Macrophyte beds in a subtropical reservoir shifted from a nutrient sink to a source after drying then rewetting

1. Macrophytes play a key role in assimilating and storing nutrients in shallow aquatic ecosystems, but their capacity to act as a long‐term nutrient sink can be affected by water level fluctuations. Water level drawdown in reservoirs followed by rewetting may mobilise a significant nutrient pool. These nutrients can be stored in the littoral zone in dead or dormant macrophytes, and in the desiccated sediments within the macrophyte beds. However, the contribution of desiccated macrophyte beds to nutrient release upon rewetting has not been well quantified.
2. Our study examined the effect of rewetting the previously desiccated waterlily Nymphoides indica (Menyanthaceae) in treatments (1) without sediments (N.i.?Sed), and (2) with sediments in N. indica beds (N.i.+Sed) on water quality.
3. We found that longer drying duration increased dissolved nutrients (nitrate/nitrite, ammonium and total dissolved organic nitrogen/phosphorus) and organic carbon release from N.i.+Sed and N.i.?Sed treatments after rewetting. In the N.i.+Sed treatment with <4 weeks of desiccation, all N. indica plants regenerated from roots after subsequent rewetting. In addition, the resulting nutrient/carbon release was not significantly different to the control treatment which did not have desiccation. A significant increase in dissolved nutrient and carbon concentrations in the water column was found in treatments with more than 10 weeks of desiccation followed by rewetting. This coincided with the sediment reaching its minimum moisture content. Furthermore, chlorophyll a (Chl‐a) concentrations in the overlying water also increased with more than 10 weeks of desiccation, presumably in response to the increased nutrient availability and the removal of competition for nutrients from macrophytes.
4. On the basis of our laboratory experiments, we calculated the potential effect of desiccated and rewetted N. indica beds on water quality in a local N. indica‐dominated reservoir after water level drawdown. We also separated the contribution of N. indica plants from their macrophyte beds on water quality changes. Fourteen days after rewetting, the total dissolved nutrients released from N. indica alone (N.i.?Sed) could contribute 0.5% of the total nitrogen and 29% of total phosphorus to the water column concentrations in the whole reservoir. In contrast, N. indica beds (N.i.+Sed) contributed more total dissolved nitrogen (4.3%) but less total dissolved phosphorus (0.3%) release into the water column. The higher nitrogen release for the N.i.+Sed treatment was likely due to the organic matter decomposition in the sediment in macrophyte beds. In contrast, the less dissolved phosphorus release, compared with the N. indica alone, was likely the result of phosphate adsorption by previously desiccated soil particles and/or assimilation by phytoplankton, since the phytoplankton biomass (as measured by Chl‐a concentrations) was significantly higher in the N.i.+Sed treatment.
5. This study highlights the importance of managing both the duration and rate of water level drawdown in reservoirs to prevent rooted macrophytes, like N. indica, from becoming a source of nutrients, which may cause deterioration in water quality.

     

Upslope development of a tidal marsh as a function of upland land use

To thrive in a time of rapid sea‐level rise, tidal marshes will need to migrate upslope into adjacent uplands. Yet little is known about the mechanics of this process, especially in urbanized estuaries, where the adjacent upland is likely to be a mowed lawn rather than a wooded natural area. We studied marsh migration in a Long Island Sound salt marsh using detailed hydrologic, edaphic, and biotic sampling along marsh‐to‐upland transects in both wooded and lawn environments. We found that the overall pace of marsh development was largely unaffected by whether the upland being invaded was lawn or wooded, but the marsh‐edge plant communities that developed in these two environments were quite different, and some indicators (soil salinity, foraminifera) appeared to migrate more easily into lawns. In addition, we found that different aspects of marsh structure and function migrated at different rates: Wetland vegetation appeared to be a leading indicator of marsh migration, while soil characteristics such as redox potential and surface salinity developed later in the process. We defined a ‘hydrologic migration zone’, consisting of elevations that experience tidal inundation with frequencies ranging from 20% to 0.5% of high tides. This hydrologically defined zone – which extended to an elevation higher than the highest astronomical tide datum – captured the biotic and edaphic marsh‐upland ecotone. Tidal inundation at the upper border of this migration zone is highly variable over time and may be rising more rapidly than mean sea level. Our results indicate that land management practices at the upland periphery of tidal marshes can facilitate or impede ecosystem migration in response to rising sea level. These findings are applicable to large areas of tidal marsh along the U.S. Atlantic coast and in other urbanized coastal settings.     

