Global aquatic biodiversity impacts of nitrogen and phosphorus fertiliser use for major crops

Abstract

Purpose
The intensive application of nitrogen and phosphorus fertilisers on agricultural land to fertilise crops has caused eutrophication, the nutrient enrichment of waterbodies leading to excessive growth of algae, deoxygenation and loss of aquatic biodiversity. Life cycle impact assessments (LCIA) are often used to determine the environmental impacts of fertiliser use. However, the lack of suitable methodologies to estimate the fate and transport of nutrients from soils makes crop and regional impact comparisons challenging. Using a newly devised, spatially explicit nutrient fate and transport model (fate factor, FF) within an LCIA framework, this study estimates the global spatial-variability of nutrient loss from fertilisation of crops and their relative impact on aquatic biodiversity, specifically species richness.

Method
The newly devised FFs are based on the global spatially explicit nutrient model IMAGE-GNM. The FF’s enable us to assess N and P’s fate and transport from indirect soil emissions (arable land, grassland and natural land) to freshwater environments. We additionally improve the spatial resolution of existing soil FFs for N within marine environments from basin scale to 5 arcmin resolution. We applied our FF’s within current LCIA methodologies to assess the nutrient loading (midpoint indicator) and final aquatic biodiversity impact (endpoint indicator) from 17 crops.

Results and discussion
Our results identify strong variability in inputs, loadings and impacts due to differences in the fate, transport and impact of nutrients within the local environmental context. Such variability is translated into large differences between the popularly used nutrient use efficiency (NUE) indicator and final aquatic impacts caused by specific crops. Heavily produced crops (maize, rice, wheat, sugarcane and soybean) with the highest loading rates to receptors did not necessarily have the highest aquatic impacts. We identified rank variability existed at different metric stages (fertiliser inputs, receptor loadings, aquatic impacts) specifically for wheat and sugarcane. Our results showed high spatial variability in high aquatic impacts with significant biodiversity loss outside of the highest production regions.

Conclusion
Our study identified global hotspots for biodiversity impacts depend on the local context that exist beyond the field (e.g. the fate and transport of nutrients to receptor environments, and the receptor environment's vulnerability). Aquatic impacts from fertiliser use for specific crop commodities should be considered in strategic fertiliser pollution control decision-making and environmentally sustainable crop-commodity trade sourcing decisions. The development of the improved FFs should be used to aid spatially explicit and site-specific LCIA nutrient studies from soils.

Here we provide the fate factors developed and used in our study to estimate biodiversity impacts from nitrogen and phosphorus fertiliser use.