Lignocellulosic crops have shown in recent decades a more efficient pathway to produce energy and also lower environmental impacts. Life cycle assessments show that fertilizers efficiency is critical as well as expected yields and extractions levels when producing energy crops. We discuss implications and criteria to apply.
The problem of fertilizers and energy crops has been clearly an important concern for those developing liquid biofuels such as ethanol and biodiesel during the past 20 years. Most companies and researchers in developing countries still discuss about environmental issues on energy crops as well as energy yields (energy output / fossil energy inputs during the energy production cycle). Sometimes we see that most researchers discussions are similar to those in Europe some years ago when all stakeholders started to make life cycle assessments of rapeseeds and sunflower for biodiesel, and sugarbeet and cereal grains for ethanol production extensively. Energy balances and emissions in those cases have a strong component in the processing (fermentation, transesterification, etc.); biomass to energy processes after farm stages. The lower energy balances of first generation biofuels, are mostly derived from a low efficiency of both ethanol and biodiesel processes converting biomass to bioliquids qith potential use for transport. In particular this resulted to be worse if marginal lands and lower yields are considered. As lower yields have to be allocated to same fossil energy uses, a lower efficiency occurs and more energy is required to obtain a liter of biofuel.
Some biodiesel alternatives are being also studied to obtain better results. Recently oil trees (Jatropha, Milletia and even palm oil) have shown that energy balances and emission savings could be higher of those found in annual cash crops (like rapeseeds) for biodiesel. There is still a debate analyzing soybean for biodiesel, a legume with no fertilizers that produce almost all biodiesel exported from one of the main suppliers today: Argentina. In particular soybean, has a critical energy allocation when a life cycle assessment has to be done. Some parts of the inputs should be allocated to grains for food (protein), and some other outputs (cake and oil) can be used to produce biodiesel. Gylcerine as by-product should be analyzed as well. All in all, it is a production scheme with almost no fossil energy for nitrogen as main top fertilizer during the crop cycle. This is bsaed on the rhizobium fixation that occurs on legumes and the fact nitrogen fertilizer is avoided by farmers (which also implies severe cost reductions). Additionally, in Argentina most farmers grow soybean with very low inputs as they produce the crop using non-tillage agricultural methods that implies much less diesel for land preparation even if they need to use some more glyphosate and other chemicals. Direct sowing a single crop or double cropping system after a winter cereal, determine a much more positive energy balance.
In the case a company uses perennial species for 2nd generation bioethanol, combustion (as pellets for cofiring or bales for combined heat and power) or even gasification of biomass, there is strong evidence of lower environmental impacts compared to first generation biofuels. Scientific research is already done and available. Assuming 7-12 years of perennial herbaceous crops (like most grasses and grasslands as energy plantations) and 15-20 years in the case of some woody crops in short rotation coppice, all dosages of nitrogen would not affect so much to the energy balances as they are assumed sometimes. This is based on the efficiency of biomass to energy by combustion. Cofiring pellets and the energy costs along the bioenergy chain are not so high as the process to make ethanol or biodiesel.
Additionally, changing the technology, biomass could determine lower impacts. Assume for a moment you change the use of pellets and consider now gasification instead of direct combustion (resulting of course in a greater efficiency) or a more efficient system like CHP (combine d heat and power). Results will change a lot with similar inputs at the farm. And what about a biorefinery? Producing second generation bioethanol from lignocellulosic raw materials and simultaneously using cogeneration to power the facilities will be probably much more efficient that with no electricity or heat produced from biomass. And this is true if ypou consider same biomass has been produced using same nitrogen dosages is in the life cycle. Energy balances and emissions savings when replacing coal will be completely different in each case. Now we could assume pyrolysis and biochar production as final by-product residue which can bring carbon back to the soil as a carbon negative solution at the time the facility produces electricity/heat; carbon cycle will be also different as well as energy balances and emissions saved compared to coal, gas or whatever we are replacing.
In terms of energy, the typical nitrogen fertilizer rates used in energy plantations will range from 50 to 250 kg N / ha per year in the form of NPK as base dressing (NPK could have about 7-9 MJ/kg of input) and top fertilization such as ammonium nitrate, urea, ammonium sulfates, etc. Some fertilizers as calcium ammonium nitrate (CAN) have different energy content in megajoules per kilo (MJ/kg) compared to others (like urea or NPK). You might choose different fertilizers to change the final footprint. Additionally, nitrogen use efficiency changes dramatically depending on several aspects during spreading application. Best suggestion: consider expectations to fertilizer and decide the best way to maximize fertilizer use efficiency. All those inputs need to be comapred with the total output (multiplying biomass calorific values per kg and the yields per hectares produced).
Another way to reduce potential negative effects from fertilizers in energy plantations will be the use of legumes. Using legumes as previous crop (or even in agroforestry or mixing grasses with legumes) could be other way to avoid emissions from fertilizers. It is clear in literature and our experience that a farmer can reduce nitrogen application in subsequent crops just using a legume in the rotation (more than 50-120kg of N could be available for next crop in several examples you will find in the literature). All depends on mineralization levels, temperature, rainfall and many other factors affecting bacterial activity and symbiosis with the legumes.
