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Article

Effects of Long-Term Organic Substitution on Soil Nitrous Oxide Emissions in a Tea (Camellia sinensis L.) Plantation in China

by
Zhidan Wu
1,
Wei Hua
2,
Kang Ni
2,
Xiangde Yang
2,* and
Fuying Jiang
1,3,*
1
Tea Research Institute, Fujian Academy of Agricultural Sciences, Fuzhou 350013, China
2
Tea Research Institute, Chinese Academy of Agriculture Sciences, Key Laboratory of Biology, Genetics and Breeding of Special Economic Animals and Plants, Ministry of Agriculture and Rural Affairs, Hangzhou 310008, China
3
National Agricultural Experimental Station for Soil Quality, Fu’an 355015, China
*
Authors to whom correspondence should be addressed.
Agronomy 2025, 15(2), 288; https://doi.org/10.3390/agronomy15020288
Submission received: 21 December 2024 / Revised: 19 January 2025 / Accepted: 22 January 2025 / Published: 24 January 2025
(This article belongs to the Special Issue Effect of Fertilizer Application on Greenhouse Gas Emissions and Soil Carbon Sequestration)
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Abstract

:
Nitrous oxide (N2O) is a major greenhouse gas (GHG) responsible for global warming. Improper fertilization in agricultural fields, particularly excessive nitrogen (N) application, accelerates soil N2O emissions. Though partial substitution with organic fertilizer has been implemented to mitigate these emissions, the effect on perennial systems, such as tea plantations, remains largely unexplored. Therefore, the present study monitored soil N2O emissions for a year in a tea plantation in South China under the following treatments: no N fertilizer (control, CK), chemical fertilizer alone (CF), replacing 40% of chemical fertilizer with organic fertilizer (CF + OF), and organic fertilizer alone (OF). Our results showed that the annual cumulative N2O emissions from the plantation soil ranged from 1.03 to 3.43 kg N2O-N ha−1. The cumulative N2O emissions, the yield-scaled N2O emissions (YSNE), and the N2O-N emission factor (EF) from the soil were the highest under the CF + OF treatment but the lowest under the OF treatment. Further analysis revealed that fertilization, mainly chemical fertilization, increased the soil ammonium (NH4+-N) and nitrate (NO3−-N) levels by 182–387% and 195–258%, respectively, and tea yields by 120–170%. However, tea yield decreased gradually with increasing organic substitution. These results prove that complete organic substitution reduces soil N2O emissions and tea yield and suggest adopting an appropriate substitution rate for optimal effect. Further random forest (RF) modeling identified water-filled pore space (WFPS; 20.27% of total variation), soil temperature (Tsoil; 19.29%), and NH4+-N (18.27%) as the key factors significantly contributing to the changes in soil N2O flux. These findings provide a theoretical foundation for optimizing fertilization regimes for sustainable tea production and soil N2O mitigation.

1. Introduction

Nitrous oxide (N2O) is a potent and long-lived gas that accumulates in the Earth’s atmosphere and causes several detrimental effects, mainly due to the greenhouse gas (GHG) effect. Its global warming potential (GWP) is nearly 300 times that of carbon dioxide (CO2) [1]. In addition, it plays a significant role in ozone layer depletion, as it can ascend from the troposphere to the stratosphere [2]. Evidence showed that most N2O (almost 60% of total global emissions) is emitted from agricultural soil [3]. The widespread and excessive use of N fertilizers, particularly chemical N fertilizers, has led to this undesirable consequence [4]. In China, which the largest consumer of fertilizers, chemical fertilizers account for approximately 57% of total GHG emissions from agricultural inputs [5,6]. Therefore, managing N fertilizer input is crucial for mitigating N2O emissions from agricultural soils and reducing the risks of climate change both in China and worldwide.
Chemical fertilizers have been substituted with organic fertilizers to mitigate environmental harm and enhance soil fertility [7,8], which could provide organic and inorganic carbon (C) and N and thereby alters the C/N ratio of the agricultural soil [9]. Studies have demonstrated the effects of organic substitution on soil N2O emissions. For example, the combined application of manure and urea increased N2O emissions in maize fields due to enhanced denitrification [10]. On the contrary, replacing synthetic fertilizers with manure promoted soil N fixation, reduced inorganic N availability, improved the conversion of N2O to N2, and ultimately decreased N2O emissions. However, Zhang et al. (2020) [11] and Meijide et al. (2007) [12], using meta-analyses, found no significant difference in N2O emissions with substitution. The slight variation in N2O emissions observed with the combined use of organic and inorganic fertilizers was attributed to the organic fertilizer type, the substitution rate, and the soil characteristics. These earlier studies on soil N2O emissions mostly focused on fields of major annual grain crops such as rice, wheat, and corn, while studies on emissions in perennial systems, such as tea plantations, are limited.
The tea plant (Camellia sinensis L.) is an important cash crop broadly cultivated in the acidic soils of tropical and subtropical regions [13]. Evidence has shown that tea-planted soils are global hotspots for N2O emissions from croplands [14]. By using an exponential model, Wang et al. (2022) estimated that total global N2O emissions from tea plantations in the 2010s were 35.6 Gg N yr−1 [15]. In addition, the response of N2O emissions was more sensitive to N inputs on acidic tea plantation soils, in which the global direct N2O emissions from tea plantations was 46.5 Gg N yr−1 when the data of N application was considered [15]. Meta-analysis showed that the three largest tea plantation N2O emitters were China, India, and Japan, with China accounting for 90% of the global estimate. Except for Kenya and Turkey, the other countries with high emissions were mainly concentrated in Asia. Additionally, pruning is another essential practice that influenced N2O emissions in tea plantations. Evidence showed that both high N application and pruned litter return alter the soil’s microbial communities, influence soil nitrification and denitrification, and ultimately increase N2O emissions [16]. Currently, the area under tea plantations in China exceeds 50 million acres, accounting for approximately 63% of the global tea plantation area, and according to our previous investigation, nearly 500 kg of pure N per hectare is applied to plantations across the main tea-producing areas of China each year [17], which is higher than that used by grain crops, such as rice and wheat (<200 kg N ha−1 yr−1) [14]. Therefore, it is crucial to assess the N2O emission patterns from the tea plantation soil and identify its influencing factor. This will offer a theoretical foundation for developing field management practices, especially rational N fertilizer strategies, aimed at reducing N2O emissions in tea-plantation soils.
There is limited research on N2O emissions from tea plantation systems, and some existing N2O emissions data were obtained by indirect approach, such as emission factor approach and process-based models; this may differ to some extent from the actual emissions. However, chamber-based methods are widely adopted since they can be utilized for small plots and narrow flux ranges [18]. In addition, the responses of N2O emissions to organic substitution have been found inconsistent across studies. For example, He et al. (2019) [19] observed a decline in soil N2O emissions in a tea plantation after a year (41.4–49.6%) of using cow manure, pig manure, and chicken manure. In contrast, Han et al. (2021) [20] detected a 17% rise in N2O emissions in tea plantation soil over two years of monitoring after total organic substitution. Meanwhile, through a meta-analysis, Yu et al. (2023) [21] reported an initial increase and a subsequent decrease in N2O emissions with increased organic substitution rates. These results reveal inconsistency in soil N2O emission patterns and influencing factors across tea plantations under organic fertilizer application. These inconsistencies may be related to the type and application rate of organic fertilizers and the climate of the experimental region. Therefore, the response of soil N2O emissions in tea plantations to organic fertilizer supply and the factors controlling the response should be comprehensively investigated.
Therefore, the present study assessed the impact of various fertilization strategies (CK, CF, CF + OF, and OF treatments) on soil N2O emissions and explored the key factors influencing soil N2O flux within a perennial tea plantation system in China throughout 2023. The objectives of this study were to (i) monitor N2O emissions from the tea plantation soil under different fertilization strategies and (ii) identify the factors driving soil N2O flux. The study’s findings will help develop a new strategy to mitigate high N2O emissions in acidic tea plantation ecosystems.

