Modification of Soil Physical Properties by Maize Straw Biochar and Earthworm Manure to Enhance Hydraulic Characteristics under Greenhouse Condition

Abstract

The deterioration of soil physical properties had led to a decrease in soil–water availability in facility agriculture. Thus, an experiment was set up with five soil treatments of 0% (CK, No additives), 3% biochar (BA₃, Mass ratio), 3% earthworm manure (QA₃), 5% biochar (BA₅), and 5% earthworm manure (QA₅) to investigate the effects on soil physical properties and hydraulic characteristics under greenhouse conditions. The physical properties of soil including the soil bulk density (BD) and total porosity (TP) were measured; the results showed that BA₅ provided the lowest soil BD (1.24 g·cm⁻³) and the highest TP (53.09%) and was 13.8% higher than CK. More importantly, the saturated hydraulic conductivity (K_S), field capacity (FC), permanent wilting point (PWP), and available water content (AWC) of the soils treated with biochar and earthworm manure were significantly higher than those of CK. At the same application rate, the effect of biochar on soil–water permeability and water-retention capacity was significantly higher than that of earthworm manure, in which the soil–water-characteristic curve (SWCC) showed that as BA₅ > BA₃ > QA₅ > QA₃ > CK, the FC and AWC increased from 28.90% and 14.13% under CK, respectively, to 40.73% and 21.91% under BA₅, respectively; and the K_S, FC, PWP and AWC of BA₅ increased by 45.93%, 40.91%, 27.46% and 54.96% compared with CK, respectively. The results revealed that the improvement of the soil TP was conducive to the enhancement of the soil K_S and FC, enhanced the soil–water permeability and the water-retention capacity, and ultimately increased the AWC. From the perspective of improving the facility soil and economic benefits, the application of 5% biochar is considered to be the most beneficial.

1. Introduction

In the current context of increasing land resource constraints, the soil properties under greenhouse conditions not only directly determine the yield and quality of vegetables, but also influence the sustainability of facility vegetable cultivation [1,2]. However, with the increase in planting years, the soils of plastic greenhouses in facility horticulture also suffer from soil secondary salinization, which leads to the deterioration of the physical properties of the soil and a reduction in its hydraulic properties, which in turn affects its water-holding capacity and leads to the decrease in soil–water availability [3,4]. How to improve the soil physical properties of facility horticulture and increase the water-holding capacity of facility soils to ensure the sustainable development of facility horticulture has attracted the attention of scholars [5,6,7]. Biochar and earthworm manure have received wide attention for their beneficial effects on improving soil quality [8,9,10].

Biochar is a highly aromatized carbonaceous product with high biochemical stability obtained from high-temperature pyrolysis of a biomass such as straw under oxygen deficit conditions [11,12]. Biochar has a rich pore structure and a huge specific surface area (SSA) with excellent adsorption capacity, which has a positive effect on improving soil physical properties [13,14,15,16]. Soil bulk density (BD) and total porosity (TP) are key indicators of soil physical quality [17], and the well-developed porous structure of biochar increases the TP of the soil surface layer, and with increasing biochar application, the soil BD tends to decrease and the soil structure is improved [18,19]. Earthworm manure is a black, fine particulate matter obtained from the disposal of biological waste under the digestive system of earthworms with the combined action of various enzymes and microorganisms [20,21]. Earthworm manure has a large specific surface area, high drainage, high-water-holding capacity, and good aeration, which increases the number and activity of microorganisms in the soil, easily forming granular structures, reducing the soil BD, and increasing the TP [22,23].

