Optimal nutrient management in rice is important for food security, climate change mitigation, adaptation and transformation, and attainment of several sustainable development goals (Cakmak, 2002; Kanter et al., 2019; Lal et al., 2020). Fertilizer use has reduced agriculture expansion into natural ecosystems by increasing crop productivity on existing land. However, while yields increased with fertilizer use in the 1960s, they stagnated in intensive rice systems in the mid-1980s despite the development of varieties with greater yield potential (Dawe and Dobermann, 1999). This resulted in large yield gaps. This was largely due to excessive or imbalanced fertilizer use based on increased reliance on blanket fertilizer application, coupled with a rapid decline in the efficiency of fertilizer uptake by plants, indicating that increased fertilizer use outpaced yield improvements (Cassman and Pingali, 1995; Tilman et al., 2002). The orientation of producing more food, associated with fertilizer overuse, particularly nitrogen (N), has caused a deterioration in soil physical, chemical, and microbiological properties and functions and increased soil and water pollution (Pingali, 2012; Srivastava et al., 2020). With increasing pressure to meet global food demand while fostering environmental sustainability, a paradigm shift is needed to a more judicious use of N fertilizer.

About 50% of global N fertilizer is applied to major cereals: Zea mays (maize; 17%), Triticum aestivum (wheat; 18%), and Oryza sativa (rice; 16%) (Heffer et al., 2017). However, globally, N use efficiency, a measure of the short-term balance between N used for grain production and N lost to the environment, has remained below 40% (Omara et al., 2019), indicating that more than 60% of applied N remains unused or is lost from soil (Dobermann, 2000; Ladha et al., 2005). Increasing N use efficiency in rice agri-food systems becomes all the more important, given that the commodity is a staple food for more than half the global population (GRiSP, 2013). More than 90% of the rice is produced in Asia, mostly by smallholder farmers. Due to high subsidy on urea fertilizer, farmers tend to apply large quantities of N fertilizer in excess of plant requirements (Ladha et al., 2005). However, grain yield response diminishes as N fertilizer rate increases and may cause lodging and susceptibility to pest and disease damage when overapplied (Balasubramanian et al., 1998; Duy et al., 2004). Excess reactive N has detrimental effects in agroecosystems, such as nitrous oxide emissions, increased soil acidity, decreased biodiversity, and groundwater contamination (Galloway et al., 2003).

Data from 46 published articles with studies conducted between 2001 and 2020 were compiled in a database (Table 1). These studies were conducted in 11 countries: eight in Asia and three in Africa. Using Web of Science, Science Direct, and Google Scholar, we used the following search terms: SSNM, SSNM rice, SSNM maize, SSNM wheat, SSNM cereals, and SSNM vs. farmers' fertilizer practice (FFP). The studies included peer reviewed journal publications, book chapters, and technical reports that show direct comparison between SSNM and FFP in the same fields. We excluded studies when SSNM was compared with other treatments such as the no input control, blanket fertilizer recommendation, government or institute recommendation, or soil test-based recommendations. We included only studies that followed the generic SSNM approach and excluded studies where factors other than fertilizer management differed between SSNM and FFP. When multiple publications reported the data from the same experiments, we used the paper with the most complete dataset. Studies were excluded if the experimental method was vague and when there are varied factors other than fertilizer management between SSNM and FFP treatments. Hence, agronomic practices except nutrient management were the same or similar in both treatments. Of the 46 studies, 23 of them conducted N omission treatment thus enabling them to calculate and report AEN (Equation 1). In some cases, AEN values were extracted from the papers. Partial factor of productivity of N (PFP N) was calculated for all studies (Equation 2).

Where GY_N is the grain yield in a treatment with N application.

GY₀ is the grain yield in a treatment without N application.

Averages of yields and AEN of both SSNM and FFP were obtained from each study, while in some instances, manual estimation from the figures was performed when these data were only presented in figures. Individual studies have variable number of replicates with 323 as the highest number. Also, nutrient rates among the fields varied and were reported as a range because there is a high spatial variability even for neighboring fields. Thus, the average between the minimum and maximum values reported was determined across the replicates and was used as nutrient rate for the reported grain yield. The most common sources of fertilizers used in the studies were urea for nitrogen, DAP for P, and muriate of potash (KCl) for potassium.

The cost benefits were reported as gross return above fertilizer cost (GRF) in this paper, which was derived from the other two economic performance metrics: total fertilizer cost (TFC; Equation 3) and gross return (Equation 4). GRF was calculated as in Equation 5. Fertilizer prices of the most common sources: urea (46-0-0) for N, DAP (18-46-0) for N and P, and KCl (0-0-60) for K (urea, DAP, and KCl), were estimated from the 10-year average across countries listed in the database (Indexmundi, 2020; https://www.indexmundi.com). The prices were calculated as per unit of nutrient leading to US$0.642 kg⁻¹ N, US$2.151 kg⁻¹ P, and US$0.633 kg⁻¹ K. Farm gate price of paddy rice was used at US$0.25 kg⁻¹ paddy rice based on the trend of the market price for the past 25 years (Indexmundi, 2020; https://www.indexmundi.com).

Where pN, pP, pK = prices of N, P and K fertilizers, respectively (US $ kg⁻¹).

N_rate, P_rate, K_rate = amount of N, P, and K applied (kg ha⁻¹).

FGP = farm gate price of paddy rice, maize, or wheat (US$ kg⁻¹).

GY = grain yield of paddy rice, maize, and wheat (kg ha⁻¹).