Hydrological regulation drives regime shifts: evidence from paleolimnology and ecosystem modeling of a large shallow Chinese lake

Quantitative evidence of sudden shifts in ecological structure and function in large shallow lakes is rare, even though they provide essential benefits to society. Such ‘regime shifts’ can be driven by human activities which degrade ecological stability including water level control (WLC) and nutrient loading. Interactions between WLC and nutrient loading on the long‐term dynamics of shallow lake ecosystems are, however, often overlooked and largely underestimated, which has hampered the effectiveness of lake management. Here, we focus on a large shallow lake (Lake Chaohu) located in one of the most densely populated areas in China, the lower Yangtze River floodplain, which has undergone both WLC and increasing nutrient loading over the last several decades. We applied a novel methodology that combines consistent evidence from both paleolimnological records and ecosystem modeling to overcome the hurdle of data insufficiency and to unravel the drivers and underlying mechanisms in ecosystem dynamics. We identified the occurrence of two regime shifts: one in 1963, characterized by the abrupt disappearance of submerged vegetation, and another around 1980, with strong algal blooms being observed thereafter. Using model scenarios, we further disentangled the roles of WLC and nutrient loading, showing that the 1963 shift was predominantly triggered by WLC, whereas the shift ca. 1980 was attributed to aggravated nutrient loading. Our analysis also shows interactions between these two stressors. Compared to the dynamics driven by nutrient loading alone, WLC reduced the critical P loading and resulted in earlier disappearance of submerged vegetation and emergence of algal blooms by approximately 26 and 10 years, respectively. Overall, our study reveals the significant role of hydrological regulation in driving shallow lake ecosystem dynamics, and it highlights the urgency of using multi‐objective management criteria that includes ecological sustainability perspectives when implementing hydrological regulation for aquatic ecosystems around the globe.     

A meta‐analysis of soil salinization effects on nitrogen pools,cycles and fluxes in coastal ecosystems

Salinity intrusion caused by land subsidence resulting from increasing groundwater abstraction, decreasing river sediment loads and increasing sea level because of climate change has caused widespread soil salinization in coastal ecosystems. Soil salinization may greatly alter nitrogen (N) cycling in coastal ecosystems. However, a comprehensive understanding of the effects of soil salinization on ecosystem N pools, cycling processes and fluxes is not available for coastal ecosystems. Therefore, we compiled data from 551 observations from 21 peer‐reviewed papers and conducted a meta‐analysis of experimental soil salinization effects on 19 variables related to N pools, cycling processes and fluxes in coastal ecosystems. Our results showed that the effects of soil salinization varied across different ecosystem types and salinity levels. Soil salinization increased plant N content (18%), soil NH₄⁺ (12%) and soil total N (210%), although it decreased soil NO₃^? (2%) and soil microbial biomass N (74%). Increasing soil salinity stimulated soil N₂O fluxes as well as hydrological NH₄⁺ and NO₂^? fluxes more than threefold, although it decreased the hydrological dissolved organic nitrogen (DON) flux (59%). Soil salinization also increased the net N mineralization by 70%, although salinization effects were not observed on the net nitrification, denitrification and dissimilatory nitrate reduction to ammonium in this meta‐analysis. Overall, this meta‐analysis improves our understanding of the responses of ecosystem N cycling to soil salinization, identifies knowledge gaps and highlights the urgent need for studies on the effects of soil salinization on coastal agro‐ecosystem and microbial N immobilization. Additional increases in knowledge are critical for designing sustainable adaptation measures to the predicted intrusion of salinity intrusion so that the productivity of coastal agro‐ecosystems can be maintained or improved and the N losses and pollution of the natural environment can be minimized.     