2. Materials and Methods

2.1. Experimental Site

The field trial of this study was conducted at Fu’An in Fujian, China (22°22′ N, 119°57′ E, altitude of 46 m), affiliated with the Tea Research Institute of Fujian Academy of Agricultural Sciences. A subtropical monsoon climate, with a mean annual precipitation (MAP) of 1426 mm and a mean annual temperature (MAT) of 26 °C, was recorded in this site during the 2013–2022 period. The soil of this site is classified as Alisol (IUSS, 2015; developed from the Quaternary eolian red deposit) [22] and has a loamy clay texture.
In 2009, an experiment was initiated in this region in a tea plantation with sandy red soil and cement ponds (200 cm × 90 cm × 90 cm) between adjacent plots to minimize unnecessary nutrient flow (Figure 1). The basic soil properties of this tea plantation were as follows: 4.85 pH (1:2.5, soil/water), 3.69 g kg−1 organic carbon, 0.30 g kg−1 total N, 26.4 mg kg−1 alkaline hydrolyzed N, 4.8 mg kg−1 available phosphorus, and 60.3 mg kg−1 available potassium. The local tea cultivar “Yucui #4” seedlings were planted in the garden in March 2010 at the rate of five plants per plot and an intra-row spacing of 0.33 m.
The garden was treated with 225.0 kg of N fertilizer, 150 kg of phosphorus (P) fertilizer (P2O5), and 300 kg of potassium (K) fertilizer (K2O) every year during the 2011–2022 period. Of this, 40%, 30%, and 30% of N and K fertilizers were applied as base fertilizer (early November to early December), spring tea topdressing (late February to early March), and autumn tea topdressing (mid-July to late August), respectively, while 100% of the P fertilizer was applied immediately after base fertilizer application. All fertilizers were applied evenly on the soil surface and incorporated to a depth of 5 cm by plowing.
N2O emissions from the soil were monitored from December 2022, and at this stage, the surface (0–20 cm) soil properties were as follows: 4.82 pH, 11.43 g kg−1 organic carbon, 1.16 g kg−1 total N, 22.39 mg kg−1 alkaline N, 173.26 mg kg−1 available phosphorus, and 189.09 mg kg−1 available potassium.

