Insight into litter decomposition driven by nutrient demands of symbiosis system through the hypha bridge of arbuscular mycorrhizal fungi

Influence of Arbuscular Mycorrhizal Fungi on Nitrogen Dynamics During Cinnamomum camphora Litter Decomposition

Arbuscular mycorrhizal fungi (AMF) can preferentially absorb the released ammonium (NH4+) over nitrate (NO3-) during litter decomposition. However, the impact of AMF's absorption of NH4+ on litter nitrogen (N) decomposition is still unclear. In this study, we investigated the effects of AMF uptake for NH4+ on litter N metabolic characteristics by enriching NH4+ via AMF suppression and nitrification inhibition in a subtropical Cinnamomum camphora forest. The results showed that AMF suppression and nitrification inhibition significantly decelerated litter decomposition in the early stage due to the repression of NH4+ in extracellular enzyme activity. In the late stage, when soil NH4+ content was low, in contrast, they promoted litter decomposition by increasing the extracellular enzyme activities. Nitrification inhibition mainly promoted the utilization of plant-derived N by promoting the degradation of the amide I, amide II, and III bands by increasing protease activity, and it promoted ammonification by increasing urease activities, whereas it reduced the utilization of microbial-derived N by decreasing chitinase activity. On the contrary, AMF suppression, which significantly reduced the ammonification rate and increased the nitrification rate, only facilitated the degradation of the amide II band. Moreover, it intensified the microbial-derived N decomposition by increasing chitinase activity. The degradation of the amide I and II bands still relied on the priming effects of AMF on soil saprotrophs. This was likely driven by AMF-mediated phosphorus (P) mineralization. Nutrient acquiring, especially P by phosphatase, were the main factors in predicting litter decomposition and protein degradation. Thus, AMF could relieve the end-product repression of locally enriched NH4+ in extracellular enzyme activity and promote early-stage litter decomposition. However, the promotive effects of AMF on litter protein degradation and NH4+ release rely on P mineralization. Our results demonstrated that AMF could alleviate the N limitation for net primary production via accelerating litter N decomposition and reducing N loss. Moreover, they could restrict the decomposition of recalcitrant components by competing with saprotrophs for nutrients. Both pathways will contribute to C sequestration in forest ecosystems, which advances our understanding of AMF's contribution to nutrient cycling and ecosystem processes in subtropical forests.

Alleviating the adverse effects of Cd-Pb contamination through the application of silicon fertilizer: Enhancing soil microbial diversity and mitigating heavy metal contamination

The use of silicon fertilizer (SF) as a means of remediating cadmium (Cd) and lead (Pb) pollution has proven to be beneficial. However, the mechanism via which SF enhances soil quality and crop productivity under Cd- and Pb-contaminated soil (S) remains unclear. This study investigated the impacts of chemical fertilizer, mineral SF (MSF), and organic SF (OSF) on microbial community structure, activity of nutrient acquisition enzymes, and growth of tobacco in the presence of S condition. SF significantly reduced the contents of Cd and Pb in soil under S condition by 6.92-42.43% and increased plant height and leaf area by 15.27-81.77%. Moreover, the use of SF was observed to increase the efficiency of soil carbon and phosphorus cycling under S condition by 6.88-23.08%. Concurrently, SF was found to play a crucial role in facilitating the establishment of a complex, efficient, and interdependent molecular ecological network among soil microorganisms. In this context, Actinobacteriota, Bacteroidota, Ascomycota, and Basidiomycota were observed to be integral components of this network. SF was found to have a substantial positive impact on the metabolic functions and organismal systems of soil microorganisms. Moreover, the combined utilization of the Mantel test and partial least squares path model provided empirical evidence supporting the assertion that the administration of SF had a positive impact on both soil nutrient acquisition enzyme activity and tobacco growth, which was attributed to the enhancement of soil microbial diversity resulting from the application of SF. Furthermore, compared with MSF, OSF has advantages in reducing soil Pb and Cd content, promoting tobacco agronomic traits, increasing the number of key microbial communities, and maintaining the structural stability of microbial networks. The aforementioned findings, therefore, suggest that the OSF played a pivotal role in alleviating the adverse impacts of S, thereby demonstrating its efficacy in this particular process.

