# Effects of Manure Waste Biochars in Mining Soils
## 4 Discussion
### CO\({}_{2}\) Emissions, qCO\({}_{2}\) Quotient, and GMea Index  
The CO\({}_{2}\) emissions of soil after the application of biochars depends on different factors. For example, [PERSON] et al. [21] identified diverse biochar properties that can have an influence on the soil carbon mineralization after biochar addition to soil, including labile carbon content, metal biochar content, or surface biochar properties. For the Zarandas-Andalusia area, the application of the four biochars increased the CO\({}_{2}\) emissions in Z1 and Z2 soils, being lower in soils treated by biochar prepared at lower temperatures due to their lower C-oxi and SOC. Moreover, the C sequestration in soil Z3 after the application of biochars BPL600 and BRM600 could be due to part of the CO\({}_{2}\) evolved being fixed on the surface area of biochars BPL600 and BRM600 (Table 1). For the Mijarojos-Cantabria area, in general, the application of biochar did not increase the CO\({}_{2}\) emissions, which could be associated with the reduction of the microorganism activity in this area after biochar addition (see GMea index in Figure 4). Finally, the increment of CO\({}_{2}\) emissions in the majority of the treatments from the Portman-Murcia area could be related to the factors mentioned above with the sandy texture of the soils in this area.  
qCO\({}_{2}\) is generally used to evaluate how the microbial biomass is using the available carbon for its biosynthesis in an efficient way [63]. According to [PERSON] et al. [64], high qCO\({}_{2}\) values are indicative of microbial populations diverting the energy destined for growth and production toward their maintenance and reparation of damages caused by disturbances in the environment. There was, in general, a reduction of the qCO\({}_{2}\) parameter in the area of Mijarojos-Zarandas due to the important pH variations in Zarandas soils after biochar addition, which could greatly disturb the environment soil microbial conditions. According to [PERSON] et al. [65], this may indicate three different situations: a possible degradation efficiency from the microbial community, a possible ability of biochar to protect microorganisms against disturbances or stress, and a possible low microbial activity due to the presence of recalcitrant carbon. Therefore, biochar amendments have different effects on the soil respiration and qCO\({}_{2}\) depending on the characteristics of soil and biochar used.  
GMea is an index that was used in previous works to measure changes in soil quality after biochar addition to soils [38; 66; 67]. In this study, GMea behavior was greatly variable depending on the soil characteristics and biochar added. Similarly, a previous study found a contrasting effect of P-rich biochars on GMea [68]. According to [PERSON] et al. [38], the reduction in GMea index after the addition of biochar could be due to the absorption of substrates and enzymes on the biochar surface. The most positive results of GMea index were found for three soils (P1, P2, and P3) of the Portman-Murcia area where GMea index increased after the addition of BPM450 and BPL450. This fact was probably due to the higher content in labile carbon according to both organic carbon (C-oxi) and soluble organic carbon (SOC) contents and P-Olsen of the biochars prepared at lower temperature [35].