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The over-reliance on synthetic nitrogen (N) in current farming is a major concern because of its adverse effects on soil quality, the environment, and crop production. Organic fertilizers such as seaweed extract (SE) and microbial inoculants (MI) provide alternatives to chemical fertilizers that could decrease the amount of synthetic N needing to be applied and improve crop growth productivity. This study evaluated the combined effect of SE and MI with reduced N rates on the growth, biomass accumulation, yield, and yield components of an N-efficient rice cultivar (Baixiang 139-A) and N-inefficient rice cultivar (Guiyu 9-B). Field experiments were conducted in the early and late growing seasons at different sites in Guangxi province, China, in 2019. A total of five treatments, such as T1: N 180 + SE 0 + MI 0 (kg ha-1) (control); T2: N 180 + SE 3 + MI 3 (kg ha -1); T3: N 144 + SE 3 + MI 3 (kg ha-1); T4: N 126 + SE 3 + MI 3 (kg ha-1); and T5: N 108 + SE 3 + MI 3 (kg ha-1) were used. The leaf area index (LAI), effective panicle number, grain per spike, grain filling rate, and 1000-grain weight were significantly increased in T2 and T3 compared with the control. T2 and T3 enhanced the biomass accumulation and grain yield of rice compared with the control. Furthermore, differences in the growth, yield, and yield components among the different cultivars were significant; however, there were no significant differences among the different locations. T3 increased the LAI, grain filling rate, biomass accumulation, and grain yield of rice by 4.5%, 5.9%, 6.6%, and 5.2%, respectively, compared with the control. Improvements in grain yield were mainly attributed to the enhanced growth and yield components. The correlation analysis also confirmed that LAI, productive tillers, grain filling rate, and biomass accumulation were positively correlated with grain yield. In sum, T3 (N144 + SE 3 + MI 3 (kg ha-1)) could achieve higher grain yield despite a reduction in the usage of chemical N. Generally, this study provides a sustainable nutrient management plan that increases crop production while minimizing costs of chemical N fertilizer application. Introduction Ac ce pt ed p ap er Rice (Oryza sativa L.) provides staple food for more than 60% of China’s population (Yuan 2014). Approximately 30.18 million hectares of rice were cultivated in China in 2018, accounting for 26% of the grain cultivated area nationally, and a total of 21.12 million tons of rice were produced (NBSC, 2018) China needs to produce 23,108 tons of rice by 2030 to meet the expected increase in per capita demand (Normile, 2008). Nitrogen (N) is important for plant growth, as N regulates crop yield by supporting the photosynthetic and sink properties of plants (Wang et al. 2016; Tubiello et al. 2015). The current farming system relies heavily on chemical N fertilizers to achieve higher yields. However, crop yield does not improve linearly with N fertilizer input (McGuire et al. 2015; Pathak 2011), and extreme N fertilization poses major environmental concerns, such as increased greenhouse gas emissions, groundwater pollution, and soil quality deterioration (Mahajan et al. 2008; Cai et al. 2018). Furthermore, the long-term use of nitrogenous fertilizers has been shown to increase the acidification, deterioration, and compaction of arable soils, thus limiting plant growth and production (Guo et al. 2017; Yue et al. 2017). This continued dependency on chemical N fertilizer for agricultural crop production is not sustainable. There is thus, a need to implement sustainable management strategies that can feed the rising population and reduce environmental costs. The amount of N fertilization in China was estimated to be approximately 29.619 million tons, accounting for 21% of agricultural N applications globally (FAO. FAOSTAT 2014). The average annual N fertilization rate is 180 kg/hm2, which is 75% greater than the worldwide average annual N application rate for rice (Ma et al. 2008; Peng et al. 2009; Chen et al. 2014) and exceeds the N fertilizer input for high-yield paddy fields (Peng et al. 2006). China is a major consumer of N fertilizer, but the N utilization rate is not high (30–50%) (Zhao et al. 2014). Moreover, more than 45% of N fertilizers are lost to the ecological environment (Ju et al. 2003; Ligeng et al. 2002). There is a growing interest in improving N-use efficiency to reduce costs without compromising crop productivity. Currently, the most challenging issue is to enhance grain yield, to feed the population on a sustainable basis with the least cost to the environment(Mueller et al. 2012; Morone et al. 2019). Several N fertilizer management techniques have been used, including optimal N Ac ce pt ed p ap er dosage (Chen et al. 2015), side-deep placement (Yao et al. 2015), and slow-release N fertilization (Yang et al. 2012). However, the implementation of these approaches has limitations because they are labor-intensive and lack of advanced technology (Anadon et al. 2016). In contrast to traditional synthetic N