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Rapid demographic ageing substantially affects socioeconomic development1–4and presents considerable challenges for food security and agricultural sustainability5–8, which have so far not been well understood. Here, by using data from more than 15,000 rural households with crops but no livestock across China, we show that rural population ageing reduced farm size by 4% through transferring cropland ownership and land abandonment (approximately 4 million hectares) in 2019, taking the population age structure in 1990 as a benchmark. These changes led to a reduction of agricultural inputs, including chemical fertilizers, manure and machinery, which decreased agricultural output and labour productivity by 5% and 4%, respectively, further lowering farmers’ income by 15%. Meanwhile, fertilizer loss increased by 3%, resulting in higher pollutant emissions to the environment. In new farming models, such as cooperative farming, farms tend to be larger and operated by younger farmers, who have a higher average education level, hence improving agricultural management. By encouraging the transition to new farming models, the negative consequences of ageing can be reversed. Agricultural input, farm size and farmer’s income would grow by approximately 14%, 20% and 26%, respectively, and fertilizer loss would reduce by 4% in 2100 compared with that in 2020. This suggests that management of rural ageing will contribute to a comprehensive transformation of smallholder farming to sustainable agriculture in China.

Farm size affects nitrogen fertilizer input and agricultural practices, which are key determinants of ammonia (NH3) emissions from croplands. However, the degree to which NH3emissions are associated with changes in farm size is not well understood yet despite its crucial role in achieving agricultural sustainability in China, where agricultural production is still dominated by smallholder farms. Here we provide a first analysis of the relationship between farm size and NH3emissions based on 863 000 surveys conducted in 2017 across China. Results show that NH3emissions (kg ha–1) on average decrease by 0.07% for each 1% increase in average farm size. This change occurs mainly due to a reduction in nitrogen fertilizer use and the introduction of more efficient fertilization practices. The largest reduction in NH3emissions is found in maize, with less pronounced changes in rice cultivation, and none for wheat production. Overall lower NH3emissions factors can be observed in the north of China with increasing farm size, especially in the northeast, the opposite pattern was found in the south. National total NH3emissions could be approximately halved (1.5 Tg) in a scenario favoring a conversion to large-scale farming systems. This substantial reduction potential highlights the potential of such a transition to reduce NH3emissions, including benefits from a socioeconomic point of view as well as for improving air quality.

Freshwater is a scarce resource, and maintaining water quality is of great importance in dryland Australia. How water quality is affected by the dry climate and socio-economic influences in Australia remains widely unknown. Here, we find that agriculture activity dominates reactive nitrogen (Nr) emissions to water bodies. Such emissions not only contribute to deteriorating water quality in Southeastern Australia but also harm marine ecosystems, including the Great Barrier Reef, a World Natural Heritage site. A dry and warm climate reduces the share of Nremitted directly to water bodies; however, it increases the Nrconcentration in surface water due to reduced water volume, leading to a 3-fold higher water Nrconcentration compared to major rivers globally, e.g., in the US or China. Business-as-usual socioeconomic development would increase the total Nremitted to surface water by at least 43% by 2050, while effective mitigation measures could reduce N runoff by about 27%. Advanced agricultural management strategies should be considered to reduce future environmental pressures due to N runoff in Australia.

Reducing ammonia (NH3) volatilization from croplands while satisfying the food demand is strategically required to mitigate haze pollution. However, the global pattern of NH3volatilization remains uncertain, primarily because of the episodic nature of NH3volatilization rates and the high variation of fertilization practices. Here, we improve a global estimate of crop-specific NH3emissions at a high spatial resolution using an updated data-driven model with a survey-based dataset of the fertilization scheme. Our estimate of the globally averaged volatilization rate (12.6% ± 2.1%) is in line with previous data-driven studies (13.7 ± 3.1%) but results in one-quarter lower emissions than process-based models (16.5 ± 3.1%). The associated global emissions are estimated at 14.4 ± 2.3 Tg N, with more than 50% of the total stemming from three stable crops or 12.2% of global harvested areas. Nearly three-quarters of global cropland-NH3emissions could be reduced by improving fertilization schemes (right rate, right type, and right placement). A small proportion (20%) of global harvested areas, primarily located in China, India, and Pakistan, accounts for 64% of abatement potentials. Our findings provide a critical reference guide for the future abatement strategy design when considering locations and crop types.

