since both values increase significantly from 2010 to 2011. In both
years, the abundance of earthworms varied significantly among the
treatments (N0 < N300 < PS45) (Table 6). This agrees with previous
reports showing that slurry treatments have long-term beneficial
effects on earthworms [60], although the NH4
þ
eN content may be
toxic to earthworms in the shorter term [61].
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 volatilization
[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 indicators
of soil quality, given that the abundance, activity and diversity
of soil microbes are closely linked to the OM content [64].