fertilization, organic fertilizers are considered effective and environmentally friendly alternative fertilizers to improve crop productivity and soil health (Iqbal et al., 2019; Iqbal., 2020a; Nkoa 2014). Seaweed extract (SE) is a bio-organic fertilizer derived from large algae in the ocean (Ullah et al. 2008). In this context, SE biostimulants are a greener alternative towards sustainably increasing crop yields (Ghosh et al., 2015). Unlike the traditional organic inputs employed in rice production which usually result in increased environmental impacts, at least in the short term (Hokazono and Hayashi, 2012), these biostimulants are unique wherein stable yield improvements are observed when used along with conventional fertilizers. Furthermore, its contains several mineral components and biologically active substances that can benefit crops and soil (Adekiya et al. 2019; Rathore et al., 2009). Seaweed extract contains a variety of growth regulators, such as cytokinin, auxin, and gibberellin required for plant growth and development (Durand et al. 2003). Seaweed extract can be naturally degraded, is non-toxic, does not pollute the environment, and provides an innocuous alternative to chemical fertilizers, which can induce much harm when applied in excess (Dhargalkar et al. 2005). Microbial agents are natural products composed of beneficial microorganisms and active microbial strains (Alori et al. 2018). They can stimulate microbial activity directly or indirectly and improve soil nutrient mobility (Suyal et al. 2016). They can be used as biological fertilizer in soil or plants and can promote plant growth, improve soil fertility, and enhance crop productivity (Jacoby et al. 2017). Microbial agents are products containing specific living microorganisms that can decompose organic and inorganic substances in the soil, activate soil nutrients, and enhance the ability of plants to absorb mineral nutrients (Muhammad et al. 2016). Application of MI can benefit the soil microbial composition and activity (Yu et al., 2010; Liu et al., 2015), may increase crop yield, improve soil chemical and physical properties including, the content and mineralization rate of soil Ac ce pt ed p ap er organic matter, the contents of essential nutrients and the structure of soil aggregates (Zhang et al., 2005; Liu et al., 2015). Moreover, application of MI may offer an effective way for incorporation of large straw loads into cropping soils instead of burning, and has been increasingly adopted in recent years in China. The application of SE and microbial inoculants (MI) can promote an increase in rice biomass accumulation and rice production (Chen 2017). Indeed, microbes are critical drivers of soil functions and agricultural crop productivity (Singh and Trivedi, 2017). One possible approach for reducing N input without compromising yield is to use a new green organic fertilizer as a supplementary fertilizer. Rice leaf area index (LAI), dry matter accumulation (DMA), and panicle grain number are strongly associated with grain yield (Iqbal et al. 2019). The LAI is an important indicator reflecting leaf coverage and crop growth (Katsura et al. 2007). Some studies have suggested that increases in LAI are beneficial for increasing grain yield (Haboudane et al. 2004; Hu et al. 2019). DMA after the heading stage is also closely related to rice grain yield, and the yield is affected by the proportion of dry matter distributed to the ear and the amount of material transferred to the ear in the later stage (Haiming et al. 2018; Tang et al. 2015). Previous studies have focused on the effects of N application rate and fertilization timing on different rice cultivars. However, our understanding of how variation in N utilization efficiency affects the growth and biomass of different rice cultivars and its relationship to rice grain yield under combined organic and inorganic amendments is poor. Here, organic fertilizer combined with N fertilizer was hypothesized to improve soil functionality and thus increase leaf area, dry matter, and grain yield. The objectives of this study were twofold: (1) to evaluate the joint effect of SE and MI with reduced N fertilizer on the growth, yield, and yield traits of different rice cultivars and (2) to characterize the relationships between leaf physiological characteristics, biomass accumulation, and grain yield. Materials and methods Experimental site and climatic conditions Ac ce pt ed p ap er The experiment was conducted in Guangxi province, southern China at three different sites (i.e., Binyang County, Liucheng County, and Yulin City) during the early (March–July) and late seasons (August–November). Table 1 shows the soil chemical properties of the experimental sites. Soil samples were collected at the depth of 0–20 cm before the commencement experiment. The soil was air-dried and crushed for initial characterization of soil chemical properties.

italian journal of agronomy

Synthetic nitrogen coupled with seaweed extract and microbial inoculants improves rice (&lt;em&gt;Oryza sativa&lt;/em&gt; L.) production under a dual cropping system