Introduction

The cycling of carbon (C) and nitrogen (N) is integral to the functionality of agricultural ecosystems and plays a crucial role in the broader terrestrial biogeochemical cycles1. The dynamics of C and N, including their migration and transformation, are essential for the stability and environmental impact of agricultural systems2,3. The dissolved C and N are particularly prone to mobility, making them susceptible to losses during these cycling processes. Such fluctuations in their concentrations are indicative of changes in soil C and N stocks and are sensitive to environmental variables4. The dissolved C and N in farmland are susceptible to leaching and runoff processes, thereby resulting in positive losses of C and N in agricultural ecosystems5. In agricultural landscapes, dissolved C and N are vulnerable to leaching and runoff, leading to net losses that can reduce soil fertility and contribute to wider environmental issues such as the eutrophication of water bodies6,7. Thus, reducing the losses of dissolved C and N in farmland is essential for curbing environmental problems while promoting sustainability and enhancing the quality of agricultural ecosystems.

Rice paddies are a vital component of agricultural ecosystems and play a pivotal role in cereal production worldwide8. Specifically, the cultivation area dedicated to rice production in China represents approximately 25% of the total arable land allocated for cereal crops, contributing to roughly 30% of the nation’s aggregate grain output9. The double-cropped rice system, prevalent in southern China, is responsible for 27% of the country’s rice output and encompasses 32% of its rice-growing area10. Characterized by a warm climate conducive to rice growth and frequent episodes of intense precipitation, southern China experiences higher losses of C and N compared to other regions2. The C and N losses occur predominantly via surface runoff and subsurface leaching, presenting significant challenges to nutrient retention and ecosystem sustainability within rice paddies. Soil acidification is one of the key factors affecting rice yield in the double-cropped rice system11. Numerous studies have indicated that lime application effectively mitigates soil acidification and enhances rice yields12,13. Specifically, liming promotes root growth and the absorption capacity for mineral nutrients in rice, thereby promoting the return of litter to the rice fields and enhancing C availability14,15. Liming enhances the activity of enzymes associated with soil C and N metabolism (e.g., cellulase and protease), promotes the mineralization of soil organic matter and plant litter, thereby accelerating the release of mineral nutrients from the soil12,13,15. Lime-induced elevation of soil pH enhances negative surface charge density, reduces cation retention capacity (e.g., NH4+) and promotes nitrate (NO3−-N) leaching via electrostatic repulsion16,17. Elevated pH favors Ca2⁺ displacement of Al3/Fe3⁺, disrupting soil aggregate stability and increasing macroporosity, thereby accelerating preferential flow pathways for C and N18. Finally, liming-induced increase in soil pH may enhance microbial activity associated with nitrification processes, thereby increasing soil NO3−-N concentration and elevating the risk of leaching losses12,19. Previous research has shown elevated levels of dissolved C and N in leachates from dryland soils following liming20,21. In contrast, the impact of liming on the leaching losses of C and N remain unexplored in rice fields with acidic red soil. Here, we hypothesized that: (1) Lime application enhances the leaching of dissolved C and N in double-cropped rice systems and (2) Liming effects on C and N mineralization in paddy fields vary seasonally.

To investigate the effects of liming on the leaching of dissolved C and N, we conducted a field experiment in a double-cropped rice field with acidic red soil. Our results may provide recommendations for optimizing lime application strategies and mitigating nonpoint-source pollution in the double-cropped rice systems.

Materials and methods

Site description

Field experiments were initiated prior to the transplantation of early rice in 2015 at a site located in Zengjia Village, Shanggao County, Jiangxi Province, China (28° 31′ N, 115° 09′ E). The experimental site experiences a southern monsoon climate characteristic of the northern subtropical zone, exhibiting a suitable combination of temperature, light, and moisture conditions (mean annual temperature of 17.5 °C, mean annual sunshine duration of 1700 h, and mean annual rainfall of 1650 mm), along with well-defined seasonal variations. Rice is the primary cereal crop in the experimental area, following a double cropping system that includes early rice (planted from April to July) and late rice (planted from July to November). This system is complemented by a winter fallow period from November to April of the following year. The soil at the experimental site is classified as Typic Stagnic Anthrosol and consists of Quaternary red clay. Before initiating the experiment, we evaluated the basic physical and chemical properties of the plow layer (0–15 cm depth), which showed a bulk density of 1.1 g cm−3, organic matter content of 18.1 g kg−1, total N concentration of 1.1 g kg−1, alkaline hydrolyzed N of 115 mg kg−1, clay content of 17.0% (< 0.002 mm), and a soil pH of 5.2.