In addition, biochar can adsorb and retain water, which in turn affects the soil–water holding capacity and hydraulic conductivity characteristics, and can enhance soil–water infiltration, but the specific mechanism needs to be further investigated [24,25,26]. Soil–water transport and storage play an important role in nutrient transport and plant growth, and soil–water characteristics mainly include the soil–water holding capacity (FC, PWP, AWC, etc.) [27,28]. The effect of biochar on soil-moisture characteristics has become one of the hot topics of recent research [29,30,31]. The results of Lydia et al. [32] showed that biochar could improve soil physical structure and porosity, thus increasing the soil–water holding capacity, soil–water effectiveness, and improving soil-moisture conditions. The addition of biochar can increase the contact between soil particles and increase the small porosity, thus increasing the soil–water retention capacity [33]. Devereux et al. [34] set up incubation tests with different mass ratios of biochar and sandy loam (0.0%, 1.5%, 2.5%, 5.0%) and revealed that the soil–water content increased with increasing the biochar application at a certain water potential (10⁻²–10⁴ kPa). Water infiltration mainly depends on the soil–water potential and K_S, and the effect of biochar on K_S of different soil textures is inconsistent, though usually biochar can reduce the K_S of sandy soils [35,36] and increase the K_S of clayey soils [37,38]. Application of biochar to the soil increases the adsorption between the biochar particles and water to increase the FC and AWC of the soil [39,40]. It was also suggested that the AWC of the soil increased when the PWP of the soil was lowered or the FC of the soil was increased [41]. Earthworm manure has advantages in physical properties, which can effectively improve soil structure and soil–water permeability [42,43]. Earthworm manure has a granular structure and is rich in polysaccharides, which are conducive to the formation and maintenance of aggregates, and they determine the hydraulic characteristics of the soil, affecting the aeration, water storage and water permeability [44,45]. Earthworm manure is rich in humus, which is a hydrophilic colloid with a high water-holding capacity and helps to enhance the water-holding capacity of the soil [46,47].

At present, most of the studies on additives for improving physical or hydraulic characteristics of facility soils have focused on a single application of biochar or earthworm manure to the soil, while limited data are available on the synergistic effects of earthworm manure on improving soil physical and hydraulic characteristics, and comparative studies between biochar and earthworm manure are lacking. In the current context of water scarcity, the effects of biochar and earthworm manure on the physical properties and hydraulic characteristics of soil in facilities need to be further investigated. Therefore, this study conjectured that adding maize straw biochar and earthworm manure would effectively affect the soil BD and TP, regulating the soil physical properties and hydraulic characteristics together with the availability of water in the soil. Additionally, the study presupposes that the addition of biochar and earthworm manure to soil can reduce the soil BD and increase the TP, and affect the K_S and SWCC of the soil, leading to an increase in the FC and AWC. To verify the above presumption, we thus explored the effect of increasing biochar and adding earthworm manure on the BD, TP, K_S, SWCC, PWP, FC, and AWC of soil amendment in a greenhouse experiment, with the aim of proposing suitable regulation measures to improve the soil–water retention capacity in facility agriculture.

2. Materials and Methods

2.1. Soil Sampling

The experimental soil samples were collected from the top 10–20 cm of the farmland soil of the water-saving Park of Hohai University (31°57′ N, 118°50′ E), located in Jiangning District, Nanjing City, Jiangsu Province, China. The soil sample was air dried and passed through a 6.3 mm sieve. The physicochemical properties of the soil samples were determined according to standard procedures as follow: the soil texture was typical yellow-brown loam according to the Chinese classification [48], soil texture was measured by Bouyoucos hydrometer (TM-85, SHTG, Shanghai, China) [49], with the clay: 26.5%, silt: 34.9%, sand: 38.6%. Bulk density (BD) of 1.41 g·cm⁻³, total porosity (TP) of 46.32%, field capacity (FC) of 28.73%, pH value of 7.07 were measured by the Remag pH meter in a 1:5 soil and water extract [50]. Organic matter of 1.04% was determined by the high-temperature oxidized organic carbon method [51]. Available soil nitrogen of 11.1 mg·kg⁻¹ was measured using a UV-Vis spectrophotometer (L007, 7522112059A; Essence Technology Instruments, Shanghai, China) [52,53]. Available soil phosphorus of 5.81 mg·kg⁻¹ was determined by the UV-Vis spectrophotometric method [54]. Available soil potassium of 101 mg·kg⁻¹ was measured using the flame photometric method [55].

2.2. Biochar and Earthworm Manure Preparation

The tested biochar is classified as maize straw biochar (purchased from Henan Lize Environmental Protection Technology Co., Ltd., Xinxiang, China). The experimental earthworm manure is obtained by fermenting pure cow dung through the digestive system of earthworms. The specific physical and chemical properties of biochar and earthworm manure were determined by the methods as detailed before, which are shown in

Table 1. Specific physicochemical properties of biochar and earthworm manure.

