The impact of the unharvested reeds on the nitrogen and phosphorus cycles in Lake Dongting

Nutrient regeneration patterns for initiating and maintaining algae blooms-a case study of in Lake Taihu

Temporal and spatial variations in kinetics of alkaline phosphatase in sediments of a shallow Chinese eutrophic lake (Lake Donghu)

Surface, overlying, and interstitial waters were collected at monthly intervals at three experimental stations in a shallow Chinese eutrophic lake (Lake Donghu) to assess the occurrence, distribution, and status of UV-sensitive phosphorus compounds (UVSP) and phosphatase hydrolyzable phosphorus (PHP), coupled with kinetics of alkaline phosphatase activity (APA). Orthophosphate (o-P) concentrations were generally the highest at Station I, where chlorophyll α (chl α) was a function of o-P at temporal scale. The Vmax/Km of APA obtained by Michaelis-Menten approach paralleled the chlorophyll data at two stations. These facts imply that the development of phytoplankton may be attributed to APA induced by PHP. The potentially available UVSP and PHP peaked in interstitial, overlying, and surface water simultaneously sometimes in 1995 to 1996 and 1997 to 1999. It is postulated that they may arise from the bottom. UVSP peaked in interstitial water at the 12–16 cm layers in sediment cores. Moreover, in interstitial water, UV irradiation resulted in an elevated o-P concentration and decreased APA in a timeseries analysis. Therefore, the mechanism that APA involved in the process of photorelease of o-P was not demonstrated. UVSP is most likely a functional group of labile phosphorus distinct from the enzymatic substrate in this shallow eutrophic lake.

As a large river connected lake, Dongting Lake is influenced by anthropogenic activities and the discharge from its upstream tributaries in the lake basin and by the water recharge via a connection to the Yangtze River (YR) outside the basin. This makes the lake phosphorous cycle more complex than that in other disconnected lakes. Here, we calculated section fluxes and ran a hydrodynamic model to investigate the phosphorus (P) variations in response to the changing interactions in the water and sediment between the YR, four tributaries, and the lake. Results show that particulate P was the dominant form with a significant linear relationship with suspended sediment (r 2=0.906). The sediment input reduction from the YR through three water inlets, which is closely related to the Three Gorges Reservoir operation since 2003, led to a decrease in the total P (TP) concentration in the western Dongting Lake. However, the impact and range of this decrease were fairly limited. Compared with the limited effect of the YR, the raised TP flux from the Yuanjiang tributary controlled the TP concentration at the outlet of the western Dongting Lake. Apart from the influence of the YR and the tributaries, anthropogenic activities (sand dredging) in the eastern Dongting Lake also contributed to a high TP concentration around the S10 area through sediment resuspension. We suggest that, compared with the reduction in TP flux and sediment load from the connected Yangtze River outside the basin, the elements within the basin (increased TP input from tributaries and sand dredging) have a greater effect on the variations of TP in Dongting Lake.

Nutrients regeneration pathway, release potential, transformation pattern and algal utilization strategies jointly drove cyanobacterial growth and their succession

The sediments in mangrove forests play an important role in the global carbon cycle due to high inputs of organic matter (OM) and low decomposition rates, making them highly efficient at sequestering carbon. The balance between OM sequestration and decomposition in these systems is influenced by a complex interplay of environmental factors. However, there is a large amount of uncertainty surrounding decomposition rates from mangrove forests, particularly at regional scales. We used standardized decomposition assays of a labile and recalcitrant substrate in 30 estuaries, spanning a gradient in human land use intensity, to identify dominant drivers of OM decomposition in temperate mangrove forests. Our results reveal that, while labile OM decomposition is strongly driven by eutrophication, recalcitrant OM decomposition is primarily influenced by increases in the minimum sediment temperature. Furthermore, we demonstrate that nutrient enrichment from human land use, in combination with increased sediment temperature, synergistically accelerates the decomposition of labile OM, thereby threatening the carbon sequestration potential of these ecosystems. This suggests that coastal eutrophication can exacerbate the effects of warming on decomposition, leading to heightened vulnerability of carbon storage and potential feedbacks between local and global stressors.

<p>Predicting the pattern of soil organic matter (SOM) decomposition as a feedback to climate change, via release of CO<sub>2</sub>, is extremely complex and has received much attention. However, investigations often do not differentiate between the extracellular and intracellular processes involved and work is needed to identify their relative temperature sensitivities. Samples were collected from a grassland soil at Sonning, UK with average daily maximum and minimum soil temperature of 15 °C and 5 °C. We measured potential activities of β-glucosidase (BG) and chitinase (NAG) (extracellular enzymes) and glucose-induced CO<sub>2 </sub>respiration (intracellular enzymes) at a range of assay temperatures (5 °C, 15 °C, 26 °C, 37<sup>  </sup>°C, and 45 °C). The temperature coefficient Q<sub>10</sub> (the increase in enzyme activity that occurs after a 10 °C increase in soil temperature) was calculated to assess the temperature sensitivity of intracellular and extracellular enzymes activities. Between 5 °C and 15 °C intracellular and extracellular enzyme activities had equal temperature sensitivity, but between 15 °C and 26°C intracellular enzyme activity was more temperature sensitive than extracellular enzyme activity and between 26 °C and 37 °C extracellular enzyme activity was more temperature sensitive than intracellular enzyme activity. This result implies that extracellular depolymerisation of higher molecular weight organic compounds is more sensitive to temperature changes at higher temperatures (e.g. changes to daily maximum summer temperature) but the intracellular respiration of the generated monomers is more sensitive to temperature changes at moderate temperatures (e.g. changes to daily mean summer temperature). We therefore conclude that the extracellular and intracellular steps of SOM mineralisation are not equally sensitive to changes in soil temperature. The finding is important because we have observed greater increases in average daily minimum temperatures than average daily mean or maximum temperatures due to increased cloud cover and sulphate aerosol emission. Accounting for this asymmetrical global warming may reduce the importance of extracellular depolymerisation and increase the importance of intracellular catalytic activities as the rate limiting step of SOM decomposition.</p>