2.2. Experimental Design and Field Management

The experiment was conducted in a completely randomized design with four treatments and three replicates per treatment; thus, the experiment had a total of 12 plots, each measuring 200 cm × 90 cm. The four treatments were as follows (Figure 1):
(1)
CK: no application of N fertilizer (0 kg ha−1) and annual application of 150 kg ha−1 P fertilizer (P2O5) and 300 kg ha−1 K fertilizer (K2O). Here, 100% of the P fertilizer and 40% of the K fertilizer were applied as base fertilizers, and the remaining K fertilizer was applied as spring tea (30%) and autumn tea (30%) topdressing.
(2)
CF: annual application of 225 kg ha−1 N fertilizer, 150 kg ha−1 P fertilizer (P2O5), and 300 kg ha−1 K fertilizer (K2O). Here, 40% of N fertilizer, 40% of K fertilizer, and 100% of P fertilizer were applied as base fertilizers; the remaining N and K fertilizers were applied as spring tea (30%) and autumn tea (30%) topdressing.
(3)
CF + OF: 40% of the chemical N fertilizer was replaced with organic fertilizer, which was applied only once as the base fertilizer. As in CF, the chemical fertilizer was applied as topdressing to spring and autumn tea.
(4)
OF: 100% chemical N fertilizer was replaced with organic fertilizer, of which 40% was applied as base fertilizer, 30% as spring tea topdressing, and 30% as autumn tea topdressing.
In 2022, the base fertilizer was applied on December 8th. In 2023, the spring tea topdressing was performed on 7th March, the autumn tea topdressing on August 10th, and the base fertilization on November 7th. Urea (46% pure N), superphosphate (P2O5 12%), and potassium sulfate (K2O 51%) were used as the N, P, and K fertilizers, respectively. The commercial organic fertilizer used in this study was a mixture of sheep and pig manure marketed by Lvming Biotechnology Co., Ltd. (Jian’Ou, China). The organic fertilizer had an organic matter content of 44.92%, a total N content of 2.22%, a carbon–nitrogen ratio (C/N) of 11.74, a total P content (P2O5) of 5.99%, and a total K content (K2O) of 1.94%. All fertilizers were applied evenly on the soil surface and plowed to a depth of 5 cm. In addition, pruning was conducted twice yearly in August and November, according to the local farming guidelines, and the pruned leaves and branches were returned entirely to the tea plantation.

2.3. Sampling and Laboratory Analysis

2.3.1. Sampling of Tea Shoots

Shoots with one bud and two leaves were picked by hand from the spring and autumn tea plants, and the fresh weight was measured. Spring tea was collected from March 28th to April 25th, and autumn tea was collected from September 10th to October 7th. Finally, the sum of spring and autumn tea yields was calculated to determine the annual tea yield.

2.3.2. Measurement of N2O Emissions and Calculation of Indicators

The N2O emissions from the tea plantation soil were monitored monthly from January to December 2023 to comprehensively analyze the impact of different fertilization strategies. The frequency of monitoring was increased following fertilization and precipitation. The soil N2O flux was measured using an infrared, laser-based gas analyzer (Li-7820, Li-COR Biosciences, Lincoln, NE, USA) with an optical feedback cavity-enhanced absorption spectroscopy (OF-CEAS) apparatus connected to a Smart Chamber (8200-01 S, Li-COR Biosciences, Lincoln, NE, USA). The Smart Chamber was positioned on the top of the PVC soil collar (5.0 cm × 20 cm; height × diameter) (Figure S1). At least two minutes were required per plot to make the measurement, and approximately 20 s were allowed between two measurements. All measurements were taken between 9:00 a.m. and 11:00 a.m. Finally, the soil N2O flux was calculated following the method described by Agusto et al. (2022) [23].
The annual cumulative N2O emission (kg N2O-N ha−1) was calculated using a linear interpolation approach. This method estimated the N2O emissions on the non-sampling days between two consecutive sampling days. Then, the total of the daily emissions was calculated to determine the annual cumulative N2O emission.
Other parameters, namely yield-scaled N2O emission (YSNE; g N2O-N kg−1 tea shoots) and N fertilizer-induced direct N2O emission factor (EF; %), were calculated using the following equations:
YSNE = E/Y,
where E represents the annual N2O emission in kg N2O-N ha−1, and Y represents the tea yield in kg ha−1.
EF = (EN − E0)/RN × 100,
where EN and E0 represent the annual N2O emissions from the treatment (fertilization) and CK plots, respectively (kg N2O-N ha−1), and RN represents the N fertilization rate (kg N ha−1).

2.3.3. Measurement of Environmental Factors

The data on air temperature and precipitation in the experimental site were obtained from a meteorological observatory at the experimental station. The soil temperature was measured using a digital thermometer following N2O monitoring. The soil sample was collected from 0–10 cm depth using a stainless steel corer (inner diameter 5 cm), and three samples were collected from each plot. Each collected sample was dried in an oven at 105 °C for 48 h to determine the soil gravimetric water content (GWC), and the GWC was converted to water-filled pore space (WFPS) as follows:
WFPS (%) = GWC × bulk density/(1 − bulk density/2⋅65) × 100%,
where GWC represents the gravimetric water content (%), bulk density is the bulk density of the soil of each plot, and 2.65 represents the accepted particle density of the soil.
In addition, to assess the inorganic N concentration, the soil sample was collected from the area around the PVC collar and at a depth of 10 cm following N2O monitoring (Figure S1), sieved through a 2 mm sieve, and treated with 1 mol L−1 KCl. The concentrations of ammonium (NH4+-N) and nitrate (NO3−-N) nitrogen in the obtained soil extract were measured by continuous flow analysis (Seal Analytical AA3, Norderstedt, Germany).

2.4. Statistical Analysis

One-way analysis of variance (ANOVA) combined with Tukey’s post hoc test was used to compare the values of Tsoil, WFPS, NH4+-N content, NO3−-N content, cumulative N2O emission, and tea yield under different fertilization strategies. We also tested the normality of the data before analysis using the Shapiro–Wilk test, which showed that the data followed a normal distribution. A Pearson correlation analysis was performed to assess the relationship of N2O flux with various environmental factors (Tsoil, WFPS, NH4+-N, and NO3−-N). Finally, random forest (RF) modeling was used to determine the most important soil variables to predict the N2O emission from the soil under fertilization.
All statistical analyses were conducted in the R platform (version 4.1.2); the ‘stats’ package was used for one-way ANOVA and Pearson correlation analysis, and the ‘randomforest’ package for RF modeling.

