Temporal Shifts in Plant Diversity Effects on Carbon and Nitrogen Dynamics During Litter Decomposition in a Mediterranean Shrubland Exposed to Reduced Precipitation

Interspecific Association and Environmental Interpretation of Dominant Species in Shrub Layer of Pinus massoniana Community on Chinese Islands

Understanding the factors driving species coexistence and competition in the shrub layer of semi-natural forests is crucial for effective forest management and conservation. However, there is limited knowledge about the interspecific associations of the main species in the shrub layer of Pinus massoniana communities in the semi-natural forest of Sandu Gulf, Ningde, Fujian Province, China. Therefore, this study aimed to investigate the influence of the abiotic environment on plant communities within the semi-natural forest of P. massoniana on the islands of Sandu Gulf. By exploring these interspecific associations, we sought to provide a more accurate understanding of the causes and processes of species coexistence and competition. The ultimate goal of this project was to offer a reference basis for optimizing the shrub layer structure in P. massoniana (plantation) forests. We found that (1) Heptapleurum heptaphyllum was the most dominant species in the shrub layer, while Smilax china demonstrated the broadest range of environmental adaptability and correspondingly broader niche than other species. (2) Our analysis revealed a predominance of positive associations among the dominant species in the shrub layer. However, the overall association was not significant, with relatively small positive and negative associations between species pairs. The significant test rate was low, and the NRI exhibited a non-significant aggregation. These findings suggest that the plant community in the shrub layer has not yet reached its most stable stage. (3) We also observed that the distribution of dominant species in the shrub layer was primarily affected by factors such as total potassium, pH, available potassium, and light (canopy density). (4) Soil pH value decreased gradually as sampling points moved inward away from the coastline, and island isolation, temperature, and precipitation indirectly affected the species' importance in the shrub layer. Considering the intense competition among the understory species, it is crucial for conservation efforts to prioritize species pairs with reduced ecological niche overlap or significant positive associations. This approach will effectively reduce competition and contribute to the maintenance of stability in the shrub layer. This study provides a theoretical basis for environmental and vegetation restoration, optimizing the species composition of island plantation forests, rationalizing plant composition, and implementing effective operation and management practices for local P. massoniana forests.

Fauna access outweighs litter mixture effect during leaf litter decomposition

Decomposition rates of litter mixtures reflect the combined effects of litter species diversity, litter quality, decomposers, their interactions with each other and with the environment. The outcomes of those interactions remain ambiguous and past studies have reported conflicting results (e.g., litter mixture richness effects). To date, how litter diversity and soil fauna interactions shape litter mixture decomposition remains poorly understood. Through a sixteen month long common garden litter decomposition experiment, we tested these interaction effects using litterbags of three mesh sizes (micromesh, mesomesh, and macromesh) to disentangle the contributions of different fauna groups categorized by their size at Wuhan botanical garden (subtropical climate). We examined the decomposition of five single commonly available species litters and their full 26 mixtures combination spanning from 2 to 5 species. In total, 2325 litterbags were incubated at the setup of the experiment and partly harvested after 1, 3, 6, 9, and 16 months after exposure to evaluate the mass loss and the combined effects of soil fauna and litter diversity. We predicted that litter mixture effects should increase with increased litter quality dissimilarity, and soil fauna should enhance litter (both single species litter and litter mixtures) decomposition rate. Litter mass loss ranged from 26.9 % to 87.3 %. Soil fauna access to litterbags accelerated mass loss by 29.8 % on average. The contribution of soil mesofauna did not differ from that of soil meso- and macrofauna. Incubation duration and its interactions with litter quality dissimilarities together with soil fauna determined the litter mixture effect. Furthermore, the litter mixture effect weakened as the decomposition progresses. Faunal contribution was broadly additive to the positive mixture effect irrespective of litter species richness or litter dissimilarity. This implies that combining the dissimilarity of mixture species and contributions of different soil fauna provides a more comprehensive understanding of mixed litter decomposition.

Mixing effects of three Eurasian plants on root decomposition in the existence of living plant community in a meadow steppe

In grasslands, roots of different plant species decay in combination in the presence of living plants, besides, most root decomposition studies are conducted on how roots of plants decomposed alone or in artificial compositions in the absence of living plants. Therefore, we evaluated how roots of different perennial plants induced effects on decomposition process under living plants and their associated mechanisms. By using litter bag technique, we determined the root decomposition process of three perennial plants, Leymus chinensis, Phragmites australis, and Kalimeris integrifolia grown in monocultures, bi- and tri-species mixtures, after 12 months of incubation under living plants and bare soil communities. We found both additive and non-additive effects on decomposition dynamics indicating that root mass losses of compositions cannot be calculated from decaying rates of individual species. The rich-nutrient roots of K. integrifolia in monocultures and in mixtures with other plant species decayed faster. Compared with bare soil, microbial activities were enhanced under living plant communities and hence stimulated decomposition rates. Our results indicated that microbial activities are important but secondary factors to root physico-chemical properties impacting root decomposition rates. In conclusion, the empirical relationships developed here are helpful to better understand the effects of root properties and microbial activities on decay rates.

Litter nitrogen concentration changes mediate effects of drought and plant species richness on litter decomposition

Biodiversity loss, exotic plant invasion and climatic change are three important global changes that can affect litter decomposition. These effects may be interactive and these global changes thus need to be considered simultaneously. Here, we assembled herbaceous plant communities with five species richness levels (1, 2, 4, 8 or 16) and subjected them to a drought treatment (no, moderate or intensive drought) that was factorially combined with an invasion treatment (presence or absence of the non-native Symphyotrichum subulatum). We collected litter of these plant communities and let it decompose for 9 months in the plant communities from which it originated. Drought decreased litter decomposition, while invasion by S. subulatum had little impact. Increasing species richness decreased litter decomposition except under intensive drought. A structural equation model showed that drought and species richness affected litter decomposition indirectly through changes in litter nitrogen concentration rather than by altering quantity and diversity of soil meso-fauna or soil physico-chemical properties. The slowed litter decomposition under high species diversity originated from a sampling effect, specifically from low litter nitrogen concentrations in the two dominant species. We conclude that effects on litter decomposition rates that are mediated by changing concentrations of the limiting nutrient in litter need to be considered when predicting effects of global changes such as plant diversity loss.

