Plant roots stimulate the decomposition of complex, but not simple, soil carbon

Unveiling the significance of rhizosphere: Implications for plant growth, stress response, and sustainable agriculture

In the rhizosphere, the activities within all processes and functions are primarily influenced by plant roots, microorganisms present in the rhizosphere, and the interactions between roots and microorganisms. The rhizosphere, a dynamic zone surrounding the roots, provides an ideal environment for a diverse microbial community, which significantly shapes plant growth and development. Microbial activity in the rhizosphere can promote plant growth by increasing nutrient availability, influencing plant hormonal signaling, and repelling or outcompeting pathogenic microbial strains. Understanding the associations between plant roots and soil microorganisms has the potential to revolutionize crop yields, improve productivity, minimize reliance on chemical fertilizers, and promote sustainable plant growth technologies. The rhizosphere microbiome could play a vital role in the next green revolution and contribute to sustainable and eco-friendly agriculture. However, there are still knowledge gaps concerning plant root-environment interactions, particularly regarding roots and microorganisms. Advances in metabolomics have helped to understand the chemical communication between plants and soil biota, yet challenges persist. This article provides an overview of the latest advancements in comprehending the communication and interplay between plant roots and microbes, which have been shown to impact crucial factors such as plant growth, gene expression, nutrient absorption, pest and disease resistance, and the alleviation of abiotic stress. By improving these aspects, sustainable agriculture practices can be implemented to increase the overall productivity of plant ecosystems.

Vegetation restoration of abandoned cropland improves soil ecosystem multifunctionality through alleviating nitrogen-limitation in the China Danxia

The microbial requirement for nutrient resources can be estimated by soil extracellular enzyme stoichiometry (EES) and their stoichiometries. Implementing the Grain for Green Program has significantly impacted land use and soil nutrient management in the China Danxia. However, drivers of soil microbial nutrient limitation changes in abandoned cropland (AC) remained unclear after vegetation restoration. Here, according to vector analysis, we evaluated microbial nutrient limitation by studying soil EES across vegetation restoration types (naturally restored secondary forests (NF) and artificially planted forests (AF)) with AC as a control. Results showed both NF and AF soils averaged higher C- and P- acquiring enzyme, indicating rapid C and P turnover rates after vegetation restoration. However, vegetation restoration resulted in higher C requirement for microorganisms with higher enzyme C:N and vector length. In addition, microorganisms shifted from N- (< 45°) to P-limited (> 45°) conditions with enzyme N:P less than 1 after vegetation restoration, and NF exacerbated microbial P limitation compared to AF. Decreased N limitation following vegetation restoration could be contributed to improving soil ecosystem multifunctionality. The greater variation of EES was explained by the interaction of pH, soil nutrient, and microbial biomass than by any one of these factors alone, suggesting that both abiotic and biotic factors regulate microbial nutrient limitation and microbial process. Overall, our results revealed vegetation restoration could alleviate N limitation in the China Danxia, and thus enhance soil ecosystem by regulating lower microbial N limitation, which provide insight into nutrient management strategies under ecological restoration of degraded areas.

Utilization of Indole Acetic Acid with Leucadendron rubrum and Rhododendron pulchrum for the Phytoremediation of Heavy Metals in the Artificial Soil Made of Municipal Sewage Sludge

The development of phytoremediation by garden plants is an effective way to deal with the dilemma of municipal sewage sludge disposal. In this study, two ornamental plants were used as phytoremediation plants to rehabilitate heavy-metal-contaminated municipal sewage sludge in field experiments, and the role of exogenous phytohormone IAA was also tested. Ornamental plants Loropetalum chinense var. rubrum (L. rubrum) and Rhododendron pulchrum (R. pulchrum) adapted well to the artificial soil made of municipal sewage sludge, and the concentrations of Cu, Zn, Pb, and Ni were decreased by 7.29, 261, 20.2, and 11.9 mg kg−1, respectively, in the soil planted with L. rubrum, and 7.60, 308, 50.1, and 17.7 mg kg−1, respectively, in the soil planted with R. pulchrum, accounted for 11−37% of the total amounts and reached significant levels (p < 0.05), except Cd. The concentration of Pb in all parts of the two ornamental plants was increased, as well as most heavy metals in L. rubrum root. As a result, three months after transplant, the phyto-extraction amounts in L. rubrum were 397, 10.9, and 1330 μg for Ni, Cd, and Pb, respectively, increased by 233% to 279%. The phyto-extraction amount in R. pulchrum were 1510, 250, and 237 μg for Zn, Pb, and Cu, respectively, increased by 143% to 193%. These results indicated a potential to remediate heavy metals of the two ornamental plants, especially L. rubrum. The results of correlation analysis implied that the interaction of heavy metals in the plant itself played an important role in the uptake of heavy metals. This seemed to explain why applying IAA in the experiment had little effect on plant growth and phytoremediation of heavy metals. This study provided a green and feasible idea for the proper disposal of municipal sewage sludge.