3. Results

3.1. Temperature, Precipitation, and Soil WFPS in the Tea Plantation

The study initially analyzed the climatic features of the tea plantation during the experimental period. The average daily air temperature was 19.72 °C, and the annual precipitation was 1551 mm (Figure 2). The monthly average air temperature was above 25 °C from June to September and below 10 °C in late December and January (Figure 2). Precipitation was recorded for almost six months, specifically from March to August, which constituted 69% of the total annual rainfall of this region (Figure 2). The monthly rainfall exceeded 100 mm from March to May and surpassed 200 mm from June to September. However, rainfall during the remaining six months (September to February) was less than 100 mm; it was less than 50 mm from October to December.
The soil temperature (Tsoil; 10 cm depth) of this region was consistent with the air temperature during the different months (Figure 3a). The Pearson correlation analysis showed a significant positive association between air temperature and Tsoil (0–10 cm), with R2 ranging from 0.76 to 0.86 (p < 0.01). Generally, the Tsoil was slightly higher (0.4 °C) than the air temperature due to the heat retention effect of soil fertilization. However, no significant difference was found in the mean Tsoil among the different treatments in the tea plantation (Figure S2a). Meanwhile, soil WFPS in the tea plantation decreased considerably after fertilization (Figure 3b). The CF, CF + OF, and OF plots recorded 29.2% (p < 0.05), 21.2% (p < 0.05), and 25.1% (p < 0.05) lower soil WFPS than the CK plot (Figure S2b). Higher soil WFPS was observed from March to August, similarly to precipitation, and the changes in soil WFPS were positively related to rainfall, with R2 ranging from 0.30 to 0.34 (p < 0.01).

3.2. Mineral N Levels in the Soil of Tea Plantation Under Different Fertilization Strategies

Fertilization significantly increased soil inorganic N concentration of the tea garden (Figure 4). Among the various treatments, CF resulted in the highest inorganic N levels in the soil. The NH4+-N concentration under CF and CF + OF treatments was 4.87 and 2.82 times higher than CK, while no significant difference was observed under OF treatment (Figure 4a). Similarly, the NO3−-N content under CF, CF + OF, and OF treatments was 3.56, 2.95, and 2.90 times that under CK (Figure 4b).
Further, the changes in inorganic N levels with time were analyzed. In the control (CK) plot, the inorganic N (NH4+-N and NO3−-N) showed less variation with season. The NH4+-N content during the experimental period ranged from 1.29 to 29.28 mg kg−1 (average = 8.48 mg kg−1), and the NO3−-N concentration ranged from 0.16 to 26.79 mg kg−1 (average = 7.36 mg kg−1) (Figure 4). On the contrary, the inorganic N concentration of the fertilized soil exhibited significant variation with season. Under the CF and CF + OM treatments, the content of NH4+-N in the 0–10 cm soil layer peaked on day 7 after spring tea topdressing (Figure 4a), while NO3−-N concentration peaked after 30 days (Figure 4). In the OF plot, NH4+-N and NO3−-N exhibited gradual changes with no distinct peaks after the treatment (Figure 4). After the autumn tea topdressing, NH4+-N concentration of the CF and CF + OF soil increased gradually and peaked at around day 2–4, while NO3−-N concentration peaked around day 10. However, no significant peak was observed in the NH4+-N and NO3−-N concentration after OF treatment. Following base fertilization in November, the maximum NH4+-N and NO3−-N levels in the CF-treated soil were detected on the 10th day and 38th day, respectively. During this stage, the content of NH4+-N in the 0–10 cm soil layer remained relatively stable under CF + OF and OF treatments, while NO3−-N remained high up to 50 days after fertilization.

3.3. N2O Flux and Cumulative N2O Emissions from the Soil of the Tea Plantation Under Different Fertilization Strategies

After fertilization, the N2O flux in the tea plantation soil remained relatively consistent throughout the experimental period, with three peaks per year (Figure 5). Following spring tea topdressing on March 7th, soil N2O flux was minimal to mid-March in all treatments due to seasonal drought. However, after receiving rainfall on March 19th (18.2 mm) and March 23rd (41.2 mm), the soil N2O flux from all treatment plots rapidly increased and peaked from April 3rd to April 7th. Notably, the peak in soil N2O emission immediately after spring tea topdressing was significantly higher than the other peaks (Figure 5). Following autumn tea topdressing and base fertilizer application, the emissions peaked 4–11 days after fertilization, with 2–3 peaks in each period due to favorable soil moisture conditions. In CK, the N2O flux from the soil remained relatively low throughout the period and ranged from 0.006 to 0.516 nmol m−2 s−1, with a small peak after rainfall and warming. The occurrence of peaks in different treatments suggests the influence of environmental conditions on N2O emissions from soil.
Fertilization significantly increased the cumulative N2O emission from tea garden soil (Figure 5). The cumulative N2O emissions from the soil in the CF, CF + OF, and OF treatments were 2.34, 2.64, and 1.68 times that from the CK plot. Among these fertilization treatments, OF showed the lowest cumulative N2O emission (2.01 kg N2O-N ha−1), which was significantly different from the values under CF (2.98 kg N2O-N ha−1) and CF + OF (3.16 kg N2O-N ha−1) treatments. The cumulative N2O emissions from the CF and CF + OF soil were almost equivalent.

