Short Communication

Linkages between plant litter diversity, soil microbial biomass and ecosystem function in temperate grasslands

While most studies evaluating plant species diversity effects on in-stream fungal decomposers have focused on dominant species, our study simulated different rare species extinction scenarios. We assessed whether the loss of these species altered the fungal biomass and aquatic hyphomycete sporulation, diversity and taxonomic composition in two experiments: even experiment, where we used the same biomass for all species (i.e., even litter mixtures); and natural proportion experiment, where we kept proportions of plant species as found in stream leaf litter (i.e., natural litter mixtures). We found that the loss of litter from rare plant species reduced fungal biomass and the reduction was greater for natural litter mixtures, indicating that evenness modulated the effect of diversity on fungal biomass, possibly through resource dissimilarity. Our findings reveal the relevance of litter from rare plant species and of the maintenance of natural species proportions in tropical riparian forests for the functioning of stream ecosystems.

Soil organic carbon (SOC) storage plays a crucial role in mitigating global climate change while maintaining forests and sustaining environmental conditions. Recent studies have shown that plant species diversity increases SOC storage in experimental grasslands and natural forests. Here, we aim to show how an integrating tree-size and trait-based ecological approach leads to mechanistic insights into SOC storage that may be overlooked in previous studies. We hypothesized that tree functional diversity effects on SOC storage would change with tree-sized-based species' functional strategies driven by soil properties in natural temperate deciduous forests. We used the forest inventory data, site-specific measured traits and soil properties from 99 temperate deciduous forest plots in northwestern Iran. We investigated the joint effects of topographic factors, soil chemical properties, and the functional attributes (multi-trait functional diversity indices and community-weighted trait mean indices) of big-sized trees (BsT; individual tree diameter ≥ 50 cm) and remaining-sized trees (RsT) on topsoil (0–30 cm) SOC storage. The multiple linear regression models showed that SOC storage increased with increasing soil available phosphorus (r = 0.36), soil exchangeable potassium (r = 0.27 to 0.31) and soil total nitrogen (r = 0.27). In contrast, SOC storage declined with increasing the community-weighted mean of specific leaf area (r = −0.38 to −0.40). The structural equation model showed that SOC storage was directly driven by the positive effect of soil nutrients (i.e. a latent variable of soil total nitrogen and available phosphorus; r = 0.42) and the negative effect of the community-weighted mean of specific leaf area (r = −0.31). The topographic factors as well as the other several functional attributes of BsT and RsT showed non-significant direct and indirect effects on SOC storage in both types of statistical modelling. Our study suggests that the mass ratio and soil fertility rather than the niche complementarity effects along with the big-sized trees effect regulates SOC storage through the acquisitive trait identity of BsT species as compared to RsT species in temperate deciduous forests. We argue that maintaining high levels of species diversity with the most efficient trait identity across tree sizes is important for biodiversity conservation and multiple forest functions such as SOC storage which should be a priority for plantation management plans or silvicultural practices in natural forests.

Forage grasses used in cropping no-till systems in tropical regions alter soil chemical properties, but their long-term impact on soil microbial processes of the nitrogen (N) cycle and microbial community abundance, composition and structure are unknown. Here, microbial functions related to nitrogen fixation, nitrification and denitrification as well as bacterial, archaeal and fungal populations were evaluated in a long-term field experiment in which tropical forage grasses palisade grass (Urochloa brizantha (Hochst. Ex A. Rich.) R.D. Webster) and ruzigrass (U. ruziziensis (R. Germ. and C.M. Evrard) Crins) were cultivated with or without N fertilization. Uncultivated soil was used as a control. Forage grasses, especially palisade grass, increased soil bacterial and fungal abundances, whereas the archaeal population was highest in uncultivated soil. In soils cultivated with forage grasses, N fertilization favored N-cycle-related genes; however, cultivation of palisade grass increased the abundances of amoA bacteria (AOB) and amoA archaea (AOA) genes associated with soil nitrification and decreased the abundances of genes nirS, nirK and nosZ genes related to denitrification, compared to ruzigrass and control, regardless of N input. In addition, abundances of total bacteria and total fungi were associated with the N cycle and plant biomass in soils cultivated with forage grasses. Forage cultivation clearly benefitted the soil nutrient environment (S-SO₄^2-, Mg²⁺, total-C and -N, N-NO₃^- and N-NH₄⁺) and microbiome (bacteria and fungi) compared with uncultivated soil. In soil cultivated with palisade grass, the microbial community composition was unresponsive to N addition. The high N uptake by palisade grass supports the competitive advantage of this plant species over microorganisms for N sources. Our results suggest that palisade grass has advantages over ruzigrass for use in agriculture systems, regardless of N input.