Property	Unit	Biochar	Earthworm Manure
BD	g·cm−3	0.42	-
PD	g·cm−3	0.94	-
TP	%	55.3	-
pH	value	9.40	8.17
OMC	%	41.1	44.9
N	mg·kg−1	390	564
P	mg·kg−1	56.4	461
K	mg·kg−1	58,513	1892

Note: Values are the average of three replicates of each propriety; BD, PD, TP, OMC, N, P, and K donate bulk density, particle density, total porosity, organic matter content, available nitrogen, available phosphorus, and available potassium, respectively.

.

2.3. Greenhouse Incubation Experiment

The main treatments were biochar and earthworm manure. This experiment included the following five treatments: CK (0% rate, 12 kg soil + no addition), BA₃ (3% rate, 12 kg soil + 360 g biochar), QA₃ (3% rate, 12 kg soil + 360 g earthworm manure), BA₅ (5% rate, 12 kg soil + 600 g biochar), QA₅ (5% rate, 12 kg soil + 600 g earthworm manure) on a mass basis. Each treatment was set up with eight replicates. Experimental pots were conducted in an open shelter covered with plastic greenhouse 5 m wide and 20 m long without temperature control under natural light conditions and were arranged in a completely randomized block design (Figure 1). Each pot with a top diameter of 32.5 cm, a bottom diameter of 28.0 cm, a height of 38.5 cm was filled with quartz sand to a height of 8 cm, considering the water retention and breathability in the bottom. Biochar and earthworm manure were mixed with soil thoroughly in proportion and added to each treatment with a natural bulk density. Each pot was irrigated with tap water to maintain the water content in the soil at the FC as follows: irrigation was based on the difference in the daily weights of all pots to compensate for the loss of water due to evaporation. The soil-moisture content of all pots was maintained at the FC throughout the experiment period and continued for 170 days. At the end of the experiment, four pots were selected from each treatment as four replicates, and soil cores and samples were collected for the determination of the physical properties and hydraulic characteristics of the soil.

2.4. Measurement Items and Methods

At the end of the experiment, soil samples were collected from 0–5 cm depth using a ring knife with 5.05 cm diameter and 5 cm height, and the dry weighing method [56] was used to measure the soil BD as follows:

$$BD=\frac{M}{V}$$

(1)

where $BD$ is the bulk density (g·cm⁻³); $M$ is the dry weight of the soil sample in the ring knife (g); $V$ was the volume of the ring knife (cm³).

The soil TP was measured using the ring knife weighing method [57] as follows:

$$TP=(1-\frac{BD}{PD})\times 100$$

(2)

where $TP$ is the soil total porosity (%); $PD$ is the soil particle density, and it was generally assessed that the soil particle density is 2.65 g·cm⁻³; $BD$ is the bulk density (g·cm⁻³).

The soil–water-characteristic curve (SWCC): Determination was carried out using a 1500 pressure plate apparatus according to Hu [58]: the soil sample was soaked to saturation in the ring knife and the pressure membrane plate was soaked to saturation with deionized water. The air pressure was set at a 0.01, 0.03, 0.1, 0.33, 1, 3, 5, 10, 15 bar, respectively; and the soil sample was taken out after each stage was equilibrated until no water was discharged from the chamber, and the mass of the soil sample was weighed and recorded ($w_i$). Finally, the soil samples were dried in an oven at 105 °C for 8 h and the dry soil mass was weighed and recorded ($w_0$). The volumetric water content of the soil at each pressure was calculated from the soil mass before and after drying, and the soil density. The soil–water content under each pressure level was calculated as follows:

$${\theta }_{mi}=\frac{w_i-w_0}{w_0}$$

(3)

where ${\theta }_{mi}$ is the soil-moisture content (g·g⁻¹); $w_i$ is the mass of the soil sample under different pressures (g); $w_0$ is the mass of the dry soil (g).

