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Even as more food needs to be produced, usable land for agricultural practices is inherently limited to roughly 20—30% of the world's land surface. The availability of agricultural land is decreasing, and there is a shortage of suitable land where it is most needed, i.e. particularly near population centres. Soil degradation is a major contributor to this decline and can generally be categorized in two ways: displacement (wind and water erosion) and internal soil chemical and physical deterioration (loss of nutrients and/or organic matter, salinization, acidification, pollution, compaction and waterlogging). Estimating total natural and human-induced soil degradation worldwide is fraught with difficulty given the variability in definitions, severity, timing, soil categorization, etc. However, it is generally agreed that its consequences have resulted in the loss of net primary production over large areas (Esch et al. 2017), thus restricting increases in arable and permanently cropped land to 13% in the four decades from the early 1960s to late 1990s (Bruinsma 2003). More importantly in relation to population growth during that time period, arable land per capita declined by about 40% (Conforti 2011). The term 'arable land' implies availability of adequate nutrients to support crop production. To counteract nutrient depletion, worldwide fertilizer consumption has risen from 90 kg/ha in 2002 to 135 kg in 2013 (Pocketbook 2015). Yet the increased use of fertilizers often results in excesses of nitrate and phosphates ending up in aquatic ecosystems (Bennett et al. 2001), causing algal blooms and eutrophication when decaying algal biomass consumes oxygen and limits the biodiversity of aquatic life. Largescale nitrate and phosphate-induced environmental changes are particularly evident in watersheds and coastal zones.
Nitrogen, potassium and phosphorus are the three major nutrients essential for plant growth. Even though demand for phosphorus fertilizers continues to grow exponentially, rock phosphate reserves are limited and estimates suggest they will be depleted within 50—100 years (Cordell et al. 2011; Steen 1998; Van Vuuren et al. 2010). Additionally, anthropogenic nitrogen input is expected to drive terrestrial ecosystems towards greater phosphorous limitations, although a better understanding of the processes is critical (Deng et al. 2017; Goll et al. 2012; Zhu et al. 2016). Currently, there are no substitutes for phosphorus in agriculture, thus putting constraints on future agricultural productivity that relies on key fertilizer input of mined phosphate (Sverdrup and Ragnarsdottir 2011). The 'P-paradox', in other words, an excess of P impairing water quality, alongside its shortage as a depleting non-renewable resource, means that there must be substantial increases in recycling and efficiency of its use (Leinweber et al. 2018).
Modern intensive agricultural practices, such as the frequency and timing of tillage or no-till, application of herbicides and pesticides, and infrequent addition of organic matter containing micronutrients can alter soil structure and its microbial biodiversity such that the addition of fertilizers no longer increases productivity per hectare. Given that changes in land usage have resulted in losses of soil organic carbon estimated to be around 8%, and projected losses between 2010 and 2050 are 3.5 times that figure, it is assumed that soil water-holding capacity and nutrient losses will continue, especially in view of global warming (Esch et al. 2017). Obviously there are trade-offs between satisfying human needs and not compromising the ability of the biosphere to support life (Foley et al. 2005). However, it is clear when modelling planetary boundaries in relation to current land use practices that it is necessary to improve N and P cycling, principally by reducing both nitrogen and phosphorus emissions and runoff from agricultural land, but also by better capture and reuse (Conijn et al. 2018).
One of the principal benefits of aquaponics is that it allows for the recycling of nutrient resources. Nutrient input into the fish component derives from feed, the composition of which depends on the target species, but feed in aquaculture typically constitutes a significant portion of input costs and can be more than half the total annual cost of production. In certain aquaponics designs, bacterial biomass can also be harnessed as feed, for instance, where biofloc production makes aquaponic systems increasingly self-contained (Pinho et al. 2017).
Wastewater from open-cage pens or raceways is often discharged into waterbodies, where it results in nutrient pollution and subsequent eutrophication. By contrast, aquaponic systems take the dissolved nutrients from uneaten fish feed and faeces, and utilizing microbes that can break down organic matter, convert the nitrogen and phosphorous into bioavailable forms for use by plants in the hydroponics unit. In order to achieve economically acceptable plant production levels, the presence of appropriate microbial assemblages reduces the need to add much of the supplemental nutrients that are routinely used in stand-alone hydroponic units. Thus aquaponics is a near-zero discharge system that offers not only economic benefit from both fish and plant production streams, but also significant reductions in both environmentally noxious discharges from aquaculture sites. It also eliminates the problem of N- and P-rich runoff from fertilizers used in soil-based agriculture. In decoupled aquaponic systems, aerobic or anaerobic bioreactors can also used to treat sludge and recover significant macro- and micronutrients in bioavailable forms for subsequent use in hydroponic production (Goddek et al. 2018) (see Chap. 8). Exciting new developments such as these, many of which are now being realized for commercial prodution, continue to refine the circular economy concept by increasingly allowing for nutrient recovery.