黄土丘陵区撂荒群落演替序列种根系对氮素施肥方式和水平的形态响应

采用盆栽试验,研究了黄土丘陵区撂荒群落演替序列种(即,黄土丘陵区摞荒群落演替主要阶段的优势种)根系对氮素施肥方式和水平的形态响应,对了解我国氮沉降增加背景下的群落生态效应及人为施肥干扰促进植被恢复具有较好的理论和实践意义。测试并分析了6个演替序列种在不同施氮方式(匀质和异质施氮)和水平(高、低和无氮对照)条件下植株个体生物量指标(地上及地下生物量和根冠比)、根系形态指标(根长、直径、表面积、比根长和比表面积)的变化及其差异显著性;并且利用根钻法和单样本T检验比较了异质施氮方式下施氮斑块与不施氮斑块根系形态指标的差异。结果表明:1)6种演替序列种地上、地下生物量和根冠比存在种间固有差别,施氮方式和水平整体上对三者无显著影响;施氮方式和植物种类对根冠比存在显著交互作用,说明个别种的根冠比对施氮方式响应明显,其中猪毛蒿根冠比在异质施氮方式下显著高于匀质施氮。2)6种演替序列种根系塑形指标包括比根长、比表面积和直径存在种间差别,并且施氮水平对比根长影响显著,高、低施氮水平下比根长都显著低于不施氮对照。3)狗尾草和铁杆蒿分别在异质高氮和异质低氮条件下施氮斑块根系生物量密度显著高于未施氮斑块;猪毛蒿在异质高氮条件下施氮斑块发生了更多的伸长生长,其根长、根表面积、比根长和比表面积在施氮斑块中的密度显著高于未施氮斑块;猪毛蒿和狗尾草在异质高氮条件下,以及白羊草在异质低氮条件下,其根系直径在施氮斑块显著小于未施氮斑块。从根系形态变化敏感性和施氮对促进植物生长来看,演替过程中演替序列种对施氮响应的敏感性总体上呈降低趋势,前期种对施氮响应更敏感,从施氮获利也更多,因而恢复前期进行人为干扰促进植被恢复效果也会更好。     

Nucleotide sequence and characterization of a maize cytoplasmic ribosomal protein S11 cDNA

We isolated a Zea mays cDNA encoding the 40S subunit cytoplasmic ribosomal protein S11. The nucleotide sequence was determined and the derived amino acid sequence compared to the corresponding Arabidopsis thaliana protein showing an homology of 90%. This ribosomal protein is encoded by a small multigene family of at least two members. The mRNA steady-state level is about one order of magnitude higher in rapidly growing parts of the plant such as the roots and shoots of seedlings compared to fully expanded leaf tissue.     

Effects of the supply levels and ratios of nitrogen and phosphorus on seed traits of Chenopodium glaucum

全球氮沉降不仅改变土壤氮和磷的有效性, 同时也改变氮磷比例。氮磷供应量、比例及其交互作用可能会影响植物种子性状。该研究在内蒙古草原基于沙培盆栽实验种植灰绿藜(Chenopodium glaucum), 设置3个氮磷供应量水平和3个氮磷比例的正交实验来探究氮磷供应量、比例及其交互作用对灰绿藜种子性状的影响。结果发现氮磷供应量对种子氮浓度、磷浓度和萌发率影响的相对贡献(15%-24%)大于氮磷比例(3%-7%), 而种子大小只受氮磷比例的影响。同时氮磷供应量和比例之间的交互作用显著影响种子氮浓度和磷浓度。同等氮磷比例情况下, 低量养分供应提高种子氮浓度、磷浓度和萌发率。氮磷比例只有在养分匮乏的环境中才会对种子大小和萌发率产生显著影响。总之, 灰绿藜种子不同性状对氮或磷限制的敏感性不同, 同时种子性状也对养分限制表现出适应性和被动响应。     

Trophic functional groups and trophic levels of the macrobenthic community at the eastern tidal flat of Lingkun Island, China

A total of 33 taxa, including 21 families, 8 classes and 7 phyla, were found at the eastern tidal flat of Lingkun Island, Wenzhou Bay. Most of them were mollusks, crustaceans and polychaetes. The species numbers found at 3 tidal flats (high, middle and low) were 25, 30 and 14, respectively. It was shown that the high tidal flat had the highest value of diversity indices, followed by middle and low tidal flats. The species belonged to 5 different trophic functional groups: Phytophagous, Camivorous, Omnivorous, Planktophagous and Detritivorous in qualitative samplings. Then 3, 8, 5, 3 and 2 species belonged to Planktophagous, Phytophagous, Camivorous, Omnivorous and Detritivorous groups, respectively, in quantitative samplings. The density and species number of each functional group did not display seasonal variation by the two-way ANOVA method. Regardless of seasonal variation, density of the Phytophagous group and Camivorous group had distinct difference among different zones of tidal flats, and the species number of the Phytophagous group and Omnivorous group had obvious difference among different zones of tidal flats. Results show that there was variation of functional groups among habitats. Change in components of the macrobenthic community reflected different food sources in different zones of tidal flats.