Effects of amendments on base cation and micronutrient availabilities in soils planted with tomato in a solar greenhouse

ABSTRACT

In order to obtain high tomato production, chemical fertilizers have been intensively applied in solar greenhouse in Northern China. This unreasonable use of chemical fertilizers always leads to soil degradation. One of the most serious problems is the loss of exchangeable base cations: calcium (Ca), magnesium (Mg), and potassium (K), and available micronutrients: iron (Fe), manganese (Mn), copper (Cu), and zinc (Zn). We collected soils with different tomato cultivation durations: 0, 5, and 20 years, to investigate the effects of different fertilizers on the dynamics of soil base cations and micronutrients which extracted by 1 M ammonium acetate and 0.005 M diethylenetriaminepentaacetic acid, respectively. Five treatments were included: no fertilizer application as control (Con), chemical nitrogen, phosphorus and potassium fertilizers (CF), rice straw (FS), chicken manure (CM), and vermicompost (VM). Our results show that soil exchangeable Ca significantly decreased with the increase of cultivation duration, whereas soil exchangeable K increased due to the fertilization of K. Soil exchangeable Ca and K exhibited decreasing trends during the growth season of tomato, but not exchangeable Mg. Long-term tomato cultivation significantly increased soil available micronutrients. Soil available Fe and Mn fluctuated sharply during the growing season of tomato plant. Except Zn, fertilizers exhibited no constant significant effect on base cations and micronutrients. After long term planting of tomato, the loss of exchangeable Ca, but not Mg and micronutrients, might potentially constrain the productivity of crop under intensive chemical fertilization.

KEY WORDS:

• Vermicompost
• chicken manure
• soil organic matter
• soil microbial biomass
• rice straw

1. Introduction

Solar greenhouse cultivation developed rapidly in the last decade for its high land-use efficiency, short production cycle, and nice incomes in northeast China. However, in order to get maximal yield of tomato, chemical fertilizers were intensively used in solar greenhouses. For example, the application of chemical N is at the rate of > 1200 kg N ha⁻¹ (Guo et al. 2010). These high chemical fertilizer inputs give rise to soil acidification, macronutrient (N, P, and K) enrichment and imbalance (Han et al. 2015). Soil degenerated faster under this condition than open-field agricultural systems (Ju et al. 2006). At the same time, the yields of tomato decreased even if higher chemical fertilizers were applied (Fu et al. 2017).

Base cations and micronutrients play essential roles in the sustainability of agriculture (Alloway 2008). The decrease of tomato yield may resulted from lack of base cations (i.e., exchangeable Ca, Mg, and K) and micronutrients (i.e., Fe, Mn, Cu, and Zn) in solar greenhouse. Unlike enrichment of macronutrients (N and P), deficiencies of base cations and micronutrients have been frequently observed in agricultural soil (Kopittke et al. 2017). A meta-analysis including 107 independent studies indicated that chemical N additions induced 24% loss of base cations in terrestrial ecosystems over time periods less than five years (Lucas et al. 2016). Short-term (1–2 year) and mid-term (4–5 year) chemical N addition mobilized base cations into solution by exchanging with H⁺, and subsequent leaching down the soil profile (Cusack et al. 2016). Intensively cultivated soils always exhibit deficiencies of micronutrients, which resulted from removing of crop production (Bitondo et al. 2013), meanwhile lower micronutrient inputs from chemical fertilizers (Fageria et al. 2003).

Organic fertilizers, such as manure and crop residue, have gained increased attention in improvement of soil fertility and soil structure (Yang et al. 2013; Peng et al. 2016). Because of their balanced nutrition content and enhancing soil retention ability of nutrients (Wang et al. 2017a), application of organic fertilizers has been shown to increase the availability of base cations and micronutrients in soil (Wei et al. 2006; Zingore et al. 2008; Cai et al. 2015). However, most of the manure contains high level of micronutrients, especially Cu and Zn, and applications of animal manures or manure composts can give rise to excessive accumulation of those metals in soil, leading to adverse effects on soil quality (Hang et al. 2009; Yang et al. 2017). Vermicompost is the cast of earthworm by feeding with organic materials, such as animal manure and rice straw. Previous studies indicated that heavy metal (including Cu) concentrations were declined significantly in vermicompost compared to original manure (Wang et al. 2017; Zhu et al. 2018). As a natural organic fertilizer, vermicompost has been widely used in organic farm for the past few years (Lim et al. 2015). However, whether vermicompost is superior to conventional organic fertilizer in improving soil base cation and micronutrient availabilities should be studied.

