Litter quality and decomposers are critical to carbon and nutrient cycling through litter decomposition. However, how relationships between litter quality and invertebrate detritivores change litter mass loss through time is poorly known. Species' initial leaf litter quality, as a legacy of their position on the "leaf economics spectrum" (LES), may determine the invertebrate contribution to litter mass loss. This contribution may change through time, as both population peaks of invertebrate detritivores and litter quality of given species will change through time.
Here we introduce invertebrate phenology into a conceptual model of drivers of litter mass loss. We hypothesized that in the early decomposition period, LES can predict litter decomposability with or without a strong invertebrate contribution, i.e., litter with higher nutrient content would decompose faster. But in the later decomposition period, when higher quality litter will already have decomposed too much and lower quality litters have still been less degraded, a strong invertebrate peak would coincide with relatively more consumption of initially lower quality litters; this would lead to a hump-back relationship between leaf litter mass loss and initial LES position in this period.
We tested our hypothesis through a one-year field decomposition experiment using leaf litter of 41 woody species in each of two sites in subtropical forest in China; only one of these sites had a strong late peak of leaf litter-feeding moth larvae in the litter layer.
LES score of litter species had a positive linear relationship with litter mass loss before the key invertebrate consumer peaks in the litter layer. However, with the invertebrates peaking later into the decomposition process, the invertebrate consumption peaked at initially lower quality litters, which altered the species' decomposability trajectory on the LES, consistent with the hypothesized hump-back relationship between leaf litter mass loss and LES. This phenomenon resulted in a strongly reduced slope of cumulative mass loss on initial LES score across species.
Our finding highlights the importance of considering interactions between the timing of detritivore activities and the timing of litter quality for better understanding the relationships between soil animals and ecosystem carbon and nutrient cycling.