Effects of Biochar Application on Vegetation Growth, Cover, and Erosion Potential in Sloped Cultivated Soil Derived from Mudstone

: Soil degradation is a crucial problem, particularly in tropical and subtropical areas. Pre ‐ vention or reduction of soil erosion requires strategies based on thorough rapid vegetation cover (VC) and favorable soil quality in subtropical and tropical areas. This study applied wood biochar (WB) and rice husk biochar (RHB) in a mudstone soil, which is widely distributed in Southern Tai ‐ wan, to investigate the effects of biochar application on soil erosion and vegetation restoration. The standard erosion unit plots (22.13 m in length and 9% in slope gradient) were set up to determine the relationship among soil losses, VC, and natural rainfall characteristics with and without biochar application. The results indicated that biochar application increased the growth rate (identified by cover ratio) of Bahia grass ( Paspalum notatum Flüggé) by 2–2.6 times within 40 days compared with control (without biochar application) and increased VC by 20% after 120 days of treatment. The biochar application could effectively reduce soil losses by 60% at least in the mudstone soil. A well ‐ predicted regression function of soil loss with VC and rainfall kinetic energy was established (amount of soil lost = − 0.435 × ln VC + 0.54 × RKE, r = 0.89, p < 0.01). analysis by using a one ‐ way analysis of variance at a significance level of p = 0.05. Differences between the mean values under each treatment were determined using Duncan’s test.


Introduction
Soil erosion is a major environmental problem worldwide, especially that due to water from heavy rainfall in tropical and subtropical areas [1]. Approximately one-sixth of the land area in the world is affected by soil degradation, and 55.6% of the affected land area is damaged by water erosion [2]. Soil degradation caused by water erosion severely affects the productivity of agricultural lands and threatens the world's food security with the increasing global population [3,4]. Additionally, the increasing demand for food and land resources has become a critical problem because of climate change. Soil degradation through erosion, hard setting, and desertification is a severe problem that affects the productivity of agricultural lands and can threaten the world's food security [5,6] because of the increasing global population. New land management techniques to effectively use soil resources and achieve land sustainability must be developed, particularly for subtropical and tropical regions.
In hilly and mountainous landscapes, the management of vegetation cover is critical for sustainability [7] because it can improve soil erosion and stabilize changing hydrological conditions. Vegetation is a sensitive factor that affects soil erosion, and the implementation of vegetation-based mitigation measures can effectively control soil erosion caused by water [8]. Thus, the development of new land management techniques to effectively use soil resources and achieve land sustainability is crucial for meeting the Sustainable Development Goals (SDGs) of the United Nations, particularly in subtropical and tropical regions with a high population density.
Biochar can improve the physical properties of soil, particularly those of degraded soils in subtropical and tropical regions [9][10][11][12][13], maintain its fertility, and facilitate vegetation growth on slopelands. Numerous studies have indicated that biochar effectively improves the structure [14,15], aggregate stability [16][17][18][19][20], and porosity [21] of soil because of its high specific surface area and inner porous structure. In addition, studies have reported that fertilizer use efficiency and crop production increased following the incorporation of biochar [10,[22][23][24]. Biochar consists of recalcitrant carbon resulting from microbial degradation and a charged surface with organic functional groups; therefore, biochar can increase soil organic carbon (SOC) and carbon levels [25][26][27][28]. The application of biochar can provide agronomic benefits, particularly fertility maintenance, SOC sequestration, and crop production [29][30][31]. However, few studies have examined the beneficial effects of biochar application on the hydraulic properties and erosion control of soil. Hseu et al. [32] recommended biochar application as an innovative management strategy for soils in mudstone areas.
Global warming increases the average annual temperature and precipitation, as well as the intensity of heavy rainfall, and these impacts could lead to more serious hazards in our study area dominated by mudstone soils. Rapid and homogenous vegetation cover is known as one of the important factors to affect soil quality and prevent the soils from erosion. This study investigated the effects of wood biochar (WB) and rice husk biochar (RHB) on soil erosion and vegetation growth at a mudstone cultivated slopeland in Taiwan. In addition, this study determined the relationship between vegetation cover and soil erosion potential at this mudstone landscape.