Availabilities of base cations and micronutrients in soil are affected by several soil attributes, such as pH, SOM, redox potential, cation exchangeable capacity (CEC), moisture, microbial activity, and crop assimilate capacity (Perakis et al. 2013; Dotaniya and Meena 2015; Gruba and Mulder 2015; Wang et al. 2017b). Thus, the availabilities of base cations and micronutrients are codetermined by soil properties and crop growth stage. Soil microbial biomass and root assimilate ability differ sharply during growth stage of plant (Li et al. 2016; Wu et al. 2018). Even SOM, the relatively stable fraction in soil, has been showed seasonally transient (Turner et al. 2015). Thus, seasonal patterns in base cations and micronutrients availability are expected to occur during the plant growth period. However, most of the previous studies focused on long-term or annual change of base cations and micronutrients, seasonal change have been rarely explored.

In this study, we invested the effects of organic and inorganic fertilizers on availabilities of soil base cations and micronutrients during different growth stages of tomato in soils with different cultivation durations. We hypothesized that (1) long-term cultivation of tomato will cause decrease of base cations and micronutrients in solar greenhouse due to low input and plant uptake, (2) addition of organic fertilizers may increase availabilities of base cations and micronutrients due to improvement of soil fertility, and (3) availabilities of base cations and micronutrients will decrease because of uptake of tomato during tomato growth.

2. Materials and methods

2.1. Study sites and experimental design

Three soils that were classified as meadow brown earth (Mollic Gleysols, FAO-UNESCO system) were collected from Guanghui township of Yuhong District, Shenyang City, Liaoning Province. These soils were cultivated with tomato for different years: 0 year (collected from a field adjacent to the tomato-planting greenhouses), 5 years (in a solar greenhouse), and 20 years (in a solar greenhouse). Field managements were similar in 5-year and 20-year solar greenhouse, and both fertilized with chemical fertilizers. The basic properties of these soils are shown in Table 1. The collected soils were transferred to polyethylene plastic pots with a diameter of 30 cm and a height of 28 cm. Each pot was filled with 15 kg of air-dried soil which had yet been mixed uniformly with fertilizers. Five treatments for each soil were included: no fertilization as control (Con), chemical fertilizer (CF), rice straw (FS), chicken manure (CM), and vermicompost (VM). Concentrations of added chemical fertilizers under CF treatment were as follows: 0.40 g N, 0.11 g P, and 0.33 g K kg⁻¹ soil as urea, superphosphate, and potassium sulfate, respectively. The added N, P, and K were equivalent to rates of 900 kg N, 250 kg P, and 747 kg K per hectare in field area. Applied rates of rice straw, chicken manure, and vermicompost were 40.8 g, 8.3 g, and 13.1 g kg⁻¹ soil (about 30 t vermicompost per hectare), respectively. Extra chemical N, P, or K were added to FS, CM, and VM treatments to ensure that the concentrations of added N, P, and K were equal with CF treatment. All the fertilizers were applied as base fertilizer in a one-time application. Chemical properties of all the fertilizers are shown in Table 2. The tested tomato variety, ‘Gold Crown No. 9’, was grown until all the fruits were harvested. All the treatments were replicated four times, and a randomized block design was applied. The pot experiment was conducted from March to June in 2016 in the greenhouse (123°57ʹ E, 41°83ʹ N) of Shenyang Agricultural University in Shenyang, Liaoning Province. The greenhouse temperature ranged from 15 to 35°C. Natural light was supplied with no supplementary light.

Table 1. Chemical characteristics of the original soils. The parameters include pH, contents of total N (TN), total P (TP), available base cations (ACa, AMg, and AK), and available micronutrients (AFe, AMn, ACu, and AZn).