ABSTRACTAir pollution presents a major risk to human health, resulting in premature deaths and reduced quality of life. Quantifying the role of vegetation in reducing air pollution concentrations is an important contribution to urban natural capital accounting. However, most current methods to calculate pollution removal are static, and do not represent atmospheric transport of pollutants, or interactions among pollutants and meteorology. An additional challenge is defining urban extent in a way that captures the green and blue infrastructure providing the service in a consistent way. We developed a refined urban morphology layer which incorporates urban green and blue space. We then applied an atmospheric chemistry transport model (EMEP4UK) to calculate pollutant removal by urban natural capital for pollutants including PM2.5, NO2, SO2, O3. We calculated health benefits directly from the change in pollutant concentrations (i.e. exposure) rather than from tonnes of pollutant removed. Urban natural capital across Britain removes 28,700 tonnes of PM2.5, NO2, SO2, O3. The economic value of the health benefits are substantial: £136 million in 2015, resulting from 900 fewer respiratory hospital admissions, 220 fewer cardiovascular hospital admissions, 240 fewer deaths and 3600 fewer Life Years Lost.

Managing reactive nitrogen (Nr) to achieve a sustainable balance between production of food, feed and fiber, and environmental protection is a grand challenge in the context of an increasingly affluent society. Here, we propose a novel framework for national nitrogen (N) assessments enabling a more consistent comparison of the uses, losses and impacts of Nrbetween countries, and improvement of Nrmanagement for sustainable development at national and regional scales. This framework includes four key components: national scale N budgets, validation of N fluxes, cost-benefit analysis and Nrmanagement strategies. We identify four critical factors for Nrmanagement to achieve the sustainable development goals: N use efficiency (NUE), Nrrecycling ratio (e.g., ratio of livestock excretion applied to cropland), human dietary patterns and food waste ratio. This framework was partly adopted from the European Nitrogen Assessment and now is successfully applied to China, where it contributed to trigger policy interventions toward improvements for future sustainable use of Nr. We demonstrate how other countries can also benefit from the application our framework, in order to include sustainable Nrmanagement under future challenges of growing population, hence contributing to the achievement of some key sustainable development goals (SDGs).

AbstractFrom a complexity science perspective, urban health and well-being challenges emerge due to the complexity of urban systems. Adverse urban health outcomes emerge from failing to respond to that complexity by taking a systems approach in knowledge and action which would open opportunity spaces for human agents to create benefits which in turn would generate salutogenic health and well-being outcomes. Lessons learned from complexity science suggest that adverse urban health outcomes emerge from a poor understanding of their complexity and from not engaging with them in a transdisciplinary, integrated fashion. A conceptual framework is presented which combines systems models from the natural and social sciences and explains how opportunities for advancing health and well-being can be co-created. The framework demonstrates that taking a systems approach is a necessary cognitive response from learning the lessons of complexity science and from understanding that humans are an inextricable part of the systems they aim at understanding and managing. Such response is at the core of systems intelligence. The implications are far reaching for the science of urban health and well-being.

CITY KNOW-HOWHuman health and planetary health are both influenced by city lifestyles, city leadership, and city development. For both, worrying trends are leading to increasing concern. It is imperative that both become core foci in urban policy. Changing the trajectory will require concerted action. The journal Cities & Healthjournal is dedicated to supporting the flow of knowledge, in all directions to help make this happen. We want to support communication between researchers, practitioners, policy-makers, communities and decision-makers in cities. This is the purpose of this City Know-how section of the journal. ‘Research for city practice’ disseminates lessons from research, explaining the key messages for city leaders, communities and the professions involved in city policy and practice. ‘City shorts’ provide glimpses of what is being attempted or achieved. ‘Case studies’ are where you will find evaluations of interventions and ‘Commentary and debate’ helps extend the conversations we are having and develop much needed new thinking. Join in these conversations. In service to strengthening the community of interest, we would like to include many and varied voices, including those from younger practitioners and researchers, connected with supporting health and health equity in everyday urban lives.

Digital technology is having a major impact on many areas of society, and there is equal opportunity for impact on science. This is particularly true in the environmental sciences as we seek to understand the complexities of the natural environment under climate change. This perspective presents the outcomes of a summit in this area, a unique cross-disciplinary gathering bringing together environmental scientists, data scientists, computer scientists, social scientists, and representatives of the creative arts. The key output of this workshop is an agreed vision in the form of a framework and associated roadmap, captured in the Windermere Accord. This accord envisions a new kind of environmental science underpinned by unprecedented amounts of data, with technological advances leading to breakthroughs in taming uncertainty and complexity, and also supporting openness, transparency, and reproducibility in science. The perspective also includes a call to build an international community working in this important area.

The Quadrennial Ozone Symposium 2016