The After-Effect of Organic Fertilizer Varies among Climate Conditions in China: A Meta-Analysis

Abstract

Organic fertilizer is utilized to improve the organic carbon levels in arable soils, which is helpful for soil quality improvement and crop yield increase. However, the after-effect of organic fertilizer varies among regions with different temperature and precipitation conditions, and the extent of the impact remains unknown. This study aimed to investigate the impact of varying temperature and rainfall conditions on the accumulation of soil organic carbon after organic fertilizer application. A meta-analysis of 168 peer-reviewed studies published between 2005 and 2022 involving a total of 464 trials was conducted. The following was discovered: (1) In the major grain-producing areas of China, there was a significant positive correlation (p < 0.01) between latitude and soil organic carbon content. Meanwhile, temperature and precipitation had a significant negative correlation (p < 0.01) with soil organic carbon content. (2) The increase in temperature inhibited the increase in soil organic carbon storage. The improvement effect of organic fertilizer application in the low-temperature areas was significantly increased by 60.93% compared with the mid-temperature areas, and by 69.85% compared with the high-temperature areas. The average annual precipitation affected the after-effect of organic fertilizer as follows: 400–800 mm > 400 mm > more than 800 mm. (3) The influence of climatic conditions on the after-effect of organic fertilizer was more significant depending on the specific tillage practice. To increase organic fertilizer use efficiency and eliminate greenhouse gas emissions, liquid organic fertilizers with abundant trace nutrients and amino acids, which take advantage of releasing nutrients more swiftly and have a better fertilization effect, could be an alternative to traditional organic fertilizers.

1. Introduction

From 1990 to 2020, China’s fertilizer application increased by 174.96%, while the grain growth rate was only 48.03% [1]. The reason for the higher growth rate of fertilizer application compared to grain yield is that fertilizers only provide readily available nutrients for seasonal crops without adding slow-release nutrients such as organic matter to the soil [2]. The long-term application of fertilizers can even damage the soil structure and cause nutrient loss in the soil, forming a vicious cycle [3,4,5,6]. Soil organic carbon (SOC) is a unique form of carbon which plays an important role in soil improvement and crop growth [7]. The exchange of energy and carbon between soil organic matter and the soil environment is crucial for agricultural productivity and climate [8]. Soil organic matter often contains biocolloids and porous structures. Biocolloids are tiny biological particles present in soil that play a key role in regulating soil structure, filtering and retaining nutrients and pollutants, and influencing the structure of soil microbial communities [9]. Biocolloids can effectively bind soil inorganic components, while porous structures can provide attachment space for microorganisms and prevent soil compaction. Soil organic carbon content is significantly related to soil physical properties such as water-holding capacity, bulk density, and aggregate structure [10]. Soil organic carbon content is also associated with microbial gene abundance, biomass, and biological activity [11]. Increasing soil organic carbon content can effectively promote the formation of organic–inorganic aggregates and thereby reduce the content of gravel in the soil [12]. The increase in soil water-holding capacity and bulk density is conducive to the accumulation of inorganic nutrients and soil organic carbon [13]. The increase in SOC content in the soil can significantly optimize the characterization of soil aggregate and particle-size distributions [14], increase soil aggregate stability, and have a significant optimization effect on soil physical properties [15]. Organic matter in the soil needs to go through inorganic pathways before being utilized by plants, and these pathways are highly dependent on the microbial environment. The increase in soil organic carbon content not only increases the total amount of organic matter available to microbes but also provides the necessary environment for their survival. A good microbial community can effectively provide nutrients for crops and thus improve grain yield and quality [16].

The most effective way to increase SOC content is to apply organic fertilizers or soil amendments during the fertilization stage. Organic fertilizers have many characteristics such as a high carbon element, high similarity between C/N and the microbial physiological environment, a multi-pore structure, and high intermolecular adhesion compared with inorganic soil amendments. Therefore, organic fertilizers are more helpful in balancing soil element ratios and optimizing the microbial environment, thus improving the SOC content in the soil [17]. Additionally, the bacterial populations, polysaccharides, and mineral complexes carried by organic fertilizers have a significant impact on micro-aggregates. Micro-aggregates are soil aggregates with a diameter of less than 250 micro-meters, which are made up of a variety of mineral, organic and biological components combined by physical, chemical, and biological processes to form a composite structure capable of resisting strong mechanical and physicochemical stresses [18]. Previous studies have shown that the quantity of fungi has a significant positive influence on the formation and stability of soil aggregates [19]. Organic fertilizers, characterized by their long decomposition cycle and richness of trace elements, can optimize soil aggregate distribution, balance soil acidity and alkalinity, increase nutrient content, and thus improve soil physical and chemical properties [20,21,22,23]. The use of organic fertilizers can increase the crop yield and the content of functional compounds such as anthocyanins in crop seeds [24].

