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Opinion: A “more ammonium solution” to mitigate nitrogen pollution and boost crop yields

Two of the world’s great agricultural challenges require bold new approaches and could share a solution. Nitrogen (N) pollution, affecting water, air, and the climate, presents one massive challenge. Ninety percent of increased reactive N originates as synthetic fertilizer applied to agricultural fields or N fixed in them (1). Because crops take up only 42 to 47% of the total applied N, more than half is lost to the environment in some way (23).

G. V. Subbarao and Timothy D. Searchinger; PNAS; June 1, 2021 118 (22) e2107576118

 

Two of the world’s great agricultural challenges require bold new approaches and could share a solution. Nitrogen (N) pollution, affecting water, air, and the climate, presents one massive challenge. Ninety percent of increased reactive N originates as synthetic fertilizer applied to agricultural fields or N fixed in them (1). Because crops take up only 42 to 47% of the total applied N, more than half is lost to the environment in some way (23). Despite some recent regional improvements in nitrogen use efficiency (NUE), global average NUE has not increased since 1980. Yet even if by 2050 the world increased NUE by 50% (to ∼70%), likely 50% increases in food production would maintain N losses to the environment at roughly their present, unacceptable levels.

Figure: We can address pollution and boost crop yields by exploiting new tools that keep a higher share of soil nitrogen as ammonium while selecting and breeding crops to exploit an ammonium/nitrate balance. Nitrification-inhibiting traits originally discovered in some tropical grasses can be enhanced in cereal crops too. Image credit: Flickr/CIAT.

 

A second challenge is to increase crop yields at a more rapid (linear) rate in coming decades to meet rising food demands without clearing more forests and releasing their carbon (3). Just as yield growth has historically resulted from synergies of crop breeding and management changes, future gains must rely on doing both in even smarter ways. Large yield growth by adding fertilizer or doubling irrigation is no longer possible or environmentally acceptable in most of the world (35).

 

The scope of these challenges requires multiple new approaches, and here we make the case for a “more ammonium solution.” With the exception of paddy rice, nitrate dominates the inorganic N in modern agricultural soils, leading to most N pollution problems. This solution would exploit new tools to keep a higher share of soil N as ammonium and select and breed crops to exploit an ammonium/nitrate balance.

 

First, it’s important to ask how more ammonium (NH4+) and less nitrate (NO3) would address nitrogen pollution. Although 20% of total N losses from field-applied N occurs through volatilization of ammonia (NH3SI Appendix and Table 3 in 6), the great majority of N losses occur after microbial reactions transform ammonium in soils into nitrate, typically in fewer than 10 days (7). As an anion, NO3 does not bind to soil (with limited exceptions) and thus easily leaches with water into groundwater and waterways. The formation and breakdown of NO3 by bacteria and archaea also release nitrous oxide, a powerful greenhouse gas, as well as more NOx, which contributes to air pollution problems. Once N nitrifies into NO3, unless crops or grasses quickly take it up, it has a good chance of polluting the environment.

 

Most promising strategies to reduce N pollution can only do so much because they focus on the “front end” by reducing fertilizer application. Strategies include educating farmers to restrict fertilizer to economically optimal rates, tools to help farmers apply most N during the growing season in amounts that account for N mineralized from soils (8), and possibly even bacteria that help cereals fix nitrogen (9). These measures could increase NUE, but substantial losses will continue in part because half of global, field-applied N occurs not in fertilizer but in manure, crop residues, air deposition, and irrigation water (2). More fundamentally, applying N more carefully cannot, by itself, prevent N from continuing to leak at the back end, which will continue so long as N turns into nitrate.

 

Significant back-end losses will continue because N continues to mineralize into nitrate from soil organic matter (SOM) even late in growing seasons—releasing N absorbed into soils in previous years (SI Appendix). Because this mineralization occurs when crops no longer take up N, it can easily turn into nitrate and escape. In some regions most N leaching occurs in winter when rains leach out this mineralized, inorganic N (1011). Cover crops provide one promising back-end strategy for annual cropping systems, but adoption rates are low and cover crops face various practical challenges (SI Appendix). To solve the N problem, agriculture needs additional tools to reduce this leakiness at the back end.

 

See: https://www.pnas.org/content/118/22/e2107576118

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