Interactive effects of plant litter type and yak excrement on litter decomposition in a shrub-encroached alpine meadow

In the plant community with a shrub-grass mosaic, the main types of litter
include herbaceous litter and its mixed forms with shrub leaves and stems.
However, the quantitative relationship between the litter composition type
and the litter decomposition, as well as how these processes are
influenced by grazing activities (e.g., excrement deposition), remains
largely unexplored.  Herein, a three-year litter incubation
experiment was conducted in a shrub-encroached alpine meadow. We aimed to
elucidate the individual and interactive effects of litter type (solely
grass litter (L), a mixture of grass litter and shrub leaves (L1), and a
mixture of grass litter, shrub leaves, and shrub stems (L2)) and yak
excrement addition (no addition, addition of dung only, addition of urine
only, addition of both dung and urine) on the litter decomposition rate
and nutrients release. Additionally, the initial chemical quality of
litter, soil microclimate, soil nutrients, and soil microbial attributes
were analyzed to ascertain the underlying driving mechanisms.  We
found that litter type and yak excrement had significant individual and
interactive effects on litter decomposition. Specifically, litter type
primarily affected litter decomposition by increasing initial carbon and
lignin concentration and reducing the nitrogen and phosphorus
concentration of the litter. In contrast, yak excrement positively
impacted litter decomposition through enhancing soil pH and soil microbial
activity. Crucially, low-quality litter (L2) mitigated the positive
effects of yak excrement on litter decomposition. While the addition of
yak excrement did not alter the overall direction of litter decomposition,
it influenced the magnitude of the effects caused by litter mixing.
Notably, temporary nitrogen immobilization was observed during the early
stages of decomposition, and net phosphorus release patterns were evident
throughout the decomposition process. However, the duration of nitrogen
immobilization differed depending on the litter type and yak excrement
treatment.  Our findings provide localized evidences that
elucidates the interactive effects of litter type and yak excrement
addition on litter decomposition and nutrient release processes. These
results contribute to a deeper understanding of how grazing activity
modulate nutrient cycling in shrub-encroached grassland ecosystems.