The soil pore structure encountered by roots affects plant-derived carbon inputs and fate

Effects of different irrigation amounts on soil structure in newly cultivated land on the Loess Plateau

It is imperative to promote water-saving irrigation technology and develop newly cultivated land in the Loess Plateau. This study focused on the interaction between roots and soil to examine the effects of different amounts of irrigation on soil structure of newly cultivated land. Herein, five irrigation levels were set, i.e., sufficient irrigation (W100), mild deficit irrigation (W80), moderate deficit irrigation (W60), severe deficit irrigation (W40), and rain-fed (RF). Physical properties and structural stability indexes of the rhizosphere soil were measured, and their relationship with plant root morphology were analyzed. The results showed that the soil structure under the high irrigation amount group (W80 and W100) was relatively stable. The average particle density of soil in each plot decreased significantly after the experiment, while the soil total porosity remained unchanged in W80 and W100 treatments. The proportion of large aggregates, the mean weight diameter, and the geometric mean diameter of soil significantly reduced in the low irrigation amount group (RF, W40, and W60). In contrast, the W100 and W80 treatments inhibited the decline in soil aggregate stability. Change in the generalized soil structure index (GSSI) and soil three-phase structure distance (STPSD) of W100 and W80 treatments were not significant, before and after the experiment. These results suggested that the soil in newly cultivated land treated with sufficient and mild deficit irrigation was closer to the ideal state for crops growing. Path analysis identified the average soil moisture content had the greatest negative effect on STPSD primarily through the root length, root surface area, and root dry weight. In conclusion, irrigation amount occupies a dominant position among all factors influencing soil structure considered in the study. And the mild deficit irrigation is suitable for agricultural cultivation in the Loess Plateau area, from the soil structure protection and water-saving perspectives.

A Global Meta-Analysis of Land Use Change on Soil Mineral-Associated and Particulate Organic Carbon

Separating soil organic carbon (SOC) into mineral-associated organic carbon (MAOC) and particulate organic carbon (POC) enables accurate prediction of SOC vulnerability to land use change (LUC). Here, we synthesize the responses of soil MAOC and POC to LUC, including land restoration and degradation, from 693 soil observations globally. We observed a large increase in soil MAOC and POC after restoration and a greater decline after degradation, but the magnitude and proportion of these two carbon fractions (fMAOC and fPOC) varied with LUC. POC, in comparison with MAOC, responded more sensitively to LUC, suggesting that POC was more vulnerable to environmental change. Using observed duration relationships, we found that the fraction of POC (fPOC) was higher at the early stage of restoration but lower at the late stage, projecting that soil carbon stability declined after short-term restoration but gradually increased after long-term restoration. Further analysis showed the context-dependent effects of LUC on carbon fractions: in arid or carbon-poor topsoil, restoration greatly increased soil carbon fractions and fPOC, while in humid or carbon-rich topsoil, degradation resulted in large decreases in POC and MAOC, especially POC. Overall, we highlight the importance of soil fractions, particularly POC, in predicting soil carbon stability and suggest that incorporating climate and initial carbon status in models of soil carbon dynamics helps to accurately predict future carbon sink potential.

Light and substrate composition control root exudation rates at the initial stages of soilless lettuce cultivation

Plant root exudation is an inherent metabolic process that enhances various functions of the root system like the mobilization of nutrients and interactions with surrounding microbial communities. In soilless crop production, roots are temporally submerged in a nutrient solution affecting the root exudation process. In this study, we asked whether root exudation in soilless cultures is affected by culturing method and substrate composition, important factors determining the root microbial ecosystem. Exploration of different growth conditions revealed that the effect of light quality depended on the substrate used. The impact of light quality and substrate was assessed by growing soilless lettuce in 100 % red light (660 nm), 100 % blue light (450 nm), and white light (full-light spectrum) in deep flow culture, or in 100 % perlite, 100 % potting soil, or mixtures of both growing media. Root exudates were collected at different time points after transplanting. The root exudation rate declined with plant age in all culturing conditions, underscoring its importance during the early stages of development. The total carbon root exudation rate was influenced by light conditions and substrate composition at the earliest timepoint of the culture but not at later growth stages. The total carbohydrate exudation rate was significantly higher under pure blue and red light compared to white light. The impact of light depended on the presence of perlite in the substrate. The total phenolic compound exudation rate was most strongly influenced by the substrate composition and reached the highest level in either pure potting soil or pure perlite. Light and growing media influence the exudation rate at the early stage, suggesting that exudation is an adaptive process of the soilless lettuce culture.