Experimental design

A completely randomized block design was employed with two treatments and three replicates. Detailed information of the experimental design can be found in Liao et al.13. The treatments included liming and the unlimed control. Each experimental plot was assigned a space of 25 m2 (5 m × 5 m) and separated by 20 cm thick levees with plastic film coverings for each replication. In the case of liming treatment, Ca(OH)2 was applied at a rate of 2.1 t and 2.0 t per hectare prior to the transplantation of early rice in 2015 and after the late rice harvest of 2018, respectively13 The rate of liming was determined through a standardized procedure. In brief, 10.0 g of air-dried surface soil (0–15 cm depth, 2-mm-sieved) was mixed with 40 mL of 0.2 mol L−1 CaCl₂ solution, followed by incremental titration with 0.15 mol L−1 Ca(OH)2 until the soil–water suspension reached the target pH of 7.012. In both limed and unlimed plots, the rice straw was returned into plowed soils. The rates of mineral N, P, and K were 120 kg ha−1, 33 kg ha−1, and 31 kg ha−1 for the early rice and 150 kg ha−1, 33 kg ha−1, and 31 kg ha−1 for the late rice, respectively. Urea served as the N fertilizer, whereby an application ratio of 5:2:3 was employed for basal fertilizer, tiller fertilizer, and panicle fertilizer. For the P fertilizer, calcium magnesium phosphate was employed as the basal fertilizer for one-time application. As for the K fertilizer, potassium chloride was used at a ratio of 5:5 for basal fertilizer and panicle fertilizer. We planted a hybrid indica rice ‘Qiliangyou 2012’ in the early rice season, and an inbred indica rice ‘Meixiangzhan 2’ in the late rice season in 2019. In both early and late season, the level of water was maintained at approximately 3 cm after transplanting rice. During the mid-tillering stage, the procedures of drainage and sun drying were implemented. Alternating dry and wet irrigation was consistently maintained after the rehydration process until the water was cut-off about 7 days before rice harvest. Chemicals were applied to control weeds, diseases, and pests during rice production based on local high-yielding practices. The daily mean air temperature and precipitation during the rice growth seasons in 2019 are showed in Fig. 1.

Fig. 1
figure 1

The daily mean air temperature and precipitation during the rice growth season in 2019.

Full size image

Sampling and measurement

Before the application of basal fertilizer, we conducted a soil profile excavation with a depth of 40 cm for each field. A ceramic attachment positioned at the base of the Rhizon Soil Moisture Sampler (SMS, Eijkelkamp, Netherlands) was inserted horizontally into the compacted soil. The excavated soil was then replaced back into its original position. At the same time, we connected a three-way valve to the upper end of the rhizosphere solution sampler to collect the leachates16,22,23. At the critical growth stage of rice, we collected leachate samples using a 50 mL sterile centrifuge tube. We filtered the leachates samples using a 0.45 µm microporous membrane, and measured the concentrations of DOC and dissolved total organic N (TON) by multi N/C 2100 (Jena GmbH, Germany). To measure NH4+-N and NO3−-N concentrations of the leachate samples, the indigophenol blue colorimetry and ultraviolet–visible spectroscopy methods were employed, respectively. The concentration of DON in the leachates was derived by subtracting the NH4+-N and NO3−-N from TON24,25.

At maturity, the grain yield was measured from a 5 m2 sampling area in each plot and adjusted to 14% moisture content. Additionally, five soil cores (3 cm in diameter) were collected to a depth of 15 cm from each plot at the maturity stage of late rice. These cores were combined into a composite sample per plot, air-dried, and then sieved through a 2 mm mesh for analysis. Soil pH was measured using a pH meter in a 1:2.5 (w/w) soil-to-water mixture. Alkaline hydrolyzable-N was determined using the microdiffusion method with NaOH. Total soil N was measured by dry combustion with an elemental analyzer (Elementar, Vario Max, Germany), while organic C was quantified using the dichromate oxidation method24.

Statistical analyses

All statistical analyses were conducted in SPSS 18.0 (IBM Inc., NY, USA). The t-test was used to examine the effects of liming on grain yield, leaching losses of C and N, and soil properties in 2019.

Results

Grain yield

Liming significantly increased grain yield by 12.7% and 12.3% in the early and late rice season relative to control, respectively (Fig. 2).

Fig. 2
figure 2

Effects of liming on grain yield in the double-cropped rice field with acidic red soil. Error bars represent the standard deviation of the mean (n = 3). Significant treatment effects are showed by *(0.01 < p ≤ 0.05).

Full size image

Ammonium nitrogen

The NH4+-N concentration in the leachates of double-cropped rice fields was low (< 51.0 μg L−1), and no significant difference was found among treatments (Fig. 3). The NH4+-N concentration of leachates at the seedling stage and mid-tillering stage was higher than that at the panicle initiation stage, heading stage, and grain filling stage of early rice.

