**4. Discussion**

Compared with the original soil (control on day 0), the pH value of the fertilized soil decreased from 6.8 to 6.1, which could be attributed to the acidic properties of the two fertilizers, namely (NH4)2SO4 and Ca(H2PO4)2. The pH value of the control decreased to 4.6 on day 42, and those of the WB300- and WB600-treated samples were considerably higher (Table 3). The results indicate that both WB300 and WB600 could alleviate soil acidification. When biochar undergoes pyrolysis at a higher temperature, it generally has a higher pH [42]. Singh et al. [43] revealed that the CaCO3 equivalence of biochar increased with the pyrolysis temperature. Accordingly, the application of biochar could engender liming effects. For biochar, a higher application rate or higher pyrolysis temperature could increase the pH or alkalinity of the biochar-treated soil [11]. The alleviation effects of biochar on soil

acidification could contribute to the retardation of the movement of several nutrients and pollutants from soils to groundwater and lower water bodies.

Applying biochar could decrease the *D*<sup>B</sup> of a soil sample [13]. However, this phenomenon was not observed in this study; a possible reason is that the reorganization of the soil structure for mixing the materials and packing the soil columns neutralized the soil properties. A similar condition can be achieved in the field through intensive tilling. For a less- disturbed sandy loam soil, the application of biochar could improve the soil structure, increase the infiltration rate, and reduce runoff water and soil erosion, thus improving soil and water conservation [7,13,41]. The volumes of leachates increased with repeated leaching for all treatments in this study. Although the differences in leachate volume between the control and biochar-treated samples decreased gradually in this study, the biochar-treated samples still retained a significantly higher amount of water in the soil columns on day 42 (Table 4). The WB300 and WB600 treatments reduced the level of water loss by 9.2% and 13.7%, respectively. Our results demonstrate that the biochar materials, particularly WB600, exhibited a strong ability to conserve water in the soil samples when applied at a rate of 2%.

The OC contents in both of the biochars were much lower than that of other organic materials commonly used in farmland. The biochar applications displayed significantly affect the SOC in the WB300 treatment only in this study (Table 3). Accordingly, the effects of biochar on DOC leaching are the results of the sorption of organic carbon onto the biochar, either within the pores of the biochar or onto the external biochar surface [7]. The biochar-applied soils retained more water than the control (Figure 4), as illustrated earlier, which could also contribute to DOC retention since DOC move down and leach out from the soil column along with the soil water. Our results illustrated that the highest amounts of DOC and leachate were found in CK indicating biochar addition could effectively retain DOC in the soils. However, the highest DOC concentration (47.0 mg/L, DOC amount/leachate volume) was found in the leachate of WB300 treatment indicated that biochar might release solube C components into soil solution, particularly in biochar pyrolized with low temperatures [7,44]. The results implicated that soluable C components onto the biochar itself might increase DOC concentration in output water (runoff or eluate from soil pedon) duing rainfall events in the biochar-amended soils. The WB300 treatment revealed the efficiency of 9.2% for water retention and the lower efficiency of 6.5% for DOC retention (Figures 4 and 5). Under WB600 treatment, the retention efficiencies were 13.7% for water and a higher of 20.0% for DOC. This result indicates that WB600 had a stronger affinity to DOC than WB300. Kasozi et al. [45] reported that the organic matter sorption onto biochar surfaces is kinetically limited by slow diffusion into the subnanometer-sized pores dominating biochar surfaces. The various organo-mineral interactions lead to aggregations of soil and organic materials, which stabilizes both soil structure and the carbon compounds within the aggregates. Furthermore, the increase in the diversity and density of carbon groups within WB300 biochar may result in the slightly increased SOC but did not prevent the leaching of DOC as effective as the WB600 biochar.

Although the NH4 <sup>+</sup>-N and NO3 −-N concentrations in the soil columns were low on day 42 (Table 3), the WB300- and WB600-treated samples, especially the WB300-treated sample, exhibited higher inorganic N concentrations than the control did. These results could be attributed to the high surface area and diverse functional groups, such as carboxyl C, O-alkyl-C, and alkyl C, of WB300 (Figure 3). Obvious higher CEC was also fould in WB300 than in WB600 in this study indicating that more NH4 <sup>+</sup>-N and K could be retained in the soil treated with WB300, and which were demonstrated by our results in Figures 6 and 9 [46,47]. Addtionally, improvement in soil physical properties such as promotion of soil aggregation and increasing of water holding capacity might also bring positive effects in nutrient leaching. Yoo et al. [31] suggested that increasing formation of aggregates by biochar addition could effectively promoted retention of NO3 −. Furthermore, they also indicated that increased water holding capacity after biochar addition was also a factor to reduce N leaching.

