Decrease in macrofauna density increases the sediment phosphorus release and maintains the high phosphorus level of water column in Lake Taihu: A case study on Grandidierella taihuensis.

Internal phosphorus (P) loading can increase the P level in the water column and further sustains cyanobacterial blooms. This study focused on the role of benthic fauna bioturbation in affecting the sediment P release and the P level of water column in a eutrophic lake, Lake Taihu. The macrofauna density decreased from 4766.56 ± 10541.80 ind/m2 in 2007 to 345 ± 447.63 ind/m2 in 2020 due to the frequent bottom-water hypoxia in Lake Taihu. The reduced macrofauna density majorly resulted from Grandidierella taihuensis, Limnodrilus hoffmeisteri, and Tanypus chinensis larvae, and their total density decreased by approximately 97% in 2020 compared to 2007. G. taihuensis, one of the major benthic faunas, was further used as a representative to investigate the effects of bioturbation on sediment P release using high-resolution sampling and imaging techniques. The results show that G. taihuensis can increase the O2 penetration depth by more than 20 mm through bio-irrigation, and causes the redox conditions in burrows and surrounding sediments to change dramatically within a few minutes due to the intermittent ventilation. Subsequent oxidation of the soluble Fe(II) led to the formation of Fe-oxide bound P in the surface sediments, thereby increasing the P retention in the sediments. When the G. taihuensis density was 1563 ind/m2 at the sampling site, approximately 0.12 g m-2 yr-1 P can be retained in sediments. As previous studies have shown that L. hoffmeisteri and T. chinensis played a similar role in increasing the P retention in sediments through their bioturbation activities, the sharp decline in benthic fauna density and burrowing activities in Lake Taihu should be an important reason for maintaining the high P level in the water column by decreasing the P retention in sediments.

Phosphorus acquisition strategy of Vallisneria natans in sediment based on in situ imaging techniques.

Phosphorus (P) availability is closely related to the distributions of pH, O2 and phosphatase activities in the rhizosphere of plants growing in soils and sediments. In this study, the P uptake processes and mechanisms of Vallisneria natans (V. natans) during two vegetation periods (i.e., week three and six) were revealed using three noninvasive 2D imaging techniques: planar optode (PO), diffusive gradients in thin films (DGT) and zymography. The results showed that increased phosphatase activity, O2 concentration and root-induced acidification were observed together in the rhizosphere of root segments and tips. In week three, when V. natans was young, the flux of DGT-labile P accumulated more in the rhizosphere in comparison with the bulk sediment. This was because increased phosphatase activity (of up to 35%) and root-induced acidification (with pH decreasing by up to 0.25) enhanced P acquisition of V. natans by the third week. However, the flux of DGT-labile P turned to depletion during weeks three to six of V. natans growth, after Fe plaque formed at the matured stage. The constant hydrolysis of phosphatase and acidification could not compensate for the P demand of the roots by the sixth week. At this stage, Fe plaque become the P pool, due to P fixation with solid Fe(III) hydroxides. Subsequently, V. natans roots acquired P from Fe plaque via organic acid complexation of Fe(III).

O2 distribution and dynamics in the rhizosphere of Phragmites australis, and implications for nutrient removal in sediments.

Root-triggered microscale variations in O2 distribution in the rhizosphere of young Phragmites australis are important for nutrient removal in sediments. In this study, the micro-scale O2 dynamics and the small-scale changes of soluble reactive phosphorus (SRP) and ammonium (NH4+) in the rhizosphere of P. australis were investigated using planar optodes and high-resolution dialysis (HR-Peeper), respectively. Results suggested that root O2 leakage has a highly variable distribution depending on the stage of root growth, the site of O2 leakage gradually shift from the entire emerging main roots to the main root tip and subsequently shifted the emerging lateral roots. The O2 concentration increased in the rhizosphere with increasing light intensity and O2 levels in the overlying water. Continuous O2 release from the lateral roots causes the formation of iron plaque on the surface of lateral roots, which reduce the mobility of P by adsorption of iron plaque in the rhizosphere. The oscillation of oxic-anoxic root zones improves nitrogen removal through the processes of anammox, heterotrophic denitrification and nitrification. This work from the micro-scale demonstrates that the O2 concentration is the spatio-temporal variations in the rhizosphere, and it presents an important role for nutrient removal in sediments.

Divergent patterns of soil phosphorus discharge from water-level fluctuation zone after full impoundment of Three Gorges Reservoir, China.

Phosphorus (P) discharged from soils in the water-level fluctuation (WLF) zone becomes increasingly important to the water quality control of the Three Gorges Reservoir (TGR) as the decrease in P input from upstream reaches and point-source pollution. To investigate the amount of soil P discharge from the WLF zone since the full impoundment of the TGR in 2010, soil and sediment samples were collected along the altitudinal gradients (140, 150, 160, 170, and 180 m above sea level) in three transects in the middle reaches of the TGR. Soil P composition was determined by a sequential extraction procedure. Different amounts of P discharge from the WLF zone were found among three soil types because of their difference in the initial P content before impoundment, with an order of yellow earth (171.1 g m-2), fluvo-aquic soil (141.7 g m-2), and purple soil (73.8 g m-2). An altitudinal pattern of soil P discharge was observed with the maximum at the 170-m sites. The downward transport of exchangeable P and clay-bound P with runoff was the major path of the soil P discharge at the 170-m sites with a slope gradient > 15°. Considerable P discharge with erosion at the upper section of the WLF zone was facilitated by the longer exposure period compared with that at bottom section (150-m sites) because of the annual anti-seasonal impoundment-exposure cycles of the TGR. The transformation of Al/Fe-P and subsequent release to water was a main mechanism of the soil P discharge during the impoundment period. The altitudinal pattern of P discharge was a result of joint effects of slope gradient, soil P forms, and the anti-seasonal hydrological regime of the TGR. The results highlight the critical role of the upper section (165-175 m) in controlling the P output from the WLF zone into the water of the TGR.