Because most of the nutrients in organic fertilizer need to be absorbed by plants through inorganic reactions, this process is relatively slow [25]. Therefore, organic fertilizers cannot provide enough available nutrients for crop growth [26]. Although the mixed application of organic and chemical fertilizers is an effective method to improve soil physicochemical properties in the case of ensuring the output of farmers, the application number of chemical fertilizers in China was 51.91 million tons by 2020, and at the same time, only 15.15 million tons of organic fertilizers had been used [1]. Therefore, in recent years, China has widely promoted the combined application of organic and chemical fertilizers to replace the use of pure chemical fertilizers, aiming to improve the quality of farmland and crop quality. In a previous study, the application of organic fertilizers significantly increased SOC storage compared with non-fertilizer treatments in the wheat cultivation system. The wheat yields from the organic fertilizer treatment group continued to grow over the five-year trial [27]. The maize yield of the organic fertilizer treatment group also continuously increased over the three-year experimental period. However, the SOC storage in the maize cultivation system was significantly reduced compared with the wheat cultivation system [27]. Verma et al. (2010) reported that SOC storage was significantly reduced by organic fertilizer treatment in the dry land wheat–maize rotation system [28]. Organic fertilizer treatment significantly increased SOC storage in maize and wheat rotation systems [29]. In the study of rice paddies, both chemical and organic fertilizer applications significantly increased SOC storage [30,31]. The role of organic fertilizers in increasing SOC storage was found to be insignificant in paddy fields, and the application of low-concentration organic fertilizers significantly reduced SOC storage [28,32]. The difference is probably caused by the different geographical conditions of the test area. In the study of organic fertilizer application, the effects of precipitation and temperature on SOC storage in the crop growth stage have not been clearly stated, and the mechanism of action is still unclear [33,34,35].

However, organic fertilizer efficiency is influenced by many climatic and soil background factors. Many studies on the effect of organic fertilizer application on the improvement in cultivated land quality and soil improvement have been conducted in different regions, while different studies have not been systematically compared to understand the effect of climatic factors on the after-effect of organic fertilizer. This study intends to clarify the influence of organic fertilizer application on soil organic matter storage through a meta-analysis of existing literature data, which will provide theoretical guidance and data support for the strategy construction of organic fertilizer in different regions of China.

2. Materials and Methods

2.1. Data Collection

For this meta-analysis, we collected 168 peer-reviewed studies published from 2005 to 2022, 98 of which were valid, involving a total of 464 trials (Figure 1). The data were collected from the ISI Web of Science (https://apps.webofknowledge.com/ (accessed on 30 March 2023) and the CNKI database (https://www.cnki.net/ (accessed on 30 March 2023)). The main search terms were “Soil after-effect”, “Farmland”, and “organic fertilizer”. The search area was limited to “PEOPLES R CHINA”. The means, standard deviation (SD), and replicate numbers (n) of the percentage of soil aggregates in the analysis were recorded, and some data were extracted from published figures using Get-Data software (ver.2.20). For papers that used standard error (SE), Equation (1) was used to convert it to standard deviation (SD).

$$SD=SE\times \sqrt{n}$$

(1)

where n represents the number of replicates. If SE and SD could not be reported, we reassigned the SD as 1/10 of the mean [36].

To avoid bias in the publication selection process, the following criteria were used:

• Studies were performed in China’s farmland soils, involving the studies of wheat, maize, sorghum, and rice. The data on soil fertility were based on the 0–20 cm soil level (tillage layer).
• Studies must be field trials and need to be effectively replicated annually.
• If multi-year data were available at the same site, only the most recent year data were used.
• Studies contained a control blank group (CK) with zero fertilizer and an experimental group (EG) with organic fertilizer and chemical fertilizer. The experimental conditions of both groups were consistent.
• We collected the average annual data of different regions for grouping, which involved three groups of high temperature (>20 °C), medium temperature (10–20 °C), and low temperature (<10 °C).

The calculation formula for soil organic carbon storage (SOCS) is as follows:

SOCS = (SOC after cultivation) − (SOC before cultivation)

(2)

From each published study, we extracted the geographical location information (longitude, latitude, and altitude), climatic conditions information (mean annual precipitation and mean annual temperature), information on fertilizer application, and study duration. When the MAP and MAT of the experimental site were not available in the article, we used the numbers from the global climate database (http://www.worldclim.org/ (accessed on 4 May 2023) according to the geographical coordinates. The experimental sites are shown in Figure 1.

2.2. Meta-Analysis

In the meta-analysis, the inverse variance statistical model and random-effect analysis model were used to analyze the mean difference between the experiment group (EG) and the control group (CK). The random-effect model assumes that there was variability in the true effects across studies, in addition to the variability due to sampling error. The model estimated the average effect size across studies as well as the amount of variation in effect sizes due to differences between different studies beyond chance. The formula for calculating the weighted mean effect size ($\theta$) using the random-effect model is as follows:

$$\theta =\frac{\sum _{i=1}^k{w}_i{y}_i}{\sum _{i=1}^k{w}_i}$$

(3)

where $k$ is the number of studies, and $y_i$ is the effect size of the $i$th study (here, the researchers use the mean of SOCS). The $w_i$ can be obtained from Formula (4):

$$w_i=\frac{{\sigma }_i^2+{\tau }^2}{{\sigma }^2}$$

(4)

${\sigma }_i$ is the standard error of the effect size, and ${\tau }^2$ is the between-study variance. The random-effect model assumes that the true effect sizes follow a normal distribution with mean $\theta$ and variance ${\tau }^2$.