Water-Holding Characteristics of Litter in Meadow Steppes with Different Years of Fencing in Inner Mongolia, China

As a main restoration measure to address degraded grasslands, the installation of fences is often accompanied by accumulation of organic litter. This accumulated litter is a layer of physical moisture which intercepts rainfall and may inhibit plant growth and development. One of the important means to judge a reasonable length of time of fencing (the time a fence is present) is through assessing the water-holding mechanism and capacity of the litter. In this study, four meadows in the Chinese Hulunbuir grassland with different years of fencing duration were investigated in order to obtain data on organic community and litter accumulation. A soaking method was used to study water-holding characteristics of the litter and was divided into three parts of stem, leaves and decomposed parts as a means to summarize the water-holding mechanism within the litter. The results showed that: (1) Compared with the light grazing meadows, the diversity and uniformity of communities in meadows of fencing displayed a downward trend, while the accumulation of litter increased. (2) The stems, leaves, and decomposed components of litter in different communities showed a highly positive linear correlation with their maximum water-holding capacity (WHC). This indicates that the stem/leaf mass ratio and decomposition degree of litter are key factors in regulating WHC. (3) Based on this understanding, we established a model based on stem and leaf mass to predict the water-holding potential of litter in real world situations.

Diversity-decomposition relationships in forests worldwide

Plant species diversity affects carbon and nutrient cycling during litter decomposition, yet the generality of the direction of this effect and its magnitude remains uncertain. With a meta-analysis including 65 field studies across the Earth's major forest ecosystems, we show here that decomposition was faster when litter was composed of more than one species. These positive biodiversity effects were mostly driven by temperate forests but were more variable in other forests. Litter mixture effects emerged most strongly in early decomposition stages and were related to divergence in litter quality. Litter diversity also accelerated nitrogen, but not phosphorus release, potentially indicating a decoupling of nitrogen and phosphorus cycling and perhaps a shift in ecosystem nutrient limitation with changing biodiversity. Our findings demonstrate the importance of litter diversity effects for carbon and nutrient dynamics during decomposition, and show how these effects vary with litter traits, decomposer complexity and forest characteristics.

Shifts in plant functional community composition under hydrological stress strongly decelerate litter decomposition

Litter decomposition is a key process of nutrient and carbon cycling in terrestrial ecosystems. The decomposition process will likely be altered under ongoing climate change, both through direct effects on decomposer activity and through indirect effects caused by changes in litter quality. We studied how hydrological change indirectly affects decomposition via plant functional community restructuring caused by changes in plant species' relative abundances (community-weighted mean (CWM) traits and functional diversity). We further assessed how those indirect litter quality effects compare to direct effects. We set up a mesocosm experiment, in which sown grassland communities and natural turf pieces were subjected to different hydrological conditions (dryness and waterlogging) for two growing seasons. Species-level mean traits were obtained from trait databases and combined with species' relative abundances to assess functional community restructuring. We studied decomposition of mixed litter from these communities in a common "litterbed." These indirect effects were compared to effects of different hydrological conditions on soil respiration and on decomposition of standard litter (direct effects). Dryness reduced biomass production in sown communities and natural turf pieces, while waterlogging only reduced biomass in sown communities. Hydrological stress caused profound shifts in species' abundances and consequently in plant functional community composition. Hydrologically stressed communities had higher CMW leaf dry matter content, lower CMW leaf nitrogen content, and lower functional diversity. Lower CWM leaf N content and functional diversity were strongly related to slower decomposition. These indirect effects paralleled direct effects, but were larger and longer-lasting. Species mean traits from trait databases had therefore considerable predictive power for decomposition. Our results show that stressful soil moisture conditions, that are likely to occur more frequently in the future, quickly shift species' abundances. The resulting functional community restructuring will decelerate decomposition under hydrological stress.

Mass loss and nutrient release during the decomposition of sixteen types of plant litter with contrasting quality under three precipitation regimes

Mass loss and nutrient release during litter decomposition drive biogeochemical cycling in terrestrial ecosystems. However, the relationship between the litter decomposition process and the decomposition stage, precipitation, and litter quality has rarely been addressed, precluding our understanding of how litter decomposition regulates nutrient cycling in various ecosystems and their responses to climate change. In this study, we measured mass loss as well as carbon and nutrient releases during the decomposition of 16 types of leaf litter under three precipitation treatments over 12 months in a common garden experiment (i.e., using standardized soil and climatic conditions). Sixteen types of leaves were divided into three functional groups (evergreen, deciduous, and herbaceous). The objectives were to understand the effects of decomposition stages and precipitation regimes on litter decomposition and to examine the relationship between this effect and chemical properties. The mass loss and release of nitrogen and potassium were significantly higher in the 6- to 12-month stage of decomposition (high temperature and humidity) than in the 0- to 6-month stage. Phosphorus was relatively enriched in evergreen leaves after 6 months of decomposition. The rates of mass loss and nutrient release were significantly greater in herbaceous than in deciduous and evergreen leaves. Increasing precipitation from 400 to 800 mm accelerated mass loss and potassium release but decreased phosphorus release in the 0- to 6-month stage of decomposition. These results highlighted the contribution to and complexity of litter chemical properties in litter decomposition.