Fig. 3
figure 3

Effects of liming on the leaching losses of NH4+-N in the double-cropped rice field with acidic red soil. Error bars represent the standard deviation of the mean (n = 3). SD, MT, PI, HD, and GF indicate seedling, mid-tillering, panicle initiation, heading, and grain filling stages, respectively.

Full size image

Nitrate nitrogen

As a result of lime application, the concentration of NO3−-N exhibited an increase in the leachates at various growth stages of early rice, showing a significant 29.6% increase at the mid-tillering stage (Fig. 4a). Liming increased the NO3−-N concentration of leachates by 14.1% at the seedling stage of late rice (Fig. 4b). There were no significant differences between the treatments at the mid-tillering stage, panicle initiation stage, heading stage, and grain filling stage of late rice.

Fig. 4
figure 4

Effects of liming on the leaching losses of NO3−-N in the double-cropped rice field with acidic red soil. Error bars represent the standard deviation of the mean (n = 3). SD, MT, PI, HD, and GF indicate seedling, mid-tillering, panicle initiation, heading, and grain filling stages, respectively. Significant treatment effects in the rice growth stage are showed by * (0.01 < p ≤ 0.05).

Full size image

Dissolved organic nitrogen

Liming had a positive effect on the DON concentration in the leachates at the seedling stage of early rice (+ 48.9%; Fig. 5a). In the late rice season, liming did not affect the concentration of DON in leachates (Fig. 5b).

Fig. 5
figure 5

Effects of liming on the leaching losses of dissolved organic nitrogen (DON) in the double-cropped rice field with acidic red soil. Error bars represent the standard deviation of the mean (n = 3). SD, MT, PI, HD, and GF indicate seedling, mid-tillering, panicle initiation, heading, and grain filling stages, respectively. Significant treatment effects in the rice growth stage are showed by ** (0.001 < p ≤ 0.01).

Full size image

Dissolved organic carbon

Except at the grain filling stage, liming increased the concentration of DOC in the leachates at seedling stage (+ 13.3%), mid-tillering stage (+ 22.5%), panicle initiation stage (+ 28.9%), and heading stage (+ 13.0%) of early rice (Fig. 6a). No significant variation was found on DOC concentration of the leachates in the late rice season (Fig. 6b).

Fig. 6
figure 6

Effects of liming on the leaching losses of dissolved organic carbon (DOC) in the double-cropped rice field with acidic red soil. Error bars represent the standard deviation of the mean (n = 3). SD, MT, PI, HD, and GF indicate seedling, mid-tillering, panicle initiation, heading, and grain filling stages, respectively. Significant treatment effects in the rice growth stage are showed by * (0.01 < p ≤ 0.05) and ** (0.001 < p ≤ 0.01).

Full size image

Dissolved organic carbon seasonal average leaching losses of carbon and nitrogen

In the early rice season, the application of lime had a significant increase in the concentration of DOC in the leachates (+ 16.6%; Fig. 7a). However, no significant effects were observed in the concentrations of NH4+-N, NO3−-N, and DON. Liming did not influence the average losses of C and N in the leachates in the late rice season (Fig. 7b).

Fig. 7
figure 7

Effects of liming on the mean seasonal concentration of dissolved C and N in leachates in the double-cropped rice field with acidic red soil. Error bars represent the standard deviation of the mean (n = 3). Significant treatment effects in the rice growth stage are showed by *** (p ≤ 0.001).

Full size image

Soil properties

Compared to control, liming raised soil pH, whereas no significant effects were found on soil alkaline hydrolyzable-N (AHN), soil organic carbon (SOC), and total nitrogen (N) at maturity of late rice (Table 1).

Table 1 Effects of liming on soil properties at maturity of late rice.
Full size table

Discussions

Liming increased rice yield, consistent with our previous findings from the first lime application cycle at the same experimental site12. The reasons are primarily as follows: The application of lime improves soil pH (Table 1), promotes the mineralization of soil organic matter, and enhances the activity of enzymes related to soil C and N metabolism, which facilitates the release of soil mineral nutrients13,26. In addition, lime application alleviates soil acidification, promotes the growth of rice roots, and enhances the absorption of soil mineral nutrients by rice, which benefits photosynthesis and the transport of assimilates to the grains, ultimately increasing rice yield12.

