Does species richness of subtropical tree leaf litter affect decomposition, nutrient release, transfer and subsequent uptake by plants?

- Litter species identity effects propagated through all steps of nutrient dynamics.
- Litter species diversity effects were species specific and time dependent.
- Litter species diversity effects did not propagate through all process steps.

During leaf litter decomposition, nutrients are released, can be transferred among different litter species, are metabolized by soil organisms and are taken up by plants again. However, it remains unclear to which extent leaf litter species richness affects these processes of nutrient cycling, and whether effects on one of those processes propagate to the subsequent one. We established a common garden decomposition experiment in a Chinese subtropical secondary forest, to trace two essential nutrients during decomposition and their uptake by plants along a litter species diversity gradient. Unlabelled, and 15N and Li (as surrogate for K) labelled leaves of three native tree species were used to create replicated 1-, 2- and 3-species mixtures, each with one species labelled per mixture. Litter mixtures were placed in mesocosms with one growing herbaceous phytometer plant. Over six months, litter and phytometer plants of each mixture were sampled at four points in time and the different process steps of nutrient dynamics were determined. Our results showed species and nutrient specific decomposition dynamics, which propagated through the processes of mass loss, nutrient release and transfer among species, and nutrient uptake dynamics of phytometer plants. However, we found no litter species diversity effects along the different litter decomposition processes. Rather specific diversity effects occurred in few cases at different points in time for mass loss, Li release and transfer dynamics. These effects were not caused by nutrient transfer from labelled to unlabelled litter, suggesting that species identity effects on decomposer dynamics may outweigh effects of nutrient transfer among litter species in mixtures. Further, the observed litter species diversity effects did not affect the 15N uptake of phytometer plants. Hence, the influence of species diversity on nutrient cycling and plant available nutrient stocks is mainly determined by the amount and variety of chemical compounds that different species exhibit and release to the soil.

Example of an experimental unit (mesocosm PVC ring) with a 2-species litter mixture and one phytometer plant used for evaluating litter species diversity effects on nutrient dynamics. One litter species per mixture was labelled with the tracer elements 15N and Li (as a surrogate for K). We examined the effects of litter species identity and richness on litter decomposition rates, nutrient release, nutrient transfer among litter species and the 15N uptake by plants. The yellow letters indicate effects of litter species identity (I), composition (C) and richness (R) and on the respective steps of nutrient cycling. Brackets indicate marginal effects (P < 0.1).293