This study evaluates the role of biochar on soil quality, carbon storage, and crop yield (tea and rice) in Northern Vietnam. Three biochars, including wood biochar (WBC), rice husk biochar (RBC), and bamboo biochar (BBC), were produced under limited oxygen conditions using Top-Lid Updraft Drum technology at temperatures around 550oC. After that, the three biochars were mixed well with ratio of weight (1:1:1), and added into FYM (Farm Yard Manure) for composting (5% biochar in weight). Then, the field trials (three replications) were conducted for tea and rice with three treatments including NPK (T1 = control), 80%NPK + 2 tons biochar/ha (T2), and 10 tons of compost + NPK (T3). The results indicated that the biochar products had positive impact on soil quality, carbon storage, and crop yield. Particularly, soil organic carbon (SOC) in tea-cultivated soil applied biochar products was increased by 19 to 20% for topsoil (0-20 cm) and 6.41 to 13.16% for subsoil (20-40 cm) compared with the control. Meanwhile, SOC of rice-plated soil applied biochar products was increased by 9.52 to 10.68 % for topsoil (0-20 cm) and 15.38 to 18.92 % for subsoil (20 -40 cm). In addition, crop yield of the treatments applied the biochar products was increased by 10.7 to 17.2% for rice and 33.0 to 48.4% for tea compared to control.
Keywords: biochar, rice, tea, physicochemical characterization, SOC storage, composting, yield
Biochar is the carbonized product gained by pyrolysis of biomass under restricted or absentoxygen conditions (Mukherjee and Zimmerman, 2013). The justification for carbonization through pyrolysis is to avoid the negative influences on human health and the environment that result from open burning of biomass residues which releases carbon dioxide, one of the most important greenhouse gases (Al-Wabel et al., 2013). The benefits of biochar were demonstrated by several previous research efforts. For instance, wood biochar applied into a Colombian savanna Oxisol increased its available Ca and Mg concentrations and pH, and reduced toxicity of Al (Major et al., 2010). In addition, biochar improved soil structure (Jones et al., 2011), created a carbon sink in soil, and reduced CH4 emissions (Liu et al., 2011). Liu et al. (2011) reported that biochar can absorb NH4+-N via cation exchange, thereby reducing the leaching of nitrogen fertilizer from soil (Huang et al., 2014).
Interestingly, biochar can be recalcitrant to microbial attack (Lehmann and Rondon, 2006), while compost or plant biomass are rapidly decomposed to greenhouse gases (CH4, CO2), especially in tropical regions (Jenkinson and Ayanaba, 1977). Thus, the application of biochar has a positive effect on soil fertility, particularly in tropical regions (Lehmann and Joseph, 2015). However, several studies have found no or negative influences of biochar application. For example, in calcareous soils, biochar did not improve pH or available P and cations (Lentz and Ippolito, 2012). Additionally, greenhouse gas emission (CO2) was increased for an Inceptisol type soil to which wood biochar produced at 350 OC was applied (Ameloot et al., 2013).These disadvantages may relate to biomass type and conditions of biochar production, and soil type.
Increasing crop and/or biomass yield is often reported for biochar application. Jeffery et al. (2017) reviewd from 109 various studies and found that crop yield incresed around 13% after biochar application, but a higher effect was seen for acidic soils (e.g. 40% increase for soils with pH <5) compared with neutral or alkaline soils where there were no significant effects. However, most studies reported the effect of biochar and/or biochar-based compost on crop yield. Thus, the aim of this study was to investigate the effect of biochar or biochar-based compost combined with chemical fertilizers (NPK) on soil quality, soil carbon storage, and crop yield (rice and tea) on Acrisol and Ferrasol soils known as low pH.
MATERIALS AND METHODS
Biochar production: The biomass residues (wood chip, rice husk, and bamboo) used for biochar production in this study were collected from Thanh Cong and Hong Tien communes, Phor Yen district, Thai Nguyen province, Vietnam. These biochars were produced by TLUD (Top-Lit Updraft Drum) technology under limited oxygen condition at temperature around 550 oC. The technology was inherited from the project “Piloting Pyrolytic Cookstoves and Sustainable Biochar Soil Enrichment in Northern Vietnam Uplands,” funded by EEP Mekong.
Compost production: the three biochars were ground into small pieces and then mixed well with ratio of 1:1:1 (w:w:w). After that, the mixed biochar (5% in weight) was added into farm yard manure (FYM) for composting. The compost was then applied for rice and tea.
Crop varieties: Khang Dan 18 for rice and LDP1 for tea
Soil types: Acrisols for rice and Ferrasols for tea
Field trials: the trials (rice and tea) were carried out at Thanh Cong (tea trial) and Hong Tien (rice) communes, Pho Yen district, Thai Nguyen province, Vietnam. Each the trial consisted of three treatments, designed in a large plot (100 m2) with three replications. Each field was one replication. The experimental treatments were described in Table 1.
