Graham K. MacDonald

Agronomic phosphorus imbalances across the world's croplands

Increased phosphorus (P) fertilizer use and livestock production has fundamentally altered the global P cycle. We calculated spatially explicit P balances for cropland soils at 0.5° resolution based on the principal agronomic P inputs and outputs associated with production of 123 crops globally for the year 2000. Although agronomic inputs of P fertilizer (14.2 Tg of P·y−1) and manure (9.6 Tg of P·y−1) collectively exceeded P removal by harvested crops (12.3 Tg of P·y−1) at the global scale, P deficits covered almost 30% of the global cropland area. There was massive variation in the magnitudes of these P imbalances across most regions, particularly Europe and South America. High P fertilizer application relative to crop P use resulted in a greater proportion of the intense P surpluses (>13 kg of P·ha−1·y−1) globally than manure P application. High P fertilizer application was also typically associated with areas of relatively low P-use efficiency. Although manure was an important driver of P surpluses in some locations with high livestock densities, P deficits were common in areas producing forage crops used as livestock feed. Resolving agronomic P imbalances may be possible with more efficient use of P fertilizers and more effective recycling of manure P. Such reforms are needed to increase global agricultural productivity while maintaining or improving freshwater quality.

Climate variation explains a third of global crop yield variability

Many studies have examined the role of mean climate change in agriculture, but an understanding of the influence of inter-annual climate variations on crop yields in different regions remains elusive. We use detailed crop statistics time series for ~13,500 political units to examine how recent climate variability led to variations in maize, rice, wheat and soybean crop yields worldwide. While some areas show no significant influence of climate variability, in substantial areas of the global breadbaskets, >60% of the yield variability can be explained by climate variability. Globally, climate variability accounts for roughly a third (~32–39%) of the observed yield variability. Our study uniquely illustrates spatial patterns in the relationship between climate variability and crop yield variability, highlighting where variations in temperature, precipitation or their interaction explain yield variability. We discuss key drivers for the observed variations to target further research and policy interventions geared towards buffering future crop production from climate variability.

Leverage points for improving global food security and the environment

How to optimize global food production Keeping societies stable and managing Earths resources sustainably depend on doing a good, steady job producing and distributing food. West et al. asked what combinations of crops and regions offer the best chance of progress. Their analysis focused on reducing greenhouse gas emissions, nutrient pollution, water use, and food waste. They identify regions that are likely to yield the best balance between applying fertilizer to increase crop yields versus the resulting environmental impact. Science, this issue p. 325 A limited set of interventions could disproportionately improve crop production and environmental sustainability. Achieving sustainable global food security is one of humanity’s contemporary challenges. Here we present an analysis identifying key “global leverage points” that offer the best opportunities to improve both global food security and environmental sustainability. We find that a relatively small set of places and actions could provide enough new calories to meet the basic needs for more than 3 billion people, address many environmental impacts with global consequences, and focus food waste reduction on the commodities with the greatest impact on food security. These leverage points in the global food system can help guide how nongovernmental organizations, foundations, governments, citizens’ groups, and businesses prioritize actions.

An index-based framework for assessing patterns and trends in river fragmentation and flow regulation by global dams at multiple scales

