Soil Soluble Carbon Research Articles

Changes in soil properties and microbial activity unveil the distinct impact of polyethylene and biodegradable microplastics on chromium uptake by peanuts.

Microplastics (MPs) are emerging persistent pollutants, and heavy metals are typical environmental pollutants, with their coexistence potentially compounding pollution and ecological risks. However, the interactive impacts and the relevant mechanisms of heavy metal and different types of MPs in plant-soil systems are still unclear. This study investigated the differential impacts of polyethylene MPs (PE MPs) and biodegradable polybutylene adipate MPs (PBAT MPs) on chromium (Cr) uptake in peanuts, focusing on plant performance and rhizosphere soil microenvironment. Compared with nondegradable PE-MPs, biodegradable PBAT MPs produced less significant influences on plant phytotoxicity, soil Cr bioavailability, and soil properties such as pH, CEC, DOC, and MBC, with the exception of MBN in Cr-contaminated soils. Compared to the control, soil pH and cation exchange capacity (CEC) decreased by MPs, while soil-soluble carbon (DOC), microbial biomass carbon, and nitrogen (MBC and MBN) increased by MPs. Compared to the control, soil-bioavailable Cr increased by 11.8-177.8% under PE MPs treatments, while increased by 5.1-156.9% under PBAT MPs treatments. The highest Cr content in shoots and roots was observed at 500.0mg·kg-1 Cr level, which increased by 53.1% and 79.2% under 5% PE MPs treatments, respectively, as well as increased by 38.3% and 60.4% under 5% PBAT MPs treatments, respectively, compared with the control. The regression path analysis indicated that pH, MBC, MBN, and soil-bioavailable Cr played a vital role in the changes of soil properties and Cr uptake by peanuts induced by MPs. Soil bacterial community analysis revealed that Nocardioides, Proteobacteria, and Sphingomonas were reduced by the inhibition of MPs, which affected Cr uptake by peanuts. These results indicated that the peanut soil microenvironment was affected by PBAT and PE MPs, altering the Cr bioavailability and plant Cr uptake in Cr-contaminated soil.

Long-Term Nitrogen Addition Decreases Soil Carbon Mineralization in an N-Rich Primary Tropical Forest

Anthropogenic elevated nitrogen (N) deposition has an accelerated terrestrial N cycle, shaping soil carbon dynamics and storage through altering soil organic carbon mineralization processes. However, it remains unclear how long-term high N deposition affects soil carbon mineralization in tropical forests. To address this question, we established a long-term N deposition experiment in an N-rich lowland tropical forest of Southern China with N additions such as NH4NO3 of 0 (Control), 50 (Low-N), 100 (Medium-N) and 150 (High-N) kg N ha−1 yr−1, and laboratory incubation experiment, used to explore the response of soil carbon mineralization to the N additions therein. The results showed that 15 years of N additions significantly decreased soil carbon mineralization rates. During the incubation period from the 14th day to 56th day, the average decreases in soil CO2 emission rates were 18%, 33% and 47% in the low-N, medium-N and high-N treatments, respectively, compared with the Control. These negative effects were primarily aroused by the reduced soil microbial biomass and modified microbial functions (e.g., a decrease in bacteria relative abundance), which could be attributed to N-addition-induced soil acidification and potential phosphorus limitation in this forest. We further found that N additions greatly increased soil-dissolved organic carbon (DOC), and there were significantly negative relationships between microbial biomass and soil DOC, indicating that microbial consumption on soil-soluble carbon pool may decrease. These results suggests that long-term N deposition can increase soil carbon stability and benefit carbon sequestration through decreased carbon mineralization in N-rich tropical forests. This study can help us understand how microbes control soil carbon cycling and carbon sink in the tropics under both elevated N deposition and carbon dioxide in the future.