3.4. Tea Yield, YSNE, and N2O EF

In this study, fertilization showed a significant impact on tea yield. Spring tea and autumn tea exhibited consistent changes in yield after fertilization (Figure 6), with 170%, 121%, and 107% rise recorded under CF, CF + OF, and OF treatments, respectively. In addition, the different treatments resulted in different levels of increase in tea yield compared to CK. Spring and autumn tea yields increased by 152.65% and 208.83% under CF, 116.33% and 130.61% under CF + OF, and 102.55% and 117.33% under OF compared to CK. The higher proportion of organic fertilizer substitution had a slightly lower annual tea yield than the lower proportion; however, the differences among these treatments with different organic fertilizer substitution rates were insignificant (Figure 6).
Different fertilization strategies significantly influenced YSNE and N2O EF (Table 1). The CF + OF treatment yielded the highest YSNE (1.28 g N2O-N kg−1 tea yield). However, YSNE values under CF and OF treatments were lower than those under CK treatment. In addition, the highest N2O EF was observed in the CF + OF treatment, while the lowest was in the OF treatment. However, no significant difference was found in N2O EF between the two chemical fertilizer treatments (CF and CF + OF).

3.5. Relationship Between Environmental Factors and N2O Flux in Tea Plantation Soil Under Different Fertilization Strategies

Further, we analyzed the relationship between soil N2O flux and various environmental variables to identify the major factors driving the emissions. The soil N2O flux positively correlated with Tsoil, WFPS, NH4+-N, and NO3−-N content under different treatments (Figure 7a). Among these environmental factors, WFPS was the only one positively correlated with N2O flux across all treatments (r = 0.210–0.574, p < 0.01). Similarly, Tsoil was positively correlated with N2O flux in all treatments except CF + OF (r = 0.16, p > 0.05). In contrast, NH4+-N showed a significant positive association with N2O flux in the CF + OF treatment (r = 0.245, p < 0.01), while NO3−-N showed no correlation with N2O flux in any treatment. The correlation between the inorganic N (NH4+-N and NO3−-N) levels and N2O flux was weak across all treatments. Moreover, there is significant correlation between WFPS, Tsoil, NH4+-N concentration, and NO3−-N concentration of tea plantation soil (Figure S3).
Finally, the key soil factors influencing soil N2O flux in the tea plantation were screened using RF regression. The modeling indicated that the four environmental factors (Tsoil, WFPS, NH4+-N, and NO3−-N) greatly explained the variations in N2O flux (R2 = 31.1%, p < 0.001; Figure 7b) and were identified to affect N2O flux (p < 0.05; Figure 7b). The N2O flux was primarily influenced by WFPS (%IncMSE = 20.27%, p < 0.01), followed by soil Tsoil (19.29%, p < 0.01), NH4+-N (18.27%, p < 0.01), and NO3−-N (8.66%, p < 0.05).

4. Discussion

4.1. N2O Emissions from Tea Plantation Soil

Previous studies have identified tea plantations as N2O emission hotspots with elevated N2O flux and N2O-EF. Yao et al. (2018) [24] reported that N2O emission from tea plantation soil ranges from 14.4 to 32.7 kg N ha−1 yr−1, 2.67 times higher than that of the adjacent farmland systems. Similarly, Jumadi et al. (2005) reported up to 32.41 kg N ha−1 yr−1 N2O emissions from the acidic soils of Indonesian tea plantations [25], which is significantly higher (10−1000 times) than that from a typical farmland soil. These elevated levels of N2O emissions from tea plantation soils have been attributed to increased N application rates [14,26]. Thus, tea plantations are ‘hotspots’ of soil N2O emissions. Our previous investigation found that the average annual N application rate across tea-producing regions in China is 491 kg N ha−1 [17], while other crop systems remain below 200 kg N ha−1 yr−1 [14]. Additionally, Meng et al. (2005) [27] have found that N fertilizer input accounts for 70% of total N2O emissions from farmlands. In the Japanese tea plantations, N fertilizer application increased the annual N2O emissions from 5.83 kg N ha−1 to 69.10 kg N ha−1 [28]. Moreover, a previous meta-analysis confirmed that N2O emissions and N2O-EFs increased linearly or exponentially following N fertilizer input in tea plantations worldwide [14,15]. Thus, we speculated that the substantial use of N fertilizers is the primary driver of elevated N2O emissions in the tea plantation.
Our present study found that the annual N2O emissions from the tea plantation during the experimental period ranged from 1.03 to 3.43 kg N2O-N ha−1 (Figure 5), and N2O-EFs ranged from 0.36−0.87% (Table 1). However, these values, especially under fertilization, are lower than those previously reported for other tea plantation systems [20,29], likely due to the relatively lower amount of N fertilizer applied in this study (225 kg N ha−1 yr−1) than the average reported in prior surveys (491 kg N ha−1 yr−1). He et al. (2019) [19] observed similar levels of N2O emissions (ranging from 1.86 to 5.37 kg N2O-N ha−1 cumulatively) and N2O-EFs (0.08% to 2.34%), with a low N fertilizer rate of 150 kg N ha−1 yr−1. Thus, we believe that N fertilizer application rates significantly influenced N2O emissions from the soil of tea plantations [15]. Further studies should be carried out to investigate N2O emissions from tea plantations utilizing a broader range of N application rates and validate the results.

