Nitrogen fertilizers are responsible of most biofuels sustainability limitations in life cycle assessments and carbon footprint. But some very realistic strategies and new findings would help to reduce their impacts.

Biofuels sustainability, certifications and policies are very much related to carbon foot print, green-house gases (GHG) emissions savings  when replacing fossil energy sources, and energy balances and environmental impacts derived from their use.

As we often show in this blog, liquid biofuels derived from food production have a strong debate regarding sustainability while most second generation biofuels and biomass from residues or energy crops to produce heat, power, biogas or syngas, have lower impact and more benefits. Also into the future, mineral fertilisers will remain important in meeting the world’s food needs so reducing its use for bioenergy purposes would lead to potential sustainability improvements.

Not al fuels are the same.

We already did a very much visited post in this blog, about how much difference we find in sustainability of different forms of biofuels.

Life Cycle Assessment (LCA) include several techniques involving accounting of most fossil energy used for the production of energy.

From machinery to fertilizers, fuel used in biomass transportation, loigistic and of course the final conversion and use in the car or biomass burning in a power station, is considered.

When analyzing biofuels and biomass uses, most researchers compare energy balances, GHG emissions and several impact categories (water, air, soil, etc.).  And the fact is that a liter of biodiesel from rapeseeds, a liter of E85 bioethanol from corn or a MWh of biopower will always imply some fossil energy consumption. Energy balances and emission savings from lignocellulosic biofuels (like cellulosic ethanol) or biopower and heat production from biomass, has often 5 to 10 more times efficiency compared to liquid biofuels for our cars.

But how we could eventually reduce fossil energy consumption in the production of biofuels?

We know that raw materials are only a percentage of the total energy consumption and GHG emissions that a liter of biofuel requires. However, we see that in some biofuels, fertilizers can be a large contribution and sometimes, the most relevant contribution in the total LCA.

Therefore utilizing biomass for biofuels would save nonrenewable energy, and reduce greenhouse gases. However, unless additional measures such as planting cover crops were taken, utilization of biomass for biofuels would also tend to increase acidification and eutrophication, primarily because larg


What opportunities we have to reduce fossil energy use from nitrogen fertilizers in energy crops biomass and biofuels?

There is no doubt that nitrogen is needed to produce biomass. But the true is that most nitrogen-based fertilizers us used during farming to produce raw materials. Most crops like rapeseeds, sunflowers, corn and also perennial grasses and most woody crops require nitrogen.

It is clear to us that energy crops shouldn’t use organic nitrogen from soil only. That would lead to eventual soil deployment and a not sustainable continous nitrogen extraction and organic matter reductions.

However there are several things that could be considered to obtain a significant reduction in fossil fertilizer uses and in particular nitrogen fertilizers:

Fixing nitrogen legumes could avoid lots of fossil energy used to produce biofuels in other species, but it is not enough well considered in standards and certifications.


There are thousands of species that would produce soil benefits producing biofuels and biomass for energy. The nitrogen fixing capacity of legumes is really well studies and we know that fossil nitrogen can be eventually avoided by almost 100%  producing oil seeds (like in soybean), straw (like in several legume forages) and wood (as happens in red alder, acacias, Leucaena and many other short rotation woody species).

Despite of land use changes, that should treated separately since any option could imply impacts.

Some good examples of projects and publications showing nitrogen fixing biomass options that produce energy are shown here:

  1. Impact of fertilizers can be reduced from 65% to 4% if soybean (Argentina) is compared to rapeseed (Spain). A methodological proposal for Life Cycle Inventory of fertilization in energy crops: The case of Argentinean soybean and Spanish rapeseed (2013)
  2. Use of soybean as a biodiesel feedstock was more energetically efficient than canola primarily due to reduced nitrogen fertilizer requirement.  Net energy balance of small-scale on-farm biodiesel production from canola and soybean
  3. Tropical and sub-tropical legume tree Pongamia pinnata produces large seeds (∼1.5–2 g) that contain about 40% oil with lower nitrogen inputs because of nitrogen fixation.  Tree legumes as feedstock for sustainable biofuel production: Opportunities and challenges
  4. Fixing legume trees alone and in agroforestry systems can produce low cost and low input biomass with energy purposes in tropics and subtropics, in particular in marginal areas . Some examples are here: a) Ethanol technical potential in Hawaii based on sugarcane, banagrass, Eucalyptus, and Leucaena. b) Species screening and biomass trials of woody plants in the semi-arid southwest United States. c) Potential for rural electrification based on biomass gasification in Cambodia
  5. Legumes mixed with grasses producing energy are very well documented. A good example for Napier grass is avabilable here.