Funneliformis mosseae influences leaf decomposition by altering microbial communities under saline-alkali conditions

Recent studies have indicated that arbuscular mycorrhizal fungi (AMF) can influence decomposition of organic materials. However, the underlying mechanisms remain unclear. Here we investigated whether AMF influence the decomposition of leaf litters and change the associated litter bacterial and fungal communities and whether this effect is altered by the level of soil saline-alkali. A pot experiment was conducted using Trifolium repens as host plant without or with AMF (Funneliformis mosseae) and with two levels of soil saline-alkali (0 and 200 mmol/L). Litterbags with different mesh size were used to measure the effect of AMF on decomposition. Our study found that AMF significantly accelerated leaf litter decomposition under both non-saline-alkali and saline-alkali conditions. The composition of bacterial and fungal communities was also altered by AMF independent of soil saline-alkali conditions. For bacterial community, AMF increased the richness but not the diversity and increased the relative abundance of Firmicutes and Nitrospirota. For fungal community, the richness and diversity were higher in AMF than in non-AMF treatment. AMF significantly resulted in a decrease of the relative abundance of Ascomycota but an increase of the relative abundance of Basidiomycota, Chytridiomycota, Mortierellomycota and Rozellomycota. Structural equation modeling (SEM) showed that AMF increased leaf litter decomposition under saline-alkali conditions primarily by affecting bacterial community composition. Together, we show that AMF increase decomposition and alter the bacterial and fungal communities, and that these effects are not modulated by the level of soil saline-alkali.

Changes in Soil Organic C Fractions and C Pool Stability Are Mediated by C-Degrading Enzymes in Litter Decomposition of Robinia pseudoacacia Plantations

Litter decomposition is the main source of soil organic carbon (SOC) pool, regarding as an important part of terrestrial ecosystem C dynamics. The turnover of SOC is mainly regulated by extracellular enzymes secreted by microorganisms. However, the response mechanism of soil C-degrading enzymes and SOC in litter decomposition remains unclear. To clarify how SOC fraction dynamics respond to C-degrading enzymes in litter decomposition, we used field experiments to collect leaf litter and SOC fractions from the underlying layer in Robinia pseudoacacia plantations on the Loess Plateau. Our results showed that SOC, easily oxidizable organic C, dissolved organic C, and microbial biomass C increased significantly during the decomposition process. Litter decomposition significantly decreased soil hydrolase activity, but slightly increased oxidase activity. Correlation analysis results showed that SOC fractions were significantly positively correlated with the litter mass, lignin, soil moisture, and oxidase activity, but significantly negatively correlated with cellulose content and soil pH. Partial least squares path models revealed that soil C-degrading enzymes can directly or indirectly affect the changes of soil C fractions. The most direct factors affecting the SOC fractions of topsoil during litter decomposition were litter lignin and cellulose degradation, soil pH, and C-degrading enzymes. Furthermore, regression analysis showed that the decrease of SOC stability in litter decomposition was closely related to the decrease of soil hydrolase to oxidase ratio. These results highlighted that litter degradation-induced changes in C-degrading enzyme activity significantly affected SOC fractions. Furthermore, the distribution of soil hydrolases and oxidases affected the stability of SOC during litter decomposition. These findings provided a theoretical framework for a more comprehensive understanding of C turnover and stabilization mechanisms between plant and soil.