4.2. Impact of Organic Fertilizer on N2O Emissions from the Tea Plantation Soil

In soil, N2O is generated through denitrification (NO3− reduction) and nitrification (NH4+ oxidation) processes, which often occur simultaneously [30]. Fertilizers serve as the primary sources of substrates for both nitrification and denitrification; consequently, applying exogenous fertilizers leads to increased soil N2O emissions in agricultural systems [31]. The present study observed a significant increase in cumulative N2O emissions from tea plantation soil after applying both chemical and organic fertilizers (Figure 5). In addition, a positive correlation was detected between N2O flux and the NH4+-N (r = 0.152, p < 0.01) and NO3−-N (r = 0.135, p < 0.01) levels (Figure 6). This finding is consistent with Liu et al. (2023) [32], who found a positive correlation between N2O flux and inorganic N in maize fields (R2 = 0.411 and 0.574).
Typically, organic fertilizers introduce N into the soil and alter the soil C/N ratio. Several studies have suggested that organic fertilizers enhance denitrification more effectively than chemical fertilizers when supplied with an equivalent amount of N, primarily due to organic molecules, such as labile C compounds [33]. However, the application of organic fertilizers increases soil organic matter content, leading to increased oxygen consumption during the decomposition process [34]. This reduction in soil oxygen levels inhibits the oxidation of NH3 into NO2− and NO3−, thereby decreasing N2O emissions [35]. A few researchers have found that substituting chemical fertilizers with organic fertilizers mitigates soil N2O emissions [36,37]. Recent studies indicate that changes in the soil C/N ratio due to organic fertilizer input significantly influence soil N2O emissions. Specifically, organic fertilizers with a low C/N ratio supply more mineral N, meet microbial N requirements and lead to net N mineralization, consequently increasing N2O emissions [10]. In acidic soils, N2O emissions shifted from nitrification to denitrification following the application of organic fertilizers with a low C/N ratio [38]. Similarly, He et al. (2019) [19] found an increase in N2O emissions after the application of organic fertilizers with a low C/N ratio (8.60) in acidic soils.
In the present study, replacing 40% of chemical fertilizer with organic fertilizer (CF + OF treatment) resulted in a 13.34% increase in cumulative N2O emissions (Figure 5) and a 22.54% increase in N2O-EFs (Table 1) from the tea plantation soil, compared to the application of chemical fertilizer alone (CF). Conversely, the use of organic fertilizer alone (OF) led to a 27.94% reduction in cumulative N2O emissions and a 49.30% decrease in N2O-EFs. The experiment was conducted in a field with a surface soil C/N ratio of 9.85 and using organic fertilizer with a C/N ratio of 11.74. Thus, it is speculated that the increase in soil N2O emissions after the partial replacement of chemical fertilizer with organic fertilizers (CF + OF) is related to the decrease in soil C/N ratio. However, the application of organic fertilizer alone (OF) reduced cumulative N2O emissions due to an increase in the soil C/N ratio. Additionally, we found that as the substitution of organic fertilizer increased, tea yield decreased, with excessive substitution adversely affecting the yield (Figure 6). These observations suggest that selecting organic fertilizers with appropriate C/N ratios and determining suitable substitution rates are crucial for optimizing yield and reducing soil N2O emissions in tea plantations.

4.3. Effects of Environmental Factors on N2O Emissions from the Soil in Tea Plantations

Research has highlighted the impact of soil moisture on N2O emissions in tea plantations [39]. Soil moisture influences nitrification, denitrification, and eventually N2O emissions by affecting various other soil factors, including aeration, redox potential, available N (NH4+ and NO3−), microbial activity, and physicochemical properties [40]. Precipitation and irrigation events, which regulate soil moisture levels, also influence the production and emission of soil N2O; precipitation often leads to a substantial increase in N2O emissions. Consistent with these earlier findings, the present study identified a strong positive correlation between N2O flux and WFPS across fertilization treatments (Figure 7). In early to mid-March, low soil moisture content due to drought in the tea plantation resulted in a reduced N2O emission rate; however, emissions increased rapidly following rainfall-induced improvement in soil moisture levels in late March (Figure 2 and Figure 3). These observations suggest that implementing appropriate water management practices and water-fertilizer coupling measures is crucial for mitigating N2O emissions in tea plantations.
Temperature is another factor that significantly influences soil N2O emissions; it primarily regulates biochemical reactions and microbial activity within the soil and influences N2O emissions. It impacts nitrification and denitrification, affecting N2O emissions [41]. The optimal temperatures for soil nitrification and denitrification generally range from 25 °C to 35 °C and 30 °C to 37 °C, respectively [42,43]. Recent research by Lv et al. (2019) [44] in tea plantations in subtropical hilly areas has shown that N2O emissions are positively correlated with the soil temperature when it is below 15 °C but associated with various other factors such as soil moisture, NH4+-N content, and NO3−-N content when the soil temperature exceeds 15 °C. Another study indicated that low soil temperature significantly reduces soil nitrification rates but does not influence the denitrification rates [45]. In the present study, a highly significant positive correlation was found between soil N2O flux and Tsoil in control (CK, no N fertilizer) and organic fertilizer (OF) treatments (Figure 7). In contrast, in soils treated exclusively with chemical fertilizer (CF), N2O flux was positively correlated with Tsoil (r = 0.400, p < 0.01). Meanwhile, when chemical fertilizer was partially replaced with organic fertilizer (CF + OF), no significant correlation was observed between soil N2O flux and Tsoil. These findings prove that CF and CF + OF treatments rapidly change NH4+-N and NO3−-N content in the soil (Figure 4), leading to a swift increase in soil N2O flux, which may overshadow the influence of Tsoil.

