The sensitivity of soil microbial respiration declined due to crop straw addition but did not depend on the type of crop straw

Microbial-driven mechanisms for the effects of heavy metals on soil organic carbon storage: A global analysis

Heavy metal (HM) enrichment is closely related to soil organic carbon (SOC) pools in terrestrial ecosystems, which are deeply intertwined with soil microbial processes. However, the influence of HMs on SOC remains contentious in terms of magnitude and direction. A global analysis of 155 publications was conducted to integrate the synergistic responses of SOC and microorganisms to HM enrichment. A significant increase of 13.6 % in SOC content was observed in soils exposed to HMs. The response of SOC to HMs primarily depends on soil properties and habitat conditions, particularly the initial SOC content, mean annual precipitation (MAP), initial soil pH, and mean annual temperature (MAT). The presence of HMs resulted in significant decreases in the activities of key soil enzymes, including 31.9 % for soil dehydrogenase, 24.8 % for β-glucosidase, 35.8 % for invertase, and 24.3 % for cellulose. HMs also exerted inhibitory effects on microbial biomass carbon (MBC) (26.6 %), microbial respiration (MR) (19.7 %), and the bacterial Shannon index (3.13 %) but elevated the microbial metabolic quotient (qCO2) (20.6 %). The HM enrichment-induced changes in SOC exhibited positive correlations with the response of MBC (r = 0.70, p < 0.01) and qCO2 (r = 0.50, p < 0.01), while it was negatively associated with β-glucosidase activity (r = 0.72, p < 0.01) and MR (r = 0.39, p < 0.01). These findings suggest that the increase in SOC storage is mainly attributable to the inhibition of soil enzymes and microorganisms under HM enrichment. Overall, this meta-analysis highlights the habitat-dependent responses of SOC to HM enrichment and provides a comprehensive evaluation of soil carbon dynamics in an HM-rich environment.

Effects of exogenous Fe addition on soil respiration rate and dissolved organic carbon structure in temperate forest swamps of northeastern China

Fe as an important redox-active transition metal plays a key role in the carbon cycle of ecosystems. To date, the mechanisms by which Fe affects organic carbon (soil respiration rate [Rs] and dissolved organic carbon [DOC] structure) remain unclear, because most studies only focused on the effect of Fe on soil organic carbon content. To understand these effects, a 30-day laboratory incubation experiment was conducted using forest swamp soils from northeastern China amended with different concentrations of exogenous Fe (no exogenous Fe added [L0], add exogenous Fe at 1 time the soil background value [L1], add exogenous Fe at 2 times the soil background value [L2]). Our results showed that exogenous Fe addition reduced the soil respiration rate by 54.8% during the incubation time. The DOC concentration decreased by 40.5% with exogenous Fe addition during the incubation time. The dissolved organic matter (DOM) characteristic parameters showed apparent variations (p < 0.05), including significant increases in the fluorescence and biological index and significant decreases in the humification index, which indicate that exogenous Fe addition reduced humification, which may lead to the increased fixation of dissolved organic carbon. In addition significant increases in tryptophan-like DOM was observed when exogenous Fe addition resulted in a soil Fe concentration of twice the background value (p < 0.05). These findings indicate that exogenous Fe addition promotes the production of endogenous soil DOC by microorganisms. Overall, Our study uses three-dimensional fluorescence spectroscopy techniques combined with the parallel factor analysis (PARAFAC) to characterize the dissolved organic matter components in soil samples under exogenous Fe addition conditions, with a view to exploring the differences in the effects of Fe on the DOC concentration and structure of wetland soils, providing a theoretical basis for the mechanisms of soil carbon fixation and soil organic matter transformation in wetland soils.

Soil Storage Conditions Alter the Effects of Tire Wear Particles on Microbial Activities in Laboratory Tests

In this study, we focused on the fact that soil storage conditions in the laboratory have never been considered as a key factor potentially leading to high variation when measuring effects of microplastics on soil microbial activity. We stored field-collected soils under four different conditions [room-temperature storage, low-temperature storage (LS), air drying (AD), and heat drying] prior to the experiment. Each soil was treated with tire wear particles (TWPs), and soil microbial activities and water aggregate stability were investigated after soil incubation. As a result, microbial activities, including soil respiration and three enzyme activities (β-glucosidase, N-acetyl-β-glucosaminidase, and phosphatase), were shown to depend on soil storage conditions. Soil respiration rates increased with the addition of TWPs, and the differences from the control group (no TWPs added) were more pronounced in the AD TWP treatment than in soils stored under other conditions. In contrast, phosphatase activity followed an opposing trend after the addition of TWPs. The AD soil had higher phosphatase activity after the addition of TWPs, while the LS soil had a lower level than the control group. We suggest that microplastic effects in laboratory experiments can strongly depend on soil storage conditions.