The pressure units were also converted into centimetres of water column to obtain the soil volumetric water content (${\theta }_{\left(h\right)}$) corresponding to each soil water suction force. The soil-moisture-characteristic curve was fitted by the van Genuchten model (VG model), which accurately quantified the relationship between the soil–water content and the soil matrix suction [59]. The model equation was as follows:

$${\theta }_{\left(h\right)}={\theta }_r+\left({\theta }_s-{\theta }_r\right)[{\left(1+{\left(\alpha h\right)}^n\right]}^m$$

(4)

where ${\theta }_{\left(h\right)}$ is the soil volumetric water content, cm³/cm³; ${\theta }_s$ is the saturated volumetric water content, cm³/cm³; ${\theta }_r$ is the residual volumetric water content, cm³/cm³; $h$ is the matrix potential, kPa; $\alpha$ is the correlation coefficient; $n$ and m are the shape parameters of the curve, $m$ = 1 − 1/$n$.

Saturated hydraulic conductivity (K_S): After saturating the moisture ring knife soil samples into a WS-55 type permeability instrument, the seal, permeability stone, the upper cover were put on and the screws were tightened, and there was no leakage of water and air. The inlet pipe mouth and water supply device were connected, and the instrument was put flat on the water exhaust, in the water head not more than 200 cm for a certain time, in order to determine when the water would start to escape from the mouth of the outlet pipe. During the test, the head pipe flushed the water to the required height after closing the pipe clamp to open the stopwatch, while measuring and recording the starting head $h_1$, after the time t ($t_2-t_1$), and then measuring and recording the end of the water soil $h_2$. The variable head method [60] was calculated as follows:

$$K_s=2.3\frac{a\times L}{A\times \left(t_2-t_1\right)}\log \frac{h_1}{h_2}$$

(5)

where $K_s$ is saturated hydraulic conductivity (%); $a$ is the cross-sectional area inside the pressure measuring tube (cm²); $L$ is the the height of the specimen, type 55 penetrometer (cm); $\mathrm{A}$ is the the specimen area, type 55 penetrometer (cm²); $h_1,h_2$ corresponds to the difference between the water level in the pressure measuring tube and the water level at the outlet at moments t₁ and t₂, respectively (cm).

The soil field capacity (FC, %) and permanent wilting point (PWP, %) were referred to the soil-moisture content at 33 kPa and 1500 kPa using a pressure extractor plate, respectively [61].

The available water content (AWC, %) was referred to the difference between the FC and PWP as follows:

$$AWC=FC-PWP$$

(6)

2.5. Data Processing and Analysis

The experimental results were recorded and processed using Excel 2010, and GraphPad Prism 8.0 was used for one-way ANOVA and graphing. Analysis of variance (ANOVA) was performed using SPSS 26.0, where Duncan’s multiple range test was applied to compare the means at a significance level of 0.05, and statistical significance was considered at p ≤ 0.05. Pearson correlation analysis was performed between the soil physical properties and hydraulic characteristics to determine the degree of relationship.

3. Results and Analysis

3.1. Soil Bulk Density and Total Porosity

The application of biochar and earthworm manure changed the soil BD, with significant differences (p ≤ 0.05) between the different application rates (Figure 2a). Compared to CK, the soil BD of BA₃ and BA₅ significantly decreased by 10.09% and 12.04%, respectively. The fitting revealed that there was an excellent quadratic function relationship between the BD and biochar application in CK, BA₃, and BA₅ ($BD=67.5x_B^2-6.775x_B+1.4125$, where BD is the bulk density, g·cm⁻³; $x_B$ is the biochar application, g·g⁻¹; $R^2=0.9082$). Meanwhile, the soil BD decreased by 7.43% and 5.66% in QA₃ and QA₅ compared to CK, respectively, and there were statistically significant differences between BA₃, QA₃, BA₅ and QA₅.

The change in the TP was negatively correlated with the soil BD, i.e., the TP increased with the decrease in BD. The TP of biochar and earthworm manure treatments were significantly higher than that of CK (Figure 2b), and BA₃ and BA₅ were significantly higher than QA₃ and QA₅, and the maximum TP was observed with BA₅ (53.1%). The results indicated that the application of biochar with the same application was more effective than earthworm manure in reducing the soil BD and increasing the TP.