Arbuscular Mycorrhizal Fungi and Dry Raw Garlic Stalk Amendment Alleviate Continuous Monocropping Growth and Photosynthetic Declines in Eggplant by Bolstering Its Antioxidant System and Accumulation of Osmolytes and Secondary Metabolites

Vegetable production under plastic sheds severely threatens regional eco-sustainability via anthropogenic activities (excessive use of agrochemicals, pesticides) and problems associated with replanting. Long-term successive cropping across growing seasons induces continuous cropping stress, whose effects manifest as diminished plant growth. Therefore, it is imperative that we develop environmentally sustainable approaches, such as replacing agrochemicals with vegetable waste like dry raw garlic stalk (DRGS) or use biofertilizers like arbuscular mycorrhizal fungi (AMF) (e.g., Diversispora epigaea). In this study, the influence of AMF on the growth, biochemical attributes, antioxidant defense system, phytohormones, accumulation of osmolytes, phenols, and mineral elements in eggplant grown on DRGS-amended soils under continuous monocropping (CMC) was studied. The results showed that inoculation with AMF or the DRGS amendment could improve the pigments' content, photosynthesis, and antioxidant defense system; augmented phytohormones synthesis (except for ABA), and increased the leaves' mineral nutrients. These parameters were enhanced most by the combined application of AMF and DRGS, which also increased the concentration of osmolytes, including proline, sugars, and free amino acids in eggplant when compared with the control. Furthermore, either AMF and DRGS alone, or in combination, ameliorated the induced stress from continuous cropping by reducing the incidence of Fusarium wilt and production of ROS (reactive oxygen species); lipid peroxidation underwent maximal reduction in plants grown under the combined treatments. The AMF, DRGS, and AMF + DRGS exhibited a lower disease severity index (33.46, 36.42, and 43.01%), respectively, over control. Hence, inoculation with AMF coupled with DRGS amendment alters the photosynthetic attributes in eggplant through the upregulation of its antioxidant system and greater accumulation of osmolytes, which led to the improved growth and yield of eggplant.

Shelterbelt farmland-afforestation induced SOC accrual with higher temperature stability: Cross-sites 1 m soil profiles analysis in NE China

Shelterbelt farmland afforestation has been well-reported in its wind-break and climate regulation function, but less is on underground-soil organic carbon (SOC) sequestration and environmental stability. In this paper, we collected 180 soil samples from soil depths of 1 m (0-20, 20-40, 40-60, 60-80, 80-100 cm) in the farmland and neighbor shelterbelts in Songnen Plain, northeastern China. The sample plots covered six regions in the study area. SOC concentration and respiration decomposition rate, Q₁₀ (temperature sensitivity), H_s (humidity sensitivity) were determined in the laboratory cultivation. Soil properties (N, P, K, electrical conductivity-EC, pH) and geographic-climate factors (multiple-year mean annual temperature and precipitation, MAT&MAP; temperature and precipitation during sampling month, MT &MP) were used to reveal the underlying reason for the changes in soil carbon sequestration. The results showed no significant difference in SOC respirational decomposition rate between farmland and shelterbelt forests but a 15.8% higher SOC concentration in shelterbelt forests (p < 0.05). The poplar shelterbelts reduced the Q₁₀ value by 15.4% (p < 0.05), with deeper soils a more significant reduction in Q₁₀. With soil moisture increases, both shelterbelt forests and farmland showed an obvious respiration pattern of first-increasing-then-decreasing. No significant H_s (linear gradients) differences were found in farmland and shelterbelt forests. Partitioning of the RDA ordination-based variation showed that SOC stability (H_s and Q₁₀) of farmland was more affected by geo-climate. In contrast, the SOC stability of shelterbelt forests was greatly influenced by soil properties. Our findings manifest that the above-mentioned SOC changes can improve shelterbelt forest carbon sequestration function by prolonging the SOC lifespan in soil by at least 7% and SOC concentration by >15%. This should be included in the future to assess the underground soil carbon impact of Three-North shelterbelts in China and provide data supports for the estimation of similar forest stands in other parts of the world.