Effects of precipitation exclusion and warming on soil soluble carbon and nitrogen in a young Cunninghamia lanceolata plantation

Soil soluble carbon and nitrogen play important roles in soil carbon and nutrient cycles and are highly sensitive to climate change, as they can be directly used by microorganisms. We used Tension Lysimeter to collect soil solution in 50% precipitation exclusion (P) and warming (5 ℃) plus 50% precipitation exclusion (WP) treatments in a 2 year-old Cunninghamia lanceolata plantation in subtropics, to examine the effects of precipitation and temperature on soil soluble carbon and nitrogen concentrations in soil profile. Results showed that neither P treatment nor WP treatment changed seasonal dynamics of soil dissolved organic carbon (DOC) concentration, with maximum value at October among all treatments. DOC concentration was increased in both P and WP treatments in the whole soil profile, especially in 60 cm soil depth. Compared with the control, DOC concentration was increased by 30.4%-88.7% and 32.8%-137.6% in P and WP treatments, respectively, with the most obvious difference being found in October. DOC concentration was decreased with the increases of soil depth in the control, but no significant difference among different soil layers were found in P and WP treatments. NO3--N concentration was increased by 221.1%-931.0% in WP treatment. Therefore, precipitation reduction might increase losses of C and N from soil solution in subtropical forest, due to improvement of soil permeability by the increases of fine roots grown into deep soil, which might stimulate soil microorganism activities and soil organic matter decomposition. Furthermore, warming would exacerbate the risk of C and N losses.

Microbial mineralization of biochar and wheat straw mixture in soil: A short-term study

A short-term incubation study was carried out to investigate the effect of biochar addition to soil on CO 2 emissions, microbial biomass, soil soluble carbon (C) nitrogen (N) and nitrate–nitrogen (NO 3–N). Four soil treatments were investigated: soil only (control); soil + 5% biochar; soil + 0.5% wheat straw; soil + 5% biochar + 0.5% wheat straw. The biochar used was obtained from hardwood by pyrolysis at 500 °C. Periodic measurements of soil respiration, microbial biomass, soluble organic C, N and NO 3–N were performed throughout the experiment (84 days). Only 2.8% of the added biochar C was respired, whereas 56% of the added wheat straw C was decomposed. Total net CO 2 emitted by soil respiration suggested that wheat straw had no priming effect on biochar C decomposition. Moreover, wheat straw significantly increased microbial C and N and at the same time decreased soluble organic N. On the other hand, biochar did not influence microbial biomass nor soluble organic N. Thus it is possible to conclude that biochar was a very stable C source and could be an efficient, long-term strategy to sequester C in soils. Moreover, the addition of crop residues together with biochar could actively reduce the soil N leaching potential by means of N immobilization.

Soil soluble carbon dynamics of manured and unmanured grasslands following chemical kill and ploughing

Better understanding of soluble carbon (C) dynamics in managed grasslands fertilised with animal manures is required to effectively estimate nutrient transport potential and C loss during land-use change. Two adjacent grassland plots, one amended with 100 m 3 ha − 1 of liquid swine manure annually since 1978 and another unamended for the same period, were either killed by glyphosate in the autumn or left with vegetation intact. Those killed were either (i) left as an undisturbed chemical fallow, (ii) ploughed (20 cm depth) by full inversion tillage (FIT) in the autumn, or (iii) ploughed (20 cm depth) in the following spring. Microbial biomass C (MBC) and soluble C fractions: hot-water extractable organic C (HWEOC), cold-water extractable organic C (CWEOC), and dissolved organic C from tension lysimeters (DOC) were monitored during one full year following herbicide application and ploughing. On both unmanured and manured sites, HWEOC was positively correlated with MBC. However, on the manured grassland the slope of the HWEOC:MBC relationship was 55% higher than on the unmanured site indicating that the manured grassland accumulated more potentially soluble C per unit MBC than the unmanured grassland. Compared to the plots with intact vegetation and the chemical fallow, full inversion tillage reduced HWEOC and CWEOC by 10 to 30%, on both manured and unmanured soils, and reduced DOC in the soil solution of the manured soils. Peak DOC concentrations measured in the field occurred when soils were water-logged, particularly on the manured grassland. Our results suggest that manured grasslands on poorly-drained soils accumulate important quantities of readily soluble C which are prone to export as DOC when the soil becomes water-saturated. However, full inversion tillage of these grassland soils attenuated the amounts of readily soluble C and, in particular, the peaks in DOC.

