Effect of urban form on PM2.5 concentrations in urban agglomerations of China: Insights from different urbanization levels and seasons.

Planned urban form has become an important strategy to improve air quality in urban agglomerations (UAs), especially pollution due to PM2.5, but the influencing mechanisms are not yet clear. This study explores the relationship between four metrics of urban form (size, fragmentation, shape, and dispersion) as determined by analysis of remotely sensed images at 30-m resolution and PM2.5 concentrations in 19 Chinese UAs. The influence of level of urban development and season is examined. Five control variables, including population density, temperature, precipitation, wind speed, and the normalized difference vegetation index (NDVI) are selected for use in multiple linear regression models. Size, fragmentation, and shape of urban form, but not dispersion, were found to have significant effects on PM2.5 concentrations of different urbanization-level UAs. Urban size and fragmentation have stronger impacts on PM2.5 concentrations in UAs with lower urbanization levels while urban shape has a greater impact in higher-level UAs. In terms of seasonal variation in all UAs, urban form is more pronouncedly associated with PM2.5 concentrations during spring and autumn than summer and winter. Urban size and fragmentation are positively associated with PM2.5 concentrations whereas urban shape and dispersion are on the contrary. The relationships between urban form and PM2.5 uncovered here underscore the importance of urban planning as a tool to minimize PM2.5 pollution. Specifically, local government should encourage polycentric urban form with lower fragmentation in urban agglomerations. UAs with lower urbanization levels should control the disordered expansion of construction land and higher-level UAs should promote the mix of green land and construction land. Moreover, measures to control air pollution from anthropogenic activities in spring, autumn and winter are likely to be more effective in decreasing PM2.5 concentrations in UAs.

The influence of urban form compactness on CO2 emissions and its threshold effect: Evidence from cities in China.

Although compact urban form plays an important role in constraining emissions of carbon dioxide (CO2), the boundary for the impact of compact urban form on these emissions has nevertheless received little attention. We consequently applied the entropy weight method and several key landscape metrics to a dataset from 295 cities in China to quantify urban form compactness (UFC) between 2000 and 2015. The STIRPAT model then was employed to estimate the impact of UFC on CO2 emissions, and a panel threshold regression model was used to estimate threshold effects capable of limiting the impact of compact urban form on emissions. Although CO2 emissions increased sharply over the 15-year study period, a significant negative relationship between UFC and CO2 emissions was detected. Two thresholds of UFC were detected, and this allowed three categories to be differentiated: before the first threshold, between the two thresholds, and after the second threshold. These categories were respectively associated with no impact, strong impact, and weak impact of UFC on reduction of carbon emissions in the 295 cities. Carbon emissions reduction consequently becomes effective when the UFC exceeds the first threshold and effectiveness persists but at a reduced level when the UFC exceeds the second threshold. Further temporal analysis confirmed that an increasing number of mostly small- and medium-sized cities could constrain their future carbon emissions by adopting a compact urban form. Thus, government policies should emphasize UFC as a strategy to reduce CO2 emissions. Moreover, by defining the range of compact urban form that has the greatest impact on CO2 emissions, our study deepens the overall understanding of the influence of UFC on carbon emission reductions, so as to make contributions to the design of low-carbon cities.

Early Detection and Mitigation of Resistance to Bt Maize by Western Corn Rootworm (Coleoptera: Chrysomelidae).

Transgenic Bt maize that produces less than a high-dose has been widely adopted and presents considerable insect resistance management (IRM) challenges. Western corn rootworm, Diabrotica virgifera virgifera LeConte, has rapidly evolved resistance to Bt maize in the field, leading to local loss of efficacy for some corn rootworm Bt maize events. Documenting and responding to this resistance has been complicated by a lack of rapid diagnostic bioassays and by regulatory triggers that hinder timely and effective management responses. These failures are of great concern to the scientific and agricultural community. Specific challenges posed by western corn rootworm resistance to Bt maize, and more general concerns around Bt crops that produce less than a high-dose of Bt toxin, have caused uncertainty around current IRM protocols. More than 15 years of experience with IRM has shown that high-dose and refuge-based IRM is not applicable to Bt crops that produce less than a high-dose. Adaptive IRM approaches and pro-active, integrated IRM-pest management strategies are needed and should be in place before release of new technologies that produce less than a high-dose. We suggest changes in IRM strategies to preserve the utility of corn rootworm Bt maize by 1) targeting local resistance management earlier in the sequence of responses to resistance and 2) developing area-wide criteria to address widespread economic losses. We also favor consideration of policies and programs to counteract economic forces that are contributing to rapid resistance evolution.

