Recent studies on perennial agriculture to produce biomass for energy have found strong evicence of soil health benefits and carbon sequestration.

Solid biomass such as straw or woody chips has been promoted in Europe to produce heat, power and sencond generation biofuels. The EU comission expects a huge growth for biobased industries that will require massive amounts of biomass feedstock in the next few decades. Biomass is already a very considerable share of total renewable energy in Europe and most trends show a huge increment and potential for 2030.

In this article we show how residues for biomass are not the right approach in Europe to promote a biobased industry. We offer sound evidence that EU soils require perennial cropping systems to increase soil fertility and give several examples of energy crops that have been studied for many years and meeting that requirement succesfully.

 

Why Europe should promote crops for biomass and cannot use only residues to promote a biobased industry?

Today, most policy makers and renewable energy organizations promote biobased products and renewable energy. However, even if goals and policy are clearly increasing the interest for solid biomass wastes (agriculture and forestry residues) to produce heat, power or second generation biofuels, then a wrong approach considering soil sustainability is evident. Below a TED video show some of our vision that is 100% coincident with most highly reputed researchers and ecologist combating climate change and creating soils worldwide.

What is the effect of residual removals on soil fertility?

Cereal straw, which is most often returned to the soil in arable cropping systems, is of renewed interest as a potential source of bioenergy. However, the sustainability of this practice which implies systematic removal of aerial biomass of cereal crops is a controversial issue, particularly in soils having a low soil organic carbon (SOC) content.

Soils in Southern European countries require perennial agriculture, green covers and increments in organic matter that only biomass for energy markets could become real. Most livestock and grazing agriculture have been diminished by Common Agricultural Policy and there is no expectation to increase extensive grassland or perennial grasses for fodder production.

There is sound evidence that soil can be affected and damaged seriously from straw collection in large scale management at farm level. Straw coverage increases topsoil C and N contents, moisture, and total porosity as well as reduces variability of topsoil temperature and in turn moderated microbial biomass and activity. Additionally, soil carbon changes in the soil can determine large fossil inputs required from nitrogen fertilizers as organic matter reductions can occur in the topsoil and this is extensively proven by scientist in Europe and many other regions worldwide. A short review of most sound scientific evidence and long term studies is available in the literature (Graham et al., 1986; Amir and Sinclair 1996; Børresen, 1999; Turley et al., 2003; Blanco-Canqui, and Lal, 2007; Saffih-Hdadi and Mary, 2008; Tarkalson et al., 2009; van Groenigen et al., 2011; Yoong et al., 2012; Nguyen et al, 2013; Weiser et al., 2013).

 

Is mulch agriculture what our soils need?

Experimental fields of Erosion Research group of CSIC in “Santa Olalla”, Toledo, Spain.

Maybe before that question we need to ask ourselves why perennials improve so much the organic matter? In literature the background on mulch agriculture is overwhelming and full of examples.

The truth is that several perennial crops for woody energy plantations like poplars or willows buy also perennial grasses or even broadleaf crops like cardoon, produce huge amounts of leaves and biomass residues going from air CO2 to become carbon in the soil, even in non oxidable forms lasting many decades in the soil as this recent study showed in poplars in Spain. In the mentioned study and others values from 1 to 7 tons of dry matter have been recorded as net increments in soil.

Some perennials are very much viable options to produce low cost biomass in marginal lands (such as semiarid and extremely warm or cold conditions). Read more here.

 

Harvest losses and falling leaves have been recorded in Europe adding from 1 to 7 tons of dry matter each year. Minerals and organic nitrogen as well as carbon sequestration are being measured every year.

Is biomass from energy crops compatible with increments in soil fertility ? 

The fact most perennial crops require less fertilizer is explained mainly because of the net increment in carbon and organic matter in the soil. The approach in most research centers in Europe, is that soil have to be stable if land is consisdered to produce biomass for energy.