Nitrous oxide emissions from soil during soybean [(Glycine max (L.) Merrill] crop phenological stages and stubbles decomposition period

The purpose of this study was to evaluate, during the phenological stages of inoculated soybean crop [Glycine max (L.) Merrill], the effect of different N fertilization levels and inoculation with Bradyrhizobium japonicum on N2O emissions from the soil. Gas emissions were evaluated at field conditions by the static-chamber method. Nitrogen fertilization increased N2O emissions significantly (P < 0.05). The variable that best explained cumulative N2O emissions during the whole soybean growing season was the soil nitrate level (r 2 = 0.1899; P = 0.0231). Soil moisture presented a greater control on N2O emissions between the grain-filling period and the crop commercial maturity (r 2 = 0.5361; P < 0.0001), which coincided with a positive balance of the available soil N, as a consequence of the decrease in crop requirements and root and nodular decomposition. Only soil soluble carbon (r 2 = 0.29; P = 0.019) and moisture (r 2 = 0.24; P = 0.039) were correlated with N2O emissions during the residue decomposition period. The relationship between soil variables and N2O emissions depended on crop phenological or stubbles decomposition stages.

Relationship between plant and soil microbial communities along a successional gradient in a chalk grassland in north-western France

Plant and microbial communities were compared through their structure and their functions in a chalk grassland in north-western France. Multivariate statistical analyses allowed us to establish floristic patterns and to identify four stages of succession from short grasslands to early stages of forest. In each stage, several measurements were made on vegetation (floristic data, biomass weight and lignin content), on soil (total carbon, soluble organic carbon, total nitrogen, polysaccharides and phenols contents, lignin content in litter) and on soil microbial communities (microbial biomass, functional structure assessed by using BIOLOG analysis, genetic structure of bacterial and fungal communities assessed by using Amplified Ribosomal DNA Restriction Analysis (ARDRA) for 16S and 18S rDNA: ARDRA 16S and 18S). Although the organic matter resources that support soil microbial communities are larger in older stages of plant succession (high plant and litter biomass and nitrogen content in plant leaves in forest stages), results show that biomass and potential level of enzymatic activity of microbial communities are negatively correlated with the presence of shrubs and positively associated with grassland habitats. These microbiological variables also present complex interactions with other factors and are positively correlated with soil humidity, soil soluble carbon and polysaccharides contents and negatively correlated with the presence of lignin in plant biomass and litter. The results show out that microbial communities are independent from the successional gradient of vegetation, in terms of functional (BIOLOG analysis) and genetic structure (ARDRA 16S and 18S). This study provides a first step in understanding the link between the biodiversity (number and type of plant species, genetic structure of microbial community) and function (biomass production, enzymatic catabolism) in chalk grassland ecosystems.

Spatial and temporal controls of soil respiration rate in a high-elevation, subalpine forest

We examined soil respiration to determine what measurable environmental variables can be used to predict variation in soil respiration rates, spatially and temporally, at a high-elevation, mixed conifer, subalpine forest site at the Niwot Ridge Ameriflux Site in Colorado. For three summers, soil respiration rates were measured using soil collars and a portable gas-exchange system. Transects of the collars were established to ensure spatial characterization of the litter-repleted areas beneath tree crowns and the litter-depleted open spaces between tree crowns. Soil temperature and soil moisture were both identified as important drivers of soil respiration rate, but were found to confound each other and to function as primary controls at different scales. Soil temperature represents a primary control seasonally, and soil moisture represents a primary control interannually. Spatially, organic layer thickness, ammonium concentration, water content, and the microbial and soil soluble carbon pools were found to predict variation from point to point. Soil microbial biomass strongly correlated to soil respiration rate, whereas root biomass was identified as a weak predictor of respiration rate and only when controlling for other variables. Spatial variation in soil respiration rate is highly determined by the depth of the soil organic horizon, which in this ecosystem varies predictably according to distance from trees. The conclusions that can be drawn from the study provide the foundation for the development of future models of soil respiration driven by fundamental variables of the climate and soil microenvironment.