			TN	TP	TK		ACa	AMg	AK		AFe	AMn	ACu	AZn
	Duration	pH	g kg^–1				g kg^–1				mg kg^–1
Soils	0 year	8.14		0.52	14.72		1.4	0.08	0.15		11.9	16.2	1.4	2.1
	5 years	7.85	1.84	1.21	15.29		1.1	0.10	0.25		45.2	23.2	3.1	7.4
	20 years	6.17	3.07	3.00	17.16		1.0	0.10	0.66		88.9	68.1	28.5	18.7

Table 2. Chemical characteristics of the fertilizers. The parameters include pH, contents of total N (TN), total P (TP), total base cations (TCa, TMg, and TK), and total micronutrients (TFe, TMn, TCu, and TZn).

	Straw	Chicken manure	vermicompost	Urea	Superphosphate	Potassium sulfate
pH		6.5	6.3	7.0	5.9	6.8
Total N (g kg^–1)	9.8	30.0	7.7	460
Total P (g kg^–1)	1.6	30.1	19.1		52
Total Ca (g kg^–1)	7.3	7.7	25.0		0.07
Total Mg (g kg^–1)	1.2	2.0	2.9	0.005	1.4	0.032
Total K (g kg^–1)	6.3	7.2	4.1			415
Total Fe (mg kg^–1)	353.7	318.5	146.2		3751
Total Mn (mg kg^–1)	130	31.3	22.7		31
Total Cu (mg kg^–1)	5.9	65.5	25.0		30.6
Total Zn (mg kg^–1)	24.1	683.6	209.3	4.3	27.1	1.2

2.2. Soil sampling and chemical analysis

Soil samples were collected from each pot on March 27 (flowering period, FP), April 26 (fruit set period, FSP), May 26 (early harvest period, EHP), and June 25 (late harvest period, LHP) of 2016 during tomato growth. Soils were passed through a 2-mm sieve and air-dried for the determination of chemical properties.

The soil pH (soil: water (w/v), 1:5) was measured using a Thunder Magnetic SJ-3F pH Meter (INESA, Shanghai, China). Soil available base cations were extracted with ammonium acetate according to the method described by Ochoa-Hueso et al. (2014). Briefly, 5.0 g soil was mixed with 1 M ammonium acetate (pH 7.0) in a ratio of 1:10 (w/v). The mixture was shaken at 150 rpm for 30 min and then filtered. The concentrations of base cations in the filtration were analyzed by atomic absorption spectrometry (AAS, Shimadzu, Japan). Soil available Fe, Mn, Cu, and Zn were detected by the diethylenetriaminepentaacetic acid (DTPA) extraction method according to Wang et al. (2017b). Briefly, 10 g soil was extracted by 20 mL 0.005 M DTPA + 0.01 M CaCl₂ + 0.1 M triethanolamine (TEA; pH 7.0). The slurry was shaken at 180 rpm for 2 h and the filtered solution was used to determine the concentrations of available micronutrients by AAS. Although the DTPA method was highly recommended by the book of Soil Fertility and Fertilizers: An introduction to Nutrient Management for the measurement of available micronutrients, the secondary mineral forms of the micronutrients cannot be detected by this method. Soil organic matter (SOM) was determined by the method of dichromate oxidation and titration with ferrous ammonium sulfate (Lu 2000). Soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were measured by the fumigation-extraction method according to Vance et al. (1987) within 7 days after soil sampling. Soil, rice straw, chicken manure and vermicompost were digested by Nitric acid (HNO₃)-hydrofluoric acid (HF)-perchloric acid (HClO₄). Briefly, 7 mL HF and 1 mL HNO₃ was added to a 30 ml teflon crucible which contains about 0.1 g soil or 0.5 g organic materials. Then the teflon crucible was heated at about 200°C until the acid liquor almost dry out. Two milliliter HClO₄ was added to the same teflon crucible after the teflon crucible cool down. Once again, the teflon crucible was heated at about 200°C until the acid liquor became clear. Total N in the digested solution was measured by the Kjeldahl method, total P was measure by the molybdenum blue colorimetric method, total K was determined by flame photometer, base cations and micronutrients were measured by AAS (Lu 2000). Mineral fertilizers were digested by nitric acid-hydrofluoric acid-perchloric acid, concentrations of base cations and micronutrients were measured by AAS (Lu 2000).

