55 woody species of different clades and
mycorrhizal types in three contrasting biomes, namely a temperate forest,
a subtropical forest and a tropical forest. We found that microbial
decomposition activities in rhizosphere soils responded linearly to those
in their corresponding bulk soils at the species level. Thereafter, we
found that REs (parameters in rhizosphere soils minus those in
corresponding bulk soils) of microbial decomposition activities had
negative linear correlations with microbial decomposition activities in
corresponding bulk soils. A multiple factor analysis revealed that soil
organic carbon, total nitrogen and soil water content favored bulk soil
decomposition activities in all three biomes, showing that the magnitude
of REs varied along a fast-slow nutrient cycling spectrum in bulk soils.
The species of fast nutrient cycling in their bulk soils tended to have
smaller or even negative REs. Therefore, woody plants commonly utilize
both positive and negative REs as a nutrient-acquisition strategy. Based
on the trade-offs between REs and other nutrient-acquisition strategies,
we proposed a push and pull conceptual model which can bring plant
nutrient-acquisition cost and plant carbon economics spectrum together in
the future. This model will facilitate not only the carbon and nutrient
cycling but also the mechanisms of species co-existence in forest
ecosystems.