Experimental Sites and Soil
The field experiment was performed at the Livestock Research Institute (LRI) in Southern Taiwan (23 o 3′37.4″ N, 120 o 20′23.9″ E; Figure 1). We established three standard erosion unit plots (22.13 m in length and 6 m in width with a 9% slope gradient equipped with collection tanks for sediment and runoff water) [33]. Each large erosion unit plot was further divided into three subplots to perform the experiments in triplicate. The three treatments (control, WB, and RHB) were random arranged in nine subplots. The standard erosion unit plots were established in July 2018. At the study site, the major soil type was mudstone, which is poorly cemented and covers an area of more than 1000 km 2 in southwestern Taiwan [34]. Figure 1 illustrates mudstone outcrops in the Chyai, Tainan, and Kaoshiung counties of southwestern Taiwan. The total thickness of these late Miocene to Pleistocene sedimentary rocks is several thousand meters. The stratigraphy of these rocks is monotonic, and they consist mainly of massive mudstone or an alternation between mudstone and sandstone. These mudstone soils were classified as Typic Eutrustept based on the soil taxonomy [35].
The topsoil (0-15 cm) was collected without gravel. The collected soil was air dried, ground to pass through a 2 mm sieve, and thoroughly mixed for use in the subsequent incubation experiment. Table 1 lists the physical structure and carbon level of the soil.

Biochars
WB and RHB were produced at a temperature of 600 °C at the Industrial Technology Research Institute (ITRI) in Taiwan ( Figure 2). WB was produced from the biomass of White Popinac, Leadtree (Leucaena leucocephala (Lam.)). For pyrolysis, the samples were placed in a tubular furnace (ITRI, Tainan, Taiwan) equipped with a corundum tube (32 mm diameter and 700 mm length) with a N2 purge (flow rate of 1 L/min) to ensure an oxygen-free atmosphere. The samples were heated at temperatures of 600 °C at a rate of 5 °C min −1 . The temperature was maintained for 2 h before cooling to an ambient temperature under N2 flow.

Field Erosion Experiment
This study applied 2% (w/w) WB and RHB in an erosional subplot, and a control subplot was maintained at each large standard erosional plot during the experiment (Figure 3). The soil and biochar were uniformly mixed before the erosion experiment. The biochar was grounded to pass through a 0.25 mm sieve, thus increasing its area of contact with the soil and facilitating soil incubation. Three treatments were performed in triplicate: (a) control without the biochar, (b) 2% RHB, and (c) 2% WB. In each subplot, the seed of Bahia grass (Paspalum notatum Flüggé) was sown by a rate of 4g/m 2 for observation of vegetation growth and coverage analysis. The coverage ratio was calculated using the kmeans method [36] by Image J (National Institutes of Health, Bethesda, Maryland, USA). The field erosion experiment was performed from July to October 2018. Data regarding the amount of rainfall during each event were obtained from the meteorological station at the LRI ( Figure 4). In this study, rainfall kinetic energy (RKE) was calculated using the empirical equation developed by Wischmeier and Smith (1965) [33] as follows: where ei is RKE (MJ ha −1 mm −1 ) and I is the rainfall intensity (mm/h). During the experiments, we collected soils (five replicates were collected and mixed to obtain a representative soil sample) from each subplot during the initial biochar application and at the end of the erosion experiment (4 months later). The topsoil (0-15 cm) was collected without gravel. The collected topsoil sample was air dried, ground to pass through a 2 mm sieve, and thoroughly mixed for analyses.