The response of soil microbial decomposition of soil organic carbon (C) to temperature variation against an average warming background is of great importance to understand how climate change affects the ecosystem C cycling. In this study, a warming and step-wised stop-warming experiment was conducted to examine whether the response of soil respiration (Rs) and heterotrophic respiration (Rh) persists post-warming and to understand the underlying mechanisms. The treatment plots (10 plots) were warmed (~1.5 °C at 10 cm soil depth) in 2017, then warming was stopped in one group (5 plots) in 2018 (WS18) and stopped in another group (the remaining 5 plots) in 2019 (WS19). Plant biomass, soil microbial biomass, and soil microbial community composition were measured from 2017 to 2019.On average, warming increased Rh by 28% in 2017. The Rh in WS18 was stilled increased by 26% in 2018, which was lower than the warming induced increase in Rh in WS19 at the same period. The Rh in WS18 showed no difference with the control and that in WS19 was higher than the control in only June in 2019. Aboveground biomass of WS18 and WS19 increased by 20% and 29%, respectively in 2017, and they were still increased by 12% and 17% in 2019 even the warming stopped one two years and one years, respectively. Belowground biomass, microbial biomass, and diversity indices showed no significant differences among treatments in 2018 or 2019. The fungal community was significantly different among WS18, W19, and the control in both 2018 and 2019. The relative abundance of Ascomycetes, which made the largest contribution to the differences in the fungal community, was negatively correlated with Rh (r = −0.4, n = 30, p < 0.05). Our results indicate a warming legacy effect on the microbial decomposition of soil organic C, resulting from the increase in plant productivity and fungal community change when warming stopped.

Soil microbial communities and plants are intimately associated and each can regulate the growth and specific composition of the other through relationships such as competition and symbiosis. Such links between the above and belowground components of soil ecosystems are important as they determine the functioning of key ecosystem processes, including decomposition and nutrient cycling. In the present study, we used structural equation models to investigate the direct and indirect effects of plant community properties (richness, evenness and net primary productivity) and of soil nutrient pools (C, N and P) on the biomass of rhizosphere and non-rhizosphere bacteria and fungi. The biomass of rhizosphere and non-rhizosphere bacteria and fungi was mainly determined by the organic matter content. More importantly, nutrient pools were not modulated by plant communities and we did not find any evidence of a link between aboveground and belowground components of soil systems in this respect. The findings indicate that aboveground and belowground components of the soil system are not directly linked and that any potential relationships will be mediated by the effects of aboveground components in nutrient pools.

Litter decomposition is a major component of the nutrient cycle in forest ecosystems. It is generally accepted that higher initial quality and litter mixture contribute to increased mass loss, and the effects of soil fauna on litter decomposition is context-dependent. Meanwhile, the formation of gaps alters the hydrological and thermal conditions of the forest through the redistribution of light and precipitation, ultimately influencing the distribution and activity of the soil biota. Previous studies have focused on the effects of either litter species or forest gaps on soil microbes. However, the combined effects of litter species and gap size on the composition and structure of the soil fauna during litter decomposition have not been thoroughly examined. By analyzing litter collected over the course of one year from a Pinus massoniana plantation, shifts in the soil fauna, including indicator groups and guilds based on feeding habits, were correlated with changes of litter substrate, gap size, and incubation time. It was also observed that mesofauna (including microarthropods, nematodes, etc.) were more susceptible to these examined factors than macrofauna. Furthermore, litter type has higher influence on the abundance and richness of soil fauna than litter diversity. In general, small to intermediate sized gaps improved soil fauna diversity, richness, and functional diversity of indicator taxa. The relative abundance of guilds varied with gap size, litter species, and incubation time. The relative abundance of carnivores increased with gap size, whereas fungivores and detritivores increased with incubation time. The observed relative abundance of herbivores was higher in P. massoniana leaf litter than in Cinnamomum camphora and mixed-species leaf litters. Overall, the data presented here indicated that the creation of small to intermediate sized gaps will increase the diversity of soil fauna and accelerate litter mass loss in P. massoniana plantations. The subsequent effects on the return of bioavailable nutrients and soil carbon storage require additional long-term studies.