High concentrations of polyethylene microplastics restrain the growth of Cinnamomum camphora seedling by reducing soil water holding capacity

The accumulation of microplastics (MPs) in soils due to anthropogenic activities affects the growth and development of plants and thereby endangering the diversity and function of ecosystems. Although there is an increasing number of studies exploring the effects of MPs on plants in recent days, most of them focus on crops only. However, few studies have been conducted on woody plants that play a prominent role in ecosystems, while crucial edaphic factors which potentially restrain plant growth in MP-contaminated soils are yet to be revealed. In the current study, a 6-month pot experiment was conducted to investigate the inhibitory effect of soil polyethylene microplastics (PE-MPs) (average size of 6.5 µm) with increasing concentrations (0, 0.1 %, 0.5 %, 1 %, and 2 % w/w) on the growth of Cinnamomum camphora seedlings. The relationships between seedling growth and soil properties were also explored. The results showed that low concentrations of PE-MPs (not larger than 0.5 % in soils) did not restrain seedling growth, while the PE-MP concentrations of 1 % and 2 % decreased the net growth of ground diameter by 38.8 % (p < 0.05) and biomass by 29.6 % (p < 0.05), respectively. Similarly, the concentration of PE-MPs in soils not larger than 0.5 % showed little effect on soil physical properties, while the 1 % and 2 % MP accumulations decreased the soil capillary porosity by 8.9 % and 22.2 % (p < 0.05), respectively, thereby reducing the soil water content by 29.8 % (p < 0.05) and 34.1 % (p < 0.05) accordingly. Furthermore, high concentrations of PE-MPs (1 % and 2 %) greatly decreased soil alkali-hydrolysable nitrogen content and decreased bacterial diversity. The structural equation model clearly indicated that the inhibitory effect of soil PE-MPs with high concentrations on seedling growth was mainly derived from the decrement of soil water holding capacity. Our findings help replenish the regulation mechanism of MPs on plant growth and suggest that C. camphora is a potentially good candidate for the phytoremediation of the low-level PE-MP-contaminated soil.

The distribution of particulate organic matter in the heterogeneous soil matrix - Balancing between aerobic respiration and denitrification

Denitrification, a key process in soil nitrogen cycling, occurs predominantly within microbial hotspots, such as those around particulate organic matter (POM), where denitrifiers use nitrate as an alternative electron acceptor. For accurate prediction of dinitrogen (N2) and nitrous oxide (N2O) emissions from denitrification, a precise quantification of these microscale hotspots is required. The distribution of POM is of crucial importance in this context, as the local oxygen (O2) balance is governed not only by its high O2 demand but also by the local O2 availability. Employing a unique combination of X-ray CT imaging, microscale O2 measurements, and 15N labeling, we were able to quantify hotspots of aerobic respiration and denitrification. We analyzed greenhouse gas (GHG) fluxes, soil oxygen supply, and the distribution of POM in intact soil samples from grassland and cropland under different moisture conditions. Our findings reveal that both proximal and distal POM, identified through X-ray CT imaging, contribute to GHG emissions. The distal POM, i.e. POM at distant locations to air-filled pores, emerged as a primary driver of denitrification within structured soils of both land uses. Thus, the higher denitrification rates in the grassland could be attributed to the higher content of distal POM. Conversely, despite possessing compacted areas that could favor denitrification, the cropland had only small amounts of distal POM to stimulate denitrification in it. This underlines the complex interaction between soil structural heterogeneity, organic carbon supply, and microbial hotspot formation and thus contributes to a better understanding of soil-related GHG emissions. In summary, our study provides a holistic understanding of soil-borne greenhouse gas emissions and emphasizes the need to refine predictive models for soil denitrification and N2O emissions by incorporating the microscale distribution of POM.