Algae, shrimp grazing, and fecal pellets synergistically increase microbial activity and enhance N immobilization during Typha angustifolia leaf litter decomposition

Algae play an important role in ecological processes of aquatic ecosystems. Understanding the interactive effects of algae with invertebrates in litter decomposition is important for predicting the effects of global change on aquatic ecosystems. We manipulated Typha angustifolia litter to control exposure to shrimp fecal pellets and/or grazing, and the green alga Chlorella vulgaris were added to test their interactive effects on T. angustifolia litter decomposition. Our results showed that algae largely shortened microbial conditioning time and improved litter palatability (increasing litter quality), resulting in greater decomposition and higher fecal pellet production. Fecal pellets enhanced grazing effects on decomposition by increasing litter ash content. The effects of algae and especially fecal pellets on decomposition were dependent on or mediated by grazing. Without grazing, algae slightly promoted decomposition and marginally offset the negative effect of fecal pellets on litter decomposition. Shrimp grazing dramatically decreased microbial activity (extracellular enzyme activity and microbial respiration) at microbial conditioning stage while enhanced microbial activity after 84 days especially with both algae and fecal pellets present. Algae significantly upregulated N- and P-acquiring and slightly downregulated C-acquiring enzyme activity. Fecal pellets significantly depressed recalcitrant C-decomposition enzyme activity. Nevertheless, the three factors synergistically and significantly increased C loss and most enzyme activities, microbial respiration, and N immobilization, resulting in the decrease of litter C:N. Our results reveal the synergistic action of different trophic levels (autotrophs, heterotrophs, and primary consumers) in the complicated nutrient pathways of litter decomposition and provide support for predicting the effects of global changes (e.g., N deposition and CO₂ enrichment), which have dramatically effects on alga dynamics and on ecological processes in aquatic ecosystems.

Subsoil Arbuscular Mycorrhizal Fungi for Sustainability and Climate-Smart Agriculture: A Solution Right Under Our Feet?

With growing populations and climate change, assuring food and nutrition security is an increasingly challenging task. Climate-smart and sustainable agriculture, that is, conceiving agriculture to be resistant and resilient to a changing climate while keeping it viable in the long term, is probably the best solution. The role of soil biota and particularly arbuscular mycorrhizal (AM) fungi in this new agriculture is believed to be of paramount importance. However, the large nutrient pools and the microbiota of subsoils are rarely considered in the equation. Here we explore the potential contributions of subsoil AM fungi to a reduced and more efficient fertilization, carbon sequestration, and reduction of greenhouse gas emissions in agriculture. We discuss the use of crop rotations and cover cropping with deep rooting mycorrhizal plants, and low-disturbance management, as means of fostering subsoil AM communities. Finally, we suggest future research goals that would allow us to maximize these benefits.

Effect of Soil Layer and Plant–Soil Interaction on Soil Microbial Diversity and Function after Canopy Gap Disturbance

Gaps by thinning can have different microclimatic environments compared to surrounding areas, depending on the size of the gap. In addition, gaps can play important roles in biological dynamics, nutrient cycling, and seedling regeneration. The impacts of gap size on soil microbial communities and enzyme activities in different soil layers in Chinese pine plantations are not well understood. Here, we created gaps of 45 m2 (small, G1), 100 m2 (medium, G2), and 190 m2 (large, G3) by thinning unhealthy trees in an aged (i.e., 50 years old) monoculture Chinese pine plantation in 2010. Soil samples were collected in 2015. The total, bacterial, Gram-positive (G+), and Gram-negative (G−) phospholipid fatty acid (PLFA) profiles were highest in medium gaps in both the organic and mineral layers. These indicesdecreased sharply as gap size increased to 190 m2, and each of the detected enzyme activities demonstrated the same trend. Under all the gap size managements, abundances of microbial PLFAs and enzyme activities in the organic layers were higher than in the mineral layers. The soil layer was found to have a stronger influence on soil microbial communities than gap size. Redundancy analysis (RDA) based on the three systems with different gap sizes showed that undergrowth coverage, diversity, soil total nitrogen (TN), total organic carbon (TOC), and available phosphorus (AT) significantly affected soil microbial communities. Our findings highlighted that the effect of gap size on soil microenvironment is valuable information for assessing soil fertility. Medium gaps (i.e., 100 m2) have higher microbial PLFAs, enzyme activity, and soil nutrient availability. These medium gaps are considered favorable for soil microbial communities and fertility studied in a Chinese pine plantation managed on the Loess Plateau.