4.4. Research Limitations

This study analyzed the impact of replacing chemical fertilizers with organic fertilizers on soil N2O emissions within a perennial tea plantation system. However, the research was confined to a specific location and timeframe. Therefore, future studies should examine regional variations in soil N2O emissions and extend the duration of field monitoring. Furthermore, a comprehensive investigation of soil microbial communities, particularly those related to N-cycling, should be carried out to elucidate the mechanisms underlying N2O emissions. Observations on the microbes associated with these processes will help validate our findings based on correlation analyses such as RF modeling. In addition, researchers should investigate a broad range of substitution rates and organic materials, including animal- and plant-based fertilizers. Studies based on such analyses will aid in optimizing field management strategies.

5. Conclusions

In tea plantations under the same level of N fertilizer, yield declined as the organic substitution rate increased. Substituting 40% of chemical fertilizer with organic fertilizer resulted in a 13.34% increase in cumulative N2O emissions, a 22.54% increase in N2O-EFs, and a 45.45% increase in YSNE compared to chemical fertilizer alone. Conversely, a 100% substitution led to a 27.94% reduction in cumulative N2O emissions, a 49.30% decrease in EFs, and a 5.68% decline in YSNE. Thus, the present study proved that the organic substitution significantly influences yield and soil N2O emissions in tea plantations. Further, random forest modeling identified WFPS and soil temperature as critical factors regulating soil N2O flux in the tea plantation. These findings underscore the importance of selecting an appropriate organic fertilizer substitution rate and adopting water–fertilizer coupling strategies to optimize yield while minimizing soil N2O emissions in tea plantations.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy15020000/s1, Figure S1: Photos of monitoring equipment of N2O emission and the location of PVC collars (5.0 cm × 20 cm; height × diameter) in the experimental plots of tea plantation. Figure S2: The changes in soil temperature (Tsoil; a) and water-filled pore space (WFPS; b) in the tea plantation under different fertilization strategies. Error bars represent the standard errors (n = 3). Lowercase letters above the bars indicate significant differences in tea yield among the different fertilization strategies based on Tukey’s post hoc test (p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer. Figure S3: Heatmap shows the Pearson correlation between Tsoil, WFPS, NH4+-N content, and NO3−-N content of the tea plantation soil. * p < 0.05, ** p < 0.01.

Author Contributions

Z.W.: experimental design, investigation, writing—original draft; W.H.: investigation, data curation; K.N.: data analysis; X.Y.: supervision, funding acquisition, writing—review & editing; F.J.: supervision, funding acquisition, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the Technology Plan Project of Fujian Province (2022R1029006), the China Agriculture Research System of MOF and MARA (CARS-19), the Modern Agricultural Industry System in Fujian Province, the Science and Technology Specific Project of Fujian Academy of Agricultural Sciences (DKBF-2024-08), and the National Natural Science Foundation of China (42407459).

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding authors.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

GHG, greenhouse gas; N2O, nitrous oxide; CK, no nitrogen fertilizer; CF, chemical fertilizer alone; CF + OF, replacing 40% of chemical fertilizer with organic fertilizer; OF, organic fertilizer alone; MAP, mean annual precipitation; MAT, mean annual temperature; YSNE, yield-scaled N2O emissions; N2O-EFs, N2O emission factors; NH4+-N, ammonium; NO3−-N, nitrate; RF, random forest; WFPS, water-filled pore space; Tsoil, soil temperature; ANOVA, one-way analysis of variance.