SPSS 22.0 (Stanford University, Stanford, CA, USA) was used to perform the variance analysis on the data, and the least significant difference (LSD) method was used to perform multiple comparisons at the 0.05 level to determine the statistical significance of different treatments. Origin Pro 2023 (OriginLab. Inc. Northampton, MA, USA) was used for graphing. Randomization tests were then used to determine whether there was a significant difference in inter-group heterogeneity among different treatments.

3. Results

3.1. Effects of Climate Factors on Soil Background Organic Carbon Storage

The experiment collected information on soil background organic matter content in 98 major grain-producing areas in China (Figure 2). The overall soil background organic matter content in China’s main grain-producing areas showed a trend of being higher in the north and lower in the south. The soil background organic matter content in the eastern coastal areas and western inland areas is generally higher than that in the central plain areas. The soil background organic matter content is the highest in the Heilongjiang region, while it is the lowest in the Hainan region.

In the regression analysis of latitude and soil background organic carbon content (Figure 3), soil background data ranged from 18 degrees north latitude to 48 degrees north latitude, covering tropical, subtropical, and temperate climate types. Among all of the survey data, soil organic carbon content was between 4.81 and 31.41 g·kg⁻¹. In more than 90% of the areas, the soil organic carbon content was between 6.00 and 16.00 g·kg⁻¹. Through regression analysis, it was found that there was a significant positive correlation (p < 0.01) between soil background organic carbon content and latitude in China. As the latitude increased, the soil background organic carbon content showed a gradually increasing trend. The average soil background organic carbon content increased by 0.36 g·kg⁻¹ for each degree of latitude.

In the regression analysis of average annual temperature and soil background organic carbon accumulation (Figure 4), among all of the data collected, the minimum annual mean temperature of each experimental site was 2.1 °C, and the maximum annual mean temperature was 26 °C, of which 31.34% was below 10 °C, 61.19% was between 10 and 20 °C, and 7.46% was above 20 °C. Through the regression analysis, it was found that there was a significant negative correlation (p < 0.01) between soil background organic carbon content and temperature in China. As the temperature increased, the soil background organic carbon content showed a gradually decreasing trend. The average soil background organic carbon content decreased by 0.37 g·kg⁻¹ with one degree increase in temperature.

In the regression analysis of annual average precipitation and soil background organic carbon accumulation (Figure 5), the collected data showed that the lowest average annual rainfall was 201 mm, and the highest was 2065 mm, and in more than 90% of the areas, the annual average precipitation exceeded 400 mm. Through the regression analysis, it was found that there was a significant negative correlation (p < 0.01) between soil background organic carbon content and annual precipitation in China. As the annual precipitation increased, the soil background organic carbon content showed a gradually decreasing trend. The average soil background organic carbon content decreased by 0.29 g·kg⁻¹ for 100 mm of annual precipitation.

3.2. Effects of Climate Conditions on Soil Organic Carbon Storage after Organic Fertilizer Application

The random-effect analysis was performed on soil organic carbon storage in high-temperature areas (average annual temperature above 20 °C), medium-temperature areas (average annual temperature above 10 °C and below 20 °C), and low-temperature areas (average annual temperature below 10 °C), respectively (Figure 6). The results showed that the effect of organic fertilizer on soil organic carbon storage decreased significantly with increasing temperature.

In high-temperature areas, the application of total fertilizers (organic fertilizers and chemical fertilizers) significantly increased soil organic carbon storage by an average of 2.83 g·kg⁻¹. Compared with chemical fertilizers, the application of organic fertilizers significantly increased soil organic carbon storage by 3.25 g·kg⁻¹, while chemical fertilizers increased soil organic carbon storage by 2.39 g·kg⁻¹. However, there was no significant difference between the improvement effect of organic fertilizers and chemical fertilizers on soil organic carbon storage.

In medium-temperature areas, the application of total fertilizers significantly increased soil organic carbon storage by an average of 2.51 g·kg⁻¹. Compared with chemical fertilizers, the application of organic fertilizers significantly increased soil organic carbon storage by 3.43 g·kg⁻¹, while chemical fertilizers increased soil organic carbon storage by 1.49 g·kg⁻¹. The improvement effect of organic fertilizers on soil organic carbon storage was 1.3 times higher than that of chemical fertilizers.

In low-temperature areas, the application of total fertilizers significantly increased soil organic carbon storage by an average of 3.57 g·kg⁻¹ with the application of the two types of fertilizers. Compared with chemical fertilizers, the application of organic fertilizers significantly increased soil organic carbon storage by 5.52 g·kg⁻¹, while chemical fertilizers increased soil organic carbon storage by 1.84 g·kg⁻¹. The improvement effect of organic fertilizers on soil organic carbon storage was two times higher than that of chemical fertilizers. By analyzing the effects of organic fertilizer application on soil organic carbon accumulation in different temperature regions, it was found that the increasing effect of organic fertilizer application on soil organic carbon accumulation in medium-temperature regions was 5.54% higher than that in high-temperature regions. The improvement effect of organic fertilizer application in low-temperature areas was significantly increased by 60.93% compared with medium-temperature areas, and by 69.85% compared with high-temperature areas.

