Spatiotemporal characteristics of summer hypoxia in Mirs Bay and adjacent coastal waters,South China

Mirs Bay is a semi-enclosed bay neighboring the Zhujiang(Pearl)River estuary,one of the largest estuarine systems in the world.The long-term historical observational data(1994-2017)of temperature,salinity,dissolved oxygen(DO),and biochemical parameters were used to examine the spatiotemporal distribution of hypoxia in Mirs Bay and adjacent coastal waters.Results show that bottom hypoxia varied seasonally and interannually.Hypoxia mainly occurred from June to September in Mirs Bay and the transition zone in the southern waters of Hong Kong,and the recorded hypoxia events have increased from 2007.The density difference between the bottom and surface layers was positively related to the bottom apparent oxygen utilization(AOU)(R=0.620,P<0.001)and negatively related to the bottom DO(R=0.616,P<0.001),indicating that water column stratification was an essential prerequisite for the formation of bottom hypoxia in summer.The bottom oxygen consumption and hypoxia had higher positive correlation with the seasonal thermocline(R=0.683,P<0.001)than the halocline(R=0.540,P<0.001),including in the area was affected by freshwater plume.The insignificant relationship between AOU and nutrients indicated that local eutrophication was not the only important factor in the formation of the hypoxic zone during summer.The decrease in phosphorous owing to the pollutant reduction policy and the increase in nitrate may have led to an increase in hypoxia events in the bay where waters therein are characterized by nitrogen-limitation.The increase in chemical oxygen demand in wastewater also promoted oxygen consumption.Compared to the adjacent coastal waters influenced by Zhujiang River plume water,the Mirs Bay experienced more hypoxia events.The high concentrations of ammonium and total Kjeldahl nitrogen in the sediment of Mirs Bay increased the oxygen depletion in the bottom water.The long residence time of the near-bottom water in Mirs Bay increased the risk of bottom hypoxia events,although the nutrient concentrations were lower than those in the transition zone.These factors lead to differences in hypoxia occurrence in Mirs Bay and adjacent coastal waters.

Soil microbial communities regulate the threshold effect of salinity stress on SOM decomposition in coastal salt marshes

Salinity stress is one of the critical environmental drivers of soil organic matter(SOM)decomposition in coastal ecosystems.Although the temperature sensitivity(Q_(10))of SOM decomposition has been widely applied in Earth system models to forecast carbon processes,the impact of salinity on SOM decomposition by restructuring microbial communities remains uncovered.Here,we conducted a microcosm experiment with soils collected from the coastal salt marsh in the Yellow River Estuary,which is subjected to strong dynamics of salinity due to both tidal flooding and drainage.By setting a gradient of salt solutions,soil salinity was adjusted to simulate salinity stress and soil carbon emission(CO_(2))rate was measured over the period.Results showed that as salinity increased,the estimated decomposition constants based on first-order kinetics gradually decreased at different temperatures.Below the 20‰salinity treatments,which doubled the soil salinity,Q_(10)increased with increasing salinity;but higher salinity constrained the temperature-related response of SOM decomposition by inhibiting microbial growth and carbon metabolisms.Soil bacteria were more sensitive to salinity stress than fungi,which can be inferred from the response of microbial beta-diversity to changing salinity.Among them,the phylotypes assigned to Gammaproteobacteria and Bacilli showed higher salt tolerance,whereas taxa affiliated with Alphaproteobacteria and Bacteroidota were more easily inhibited by the salinity stress.Several fungal taxa belonging to Ascomycota had higher adaptability to the stress.As the substrate was consumed with the incubation,bacterial competition intensified,but the fungal co-occurrence pattern changed weakly during decomposition.Collectively,these findings revealed the threshold effect of salinity on SOM decomposition in coastal salt marshes and emphasized that salt stress plays a key role in carbon sequestration by regulating microbial keystone taxa,metabolisms,and interactions.

Disentangling the contributions of arbuscular mycorrhizal fungi to soil multifunctionality

Soil multifunctionality represents a range of soil processes driven by the interactions between soil abiotic and biotic components.As a group of ubiquitous fungi that form mutualistic symbiotic associations with a vast array of terrestrial plants,arbuscular mycorrhizal(AM)fungi may play a critical role in maintaining soil multifunctionality,but the characteristics of their contributions remain to be unraveled.This mini review aims to disentangle the contributions of AM fungi to soil multifunctionality.We provide a framework of concepts about AM fungi making crucial contributions to maintaining multiple soil functions,including primary productivity,nutrient cycling,water regulation and purification,carbon and climate regulation,habitat for biodiversity,disease and pest control,and pollutant degradation and detoxification,via a variety of pathways,particularly contributing to soil and plant health.This review contends that AM fungi,as a keystone component of soil microbiome,can govern soil multifunctionality,ultimately promoting ecosystem services.

