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Soil enhancement with organic materials is a common component of soil fertility management for crop production, with the aim of providing essential plant nutrients and improving overall soil physical, chemical, and biological quality (Diacano and Montemurro, 2010). Marine macro-algae, or seaweed, has been historically used as a soil enhancement material, and may have application for modern agriculture as a low cost source of nutrient-rich biomass (Angus and Dargie, 2002; Cuomo et al., 1995). While seaweed compost and extract products have been widely evaluated for agricultural applications (Woznitza and Barrantes, 2005; Khan et al., 2010), evaluation of unprocessed seaweed biomass as an enhancement material is limited, particularly with regard to soil quality. Application of seaweed material may uniquely affect soil quality parameters as a result of its chemical characteristics, including carbon (C) and nitrogen (N) composition, and salt, sulfur (S), heavy metal, and trace element content. In this study, the putative benefits of seaweed enhancement for crop growth and production were assessed on various crops in field experiment, including analysis of soil physical, biological, and chemical properties.
1.2 Historical use of seaweed in agriculture.
In coastal regions, collection and application of seaweed is a traditional soil fertility management strategy, especially where agriculture relies on use of local resources (Cuomo et al., 1995). As a readily-available, low-cost material to supplement soil fertility, application of seaweed biomass is often an integral component of traditional, small-scale, diversified agriculture (Angus and Dargie, 2002). For instance, agriculture in the Machair region of the Scottish Outer Hebrides Islands involves a rotation-intensive system that integrates the application of locally available seaweed biomass (Angus and Dargie, 2002; Kent et al. 2003). Traditional agriculture of the Machair, practiced for at least 1,000 years before present (YBP), relies on a “crofting” system that generally includes an intensive rotation of livestock grazing, field crop cultivation, and two years of fallow, with hypothesized effects on soil biodiversity (Angus and Dargie, 2002; Vink et al., 2009). Soil fertility is still largely maintained by the traditional practice of application of manure and seaweed, primarily the brown alga Laminaria digitata (Angus and Dargie, 2002), which is collected and piled onshore for 1-2 weeks prior to application. Promotion of seaweed application as a part of sustaining small-scale, diversified agriculture is supported by Scottish Natural Heritage, a governmental conservation organization, as well as local conservation group efforts (Angus and Dargie, 2002).In addition to the Machair region, historical accounts of seaweed use in agriculture range from the British Isles, to coastal mainland Europe, to the northeastern region of the United States, including New York, Maine, and Rhode Island (Fussel, 1973; Smith et al., 1989; Cuomo et al., 1995). For example, prior to the adoption of synthetic fertilizer, potato production in Rhode Island incorporated seaweed collection as a means of maintaining soil fertility, including for agricultural research at the University of Rhode Island Agricultural Experiment Station (R. Casagrande, personal communication). Seaweed in the modern agricultural context In organic or reduced-input cropping systems, both in the U.S. and worldwide, seaweed-based agricultural products (e.g. extracts for foliar application and composts) are commonly employed (Khan et al., 2009). However, application of unprocessed biomass is less prevalent. To reduce dependence on application of inorganic fertilizers, make use of an abundant (sometimes over-abundant) resource, and improve soil quality, the traditional practice of seaweed application may have modern application in coastal regions. Because adding seaweed to soil can increase plant macro and micronutrients, and may improve soil biological, chemical and physical properties (Khan et al., 2009), the practice may be an additional strategy to manage soil fertility and quality that addresses the dual problems of reliance on inorganic chemical fertilization and wasting of valuable, nutrient-rich biomass. Inorganic fertilizer inputs account for a large fraction of conventional farm expenses, energy consumption, and carbon emissions (Lal, 2004). Application of inorganic fertilizers without addition of organic enhancements, cover crop use, or use of alternative tillage practices can result in depletion of soil organic matter (SOM), with concomitant negative effects on many soil properties important for crop productivity (e.g. nutrient retention, moisture-holding capacity, aggregate formation, and microbial activity) (Brock et al., 2012; Franzluebbers, 2012). Furthermore, levels of nutrient elements other than N, P, and K (e.g. Ca, Mg, Mo, B, and S) are generally low in inorganic fertilizers, and are of increasing concern for crop quality and nutritional value (Welch and Graham, 2012). Consequently, reliance on inorganic fertilizer as a sole source of fertility is often questioned as a sustainable management strategy, and 4diversification of inputs is encouraged, particularly inputs that provide not only primary nutrients (i.e. N, P and K), but also organic matter and trace elements (Lal, 2004). Organic enhancements used to improve soil fertility include traditional (e.g. animal manure) and non-traditional (e.g. industrial by-products) materials (Power et al., 2000). Seaweed, which contains primary nutrients, organic C, and other nutrient elements, is thus a good candidate organic enhancement material as part of a diversified soil fertility management strategy.
In addition to the potential crop nutrition benefits of seaweed enhancement, the prevalence of seaweed biomass in coastal areas as a result of both natural phenomena and anthropogenic impacts may allow for use of seaweed with minimal cost. Nutrient (N and P) enrichment of coastal waters – sometimes attributed to fertilizer runoff from agriculture and home use – can cause excessive seaweed growth (Morand and Merceron, 2005). In addition to detrimental ecological impacts (e.g. oxygen depletion), the accumulation of seaweed biomass on beaches can have negative economic consequences (RI DEM, 2010). For instance, in the summer of 2012, accumulation of the red seaweed Polysiphonia sp. on Massachusetts beaches required mechanical removal and disposal in order to maintain beaches for public use, costing money for equipment use and labor, as well as preventing beach use. Beach-cast biomass is often removed and disposed of in landfills. Although the species composition and properties of beach-cast seaweed varies based on location and environment (e.g. estuarine vs. marine), the coordination of accumulated seaweed biomass removal with agricultural application may provide a low-cost, locallyavailable resource for soil fertility management. To initiate this arrangement for 5 coastal regions, characterization of seaweed biomass in terms of location and abundance, species composition, and chemical characteristics relevant to soil quality and plant nutrition is required. Additionally, quantification of seaweed biomass effects on soil quality and crop production is required to validate putative benefits or negative effects of seaweed enhancement practices
Marine algae is estimated to contribute about 70 % to 80 % of earth’s atmospheric oxygen, amounting to about 330 billion tonnes of oxygen per year (Hall, 2008). This is an indication of how important algae are to the environment. Algae are simple, autotrophic organisms that are either microscopic or macroscopic. Specifically, seaweeds are macroscopic algae that thrive in benthic marine waters. Just like terrestrial plants, these groups of multicellular organisms are autotrophic and thus have the ability to carry out photosynthesis. However, they do not posses several distinct organs such as true leaves, roots, flowers and seeds that typify terrestrial plants (Sumich &Morrissey, 2004). There are roughly 10000 different species of seaweeds recorded. Generally, seaweeds can be divided into three groups, namely Rhodophyceae (6000 species), Chlorophyceae (2000 species) and Phaeophyceae (2000 species) based on their colour pigment (Guiry & Guiry, 2011). The genus being studied, Sargassum, belongs to the group Phaeophyceae, which obtains its distinctive brown colour from the xantophyll pigment of fucoxanthin. Cell walls of these algae are mainly composed of cellulose and alginic acid, a valuable component that adds commercial value to Sargassum species. In Asia, seaweeds are commonly used as fertilizers and as food for both humans and animals. Trono (1999), McHugh (2003) and Phang (2006) are among the many authors who have listed down the beneficial usages of seaweeds which include Sargassum as raw products for cosmetic and pharmaceutical industry.
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