The random-effect analysis was performed on soil organic carbon storage in areas with annual rainfall of the experimental year < 400 mm, between 400 and 800 mm, and >800 mm, respectively (Figure 7). In the areas with rainfall < 400 mm, the application of total fertilizers (organic fertilizers and chemical fertilizers) significantly increased soil organic carbon storage by an average of 3.03 g·kg⁻¹. Compared with total fertilizers, the application of organic fertilizer significantly increased soil organic carbon storage by 5.95 g·kg⁻¹, while the application of chemical fertilizer increased soil organic carbon storage by 0.53 g·kg⁻¹. The improvement effect of organic fertilizer on increasing soil organic carbon storage was 10.23 times higher than that of chemical fertilizer.

In the areas with rainfall between 400 and 800 mm, the application of total fertilizers (organic fertilizers and chemical fertilizers) significantly increased soil organic carbon storage by an average of 4.87 g·kg⁻¹. Compared with total fertilizers, the application of organic fertilizer significantly increased soil organic carbon storage by 6.65 g·kg⁻¹, while the application of chemical fertilizer increased soil organic carbon storage by 3.50 g·kg⁻¹. The improvement effect of organic fertilizer on increasing soil organic carbon storage was 0.9 times higher than that of chemical fertilizer.

In the areas with rainfall > 800 mm, the application of total fertilizers (organic fertilizers and chemical fertilizers) significantly increased soil organic carbon storage by an average of 2.28 g·kg⁻¹. Compared with total fertilizers, the organic fertilizer application significantly increased soil organic carbon storage by 3.95 g·kg⁻¹, while the application of chemical fertilizer increased soil organic carbon storage by 0.52 g·kg⁻¹. The improvement effect of organic fertilizer on increasing soil organic carbon storage was 6.6 times higher than that of chemical fertilizer.

3.3. Effects of Temperature and Precipitation on the After-Effect of Organic Fertilizer with Different Tillage Methods

The overall trend obtained from the previous statistics was analyzed by considering different farming methods in various regions as a whole. To avoid differences in the effects of different tillage methods on the after-effect of organic fertilizer, this analysis separated the paddy field, dry land, and rice–crayfish farming model in all of the survey data. The impact of the average annual temperature and annual precipitation under various farming methods on the after-effect of organic fertilizer application was studied.

With the paddy field farming method, the random-effect analysis of the application of organic fertilizer in the high-temperature areas (HT), mid-temperature areas (MT), and low-temperature areas (LT) on soil organic carbon storage was conducted (Figure 8a). Overall, the application of organic fertilizer with the paddy field farming method significantly increased soil organic carbon storage (4.44 g·kg⁻¹). In different temperature areas, the increase in soil organic carbon storage from the application of organic fertilizer in the high-temperature areas was the lowest, only 2.55 g·kg⁻¹, while soil organic carbon storage was significantly higher than that in the high-temperature areas (p < 0.05) in the mid-temperature areas, being 4.10 g·kg⁻¹. The application of organic fertilizers in the low-temperature areas resulted in the most significant increase in soil organic carbon storage, as high as 9.10 g·kg⁻¹. Soil organic carbon storage in the mid-temperature areas significantly increased by 60.78% compared with the high-temperature areas, while the application of organic fertilizer in the low-temperature areas increased soil organic carbon storage by 2.57 times compared with the high-temperature areas.

For the paddy field farming method, fertilizer was mainly distributed in areas with an annual average precipitation above 800 mm. Therefore, this study only involves areas with an average annual precipitation of less than 800 mm and areas with an average precipitation above 800 mm. In this condition, there is an interactive effect when analyzing the impact of organic fertilizer application on soil organic carbon storage (Figure 8b). The results show that in different areas, according to the annual average precipitation, the application of organic fertilizers improved the average soil organic carbon storage (7.91 g·kg⁻¹). Specifically, compared with areas with precipitation > 800 mm, the application of organic fertilizer in areas with precipitation < 800 mm increased soil organic carbon storage more significantly, reaching 10.68 g·kg⁻¹. In contrast, the application of chemical fertilizer in areas with precipitation over 800 mm increased soil organic carbon storage (3.94 g·kg⁻¹). The application of organic fertilizer in areas with precipitation less than 800 mm increased soil organic carbon storage by 1.71 times compared with areas with precipitation over 800 mm.

In the dry land cropping system, the random-effect analysis was performed on soil organic carbon storage with the application of organic fertilizer in the medium-temperature areas (MT) and the low-temperature areas (LT) (Figure 9a). Generally speaking, the application of organic fertilizer increased soil organic carbon storage (2.85 g·kg⁻¹). The application of organic fertilizer in the LT areas significantly increased soil organic carbon storage, reaching 3.66 g·kg⁻¹, which was higher than that in the MT areas where the organic fertilizer application resulted in a 2.00 g·kg⁻¹ increase in soil organic carbon storage. The application of organic fertilizer in the LT areas resulted in a significant 83% increase in soil organic carbon storage compared with the MT areas. The data were divided from all dry land cropping patterns into two groups according to the annual average precipitation: areas with annual precipitation < 400 mm and areas with annual precipitation > 400 mm. We then compared the impact of the application of organic fertilizer on soil organic carbon storage in these two groups (Figure 9b). The application of organic fertilizer in areas with different annual precipitation increased soil organic carbon storage by an average of 2.12 g·kg⁻¹. In the areas with annual precipitation < 400 mm, the application of organic fertilizer significantly increased soil organic carbon storage (3.56 g·kg⁻¹), more than in the areas with precipitation > 400 mm, where soil organic carbon storage only increased by 0.57 g·kg⁻¹ due to organic fertilizer application. In comparison, the application of organic fertilizer in areas with annual precipitation < 400 mm increased soil organic carbon storage 5.2 times more than in areas with annual precipitation > 400 mm.