Our mini-review shows SSNM as an effective N management strategy for improving rice productivity and profit for farmers while increasing N use efficiency, thus attaining environmental benefits. On average, optimized nutrient management reduced N fertilizer inputs, improved yields, and hence, increased N use efficiency. While N use efficiency has been used to indicate the balance between N used for grain production and losses to the environment (Dobermann, 2007; Omara et al., 2019), only a few studies have quantified N losses under SSNM. For example, a recent study using Nutrient Expert showed both agronomic and environmental benefits of SSNM in a rice–maize cropping systems in China (Wang et al., 2020). In that study, N losses and GHG emissions with SSNM were 10.1 and 6.6% lower than FFP for rice and 46.9 and 37.2% for maize, respectively. The reduced losses arose from increased N use efficiency. Nutrient Expert was also used for winter wheat in North China where N fertilizer rates were 41.4% lower with SSNM than FFP, leading to a 70% increase in agronomic N use efficiency and 55% lower emissions of N₂O (Zhang et al., 2018).

Similarly, using Nutrient Expert in a wheat cropping system in India, GHG emissions were 16–42% lower under SSNM than FFP, both under conventional and no-till, but with greater benefits under no-till (Sapkota et al., 2014). Sapkota et al. (2021) observed a 2.5% reduction in global warming potential associated with reduced GHG emissions, increased rice yields and profit in India when Nutrient Expert was compared to FFP. Earlier studies in the Philippines and Vietnam (Pampolino et al., 2007) and in China (Wang et al., 2007) also showed increased N use efficiency with reduced N losses through leaching, runoff, and N₂O emissions with SSNM compared to FFP. SSNM, hence, provides a climate mitigation nutrient management option compared with the FFP. Considering the wide range of conditions where rice is grown, there is need to evaluate the benefits of SSNM in more locations and using different digital tools. However, adoption of SSNM recommendations has been low, highlighting the need to strengthen extension systems through public and private partnerships.

Currently, the SSNM-based digital tools, including RCM, provide pre-season nutrient management recommendations and focus on balanced application of N, P, and K with little emphasis on micronutrients. On the other hand, farmers generally lack awareness of the importance of micronutrients and there are less obvious and/or immediate yield gains and profit associated with micronutrient fertilization. This has resulted in mining of these micronutrients from rice soils, while malnutrition, particularly due to zinc and iron deficiency, is common for communities relying on rice-based diets (Palanog et al., 2019). While much of the research on improving micronutrient nutrition in rice have been through breeding (Dixit et al., 2019), there is also need to optimize micronutrient management, along with major nutrients, for the production of healthier rice through agronomic bio-fortification as soil or foliar application (Hakoomat et al., 2014). Given that more iron and zinc is needed during the early growth stages of rice, application in the soil is more practical than foliar application, which requires the plant leaves to have been developed significantly in order to effectively take up the foliar applied nutrients. However, zinc has been shown to convert to unavailable forms immediately after the application of zinc sulfate in flooded soils (Bunquin et al., 2017). Thus, soil application of zinc is not highly effective in flooded rice fields. Although foliar application of zinc at later growth stages of rice does not result in yield gain, it has been shown to increase grain zinc content and is therefore important for the production of healthier high-zinc rice (Hakoomat et al., 2014; Rubianes et al., 2018).

While micronutrient fertilization improves the nutrition value of rice grain, farmers are often not compensated for the extra input costs. They are unwilling to invest in micronutrient management in the absence of other incentives. Policy shifts are needed to reward farmers for the production of more nutritious and healthier rice; some of them necessarily require active public–private partnerships. For example, the Sustainable Rice Platform, which is promoting a premium price for sustainably produced rice (SRP, 2019). Such efforts can foster environmental sustainability, ensuring that rice systems in the Global South deliver essential ecosystem services while also improving farmers' livelihoods. The changing climate and other driving forces like shortage of labor and water dictate shifts from continuous intensive rice systems to changes in agronomic management practices, such as direct seeded rice, non-puddled rice, and water saving technologies. Increasing rice productivity while minimizing adverse environmental consequences require the adoption of integrated nutrient and crop management practices that increase system-level efficiency.

Our mini-review clearly shows that SSNM in rice cropping systems increases rice yield, profit, and N use efficiency while reducing N losses and GHG emissions when compared with the farmer practice. AEN and PFP were 38.2 and 18.2% greater with SSNM than the farmer practice. This was achieved using 14% lower N fertilizer. The superior performance of SSNM compared to the farmer practice is mainly due to better distribution with more splits of N fertilizer during the growing season coupled with balanced fertilization. However, SSNM has mainly focused on the major nutrients, ignoring micronutrients, and thus, impacting human nutrition for those whose diets are rice-based, while potentially mining the soils of the micronutrients. SSNM-based digital decision support tools enable dissemination of SSNM recommendations at scale, but this requires a pluralistic approach that fosters collaboration among multiple organizations and service providers with support from governments. Additionally, linking the digital tools with GIS and remote sensing tools allows fine-tuning of SSNM recommendations, addressing the huge spatial variability in smallholder farming systems. SSNM research and evaluation has focused on favorable environments in Asia, despite the increasing demand and production in Africa. More research is needed on SSNM under diverse management practices, such as direct seeding, and in marginal environments along with quantification of nutrient losses, along with inter-disciplinary approaches to enhance farmer uptake of SSNM.

PC, SS, and JH conceived the project. MB extracted data from publications. PC and MB conducted analyses. PC wrote the manuscript draft. All authors contributed to the literature search, interpretation of the results, and writing of the final paper.

The Swiss Agency for Development and Cooperation (SDC; Grant No. 81016734) through the Closing the Rice Yield Gap (CORIGAP) project provided support for this review. The SSNM approach reviewed here was initially developed under the Reversing Trends of Declining Productivity in Intensive Irrigated Rice Systems project (RTDP), also funded by SDC. SSNM was field tested and fine-tuned through subsequent SDC funded projects including Irrigated Rice Research Consortium (IRRC), a predecessor of CORIGAP.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.