2.3. Statistical analyses

Three-way ANOVAs were used to test the effects of planting time (Y), growth stage (S), and fertilizer treatment (T) on exchangeable base cations and available micronutrients. Multiple comparisons with Duncan design were conducted to determine differences in base cations and micronutrients among planting times. Pearson correlation analysis was performed to examine the relationships between base cations, micronutrients and pH, SOM, MBC, and MBN. All statistical analyses were performed with SPSS 19.0 (SPSS, Inc., Chicago, IL, USA) and statistical significance was accepted at P < 0.05.

3. Results

3.1. Soil exchangeable base cations

The concentration of exchangeable Ca decreased significantly with the increase of cultivation duration (Figure 1A), about 0.5 g kg⁻¹ exchangeable Ca lost during the past 20 year planting of tomato. No significant increase or decrease trend of exchangeable Ca was found under all the treatments compared with Con. The concentration of exchangeable Ca decreased during the growth period under CM treatment in all the three soils. Three-way ANOVAs showed that the concentration of exchangeable Ca was significantly influenced by Y and S (Table 3, P < 0.001 and 0.05, respectively), but no T effect was found. Soil exchangeable Mg concentration ranged from 60 mg kg⁻¹ to 140 mg kg⁻¹, it increased after planting of tomato for 5 years, then decreased as the planting duration up to 20 years (Figure 1B). Compared with Con, CM increased concentration of exchangeable Mg in 0- and 5-year soils in early growth period of tomato (FP and FSP), whereas CF significantly decreased concentrations of exchangeable Mg at the same period. The concentration of exchangeable Mg exhibited significant Y, S, and T effects. In contrast to exchangeable Ca, soil exchangeable K exhibited a significant increasing trend with the increase of cultivation duration, irrespective of treatment (Figure 1C). After 20 year planting of tomato, average concentration of soil exchangeable K increased by about 63%. Compared with Con, all the treatments significantly increased exchangeable K content, with the highest content was found under the CM treatment, which was up to approximately 1200 mg kg⁻¹. Differences of exchangeable K content between organic fertilizer treatments (CM and VM) and Con increased with the increase of cultivation duration. Concentrations of exchangeable K sharply decreased during the tomato growth season, especially between FSP and EHP. The concentrations of exchangeable K exhibited significant Y, S, and T effects.

Table 3. Results (F-values) of ANOVAs on the effects of cultivation year (Y), growth stage (S) and treatment (T) on soil exchangeable Ca, Mg, K, and available Fe, Mn, Cu, and Zn.

	Ca	Mg	K	Fe	Mn	Cu	Zn
Year (Y)	28.5***	41.6***	1816***	2578***	1139***	83,706***	10,891***
Stage (S)	2.74*	3.24*	1522***	799***	2015***	1358***	150***
Treatment (T)	0.90	12.37***	2575***	16.7***	207***	330***	2425***
Y × S	0.51	2.19*	148***	208***	293***	918***	27.4***
S × T	0.79	1.38	152***	26.1***	132***	124***	106***
Y × T	0.54	2.44*	57.3***	65.1***	108***	69.8***	28.0***
Y × S × T	1.09	2.69***	18.78***	35.7***	67.4***	126***	23.3***

* Significance level at P < 0.05.

** Significance level at P < 0.01.

*** Significance level at P < 0.001.

Figure 1. Effects of fertilization, growth period, and cultivation duration on soil exchangeable Ca (a), Mg (b), and K (c). Treatments include no fertilization as control (Con), chicken manure fertilization (CM), chemical fertilizer fertilization (CF), fresh rice straw fertilization (FS), and vermicompost fertilization (VM). Growth periods include flowering period (FP), fruit set period (FSP), early harvest period (EHP), and late harvest period (LHP). Cultivation durations include 0 year, 5 years, and 20 years. Data are represented as means ± standard error (n = 4). The capital letters indicate significant differences between different cultivation durations.