For the rice–crayfish farming model, we extracted 24 sets of effective data. Among them, 12 sets of data came from the experimental sites with an average annual temperature of <17 °C, 4 sets of data had an average annual temperature of 17–18 °C, and 8 sets of effective data on rice–crayfish farming collected from Hainan were classified as groups with an average annual temperature greater than 18 °C. In the temperatures of 16–17 °C, rice–crayfish farming significantly increased soil organic carbon storage (5.97 g·kg⁻¹). In the temperatures of 17–18 °C, rice–crayfish farming increased soil organic carbon storage (4.68 g·kg⁻¹). In environments with a temperature above 18 °C, rice–crayfish farming increased soil organic carbon storage (0.28 g·kg⁻¹), which is a significantly lower increase compared to the treatment temperatures of 16–17 °C and 17–18 °C.

In general, there are differences in the increase in soil organic carbon storage brought about by the application of organic fertilizer between paddy fields and dry lands. In regions with the same temperature (Figure 8a and Figure 9a), the paddy cultivation model significantly increases soil organic carbon storage with the use of organic fertilizer compared with the dry land cultivation model. However, upon increasing the temperature, the decline in the improvement effect of soil organic carbon storage brought by the application of organic carbon in the paddy cultivation model is significantly higher than that in the dry land cultivation model.

4. Discussion

4.1. Effects of Climatic Factors on the After-Effect of Organic Fertilizer

There is a significant correlation between the content of soil organic carbon and soil and climatic temperature (Figure 2 and Figure 3). At a regional scale, the correlation between SOC and soil temperature is negative between 52° N and 40° S parallels and positive beyond this region. This suggests that large SOC stocks in low-temperature areas might increase under global warming, while small SOC stocks in high-temperature areas might decrease. The effect is attributed to the temperature-dependent SOC formation (photosynthesis) and decomposition (microbial activities) processes [37]. Moreover, climate change scenarios predict a reduction in SOC content due to increased temperatures, with a significant impact depending on average temperature and annual precipitation. This suggests a potential trend of SOC migration toward higher latitude and altitude under climate change [38]. Therefore, the correlation between SOC and climate factors (temperature and precipitation) varies depending on the geographic extent [39]. This indicates the need for studies to understand the relationship between SOC and climate factors accurately. The effect of organic fertilizer application on increasing soil organic carbon storage decreases with the rise in temperature (Figure 6). This may be related to the influence of temperature change on soil organic carbon transformation in different regions. One potential explanation for this occurrence could be linked to the activity levels of crucial enzymes involved in the soil organic matter’s carbon cycling pathway, namely, cellulase (CEL) and β-1,4-glucosidase (BG). Elevated soil temperatures might contribute to the decrease in and deactivation of these soil enzymes. Consequently, the organic carbon introduced into the soil by organic fertilizers may remain unconverted and underutilized [40].

Moreover, in high-temperature areas, there is no significant difference in the enhancement of soil organic carbon storage by organic and chemical fertilizers (Figure 6). This may be because a large amount of organic carbon is added to the soil by organic fertilizers in a short period, and the fixation and utilization of soil organic carbon are extremely dependent on microbial physiological and biochemical reactions. In soils lacking microbial communities that support organic carbon fixation, organic carbon is prone to loss [41,42]. Furthermore, increased temperature boosts microbial activity, with 75% of the excess organic carbon added in the short term being used to provide energy for soil microbes instead of being fixed in the soil [43]. Additionally, elevated temperatures activate microbial (de)nitrification and respiration reactions [44]. When the temperature reaches 40 °C compared with 30 °C, the emissions of N₂O and CO₂ in the soil increase significantly by 38%, leading to the loss of organic carbon in the soil in the form of greenhouse gases [6,45]. In summary, there is a lack of suitable microbial environments for organic carbon storage in high-temperature soil, and high temperatures cause soil enzyme inactivation and denaturation, making it more difficult for soil organic carbon to be fixed and utilized in the short term. A large amount of unutilized or unfixed organic carbon results in soil nutrient waste and greenhouse gas emissions. Therefore, the application of organic fertilizers in high-temperature areas in China may not significantly increase soil organic carbon storage.

In the cultivation pattern of paddy fields (Figure 8a), the application of organic fertilizer has been found to reduce soil organic carbon storage as the temperature increases. However, it is important to note that the climatic factors affecting the decomposition and fixation of organic carbon differ between paddy fields and dry land cultivation patterns [46]. In rice fields, higher temperatures stimulate the release of substances related to organic carbon decomposition by the rice root system, leading to changes in the physical and chemical properties of the soil. This ultimately results in the outflow of soil C and N, leading to the loss of soil organic carbon in the form of N₂O and CO₂ [47,48,49]. Under the anaerobic conditions of paddy fields, increased temperature stimulates the metabolic activity of microbes [50,51]. Previous studies have demonstrated that the formation of every 7 µg of microbial structural carbon consumes 14.6 µg of soil organic carbon through respiration process [52]. Consequently, the organic carbon fixed by microbes is significantly lower than the organic carbon consumed by microbes. Similar to the case in paddy fields, organic fertilizer application significantly reduces soil organic carbon storage as the temperature rises in dry land (Figure 9a). The most probable reason is that in dry land conditions, the water environment for microbial survival is limited, and increased temperatures may reduce soil microbial activity. This can result in inadequate accumulation of structural organic carbon in microbes [53]. Dry organic carbon will also be eroded by wind and sand, ultimately leading to a reduction in soil organic carbon storage [54,55].