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Figure 1. Geographic coordinates of the study area and the treatments adopted in the experiment. Here, (a) map of China, (b) map of Fujian Province, (c) map of Fu’An city, and (d) the experimental plot.
Figure 1. Geographic coordinates of the study area and the treatments adopted in the experiment. Here, (a) map of China, (b) map of Fujian Province, (c) map of Fu’An city, and (d) the experimental plot.
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Figure 2. Daily variations in average air temperature (°C) and precipitation (mm) in a tea plantation under different fertilization strategies. The meteorological data were obtained using a WS-MC01 compact automatic weather station installed on-site.
Figure 2. Daily variations in average air temperature (°C) and precipitation (mm) in a tea plantation under different fertilization strategies. The meteorological data were obtained using a WS-MC01 compact automatic weather station installed on-site.
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Figure 3. Seasonal variations in soil temperature (Tsoil; 10 cm depth) (a) and water-filled pore space (WFPS) (b) in a tea plantation under different fertilization strategies. Error bars represent the standard errors (n = 3). Vertical arrows indicate the timing of fertilizer application. CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
Figure 3. Seasonal variations in soil temperature (Tsoil; 10 cm depth) (a) and water-filled pore space (WFPS) (b) in a tea plantation under different fertilization strategies. Error bars represent the standard errors (n = 3). Vertical arrows indicate the timing of fertilizer application. CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
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Figure 4. Variations in NH4+-N (a) and NO3−-N levels (b) in the tea plantation soil after fertilization. The left panels indicate the variations in inorganic N with time, while the right panels indicate the inorganic N levels under different fertilization strategies. Error bars represent the standard errors (n = 3). The vertical arrows indicate the timing of fertilizer application. The lowercase letters above the bars indicate significant differences in NH4+-N and NO3−-N levels among the fertilization strategies (Tukey’s post hoc test; p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
Figure 4. Variations in NH4+-N (a) and NO3−-N levels (b) in the tea plantation soil after fertilization. The left panels indicate the variations in inorganic N with time, while the right panels indicate the inorganic N levels under different fertilization strategies. Error bars represent the standard errors (n = 3). The vertical arrows indicate the timing of fertilizer application. The lowercase letters above the bars indicate significant differences in NH4+-N and NO3−-N levels among the fertilization strategies (Tukey’s post hoc test; p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
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Figure 5. Variations in N2O flux and cumulative N2O emission from tea garden soil under different fertilization strategies. The insert panel displays the cumulative N2O emissions under different fertilization strategies. Error bars represent the standard errors (n = 3). The vertical arrows indicate the timing of fertilizer application. Lowercase letters above the bars in the graph indicate significant differences in the cumulative N2O emissions among the different fertilization strategies (Tukey’s post hoc test; p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
Figure 5. Variations in N2O flux and cumulative N2O emission from tea garden soil under different fertilization strategies. The insert panel displays the cumulative N2O emissions under different fertilization strategies. Error bars represent the standard errors (n = 3). The vertical arrows indicate the timing of fertilizer application. Lowercase letters above the bars in the graph indicate significant differences in the cumulative N2O emissions among the different fertilization strategies (Tukey’s post hoc test; p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
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Figure 6. The changes in spring (a) and autumn (b) tea yield and annual tea yield (c) in a plantation under different fertilization strategies. Error bars represent the standard errors (n = 3). Lowercase letters above the bars indicate significant differences in yield among the different fertilization strategies based on Tukey’s post hoc test (p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
Figure 6. The changes in spring (a) and autumn (b) tea yield and annual tea yield (c) in a plantation under different fertilization strategies. Error bars represent the standard errors (n = 3). Lowercase letters above the bars indicate significant differences in yield among the different fertilization strategies based on Tukey’s post hoc test (p < 0.05). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
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Figure 7. The influence of soil temperature (Tsoil), WFPS, NH4+-N, and NO3−-N on N2O flux in a tea plantation under different fertilization strategies. (a) Heatmap shows the Pearson correlation between N2O flux and various soil variables. Here, * and ** indicate significance at p < 0.05 and p < 0.01. (b) Random forest regression reveals the key factors influencing soil N2O flux in the tea plantation. Here, %IncMSE indicates “The increase in the mean square error”, and R2 refers to the model’s goodness of fit; * or ** above the bars indicates that the factor significantly influences N2O flux (p < 0.05 or p < 0.01). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
Figure 7. The influence of soil temperature (Tsoil), WFPS, NH4+-N, and NO3−-N on N2O flux in a tea plantation under different fertilization strategies. (a) Heatmap shows the Pearson correlation between N2O flux and various soil variables. Here, * and ** indicate significance at p < 0.05 and p < 0.01. (b) Random forest regression reveals the key factors influencing soil N2O flux in the tea plantation. Here, %IncMSE indicates “The increase in the mean square error”, and R2 refers to the model’s goodness of fit; * or ** above the bars indicates that the factor significantly influences N2O flux (p < 0.05 or p < 0.01). CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer.
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Table 1. Yield-scaled N2O emissions (YSNE) and direct N2O emission factors (Efs; %) in a tea plantation under different fertilization strategies.
Table 1. Yield-scaled N2O emissions (YSNE) and direct N2O emission factors (Efs; %) in a tea plantation under different fertilization strategies.
TreatmentsYSNE
(g·N2O-N kg−1 Tea Yield)		N2O-EF (%)
CK1.03 ± 0.10 ab-
CF0.88 ± 0.10 b0.71 ± 0.16 a
CF + OF1.28 ± 0.29 a0.87 ± 0.06 a
OF0.83 ± 0.02 b0.36 ± 0.12 b
F-value4.7614.65
p-value<0.05<0.01
CK, no N fertilizer application as control; CF, 100% chemical fertilizers; CF + OF, 60% chemical fertilizers + 40% organic fertilizer; OF, 100% organic fertilizer. Values shown are means ± standard errors. Different lowercase letters indicate significant differences among treatments based on Tukey’s post hoc test (p < 0.05).
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Wu, Z.; Hua, W.; Ni, K.; Yang, X.; Jiang, F. Effects of Long-Term Organic Substitution on Soil Nitrous Oxide Emissions in a Tea (Camellia sinensis L.) Plantation in China. Agronomy 2025, 15, 288. https://doi.org/10.3390/agronomy15020288

AMA Style

Wu Z, Hua W, Ni K, Yang X, Jiang F. Effects of Long-Term Organic Substitution on Soil Nitrous Oxide Emissions in a Tea (Camellia sinensis L.) Plantation in China. Agronomy. 2025; 15(2):288. https://doi.org/10.3390/agronomy15020288

Chicago/Turabian Style

Wu, Zhidan, Wei Hua, Kang Ni, Xiangde Yang, and Fuying Jiang. 2025. "Effects of Long-Term Organic Substitution on Soil Nitrous Oxide Emissions in a Tea (Camellia sinensis L.) Plantation in China" Agronomy 15, no. 2: 288. https://doi.org/10.3390/agronomy15020288

APA Style

Wu, Z., Hua, W., Ni, K., Yang, X., & Jiang, F. (2025). Effects of Long-Term Organic Substitution on Soil Nitrous Oxide Emissions in a Tea (Camellia sinensis L.) Plantation in China. Agronomy, 15(2), 288. https://doi.org/10.3390/agronomy15020288

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