3.2. Soil available micronutrients

There was a significant inter-annual variation in the concentration of available Fe (Figure 2A, P < 0.05). Compared with 0-year soil, the average concentration of soil available Fe increased by 125.7% and 347.0% in 5-year and 20-year soils, respectively. However, the concentration of available Fe sharply fluctuated during different growth stages. The maximal value of available Fe under 5-year soil was larger than that of the minimum value under 20-year soil, irrespective of treatment. Similar to available Fe, average concentration of available Mn significantly increased with the extension of cultivation duration (Figure 2B, P < 0.05). The concentration of available Mn was occasionally higher in 0-year soil than 20-year soil. The average concentration of available Cu increased by 60.7% in 5-year soil compared with 0-year soil. However, the average concentration of available Cu in 20-year soil was about 11 and 7 fold higher than in 0- and 5-year soils, respectively (Figure 2C). Sharp fluctuation of the available Cu during tomato growth reason was only found on the 20-year soil. The average concentration of available Zn significantly increased with increase of cultivation duration (Figure 2D, P < 0.05). The concentration of soil available Zn was significantly higher under CF treatment than other treatments, irrespective of tomato cultivation duration and growth stage. Three-way ANOVAs analysis indicated that all the three factors, Y, S, and T, and their interaction had significant effects on the concentration of soil micronutrients (Table 3, P < 0.001).

Figure 2. Effected of fertilization, growth period and cultivation duration on soil available Fe (a), Mn (b), Cu (c), and Zn (d). Treatments include no fertilization as control (Con), chicken manure fertilization (CM), chemical fertilizer fertilization (CF), fresh rice straw fertilization (FS), and vermicompost fertilization (VM). Growth periods include: flowering period (FP), fruit set period (FSP), early harvest period (EHP), and late harvest period (LHP). Cultivation durations include 0 year, 5 years, and 20 years. Data are represented as means ± standard error (n = 4). The capital letters indicate significant differences between different cultivation durations.

3.3. Soil pH, SOM, and MBC

Soil pH was significantly lower in 20-year soil than in 0-year and 5-year soils (Figure 3A). The highest soil pH during tomato growth period was found at EHP stage, regardless of tomato cultivation duration and treatment. In general, pH in CF treatment was lower than in Con and organic fertilizer treatments. Soil organic carbon increased with the increase of planting duration (Figure 3B), and firstly increased and then decreased during tomato growth period in all the soils and treatments. All the treatments had higher SOC than Con in most of the growth stages in all the three soils. Soil microbial biomass C had increasing trend with the increase of planting duration (Figure 3C). Generally, MBC was higher in organic fertilizer treatments than in CF and Con.

Figure 3. Effected of fertilization, growth period and cultivation duration on soil pH (a), soil organic matter (SOM) (b), and soil microbial biomass (SOM) (d). Treatments include: no fertilization as control (Con), chicken manure fertilization (CM), chemical fertilizer fertilization (CF), fresh rice straw fertilization (FS) and vermicompost fertilization (VM). Growth periods include flowering period (FP), fruit set period (FSP), early harvest period (EHP), and late harvest period (LHP). Cultivation durations include 0 year, 5 years and 20 years. Data are represented as means ± standard error (n = 4). The capital letters indicate significant differences between different cultivation durations.

3.4. Relationships between base cations, micronutrients, and selected soil properties

The concentration of exchangeable Ca significantly and positively correlated with soil pH (Figure 4A, P < 0.01). No relationship was observed between exchangeable Mg and soil pH (Figure 4B). On the contrary, significant negative relationships were observed between exchangeable K, micronutrients and soil pH (Figure 4C–G, P < 0.01).

Figure 4. The relationships between soil pH and exchangeable Ca (a), Mg (b), K (c), and available Fe (d), Mn (e), Cu (f), and Zn (g) across all treatments and cultivation duration during different growth periods.

SOM was significantly and negatively correlated with exchangeable Ca (Figure 5A, P < 0.001), whereas significantly and positively correlated with exchangeable K and micronutrients (Figure 5C–G, P < 0.001).

Figure 5. The relationships between soil organic matter (SOM) and exchangeable Ca (a), Mg (b), K (c), and available Fe (d), Mn (e), Cu (f), and Zn (g) across all treatments and cultivation duration during different growth periods.

The concentration of exchangeable K significantly and positively correlated with soil MBC and MBN (Figure 6A and 6B), P < 0.001). When data were analyzed individually for each cultivation duration (0 year, 5 years, and 20 years), significant positive relationships still exist between exchangeable K and MBC and MBN (Figure 6A and 6B).

Figure 6. The relationships between soil exchangeable K and soil microbial biomass C (a) and biomass N (b) within each cultivation duration (0 year, 5 years, and 20 years) and across all the cultivation durations (total).