Microplastic fiber and drought effects on plants and soil are only slightly modified by arbuscular mycorrhizal fungi

Microplastics are increasingly recognized as a factor of global change. By altering soil inherentproperties and processes, ripple-on effects on plants and their symbionts can be expected.Additionally, interactions with other factors of global change, such as drought, can influence theeffect of microplastics. We designed a greenhouse study to examine effects of polyester microfibers,arbuscular mycorrhizal (AM) fungi and drought on plant, microbial and soil responses. We found thatpolyester microfibers increased the aboveground biomass of Allium cepa under well-watered anddrought conditions, but under drought conditions the AM fungal-only treatment reached the highestbiomass. Colonization with AM fungi increased under microfiber contamination, however, plantbiomass did not increase when both AM fungi and fibers were present. The mean weight diameter ofsoil aggregates increased with AM fungal inoculation overall but decreased when the system wascontaminated with microfibers or drought stressed. Our study adds additional support to themounting evidence that microplastic fibers in soil can affect the plant–soil system by promoting plantgrowth, and favoring key root symbionts, AM fungi. Although soil aggregation is usually positivelyinfluenced by plant roots and AM fungi, and microplastic promotes both, our results show that plasticstill had a negative effect on soil aggregates. Even though there are concerns that microplastic mightinteract with other factors of global change, our study revealed no such effect for drought.

Larger floods reduce soil CO_(2)efflux during the post-flooding phase in seasonally flooded forests of Western Amazonia

Seasonally flooded várzea forests of Western Amazonia are one of the most productive and biodiverse wetland forests in the world.However,data on their soil CO_(2)emissions,soil organic matter decomposition rates,and soil C stocks are scarce.This is a concern because hydrological changes are predicted to lead to increases in the height,extent,and duration of seasonal floods,which are likely to have a significant effect on soil C stocks and fluxes.However,with no empirical data,the impact of altered flood regimes on várzea soil C cycles remains uncertain.This study quantified the effects of maximum annual flood height and soil moisture on soil CO_(2)efflux rate(R_(s))and soil organic matter decomposition rate(k)in the várzea forests of Pacaya Samiria National Reserve,Peru.The study was conducted between May and August 2017.The results showed that R_(s)(10.6–182.7 mg C m^(-2)h^(-1))and k(0.016–0.078)varied between and within sites,and were considerably lower than the values reported for other tropical forests.In addition,R_(s)was negatively affected by flood height(P<0.01)and soil moisture(P<0.001),and it decreased with decreasing river levels post flooding(P<0.001).In contrast,k was not affected by any of the above-mentioned factors.Soil moisture was the dominant factor influencing R_(s),and it was significantly affected by maximum flood height,even after the floods had subsided(P<0.001).Consequently,we concluded that larger floods will likely lead to reduced R_(s),whilst k could remain unchanged but with decomposition processes becoming more anaerobic.

Comparative analysis of planted and unplanted controls for assessment of rhizosphere priming effect

The rhizosphere priming effect(RPE)is increasingly being considered to be an important regulator of soil organic matter(SOM)decomposition and nutrient turnover,with potential importance for the global CO_(2) budget.As a result,studies on the RPE have rapidly increased in number over the last few years.Most of these experiments have been performed using unplanted soil as the control,which could potentially lead to incorrect assessment of the RPE.Therefore,we performed a greenhouse experiment to investigate how the choice of control(i.e.,unplanted control and planted control)influenced the quantification of RPE on SOM decomposition and gross nitrogen(N)mineralization,and to link this to differences in microbial and abiotic soil properties between the two controls.In the planted control,planted seedlings were cut at soil surface 5 d before measurement of the RPE.The RPE on SOM decomposition was positive in pine soil and almost 2-fold higher when calculated from the planted control than from the unplanted control.In spruce soil,a negative RPE on SOM decomposition was found when calculated from the planted control,while the RPE was positive when calculated from the unplanted control.No RPE on gross N mineralization was found when calculated from the planted control,while a positive RPE of more than 100%was found when calculated from the unplanted control.The microbial biomass and growth rate were lower,while the inorganic N content was higher in the unplanted control than in the planted control.The microbial community composition and potential enzyme activity in the planted treatment and planted control were similar,but they differed significantly from those in the unplanted control.The results showed that the RPE varied widely depending on the choice of control;thus,we suggest that a planted control,in which the aboveground plant parts are removed only a few days before the measurement of RPE,should be used as the control when elucidating the RPE on belowground C and N cycling responses to environmental change.