1. Introduction
Land degradation has been recognized as a serious global problem. The Intergovernmental Panel on Climate Change (IPCC) [
1] reported that interactions between land degradation and climate change form a vicious cycle. The overuse of chemical fertilizer has resulted in hardened soil, decreased fertility, low soil quality, increased used of pesticides and herbicides, polluted air and water, and also produces greenhouse gasses. It also contains salt as well as other acidic materials, which are some of the most critical characteristics of chemical fertilizer and are expected to damage the soil in the long run. The excessive use of chemical fertilizers results in the degradation of land quality, soil acidification, and poor soil aggregate structures, thereby reducing crop productivity [
2]. Numerous studies have reported that organic fertilizers can effectively improve land quality, soil structure, and tilth, and increase organic matter [
3,
4], microbial activity, and diversity in the soil [
5,
6,
7,
8]. Long-term use of organic fertilizers can mitigate soil deterioration. However, because of the rapid decomposition of organic fertilizers, farmers must regularly reapply organic fertilizers, which may incur additional costs and economic burdens.
In the past decade, biochar has been widely proposed as a material for soil improvement [
5,
7,
8]. Biochar, a solid material containing abundant carbon, is created through pyrolysis of biomass in a low-oxygen environment [
9]. Adding biochar to soil improves the physical properties of the soil, including increased soil aggregation, enhanced soil water retention, and increased soil corrosion resistance, and helps retain soil nutrients [
5,
7,
8,
10]. Jindo et al. (2020) [
11] demonstrated that some biochars contains abundant potassium (K). Therefore, adding biochar to soil can satisfy crop K requirements and increase crop yield [
5]. Moreover, biochar retains soil organic matter due to its porous structure and can effectively reduce greenhouse gas emissions (e.g., CO
2 and N
2O) [
6].
Adding biochar to compost or chemical fertilizers can substantially increase soil fertility, enhance soil physical properties, sustain soil quality, and improve crop productivity [
5,
9,
12,
13,
14,
15]. Additionally, mixing biochar in fertilizers prevents the rapid decomposition of fertilizers or compost, enabling the fertilizer or compost to serve as a long-term supply of nutrients for crops [
16,
17,
18,
19]. Li et al. (2014) [
20] proposed that when biochar is added to organic or chemical fertilizers, the functional groups on the biochar surface can bind with ammonium and nitrate ions. This phenomenon increases the nitrogen (N) use efficiency (NUE) of the crops, which in turn increases crop yield.
At present, most studies have only mixed biochar and fertilizers into the soil. Few studies have researched the effectiveness of granular compound materials or explored the decomposition rate and nutrient releasing mechanisms of biochar fertilizers. Therefore, soil improvements using biochar amendment alone can have inconsistent effects on crop yields and may even lead to N deficiency in soil in the short term [
6,
7]. Therefore, joint application of compost and biochar in agricultural soil can prevent biochar from absorbing soil nutrients during the initial application stage, avoiding soil N deficiency, providing crops with a long-term supply of nutrients, and maintaining soil fertility. Use of biochars for organic/inorganic compound fertilizer can be an option to achieve high productivity and low carbon intensity along with reducing nitrogen fertilizer use in Chinese rice agriculture [
20]. Enhanced plant growth in the biochar + compost treated soil has largely been attributed to improved nutrient availability and uptake compared to biochar alone [
21,
22,
23]. Furthermore, slow-release fertilizers created by mixing biochar and compost particles can enhance the effectiveness of organic fertilizers. However, studies have yet to determine optimal proportions for mixing biochar and compost [
20].
Based on the concept of circular agriculture and sustainable environment, reuse/recycle of agricultural wastes to be a useful fertilizers to replace of chemical fertilizers might be a feasible agricultural management practice in the future. This study supposes that application of the biochar–compost composites could not only improve the soil quality but also facilitate the fertilizer use efficiency for crops. This study mixes wood biochar made using reused agriculture wastes into different proportions of bagasse compost to create innovative organic fertilizers through granulation. The biochar compound materials were subsequently applied to two common types of agricultural soil (sandy soil and clay soil) in Taiwan to evaluate the nutrient releasing efficiency in our study.
2. Materials and Methods
2.1. Biochar Source and Preparation
Waste woods including Taiwan Acadia (Acacia confuse Merr.), Casuarina (Casuarina equisetifolia Forst.), and Leucaena (Leucaena glauca (L.) Benth.), were used to make biochar in this study. These three kinds of wood are the most abundant ones in the coastal areas in Taiwan, and produce a lot of waste wood every year. Wood biochar was prepared by subjecting the waste wood materials to pyrolysis by using an open furnace (Fire Box S-119, Airburners Inc., Palm City, FL, USA). The carbonization temperature was over 700 °C. After production, the biochar was ground to a fine powder, then filtered (2 mm) for subsequent analysis and the production of particle biochar compound materials (pBCMs). The pBCMs were produced by mixing the biochar and compost in a granulator (Young & Dear, YD300, Taiwan), forming composite granules (about 1 cm in length and 0.8 cm in diameter).
2.2. Soil Collection for the Experiment
Sandy soil and clay soil were collected from fallow agricultural lands in southern Taiwan. The soil collected was mainly surface soil (0–15 cm) which was thern placed in plastic bags. The collected sandy soil was classified as Udorthent [
24], the parent material of which is freshly accumulated calcareous clay slate. The collected clay soil was classified as Paleudult. The collected soil was air dried, ground, and filtered using a 10-mesh filter (2 mm) and stored in plastic jars for subsequent property analysis and use in the incubation experiment. The analytical results are presented in
Table 1.
