Nathaniel D. Mueller

Solutions for a cultivated planet

Increasing population and consumption are placing unprecedented demands on agriculture and natural resources. Today, approximately a billion people are chronically malnourished while our agricultural systems are concurrently degrading land, water, biodiversity and climate on a global scale. To meet the world’s future food security and sustainability needs, food production must grow substantially while, at the same time, agriculture’s environmental footprint must shrink dramatically. Here we analyse solutions to this dilemma, showing that tremendous progress could be made by halting agricultural expansion, closing ‘yield gaps’ on underperforming lands, increasing cropping efficiency, shifting diets and reducing waste. Together, these strategies could double food production while greatly reducing the environmental impacts of agriculture.

Closing yield gaps through nutrient and water management

In the coming decades, a crucial challenge for humanity will be meeting future food demands without undermining further the integrity of the Earth’s environmental systems. Agricultural systems are already major forces of global environmental degradation, but population growth and increasing consumption of calorie- and meat-intensive diets are expected to roughly double human food demand by 2050 (ref. 3). Responding to these pressures, there is increasing focus on ‘sustainable intensification’ as a means to increase yields on underperforming landscapes while simultaneously decreasing the environmental impacts of agricultural systems. However, it is unclear what such efforts might entail for the future of global agricultural landscapes. Here we present a global-scale assessment of intensification prospects from closing ‘yield gaps’ (differences between observed yields and those attainable in a given region), the spatial patterns of agricultural management practices and yield limitation, and the management changes that may be necessary to achieve increased yields. We find that global yield variability is heavily controlled by fertilizer use, irrigation and climate. Large production increases (45% to 70% for most crops) are possible from closing yield gaps to 100% of attainable yields, and the changes to management practices that are needed to close yield gaps vary considerably by region and current intensity. Furthermore, we find that there are large opportunities to reduce the environmental impact of agriculture by eliminating nutrient overuse, while still allowing an approximately 30% increase in production of major cereals (maize, wheat and rice). Meeting the food security and sustainability challenges of the coming decades is possible, but will require considerable changes in nutrient and water management.

Yield trends are insufficient to double global crop production by 2050.

Several studies have shown that global crop production needs to double by 2050 to meet the projected demands from rising population, diet shifts, and increasing biofuels consumption. Boosting crop yields to meet these rising demands, rather than clearing more land for agriculture has been highlighted as a preferred solution to meet this goal. However, we first need to understand how crop yields are changing globally, and whether we are on track to double production by 2050. Using ∼2.5 million agricultural statistics, collected for ∼13,500 political units across the world, we track four key global crops—maize, rice, wheat, and soybean—that currently produce nearly two-thirds of global agricultural calories. We find that yields in these top four crops are increasing at 1.6%, 1.0%, 0.9%, and 1.3% per year, non-compounding rates, respectively, which is less than the 2.4% per year rate required to double global production by 2050. At these rates global production in these crops would increase by ∼67%, ∼42%, ∼38%, and ∼55%, respectively, which is far below what is needed to meet projected demands in 2050. We present detailed maps to identify where rates must be increased to boost crop production and meet rising demands.

Recent patterns of crop yield growth and stagnation

In the coming decades, continued population growth, rising meat and dairy consumption and expanding biofuel use will dramatically increase the pressure on global agriculture. Even as we face these future burdens, there have been scattered reports of yield stagnation in the worlds major cereal crops, including maize, rice and wheat. Here we study data from ∼2.5 million census observations across the globe extending over the period 1961-2008. We examined the trends in crop yields for four key global crops: maize, rice, wheat and soybeans. Although yields continue to increase in many areas, we find that across 24-39% of maize-, rice-, wheat- and soybean-growing areas, yields either never improve, stagnate or collapse. This result underscores the challenge of meeting increasing global agricultural demands. New investments in underperforming regions, as well as strategies to continue increasing yields in the high-performing areas, are required.

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.

Direct effects, compensation, and recovery in female fathead minnows exposed to a model aromatase inhibitor.

Background Several chemicals in the environment have the potential to inhibit aromatase, an enzyme critical to estrogen synthesis. Objectives The objective of this study was to provide a detailed characterization of molecular and biochemical responses of female fathead minnows to a model aromatase inhibitor, fadrozole (FAD). Methods Fish were exposed via water to 0, 3, or 30 μg FAD/L for 8 days and then held in clean water for 8 days, with samples collected at four time points during each 8-day period. We quantified ex vivo steroid production, plasma steroids, and plasma vitellogenin (Vtg) concentrations and analyzed relative transcript abundance of 10 key regulatory genes in ovaries and 3 in pituitary tissue by real-time polymerase chain reaction. Results Ex vivo 17β-estradiol (E2) production and plasma E2 and Vtg concentrations were significantly reduced after a single day of exposure to 3 μg or 30 μg FAD/L. However, plasma E2 concentrations recovered by the eighth day of exposure in the 3-μg/L group and within 1 day of cessation of exposure in the 30-μg/L group, indicating concentration- and time-dependent physiologic compensation and recovery. Concentration-dependent increases in transcripts coding for aromatase (A isoform), cytochrome P450 side-chain cleavage, steroidogenic acute regulatory protein, and follicle-stimulating hormone receptor all coincided with increased E2 production and recovery of plasma E2 concentrations. Conclusions Results of this research highlight the need to consider compensation/adaptation and recovery when developing and interpreting short-term bioassays or biomarkers or when trying to predict the effects of chemical exposures based on mode of action.