Of course levels of fertilizers directly imply on fossil energy INPUTS of the crops. However, as we said before, the quantity of fossil energy in lignocellulosic energy crops is considerably high since most inputs are those regarding fertilizers and diesel used. Nevertheless, in lignocellulosic perennial energy crops, the global energy balance is in general very much positive and the problem is not that relevant as it is in biofuels. Additionally, if those lignocellulosic crops are perennials the energy output (several cuttings, many years) will be much higher that INPUTS used such as land preparation, harvest and fertilizers or chemicals used.
In lignocellulosic energy crops, outputs can be 8-20 more times than inputs easily, depending on yields and many factors that will be linked to them such as crop management, soil characteristics, nitrogen leaching, rainfall, type of nitrogen fertilizer, etc. Energy outputs per unit of fossil inputs in MJ will determine the balance and of course you will be able to define emissions (CO2e units per fossil energy MJ produced). Then one can compare.
In general, with perennial grasses, this won’t be a serious problem except when yields are too low but fertilizers applied are still high anyway. This is something that might occur in semiarid agriculture of developed countries like Spain, Greece or some parts of Australia or other regions where extensive commercial farms sometimes apply fertilizes expecting rainfall that (sometimes) never arrive. They could eventually incur in high use of fossil energy and produce low biomass production per energy units used. Anyway, as a general rule, farmers apply nitrogen when yields will be increased and this is how they know their revenues are safe.
Too low energy balances could be also a problem if companies truck wet biomass like sometimes can occur in wet areas. All these factors imply fossil energy in the cycle and efficiency will be the key for optimization.
Irrigation is other important factor that is quite linked to fertilizer impacts. Many companis still consider irrigation as a key issue to reduce risks from biomass supply at least in a part of their lands. In some semiarid lands, irrigation will be required to produce feedstock as it has been shown in Spanish agriculture. If spring and summer have no rainfall, productivity can be just too low to produce most energy known crops like Miscanthus or Switchgrass. A selection of best efficient energy crops for marginal lands is something we offer. Many countries require solutions on irrigation for energy crops. Poplar in short rotation coppice using dripping irrigation to produce 10-20 t/ha per year could be something dangerous. Huge inputs are used there and if the production goes just too low (lets say under 5 or 8 t/ha per year), the net CO2 savings will be also too low as well as energy balances in the whole cycle.
But what is too low? The EU commission has a statement regarding lignocellulosic crops and solid biomass. They suggest to follow similar criteria compared to biofuels: after 2018, 60% of greenhouses gases emissions (GHG) as net saving produced when replacing the CLEANEST energy alternative (in Europe often an electricity mix based on natural gas electricity and nuclear power among other possible sources).
Back to fertilizers, for the any case information on soil analisys and in particular organic matter levels will be required to establish exact dosages for expected production. And that’s the key: the expected production in tons/ha per year, and what are extraction levels in each case. A farmer will often rotate land use, and will have food and grasslands. Some marginal areas could be occupied by crops that have more efficient patterns to produce lignocellulosic feedstock. And not only nitrogen should be considered in estimations, also phosphorous and potassium but many other micronutrients would be calculated to be returned to the soil. The rotation itself will determine organic matter incorporation to the soil. A typical harvest of tropical grasses will release about 1-2 dried tons per ha per year just during crop operations.
Some good papers supporting all these ideas about fertilizers and energy crops as well as on life cycle assessments can be found here:
- Crop residue removal and fertilizer N: Effects on soil organic carbon in a long-term crop rotation experiment on a Udic Boroll
- Total and available soil carbon fractions under the perennial grass Cynodon dactylon (L.) Pers and the bioenergy crop Arundo donax L.
- Yields and greenhouse gas emissions of cultivation of red clover-grass leys as assessed by LCA when fertilised with organic or mineral fertilisers
- Large-scale bioenergy production from soybeans and switchgrass in Argentina
- On sustainability of bioenergy production: Integrating co-emissions from agricultural intensification
- Life cycle assessment of bioenergy systems: State of the art and future challenges
- Nutrient fertilizer requirements for sustainable biomass supply to meet U.S. bioenergy goal
- Energy- and greenhouse gas-based LCA of biofuel and bioenergy systems: Key issues, ranges and recommendations
- Policy incentives for switchgrass production using valuation of non-market ecosystem services
- Modeling nitrogen loss from switchgrass agricultural systems
- LCA of bioenergy chains in Piedmont (Italy): A case study to support public decision makers towards sustainability
- LCA of second generation bioethanol: A review and some issues to be resolved for good LCA practice
- Life cycle assessment of different bioenergy production systems including perennial and annual crops
- LCA of poplar bioenergy system compared with Brassica carinata energy crop and natural gas in regional scenario
- Assessment of nitrogen fertilization for the CO2 balance during the production of poplar and rye
- Evaluation of Biomass Combustion based Energy Systems by Cumulative Energy Demand and Energy Yield Coefficient
- The Potential for Production of Biomass for Biofuel by the Cultivation of Hybrid Poplar and Hybrid Aspen in the South of Sweden
- GHG balances of bioenergy systems – Overview of key steps in the production chain and methodological concerns