Replacing industrial fertilizers with legume trees: beans for thought?

Co-written by Todd Rosenstock and Kate Tully.

Inter-cropping maize and fertilizer trees. Photo credit: ICRAF.

Inter-cropping maize and fertilizer trees. Photo credit: ICRAF.

Supplementing soils in sub-Saharan Africa (SSA) with nitrogen (N), an essential plant nutrient, through inorganic or organic fertilizers is essential for maintaining and increasing crop production, which is directly linked to improved food security and natural resource preservation. Without the application of fertilizer, smallholder farmers mine soil nutrient stocks and degrade the long-term capacity of fields and farm to produce food, fodder, and fuel.

Seed pods from leguminous trees are often used as livestock fodder. Photo credit: ICRAF.

Seed pods from leguminous trees are often used as livestock fodder. Photo credit: ICRAF.

Methods of increasing available soil N has been debated for many years. Mineral N fertilizers offer rapid results in terms of stimulating crop growth, but their limited availability and high per unit costs typically constrain their use by smallholders in SSA. For instance, the average maize farmer in Kenya applies less than 10 kilograms (kg) of N per hectare (ha). By comparison, farmers in the United States and Europe typically apply more than 200 kg N per ha. Overuse of mineral N has been linked to a number of negative environmental impacts including drinking water contamination and, more broadly, climate change.

Leguminous plants are often proposed as an alternative to increase N in African smallholder farming systems. Legumes (including soybeans, lentils, and some tree species, among others) “fix” N in the atmosphere into plant-available forms, effectively transferring the N into the soil. When grown in combination with staple crops, legumes provide a mechanism for increasing N in farming systems, thus increasing farm productivity.

However, inter-planting staple crops with legumes is not without environmental concern as well.

“Whether biologically fixed by soil bacteria or industrially fixed via the Haber-Bosch process, the origin of the N and the potential environment threats are the same” said Todd Rosenstock, Environmental Impacts Scientist at the World Agroforestry Centre (ICRAF).

Rosenstock is the lead author of a recent paper evaluating whether agroforestry with legume trees can sustainably increase soil N without stressing climate and water resources.

In general, data quantifying environmental impacts such as greenhouse gas emissions and N-leaching from legume-based agroforestry is scant. The available evidence, however, shows that legume-based agroforestry catalyzes nitrous oxide – a greenhouse gas roughly 298 times as potent as carbon dioxide over a 100-year timeframe – at similar levels to what might be expected from industrial N fertilizers. The increased N emissions may be off-set by increased carbon accumulation in soils and biomass in agroforestry systems versus monoculture cereal production.

Rosenstock pointed out that,

“The processes of carbon and nitrogen exchange between air, plants, and soils are complex and often offset one another. Thus, it is critical to take a systems approach when evaluating farming innovations in terms of their impacts on the climate system.”

Furthermore, the study did not examine greenhouse gas emissions alone. It also evaluated the potential for legume-based agroforestry to increase N-leaching, which can cause nitrate loading in groundwater, contaminating local drinking water supplies. They found that legume trees elevate soil N concentrations substantially but with such few data documenting movement below the tree root-zone, it is impossible to evaluate the risk of N leaching at this time.

When discussing the assessment, Kate Tully – a scientist at the Earth Institute and co-author on the paper, emphasized that,

“We know that soil N can be lost to the environment via multiple pathways. And oftentimes when you decrease one form of loss, another form increases. Data were insufficient to answer the question about leaching losses in legume-based agroforestry with any certainty at this time. Future research needs to examine both gaseous and water-borne loss pathways using a systems approach.”

The environmental assessment coupled with the need to introduce N into cropping system led the authors to conclude that while concerns over the impact of emissions on greenhouse gas balance are justified, legume-based agroforestry represents a viable option of sustainable development for smallholder farmers. Rosenstock went on to suggest that,

“The drivers of N loss in agricultural systems are well known and with an eye to the potential for environmental harm, systems can be designed that effective minimize the risk and maximize benefits, despite the uncertainty.”

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