Rice cultivation is an environmentally costly agricultural technology. Although chemical fertilizers, such as urea, are massively applied to maximize yield 10,26,27, it is also known to result in significant environmental burdens, such as GHG promotion 28,29. Our results provide solid experimental evidences that next generation fertilizers, such as FeONPs, can break the trade-offs between rice production (food security) and GHG emissions (climate regulation) by promoting rice production, while decreasing GHG emissions. These findings have vast consequences for the management of rice fields worldwide, and for the consecution of the Sustainable Development Goals.
Nano iron fertilization helps climate change mitigation by decreasing greenhouse gas emissions and promoting carbon sequestration
GHG emissions were evaluated after four years of experiment. Urea fertilization increased both CH4 and N2O emission rates during the rice growing season (Fig. S1), compared with negative control (i.e., No fertilization). In contrast, FeONPs fertilization decreased CH4 emission rates across contrasting crop stages (e.g., at the tillering, jointing, and booting stages (Fig. 1A). Moreover, FeONPs fertilization decreased N2O emission rates at the jointing and booting stages, compared with Urea fertilization (Fig. 1D). Consequently, FeONPs fertilization reduced more than 40% cumulative CH4 and N2O emission compared with Urea and No fertilizations (P < 0.05) (Figs. 1B and 1C). Global warming potential (GWP) 30 and greenhouse gas intensity (GHGI) 5,31 were respectively calculated and both were significantly lower under FeONPs fertilization compared with positive and negative controls (P < 0.05) (Figs. 1E and 1F).
To further investigate the reported decrease in CH4 emissions in response to FeONPs fertilization, the microbial functional genes related to methanogenesis (mcrA gene) and methanotrophs (pmoA gene) were determined across different fertilization strategies. We found that FeONPs decreased the abundance of methanogenic mrcA gene (Fig. 1G). We further found that soil enzyme activities involved in soil carbon cycling 32,33 were depressed by FeONPs (Table S1). This was especially important for the ratio of oxidases and hydrolases (Fig. 1H). Such result suggests that the small molecular organic components that serve as substrates for methanogenesis were decreased in response to FeONPs fertilization. In turn, the decrease in substrates for methane production limited the abundance of the mrcA gene. This inhibitory effect on CH4 emissions was also reflected in a decline of methane-oxidizing pmoA genes (Fig. S2), for which methane is the main substrate for methanotrophs 34.
We then further investigated N2O associated microbial processes. First, we found FeONPs fertilization did not significantly affect any of the tested 10 microbial genes involved in nitrification and denitrification (P > 0.05) (Fig. S2). Since we know that the Feammox process is involved in both Fe and N cycling in paddy soils 35–37, we further used the 15N isotope tracing method to investigate the response of Feammox activity to FeONPs fertilization. We revealed that FeONPs fertilization significantly increased Feammox activity compared with Urea fertilization (P < 0.05) (Fig. 1I). We further showed that the abundances of Geo gene (i.e., Geobacteria driving Feammox process) were enhanced under FeONPs fertilization (Fig. 1J). Moreover, we found that soil NH4+ was not affected by FeONPs fertilization compared with Urea fertilization (Fig. 1K), suggesting that additional NH4+ was oxidized by Feammox process. Importantly, the main products of the Feammox process contain NO2–, NO3–, and N2, but no N2O 38,39. In this regard, FeONPs fertilization increased soil NO3– during the booting and harvesting stages (Fig. 1L) while decreasing N2O emission.
Finally, our results also showed that FeONPs fertilization has the potential to promote carbon sequestration in soils. Particularly, FeONPs fertilization decreased both soil oxidases and hydrolases compared with no and/or Urea fertilizations (Table S1). More importantly, the ratios of oxidases to hydrolases were significantly lower under FeONPs fertilization, compared with controls (P < 0.05) (Fig. 1H). Soil hydrolases degrade complex organic matter into simpler compounds, while soil oxidases can act as the ‘latch’ to avoid soil organic matter decomposition 40,41. The decrease in the ratio between oxidases and hydrolases 33,42 supported a larger sequestration of soil organic carbon (Fig. 1M). In this regard, FeONPs fertilization can facilitate long-term increases in the soil fertility of the rice ecosystem. Together, these results suggest that FeONPs can help mitigate climate change while supporting food production compared with traditional fertilizers.
Nano iron fertilization enhances rice production
Our four-year field experiment demonstrated that FeONPs fertilization significantly enhanced rice yields compared with Urea and/or No fertilizations (Fig. 2A) (P < 0.05). FeONPs fertilization increased plant biomass, which especially noticeable during the harvesting stage (P < 0.05) (Fig. 2B exhibiting the data at the fourth year).
The positive influence of FeONPs fertilization on food production is probably associated with its capacity to generate soil fertility. FeONPs fertilization significantly increased plant N content during the jointing, booting, and harvesting stages (P < 0.05) (Fig. 2C). As evidence, the N-utilization efficiency was significantly enhanced by FeONPs fertilization (P < 0.05) (Fig. 2D). Our field experiments showed that ammonia volatilization significantly decreased during the whole rice growing season (P < 0.05) (Fig. 2F). To unravel the underlying mechanism, we conducted the infrared spectra analysis to verify whether FeONPs can absorb free ammonium ions. The results showed that a new absorption peak at 145 cm− 1, which represents N-H bond, was observed in the entrapment of ammonium by FeONPs, indicating a strong absorption ability of FeONPs to ammonium (Fig. 2G). It is corroborated that nano effects, such as the high surface area, can make FeONPs highly reactive and efficient in delivering nutrients to plants 43. Additionally, FeONPs fertilization enhanced the levels of chlorophyll (Fig. 2E), by increasing Fe bioavailability 18.
To further confirm whether FeONPs fertilization can hold more N nutrient in paddy soils, the soil available inorganic N contents were measured throughout the rice growing season. It was observed that FeONPs fertilization did not change soil NH4+ (Fig. 1K), but it increased soil NO3– during the booting and harvesting stages compared to Urea fertilization (Fig. 1L). Rice prefers NH4+-N during the initial growth stages (from transplanting to jointing stages), while it absorbs more NO3–-N after the booting stage 44,45. Therefore, such synchronization suggests that FeONPs fertilization increases the ability of nitrogen supply by increasing the adsorption capacity of ammonium, facilitating the slow and steady release of nutrients, and thereby enhancing nutrient use efficiency and plant N content in the FeONPs fertilized paddy fields.