Overall, both biochars can effectively reduce N leaching and provide a potential N source of nutrient delivery to plants [48]. Agricultural non-point source (NPS) pollution is the leading source of water quality impacts to rivers and lakes. Nitrogen from fertilizers, manure, waste and ammonia turns into nitrite and nitrate. High levels of these toxins deplete waters of oxygen, killing all of the animals and fish. Nitrates also soak into the ground and end up in drinking water. Health problems can occur as a result of this and they contribute to methemeglopbinemia or blue baby syndrome which causes death in infants. Base on our results, application biochar might be a useful managememt practice to reduce NPS pollution in watersheds, particularly in tropical/subtropical climate regions. In this study, we measured the N losses through leaching. The N losses through denitrification and ammonia volatilization, which may form NH3, N2, or N2O, were not accessed. As the soil was pH 6.8 and the soil columns were nearly water-saturated during the experiment, we used acidic fertilizer, (NH4)2SO4 and Ca(H2PO4)2 intentionally to minimize ammonium volatilization.

Applying the biochar materials engendered only a slight increase in the available P in the soil samples after the leaching experiment (Table 3). Both biochar materials, particularly WB600, could retain soil P, according to the leaching results (Figure 8). This effect also resulted in the higher concentrations of Ava. P in the soil samples after the experiment, as mentioned. P tends to precipitate with Fe under acidic conditions. However, the application of the biochar materials increased the soil pH, which could enhance the release of P. Therefore, the biochar materials were likely to have contained numerous bonding sites or other co-precipitation elements, resulting in considerably higher P retention efficiency levels compared with their water retention efficiency levels. During soil pH 4 to 6, increasing of soil pH might unlock P "adsorbed on soluble and hydrous Fe/Al oxides" into an available form, however, the unlocked P might be adsorbed again onto biochar to form a potential available form. Our results was consistence with the resuld of Laird et al. [3] who indicated that biochar addition could effectively reduce leaching of dissolved P in the soil column due to adsorption of orthophosphate and adsorption of organic P compounds by biochar.

The Ex. K concentration was higher in the biochar-treated soil samples, particularly the WB600-treated sample, than in the control at the beginning of the experiment; this could be attributed to the high mobility of K in plant ash. A higher pyrolysis temperature may result in a higher concentration of soluble K in biochar. However, the Ex. K concentrations did not differ significantly between the samples on day 42. Both biochar materials considerably reduced the total quantity of K leached from the soil samples. The efficiency levels of WB300 and WB600 in inhibiting K leaching were 29.7% and 7.7%, respectively. WB600 inhibited K leaching mainly through holding the soil solutions, and WB300 possibly had additional K sorption mechanisms mainly related to C functional groups.

Our results indicated that both WB300 and WB600 could effectively reduce the leaching of soil water, DOC, NH4 <sup>+</sup>-N, NO3 −-N, P, and K. After incubation, not only nutrient concentration but also leachate volume were found lower in biochar-treated soil compared with those in control (Table 4). Reharding the strong water retention capacity of the biochar-treated soils, two possible reasons were speculated, which were (1) high degree of evaporation (about 30 ◦C in average during summer in southern Taiwan) for the soil column after water adding; therefore, the biochar treatments might still effectively retain water after water addition; (2) the volume of adding water (700 mL) to the soil column at each date still did not match the pore volume (~2000 cm3) of soil column; therefore, the biochar treatments might still effectively retain water after water addition at day 7 and other dates. Nutrient leaching could be inhibited through increased water retention. Moreover, the inhibitory effect of the biochar materials on the mineralization of organic N, in terms of physical protection of organic matter [7,40], can reduce the quantity of nutrients released and thus reduce subsequent leaching. Our results reveal that WB300 exhibited a high NH4 <sup>+</sup>-N and K (predominately cations) retention efficiency, and WB600 exhibited a high water, DOC, and P (predominately anions) retention efficiency. These results indicate that WB300 was negatively charged, which was most likely due to the distribution of various carbon functional groups. Overall, the biochar-induced retention of soil water, DOC, and nutrients could be considered to positively affect nutrient and water conservation and to improve soil quality. Reducing the leaching of water, DOC, and nutrients from soils could conserve groundwater and connected water bodies. Therefore, biochar application can benefit both soil and

water conservation. Further research should be conducted to investigate whether these positive effects can be extended to the field and to downstream water bodies at the catchment level.
