Finally, it is noted with interest that the repeated application of PS at a dose equivalent to 32 tons ha1 of OM over 8 years was not sufficient to increase the OM content of the soil above that of the N300 plots (Table 6). This contrasts with the results of previous studies, albeit conducted under different environmental conditions [24,49,62]. One potential explanation is that the N richness of PS45 reduces the amount of OM that must be applied to achieve N fertilization requirements (Table 2). Alternatively, the lability of organic substrates in PS (and its low C/N ratio) may facilitate the mineralization of OM by microorganisms and subsequent volatili- zation [12,63]. This may explain why the highest CO2 flux was detected in the PS45 plots (Table 6). Our hot climate favors the loss of organic matter by accelerating microbial activity, thus increasing its oxidation and volatilization. Finally, the higher yield in the N300 plots increases soil carbon inputs from crop residues and root exudates, but the high C/N ratio of crop residues reduces the decomposition rate leading to an increase in the OM content. These combined factors may account for the similarity between the PS45 and N300 plots in terms of OM content and other biological indi- cators of soil quality, given that the abundance, activity and diver- sity of soil microbes are closely linked to the OM content [64].