Fertilizer in the Nitrogen Cycle
What is really going on underground?
As a water and soil science master's student at École Normale Supérieure in France, my research focuses on the infiltration and persistence of nitrogen-based synthetic fertilizers in underground aquifers and aquatic ecosystems. This issue is pressing in France, leading to waterway degradation and disruptions in aquatic ecosystems nationwide. Currently, I'm modeling potential N-fertilizer runoff in the Scorff region of northern France using ModFlow software. By considering factors like water discharge, topography, porosity, and flow rates, I aim to simulate pollutant runoff and infiltration. To deepen my understanding, I'm exploring the global history of N-based fertilizer use and its impact on the biological nitrogen-fixing cycle. Specifically, I'm interested in the synergistic relationship between nitrogen-fixing legumes and bacteria like rhizobium.
How have humans changed the nitrogen cycle?
Since the mid-1960s, an unprecedented agricultural intensification has become evident with a shift in agricultural methods, especially the overuse of synthetic nitrogen-based fertilizers. As a result, grain production has doubled in the past four decades since the Green Revolution. However, this rate of agricultural production increase is not sustainable due to diminishing yields and the impact of modern agricultural practices on the environment.
The main consequence of human activity on the nitrogen cycle is increased nitrogen availability, which overall enhances cycle productivity, leading to substantial biomass accumulation. Today, human activities have caused the fixation of 140 teragrams/year of nitrogen in terrestrial ecosystems, of which approximately 80 teragrams/year are industrially created nitrogen fertilizers.
What are the consequences of increased nitrogen availability?
In the absence of synthetic nitrogen, the productivity and dynamics of terrestrial ecosystems, including agricultural ecosystems, are constrained by the availability of biologically fixed nitrogen. Many food crops can form a symbiotic relationship with nitrogen-fixing soil bacteria called rhizobia. The result of this symbiosis is the formation of nodules on the plant root, within which the bacteria can convert atmospheric nitrogen into ammonia that can be used by the plant. However, the plant-rhizobia relationship is altered by anthropogenic nitrogen emissions (including the the combustion of fossil fuels), the production of nitrogenous fertilizers, and the cultivation of nitrogen-fixing legumes. When nitrogen becomes more accessible within the environment, plants decrease their reliance on their bacterial partners.
As human activity continues to alter biological nitrogen fixation, soil degradation is becoming a major concern. In an agricultural context, the environmental consequence of synthetic chemicals compromising symbiotic nitrogen fixation by bacteria is the increasing reliance on synthetic nitrogen fertilizers, resulting in reduced soil fertility and thus unsustainable long-term agricultural yields.
Although the use of synthetic nitrogen fertilizers is relatively well managed, particularly in Europe, the overall efficiency of synthetic nitrogen fertilizers in increasing crop yields is less than 50%. Analyses indicate that this efficiency will continue to decrease over time and will lead to increased abuse of synthetic fertilizers to compensate for yield loss. Furthermore, global demand for synthetic nitrogen fertilizers is expected to continue growing to meet soil nutritional requirements to keep up with supply and demand trends.
What can be done?
Some proposed strategies include crop rotation and the use of nitrogen-fixing legumes as intercrops to promote the biological nitrogen fixation process by rhizobia. These alternatives allow for long-term sustainability of agricultural systems as nitrogen-fixing plants offer an economically and ecologically viable alternative to synthetic nitrogen fertilizers. Biological nitrogen fixation can be a major nitrogen source in agriculture when a nitrogen-fixing system is utilized.
Soil fertility management requires an understanding of potential nitrogen losses due to leaching in the overall nitrogen biological fixation cycle. Agronomic (short-term productivity) and environmental (long-term sustainability) performances of cropping systems heavily depend on the balance between nitrogen fluxes, i.e., examining nitrogen inputs and outputs in a field crop. Organic farms often serve as a model for this balance, but unfortunately represent less than 3% of agricultural land use. Understanding nitrogen flux at the field level is a crucial indicator of shifts towards nitrogen deficits or excesses and thus potential soil degradation or losses due to leaching.
Organic crop rotation relies primarily on nitrogen fixation, through the symbiosis between legumes and rhizobia, contributing to 87% of total nitrogen input. However, crop rotation also results in higher nitrogen yields compared to conventional production of commercial crops in industrial agriculture with the use of synthetic fertilizers. Additionally, although nitrogen-fixing crop rotations provide significant nitrogen inputs and outputs, the reliance on symbiotic nitrogen fixation requires other variables such as nitrogen balancing to understand inputs and outputs to maximize yields.
Organic farming practices are considerably more sustainable and reduce environmental nitrogen losses on a large scale in agriculture. However, crop rotation and nitrogen-fixing legumes are not yet sufficient without the use of synthetic nitrogen fertilizers. Although still too few, studies have shown that it is possible to improve biological nitrogen fixation despite the anthropogenic effects of the nitrogen cycle. These improvements can be achieved through targeted management practices, especially regarding the precise use of synthetic nitrogen fertilizers.