Significant relationships between the available micronutrients and pH and SOM were occasionally observed, but no constant significant relationship was found within each cultivation duration (Table 4).

Table 4. Pearson correlation analyses (R values) of micronutrients with soil pH and soil organic matter (SOM).

		Fe	Mn	Cu	Zn
0 year	pH	−.186	−.301**	−.199	−.543**
	SOM	.085	.027	.104	−.144
5 years	pH	−.304**	−.161	.124	−.086
	SOM	−.105	−.063	.358**	.281*
20 years	pH	−.389**	−.244*	.081	−.442**
	SOM	−.143	.277*	−.064	−.069

* Significance level at P < 0.05.

** Significance level at P < 0.01.

4. Discussion

4.1. Responses of base cations and micronutrients to cultivation duration

4.1.1. Soil exchangeable base cations

Previous studies indicated that soil exchangeable base cations exhausted under long-term input of chemical fertilizers, especially N fertilizer (Hoegberg et al. 2006; Guo et al. 2010; Lu et al. 2014). In this study, exchangeable Ca significantly decreased with the increase of cultivation duration. However, exchangeable Mg and K showed no decline after 20-year cultivation (Figure 1), which rebutted our hypothesis that long-term cultivation of tomato will cause decrease of base cations because of intensive use of chemical fertilizers. Significant decrease of soil exchangeable Ca after long-term tomato planting ascribes to the preferential exchange of Ca with proton in soil colloid surface and subsequent leaching with water, and uptake by tomato plant (Lucas et al. 2016). Magnesium dynamic is more complex during the 20 year cultivation, firstly increased and subsequently decreased (Figure 1B). This phenomenon mainly results from (1) preferential uptake of Ca by tomato plant giving more space for the retention of exchangeable Mg in short-term cultivation soil. No obvious decreasing trend of soil available Mg during the growth of tomato in 0- and 5-year soils suggested that removing of Mg by tomato plant is not significant (Figure 1B); and (2) leaching with water from surface to deep layer soil after the exhaustion of Ca in long-term cultivation soil. The increase of exchangeable K (Figure 1C) after 5 and 20 years cultivation has mainly contributed to K fertilization. In order to raise the yield and quality of tomato, K has been considered to be the key nutrient to production of quality fruits is intensively used in the greenhouse in northeast China (Usherwood 1985). Annual use of K reached about 750 kg ha⁻¹ in greenhouse production in China, which has been far beyond the need of tomato production, and then accumulated in soil.

4.1.2. Soil available micronutrients

Deficiency of micronutrients due to plant uptake after long-term cultivation of crop has been recognized (Singh 2008). However, in this study, we observed the opposite result: soil available micronutrients significantly increased with the cultivation of tomato (Figure 2). The availability of micronutrients strongly depend on the balance among input (mineral weathering release and atmospheric deposition), recycling (plant biomass turnover), retention (sorption by soil surface charge or complexing with organic matter), and output (biomass remove and hydrologic leaching) (Li et al. 2008). In our study, soil available micronutrients negatively correlated with soil pH, while positively correlated with SOM (Figure 4D–G and Figure 5D–G), which implied that soil acidification after long-term N addition enhanced mineral weathering and therefore release of micronutrients (input), at the same time, the increase of SOM strengthen the retention of micronutrients by adsorption (retention). Both the two aspects work together to make the increase of soil available micronutrients.