The global number of dam constructions has increased dramatically over the past six decades and is forecast to continue to rise, particularly in less industrialized regions. Identifying development pathways that can deliver the benefits of new infrastructure while also maintaining healthy and productive river systems is a great challenge that requires understanding the multifaceted impacts of dams at a range of scales. New approaches and advanced methodologies are needed to improve predictions of how future dam construction will affect biodiversity, ecosystem functioning, and fluvial geomorphology worldwide, helping to frame a global strategy to achieve sustainable dam development. Here, we respond to this need by applying a graph-based river routing model to simultaneously assess flow regulation and fragmentation by dams at multiple scales using data at high spatial resolution. We calculated the cumulative impact of a set of 6374 large existing dams and 3377 planned or proposed dams on river connectivity and river flow at basin and subbasin scales by fusing two novel indicators to create a holistic dam impact matrix for the period 1930?2030. Static network descriptors such as basin area or channel length are of limited use in hierarchically nested and dynamic river systems, so we developed the river fragmentation index and the river regulation index, which are based on river volume. These indicators are less sensitive to the effects of network configuration, offering increased comparability among studies with disparate hydrographies as well as across scales. Our results indicate that, on a global basis, 48% of river volume is moderately to severely impacted by either flow regulation, fragmentation, or both. Assuming completion of all dams planned and under construction in our future scenario, this number would nearly double to 93%, largely due to major dam construction in the Amazon Basin. We provide evidence for the importance of considering small to medium sized dams and for the need to include waterfalls to establish a baseline of natural fragmentation. Our versatile framework can serve as a component of river fragmentation and connectivity assessments; as a standardized, easily replicable monitoring framework at global and basin scales; and as part of regional dam planning and management strategies.

Integrating legacy soil phosphorus into sustainable nutrient management strategies for future food, bioenergy and water security

Legacy phosphorus (P) that has accumulated in soils from past inputs of fertilizers and manures is a large secondary global source of P that could substitute manufactured fertilizers, help preserve critical reserves of finite phosphate rock to ensure future food and bioenergy supply, and gradually improve water quality. We explore the issues and management options to better utilize legacy soil P and conclude that it represents a valuable and largely accessible P resource. The future value and period over which legacy soil P can be accessed depends on the amount present and its distribution, its availability to crops and rates of drawdown determined by the cropping system. Full exploitation of legacy P requires a transition to a more holistic system approach to nutrient management based on technological advances in precision farming, plant breeding and microbial engineering together with a greater reliance on recovered and recycled P. We propose the term ‘agro-engineering’ to encompass this integrated approach. Smaller targeted applications of fertilizer P may still be needed to optimize crop yields where legacy soil P cannot fully meet crop demands. Farm profitability margins, the need to recycle animal manures and the extent of local eutrophication problems will dictate when, where and how quickly legacy P is best exploited. Based on our analysis, we outline the stages and drivers in a transition to the full utilization of legacy soil P as part of more sustainable regional and global nutrient management.

Embodied phosphorus and the global connections of United States agriculture

Agricultural phosphorus (P) use is intricately linked to food security and water quality. Globalization of agricultural systems and changing diets clearly alter these relationships, yet their specific influence on non-renewable P reserves is less certain. We assessed P fertilizer used for production of food crops, livestock and biofuels in the US agricultural system, explicitly comparing the domestic P use required for US food consumption to the P use embodied in the production of US food imports and exports. By far the largest demand for P fertilizer throughout the US agricultural system was for feed and livestock production (56% of total P fertilizer use, including that for traded commodities). As little as 8% of the total mineral P inputs to US domestic agriculture in 2007 (1905 Gg P) was consumed in US diets in the same year, while larger fractions may have been retained in agricultural soils (28%), associated with different post-harvest losses (40%) or with biofuel refining (10%). One quarter of all P fertilizer used in the US was linked to export production, primarily crops, driving a large net P flux out of the country (338 Gg P). However, US meat consumption relied considerably on P fertilizer use in other countries to produce red meat imports. Changes in domestic farm management and consumer waste could together reduce the P fertilizer required for US food consumption by half, which is comparable to the P fertilizer reduction attainable by cutting domestic meat consumption (44%). US export-oriented agriculture, domestic post-harvest P losses and global demand for meat may ultimately have an important influence on the lifespan of US phosphate rock reserves.

