**4.3. SOR rates affecting C‐CO2 emission and soil organic matter pools in incubated soil**

The mean C‐CO2 flux rate of incubated soils with SOR applications varied from 0.30 to 2.79 Mg ha−1 at the lowest rate (0 Mg ha−1) at the beginning of the incubation and at the highest SOR rate (16 Mg ha−1) at the end of the incubation process (**Figure 1**). At the highest SOR rate, there was an increase in the C‐CO<sup>2</sup> flux equivalent to seven times compared with the beginning of the process (0.45 Mg ha−1 C‐CO2).

**Figure 1.** C‐CO<sup>2</sup> flux in incubated soils with different rates of SOR. T1 = Control (0 Mg ha−1 SOR); T2 = 1 Mg ha−1 SOR; T3 = 2 Mg ha−1; T4 = 4 Mg ha−1; T5 = 8 Mg ha−1; T6 = 16 Mg ha−1. Source: Romaniw [60].

The mean C‐CO<sup>2</sup> fluxes observed due to the increasing SOR applications of 0, 1, 2, 4, 8 and 16 Mg ha−1 were 10.3, 12.5, 12.6, 13.0, 15.0 and 22.4 kg ha−1, respectively. Therefore, only the highest SOR rate of 16 Mg ha−1 is out of the ideal range of 9.8–19.5 kg ha−1 as evaluated by the Soil Quality Kit test in long‐term experiments [55].

With the increase in SOR rates in incubated soils, we could observe a more pronounced effect 60 days after applications (**Figure 1**), which indicates that the CO2 emission rates of microbial biomass decrease as C starts being fixed in the soil.

After the initial increase on day 45, in general, CO<sup>2</sup> emissions among the treatments tended to be similar to the control soils. After 60 days of incubation, all treatments started emitting a similar amount of CO2. Such evolution was also observed by Sánchez‐Monedero et al. [56] in an incubation experiment with composted sewage sludge at different stabilization degrees.

**4.3. SOR rates affecting C‐CO2 emission and soil organic matter pools in incubated soil**

the process (0.45 Mg ha−1 C‐CO2).

232 Organic Fertilizers - From Basic Concepts to Applied Outcomes

The mean C‐CO2 flux rate of incubated soils with SOR applications varied from 0.30 to 2.79 Mg ha−1 at the lowest rate (0 Mg ha−1) at the beginning of the incubation and at the highest SOR rate (16 Mg ha−1) at the end of the incubation process (**Figure 1**). At the highest SOR rate, there was an increase in the C‐CO<sup>2</sup> flux equivalent to seven times compared with the beginning of

**Figure 1.** C‐CO<sup>2</sup> flux in incubated soils with different rates of SOR. T1 = Control (0 Mg ha−1 SOR); T2 = 1 Mg ha−1 SOR; T3

The mean C‐CO<sup>2</sup> fluxes observed due to the increasing SOR applications of 0, 1, 2, 4, 8 and 16 Mg ha−1 were 10.3, 12.5, 12.6, 13.0, 15.0 and 22.4 kg ha−1, respectively. Therefore, only the highest SOR rate of 16 Mg ha−1 is out of the ideal range of 9.8–19.5 kg ha−1 as evaluated by the Soil

With the increase in SOR rates in incubated soils, we could observe a more pronounced effect 60 days after applications (**Figure 1**), which indicates that the CO2 emission rates of microbial

After the initial increase on day 45, in general, CO<sup>2</sup> emissions among the treatments tended to be similar to the control soils. After 60 days of incubation, all treatments started emitting a

= 2 Mg ha−1; T4 = 4 Mg ha−1; T5 = 8 Mg ha−1; T6 = 16 Mg ha−1. Source: Romaniw [60].

Quality Kit test in long‐term experiments [55].

biomass decrease as C starts being fixed in the soil.

According to **Figure 1**, there is a tendency for stabilization of the C‐CO<sup>2</sup> emissions after 80 days of incubation. This fact may be related to the availability of substrate for microbial activity as reported by Campbell et al. [57]. The balanced fertilization with SORs as a source of labile carbon supports the microbial activity, resulting in increases in C‐CO2 emissions.