Soil and crop sampling: soil samples (before and after experiments) were collected from the depths of 0-20 cm and 20–40 cm. For crop yield, rice was harvested from 5 defferent sites of each plot with 2 m2 per site, while tea leaves were collected from entrire plot.
Biochars were analyzed for pH using deionized water with ratio 1:20 (w/v) and the mixture was shacked up to 1.5h to ensure the solution and biochar mixed well. Then, the solvent was stirred again by steel spatula and the pH of the solvent was continuously measured using a Thermo Orion 3 star pH meter. Cation Exchange Capacity (CEC) was determined by saturating 1.0 g of the biochar with 50 mL of CH3COONH4 1N (pH=7) and putting on a shaker table overnight which ensured the biochar surfaces to be sufficient wetting. After shaking, other addition of 40 mL ammonium acetate was added. Next, using ethanol (80%) to discard all redundant NH4+ around the samples by three times with total volume of 60 mL. Remove the samples to glass vase and add 50 mL of 2N KCl. Keep solution in 16 h for equilibrium and NH4+ absorbed in sample are completely replaced, then immediately add the other 40 mL of 2N KCl for subsequent extraction. The solution of the extracted NH4+ was quantified to measure the content of NH4+ followed by Kjeldhal method with KVELP scientific UDK127 instrument. Total N was determined by Kjeldhal method with KVELP scientific UDK127 instrument after the biochar (1g) was digested by H2SO4 (10ml) with catalyzing by K2SO4 and CuSO4 (5g).
For total P, Ca, Mg, and K, biochar samples were combusted at the temperature of 500°C over 2h and keeping at 500°C for 8h. Firstly, 5.0 mL concentrate HNO3 were added to each vessel and digested at 120°C until dryness. Secondly, tubes were removed and allowed to cool before adding 1.0 mL HNO3 and 4.0 mL H2O2. Then, the samples were placed back into a preheated block and cooked at 120°C to dryness, and dissolved with 1.43 mL HNO3, made up with 18.57 mL deionized water to get the 5% acid concentration, sonicated for 10 min, and filtered.
Soil was analyzed for pH using KCl 1M with ratio 1:2.5 (w/v) and the mixture was shacked up to 1.5h to ensure the solution and biochar mixed well, and measured by a Thermo Orion 3 star pH meter. OC was measured by Walkey – Black method. Total N was analyzed by Kjeldahl method after soil samples were digested by H2SO4. Total P and K was measured by molypden green colour and Flame Atomic Adsorption Spectroscopy, repectively after soil samples were digested by sunfuric and pecloric acids. P available was analyzed by Bray II, and K available was identified by Flame Atomic Adsorption Spectroscopy after soil samples were axtracted by KCL 1M. CEC was analyzed by Ammonium Replacement Method (CH3COONH4, pH = 7).
The data were summaried and analyzed by Microsoft Exel and SPSS 22.
RESULTS AND DISCUSSION
Biochar products and soil charactarization
Bamboo, rice husk, and wood biochars were mixed with ratio of 1:1:1 (w:w:w) before analyzing. The mixed biochar was analyzed by pH, OC, total N, P, K, Ca, Mg, CEC, and heavy metals (Cd, Zn, Pb, Cu). Results indicated that the biochar had high organic cabon (OC) with 50.53% and alkaline pH (pHH20 = 9.44). CEC obtained 11.42 Cmol/kg, but the content of plant nutrient elements (N, P, K, Ca, Mg) was low (Table 2). The compost had higher plant nutrients than biochar. Particilarly, the content of heavy metals (Cd, Cu, Pb, Zn) of biochar and compost was in range of the IBI standard and Vietnamese standard.
Likely biochar, soil from rice and tea fields was collected and analyzed before the trials coducted. The results were described in Table 3. The data indicated that the soil of rice paddy was strong acid (pHKCL = 4.67 – 4.75), while the soil of tea field was very strong acid (pHKCL <4.5). The content of OC is medium for both soil. Meanwhile, the total nitrogen content was low for paddy soil, but the medium level was seen for tea field. Especially, total P is observed in rich for topsoil (0-20 cm) for both soils, and the rich in total K was also seen for soil of tea field for both layers. Howevwe, P and K availale of both the soils were in range of medium. The content of exchangeable Ca2+ and Mg2+ was in range of medium for both the soils, excepting the high Mg2+content was observed for toplayer of rice paddy. CEC was low for soil of rice paddy, while the medium level (topsoil) was seen for the soil of tea field.
Effect of biochar on soil quality of rice and tea fields
The soil quality of tea field one year after application of biochar:
The effects of biochar and biochar-based compost on soil fertility of rice paddy are shown in Table 4. The results are the average values of 3 replicates. The results showed that pH values were not different (P>0.05) among three treamnets one year after application of biochar products. Although biochar products applied did not impact on total N content of topsoil (0-20 cm), the total N of subsoil at T2 and T3 (biochar applied) was higher (P<0.05)compared to the control (T1 with no biochar applied). These results may relate to the role of biochar in reducing N leaching along soil profile. Interestingly, biochar product improved significantly the concentration of P and K available in comparision with the control. The same trend was also observed for exchangeable cations and CEC.
