Livestock manure is considered to be a practical organic amendment for enhancing soil productivity as substitutes to chemical fertilizers in croplands (Du et al., 2020; Hua et al., 2020). However, improper application of livestock manure can create various environmental problems. Manure-derived nitrogen (N) can be lost through leaching, runoff, volatilization, denitrification, and other processes. Significant proportions of N are often lost via ammonia (NH₃) emissions from manure-amended soils. For instance, an average of 12.8% and 17.9% of the applied N was reported to have volatilized as NH₃ gas in Chinese upland and paddy fields, respectively (Zhang et al., 2013). The emitted NH₃ can lead to increased N deposition and contribute to soil acidification, water eutrophication, and particulate matter (PM) formation (Wu et al., 2016; Ehrnsperger and Klemm, 2021).

Primary PM comprise mixed liquid droplets and solid particles in the air (e.g., soot, dust, smoke). However, secondary PM₂.₅ having diameters less than 2.5 μm may be formed through reactions of NH₃ with other gaseous acidic precursors (e.g., NO_x, SO_x). NH₃ is a reactive gas that readily combines with sulfate (SO₄²⁻) and nitrate (NO₃⁻) components in acidic cloud droplets to form particulates (Asman et al., 1998). When PM₂.₅ is inhaled, it penetrates deep into the lungs to impair respiration by irritating and corroding the alveolar walls (Xing et al., 2016). Globally, over 4.2 million premature deaths and 103 million disability-adjusted life years have been attributed to PM₂.₅ pollution (Cohen et al., 2017).

Global N losses via NH₃ emissions are estimated to be 51–54 Tg N year⁻¹ (Bouwman et al., 1997). Approximately 10%–20% of these emissions originate from natural sources, while the remaining 80%–90% are from anthropogenic sources (Zhang et al., 2018). Among the anthropogenic activities, agriculture contributes to about 80%–90% of NH₃ emissions, with 63% from livestock feeding and 37% from crop cultivation (Bouwman et al., 1997; Piwowar, 2020). In soils, NH₃ emission intensities are influenced by several factors, such as N fertilization level and soil conditions (e.g., temperature, moisture content, pH) (Kang et al., 2016; Klimczyk et al., 2021). Compared to upland soils, flooded rice paddies may provide more favorable conditions for higher NH₃ emissions. Applied N in soils can mineralize into ammonium (NH₄⁺) and then be aerobically nitrified into nitrate (NO₃⁻) (Sah and Mikkelsen, 1983). However, the nitrification rate is strongly regulated by oxygen availability (Jechalke et al., 2011; Yang et al., 2016), which generally results in the prolonged presence of inorganic NH₄⁺ in flooded rice paddies and increased volatilization into NH₃ gas (Buresh et al., 2008).

Hypothetically, stabilized organic amendments (e.g., biochar, compost) may be more effective at reducing NH₃ emissions than fresh manures. In compost, inorganic N is predominantly available in the form of NO₃⁻, which does not revert to NH₄⁺. Stabilized organic matter can slow mineralization and thereby decrease NH₃ emissions (Lü et al., 2013; Agyarko-Mintah et al., 2017). Similarly, biochar contains less inorganic N owing to the expulsion of labile compounds during the pyrolysis process (Bruun et al., 2011; Buss et al., 2022). The organic C and inorganic N in biochar are hence more stable and resistant to decomposition (Haider et al., 2020). However, there are no readily available reports on the potentials of these stabilized organic amendments to reduce NH₃ emissions in flooded rice paddy soils.

To investigate the feasibility of stabilized manure amendments in reducing NH₃ emissions in rice paddies, three types of manures (fresh, composted, and biochar) were selected as the main treatments. The NH₃ emission losses and rice yield properties were characterized during rice cultivation to identify the organic amendment with the lowest NH₃ flux intensity.

The N losses via NH₃ emissions from the field-applied amendments have garnered significant attention, owing to their impacts on reduced fertilizer effectiveness and more environmental problems (Misselbrook and Powell, 2005; Monaco et al., 2012). Flooded rice paddies possess soil conditions that are particularly conducive to NH₃ volatilization compared to upland conditions owing to the reduced oxygen levels and higher pH during the flooded cropping season (Ventura and Yoshida, 1977; Zhao et al., 2009; Zhang et al., 2013). Urea-N applied to the soil is rapidly converted to NH₄⁺ ions, which can either volatilize into NH₃ gas or undergo biological nitrification to form NO₃⁻ ions (Saggar et al., 2013; Barth et al., 2019). However, the rate of nitrification is influenced by oxygen availability (Jechalke et al., 2011; Hsiao et al., 2014). In flooded rice paddies, NH₄⁺ ions can persist due to the low-oxygen environment, leading to higher NH₃ volatilization losses.

In the present study, soil Eh values indicating the oxidation and reduction states decreased rapidly from 150 mV to –260 mV immediately after flooding (Figure 1). This decrease signifies a significant reduction in the soil oxygen concentration after flooding, creating favorable conditions for NH₃ volatilization loss. The low-oxygen environment inhibits the activity of the nitrifying bacteria, which normally convert NH₄⁺ to NO₃⁻. Consequently, the NH₄⁺ ions accumulate in the soil and are more prone to volatilization as NH₃ gas (Cui et al., 2021). Furthermore, this soil reduction increases the soil pH to 7.5–9.0, which is another favorable condition for NH₃ emission that persists throughout the flooded period (Figure 4).