Analytical Methods
The bulk density (BD) was determined using the core method [37]. The pH values of the soil samples and biochars were determined in their mixtures prepared using deionized water (1:2.5 w/v for soils and 1:20 w/v for biochars) by using a pH meter (F-74 BW, Horiba, Japan) [38]. The electrical conductivity (EC) of the saturated paste extracts of the soils was determined using a Horiba F-74 BW meter [39]. Soil particle size distribution was examined using the pipette method [40]. The cation exchange capacity (CEC) was determined using the ammonium acetate method (pH 7.0) [41]. Exchangeable K was extracted using 1 mol L −1 NH4OAc (1:10 w/v for the soils and 1:20 w/v for the biochars) and determined through atomic absorption spectrometry (Z-2300, Hitachi, Tokyo, Japan). The organic carbon content was determined using the wet oxidation method [42]. The available phosphorous content was determined using the Bray and Kurtz P-1 method [43]. Inorganic nitrogen was extracted using 2 M KCl (1:10 w/v), and the concentrations of NH4 + -N and NO3 − -N were determined through steam distillation by using MgO and Devarda's alloy [44].

Statistical Analysis
Data were analyzed using IBM SPSS Statistics 22 for Windows (IBM, New York, USA). Datasets were subjected to mean separation analysis by using a one-way analysis of variance at a significance level of p = 0.05. Differences between the mean values under each treatment were determined using Duncan's test. Table 1 lists the properties of the studied soil, WB, and RHB. The total carbon content in the WB and RHB was 43.4% and 47.1%, respectively. RHB had a higher pH value and total carbon level than WB. However, a higher content of inorganic N and available P was observed in WB than in RHB (Table 1). In this study, after 120 days, the biochar presence inhibited the increase in Bd compared with the control ( Table 2); this finding is in agreement with that reported by Busscher et al. [45], who indicated that increasing the total organic carbon content through the addition of organic amendments to soils significantly reduced Bd. The decrease in the Bd of the biochar-amended soils might be due to the alteration of porosity caused by change of soil aggregate sizes, as reported by Tejada and Gonzalez [24], who amended the soils by using organic amendments in Spain. In addition, the organic carbon content increased after biochar application in the mudstone soil and the initial incubation ( Table 2). No marked changes in inorganic nitrogen and available phosphorus content were observed under the three treatments after biochar application and incubation ( Table 2). The results indicated that the improvement in Bd and OC contents might have facilitated vegetation growth in the biochar-treated mudstone soils [15,21,32]. Lowercase letters represent comparisons between the same treatment on different days (p < 0.05); Uppercase letters represent comparisons between different treatments on the same day (p < 0.05).

