agronomy Article A Novel Compost for Rice Cultivation Developed by Rice Industrial By-Products to Serve Circular Economy Kalliopi Kadoglidou, Argyris Kalaitzidis, Dimitrios Stavrakoudis , Aggeliki Mygdalia and Dimitrios Katsantonis * Institute of plant Breeding and Genetic Resources, Hellenic Agricultural Organization—DEMETER, Thermi, Thessaloniki GR-57001, Greece * Correspondence: [email protected]; Tel.: +30-2310-471-544 Received: 18 July 2019; Accepted: 12 September 2019; Published: 15 September 2019 Abstract: Rice is the major staple crop worldwide, whereas fertilization practices include mainly the application of synthetic fertilizers. A novel compost was developed using 74% of rice industrial by-products (rice bran and husks) and tested in rice cultivation in Greece’s main rice producing area. Field experimentation was conducted in two consecutive growing seasons (2017 and 2018) and 1 comprised six fertilization treatments, including four compost rates (C1: 80, C2: 160, C3: 320 kg ha− 1 of nitrogen all in split application, C4: 160 kg ha− of nitrogen in single application), a conventional treatment, as well as an untreated control. A total of 21 morpho-physiological and quality traits were evaluated during the experimentation. The results indicated that rice plants in all compost treatments had greater height (8%–64%) and biomass (32%–113%) compared to the untreated control. In most cases, chlorophyll content index (CCI) and quantum yield (QY) were similar or higher in C3 compared 1 to the conventional treatment. C2 and C3 exhibited similar or greater yields, 7.5–8.7 Mg ha− in 1 1 2017 and 6.3–6.9 Mg ha− in 2018, whereas the conventional treatment resulted in 7.3 Mg ha− and 1 6.8 Mg ha− in the two years, respectively. No differences were observed in most quality traits that affect the rice commodity. The current study reveals that in sustainable farming systems based on circular economy, such as organic ones, the application of the proposed compost at the rate of 1 6 Mg ha− can be considered sufficient for the rice crop nutrient requirements. Keywords: rice bran; husks; paddy rice residues; organic fertilizer; plant growth; renewable inputs 1. Introduction Conventional agriculture plays the most significant role in meeting the food demands of the growing human population. However, the massive application of chemical fertilizers has disturbed field management, increasing the problem of soil, ground water, and air pollution at a global scale. Consequently, an increase in fertilizer application and a rise in the final food product prices has been observed, whereas the concerns of the farmers and the consumers are evident. Recent efforts have targeted toward the development of a healthier and nutrient rich food of high quality in sustainable comportments to ensure bio-safety. In agriculture, alternate means of soil fertilization relies on the organic inputs to boost nutrient supply and maintain the field management. The use of composts and biofertilizers is within the scope of a sustainable agricultural system that provides an ecologically healthy and economically viable crop production, especially when they are derived by low-value by-products of the cultivated plants like rice (Oryza sativa L.). Rice is a highly nutritious cereal that can be used as the source of several bioactive compounds. Specifically, brown rice is an excellent source of complex carbohydrates, vitamins, minerals, phytosterols, Agronomy 2019, 9, 553; doi:10.3390/agronomy9090553 www.mdpi.com/journal/agronomy Agronomy 2019, 9, 553 2 of 18 and dietary fibers [1]. However, because of the global preference for consumption of milled rice, most nutritional parts of the grain are lost during the milling process by the removal of the pericarp, the seed shell, and the embryo of the grain, known as rice bran (RB), which contains rice bran oil and holds approximately 10% of the overall rice yield [2]. RB is rich in protein, vitamins, essential minerals, and antioxidant compounds [3]. Previous studies indicated the possible use of RB as a natural fertilizer containing 2.5% nitrogen, 3% phosphorus, 2.3% potassium, and 1% magnesium, whereas the C/N ratio is approximately 19 [4]. It is estimated that the world annual production of RB amounts to 76 million tons [5,6]. Another notable product derived from the de-husking process in rice mills is the seed shells, known as husks or hulls. These are parts of the rice seed (palea and lemma) and they represent 20%–22% of the milling process’s residuals [7]. Rice husks (RH) are composed of 28% cellulose, 28.6% hemicelluloses, 24.4% lignin, and 18.4% extractive matter [8]. According to Vadivel and Brindha [9], the global production of RH is very significant and falls in the range of 20 million tons annually. Therefore, the utilization of the RH is of high importance. Since