The soi quality of tea field after one year applied biochar
The analysis data of soil samples at 3 treatments one year after application of biochar products were shown in Table 5. Likely the soil of tea field, pH in 2 layers (0 -20 and 20-40 cm) of soil of rice field one year after application of biochar products tended to increase but there was no difference (p>0.05) among the three treatments. Meanwhile, the content of total N, P2O5 availbable, K2O available, exchangeable Ca2+ and Mg2+, and CEC of T2 and T3 treatments increased significantly (p<0.05) compared with the control (T1). However, there was no difference (p>0.05) between T2 (Biochar) and CT3 (biochar-based compost) for the content of total N, P2O5 availbable, K2O available, exchangeable Ca2+ and Mg2+, and CEC. There was a similar trend at the 20-40 cm layer, soil fertility in the biochar treatment (CT2 & CT3) was significantly improved compared to the control (CT1).
Effect of biochar on carbon organic storage in soil of rice and tea fields
The effect of biochar on the storage of soil organic carbon in tea field is showed in Table 6. In fact, both biochar and biochar-based compost (T2 & T3) application increased (p < 0.05) the organic carbon content in the soil compared with the control (T1). Specifically, the OC content of CT2 and CT3 increased by 19 to 20% for topsoil (0 - 20 cm) and 15 to 19% for subsoil (20 - 40 cm) compared with the control (T1 with no biochar and compost applied). Interestinglly, the soil organic carbon storage of T2 (2 tons biochar/ha applied) and T3 (10 tons biochar-based compost) were not statistically different (p>0.05).
For paddy soil, the application of biochar (T2) and biochar-based compost (T3) for rice after one year increased the OC content of the soil by 9.62 to 10.58 % for topsoil (0-20 cm) and 37.50 to 40.63 % for subsoil (20-40 cm) compared with the control (T1) a. However, there was no significant defference (p>0.05) of the soil organic carbon staroge for the topsoil, but the significant defference (p<0.05) was observed for the subsoil (20-40 cm) compared with the control.
Thus, the application of biochar and biochar-based compost for rice and tea has improved the content of available nutrients, increased soil organic carbon and adsorption capacity (CEC) of the soil, thereby contributing to minimizing the loss of nutrients in the soil. Interestingly, the effect of biochar products on the storage of soil organic carbon of rice paddy (flooded soil) and tea field (dry soil) in differnet depths was different. In fact, the increase of the soil organic carbon in the subsoil (37.50 - 40.63%) of tea field was higher compared with the topsoil (9.62-10.58%). Meanwhile, both the soil depths of tea field has the same trend of soil organic carbon sequestrion (15-20%). These evidences indicated that the movement of soil organic carbon from topsoil to subsoil of rice paddy was higher than those of tea field.
Effect of biochar products on rice and tea yield
The data in Table 8 are the average yield of fresh and dried leaf tea. The results showed that the yield of tea of the treatments (T2&T3) applied biochar products + NPK was higher (p < 0.05) than that of the control (T1). In fact, the tea applied biochar-based compost + NPK (T3) had highest yield (13.90 tons/ha) which was 48% higher compared to the control (T1). Particularly,when applying 2 tons biochar/ha +80% NPK (T2, reducing 20% NPK), the tea yield (fresh) obtained 12.65 ± 0.39 tons/ha, giving an increase of 33 % compared to the control (T1). Thus, although in this study, we did not analyze the OC content in tea, but with the conversion formula 1C = 3.67 CO2 of IPCC - Intergovernmental Panel on Climate Change (Penman et al, 2013), the application of biochar products also increased the carbon storage in tea crop through photosynthesis.
The positive effects of biochar products were also observed for rice (Table 9). The results indicated that the treaments (T2 & T3) applied biocar products combined with NPK had the higher yield increase (p < 0.05) from 11 to 17% compared with the control (T1). Thus, the effect of biochar products on rice was lower than that on tea.
The rice straw yield had the same trend. In facts, the rice straw yield of T2 (2 tons biochar + 80% NPK) and T3 (10 tons compost + NPK) was higher (p<0,05) than that of T1. Accoding to Cuong et al (2014), total OC of rice straw was 43.37%, and thus total OC of T2 & T3 incresed from 0.27 to 0.41 tons/ha compared with the control (T1). According to Penman et al (2003), with 1C= 3.67 CO2, the CO2 captured in rice straw of T2 & T3 increased by 0.99 to 1.50 tons/ha compared with T1.
The application of biochar products improved soil quality, increased carbon sequestration in the soils of tea and rice fields. For tea field, the organic carbon sequestration of the topsoil (0-20cm) and the subsoil (20-40 cm) was similar. Meanwhile, the higher sequestration was observed for the subsoil compared with the topsoil of rice paddy. In addition, the biochar products had positive effect on crop yield, especially tea crop.
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