For microbes with heat-resistant genomes and those in regions with relatively abundant water, their biological activity increases with the rise in temperature [56]. This leads to greater soil organic carbon consumption, with CO₂ emissions increasing 6.6 times and CH₄ emissions increasing 3.7 times when the temperature rises from 5 °C to 15 °C [57], eventually resulting in soil organic carbon being consumed by microbes. In summary, the enhancement effect of organic fertilizer application on soil organic carbon storage in both paddy and dry land will decrease as the temperature rises. Compared with dry land, the effect of organic fertilizer on soil organic carbon storage in the paddy field cultivation pattern is more noticeably affected by temperature. The reason for this may be that the soil of dry land has a lower water content and pH, and the lower pH significantly suppresses the formation of microbial communities and inorganic reactions of organic carbon. At the same time, the number of fungi and bacteria in dry land are 41–63% and 58–69% lower than those in paddy fields. Therefore, under the same climatic conditions, the microbial environment in paddy fields is richer compared with that in dry land. When the temperature rises, the biological activity of microbes increases significantly, resulting in a higher consumption of organic carbon in paddy fields. The efficient microbial biochemical reactions are accompanied by the fixation of organic carbon, ultimately leading to significantly higher soil organic carbon storage in paddy fields than in dry lands [58,59].

Previous studies have suggested that in temperate and subtropical areas, rice–crayfish farming can improve the organic carbon content of the soil. The combination of rice and aquaculture can effectively increase dissolved organic carbon in the rice cultivation system [60]. Integrated rice–crayfish farming does not directly apply organic fertilizer. Instead, it increases organic matter input into the cultivation system by feeding on Procambarus clarkii in the fields. The feed enters the crop growth system either through direct microbial decomposition or after digestion by crayfish, with effects and components similar to those of organic fertilizer application. The present study found that compared with the paddy field cultivation model under medium-temperature conditions, soil organic carbon storage in rice–crayfish farming at 16–17 °C significantly increased, reaching 5.97 g·kg⁻¹ (Figure 10). Compared with only rice farming, rice–crayfish farming can significantly increase soil organic carbon storage. This might be because in comparison to direct organic fertilizer application, the fertilizer input into the early stage of rice–crayfish farming can fully meet the early growth needs of crops and promote the richness of microbial communities [61]. In the later stage, organic carbon is added to the farming system through feeding multiple times. The organic carbon added in this way needs to enter soil circulation under the digestive decomposition of Procambarus clarkii. The process facilitates soil in repeatedly absorbing small amounts of organic carbon, which is slow and enduring, ultimately boosting soil organic carbon storage [62].

The distribution of precipitation in China’s major grain-producing areas also has a significant impact on soil organic carbon storage. Areas with precipitation less than 400 mm and precipitation between 400 and 800 mm show more pronounced increases in soil organic carbon storage when applying organic fertilizer (Figure 7). This may be because in areas with precipitation less than 400 mm, 47.5% are in low-temperature regions, and 52.5% are in medium-temperature regions; meanwhile, in areas with precipitation between 400 and 800 mm, 55% are in low-temperature regions, and 45% are in medium-temperature regions. The temperature distribution in these two regions has similarities with seasonal changes. Additionally, the characteristics of four distinct seasons and low annual average temperatures are conducive to organic carbon storage [63]. In addition, compared with arid areas with precipitation less than 400 mm, the alternating characteristics of dry and wet conditions in the 400–800 mm region are more conducive to crop growth and the establishment of microbial communities [58,64]. Areas with precipitation between 400 and 800 mm are mainly located in the northeastern plain and central China plain.

China has been implementing a straw-returning strategy for a long time. Years of straw returning have not only ensured the increase in soil organic carbon content but have also cultivated a rich microbial community [65]. Therefore, the application of organic and chemical fertilizers in this region can significantly increase soil organic carbon storage. Areas with precipitation of more than 800 mm are mainly located on the south of the Qinling–Huaihe line. Additionally, precipitation leads to a decrease in soil oxygen concentrations. Insufficient soil oxygen concentrations will intensify the anaerobic respiration of microbes and the (de)nitrification reaction of anaerobic microbes, and when the soil moisture content reaches 80%, N₂O emissions can reach 27.6 μg N kg⁻¹ h⁻¹ [66]. In addition, when the soil moisture content increases from 70% to 100%, CH₄ emissions will increase from 4.1 μg C kg⁻¹ h⁻¹ to 6.2 μg C kg⁻¹ h⁻¹ [57]. These conditions ultimately result in the loss of soil organic carbon in the form of greenhouse gases [45,67]. Generally speaking, areas with high precipitation are mostly in high-temperature areas, where microbial communities necessary for organic carbon fixation are not well assembled [68]. Therefore, the fixation and utilization of organic carbon are restricted in this case. The absorption and fixation of organic carbon is a long-term process, and non-fixed organic carbon is easily affected by leaching [69].

