Biomimicry
for shaping future
Agriculture
Focus on one problem: Nutrient cycling
Problem statement: Disruption of -or imbalanced- nutrient cycle in agricultural systems.
Modern agriculture has had immense benefit in terms of reducing hunger and improving nutrition, especially since the second half the the 20th century through the introduction of fertilizers, pesticides and other technologies of the 'Green Revolution’, considerably increasing the yields (Stoate et al. 2001; Tilman et al., 2002; FAO, 2001; WHO, 1990). Sustaining this level of production is a major challenge of our generation but, at the same time, the associated environmental impacts of such practices have to be greatly reduced (Tilman et al., 2001; Vitousek, 1997; Carpenter, 1998). A major cause of environmental harm from this kind of agriculture is the disruption of the nutrient cycle (Conradin & Wafler, 2009). According to the United Nation’s Glossary of Environment Statistics (1997) “A nutrient cycle is a repeated pathway of a particular nutrient or element from the environment through one or more organisms and back to the environment”. This circulation is essential for life, from the physical-chemical environment to the biota and back again (Martin, 2010).
Current agricultural practices have however disrupted it through the harvesting of crops, erosion of soils, water runoff, leaching and volatilization of nutrients and inadequate cultural methods (excess of fertilizers, monocultures, livestock exclusion, etc.); leading to an unsustainable agriculture over the long term (Gruhn et al., 2000; Pedro et al. 1997).
The causes of the disrupted nutrients cycle can be divided into 2 categories: (1) the leakage of the nutrients from the system, and (2) inadequate cultural practices.
(1) Leakage of nutrients from the system
Increased addition of inputs on cultivated land is the keystone of agricultural intensification of the 20th century. The addition of nutrients (nitrogen, phosphorus, potassium, magnesium, iron, and calcium among others), through synthetic fertilizers, animal manure, use of N-fixing plants, and deposition of airborne pollutants, has doubled the natural inputs for nitrogen and increased by five times the phosphorus content in terrestrial ecosystems (Howarth et al., 2005). These additional inputs have resulted in large-scale changes in nutrient cycles in the last decades (Howarth et al., 2005). The nutrient loss from the soil decreases the soil fertility and consequently, more nutrient input is required. This positive feedback results in an excessive use of fertilizers and further degradation of the quality of the ecosystem (Carpenter et al., 1998).
Currently, a minor part of applied nutrients ends up in harvested crops, in other words, most of it simply leaks out of the system (Carpenter et al. 1998; NAE, 2015; Pimentel, 1991; Meeus, 1993). In undisturbed systems, nutrients losses from the system are usually low since there is high biological demand for nitrogen, and inputs from the atmosphere are low; therefore any that is rendered available is quickly assimilated (Kibblewhite et al., 2008). However, disruption from tillage and other agricultural practices increases losses via leaching or volatilization because mixing of the soil leads to more rapid decomposition of organic matter; the conversion of nutrients from organic to mineral forms might then exceed the biological demand (Kibblewhite et al., 2008; Lehmann et al.,1998d). Nutrients that cannot be used by the soil–plant system are leaked into other environmental compartments via soil leaching or gaseous emissions (Kibblewhite et al., 2008). Nutrients leached in the soil end up in ground- and surface-water,
affecting inland water and coastal systems due to eutrophication, that is, excessive nutrients in waters lead to blooms of algae (Howarth et al., 2005; Thompson, 2014; Altieri, 2000). These colonizations by algae deplete the water of its oxygen, creating anaerobic ‘‘dead zones’’ devoid of life forms that depend on oxygen such as fish and other animals (Howarth et al., 2005). Nutrient leaching also depends on the soil properties such as texture, clay minerals and organic matter content. The texture of the soil influences nutrient retention: the finer the soil texture the greater the ability to store nutrients (CTAHR, 2015). Because clays have large surface area and a negative charge, they attract and hold in the soil nutrients that are positively charged (Tucker, 1999). Finally, organic matter is a main source of nutrients in the soil and it helps binding the soil particles into aggregates and thereby limit the flow of water and nutrients down the soil (FAO, 2005). Depending on these properties, different soils will thus be differently affected by leaching of nutrients. Lastly, leaching is favored by a wrong timing of fertilizer application; the nutrient input is indeed often not temporally synchronized with the plant demand and end up in the groundwater (Kramer et al., 2002; The Fertilizer Institute, 2015).
Nitrogen is the only nutrient from the soil system that can be leaked to the atmosphere (Thompson, 2014). This occurs through denitrification process, during which bacteria in the soil break down nitrate (NO3-) to obtain oxygen and thereby release gaseous N2, and through the volatilization of ammonia (NH3), which is a common form of nitrogen in the soil (Thompson, 2014).