3.2. Soil–Water-Characteristic Curve and Saturated Hydraulic Conductivity

The SWCC for each treatment showed that the water content decreased as the soil–water uptake increased (Figure 3). In the low water uptake section (<1000 cmH₂O), the SWCC of each treatment decreased and changed steeply, and the earthworm manure treatment contained more macropores than biochar, so the changes in QA₃ and QA₅ were steeper. In the medium and high-water absorption section (>1000 cm H₂O), all treatments showed a slow decline and then leveled off, with CK being the most obvious, followed by QA₃, QA₅, BA₃ and BA₅, indicating that the application of biochar with the same application had a better effect on soil–water holding capacity than earthworm manure.

The results showed that the application of biochar and earthworm manure to the soil significantly (p < 0.0001) improved the K_S of soil (Figure 4). The K_S was significantly increased by 42.35%, 45.93%, 37.13%, and 39.74% for BA₃, BA₅, QA₃ and QA₅, respectively, compared to CK, but there was no significant difference (ns) between them. It indicates that the application of biochar and earthworm manure significantly improved the hydraulic conductivity of the facility soil, and the improvement effect increased with the application rate, and biochar was more effective than earthworm manure at the same application rate.

3.3. Soil Field Capacity, Permanent Wilting Percentage and Available Water Capacity

The results obtained indicated that the FC, PWP and PAW% showed significant differences (p ≤ 0.05) in the treatment of biochar and earthworm manure (Figure 5a). The highest FC, PWP and PAW% were recorded by BA₅ (40.7%, 18.8% and 21.9%), followed by BA₃ (38.5%, 17.5% and 20.9%), QA₅ (33.8%, 15.9% and 18.0%) and QA₃ (33.6%, 15.5% and 18.0%), whereas the lower values were observed with CK (28.9%, 14.8% and 14.1%); particularly at the rate of BA₅, its FC, PWP and PAW% increased by 40.91%, 27.46% and 54.96%, respectively, compared with CK. Meanwhile, BA₃ and BA₅ were significantly higher than QA₃ and QA₅, indicating that the improvement effect of biochar on hydraulic characteristics was significantly higher than that of earthworm manure.

The correlation coefficients of the three groups of FC, PWP, and AWC parameters for each treatment were found to be in the range from 0.957–1.000 (Figure 5b), indicating that the application of biochar and earthworm manure could significantly improve the hydraulic characteristics of the soil in the facility, and the high application (5%) of biochar and earthworm manure had a better effect on enhancing the hydraulic characteristics of the soil than the low application (3%).

3.4. Correlation Analysis of Soil Physical Properties and Hydraulic Characteristics

The correlation analysis between soil physical properties (BD, TP) and hydraulic characteristics (K_S, FC, PWP, AWC) are shown in

Table 2. Correlation coefficient matrix between soil physical properties and hydraulic characteristics.

Indices	$\mathit{B}\mathit{D}$	$\mathit{T}\mathit{P}$	${\mathit{K}}_{\mathit{s}}$	$\mathit{F}\mathit{C}$	$\mathit{P}\mathit{W}\mathit{P}$	$\mathit{A}\mathit{W}\mathit{C}$
$BD$	1.000
$TP$	−0.996	1.000
$K_s$	−0.868	0.871	1.000
$FC$	−0.953	0.951	0.791	1.000
$PWP$	−0.851	0.855	0.711	0.897	1.000
$AWC$	−0.930	0.925	0.770	0.973	0.772	1.000

Note: Pearson correlation coefficient 0.8–1.0 means very strong correlation, 0.6–0.8 means strong correlation, 0.4–0.6 means medium correlation, 0.2–0.4 means weak correlation, 0.0–0.2 means very weak correlation or no correlation.

. According to the correlation coefficient matrix, all the hydraulic characteristic parameters (K_S, FC, PWP, AWC) proved to be highly significant when correlated with the BD and TP (R > 0.8), indicating that the improvement of the soil physical properties was conducive to the improvement of the soil hydraulic characteristics and enhancing the soil–water availability. Every two of K_S, FC, PWP, and AWC had a highly significant positive correlation (R > 0.6). In addition, PWP and AWC showed a strongly significant correlation with FC (R > 0.8), indicating that the FC was most closely related to AWC, which was conducive to improving soil permeability and enhancing the soil–water holding capacity.