The erosion effect of precipitation and surface runoff have varying levels of damage to paddy fields and dry land, and the soil moisture conditions under paddy and dry land cultivation modes are greatly different. For the paddy field cultivation mode, in areas where precipitation exceeds 800 mm, the improvement effect of organic fertilizer in soil organic carbon storage reduces by 6.74 g·kg⁻¹ compared with areas where precipitation is less than 800 mm. This might be due to the hypoxic environment that soil microbes in paddy farming systems live in. An increase in soil moisture and the bloom of algae will decrease dissolved oxygen in the water, worsening the hypoxic environment of the soil. Since soil microbes fix organic carbon mainly through aerobic respiration, the lack of oxygen in the soil greatly limits the fixation of organic carbon in the soil, reducing the effectiveness of organic fertilizer [70]. Moreover, adding more fertilizer in anaerobic conditions significantly boosts the bioactivity and abundance of Serratia and Enterobacter, which ultimately enhances soil nitrate reduction reactions and causes the loss of organic carbon in the form of greenhouse gases [71]. According to previous reports, the tropics and subtropics produce about 511 Tg C of dissolved organic carbon annually, which is higher than in the temperate areas (244 Tg C), as 30 % of dissolved organic carbon is located in the top 10 cm of soil [72]. Therefore, high dissolved organic carbon proportions are more susceptible to erosion, runoff, and organic carbon leaching losses in the tropics and subtropics.

However, in areas where precipitation exceeds 400 mm under the dry land cultivation mode, the improvement effect of organic fertilizer in soil organic carbon storage reduces by 2.99 g·kg⁻¹ compared with the areas where precipitation is less than 400 mm. This could be attributed to the lack of soil water, which restricts microbial activity, and therefore, organic carbon is not decomposed by microbes in the regions lacking in precipitation [73]. Meanwhile, soil is rarely affected by leaching in the regions with less rainfall, and thus, organic carbon added to the soil has a long retention time. When there is more precipitation, soil water content increases and ventilation conditions become worse under the dry land cultivation mode, which is not conducive to the fixation of organic carbon by microorganisms [74]. In this condition, about 20% of the non-fixed organic carbon is washed away into the water system by leaching [72], resulting in tremendous organic carbon loss. Therefore, in temperate and subtropical regions in China, more consideration is needed for reasonable organic fertilizer use to reduce organic nutrient runoff and greenhouse gas emissions [75].

Furthermore, the after-effects of organic fertilizer for plant growth are also affected by the physical and chemical properties of soil. The effectiveness of organic fertilizers does vary in different types of soil, mainly due to differences in the physical and chemical properties of each type of soil. The pH of soil can affect the effectiveness of organic fertilizers [76]. Soil pH influences the solubility of nutrients in organic fertilizers, with certain nutrients becoming more available to plants at specific pH levels [76]. Adjusting soil pH through organic amendments can help optimize nutrient availability and uptake by crops [76]. Soils with better porosity and water-holding capacity tend to have improved responses to organic fertilizer application. These properties ensure that water and nutrients are adequately available to plant roots, facilitating the effective use of the nutrients provided by organic fertilizers [77].

4.2. Feasible Strategies for Improving Soil Fertility in High-Temperature and Rainy Regions

The application of organic fertilizers serves two main purposes in agriculture. Firstly, it directly adds organic and inorganic nutrients to the soil [78], which could meet crop growth needs by providing available nitrogen, phosphorus, potassium, trace elements, and micro-molecule organics needed by crops to improve yield and quality [7,79,80]. Secondly, organic fertilizers have a loose physical structure and contain an organic binder, which effectively improves soil structure by increasing the number of large aggregates and preventing soil crusting [73]. However, climatic factors like high temperatures and heavy rainfall can decrease the improvement effect of organic fertilizers. Therefore, it is crucial to implement more scientific and rational measures to enhance the physical and chemical properties of soil, ultimately improving soil fertility in South China.

Previous studies have shown that liquid organic fertilizers have several advantages over traditional solid organic fertilizers. Liquid organic fertilizers contain available nutrients that can be readily absorbed by crops, leading to improved crop yield and nutrient use efficiency [72]. They also have higher concentrations of trace nutrients and dissolved organic carbon, which is rich in amino acids, and exhibit high chemical and biological activity, thereby enhancing crop quality [81]. Furthermore, liquid organic fertilizers can introduce specific microbial communities which could help to establish a favorable microbial environment for soil organic carbon turnover, especially in cases where the soil microbial environment is damaged. Compared with traditional organic fertilizers, liquid organic fertilizers have a more balanced ratio of small organic molecules, such as amino acids and polysaccharides. The balance improves microbial activity and soil enzyme activity levels, accelerates microbial amino acid and protein synthesis, and shortens the soil organic carbon improvement cycle [42]. Moreover, liquid organic fertilizers facilitate the distribution of nutrients in the cultivation soil, leading to improved crop yield and nutrient utilization efficiency [82]. By using liquid organic fertilizers instead of traditional solid organic fertilizers, crops can receive more nutrients, resulting in increased yield. It is well known that the integrated system of water and fertilizer is conducive to improving water and fertilizer use efficiency, while due to the low dissolution rate of traditional solid organic fertilizers, it is difficult to achieve water and solid organic fertilizer integration. Moreover, liquid organic fertilizers can be directly added to automatic irrigation agricultural facilities without the need for additional mechanisms in the experiment. However, liquid organic carbon cannot effectively improve soil physical properties because it does not have solid materials found in traditional organic fertilizers [83].

