Efficiently allocating nitrogen (N) across space maximizes crop productivity for a given amount of N input and reduces N losses to the environment. Here we quantify changes in the global spatial efficiency of cropland N use by calculating historical trade-off frontiers relating N inputs to possible N yield assuming efficient allocation. Time series cropland N budgets from 1961 to 2009 characterize the evolution of N input-yield response functions across 12 regions and are the basis for constructing trade-off frontiers. Improvements in agronomic technology have substantially increased cropping system yield potentials and expanded N-driven crop production possibilities. However, we find that these gains are compromised by the declining spatial efficiency of N use across regions. Since the start of the Green Revolution, N inputs and yields have moved farther from the optimal frontier over time; in recent years (1994–2009), global N surplus has grown to a value that is 69% greater than what is possible with efficient N allocation between regions. To reflect regional pollution and agricultural development goals, we construct scenarios that restrict reallocation, finding that these changes only slightly decrease potential gains in nitrogen use efficiency. Our results are inherently conservative due to the regional unit of analysis, meaning a larger potential exists than is quantified here for cross-scale policies to promote spatially efficient N use.
Bibliographical noteFunding Information:
Funding for N.D.M. was provided by postdoctoral fellowships from the Harvard Center for the Environment and the USDA Agriculture and Food Research Initiative. Funding for J.S.G. is from the Gordon and Betty Moore Foundation, with additional support from the Belmont Forum/FACCE-JPI DEVIL project (NE/M02127/1). The data used for this study can be found in the supporting information for Lassaletta et al. .
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- environmental nitrogen losses
- global cropland
- nitrogen allocation
- nitrogen use efficiency