Genotypic variation in maize (Zea mays) influences rates of soil organic matter mineralization and gross nitrification

Agricultural management practices that increase soil organic matter (SOM), such as no-tillage (NT) with crop residue retention, together with crop varieties best able to source nutrients from SOM, may help reverse soil degradation and improve soil nutrient supply and uptake by plants in low-input environments of tropical and subtropical areas. Here, we screened germplasm representing genetic diversity within tropical maize breeding programmes in relation to shaping SOM mineralization. Then we assessed effects of contrasting genotypes on nitrification rates, and genotype-by-management history interactions on these rates. SOM-C mineralization and gross nitrification rates varied under different maize genotypes. Cumulative SOM-C mineralization increased with root diameter but decreased with increasing root length. Strong influences of management history and interaction of maize genotype-by-management history on nitrification were observed. Overall, nitrification rates were higher in NT soil with residue retention. We propose that there is potential to exploit genotypic variation in traits associated with SOM mineralization and nitrification within breeding programmes. Root diameter and length could be used as proxies for root-soil interactions driving these processes. Development of maize varieties with enhanced ability to mineralize SOM combined with NT and residue retention to build/replenish SOM could be key to sustainable production.

Soil carbon balance of afforested peatlands in the maritime temperate climatic zone

Drainage and conversion of natural peatlands to forestry increases soil CO₂ emissions through decomposition of peat and modifies the quantity and quality of litter inputs and therefore the soil carbon balance. In organic soils, CO₂ net emissions and removals are reported using carbon emission factors (EF). The choice of specific default Tier 1 EF values from the IPCC 2013 Wetlands supplement depends on land-use categories and climate zones. However, Tier 1 EF for afforested peatlands in the temperate maritime climate zone are based on data from eight sites, mainly located in the hemiboreal zone, and the uncertainty associated with these default values is a concern. In addition, moving from Tier 1 to higher-Tier carbon reporting values is highly desirable when large areas are affected by land-use changes. In this study, we estimated site-specific soil carbon balance for the development of Tier 2 soil CO₂ -C EFs for afforested peatlands. Soil heterotrophic respiration and aboveground tree litterfall were measured during two years at eight afforested peatland sites in Ireland. In addition, fine-root turnover rate and site-specific fine-root biomass were used to quantify belowground litter inputs. We found that drainage of peatlands and planting them with either Sitka spruce or lodgepole pine, resulted in soils being net carbon sources. The soil carbon balance at multi-year sites varied between 63 ± 92 and 309 ± 67 g C m^-2  year^-1 . Mean CO₂ -C EF for afforested peatlands was 1.68 ± 0.33 t CO₂ -C ha^-1  year^-1 . The improved CO₂ -C EFs presented here for afforested peatlands are proposed as a basis to update national CO₂ -C emissions from this land-use class in Ireland. Furthermore, new data from these sites will significantly contribute to the development of more reliable IPCC default Tier 1 CO₂ -C EFs for afforested peatlands in the maritime temperate climate zone.

Fine roots and mycorrhizal fungi accelerate leaf litter decomposition in a northern hardwood forest regardless of dominant tree mycorrhizal associations

●Fine roots and mycorrhizal fungi may either stimulate leaf litter decomposition by providing free-living decomposers with root-derived carbon, or may slow decomposition through nutrient competition between mycorrhizal and saprotrophic fungi. ●We reduced the presence of fine roots and their associated mycorrhizal fungi in a northern hardwood forest in New Hampshire, USA by soil trenching. Plots spanned a mycorrhizal gradient from 96% arbuscular mycorrhizal (AM) associations to 100% ectomycorrhizal (ECM)-associated tree basal area. We incubated four species of leaf litter within these plots in areas with reduced access to roots and mycorrhizal fungi and in adjacent areas with intact roots and mycorrhizal fungi. ●Over a period of 608 d, we found that litter decayed more rapidly in the presence of fine roots and mycorrhizal hyphae regardless of the dominant tree mycorrhizal association. Root and mycorrhizal exclusion reduced the activity of acid phosphatase on decomposing litter. ●Our results indicate that both AM- and ECM-associated fine roots stimulate litter decomposition in this system. These findings suggest that the effect of fine roots and mycorrhizal fungi on litter decay in a particular ecosystem likely depends on whether interactions between mycorrhizal roots and saprotrophic fungi are antagonistic or facilitative.

Causal hypotheses accounting for correlations between decomposition rates of different mass fractions of leaf litter

Whole-leaf decomposition rates are the sum of the decomposition rates of each chemical fraction (water-soluble, cellulose, hemicellulose, lignin), but the decomposition rates of each fraction show complicated patterns of covariation. What explains these patterns of covariation? After measuring the k values of each fraction in 42 different mixtures of tree leaf litters from five species, we tested three alternative causal hypotheses that have been proposed in the literature concerning these mixture interactions using structura equations modeling. All three hypotheses were rejected by the data. We then proposed a new hypothesis, in which rapid decomposition of the labile (water-soluble) fraction stimulates the decomposition of lignin by white-rot fungi and the decomposition of hemicellulose by brown-rot fungi. A more rapid decomposition of hemicellulose then stimulates the decomposition of cellulose. This hypothesis is both consistent with known biology and with our data and is proposed as the most viable current hypothesis.