**5. Biochar for soil fertility**

*Applications of Biochar for Environmental Safety*

explain differences in overall soil CH4 [10].

*Soil pH of different treatments. Data replotted from [24].*

emissions.

from denitrification by increasing the air content of the soil or by absorbing water from the soil, thus improving aeration of the soil [18]. Biochar amendment may increase soil pH (**Figure 4**) when applied to soil [11]. Changes in soil pH may result in changes in nitrifier or denitrifier enzymatic activity and therefore soil N2O

There is limited evidence, mostly from studies in rice paddies to suggest that biochar amendment affects soil CH4 emissions [20]. The greater uptake of methane may be attributed to the protected environment created for the CH4 oxidizers and improved soil porosity. The aerobic, well-drained soils can be a sink for CH4 due to the possible high rate of CH4 diffusion and ensuing oxidation by methanotrophs. Improved soil physical properties such as low bulk density and the associated increase in total porosity, mostly due to the relative increase in macroporosity [21], may significantly decrease CH4 emissions. Increased availability of labile C substrates following biochar addition stimulates the activities of methanogenic bacteria that may account for increased CH4 emissions [22]. However, this could be a short-term effect since labile carbon fraction in biochar could be mineralized rapidly. Karhu et al. [23] observed increased soil CH4 consumption in arable soil due to increased soil aeration following biochar application. Biochar addition to soil has been assumed to increase soil temperature and soil pH. However, the effect of biochar on soil temperature and soil pH has not been suggested as mechanisms to

Carbon dioxide is produced mainly from the decomposition of plant residues and organic matter by soil microbes and respiration from microbes and roots. Carbon dioxide can be divided into autotrophic and heterotrophic respiration based on different biological sources [10]. The effect of biochar on carbon emissions in soil are very complex and changes in CO2 emissions can be a response of diverse mechanisms. Biochar amendment affects CO2 emissions by changing the physical and chemical characteristics of the soil and of the microbial diversity [10]. CO2 emission could be reduced through the effect of biochar application on C-mineralizing enzymes [25]. Some studies have shown that biochar addition could stimulate the mineralization of soil organic carbon (SOC) [11, 12] and correspondingly increase emissions of CO2. However, conversely, the suppression of SOC mineralization has also been reported [25], thereby causing a decrease in CO2 emissions. Biochar application could also stimulate CO2 emission by enhancing soil properties [10]. As indicated in Refs. [16, 22], transforming carbon in plant residues into stable

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**Figure 4.**

The decline of soil fertility is a major problem confronting crop production and environmental sustainability. The functions of soil depend on three main properties, physical, chemical, and biological, which influence global cycles of organic C and N [26]. The adoption of sound soil management strategies such as appropriate tillage methods, crop residues practices, biochar application, and efficient N fertilization has been suggested to improve soil properties [27]. These strategies can be achieved by increased input of crop residues while minimizing C loses by erosion, decomposition, and carbon emission. While conservation agriculture systems have been noted to improve soil organic C [27], conventional plow-based farming systems could accelerate carbon mineralization and thus reduce soil C content, which are attributed to soil aggregates disruption and increased oxidization through soil disturbances [28]. The incorporation of biochar into soil varies soil structure, porosity, and bulk density. According to Oguntunde et al. [29], this may in turn have consequences for important soil functions such as soil aeration and plant growth. In Ref. [30], it is postulated that biochar application results in an increase in soil C.

The expectation of increased soil fertility attributed to biochar application emanated from the studies of the terra preta that contains high proportions of black carbon [29]. The high soil organic matter content of the terra preta provides the evidence of the enhancement due to biochar application. In contrast to mainstream chemical fertilizer, biochar also contains bioavailable elements such as selenium that has potential to assist in enhancing crop growth. It is not clear concerning the potential effects of biochar on microbial activity in soil. Assuming that plant inputs and hence microbial substrate remain unchanged, enhanced microbial activity alone would diminish soil organic matter. However, this is contrary to the observation in terra preta, where soil organic matter is generally higher than in similar surrounding soil [26]. However, a change in the balance of microbial activity between different functional groups could benefit crop nutrition, specifically enhancement of mycorrhizal fungi [11], and this could feedback into higher net primary productivity and carbon input.

There are several reasons why biochar might be expected to decrease the potential for nutrient leaching in soils, and thus enhance nutrient cycling and also protect against leaching loss. In field studies where positive yield response to biochar application has been observed [7, 13], enhanced nutrient dynamics could be the reason for the observed results.