Spatiotemporal distribution and controlling factors on ammonium in waters in the central Yangtze River Basin, China

Extracellular enzymes play a key role in microbe‐mediated organic matter decomposition in soils, and the efficiency of these enzymes in substrate decomposition depends on their mobility and specific activity in soils. In this work, we explored the influence of biochar adsorption on extracellular enzyme activity across a spectrum of environmental conditions, from simple to complex. Batch adsorption results showed that biochar adsorption of two hydrolytic enzymes—α‐amylase and amyloglucosidase (AMG)—similarly decreases with pH and follows the Langmuir isotherm, suggesting electrostatic interaction between them. Activity of AMG and alkaline phosphatase (ALP), which belong to carbon and phosphorus cycling enzymes, was measured using a novel calorimetric method. The technique demonstrated advantages over conventional enzyme assays, such as in situ real‐time measurement of reaction rate and the ability to identify potential interferences. The technique enabled the measurement of specific activity of biochar‐adsorbed AMG, which ranged from 10% to 90% of that of free AMG. The effect of substrate adsorption on activity measurement was demonstrated through the examination of two substrates for ALP, which suggested the use of effective substrate concentration (instead of nominal concentration) in calculating enzyme activity kinetics. Soil column experiments showed that biochar amendment can affect the activity of AMG in starch hydrolysis through changing the mobility of AMG (and accessibility to substrate) and its specific activity. Results from this work improve our understanding of the effects of biochar adsorption on enzyme activity and suggest the need to appropriately interpret enzyme activity data and account for confounding processes.

模拟氮沉降对温带森林土壤酶活性的影响

Tunnel construction in watershed area of urban lakes would accelerate eutrophication by inputting nutrients into them, while mechanisms underlying the internal phosphorus cycling as affected by construction events are scarcely studied. Focusing on two main pathways of phosphorus releasing from sediment (enzymatic mineralization and anaerobic desorption), spatial and temporal variations in phosphorus fractionation, and activities of extracellular enzymes (alkaline phosphatase, β-1,4-glucosidase, leucine aminopeptidase, dehydrogenase, lipase) in sediment were examined, together with relevant parameters in interstitial and surface waters in a Chinese urban lake (Lake Donghu) where a subaqueous tunnel was constructed across it from October 2013 to July 2014. Higher alkaline phosphatase activity (APA) indicated phosphorus deficiency for phytoplankton, as illustrated by a significantly negative relationship between APA and concentration of dissolved total phosphorus (DTP). Noticeably, in the construction area, APAs in both sediment and surface water were significantly lower than those in other relevant basins, suggesting a phosphorus supply from some sources in this area. In parallel, its sediment gave the significantly lower iron-bound phosphorus (Fe(OOH)∼P) content, coupled with significantly higher ratio of iron (II) to total iron content (Fe(2+)/TFe) and dehydrogenase activities (DHA). Contrastingly, difference in the activities of sediment hydrolases was not significant between the construction area and other basins studied. Thus, in the construction area, subsidy of bioavailable phosphorus from sediment to surface water was attributable to the anaerobic desorption of Fe(OOH)∼P rather than enzymatic mineralization. Finally, there existed a significantly positive relationship between chlorophyll a concentration in surface water and Fe(OOH)∼P content in sediment. In short, construction activities within lakes may interrupt cycling patterns of phosphorus across sediment-water interface by enhancing release of redox-sensitive phosphate, and thereby facilitating phytoplankton growth in water column.

Global meta-analysis on the responses of soil extracellular enzyme activities to warming

Temperature sensitivity of extracellular enzymes differs with peat depth but not with season in an ombrotrophic bog

This study aims to understand the influence of salinity and labile organic matter removal on the fate and behavior of metals in coastal technosols. Two technosol cores were collected near the Lebanese shore. The cores were sectioned into layers; each layer was characterized for pH, salinity, electric conductivity, labile and total organic matter, grain size, and total and oxalate-extractable metals. Consequently, two saline solutions were used in desorption experiments to understand the role of ionic strength and labile organic matter on metal release. The results showed that the technosol layers were highly heterogeneous; most layers were enriched with Fe, Zn, Pb, and Cu. The mineralogical investigations showed that the metals, notably Fe, were not present as crystalline minerals, rather big percentages of the metals were found in amorphous or poorly crystalline phases. Desorption experiment showed that Mg release was dependent on salinity and organic matter in both technosols, while Pb release was dependent on both factors only in one. Additionally, Zn and Cu were associated to organic matter, and their release was conditioned by the removal of labile organic matter; iron was primarily found as amorphous or poorly crystalline phases, and salinity had a major role in its release. The role of ionic strength and labile organic matter removal on the behavior of metals in technosols was demonstrated. Finally, understanding metal dynamics between the solid and liquid compartments in technosols, especially where salt deposition occurs, is important to reduce unwanted metal leaching to groundwater or seawater and transfer to biota.

Labile soil organic matter as influenced by cropping practices in an arid environment