Biochar can be used as a replacement for organic fertilizers to improve soil physical properties [84,85,86]. On the one hand, biochar can improve the environment of water-stable aggregates and the adsorption capacity of soil [87,88,89]. On the other hand, the use of biochar can significantly reduce the rate of carbon mineralization in soil and decrease its temperature sensitivity [5]. Natural organic carbon molecules can bond with biochar, making their combination more difficult to absorb by microorganisms, hence protecting soil organic carbon from microbial decomposition [90]. In addition, biochar is more difficult for microorganisms to mineralize and utilize compared with natural organic carbon. Its stability and survival time are significantly higher than those of natural organic carbon, making it possible to be preserved in complex environments [91]. This makes biochar less vulnerable to the impact of temperature and precipitation, making it a potential replacement for organic fertilizers to improve soil physical structure in the middle and low-latitude areas. Therefore, while applying liquid organic fertilizers, biochar can effectively resist the mineralization of soil organic carbon caused by climatic temperature, thereby allowing organic matter to provide nutrients more persistently for crops and to contribute to soil improvement.

Besides biochar, mineral and engineering wastes can also effectively improve the physicochemical properties of soil [92]. For example, potash feldspar, gypsum, and calcium carbonate containing available mineral nutrients are shown to be effective for acidic soil amelioration [93]. Flue gas desulfurization gypsum and humic acid decreased soil pH, sodium adsorption ratio, and exchangeable sodium percentage, and increased soil electrical conductivity and saturated hydraulic conductivity [94]. Saha et al. reported that brown coal–urea blends increased biomass yield by 27% and 23% in Tenosol and Dermosol soil [95], respectively, in silver beet cultivation, since brown coal–urea can substantially enhance the potentially mineralizable nitrogen and organic carbon content of soil, thus increasing fertilizer N availability and uptake, and reducing N₂O emissions by 29% and NH₃ emissions by 36% compared to urea alone [96]. Moreover, the above measures can effectively improve the physical and chemical properties of soil, but the improvement in the microbial environment in soil is an extremely slow process. Therefore, the construction of microbial environments needs to be considered during fertilization [97]. Furthermore, microbial inoculants can increase soil urease, invertase, alkaline phosphatase, catalase activity, available nitrogen, available phosphorus, available potassium, and organic carbon in soil [97]. Many reports have indicated that the application of microbial inoculants can influence, at least temporarily, the resident microbial communities [98], while a combination of inoculants will not necessarily produce an additive or synergic effect, but rather a competitive process [99]. The plant–soil–biota pathway varies greatly among different climatic conditions; thus, the microbial assembly conditions should be considered when using microbial inoculants [99].

The combined application of organic and chemical fertilizers is considered the most efficient way to improve soil fertility and crop production. However, the ratio of organic and chemical fertilizers should be carefully calculated under different climatic conditions with varied temperature and precipitation. The application strategy with the optimal ratio of organic and chemical fertilizers which can balance the available nutrients and improve soil quality is a sustainable method in modern agriculture. Meanwhile, the degradation of organic fertilizers and the efficacy period of chemical fertilizers are both closely related to climatic conditions, especially temperature and precipitation [100,101,102]. Thus, developing specific organic and chemical fertilizer application strategies based on climatic conditions in different regions is expected in future studies.

5. Conclusions

Soil organic carbon content, which is the most important indicator of soil fertility, varies as the temperature and precipitation changes in different regions in China. The after-effect of organic fertilizer is much lower in tropical regions than in temperate regions, which could be mainly attributed to the microbial activities under varied temperature and precipitation conditions. The application of traditional organic fertilizers has been widely recognized due to the chemical and physical improvements in the soil, but organic fertilizer has the characteristics of slow mineralization, weak temperature tolerance, and low solubility, and thus low fertilizer utilization and high greenhouse gas emissions. In contrast, liquid organic fertilizers with abundant trace nutrients and amino acids have more advantages, including releasing nutrients more swiftly and a better fertilization effect, which can effectively improve crop yield and quality. Meanwhile, the combination of liquid organic fertilizers and soil physical property amendments, such as biochar, can effectively enhance the water-stable aggregate and adsorption capacity of soil, slow the rate of organic carbon mineralization, and reduce temperature sensitivity. These measures can increase crop yield and quality by strengthening soil organic carbon resilience to precipitation and high temperatures while reducing agricultural greenhouse gas emissions, thus providing strong support for sustainable agricultural development. Certainly, liquid organic fertilizers mixed with biochar and other soil amendments have the potential to replace conventional organic fertilizers. And compared to conventional organic fertilizers, this option will be more adaptable to the environment as the formulation can be adjusted.