Herbivore Dung Promotes Plant Litter Decomposition Rate in a Semi-arid Grassland Ecosystem

The decomposition of plant litter and animal dung is the key processes of nutrient cycling and has been studied in a wide range of grassland ecosystems. However, most studies focus on the separate processes of either plant litter or animal dung decomposition, while the interactions between these two processes, which occur in grazed grassland ecosystems, are rarely studied. We conducted an experiment in a semi-arid grassland in Central Inner Mongolia, to examine the mass loss rates and chemical composition changes of two plant litter species and three herbivore dung types separately or in combination over a 2-year period. Twenty litterbag treatments were used to represent 2 treatments of plant litter only, 6 treatments of herbivore dung only at two addition levels, and 12 treatments of litter and dung combinations. We found that horse dung had the biggest mass loss rate, followed by cattle dung and sheep dung, either separately or mixed with plant litter. Dung addition promoted the mass loss, nitrogen release and fiber fraction (hemicellulose, cellulose and lignin) loss of plant litter, and the promotion was strongest for cattle dung, weakest for sheep dung, and stronger at high dung addition levels and in the early stages. However, the effect of litter addition on dung decomposition, a negative effect, was significant only when the dung proportion was low in the litter and dung mixture (that is, in the treatment of 10 g litter + 6 g dung). Furthermore, a significant positive non-additive effect of mixing litter and dung on their total mass loss was detected for the mixture at a litter:dung ratio of 10:6, the ratio in the major grazing area of the grassland. Our findings suggest that mixture of herbivore dung with plant litter, as in actual grassland ecosystems, promotes litter decomposition, and the composition of the litter and dung in mixture, though it may inhibit dung decomposition, due to the changes in chemical stoichiometry of decomposing materials and biophysical microenvironment by the mixing. It is necessary to incorporate the non-additive effect of litter and dung mixture in modeling carbon and nutrient cycling of grassland ecosystems.