Litter Decomposition in Yellowstone Grasslands: The Roles of Large Herbivores, Litter Quality, and Climate

Contribution of Litter and Root to Soil Nutrients in Different Rocky Desertification Grasslands in a Karst Area

Litter and root decomposition is an important source of soil organic matter and nutrients. To ascertain the contribution of litter and root to natural grassland nutrients in rocky desertification areas, from March 2017 to January 2018, the continuous soil column method, collector method, and litter decomposition method were used to study the soil nutrients, litter and root biomass, decomposition, and nutrient release of potential, moderate, and severe rocky desertification grasslands, as well as their responses to rocky desertification. The results showed that the litter and root decomposition rate showed a trend of being first fast and then slow, and the decomposition rate of litter and root was greater than 50% after 300 days. The annual litter decomposition rates of potential, moderate, and severe rocky desertification grasslands were 69.98%, 62.14%, and 49.79%, respectively, and the annual decomposition rates of root were 73.64%, 67.61%, and 64.09%, respectively. With a deepening degree of rocky desertification, the litter and root decomposition rate decreased. The decomposition coefficients, k, of litter in potential, moderate, and severe rocky desertification grasslands were 1.128, 0.896, and 0.668, respectively, and the decomposition coefficients, k, of root were 1.152, 1.018, and 0.987, respectively. The nutrient release processes of litter and root were different, and the release mode ultimately manifests as "release". In rocky desertification grasslands, the organic carbon (OC), total nitrogen (TN), total phosphorus (TP), and total potassium (TK) released by litter and root decomposition were 18.93-263.03 g·m-2·yr-1, 1.79-5.59 g·m-2·yr-1, 0.18-0.47 g·m-2·yr-1, and 0.66-3.70 g·m-2·yr-1, respectively. The contribution of root to soil nutrients was greater than that of litter. The degree of rocky desertification was negatively correlated with the biomass, decomposition rate, and nutrient return amount of litter and root. The results of this study provide direct field evidence and illustrate the contribution of litter and root decomposition in rocky desertification grasslands to soil nutrients.

Density-dependent plant growth drives grazer stimulation of aboveground net primary production in Yellowstone grasslands

The mechanisms by which grazing animals influence aboveground net primary production (ANPP) in grasslands have long been an area of active research. The prevailing wisdom is that grazing can increase ANPP by increasing the availability of growth-limiting resources such as nitrogen and water, but recent theory suggests that the density-dependent growth of grassland vegetation can lead to grazer-stimulation of ANPP simply by removing shoot biomass and increasing relative growth rate (RGR). We compared the relative roles of resource availability and density-dependent growth in driving positive responses of ANPP to grazing in Yellowstone National Park. We measured the effects of clipping (50% simulated grazing intensity) and natural grazing on soil nitrogen availability, soil moisture, and shoot growth over 2 months in two grassland plant communities (mesic and dry) grazed primarily by bison. Clipping increased RGR by over 100% in both grassland types but had no effect on N availability or soil moisture during the same growth periods. Clipping stimulated ANPP only at mesic grassland, and the magnitude of this effect was strongly related to the initial plant biomass at the time of clipping relative to estimated peak biomass, supporting the density-dependent framework. Bison grazing had qualitatively similar effects on ANPP and RGR to clipping with no accompanying effects on N availability or soil moisture. Our results show how grazing can stimulate ANPP independent of a direct influence on resource availability simply by exploiting the dynamics of density-dependent plant growth.

The decomposition process and nutrient release of invasive plant litter regulated by nutrient enrichment and water level change

Wetlands are vulnerable to plant invasions and the decomposition of invasive plant litter could make impacts on the ecosystem services of wetlands including nutrient cycle and carbon sequestration. However, few studies have explored the effects of nutrient enrichment and water level change on the decomposition of invasive plant litter. In this study, we conducted a control experiment using the litterbag method to compare the decomposition rates and nutrient release in the litter of an invasive plant Alternanthera philoxeroides in three water levels and two nutrient enrichment treatments. This study found that the water level change and nutrient enrichment showed significant effects on the litter decomposition and nutrient dynamic of A. philoxeroides. The increase of water level significantly reduced the decomposition rate and nutrient release of litter in the nutrient control treatment, whereas no clear relationship was observed in the nutrient enrichment treatment, indicating that the effect of water level change on litter decomposition might be affected by nutrient enrichment. At the late stage of decomposition, the increase of phosphorus (P) concentration and the decrease of the ratio of carbon to P suggested that the decomposition of invasive plant litter was limited by P. Our results suggest that controlling P enrichment in water bodies is essential for the management of invasive plant and carbon sequestration of wetlands. In addition, the new index we proposed could provide a basis for quantifying the impact of invasive plant litter decomposition on carbon cycle in wetlands.