Biofuels require lots of fossil energy from fertilizers applied in cropping systems (mainly from inorganic nitrogen sources). Its impact in footpront is huge and produce high GHG emissions, When plantations producing biomass consider some improved techniques it is feasible to reduce nitrogen doses applied. Changes in footprint derived from these practices should be addressed in sustainability criteria debate and policies as well as standard certification.


 napier1Several grasses can avoid fossil energy uses if certain practices are considered. Some experiences with Napier grass and rhizobium nitrogen fixing bacteria, have been showing promising results and are already applied at commercial scales. Here we enlist some articles showing strong evidence of potential development and implementation of symbiotic bacteria that may fix nitrogen and replace inorganic nitrogen sources in fertilizers.

  1.  Responses of Sorghum and Pennisetum Species to the N2-Fixing Bacterium Azospirillum brasilense
  2. Potential for nitrogen fixation in the roots of Pennisetum purpureum
  3. Biological nitrogen fixation in sugarcane
  4. Avoiding fossil energy from nitrogen fertilizers in sugarcane
  • BIONITROGEN: Urea from biomass as by-product of energy production

 A great example is the Florida-based BioNitrogen Corp. that granted a patent by the U.S. Patent and Trademark Office for its technology that converts biomass feedstocks into urea fertilizer.

The technology can convert 1,000 tons of residual biomass, including sugarcane bagasse, palm waste, corn stover and rice hulls to produce 520 tons of granulated urea with 46 percent nitrogen content. During the process, the biomass is dried, cleaned, ground and then gasified. The produced syngas is then cleaned and processed through catalytic reaction stages to produce the fertilizer. During the process, any byproducts of the vent streams are recycled back through the system to produce electricity and 60 tons of ash, which is sold as a concrete additive. Ernie Iznaga, vice president of operations at BioNitrogen said the amount of power generation depends upon the type of power production method and the location.



 There are several techniques to improve nitrogen use efficiencies that could lead to reductions in nitrogen fertilizer applications, nitrogen leaching and nitrogen volatilization. Best practices are being prommoted for food production globally since 80’s with great acceptance and environmental benefits and biofuels should consider them as well as most certifications and standards.

Some examples from literature and research:

  1. Splitting top fertilizer use in crops can double nitrogen use efficiency reducing nitrogen application requirement and energy used.
  2. Fert-irrigation increases nitrogen use efficiency from X to X %
  3. Rotations and cropping herbaceous species after fixing legume (as previous crop) can determine lower fossil energy requirements.  Some examples: a) Rates of nitrogen application required to achieve maximum energy efficiency for various crops: results of a long-term experiment; b)
  4. Some nitrogen fertilizers have lower impact than others
  5. Avoiding nitrogen losses such as nitrogen run-off and volatilization can be possible if improved techniques are promoted. Click here to read a good explanation on new approaches needed for food crops that would be also compatible considering several existing production systems like corn or soybean. Some more examples: a) Best Nitrogen Management Practices to Decrease Greenhouse Gas Emissions; b) Enhancing Nutrient Use Efficiency of Forage Crops; c) Better management practices in Sugarcane farming systems d) Top dress tips for nitrogen and sulphur
  6. Switchgrass cutting frequency and nitrogen management can increase nitrogen use efficiency. Values from 150kg of biomass per kg of nitrogen  applied can be double with improved management. Differences found could lead to significant GHG reductions.



Fertilizers and energy crops  Organic wastes are present in the rural sector economy both in small and large scale farming activities.  The use of manure and other nitrogen sources from organic wastes is well documented and could be also considered to produce more “organic” or “ecologic” biofuels or even lignocellulosic materials too.  Some examples regarding this are summarized here showing the reduction in GHG emissions and improvement in energy balances that could be obtained. Combinations between livestock systems, grass management and biogas are also good examples  to go further in synergies that will lead to more efficiency bioenergy systems.

  1. Utilization of waste nitrogen for biofuel production in China
  2. Integrating livestock manure with a corn–soybean bioenergy cropping system improves short-term carbon sequestration rates and net global warming potential
  3. Palm Oil Mill Effluent as an Additive with Cattle Manure in Biogas Production
  4. Benefits of supplementing an industrial waste anaerobic digester with energy crops for increased biogas production
  5. Mixing legumes and using majnure to produce grasses for bioenergy to reduce GHG emissions
  6. Soil organic matter and NPK status as influenced by integrated use of green manure, crop residues, cane trash and urea N in sugarcane-based crop sequences
  7. Bioagents to supply nitrogen in sugarcane could replace fossil nitrogen sources .