The increase in SOR rates resulted in a linear tendency with the C‐CO2 flux (**Figure 2**). This tendency is probably related to the SOR C:N ratio and structure, which provides higher surface contact with soil particles. These factors allied to ideal conditions of humidity and temperature increase the microbial activity, leading to higher C‐CO<sup>2</sup> emission rates [58]. The increase in C‐ CO2 emission with higher SOR rates at the end of the incubation period is probably related to the fast soil microbiota growth and the decomposition of higher organic material amounts. This fact indicates that such higher SOR rates could cause a higher liberation of organic materials in the soil, which easily decompose due to temperature and humidity conditions.

**Figure 2.** Accumulated C‐CO2 emissions affected by SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1 after 125 days of incuba‐ tion. Source: Romaniw [60].

Although the increase in SOR rates resulted in higher C‐CO<sup>2</sup> emissions, linear increases in the TOC were observed (*P* < 0.05) (**Figure 3**), indicating its influence over soil carbon mineraliza‐ tion. The fast mineralization at the beginning of the incubation process is mainly related to the amount of labile carbon available. As the decomposition process begins, the influence of labile fraction lessens due to its easy degradation [58, 59]. In general, all samples with SOR applica‐ tion presented higher TOC contents compared with the control.

**Figure 3.** TOC content affected by SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1 after 125 days of incubation. Source: Roma‐ niw [60].

The C‐HW content decreased at the SOR rate of 16 Mg ha−1. The high SOR rate possibly caused a reduction in soil aeration, leading to lower microbial activity and carbon mineralization (C‐ HW) (**Figure 4**).

**Figure 4.** C‐HW content affected by SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1 after 125 days of incubation. Source: Roma‐ niw [60].

The SOR rate of 8 Mg ha−1 provided increases in C‐HW and C‐OXP (**Figures 4** and **5**), mainly because of the high microbial activity due to the availability of labile carbon. The proportions between labile and recalcitrant fractions differ in the fertilizer that presents a higher concen‐ tration of soluble fraction and that with lower fiber contents [61]. The differences in biochemical composition can alter the structure of microbial biomass and affect its efficiency in C use, resulting in differences in C mineralization of different organic sources.

**Figure 5.** POXC content affected by SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1 after 125 days of incubation. Source: Roma‐ niw [60].

**Figure 3.** TOC content affected by SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1 after 125 days of incubation. Source: Roma‐

The C‐HW content decreased at the SOR rate of 16 Mg ha−1. The high SOR rate possibly caused a reduction in soil aeration, leading to lower microbial activity and carbon mineralization (C‐

**Figure 4.** C‐HW content affected by SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1 after 125 days of incubation. Source: Roma‐

The SOR rate of 8 Mg ha−1 provided increases in C‐HW and C‐OXP (**Figures 4** and **5**), mainly because of the high microbial activity due to the availability of labile carbon. The proportions between labile and recalcitrant fractions differ in the fertilizer that presents a higher concen‐ tration of soluble fraction and that with lower fiber contents [61]. The differences in biochemical

niw [60].

niw [60].

HW) (**Figure 4**).

234 Organic Fertilizers - From Basic Concepts to Applied Outcomes

The C‐CO<sup>2</sup> flux, when related to the TOC stock, expressed the amount of C‐CO<sup>2</sup> lost by each Mg ha−1 TOC produced according to the SOR rates applied (**Figure 6**). This parameter is a sensitive indicator of the environmental changes that can occur due the increasing SOR applications. It can be used to detect disturbances, reflecting the increase in C‐CO2 emission. The TOC stock was greater with the increase in SOR rates, which indicates higher potential for C‐CO2 sequestration.

**Figure 6.** Relationship between C‐CO<sup>2</sup> flux and TOC stock in incubated soils for 125 days with SOR rates of 0, 1, 2, 4, 8 and 16 Mg ha−1. Source: Romaniw [60].

The increase in C‐CO2 emissions with SOR addition produced an initial increment and variability in TOC (**Figure 6**). This variability suggests disturbance in the microorganisms' activity through the SOR addition. The SOR applications provided accumulated emissions of 1.28, 1.56, 1.58, 1.63, 1.88 and 2.79 Mg ha−1 C‐CO<sup>2</sup> and fixations of 0.24, 1.52, 1.36, 1.92, 1.92 and 3.04 Mg ha−1 TOC (reduced values from the initial TOC) in the soil after the 125‐day incubation period. These results indicate that, although there is a pronounced flux of C‐CO2 with higher SOR applications, the TOC levels also increased. The TOC fixation was higher than the C‐ CO2 flux for the 4, 8 and 16 Mg ha−1 rates.

Therefore, SOR application can be considered a promising strategy in order to provide soil C sequestration, affecting directly the quality and productivity of the system.
