First! Quantify global farmland phosphorus budgets at the year, country and crop type levels

On October 12, 2022, researchers at the University of Maryland Center for Environmental Science published a new study titled “Global trends of cropland phosphorus use and sustainability challenges” in the journal Nature.

The group has developed the first database of its kind to quantify the phosphorus budget of farmland around the world at the level of different years, countries and crop types. This database will help countries and regions assess their performance in addressing the challenges of phosphorus pollution and scarcity, and guide action towards a more sustainable future.

The corresponding author of the paper is Zhang Xin; The first author is Zou Tan; Another author is Eric Davidson.

In order to meet the growing food demand in countries and regions, while meeting the twin challenges of increased phosphorus pollution and dwindling phosphate reserves, countries and regions need to improve phosphorus use efficiency (PUE) (the ratio of phosphorus output to phosphorus input in a defined system). There has been a lot of research work to quantify historical phosphorus budgets (inputs and outputs of crop production systems) and phosphorus use efficiency (PUE), as well as to propose current challenges and potential solutions to phosphorus management. While many efforts have been made by countries and regions to improve nutrient management practices in farmland, few studies have examined the historical trajectory of PUE and its socio-economic and agricultural drivers at the level of different years, countries and crop types. In addition, there are fewer studies assessing how improvements in PUE can reduce phosphorus pollution and address the twin challenges of phosphorus scarcity at the national level.

In this work, the research group provides a database of phosphorus budgets and PUEs by country and crop type from 1961 to 2019, and studies the impact of factors such as economic development stage and crop mix on PUE at the national level, as well as the challenges that may be faced globally and nationally under different PUE changes in the future. To meet the challenges of phosphorus management, the PUE for global crop production must increase to 68-81%, and recent trends suggest that many countries have made some clear progress towards this goal. However, the challenges and opportunities for farmland phosphorus management vary widely from country to country, and countries need more specific strategies tailored to national circumstances and the current state of agricultural development.

Figure 1: Phosphorus streams in farmland mainly considered in the study. Blue arrow, the main phosphorus inputs include fertilizer and manure; green arrow, phosphorus yield (i.e. phosphorus in harvested crops); red arrow, phosphorus loss; Black arrow, phosphorus internal soil cycle.

Regionally, socio-economic and agricultural development varies from country to country, leading to different major challenges. Brazil’s surplus, for example, is driven by increased soybean planting on phosphorus-fixing soils. By contrast, by around 2010 China had increased production while also reducing PUE and increasing its phosphorus surplus due to relatively low fertilizer prices, relatively small farm sizes, and expanded production of low-PUE crops such as fruits and vegetables. Although some countries, such as China and India, have made progress in increasing PUE in recent years, many regions still need to reduce phosphorus surpluses, increase phosphorus production, and find solutions to their phosphorus scarcity. The study identified five socioeconomic and agricultural factors that are important to PUE, including nitrogen use efficiency, fertilizer-to-crop price ratio, average farm size, crop mix, and agricultural machinery use.

Figure 2: Historical phosphorus budget and PUE trends from 1961 to 2019. a, phosphorus production and PUE trends in example countries. b. PUE in example countries and phosphorus residues accumulated in soil. The grayscale in a indicates the phosphorus surplus. Data for high, middle and low income economies are blue, green and red, respectively, and PUE data greater than 200% are not shown. Pink areas indicate a negative phosphorus surplus. We used a five-year moving average here to limit the impact of factors such as weather. The seven dots on each line represent the average data for 1961, 1970, 1980, 1990, 2000, 2010, and 2019.

The main phosphorus-related challenge facing the world is to increase crop yields while reducing human interference with the phosphorus cycle to within the estimated planetary boundary. To meet the possible phosphorus pollution challenge, global farmland PUE needs to increase from 65% in 2017 to 68%-81% in 2050, while phosphorus production needs to increase from 15.1 Tg/P/yr in 2017 to 19.3 Tg/P/yr in 2050. The challenge of phosphorus pollution varies widely across regions of the world. Assuming that global farmland is tasked with reducing human interference with the phosphorus cycle on average, the average phosphorus surplus needs to be maintained below 3.5-6.9 kg p/ha/yr. In 2010, the average phosphorus surplus levels in China and India were 24 kgP/ha/yr and 10 kgP/ha/yr, respectively, both above the safety boundary. If the PUE of countries, regions and crops increases to 75th percentile globally, the number of countries with phosphorus surpluses above the safe border will be significantly reduced.

The challenge of phosphorus scarcity is not serious on a global scale, but it is acute in countries with scarce phosphorus resources. Even with the full use of domestic phosphorus resources, manure and soil residual phosphorus, and increased PUE, India, Viet Nam and many other countries will not be able to rely on domestic phosphorus resources to meet production targets by 2050. For countries facing severe phosphorus scarcity challenges (e.g., India, Mexico and Viet Nam), they need appropriate fertilizer import strategies to increase phosphorus recovery in manure, food waste and human fertilizers. Some countries currently dependent on phosphate imports, such as India and Brazil, have abundant sources of phosphate fertilizer, from manure and human fertilizers near farmland. (Source: Web of Science)

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