Our results demonstrated that liming increased the leaching losses of dissolved C and N relative to the unlimed treatment. This effect may be attributed to several factors. First, preliminary outcomes of the experiment presented that the application of lime had a positive and significant influence on enzyme activities related to soil C (e.g., sucrase and cellulase) and N cycling (e.g., protease and urease)13. Consequently, liming appears to accelerate the mineralization of organic litter and soil organic matter, which in turn increases the leaching of dissolved C and N from rice paddies with acidic soils26,27,28. Second, lime application plays a vital role in mitigating soil acidification (Table 1) and enhancing rice growth (Fig. 2), factors that can indirectly affect dissolved nutrient levels12. Increased organic material deposition from previous cultivation seasons might elevate the concentrations of dissolved C and N in both paddy soil and floodwater29. Last, the application of lime not only stimulates the mineralization of soil organic matter but also enhances the microbial nitrification process, leading to increased NO3--N levels in the soil30. The elevated NO3−-N content is susceptible to the risk of N leaching loss due to rainwater and irrigation water31. Notably, acidic paddy soils are dominated by variable charge minerals (e.g., Fe/Al oxides) whose net negative charge increases with rising pH32. Liming elevates soil pH, triggering deprotonation of functional groups on organic matter and clay minerals16. This process may reduce negative charge density, thereby diminishing cation retention capacity (e.g., NH4+), while subjecting NO3− to enhanced electrostatic repulsion that facilitates leaching17.

The timing of leaching losses for dissolved C and N was consistent with our hypothesis. Our findings that the concentration of NH4+-N, DON, and DOC in the leachates during the early growth stage was higher relative to that during the late growth stage in the early rice season. First, during the early growth stage of rice, specifically after the transplanting of seedlings, the limited physiological characteristics of root morphology result in a reduced capacity to absorb and utilize N33. Second, plowing activities in rice paddies reduce the soil’s capacity to adsorb N34. Third, in the double-cropped rice system, the presence of a winter fallow period enables the late rice straw to undergo a decomposition cycle of nearly 5 months, causing positive release of N from the rice straw. Last, the hydrological dynamics of the double-cropped rice system further modulate the leaching of dissolved C and N35. We observed that elevated leaching of NH4+-N and DON during the early growth stage of the early rice season was associated with higher rainfall events (Fig. 1), where continuous flooding enhanced solute mobility through macropores36. Additionally, during the inherent wetting–drying cycles in rice production, pH fluctuations occur due to changes in redox potential35. When fields are drained, the redox potential increases, promoting the oxidation of reduced substances and the release of H+, which leads to a decrease in pH37. Liming mitigates these pH fluctuations by neutralizing H+ released during oxidation processes, potentially prolonging the stability of NH4+-N30. Nevertheless, drainage conditions in the late growth stage reduced hydraulic conductivity, thereby mitigating leaching losses38.

In contrast, the application of lime appears to have an insignificant effect on N concentration in leachates during the late rice season. This phenomenon may be attributed to variations in the C/N ratio of rice straw between the seasons12,29. There is a short duration of about 2 weeks between the early rice harvest to late rice transplantation. During this time window, the presence of fresh straw with a high C/N ratio promoted soil microorganisms fixation of N, which effectively reduced the leaching loss of N12. Additionally, we selected urea as the N fertilizer in this experiment. The high air temperature in the late rice season leads to the production of NH4+-N, resulting from urea hydrolysis, which is prone to volatilization and diffusion into the atmosphere. This factor reduced the storage time of NH4+-N in the plowed soil, thus reducing the potential risk of its leaching loss29.

Previous studies have pointed out that the predominant form of N leaching loss in paddy soil is NO3−-N, which were consistent with our findings39,40. We observed that the concentration of NH4+-N in the leachates was low, and the main forms of N leaching loss were NO3−-N and DON. Because rice is more sensitive to the absorption of NH4+-N than NO3−-N41. Furthermore, ammonia volatilization predominantly characterizes N loss in paddy fields, which contributes to reduced NH4+-N leaching. Moreover, the mobility of NO3−-N in leachates is notably greater than that of NH4+-N, a phenomenon supported by recent studies42,43. From the perspective of soil microorganisms, the application of lime in soil increases soil pH and improves the gene abundance and community structure of some microorganisms involved in the soil nitrification process, thereby facilitating the transformation of NH4+-N to NO3−-N in soil2,44 While liming enhances leaching, other factors such as the double-cropped rice system and irrigation practices may also play a role32,45. In this study, the compressed double-cropped rice cycle affected the inter-seasonal straw decomposition duration (e.g., a 2-week fallow between early and late rice versus a 5-month winter fallow), altering the rates of microbial nitrogen immobilization and mineralization12. Irrigation practices directly influence soil water movement: Excessive irrigation increases soil hydraulic conductivity, thereby promoting the leaching of dissolved C and N45.

Conclusions

Liming significantly boosted grain yield while increased the concentrations of NO3−-N, DON, and DOC in the leachates from double-cropped rice fields with acidic red soil. The concentration of NH4+-N in the leachates remained low and unaffected by liming. To reduce the leaching losses of C and N, we recommend implementing effective measures such as optimizing soil tillage and water management, along with using high-efficiency N fertilizers when applying lime in acidic rice paddies.