RH are inedible by humans, but suitable for animal feed [10], they can be used in various non-food applications. The most important uses of RH are the production of energy (fuel) and ethanol, the pyrolysis for silicon dioxide production [7], the smoking of foods, and the acceleration of the bioremediation process into the soil [11]. Ogbo and Odo [12] confirmed the suitability of RH as a carrier for biofertilizer production. Moreover, Evans and Gachukia [13] reported that parboiled fresh husks can be used as a low cost perlite substitute in greenhouse substrates without any significant reduction in plant growth or quality, while at rates up to 40% (v/v) no significant reduction was observed impacting plant tissue N (without N depletion). Extracts from RH act not only against phytopathogenic fungi, such as root rot seedling rice [14], but also against algae [15] and bacteria [16]. Expanding the positive effects of RH on rice straw, Iranzo et al. [17] found that the characteristics (physical, chemical, microbiological properties) of the rice straw were complementary to those of the sewage sludge for their application as a compost. Despite the fact that the activity of RB and RH were investigated intensively, especially the antioxidant activity of RB and the phytochemical of RH, their simultaneous exploitation in sustainable farming systems has not yet been deeply investigated. The aim of the current study is to investigate the potential use of a natural fertilizer, produced by rice by-products such as a combination of RB and RH, in rice cultivation through the evaluation of agronomic and physiological parameters, as well as of rice productivity and quality indices, serving the principals of circular economy. Extending the research outcomes, this natural RB fertilizer can be utilized in organic rice cultivation. 2. Materials and Methods 2.1. Site and Climatic Conditions Two field experiments were conducted in two consecutive years, 2017 and 2018, at the Experimental Station of Kalochori, Thessaloniki, Greece (40◦36058.75” N, 22◦49051.16” E). The soil consisted of 22% 1 clay, 50% silt, 28% sand, with 2.84% organic matter, electrical conductivity 1.3 dS m− and pH 7.5. Temperature and relative humidity were constantly recorded throughout both cultivation periods, in order to compare the annual meteorological datasets. Table1 presents the monthly averages of temperature and relative humidity during the two growing seasons of the experimentation, with the values being the norm of a typical Mediterranean climate. Agronomy 2019, 9, 553 3 of 18 Table 1. Monthly temperature and relative humidity during the rice growing seasons in 2017 and 2018 at the experimental site. Minimum, maximum, and average values of each year are reported, averaged over each month. Temperature ( C) Relative Humidity (%) Year Month ◦ Agronomy 2019, 9, x FOR PEER REVIEWMinimum Maximum Average Minimum Maximum Average 3 of 18 June 18 32 25 50 84 61 AugustJuly 18 19 34 34 24 26 60 5398 9679 73 2017 August 18 34 24 60 98 79 SeptemberSeptember 14 14 32 32 21 21 52 5297 9778 78 October 13 13 28 28 19 19 43 4389 8969 69 JuneJune 20 20 32 32 25 25 51 5191 9172 72 JulyJuly 19 19 33 33 26 26 50 5096 9673 73 2018 August 19 34 25 62 100 87 2018 SeptemberAugust 19 16 34 31 25 22 62 68100 100 87 90 SeptemberOctober 16 11 31 26 22 18 68 51100 95 90 79 October 11 26 18 51 95 79 2.2. Compost Preparation and Properties 2.2. Compost Preparation and Properties Initially, the compost formula was developed in preliminary experiments during 2016, where the compostingInitially, mixture the compost was placed formula under was a PolyVinyldeveloped Chloride in preliminary (PVC) PVCexperiments soil cover during sheet and2016, exposed where theto an composting aerobic digestion mixture in was heaps placed of approximately under a PolyVinyl 1.5 1.5 Chloride0.5 m2 (PVC)volume PVC Figure soil1 a.cover The sheet materials and × × 2 wereexposed mixed to an at aerobic a weekly digestion basis, whereas in heaps moisture of approximately was controlled 1.5 × 1.5 at × 40%–60% 0.5 m volume by watering Figure 1(a). with The tap materials were mixed at a weekly basis, whereas moisture was controlled at 40%–60% by watering water every 5 days. The temperature was checked to be never above 60–65 ◦C at the center of the pile with tap water every 5 days. The temperature was checked to be never above 60–65 °C at the center using a hand-held thermometer. A temperature of 60–65 ◦C was not surpassed, to avoid alterations of the pile using a hand-held thermometer. A temperature+ of 60–65 °C was not surpassed, to avoid that might take place in the microflora and losses of NH4 -N. The composting process was performed + alterationsfor at least 40that days, might while take the place C/N in ratiothe microflora was being and checked losses on of a NH frequent4 -N.