4.2. Responses of base cations and micronutrients to growth stage of tomato

4.2.1. Concentrations of exchangeable base cations

Contrasted with our second hypothesis, no constant increase of soil exchangeable Ca and Mg under organic fertilizer treatments (rice straw, chicken manure and vermicompost) was found (Figure 1A and 1B). Previous long-term experiment indicates that application of organic fertilizers can maintain and restore soil exchangeable base cations, especially exchangeable Ca and Mg (Cai et al. 2015). Increase of exchangeable base cations under manure and crop straw additions ascribe to (1) alleviate of soil acidity, which can weaken base cations loss, (2) increase of SOM enhance the adsorption capacity of base cations by SOM (Oorts et al. 2003), and (3) replenishment of base cations by manure and crop straw (Xu et al. 2006; Cai et al. 2015). All of these aspects are long-term processes. In this study, the soils only been treated for one year, it is too short to have positive effect on the concentration of exchangeable Ca and Mg. However, compared with Con all treatments significantly increased soil exchangeable K (Figure 1C), which may ascribe to the K solubilizing microorganisms. The differences of soil exchangeable K content between organic fertilization treatments (CM and VM) and Con are obviously higher than the added amount of K (about 332 mg K kg⁻¹ soil), which implies that part of the exchangeable K came from another source. Numerous microorganisms in soil have been verified can promote the release of K fixed in mineral lattice (Sheng and He 2006; Lian et al. 2008; Meena et al. 2014). Those microorganisms (termed as K solubilizing bacteria) are mainly composed of bacteria belong to Pseudomonas and Bacillus genera (Gu et al. 2013; Saha et al. 2016). Our previous study revealed that the relative abundances of Pseudomonas and Bacillus genera were relatively higher in organic fertilizer treatments than in CF and Con (Zhao et al. 2019). We also observed that exchangeable K is positively related to soil microbial biomass regardless of cultivation duration (Figure 6A and 6B), increase of microbial biomass combined with the high relative abundance of K solubilizing bacteria may explain the extra increase of exchangeable K content under the addition of organic fertilizers compared with CF. Generally, concentrations of exchangeable Ca and K decreased with tomato growth, whereas Mg has no obvious decrease trend (Figure 1) indicate that Ca and K, but not Mg should be preferentially supplied in the production of tomato.

4.2.2. Concentrations of available micronutrients

Soil available Fe, Mn, and Cu exhibit no constant significant treatment effects during the tomato growth season (Figure 2A–C), which may due to short treatment time. The inputs of Fe, Mn, and Cu accompany with the additions of rice straw, chicken manure and vermicompost are 14.4–1.9, 5.3–0.3, and 0.54–0.33 mg kg⁻¹, respectively. Inputs of these macronutrients from fertilizers were relatively lower than the needs of tomato growth. However, concentration of available Zn was significantly higher under chemical fertilizers application than other treatments (Figure 2D). The amount of available Zn bring to soil by chemical fertilizers was slight, was far less than 1 mg kg⁻¹ (Table 2). We deem that the high Zn availability is mainly due to secondary mineral dissolution after soil acidification under chemical fertilizer application. Concentrations of available Fe and Mn fluctuated sharply during the growth season of tomato. Previous study suggested that availabilities of Fe and Mn greatly depend on soil redox condition which may contribute to the fluctuation of these micronutrients during the tomato growth period (Schwab and Lindsay 1983). However, it is impossible to form strict anaerobic condition in our soils because of the irrigation system used in this study was drip irrigation. We found that pH exhibited opposite development trends with soil available Fe and Mn during the growth of tomato (Figs. 2 and 3), which indicated that greater acidity released Fe and Mn from minerals may contribute to the fluctuation of these micronutrients. Combined with the previous study that the ideal ranges of Fe, Mn, Cu, and Zn for tomato growth were 100–300, 60–240, 5–20, and 20–50 mg kg⁻¹, respectively (Rosen and Eliason 2005), our results indicated that although the average concentrations of available Fe and Mn increased after long-term intensive application of chemical fertilizers, the deficiencies of these nutrients can also occurred for their lower availabilities in key growth stages. Measurement of availabilities of Fe and Mn after harvesting of crop, which has been widely used in agricultural practice is improper; their availabilities should be monitored on the key growth stages of crop.

5. Conclusion

Long-term planting of tomato with the application of chemical fertilizers will cause depletion of available Ca and Mg, whereas increased the availability of micronutrients. Soil pH and SOM are the most important two factors influencing availabilities of base cations and micronutrients. The beneficial effects of organic fertilizers on availability of base cations and micronutrients should be a long-term process, and accompanied additions of micronutrients are need. Our findings suggest that depletion of soil available Ca, but not Mg, and micronutrients are the main limit factors for the production of tomato after long-term intensive application of chemical fertilizers. The availabilities of Fe and Mn after harvesting of crop should not on behalf of the supply capacity of these nutrients for crop growth. In the production of vegetables in solar greenhouse, monitoring of micronutrients availability should be conducted on key growth stages to get high yield.

Additional information

Funding

This work was supported by the National Key Research and Development Program of China [grant number 2016YFD0201004].