In the field study, similar levels of chemical fertilizers were applied to all treatments, and the N fertilizer (urea) was split into three application times (basal, tillering, and panicle initiation stages) during rice cultivation. Regardless of the manure management conditions, the NH₃ gas emissions fluctuated in similar patterns (Figure 2), sharply increasing within 2–3 days after urea application and then rapidly decreasing to the background levels. The results indicate that the NH₃ emission rates were highly correlated with NH₄⁺ concentrations in the floodwaters rather than in the soils (Figure 6). Unlike the general notion that NH₃ volatilization loss is influenced by inorganic NH₄⁺ content in the soil (Du Plessis and Kroontje, 1964), inorganic NH₄⁺–N concentration in the floodwater was found to be the most influential factor determining NH₃ flux in the rice paddy (Figure 6).

Interestingly, while the entire manure amendment was incorporated into the surface soil as the basal fertilizer before transplanting, the addition of urea through side-dressing significantly increased NH₃ volatilization loss in the manure treatments. This suggests that the added urea may have stimulated microbial activity (Xiang et al., 2023). Under favorable conditions, the soil microorganisms can metabolize urea and organic matter more rapidly, leading to increased NH₃ production as a metabolic byproduct (Lee et al., 2021; Li et al., 2022).

For example, the same amounts of urea N were applied in all treatments at the tillering stage (15 days after transplanting), but this addition increased NH₃ emission rates from tillering to panicle initiation fertilization by an average of 60% and 12% in the fresh and compost treatments, respectively. However, under biochar manure treatment, the NH₃ emissions were comparable to those without manure (Figure 2). The applied urea-N might boost microbial activity in the soil, accelerate the decomposition of organic amendments and soil organic matter, and subsequently increase NH₃ volatilization under anaerobic and high-pH conditions (He and Suzuki, 2004; Wang et al., 2021).

Among the selected manure amendments, the fresh and compost applications significantly increased the average seasonal NH₃ fluxes by 58% and 29% over no manure application, respectively (Figure 3). Biochar had an alkaline pH (8.4 ± 0.2 in water suspension) (Table 1), but its application did not increase NH₃ volatilization loss during the cropping season; instead, it decreased the seasonal NH₃ flux by approximately 7% compared to no manure application. Biochar is a C-rich solid product produced by the low-temperature pyrolysis of biomass (Lehmann and Joseph, 2015; Tomczyk et al., 2020); it contains oxygen functional groups, such as carboxyl and keto groups, which are responsible for NH₄⁺ adsorption through hydrogen bonding and electrostatic interactions (Cai et al., 2016). The reduction in NH₃ emission loss may be related to biochar’s high negativity (Gundale and DeLuca, 2007) and low inorganic N content (Table 1). Labile N in the biomass could volatilize during pyrolysis, resulting in biochar having very low inorganic N content (Ohtsuka et al., 1994; Clark et al., 2017). Some studies have shown the effectiveness of biochar in reducing N leaching losses (Major et al., 2012), with a high C/N ratio enhancing N immobilization (DeLuca et al., 2015).

However, compost contains higher concentrations of inorganic N (NH₄⁺ and NO₃⁻) compared to fresh manure (see Table 1), which makes it a less-effective application for reducing NH₃ volatilization losses. Rice grain productivity significantly increased with compost manure application (Figure 3), likely due to the higher N content (Table 1). However, fresh and biochar applications increased the average rice productivities by 5% and 11% over the control, respectively, although this was not a statistically significant difference compared to the control. The NH₃ intensity, indicating seasonal NH₃ flux per grain yield, was 12 kg NH₃–N Mg⁻¹ grain for the control treatment. Fresh and compost manure applications significantly increase this intensity by approximately 50% and 12% over the control, respectively (Figure 3), primarily due to the higher NH₃ emissions. However, biochar application decreased NH₃ intensity during rice cultivation by around 16% compared to that of the control.

Regardless of the organic amendment management condition, NH₃ gas was mainly volatilized immediately after application of the chemical N fertilizer (urea). However, manure-based amendments, when incorporated as basal fertilizers before rice transplantation, can significantly increase NH₃ emission rates following urea application. Both fresh and compost manures, which are characterized by high contents of inorganic N, notably increase NH₃ volatilization during rice cultivation. In contrast, biochar having high material pH but low inorganic N content, does not elevate NH₃ emission loss over that of the control. Notably, NH₃ emission rates are primarily governed by the NH₄⁺ contents in the flooded waters, rather than those in the soils. Although compost manure application significantly boosts rice grain yield, fresh and biochar manure applications result in slight productivity increments without statistical significance. Consequently, the application of fresh and compost manures can markedly increase NH₃ emission intensities, indicating the seasonal NH₃ flux per grain yield, whereas biochar manure does not exhibit such intensity. In summary, biochar as an organic amendment is a more viable option compared to fresh and compost manures for mitigating NH₃ emissions and enhancing soil productivity in rice paddies.