Deer slow down litter decomposition by reducing litter quality in a temperate forest

Litter decomposition is a key process that allows the recycling of nutrients within ecosystems. In temperate forests, the role of large herbivores in litter decomposition remains a subject of debate. To address this question, we used two litterbag experiments in a quasiexperimental situation resulting from the introduction of Sitka black-tailed deer Odocoileus hemionus sitkensis on forested islands of Haida Gwaii (Canada). We investigated the two main pathways by which deer could modify litter decomposition: change in litter quality and modification of decomposer communities. We found that deer presence significantly reduced litter mass loss after 1 yr, mainly through a reduction in litter quality. This mass loss reflected a 30 and 28% lower loss of carbon (C) and nitrogen (N), respectively. The presence of deer also reduced the ability of decomposers to break down carbon, but not nitrogen. Indeed, litter placed on an island with deer lost 5% less carbon after 1 yr of decomposition than did litter decomposing on an island without deer. This loss in ability to decompose litter in the presence of deer was outweighed by the differences in mass loss associated with the effect of deer on litter quality. Additional effects of feces deposition by deer on the decomposition process were also significant but minor. These results suggest that the effects dramatic continental-scale increases in deer populations may have on broad-scale patterns of C and N cycling deserve closer attention.

Soil microbial community dynamics mediate the priming effects caused by in situ decomposition of fresh plant residues

Extreme climate events always leave numerous fresh plant materials (FOM) in forests, thus increasing the input of carbon (C) resources to soil system. The input of exogenous C may accelerate or inhibit the decomposition of soil organic carbon (SOC), which is defined as the positive or negative priming effect (PE), respectively. However, the characteristics and microbial mechanisms of PE caused by FOM remain unknown. A 110-day in situ soil incubation experiment was conducted in a subtropical forest, with ¹³C-labeled fresh leaves from four native species (Castanopsis fissa, CF; Pinus massoniana, PM; Machilus chekiangensis, MC; and Castanopsis chinensis, CC) serving as the FOM respectively. We measured the CO₂ effluxes derived from ¹³C-labeled FOM and soil, and the composition and diversity of soil bacterial and fungal communities throughout the incubation to explore the correlations between PE and microbial attributes. As a result, the PE caused by FOM inputs were negative initially but became positive after 61 d. The FOM decomposition rate was positively related to PE intensity, and there was a significant difference between coniferous and broadleaved species in the middle period of the study. More than 77% of the total C lost from FOM was emitted as CO₂, indicating that FOM-C serves as an energy resource for soil microbes. The α-diversity of the bacterial community at genus-level showed significantly positive correlation with PE at 24 d, and the composition of bacterial community at OTU-level had a marked relationship with the PE between 24-110 d. The relationship between fungal community diversity and composition with PE was only observed at 7 and 110 d, respectively. This study firstly investigated the patterns of PE resulted from numerous FOM input, and the results suggested that soil bacterial community, in particular the Actinobacteria phyla, played a more important role in triggering such PEs than fungal community.

Wetland plant litter decomposition occurring during the freeze season under disparate flooded conditions

To investigate the heterogeneity of plant litter decomposition in the freeze and freeze-free seasons and the responses to disparate flooded conditions in seasonally frozen wetlands, in situ simulation experiments of litter decomposition were performed in the Sanjiang Plain, Northeast China. The experiments were conducted using the litter bag method for representative plants, Carex lasiocarpa and Calamagrostis angustifolia, in both non-flooded and flooded areas between November 2011 and November 2013. Heterogeneous effects of the freeze season and its interaction with hydrological regimes on the decomposition of the litter of various species and organs were observed. The litter decomposition occurred during the freeze season and made a significant contribution to the loss throughout the year. The two-year mass-loss of C. lasiocarpa and C. angustifolia and their organs were ordered differently between the freeze season and the freeze-free season. The proportion of litter mass-loss during the freeze season accounting for the whole year in the flooded area were greater than that in the non-flooded area, except for the C. angustifolia root litter. The litter mass-losses of entire C. lasiocarpa and C. angustifolia during the freeze season were greater than those during the freeze-free season in the flooded area, while the pattern was opposite in the non-flooded area. The effect of environmental factors on litter decomposition might override the effects of litter substrate quality. The total N and P of the litter of the entire C. lasiocarpa and entire C. angustifolia increased significantly relative to the initial values after two years and tended to enrich more in the litter under flooded conditions than under non-flooded conditions. The results highlighted the heterogeneous effects of the freeze season and its interaction with hydrological regimes on various species and organs, which would provide management and restoration options for degraded wetlands caused by climate change.