Unraveling consequences of the co-exposure of polyethylene microplastics and acid rain on plant-microbe-soil system

Emerging microplastics (MPs) pollution and continuing acid rain (AR) co-exist in terrestrial ecosystems, and are considered as threats to ecosystems health. However, few data are available on MPs-AR interactions in plant-microbe-soil systems. Here, a microcosm experiment was manipulated to elucidate the co-exposure of polyethylene MPs (PE MPs; 1%, 5% and 10%) and AR (pH 4.0) on soil-lettuce system, in which the properties of soil and lettuce, and their links were explored. We found that 10% PE MPs increased soil CO₂ emission and its temperature sensitivity (Q₁₀) in combination with AR, while 1% PE MPs reduced soil CO₂ emission irrespective of AR. PE MPs addition did not influence lettuce production (total biomass) though its photosynthesis was affected. PE MPs exerted negative impact on soil water availability. PE MPs treatments increased NH₄⁺-N content of soil without AR, and dissolved organic carbon content of soil sprayed with AR. 10% PE MPs combined with AR reduced soil microbial biomass, while soil microbial community diversity was not affected by PE MPs or AR. Interestingly, 10% PE MPs addition altered soil microbial community structure, and promoted the complexity and connectivity of soil microbial networks. 5% and 10% PE MPs addition decreased soil urease activity under AR, but this was not the case without AR. These findings highlight the critical role of AR in regulating PE MPs impacts on plant-microbe-soil ecosystems, and the necessity to incorporate other environmental factors when evaluating the actual impacts or risks of MPs pollution in terrestrial ecosystems.

Effects of plastic mulching on soil CO2 efflux in a cotton field in northwestern China

In Northwestern China, more and more traditional cultivation system (TC) with no mulching and flood irrigation have been replaced by modern cultivation technology (MC) combining plastic film mulching with drip irrigation. Does plastic film mulching increase or reduce soil CO₂ emission in arid areas? In order to study the effects of plastic mulching on soil CO₂ efflux, a field study was conducted to compare soil CO₂ concentration, soil CO₂ efflux, soil temperature and moisture between the TC treatment and the MC treatment during a cotton growing season in Northwestern China. The seasonal patterns of soil profile temperature and soil moisture in the TC treatment were similar to that in the MC treatment. The mean value of soil profile temperature in the MC treatment was higher than that in the TC treatment. Except for soil moisture at 15 cm depth, the mean value of soil moisture at 5 cm and 10 cm depths in the MC treatment was higher than that in the TC treatment. The variation patterns of soil CO₂ concentration and soil CO₂ efflux in MC treatment were different to that in the TC treatment. Although the peak of soil CO₂ concentration in the TC treatment was earlier than that in the MC treatment, the duration of soil CO₂ concentration with high values in TC treatment was shorter than that in the MC treatment. Based on the model of Fick’s first diffusion law, soil surface CO₂ efflux in the MC and TC treatments were determined. The surface CO₂ efflux in the TC treatment calculated by Fick’s first diffusion law model was in good agreement with the value measured by chamber method. The seasonal curve of soil surface CO₂ efflux in the MC treatment indicate the similar pattern with that in the TC treatment, and the rate of CO₂ efflux was lower in the MC system. In the MC treatment, the seasonal variation of soil surface efflux was explained more by soil moisture than by soil temperature. However, in the TC treatment, the seasonal variation of soil surface efflux was explained more by soil temperature than by soil moisture. Over the completely experimental period, accumulated rates of soil CO₂ efflux were 361 g C m⁻² and 474 g C m⁻² for the MC and TC system, respectively. We concluded that converting agricultural practices from traditional cultivation to the plastic mulching cultivation could reduce soil CO₂ efflux by approximately 110 g C m⁻² year⁻¹ in agricultural land in arid areas of Northwestern China.