Effects of Biochar Application on Soil Properties, Soil Losses, and Vegetation Growing in Mudstone Soil
After 120 days of biochar application, approximately 1345 and 1206 kg of soil was lost after treatment with 2% WB and 2% RHB, respectively, which is significantly lower than the control (2195 kg) by 38.8% and 45.1%, respectively ( Figure 5). The results indicate that the biochar considerably decreased soil erosion amounts by improving soil physical properties such as Bd. The finer grain size (<0.5 cm) in RHB than in WB (1-2 cm; Figure 2) enabled the soil to reach suitable Bd for more effective growth to plants, resulting in superior vegetation coverage, which prevents water erosion. This study investigated the effects of biochar application on the growth of Bahia grass (Paspalum notatum Flüggé) by determining the coverage ratio. A coverage ratio of 80% was achieved faster after the biochar treatments than after the control treatment before 50 days of incubation ( Figure 6). The vegetation cover (VC) of the study area could be divided into three threshold zones based on the findings of Chen et al. [46]: the rapid growth period (first period; 0%-40% of VC), the transition period (second period; 40%-80% of VC), and the stagnation period (third period; 80%-100% of VC). In the rapid growth period, an increase in unit coverage markedly reduced soil erosion. Biochar application effectively facilitated grass growth during the first and second periods after WB treatment ( Figure 6). During the third period, a significantly higher VC was noted after the biochar treatments. The results indicate that biochar application improved the physical properties and increased the organic carbon content of the mudstone soil; these factors might have facilitated the growth of Bahia grass. Liao et al. [47] reported that the yield of Chinese cabbage (Brassica rapa chinensis) increased by at least 30% after the co-application of biochar and compost in a tropical highly weathered soil. Lehmann and Joseph and Sohi et al. [48,49] reported that biochar increases crop yield because it improves the soil structure owing to its effects on BD and pore and particle size distribution. Biochar application can exert beneficial biophysical effects in terms of the availability of air and water within the root zone, and thus on the germination and survival of plants [21,50]. Biochar application has been proposed for the restoration of desertified [5,6,51] and secondary salinized land [6,48,52]. However, few field studies have investigated the effects of biochar amendments on saltaffected croplands. Lashari et al. [53] used a combination of biochar and pyroligneous solution (a byproduct when biochar is produced through the pyrolysis of biomass) to improve the soil quality and wheat yield of a salt-stressed cropland in China.
Agronomical solutions including tillage, forest restoration, and soil erosion prevention in mudstone areas have been used worldwide because of the high salinity and poor physical and chemical properties of these soils. Biochar prepared from biomass waste through slow pyrolysis has attracted widespread attention because of its high carbon content, abundance of surface functional groups, porous structure, acid-neutralizing capacity, and ability to retain nutrients. In this study and our previous study [30] carried out nearby this study, we observed that biochar improved not only the chemical and biological properties of the mudstone soils, including pH, CEC, and BS, but also their physical properties, namely Bd, Ksat, aggregate stability, and erosion resistance. Our results indicate that biochar application can improve the quality of tropical degraded soil and reduce the amount of soil lost in regions with heavy rainfall. Figure 7 presents the paired data of the pre-restoration and post-restoration VC and soil erosion amount, determined through regression analysis. The results indicate a natural log function between the erosion amount and VC (Figure 7a). The soil erosion amount decreased as VC increased (p < 0.01, n = 24). We observed that soil loss tended to considerably decrease during the starting period (0-40 days) after biochar application and then increase and remain stable during the intermediate (40-80 days) and stagnant (80-120 days) periods, respectively, which corresponds to the changes in VC (Figure 6). Zhao et al. [54] reported that VC on the soil surface increased soil surface roughness and acted as a barrier reducing surface runoff and increasing infiltration time, which reduces soil erosion. We observed a significant linear relationship between the RKE and soil erosion amount (p < 0.01, n = 24; Figure 7b). Sirjani and Mahmoodabadi [55] reported that sediment load primarily depends on the capacity of the splash erosion, detachment, and transportation of soil particles based on raindrop kinetic energy. Zhao et al. [54] indicated that an increase in VC increased the resistance coefficient, thus reducing runoff and sediment loss, and that changes in rainfall intensity had little effect on the Reynolds number and runoff volume. We performed stepwise multiple regression to predict soil loss by using VC and rainfall energy as covariates. Table 3 presents the parameters for the stepwise multiple regression. Before the stepwise multiple regression, we transformed the nonlinear regression between VC and the amount of soil lost into a linear regression by using the logarithm of the VC value. The results indicate regression between soil loss and VC and RKE (amount of soil lost = −0.435 × ln VC + 0.54 × RKE). RKE and VC accounted for 43% and 14% of the variance in this equation, respectively. A low collinearity was noted between RKE and VC (variance inflation factor <10), indicating these two parameters could predict the amount of soil lost in our study area (Table 3).

Conclusions
For soil and water conservation, VC plays a crucial role in preventing soil loss at a certain value. Our study indicated that biochar significantly promoted grass growth by improving the physiochemical properties of mudstone soil in Taiwan. Mudstone soil (badland soil) relies heavily on fertilizers to grow plants and overcome its poor physical characteristics. However, synthetic fertilizers are effective only for a limited duration and do not resolve the problem of depleted soil organic carbon levels during soil degradation. Biochar can both increase fertility and soil organic carbon and improve the poor physical properties of mudstone soil. Our findings support biochar amendments as a possible management strategy for degraded soils.