If you analyze Life Cycle Assessments in literature for energy crops or biomass energy, you will learn that one of the most important factors is fossil energy used for the energy obtention process.

evaluations

Organic matter in the soil using grasses is estimated to increase carbon sequestration between 0.5 and 1.5 Mg/acre each year. Topsoil building is critical for farmers working on sustainable bioenergy crops in marginal lands and fragile areas.

In annual energy crops, diesel used and nitrogen fertilizers account around 60-80% of all energy consumed to produce a gigajoule of energy. Nitrogen fertilizers and diesel utilized by tractors and other operations are critical but a second aspect is important too: if you plow every year, less organic matter will be accumulated in the soil.  In perennial crops, increments in organic matter determine low depenance on nitrogen fertilizers and energy balances because of mineralization and lower nitrogen volatilization as ammonia or nitrates. Harvest losses return several macro elements like phosphorous, potassium or calcium. The increment in organic matter, stabilize the soil, hold moisture and increase yields in the same year, but also in subsequent years. Rotations with perennials might indeed boost our food production.

Dedicated plantations increase carbon in the soil in a higher extent compared to native unmanaged systems.  Project

Some good articles about perennial crops producing carbon sequestration, increased mesofauna and biota or increments in belowground enzymes activity are cited below (go to the bottom of this page).

 

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.

Are legumes viable options as perennial crops for energy?

Fixing nitrogen species are those plants that can have the capacity to use atmospheric nitrogen in the air and fix it in roots to take nitrogen from air space in the soil rooting system. In most cases they are simbiotic with organisms in the soil as rhizobium or rhizobacter. Legume trees are a hotspot for biomass to energy projects, yes. They can improve the soil and return carbon and nitrogen increasig fertility. Several options are viable across Europe including species like Leucaena, Robinia, Acacias and many others.

Soil sustainability is low presently, because of cereal monocultures, erosion and lack of adequate rotations with grasses, woody crops or fallow agriculture. Setaside policies in the past helped, yes, but not that much and soil degradation could only increase except with favour organic matter increments in long term. Find a sound scientific evidence that  legume bio3energy crops can boost subsequent food crop yields here.

Is CAP reform greening compatible with energy crops?

Clearly, EU wants farmers to become more and more sustainable. Most greening policies require farmers to diversify crops, promote sustainable rotations, legumes and perennial agriculture.  But, are all those measures compatible to energy crops for biomass feedstock for biobased industries?

Well, in ths review we showed clear evidence that soil would benefit from perennial energy crops. A long list of scientific reports in highly reputed journals is included below to show evidence on how perennial bioenergy crops might improve soil fertility.

 

Could we build topsoil with carbon negative solutions?

There are some options to increase carbon furthermore. Addtional to biomass harvest losses, perennial rooting systems and falling leaves, there are some biomass to energy technologies that can offer solid biomass wastes that could be used as organic fertilizer to improve soil fertility. Just to mention two examples, we want to make a brief reference to pyrolisis and anaerobic digestion (AD). In the first one, solid biomass is used to produce energy with a by product called “biochar” that is commercially used as soil amendment. Several companies offer commercial solutions for pyrolisis and biochar which benefit soils and industry cashflow at the time that farmers obtain a benefit from biomass to energy projects.

Here a good explanation on some benefits of biochar from woody biomass.

The same applies to AD and liquid or soid digestate produced from co-digestion of silage and manure or vinasse. We participate in a project which is promoting this aspect in a tropical island in the Caribbean.

So, not only perennial energy cropping systems can benefit the soil, but also residues from biobased industries will create topsoil fertility and carbon sequestration.

Producing bales with extremely low cost from this perennial hardy grass, has been demonstrated viability in several countries.

Producing bales with extremely low cost from this perennial hardy grass, has been demonstrated viability in several countries.

 

 

 

 

More on perennials? Don’t miss our section on perennial grasses for biobased products.

 

 

 

Looking for overwhelming evidence on all this? Here we attached below our review of most recent (2007-2014) scientific publications in highly reputed peer-reviewed journals: