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Article

The Responses of C, N, P and Stoichiometric Ratios to Biochar and Vermicompost Additions Differ from Alfalfa and a Mine Soil

by
Yu Zhang
1,
Huizhen Mai
1,
Qinghong Qiu
1,
Yinghua Zhu
1,
Jiayi Long
1,
Shengfu Chen
1 and
Yuanqi Chen
2,3,*
1
Hunan Province Key Laboratory of Economic Crops Genetic Improvement and Integrated Utilization, School of Life and Health Sciences, Hunan University of Science and Technology, Xiangtan 411201, China
2
Institute of Geographical Environment and Carbon Peak and Neutrality, School of Earth Science and Spatial Information Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
3
Hunan Province Key Laboratory of Coal Resources Clean-Utilization and Mine Environment Protection, Hunan University of Science and Technology, Xiangtan 411201, China
*
Author to whom correspondence should be addressed.
Agriculture 2023, 13(10), 1954; https://doi.org/10.3390/agriculture13101954
Submission received: 23 August 2023 / Revised: 1 October 2023 / Accepted: 3 October 2023 / Published: 7 October 2023
(This article belongs to the Section Ecosystem, Environment and Climate Change in Agriculture)
Browse Figures
Versions Notes

Abstract

:
The use of ecological stoichiometry is quite effective for exploring the nutrient dynamics and relationships between plants and soils. However, the way that the plant and soil stoichiometry changes with soil remediation in mining ecosystems remains unclear. Biochar and vermicompost are generally applied to remediate contaminated soil. In this study, a pot experiment was conducted with a mine soil planted with alfalfa. Biochar (B) and vermicompost (V) were added to the soil separately in three different proportions, equivalent to application rates (w/w) of 0% (control, CT), 2.5% (low rate, l), and 5% (high rate, h). This resulted in nine treatments, including control (CT), Bl, Bh, Vl, Vh, BlVl, BlVh, BhVl, and BhVh. The carbon (C), nitrogen (N), and phosphorus (P) concentrations and stoichiometric characteristics of the alfalfa aboveground parts (plant) and soil were investigated. The results showed that biochar application significantly increased the concentrations of soil organic C (SOC), soil total N (TN), soil total P (TP), soil C:N, and plant P concentration, but decreased plant N concentration, and plant C:P and N:P ratios. The effects of vermicompost addition on SOC, soil TN, TP, and stoichiometric characteristics depended on the biochar addition rates, but it increased plant N concentration and N:P, and decreased plant C:N under the condition of low biochar addition. Additionally, the plant N concentration was negatively correlated with soil N and total manganese (Mn) concentrations, whereas there was a positive correlation between plant and soil P concentrations. The soil total and available cadmium (Cd) were positively correlated with plant N concentration but negatively correlated with plant P concentration. The results indicated that the stoichiometric characteristics of plants and soil had diverse responses to biochar and vermicompost additions, and different soil heavy metal elements. Biochar and vermicompost application improved external P and N utilization by plants, respectively. Vermicompost addition enhanced biological N fixation in alfalfa. These findings suggest that vermicompost addition could be an optimal method by which to promote vegetation restoration in mine soils with poor N levels, and that biochar could be applied to low-P soils. The effects of heavy metals on plant and soil stoichiometric characteristics should be taken into consideration. Consequently, this study will provide scientific references for biochar and vermicompost applications in alfalfa planting and management, and vegetation restoration in mining areas.

1. Introduction

Carbon (C), nitrogen (N), and phosphorus (P) play vital roles in plant growing processes and physiological regulations [1,2]. C is a necessary energy element in plants. N and P are important nutrients and plant growth-limiting factors [3]. Stoichiometry has been commonly applied to study the stoichiometric characteristics (i.e., elemental concentration and ratio) of plant and soil nutrients (mainly C, N, and P) in order to explore their relationship within the plant–soil system, and to analyze the nutrient limitations, the coupling relationships among multiple nutrients, and the adaptability of plants to various environments [4,5]. Degraded mine ecosystems have been severely disturbed by anthropogenic activities [6]. Vegetation restoration is usually limited by soil nutrients. The plant stoichiometric characteristics of mine ecosystems can reflect the soil nutrient supply and identify limiting factors for plant growth, which are helpful when guiding nutrient management and selecting adaptive plants for mine ecosystem restoration [4]. A previous study on the stoichiometric characteristics of six plants in the mining area showed that leguminous plants can alleviate N limitation in the early stage of vegetation recovery to reach a stoichiometric balance; these plants were considered pioneer plants for ecological restoration in mining areas [7]. Chen et al. [8] analyzed the leaf stoichiometries of Pinus tabuliformis, Hippophae rhamnoides, and Medicago sativa in different recovery years in a mining area and found that the growth of P. tabuliformis was limited by N, while P was a limiting factor for growth of H. rhamnoides and M. sativa. However, there is little information on stoichiometric relationships between plants and soils and the effects on plant growth in mine ecosystems.
Adding exogenous organic amendments is a common and cost-effective remediation measure to improve mine soil quality [9,10,11]. As biochar has strong adsorption, high porosity, and a large specific surface area, it has potential advantages in reducing the bio-availability of heavy metals and improving soil nutrients [12,13]. Numerous studies have evaluated the effect of biochar application on improving heavy-metal-polluted soils [14,15,16], but few focused on changes in soil nutrient concentrations and their stoichiometric characteristics [17,18]. The application of biochar in an exhausted copper mine area increased soil C, N, and effective P concentrations, while promoting plant growth [19]. Guo [20] conducted a pot experiment to examine the effects of fruit wood biochar and iron-modified coconut shell biochar on the growth of Cassia occidentalis and soil nutrients under heavy metal stress conditions, and found that the two types of biochar markedly increased soil organic matter, TN, TP, and effective P concentrations, but inhibited plant growth. Vermicompost with high nutrient concentrations can supply nutrients for plant growth [21,22], and can also be applied to remediate heavy-metal-polluted soils. Lukashe et al. [23] examined the improvement effect of fly-ash-enriched vermicompost on mine-waste-contaminated soil and concluded that vermicompost reduced heavy metal solubility while increasing soil N and P availability. The application of 25% vermicompost could significantly increase soil total C, N, and P concentrations and effectively improve the soil’s physicochemical properties [24]. The improvement of soil nutrients directly affects nutrient uptake by plants. Meanwhile, plants can adjust nutrient absorption strategies to maintain the dynamic balance of nutrients (C, N, and P) [25,26,27]. Therefore, understanding the C, N, and P characteristics between the plant and soil with exogenous organic amendments will provide further information on nutrient cycling in mining area ecosystems.
Alfalfa (Medicago sativa), as a perennial leguminous forage, may enhance soil nutrients via biological N fixation, improve soil properties, and absorb heavy metals [28]. However, infertile soils, especially those with a deficiency in soil N and P, can constrain the growth of alfalfa [29,30]. The increase in soil nutrient concentrations may improve the forage quality and reduce the heavy metal stress [31]. It is thus necessary to study the effects of biochar and vermicompost addition on alfalfa and mining soil, which will be crucial to understanding how to improve soil nutrient balance and alleviate alfalfa nutrient limitations.
In the present study, a pot experiment with different levels of biochar and vermicompost additions was conducted in a manganese mining area. The C, N and P concentrations and stoichiometric ratios of aboveground portions of alfalfa and soil were analyzed. The purposes of this study were to determine: (1) the changes in nutrient concentrations in plants and soil with biochar and vermicompost additions and (2) the effects of soil nutrient concentrations and their stoichiometric characteristics on plant nutrients and stoichiometric ratios.

2. Materials and Methods

2.1. Study Site and Soil Collection

The studied manganese (Mn) mining area (112°45′–112°55′ E, 27°53′–28°03′ N) is located in Xiangtan City, Hunan, China. This region has a subtropical monsoon climate. The mean annual temperature is 17.4 °C, with a mean annual precipitation of 1431.4 mm. The soil is polluted with Mn and other heavy metals (Mn, 1793 mg kg−1; Cd, 401 μg kg−1). The soil fertility is relatively poor.
In early October 2021, the soil from 0–20 cm was collected from the manganese mining area and sieved through a 5 mm mesh screen to remove stones and biological materials. The soil collected is dryland agricultural soil, used to cultivate vegetables and dry crops. It is classified as an Ultisol, developed from sandstone [32]. The collected soil was taken back to the lab and air-dried for the pot experiment. The initial chemical properties were determined. Soil organic C (SOC), soil total N (STN), soil total P (STP), and soil pH were 22.86 ± 0.44 g kg−1, 0.98 ± 0.09 g kg−1, 1.06 ± 0.06 g kg−1, and 6.92, respectively (Table S1).

2.2. Experiment Design

Medicago sativa (alfalfa) seeds were purchased from Xi’an Qianwo Technology Co., Ltd. (Xi’an, China). Biochar was purchased from Henan Zhongxin Blue Sky Environmental Protection Equipment Co., Ltd. (Nanyang, China). This was made of chicken manure and slowly pyrolyzed at 500 °C in a carbonization furnace. Vermicompost was bought from an earthworm farm in Henan province, obtained by the composting of cow manure. The organic C (OC), total N (TN), total P (TP), and pH of the biochar were 314.73 ± 5.26 g kg−1, 36.91 ± 1.85 g kg−1, 28.66 ± 0.46 g kg−1, and 10.24, respectively. The vermicompost OC, TN, TP, and pH were 28.10 ± 0.54 g kg−1, 2.12 ± 0.09 g kg−1, 2.21 ± 0.26 g kg−1, and 7.23, respectively (Table S1). Biochar and vermicompost were sieved with a 2 mm bore diameter sieve before addition. The vermicompost was air-dried and then sterilized with steam at 121 °C.
The treatments included two factors, biochar (B) and vermicompost (V). Biochar and vermicompost treatments included three levels for each, which were equivalent to application rates (w/w) of 0% (control, CT), 2.5% (low rate, l), and 5% (high rate, h). The treatments were as follows: 0% vermicompost plus 0% biochar (control, CT), 2.5% vermicompost addition (Vl), 5.0% vermicompost addition (Vh), 2.5% biochar addition (Bl), 5.0% biochar addition (Bh), 2.5% biochar plus 2.5% vermicompost addition (BlVl), 2.5% biochar plus 5.0% vermicompost addition (BlVh), 5.0% biochar plus 2.5% vermicompost addition (BhVl), and 5.0% biochar plus 5.0% vermicompost addition (BhVh) (Table S2). Each treatment had 5 replicates, for 45 pots in total.
Each 3.79 L plastic pot was filled with 1.8 kg air-dried soil. Then, the different rates of biochar and/or vermicompost were added to the corresponding experimental treatment and were homogenously mixed. The soil in each plot was managed at 65% of the maximum field water-holding capacity of the soil. Ten alfalfa seeds were sown in each pot in October 2021, and were cultivated in the greenhouse of Hunan University of Science and Technology. After germination for one month, the seedlings were thinned, and six healthy alfalfa seedlings were kept in each pot.

2.3. Sampling and Laboratory Analyses

As the alfalfa seeds were planted in pots and grew slowly, the plant and soil were destructively sampled in April 2022. The plants were divided into aboveground and belowground parts. The nutrients of the belowground parts (roots) were not determined because of their low biomass. Additionally, the stoichiometric characteristics of the roots could not be calculated in this study. The plant and soil samples in each pot were collected respectively and transported to the laboratory. Plant samples of the aboveground parts were weighed and ground for chemical property analysis after drying in an oven at 65 °C. The soil samples were divided into two parts, one for testing soil moisture content, and the other for analyzing physicochemical properties. The root/shoot ratio was calculated using the belowground biomass divided by the aboveground biomass of alfalfa.
The soil moisture content was determined by oven-drying at 105 °C for 24 h. SOC and plant C concentrations were measured with the potassium dichromate plus concentrated sulfuric acid oxidation method [33]. The soil and plant TN and TP concentrations were determined through the Kjeldahl method and Mo-Sb anti-spectrophotometer method following concentrated sulfuric acid digestion, respectively [33]. The ratios of C, N, and P in the soil and alfalfa were calculated through their element mass ratios.
For the plant and soil samples, total Mn and Cd concentrations were digested with concentrated nitric acid, hydrofluoric acid, and hydrogen peroxide in a microwave digester (CEM corporation, Matthews, NC, USA), and then determined via flame atomic absorption spectrophotometry (FAAS). The exchangeable and acid-soluble fractions (herein called available fraction) of Mn and Cd concentrations were determined via a sequential extraction BCR method (European Community Bureau of Reference) [34].

2.4. Statistical Analysis

Two-way ANOVAs were used to estimate the impacts of biochar, vermicompost, and their interactions on the nutrient concentrations and stoichiometric characteristics of the plant and soil, plant total biomass, and root/shoot ratio. Differences in the chemical properties (C, N, and P concentrations and their stoichiometries) of the plants and soil, plant total biomass, and root/shoot ratio among different treatments were examined using one-way ANOVA and multiple comparison analysis (Fisher’s least significant difference, LSD). Pearson’s correlation analysis was performed to analyze whether there was a significant correlation between each index. Statistical analyses were conducted using SPSS 18 (SPSS, Inc., Chicago, IL, USA), with statistical significance determined at p < 0.05. The figures were drawn using Origin 2018 (Origin Lab Corp., Northampton, MA, USA).

3. Results

3.1. Plant Nutrient Concentrations and Stoichiometric Characteristics

Two-way ANOVAs implied that biochar and vermicompost treatments had no significant effect on plant C concentration (p = 0.194 and 0.905, respectively; Figure 1A), but that they affected plant N concentration and, C:N and N:P ratios (all p < 0.05; Figure 1B and Figure 2A,C). The plant P concentration and C:P ratios were significantly affected by biochar addition (p < 0.001 and p = 0.002, respectively) rather than by vermicompost addition (p = 0.477 and 0.627, respectively; Figure 1C and Figure 2B).
Based on one-way ANOVA results, the plant C concentration was higher in the biochar-addition-alone treatment groups (Bl, Bh) than in the CT (p = 0.024), but there was no significant difference in the groups with vermicompost addition (p = 0.249 and 0.931, respectively; Figure 1A). The plant N concentration was significantly lower in the biochar-addition-alone treatment groups than in CT, and in the biochar-plus-low-vermicompost groups (BlVl, BhVl) than the low-vermicompost-addition-alone group (Vl). This was also observed in the biochar-plus-high-vermicompost groups (BlVh, BhVh) compared with the high-vermicompost-addition-alone group (Vh). However, higher plant N concentration was found in the low-biochar-plus-vermicompost-addition group than in the low-biochar-addition-alone group (Bl), as well as the high-biochar-addition-alone groups. That is to say, biochar addition decreased the plant N concentration, but vermicompost addition had an opposite effect (Figure 1B). The plant P concentration was increased by biochar addition, especially high biochar addition. However, it was decreased by vermicompost addition in the treatments with low biochar addition (Figure 1C).
The results of the one-way ANOVA for plant stoichiometric ratios indicated that the plant C:N ratios were significantly higher in the treatments with biochar addition alone than in the CT, and higher in the treatments with biochar plus high vermicompost additions than in treatments with high vermicompost addition. It was, however, lower in the treatments with vermicompost plus low biochar addition than in the treatment with low biochar addition, and lower in the vermicompost-plus-high-biochar-addition groups than in the high-biochar-addition group (BhVl, BhVh vs. Bh; Figure 2A). Moreover, the biochar addition decreased the plant C:P ratio. The impact of vermicompost addition on plant C:P was related to biochar addition. When biochar addition was low, the vermicompost addition enhanced the plant C:P ratio, but it reduced plant C:P ratio under the conditions of high biochar addition (Figure 2B). The plant N:P ratio responded to biochar and vermicompost additions similarly to the plant C:P ratio (Figure 2C).

3.2. Plant Biomass and Their Allocations

Vermicompost addition significantly affected the total biomass of the alfalfa, but biochar did not (two-way ANOVAs; p = 0.019 and 0.095, respectively). That is, vermicompost addition significantly increased the total biomass of the alfalfa (Figure 3A). Biochar treatment without vermicompost significantly affected the root/shoot ratio (p = 0.003 and 0.838, respectively; Figure 3B). In other words, the root/shoot ratio was significantly increased with biochar additions. Meanwhile, significant interaction effects on plant total biomass and root/shoot ratio were detected between biochar and vermicompost additions (p = 0.005 and <0.001, respectively; Figure 3A,B).
One-way ANOVA showed that biochar addition significantly increased the plant total biomass with low vermicompost addition, as well as the plant root/shoot ratios compared with CT (p = 0.005 and <0.001, respectively; Figure 3A). The vermicompost affected the plant total biomass and root/shoot ratio. Specifically, plant total biomass was higher in the treatments with low biochar and vermicompost additions than low biochar addition alone. The root/shoot ratio was increased by vermicompost addition in the soils with low biochar addition or no biochar addition (BlVl, BhVl vs. Vl; Bl, Bh vs. CT), but was decreased in the treatments with high biochar addition (BlVh, BhVh vs. Vh; Figure 3B).

3.3. Soil Nutrients and Their Stoichiometric Ratios

Biochar treatments significantly altered the SOC, STN, STP, and soil C:P and N:P, but they did not affect soil C:N. However, vermicompost did not affect SOC, STN, STP and their stoichiometric characteristics (two-way ANOVAs, Figure 4A–C and Figure 5A–C).
One-way ANOVA showed that biochar addition significantly affected SOC, STN, STP, and soil C:P, and N:P (all p < 0.05, Figure 4A–C and Figure 5B,C), but did not affect soil C:N. Specifically, higher SOC, STN, and STP values resulted from treatments with biochar addition than were found in the referenced controls (all p < 0.05, Bl, Bh, vs. CT; BlVl, BhVl, vs. Vl; BlVh, BhVh, vs. Vh; Figure 4A–C). The soil C:P and N:P were lower in biochar addition treatments than in the corresponding controls (all p < 0.05, Figure 5B,C). The vermicompost addition alone increased SOC and STP. However, SOC, STN, C:P and N:P were lower in the treatments with vermicompost plus high biochar addition (BhVl, BhVh, vs. Bh). The soil P was decreased by vermicompost addition in the treatments with low biochar addition, but C:P and N:P ratios were increased.

3.4. Relationships of Plant and Soil Nutrients and Stoichiometric Ratios

The correlation analysis showed that there were no significant effects between plant C concentration and SOC, STN, STP and their stoichiometric ratios (p > 0.05; Figure 6A). However, plant N concentration was negatively correlated with SOC, STN, and STP, but positively correlated with soil C:P and N:P (all p < 0.01, Figure 6A). The plant P concentration was positively correlated with SOC, STN, and STP, but negatively correlated with soil C:P and N:P (all p < 0.05, Figure 6A). Additionally, the plant N concentration was negatively affected by total soil Mn concentration, not by available soil Mn concentration; However, it was positively affected by total and available soil Cd (p < 0.01, Figure 6A). The plant P concentration was negatively affected only by total and available soil Cd (p < 0.01, Figure 6A).
The stoichiometric ratios of plant C, N, and P were impacted by soil nutrients and stoichiometric ratios. Specifically, the plant C:N ratio was positively correlated with SOC, STN and STP concentrations (all p < 0.01), but negatively correlated with soil C:N, C:P and N:P (all p < 0.05, Figure 6A). Inversely, negative relationships between plant C:P and STP (p <0.01), and positive relationships between it and soil C:P and N:P ratios were detected (all p < 0.01, Figure 6A). The plant N:P ratio was negatively affected by SOC, STN and STP concentrations (all p < 0.01), and positively related to soil C:P and N:P ratios (all p < 0.01, Figure 6A). The plant C:N ratio was also positively affected by soil total Mn, and negatively affected by total and available soil Cd (p < 0.05, Figure 6A). Both plant C:P and N:P ratios were positively related to total and available soil Cd concentrations (all p < 0.01, Figure 6A).
The plant stoichiometric ratios were also related to plant nutrient concentrations. The plant C:N ratio was positively affected by plant C and P concentrations (all p < 0.05), whereas it was negatively affected by plant N concentration (p < 0.01, Figure 6B). The plant C:P and N:P ratios were positively associated with plant N concentration, whereas they were negatively correlated with plant P concentration (all p < 0.05, Figure 6B). The plant stoichiometric ratios were not significantly correlated with plant Mn and Cd concentrations, though the plant C concentration was negatively affected by plant Mn concentration (p < 0.01, Figure 6B).

4. Discussion

4.1. Effects of Biochar and Vermicompost on Plant Nutrient Concentrations

In this study, biochar application decreased plant N concentration, whereas it increased plant P concentration. The most plausible reason for biochar influencing P absorption in plants may be the abundant P concentration in the soil. Because the P concentration was high in biochar, the soil P was increased through biochar addition. Biochar-derived P could enhance the microbial abundance and enzyme activity of soil to facilitate the uptake and utilization of P by plants [35]. The increased soil N induced by biochar addition could enhance soil phosphatase activity and increase soil phosphate mineralization rate [36]. This was also demonstrated by the positive correlations between plant P and soil N and P concentrations in this study. Alternatively, this may be ascribed to the strong adsorption capacity of biochar for phosphate [37], which can improve P availability by improving the soil P supply in manganese ore area [38]. In addition, the decrease in plant N concentration may be attributed to high soil P concentration, which can cause the relative deficiency of available soil N for alfalfa. Plants may release excess N to construct extracellular phosphatases to acquire P from organic sources, and the increase in soil P may cause a reduction in excess N, decreasing plant N concentration [39]. Significant negative correlations of plant N with soil N and P concentrations were also observed. This result is in accordance with that of Lu et al. [40]. Therefore, biochar addition may regulate plant nutrient absorption by altering soil nutrients availability. The available N and P significantly affected aboveground and belowground biomass and their biomass allocation [41]. The significant effects of biochar on the root/shoot ratio of alfalfa also support the idea that the biochar altered soil nutrient availability.
Vermicompost application significantly increased plant N concentration in the treatments with low and high biochar additions. The biological N fixation in the alfalfa could be enhanced through vermicompost addition. Turp et al. [42] found that vermicompost with enriched nutrients improves the nodulation and N-fixing capacity of legume crops. This observation was supported by the enhanced plant biomass in the treatments with vermicompost addition plus low biochar addition. Unfortunately, the other relative indicators of biological N-fixing have not been studied. The enhanced plant absorption and utilization of N induced by vermicompost could also be responsible for this [43]. Moreover, soil N bioavailability could be higher than P [40]. In our study, vermicompost addition significantly changed neither the concentrations of SOC, STN, and STP nor their stoichiometric characteristics. However, it could affect plant nutrient absorption by altering soil physical properties, microbial biomass and community structure [44]. The impact mechanisms of vermicompost addition still need further study. The different nutrient responses of alfalfa aboveground parts to the changes in soil nutrients indicated that alfalfa had contrasting absorption strategies to changes in soil nutrient availability. Furthermore, it is worth noting that combined applications of biochar and vermicompost on plant nutrient concentrations are not a simple superposition effect. For instance, high biochar plus high vermicompost addition significantly increased plant P concentration, while high-biochar or high-vermicompost-alone treatments had no significant effect on plant P concentration.

4.2. Effects of Biochar and Vermicompost on Plant C, N, P Stoichiometry

The stoichiometric ratio of plants can clearly indicate their nutrient utilization strategy. Plant C:N and C:P ratios reflect the strategies of N and P utilization efficiency and the level of plant nutrient utilization [45]. In this study, the plant C:N ratio was significantly increased by Bl, Bh, and BhVh treatments, indicating that the utilization efficiency of N was increased under these treatments. The C:P ratio was decreased by Bl, BhVl, and BhVh treatments, suggesting that plant P utilization efficiency was low, though the utilization rate of soil P increased. The N:P ratio can reveal the dynamic balance of soil nutrient availability with plant nutrient requirements and is regarded as an indicator to evaluate the soil nutrient supply for plant growth [46]. It has been demonstrated that the leaf N:P ratio < 14 often revealed limited N and N:P ratio > 16, which indicates a limited P in the terrestrial ecosystems [1,47]. In our study, plant N:P ratios in all treatments varied from 5.56 to 8.78, suggesting that there was limited N in the plant growth. The soil N:P ratios ranged from 1.32 to 2.02 for different treatments, which were lower than the mean value in China (N:P = 5.2) [48], indicating that soil nutrients in the manganese ore area were poor, especially for soil N. It was reported that biochar can enhance biological N fixation and decrease soil-derived N uptake by plants [49]. However, in a heavy-metal-contaminated soil, biological N fixation was reduced by biochar addition [50]. We also found that biochar decreased the N concentration of legumes in this study. As the total biomass was not decreased, the effect of polycyclic aromatics and phenols toxicities in biochar-derived dissolved organic matter in alfalfa may thus be negligible. This was likely caused by the immobilization of biochar to microbes and available nutrients [50]. Additionally, plant growth is restricted by N-limited conditions at plant N:P < 14 and soil C:N < 30, and the soil is at risk of nitrate leaching [51]. We thus speculated that soil N may be lost in the mining areas. Moreover, alfalfa P demand is high [52], which results in an increase in plant P concentration so as to produce enough rRNA to synthesize proteins and thereby reducing the ratio of N to P [53]. Compared with other treatments, high biochar plus low vermicompost addition significantly increased plant P but did not significantly affect its N concentration, which could have a positive impact on vegetation restoration and soil quality in the mining area.
Soil nutrients can directly affect plant nutrient absorption and utilization, leading to the changes in plant stoichiometric characteristics [54,55]. Significant correlations between soil and plant stoichiometric ratios have been frequently reported [56]. In this study, the plant N:P ratio was positively correlated with soil C:P and N:P ratios, which is consistent with the work of Chen et al. [57]. Soil nutrients regulate nutrient stoichiometry in aboveground plants through roots [58]. However, nutrient characteristics and stoichiometric ratios of the underground parts of alfalfa have not been determined in this study, and the specific mechanism needs to be further studied.

4.3. Effects of Soil Mn and Cd on Plant C, N, P Concentrations and Their Ratios

Mn is a vital element for plant growth, but excess Mn can influence the absorption and utilization of other elements by plants [59]. We found that total soil Mn concentration negatively affected plant N concentration. This indicates that high Mn toxicity may inhibit plant N absorption. This is in accordance with the results of Dimkpa et al. [60]. No significant correlations of plant P with total and available soil Mn indicated that the P absorption was not sensitive to Mn stress. This is inconsistent with the observation that lower shoot P concentration was induced by Mn addition [60]. The amount of P in the soil could be responsible for this [61]. Our experiments demonstrated that plant N absorption is less tolerated than P in Mn-contaminated soil.
Additionally, total and available soil Cd concentrations (<0.30 mg kg−1) positively interacted with the N concentration of plants and negatively correlated with plant P concentration. That is, low soil Cd can promote alfalfa N accumulation but impede alfalfa P accumulation. Indeed, some studies found that low Cd concentration is not toxic to plants and may even stimulate plant growth [62]. For example, the growth of rice plants was increased with 0.1–0.5 mg kg− 1 Cd addition [63]. Similarly, plant Nwas increased by Cd addition [62]. It was demonstrated that rhizobium could be less sensitive to low Cd concentration in soils. Jach et al. [64] found that legumes and N-fixing rhizobia reduce the heavy metal toxic effects on plants. The negative impact of soil Cd on plant P was observed in a previous study [65], which agrees well with our experimental results. However, positive correlations between the P uptake of Allium fistulosum and Cd concentration (1.0–2.5 mg kg−1) were recorded by Li et al. [66]. These inconsistent results may be attributed to the diverse responses of different plant species to Cd concentration. Thus, our results indicated that different heavy metals in the soil caused divergent effects in plant nutrients.

5. Conclusions

Our study showed that biochar application significantly increased SOC, STN, STP and plant P concentrations, but decreased plant N, C:P, and N:P ratios in alfalfa. The diverse impacts of vermicompost on plant and soil N and P concentrations and their ratios were related to biochar addition rates. Moreover, plant N was negatively affected by soil N, while there was a positive correlation between plant P and soil P. Soil Cd was positively correlated with plant N concentration and negatively with plant P concentration. Plant N was negatively affected by total soil Mn. Thus, these results indicated that the responses of stoichiometric characteristics to biochar and vermicompost applications differed between plants and soil, and soil heavy metal elements also affected plant macronutrients and their ratios. Biochar and vermicompost application improved the external P and N utilization by plants, respectively. Vermicompost addition enhanced biological N-fixing in alfalfa. These findings suggest that vermicompost may be an optimal measure for improving vegetation restoration in mine soils with poor N, and that biochar could be applied to P-deficient soils. The effects of heavy metals on nutrient cycling should be considered. In summary, this study will provide scientific references for biochar and vermicompost applications in vegetation restoration and management, and in crop cultivation in mining areas.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agriculture13101954/s1. Table S1. Physiochemical properties of the soil, biochar and vermicompost. Table S2. The levels of biochar and/or vermicompost addition in all treatments.

Author Contributions

Conceptualization, Y.Z. (Yu Zhang) and Y.C.; methodology, Y.Z. (Yu Zhang), Q.Q., H.M. and Y.C.; validation, Q.Q., H.M., Y.Z. (Yinghua Zhu), J.L. and S.C.; investigation, Y.Z. (Yu Zhang) and Y.C.; data curation, Q.Q.; writing—original draft preparation, Y.Z. (Yu Zhang), Q.Q., H.M., Y.Z. (Yinghua Zhu), J.L., S.C. and Y.C.; writing—review and editing, Y.Z. (Yu Zhang), H.M., Y.C.; supervision and project administration, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Sciences Foundation of China (32271729, 31600174, and 31901194) and Scientific Research Fund of Hunan Provincial Education Department, China (21C0359).

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

The authors would like to thank Z. Qiu and Y. Liu for soil collection in the field. We gratefully acknowledge constructive comments from anonymous reviewers that improved the manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Differences in the (A) total C (TC), (B) total N (TN), and (C) total P (TP) concentrations of plants (alfalfa aboveground parts) in all treatments. The values are means ± SE, n = 5. The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters denote significant differences in the treatments of Bl, Bh, and CT; different capital letters for the treatments of Bl, BlVl, and BlVh. * Indicates significant differences between the treatments from the same group (p < 0.05).
Figure 1. Differences in the (A) total C (TC), (B) total N (TN), and (C) total P (TP) concentrations of plants (alfalfa aboveground parts) in all treatments. The values are means ± SE, n = 5. The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters denote significant differences in the treatments of Bl, Bh, and CT; different capital letters for the treatments of Bl, BlVl, and BlVh. * Indicates significant differences between the treatments from the same group (p < 0.05).
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Figure 2. The C:N ratio (A), C:P ratio (B), and N:P ratio (C) of plants (alfalfa aboveground parts). The values are means ± SE (n = 5). The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT, while different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Denotes significant differences between the treatments from the same group (p < 0.05). No annotations are present if there were no significant differences between the treatments from the same group.
Figure 2. The C:N ratio (A), C:P ratio (B), and N:P ratio (C) of plants (alfalfa aboveground parts). The values are means ± SE (n = 5). The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT, while different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Denotes significant differences between the treatments from the same group (p < 0.05). No annotations are present if there were no significant differences between the treatments from the same group.
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Figure 3. The plant total biomass (A) and ratios of belowground to aboveground biomass of alfalfa (B). The values are means ± SE, n = 5. The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT; different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Indicates significant difference between the treatments from the same group (p < 0.05).
Figure 3. The plant total biomass (A) and ratios of belowground to aboveground biomass of alfalfa (B). The values are means ± SE, n = 5. The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT; different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Indicates significant difference between the treatments from the same group (p < 0.05).
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Figure 4. Differences in the (A) organic C (OC), (B) total N (TN), and (C) total P (TP) concentrations of the soil. The values are means ± SE (n = 5). The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT; different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Indicates significant differences between the treatments from the same group (p < 0.05).
Figure 4. Differences in the (A) organic C (OC), (B) total N (TN), and (C) total P (TP) concentrations of the soil. The values are means ± SE (n = 5). The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT; different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Indicates significant differences between the treatments from the same group (p < 0.05).
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Agriculture 13 01954 g005
Figure 5. The C:N ratio (A), C:P ratio (B), and N:P ratio (C) of the soil in all treatments. The values are means ± SE, n = 5. The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT; different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Indicates significant differences between the treatments from the same group (p < 0.05).
Figure 5. The C:N ratio (A), C:P ratio (B), and N:P ratio (C) of the soil in all treatments. The values are means ± SE, n = 5. The p values of two-way ANOVAs are shown in the top left corner. Different lowercase letters indicate significant differences in the treatments of Bl, Bh, and CT; different capital letters indicate the same for the treatments of Bl, BlVl, and BlVh. * Indicates significant differences between the treatments from the same group (p < 0.05).
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Figure 6. Correlation of plant C, N, and P concentrations and their stoichiometric ratios with the C, N, and P concentrations and their ratios, and Mn and Cd concentrations of the soil (A) and plants (B). OC: organic C, TC: total C, TN: total N, TP: total P, T-Mn: total Mn, A-Mn: available Mn, T-Cd: total Cd, A-Cd: available Cd. * Indicates significant correlations (p < 0.05), ** indicates extremely significant correlations (p < 0.01).
Figure 6. Correlation of plant C, N, and P concentrations and their stoichiometric ratios with the C, N, and P concentrations and their ratios, and Mn and Cd concentrations of the soil (A) and plants (B). OC: organic C, TC: total C, TN: total N, TP: total P, T-Mn: total Mn, A-Mn: available Mn, T-Cd: total Cd, A-Cd: available Cd. * Indicates significant correlations (p < 0.05), ** indicates extremely significant correlations (p < 0.01).
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Zhang, Y.; Mai, H.; Qiu, Q.; Zhu, Y.; Long, J.; Chen, S.; Chen, Y. The Responses of C, N, P and Stoichiometric Ratios to Biochar and Vermicompost Additions Differ from Alfalfa and a Mine Soil. Agriculture 2023, 13, 1954. https://doi.org/10.3390/agriculture13101954

AMA Style

Zhang Y, Mai H, Qiu Q, Zhu Y, Long J, Chen S, Chen Y. The Responses of C, N, P and Stoichiometric Ratios to Biochar and Vermicompost Additions Differ from Alfalfa and a Mine Soil. Agriculture. 2023; 13(10):1954. https://doi.org/10.3390/agriculture13101954

Chicago/Turabian Style

Zhang, Yu, Huizhen Mai, Qinghong Qiu, Yinghua Zhu, Jiayi Long, Shengfu Chen, and Yuanqi Chen. 2023. "The Responses of C, N, P and Stoichiometric Ratios to Biochar and Vermicompost Additions Differ from Alfalfa and a Mine Soil" Agriculture 13, no. 10: 1954. https://doi.org/10.3390/agriculture13101954

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