The pH values of the soil samples and the biochars were determined in a mixture with deionized water (1:2.5
w/v for soils; 1:20
w/v for biochars), using a Horiba F-74 BW meter [
25]. Soil particle size distribution was determined with the pipette method [
26]. The exchangeable K was extracted with 1 mol L
−1 NH
4OAc (1:10
w/v for the soils; 1:20
w/v for the biochars) [
27] and was determined by the atomic absorption spectrometry method (Z-2300, Hitachi, Japan). Organic carbon content was determined by the wet oxidation method [
28]. Available phosphorous was determined by the Bray P-1 extract test [
28]. Inorganic N was extracted with 2 M KCl (1:10
w/
v), and the concentrations of NH
4+-N and NO
3−-N were determined by steam distillation, using MgO and Devarda’s alloy [
29]. Total C was determined by dry combustion method and total N was measured using the Kjeldahl procedure [
30].
2.3. Calcium Carbonate Analysis
Calcium carbonate (CaCO
3) analysis was performed using the method proposed by Leoppert et al. (1984) [
31]. First, 5 g of studied soils, biochar, compost, and compound materials were weighed and filtered with a 10-mesh filter (2 mm). The soil was subsequently placed in a beaker, 50 mL of 1 N HCl solution was added, and continuously shaken to react for 1 hr. The solution was then filtered, and phenolphthalein indicator was added. Finally, the solution was subjected to titration using standardized 0.1 N NaOH titration until the titration end point was reached.
2.4. Compost Source
This study employed bagasse compost produced by Taiwan Sugar Corporation. The compost properties are listed as follows: total N (1.5%), organic matter (55%), total phosphoric oxide (0.9%), and total K oxide (1.5%); the pH was 7.4.
2.5. Biochar Compound Material Preparation
Four granular materials were prepared: (1) BC3, composed of biochar and compost at a 1:3 ratio (
w/
w; moisture 15%); (2) BC, composed of biochar and compost at a 1:1 ratio (
w/
w; moisture 20%); (3) B, biochar-only (moisture 25%); and (4) C, compost-only (moisture 10%). Granulation equipment was used (Young & Dear 300, Taiwan). In the granulation process, deionized water was added as a binder. The properties of the granular material are listed in
Table 2.
2.6. Indoor Incubation Experiment for Evaluating Nutrient Releasing and Decomposition of pBCMs
A 6-inch (15 cm) plastic incubation pot was used for the pot experiment. The inner diameter and depth of the plot were 16 and 17 cm, respectively. Each pot was filled with 2 kg of soil. In the incubation experiment, the amount of pBCMs applied was based on the recommended amount of N fertilizer for Chinese cabbages (Brassica rapa chinensis) (300 kg ha−1) in Taiwan. The materials were packaged using tea bags and placed in the soil at a depth of approximately 15 cm. Soil moisture samplers (Rhizon SMS, Eijkelkamp, Netherlands) were buried in the incubation pots to extract soil solution on days 1, 3, 7, 30, 60, 120, and 180 to measure inorganic N, available P, and exchangeable K for evaluating nutrient release. The pots were weighed daily and supplemented with water at the field capacity of the soil. On the soil solution extraction days, the pBCMs inside the tea bags were dried in an oven at 50 °C for 4 h and then weighed to calculate the decomposition rate. Each process was performed in three replicates.
2.7. The Pot Experiment for Crop Production
The cabbage species selected for the pot experiment was the Chinese white cabbage (Brassica chinensis Linn.). The amount of pBCM applied was based on the recommended standard amount of N fertilizer for Chinese cabbages, which was 25 g (w/w) of C, 195 g of B, 45 g of BC, and 32 g of BC3, respectively, incorporated in the soil (2 kg) in the pots. The crops were cultivated for two consecutive crop periods (each crop period was 28 days); no additional fertilizer was provided between the two periods. The pBCMs were evenly spread and mixed with 5 cm of the surface soil. The Chinese cabbages were harvested monthly. Each pot was used to cultivate three Chinese cabbages, and samples were placed according to the randomized complete block design. After harvesting, the cabbage leaves were dried, weighed, and the crop yield was calculated.
2.8. Calculation and Statistical Analysis of the Nutrient Utility Rate
Nutrient utility rate represents the total biomass or economic yield generated per unit of nutrients absorbed by the plant. Moll et al. (1982) [
32] proposed using NUE to calculate plant nutrient utility rate. For NUE, the equation is as follows: NUE (%) = yield of plant/N measured in each pot. The total N contents were determined for the amended soils before crop planting and after the 1
st crop harvest for NUE calculation. NUE, also known as fertilizer absorption rate or fertilizer recovery efficiency, refers to the percentage of N in the fertilizer absorbed by the plant. Phosphorus (P) use efficiency (PUE) refers to the phosphorus concentration which was multiplied by grain yield and aboveground biomass yield to calculate the P absorbed by the grain and aboveground biomass [
33]. The equation is as follows: PUE (kg/kg) = yield of plant/amount of P absorbed from the soil [
32]. Randomized complete block design was used, and GLM procedures in the SAS software were used to perform two-way analysis of variance. The mean values of each treatment group were compared using Duncan’s new multiple range test. The significance level was set at
p < 0.05.