Dynamic nature of alterations in the endocrine system of fathead minnows exposed to the fungicide prochloraz.

The vertebrate hypothalamic-pituitary-gonadal (HPG) axis is controlled through various feedback mechanisms that maintain a dynamic homeostasis in the face of changing environmental conditions, including exposure to chemicals. We assessed the effects of prochloraz on HPG axis function in adult fathead minnows (Pimephales promelas) at multiple sampling times during 8-day exposure and 8-day depuration/recovery phases. Consistent with one mechanism of action of prochloraz, inhibition of cytochrome P450 (CYP) 19 aromatase activity, the fungicide depressed ex vivo ovarian production and plasma concentrations of 17beta-estradiol (E2) in female fish. At a prochloraz water concentration of 30 microg/l, inhibitory effects on E2 production were transitory and did not persist during the 8-day exposure phase. At 300 microg/l prochloraz, inhibition of E2 production was evident throughout the 8-day exposure but steroid titers recovered within 1 day of cessation of exposure. Compensation or recovery of steroid production in prochloraz-exposed females was accompanied by upregulation of several ovarian genes associated with steroidogenesis, including cyp19a1a, cyp17 (hydroxylase/lyase), cyp11a (cholesterol side-chain cleavage), and follicle-stimulating hormone receptor. In male fathead minnows, the 8-day prochloraz exposure decreased testosterone (T) production, possibly through inhibition of CYP17. However, as for E2 in females, ex vivo testicular production and plasma concentrations of T recovered within 1 day of stopping exposure. Steroidogenic genes upregulated in testis included cyp17 and cyp11a. These studies demonstrate the adaptability of the HPG axis to chemical stress and highlight the need to consider the dynamic nature of the system when developing approaches to assess potential risks of endocrine-active chemicals.

Global malnutrition overlaps with pollinator-dependent micronutrient production

Pollinators contribute around 10% of the economic value of crop production globally, but the contribution of these pollinators to human nutrition is potentially much higher. Crops vary in the degree to which they benefit from pollinators, and many of the most pollinator-dependent crops are also among the richest in micronutrients essential to human health. This study examines regional differences in the pollinator dependence of crop micronutrient content and reveals overlaps between this dependency and the severity of micronutrient deficiency in people around the world. As much as 50% of the production of plant-derived sources of vitamin A requires pollination throughout much of Southeast Asia, whereas other essential micronutrients such as iron and folate have lower dependencies, scattered throughout Africa, Asia and Central America. Micronutrient deficiencies are three times as likely to occur in areas of highest pollination dependence for vitamin A and iron, suggesting that disruptions in pollination could have serious implications for the accessibility of micronutrients for public health. These regions of high nutritional vulnerability are understudied in the pollination literature, and should be priority areas for research related to ecosystem services and human well-being.

Effects of a 3β-Hydroxysteroid Dehydrogenase Inhibitor, Trilostane, on the Fathead Minnow Reproductive Axis

A number of environmental contaminants and plant flavonoid compounds have been shown to inhibit the activity of 3beta-hydroxysteroid dehydrogenase/Delta(5)-Delta(4) isomerase (3beta-HSD). Because 3beta-HSD plays a critical role in steroid hormone synthesis, inhibition of 3beta-HSD represents a potentially important mode of endocrine disruption that may cause reproductive dysfunction in fish or other vertebrates. The objective of this study was to test the hypothesis that exposure to the model 3beta-HSD inhibitor, trilostane, would adversely affect reproductive success of the fathead minnow (Pimephales promelas). Results of in vitro experiments with fathead minnow ovary tissue demonstrated that trilostane inhibited 17beta-estradiol (E2) production in a concentration- and time-dependent manner, and that the effect was eliminated by providing a substrate (progesterone) that does not require 3beta-HSD activity for conversion to E2. Exposure of fish to trilostane caused a significant reduction in spawning frequency and reduced cumulative egg production over the course of the 21-day test. In females, exposure to 1500 mug trilostane/l reduced plasma vitellogenin concentrations, but did not cause significant histological alterations. In males, average trilostane concentrations as low as 50 mug/l significantly increased testis mass and gonadal somatic index. Trilostane exposure did not influence the abundance of mRNA transcripts coding for 3beta-HSD or other steroidogenesis-regulating proteins in males or females. As a whole, results of this study support the hypothesis that 3beta-HSD inhibition can cause reproductive dysfunction in fish, but did not yield a clear profile of responses at multiple levels of biological organization that could be used to diagnose this mode of action.

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