8.5 SOIL QUALITY ASSESSED BY CARBON

8.5 SOIL QUALITY ASSESSED BY CARBON MANAGEMENT INDEX: CASE STUDY OF A SUBTROPICAL ACRISOL IN BRAZIL SUBJECTED TO TILLAGE SYSTEMS AND IRRIGATION
Carbon management index (CMI), originally proposed by Blair et al. (1995), can be used to assess soil quality based on information related to soil organic C dynamics. This index expresses soil quality in terms of increments in total C content and in the proportion of labile C fraction compared to a reference soil, generally that under NV, which arbitrarily has a CMI = 100. In the proposal of Blair et al. (1995), labile C fraction was considered as that oxidised with 333 mM KMnO4 treatment, but recent reports have proposed the particulate organic matter isolated through physical fractionation based either on densimetric (Diekow et al., 2005a; Vieira et al., 2007) or granulometric approaches (Skjemstad et al., 2006) as the labile fraction to estimate the CMI.
The light fraction of SOM is basically constituted by partially decomposed plant, animal, and fungi residues (Gregorich et al., 1994), and therefore it is referred to as being a labile fraction sensitive to changes in soil management regime rather than the whole SOM pool (Gregorich et al., 1994; Freixo et al., 2002).
Several studies have shown the signiicant inluence of the soil tillage system on particulate organic matter (Cambardella and Elliott, 1992; Bayer et al., 2002; Freixo et al., 2002), so that higher stocks and concentrations of this fraction were found in NT than in conventionally tilled soils, because of the lower soil disturbance and decomposition rate due to NT management (Balesdent et al., 2000). On the other hand, irrigation by increasing the water availability in soil may possibly stimulate soil microbial activity and thus increase the decomposition of the labile organic matter fraction. However, this is only a hypothesis and the relationship between irrigation and light organic matter dynamics, not being suficiently covered by literature, has yet to be better clariied, particularly for tropical and subtropical soils subjected to different tillage systems.
De Bona et al. (2008) evaluated the inluence of sprinkler irrigation on soil quality of a southern Brazilian sandy loam Paleudult subjected to CT and NT for 8 years. According to the authors, total C stock, and thus the carbon pool index (CPI), in the 0–200 mm layer were affected neither by tillage system nor by irrigation. On the other hand, the concentration of labile C—and thus the C lability and lability index (LI)—were lower in CT than in NT, as well as in irrigated than in nonirrigated systems (Table 8.7). The effect of irrigation in decreasing the C lability was more pronounced in NT than in CT soil. A combination of residue accumulation and higher water availability on NT soil surface had probably provided suitable conditions to increase the microbial mineralization activity on the light fraction of the organic matter. The results of CMI, whose variations were caused mainly by LI, indicate that soil quality was improved with adoption of NT in substitution to CT, but not with adoption of irrigation (Table 8.7).
De Bona et al. (2008) concluded that the labile C fraction is more sensitive to the inluence of tillage systems and irrigation systems than the total C stock, so that CT and irrigation signiicantly reduced the labile C stock in comparison to NT and nonirrigated systems. The authors also reported that soil quality based on CMI was improved with the adoption of NT in substitution to CT, but was reduced with the adoption of irrigation, possibly due to increase in the decomposition rate of labile organic matter. Thus, NT soils subjected to irrigation require a higher biomass addition compared to nonirrigated soils (De Bona et al., 2008).