Direct measurements of dissolved N2 and N2O highlight the strong nitrogen (N) removal potential of riverine wetlands in a headwater stream.

Increasing levels of nitrogen (N) in aquatic ecosystems due to intensified human activities is focusing attention on N removal mechanisms as a means to mitigate environmental damage. Important N removal processes such as denitrification can resolve this issue by converting N to gaseous emissions. Here, the spatiotemporal variability of N removal rates in China's Zhongtian River, a headwater stream that contains wetlands, was investigated by quantifying gaseous emissions of the main end products, N2 and N2O, using the water-air exchange model. Excess concentrations of these gases relative to their saturations in the water column generally varied within 1.4-8.7 µmol L-1 and 8.7-20.3 nmol L-1, with mean values of 4.5 µmol L-1 and 13.7 nmol L-1, respectively, demonstrating significant N removal in the river. The reach with wetlands was characterized by higher in-stream N2 production than the non-wetland reach, especially in July, when aquatic vegetation is most abundant. High N2O emissions during the same period in the non-wetland reach indicate that environmental conditions associated with vegetation are conducive to N2 production and likely constrain N2O emission. Changes in dissolved oxygen, pH, temperature, and carbon to nitrogen ratios are correlated with the observed spatiotemporal variabilities in gaseous N production. The mean N removal rate in the wetland reach was roughly twice that in the non-wetland reach, i.e., 22.4 vs. 10.3 mmol N m-2 d-1, while the corresponding efficiency was about five times as high, i.e., 15 % vs. 3 %. This study reveals the spatiotemporal patterns of in-stream N removal in a headwater stream and highlights the efficacy of wetlands in N removal. The data provide a strong rationale for constructing artificial wetlands as a means to mitigate N pollution and thereby optimize riverine environmental conditions.

Restored riverine wetlands in a headwater stream can simultaneously behave as sinks of N2O and hotspots of CH4 production.

Wetlands can improve water quality, but they are also recognized as important sources of greenhouse gases (GHG) such as nitrous oxide (N2O) and methane (CH4). Emissions of these gases from wetland ecosystems, especially those in headwaters, are poorly understood. Here, we determined monthly concentrations of dissolved N2O and CH4 in a headwater stream of the Taihu Lake basin of China that contains both wetland and non-wetland reaches. Daily GHG dynamics in the wetland reach were also investigated. Riverine N2O and CH4 concentrations generally varied within 10-30 nmol L-1 and 0.1-1.5 µmol L-1, respectively. CH4 saturation levels in the wetland reach were about seven times higher than those in the non-wetland reach, but there was no difference in N2O saturation. In the wetland reach, saturation levels of CH4 peaked in July, coincident with a dip in N2O saturation to levels below its saturated solubility. This underscores that hotspots of CH4 production and sinks for N2O can occur occasionally in wetlands in mid-summer, when vegetative growth and microbial activities are high. Diurnal measurements indicated that CH4 saturation in water flows passing through the wetlands from midnight through the early morning can surge to levels 10 times higher than those detected at other times of the day. Simultaneously, saturation levels of N2O decreased by 75%, indicating a net consumption of N2O. Changes in nutrient supply determined by upstream inflows, as well as dissolved oxygen, pH, and other environmental factors mediated by the wetlands, correlate with the differentiated behavior of N2O and CH4 production in wetlands. Additional work will be necessary to confirm the roles of these factors in regulating GHG emissions in riverine wetlands.

Modeling phosphorus sources and transport in a headwater catchment with rapid agricultural expansion.