Arbuscular mycorrhizal symbiosis enhances the accumulation of plant-derived carbon in soil organic carbon by regulating the biosynthesis of plant biopolymers and soil metabolism

Plant-derived carbon (C) is a critical constituent of particulate organic carbon (POC) and plays an essential role in soil organic carbon (SOC) sequestration. Yet, how arbuscular mycorrhizal fungi (AMF) control the contribution of plant-derived C to SOC storage through two processes (biosynthesis of plant biopolymers and soil metabolism) remains poorly understood. Here, we utilized transcriptome analysis to examine the effects of AMF on P. communis roots. Under the AM symbiosis, root morphological growth and tolerance to stress were strengthened, and the biosynthetic pathways of key plant biopolymers (long-chain fatty acids, cutin, suberin, and lignin) contributing to the plant-derived C were enhanced. In the subsequent metabolic processes, AMF increased soil metabolites contributing to plant-derived C (such as syringic acid) and altered soil metabolic pathways, including carbohydrate metabolism. Additionally, C-acquiring soil extracellular enzyme activities were enhanced by AMF, which could affect the stabilization of plant-derived C in soil. The contents of POC (21.71 g kg-1 soil), MAOC (10.75 g kg-1 soil), and TOC (32.47 g kg-1 soil) in soil were significantly increased by AMF. The concentrations of plant-derived C and microbial-derived C were quantified based on biomarker analysis. AMF enhanced the content of plant-derived C in both POC and MAOC fractions. What's more, plant-derived C presented the highest level in the POC fraction under the AMF treatment. This research broadens our understanding of the mechanism through which plant-derived C contributes to the accumulation of POC and SOC induced by AM symbiosis, and evidences the benefits of AMF application in SOC sequestration.

Diverse factors influence the amounts of carbon input to soils via rhizodeposition in plants: A review

Rhizodeposition encompasses the intricate processes through which plants generate organic compounds via photosynthesis, store these compounds within aboveground biomass and roots through top-down transport, and subsequently release this organic matter into the soil. Rhizodeposition represents one of the carbon (C) cycle in soils that can achieve long-term organic C sequestration. This function holds significant implications for mitigating the climate change that partly stems from the greenhouse effect associated with increased atmospheric carbon dioxide levels. Therefore, it is essential to further understand how the process of rhizodeposition allocates the photosynthetic C that plants create via photosynthesis. While many studies have explored the basic principles of rhizodeposition, along with the associated impact on soil C storage, there is a palpable absence of comprehensive reviews that summarize the various factors influencing this process. This paper compiles and analyzes the literature on plant rhizodeposition to describe how rhizodeposition influences soil C storage. Moreover, the review summarizes the impacts of soil physicochemical, microbial, and environmental characteristics on plant rhizodeposition and priming effects, and concludes with recommendations for future research.

Soil pore characteristics and the fate of new switchgrass-derived carbon in switchgrass and prairie bioenergy cropping systems

Monoculture switchgrass and restored prairie are promising perennial feedstock sources for bioenergy production on the lands unsuitable for conventional agriculture. Such lands often display contrasting topography that influences soil characteristics and interactions between plant growth and soil C gains. This study aimed at elucidating the influences of topography and plant systems on the fate of C originated from switchgrass plants and on its relationships with soil pore characteristics. For that, switchgrass plants were grown in intact soil cores collected from two contrasting topographies, namely steep slopes and topographical depressions, in the fields in multi-year monoculture switchgrass and restored prairie vegetation. The 13C pulse labeling allowed tracing the C of switchgrass origin, which X-ray computed micro-tomography enabled in-detail characterization of soil pore structure. In eroded slopes, the differences between the monoculture switchgrass and prairie in terms of total and microbial biomass C were greater than those in topographical depressions. While new switchgrass increased the CO2 emission in depressions, it did not significantly affect the CO2 emission in slopes. Pores of 18-90 µm Ø facilitated the accumulation of new C in soil, while > 150 µm Ø pores enhanced the mineralization of the new C. These findings suggest that polyculture prairie located in slopes can be particularly beneficial in facilitating soil C accrual and reduce C losses as CO2.