4. Discussion

The present results showed that the different application rates of biochar and earthworm manure could significantly reduce the soil BD, and there was a linear negative correlation between the TP and the BD. Hence, the TP increased accordingly, and BA₃, BA₅ were higher than QA₃, QA₅, and applying 5% biochar had the best effect on improving the TP. Our results are consistent with the studies that showed that the effects of biochar on soil physical properties mainly include a reduction in the soil BD and an increase in the TP [62]. Biochar, due to its own low specific gravity and loose texture, could directly improve soil bulkiness and reduce the soil BD, which in turn increased the soil TP [63]. Nahidan et al. [64] also reported that earthworm manure had a large specific surface area as well as a strong adsorption capacity, which could effectively increase the TP of the soil. In our study, the SWCC was BA₅ > BA₃ > QA₅ > QA₃ > CK, which may be explained by the soil being treated with biochar having more TP and finer texture than earthworm manure, and the water kept in the small and medium pores only being released slowly within a large suction range, so that the curve drops more gently. The SWCC is the main parameter of soil hydrodynamics, which is closely related to soil structure and texture. Our results agree with the research of Wu. et al. [65] that the application of biochar altered the SWCC, and the soil–water holding capacity increased with the increase in biochar addition. Our study is also supported by Wang et al. [66], revealing that biochar was able to improve the water-holding capacity of the soil, increasing water content by 11.3% compared to the no-char treatment by a comparative study of the SWCC according to the van Genuchten model and Gardner model.

Saturated hydraulic conductivity (K_S) was related to the number of soil pores, soil texture, structure, etc. [67]. Our study showed that the K_S of BA₃, BA₅, QA₃, and QA₅ were significantly higher than CK, and the change of K_S and TP showed a consistent trend, which may be explained by the biochar with the same application rate increasing the TP more than that of earthworm manure, and the soil permeability being stronger after application, so that K_S was also higher. Our results are supported by Wang et al. [68], who found that the K_S could be increased by up to 45% after one year of biochar application.

The obtained results indicated that the FC, PWP, and AWC of BA₃, BA₅, QA₃, and QA₅ were significantly improved compared with CK, and the improvement became better as the biochar rate increased. The FC was a physical property of soil, and its size was related to the soil TP, BD, texture and OMC [69,70]. Our results are supported by Lu et al. [71], who revealed that biochar application was positively correlated with the FC, with the addition of 2%, 4% and 6% biochar to the clay soil increasing the FC by 12%, 20% and 31%, respectively. Li et al. [72] also revealed that earthworm manure could affect the SSA of soil, promote the formation of soil aggregates, and influence the water retention properties of soil. The FC increased with increasing the biochar addition rate, and the porous nature of biochar absorbed water more readily, thereby increasing the PWP [73]. The AWC of soil increased with the increase in the soil FC. Peake et al. [74] revealed that the effects of adding 0.1%, 0.5% and 2.5% (w/w) biochar to eight different soils ranged from 4.0~13.4%, 9.8~33.7%, and 0.3~48.4% of AWC, respectively. In addition, our study revealed that the AWC of BA₃ and BA₅ was significantly higher than that of QA₃ and QA₅. This may be explained by our experiment being conducted over a short period, and the effects of biochar and earthworm manure on the AWC of soil being related to their type, application rate, soil conditions and the experiment time [75]. It is necessary to further investigate the impacts of biochar and earthworm manure on soil hydraulic characteristics through long-term experiments.

5. Conclusions

In our study, both biochar and earthworm manure at different application rates positively affected the physical properties of the soil, mainly at the high application rate of 5% biochar. With increasing the biochar application rates, the soil BD was effectively reduced, the physical properties of the soil were improved, and it also contributed to the improvement of water retention and infiltration, enhanced the SWCC and K_S, eventually increased the FC and AWC. However, at the same application rate, biochar was significantly more effective in enhancing the TP than earthworm manure, resulting in significantly higher K_S, FC, and AWC in BA₃, BA₅ than in QA₃, QA₅. The AWC increased from 14.1% to 18.0%, 18.0%, 20.9%, and 21.9% under CK, QA₃, QA₅, BA₃, and BA₅, respectively. Therefore, under the present experimental conditions, maize straw biochar was more effective than earthworm manure in improving the physical properties and hydraulic characteristics of the facility soil, and 5% biochar was the most effective in improving soil–water availability.