Increasing riverine phosphorus (P) levels in headwaters due to expanded and intensified human activities are worldwide concerns, because P is a well-known limiting nutrient for freshwater eutrophication. Here we adopt the conceptual framework of the SPAtially Referenced Regressions On Watershed attributes (SPARROW) model to describe total phosphorus (TP) sources and transport in a headwater watershed undergoing rapid agricultural expansion in the upper Taihu Lake Basin, China. Our models, which include variables for land cover, river length, runoff depth, and pond density, explain 94% of the spatio-temporal variability in TP loads. Agricultural lands contribute the largest percentage (61%) of the TP loads delivered downstream, followed by forestland (21%) and urban land (18%). Future agricultural expansion to 15% of the total basin area is possible, which could lead to a 50% increase in TP loads. According to our analysis, an average of 24% of the total P export from the watershed landscape was intercepted in ponds. The exported amount was subsequently retained by tributaries and along the mainstem river, accounting for 14% and 43% of their inflowing loads, respectively. The remaining ∼6 tons yr-1 of TP was eventually transported into Tianmu Lake, in Southeastern China. The model identified several sub-catchments as hotspots of TP loss and thus logical sites for targeted management. Our study underscores the significance of agricultural expansion as a factor that can exacerbate headwater TP pollution, highlighting the importance of landscapes to buffer TP losses from sensitive hilly catchments. This also points to a need for an integrated management strategy that considers the spatial-varying P sources and associated transport of TP in precious headwater resources.

Extracellular proteins involved in soybean cultivar-specific nodulation are associated with pilus-like surface appendages and exported by a type III protein secretion system in Sinorhizobium fredii USDA257.

Several gram-negative plant and animal pathogenic bacteria have evolved a type III secretion system (TTSS) to deliver effector proteins directly into the host cell cytosol. Sinorhizobium fredii USDA257, a symbiont of soybean and many other legumes, secretes proteins called Nops (nodulation outer proteins) into the extracellular environment upon flavonoid induction. Mutation analysis and the nucleotide sequence of a 31.2-kb symbiosis (sym) plasmid DNA region of USDA257 revealed the existence of a TTSS locus in this symbiotic bacterium. This locus includes rhc (rhizobia conserved) genes that encode components of a TTSS and proteins that are secreted into the environment (Nops). The genomic organization of the TTSS locus of USDA257 is remarkably similar to that of another broad-host range symbiont, Rhizobium sp. strain NGR234. Flavonoids that activate the transcription of the nod genes of USDA257 also stimulate the production of novel filamentous appendages known as pili. Electron microscope examination of isolated pili reveals needle-like filaments of 6 to 8 nm in diameter. The production of the pili is dependent on a functional nodD1 and the presence of a nod gene-inducing compound. Mutations in several of the TTSS genes negate the ability of USDA257 to elaborate pili. Western blot analysis using antibodies raised against purified NopX, Nop38, and Nop7 reveals that these proteins were associated with the pili. Mutations in rhcN, rhcJ, rhcC, and ttsI alter the ability of USDA257 to form nodules on Glycine max and Macroptilium atropurpureum.

The effect of FITA mutations on the symbiotic properties of Sinorhizobium fredii varies in a chromosomal-background-dependent manner.

nodD1 of Sinorhizobium fredii HH103, which is identical to that of S. fredii USDA257 and USDA191, repressed its own expression. Spontaneous flavonoid-independent transcription activation (FITA) mutants of S. fredii HH103 M (=HH103 RifR pSym::Tn 5-Mob) showing constitutive expression of nod genes were isolated. No differences were found among soybean cultivar Williams plants inoculated with FITA mutants SVQ250 or SVQ253 or with the parental strain HH103M. Soybean plants inoculated with mutant SVQ255 formed more nodules, and those inoculated with mutant SVQ251 had symptoms of nitrogen starvation. Sequence analyses showed that all of the FITA mutants carried a point mutation in their nodD1 coding region. Mutants SVQ251 and SVQ253 carried the same mutation, but only the former was symbiotically impaired, which indicated the presence of an additional mutation elsewhere in the genome of mutant SVQ251. Mutants SVQ251 and SVQ255 were outcompeted by the parental strain for nodulation of soybean cultivar Williams. The symbiotic plasmids of mutants SVQ251 and SVQ255 (pSym251 and pSym255, respectively) and that (pSymHH103M) of the parental strain were transferred to pSym-cured derivatives of S. fredii USDA192 and USDA193 (USDA192C and USDA193C, respectively). Soybean responses to inoculation with S. fredii USDA192C and USDA193C transconjugants carrying pSym251 and pSymHH103M were not significantly different, whereas more nodules were formed after inoculation with transconjugants carrying pSym255. Only transconjugant USDA192C(pSym255) produced a significant increase in soybean dry weight.