Soil erosion is an important underlying cause of nutrient loss in agricultural systems, mainly promoted by wind and water (Stoate et al, 2001). They can affect in a direct way, i. e. taking away the nutrients bounded to soil particles (Ramos and Martínez-Casasnovas, 2006) and indirectly through the decrease of organic matter, leading to a reduced water and nutrient retention within the soil (Stoate et al, 2001). Erosion is greatly facilitated by some farming practices such as tilling and ploughing (Jordan et al, 2000) and by the lack of vegetation cover (Chambers et al. 1992). Other environmental elements that facilitate soil erosion are rainfall, presence of slopes, type of crop and soil characteristics (MAFF, 1999; Alstrom and Bergman 1990; Stoate et al, 2001). At the end soil erosion leads to important reductions in yield, also generating other negative impacts on the ecosystems such as the above mentioned eutrophication of water bodies (Stoate et al, 2001).
Although water runoff is highly linked with soil erosion by taking away nutrients attached to soil particles (Ramos and Martínez-Casasnovas, 2006), dissolution of these molecules in organic and inorganic form is an important factor of nutrient loss (Comin et al, 1997; Schlesinger et al, 2000). Runoff is directly linked to rain characteristics like frequency and intensity (Schreiber et al, 1976); to soil porosity, that is, larger pores facilitate water infiltration reducing the water runoff (Zhang et al, 2007), and is diminished with a higher presence of vegetation cover (Le Bissonnais et al., 2004; Durán et al., 2004). Regarding soil structure the use of heavy machinery causes soil compaction, decreasing water infiltration and facilitating water runoff (Stoate et al. 2001). Soils with low organic matter content and poor biota are especially vulnerable to this process (Makeschin, 1997).
The loss of nutrients in the soil can also be attributed to harvesting practices. Crop harvest in itself removes nitrogen, phosphorus and other nutrients from the system then, in order to sustain the production, these nutrients need to be replaced (Vitousek et al., 2009). Nevertheless, harvesting should not be considered as an actual loss as it is the main objective of the cropping system (Lindstrom et al., 1986). The harvest of crop residues is a common practice in agriculture. These residues are mainly used for feedstock or bioenergy and are treated as waste products without value if they are left on the land. However, they have a significant role in preserving the soil organic matter content and the physical and chemical properties of the soil. Studies showed that when residues were not harvested phenomena like water runoff and soil erosion were notably reduced (Gregg et al., 2010). Moreover, residue removal implies nutrient loss as they are important sources of nutrients that get back to the soil and to the next generation of plants (Lindstrom et al., 1986). Thus the lower the amount of crop residues left on the field the lower the yield. Indeed, residue removal is highly correlated to nitrogen loss in the soil. Consequently, harvesting residues can increase the demands for crop inputs such as fertilizers in order to sustain the same yields (Gregg et al., 2010).
(2) Inadequate cultural practices
The capacity of ecosystems to use and retain nutrients has also been dramatically damaged by the simplification of our landscapes into large-scale, low-diversity agricultural fields (Howarth et al., 2005). These productive systems require higher energy inputs through the use of fossil fueled machinery and the intensive use of fertilizers, which again disrupt the nutrient cycle in the soils (Howarth et al., 2005). The reduction of biodiversity caused by monocultures, both at the species and landscape levels, has resulted in nutrients losses from the agricultural systems (Bennett et al., 2012; Ewel et al., 1991; Howarth et al., 2005; Swift & Anderson, 1993; Vitousek & Hooper, 1993;). Lehmann et al. (1999) have shown that monocultures have lower nutrient efficiency compared to more diverse systems, such as agroforestry, which can be attributed to higher root abundance and higher ratio of nutrient uptake-to-leaching in the diversified system. The nutrients that are not efficiently uptaken in monocultures then leak from the soil system. This process reflects the niche complementarity of plant species; the ecological differences between species lead to more complete utilization of resources in diverse communities relative to monocultures (Hector et al., 1999). Regarding fauna diversity, monocultures have been shown to decrease that biodiversity and thus disrupt nutrients cycle by reducing microbial communities and other communities of organism responsible for decomposition (Brooks, 2006; McLaughlin & Mineau, 1995).
Vegetation cover is a noticeable factor in soil nutrient content - in the first place, the plants take nutrients up to for their biomass growth and, secondly, they decrease the erosion by stabilizing soil aggregates due to larger organic matter content (Tisdall and Oades, 1982; Benckiser, 1997; Kosmas et al, 1997) and by the binding effect of the roots (Oades, 1993).
Livestock production systems have various effects on the environment. Many of these effects occur on the land use (changes in the landscape) and nutrient cycling. It is evidenced that only a small portion of nitrogen, phosphorus and potassium derived from the livestock manure is utilized by the crops and the remaining is dissipated into the environment (Oenema et al., 2007). Carbon is released from manure in gaseous form, while nitrogen and sulphur evaporate or run off with the surface water. This make their loss pathways faster especially in intensive agriculture systems. Recently, new developed manure management techniques aim to reduce the losses from animal droppings, for example through low-protein animal feeding, air purification and manure covering during storage (Petersen et al., 2007). A sustainable recycling system of manure is required to provide the required amount of nutrients to the crops in an efficient way, increasing its productivity while reducing the needs of fertilizer (Petersen et al. 2007).
For each cause of the nutrient cycle problem, we looked for in nature at different levels