A tradeoff frontier for global nitrogen use and cereal production

Nitrogen fertilizer use across the world’s croplands enables high-yielding agricultural production, but does so at considerable environmental cost. Imbalances between nitrogen applied and nitrogen used by crops contributes to excess nitrogen in the environment, with negative consequences for water quality, air quality, and climate change. Here we utilize crop input-yield models to investigate how to minimize nitrogen application while achieving crop production targets. We construct a tradeoff frontier that estimates the minimum nitrogen fertilizer needed to produce a range of maize, wheat, and rice production levels. Additionally, we explore potential environmental consequences by calculating excess nitrogen along the frontier using a soil surface nitrogen balance model. We find considerable opportunity to achieve greater production and decrease both nitrogen application and post-harvest excess nitrogen. Our results suggest that current (circa 2000) levels of cereal production could be achieved with ∼50% less nitrogen application and ∼60% less excess nitrogen. If current global nitrogen application were held constant but spatially redistributed, production could increase ∼30%. If current excess nitrogen were held constant, production could increase ∼40%. Efficient spatial patterns of nitrogen use on the frontier involve substantial reductions in many high-use areas and moderate increases in many low-use areas. Such changes may be difficult to achieve in practice due to infrastructure, economic, or political constraints. Increases in agronomic efficiency would expand the frontier to allow greater production and environmental gains. S Online supplementary data available from stacks.iop.org/ERL/9/054002/mmedia

Land-Use Legacies Are Important Determinants of Lake Eutrophication in the Anthropocene

Background A hallmark of the latter half of the 20th century is the widespread, rapid intensification of a variety of anthropogenically-driven environmental changes—a “Great Acceleration.” While there is evidence of a Great Acceleration in a variety of factors known to be linked to water quality degradation, such as conversion of land to agriculture and intensification of fertilizer use, it is not known whether there has been a similar acceleration of freshwater eutrophication. Methodology/Principal Findings Using quantitative reconstructions of diatom-inferred total phosphorus (DI-TP) as a proxy for lake trophic state, we synthesized results from 67 paleolimnological studies from across Europe and North America to evaluate whether most lakes showed a pattern of eutrophication with time and whether this trend was accelerated after 1945 CE, indicative of a Great Acceleration. We found that European lakes have experienced widespread increases in DI-TP over the 20th century and that 33% of these lakes show patterns consistent with a post-1945 CE Great Acceleration. In North America, the proportion of lakes that increased in DI-TP over time is much lower and only 9% exhibited a Great Acceleration of eutrophication. Conclusions/Significance The longer and more widespread history of anthropogenic influence in Europe, the leading cause for the relatively pervasive freshwater eutrophication, provides an important cautionary tale; our current path of intensive agriculture around the world may lead to an acceleration of eutrophication in downstream lakes that could take centuries from which to recover.

Variability in ecosystem service measurement: a pollination service case study

Research quantifying ecosystem services (ES) - collectively, the benefits that society obtains from ecosystems -is rapidly increasing. Despite the seemingly straightforward definition, a wide varie ...

Environmental health impacts of feeding crops to farmed fish.

Half of the seafood consumed globally now comes from aquaculture, or farmed seafood. Aquaculture therefore plays an increasingly important role in the global food system, the environment, and human health. Traditionally, aquaculture feed has contained high levels of wild fish, which is unsustainable for ocean ecosystems as demand grows. The aquaculture industry is shifting to crop-based feed ingredients, such as soy, to replace wild fish as a feed source and allow for continued industry growth. This shift fundamentally links seafood production to terrestrial agriculture, and multidisciplinary research is needed to understand the ecological and environmental health implications. We provide basic estimates of the agricultural resource use associated with producing the top five crops used in commercial aquaculture feed. Aquacultures environmental footprint may now include nutrient and pesticide runoff from industrial crop production, and depending on where and how feed crops are produced, could be indirectly linked to associated negative health outcomes. We summarize key environmental health research on health effects associated with exposure to air, water, and soil contaminated by industrial crop production. Our review also finds that changes in the nutritional content of farmed seafood products due to altered feed composition could impact human nutrition. Based on our literature reviews and estimates of resource use, we present a conceptual framework describing the potential links between increasing use of crop-based ingredients in aquaculture and human health. Additional data and geographic sourcing information for crop-based ingredients are needed to fully assess the environmental health implications of this trend. This is especially critical in the context of a food system that is using both aquatic and terrestrial resources at unsustainable rates.