The mineralizable N content reflects the amount of biologically
active N in the soil [48], i.e., the N that can be progressively
mineralized by microorganisms and thus made available to
growing crops. The mineralizable N content increased significantly
when fertilizer was applied, particularly PS45. This contrasts with
the higher residual soil NO3
eN content in the N300 plots, which
increases the risk of leaching. This combination of abundant
mineralizable N but scarce mineral N suggests the N cycle is more
tightly coupled to plant requirements in the PS45 plots, with
concomitant positive effects on soil fertility and the environment.
Similar results were reported in previous studies comparing
organic and mineral fertilizers for maize production [49].
CO2 emissions and phosphatase activity were measured as
indicators of general and specific soil microbial activity, respectively.
Microbial phosphatases play a central role in the mineralization
of P by catalyzing the hydrolysis of esterified phosphoric acid
and releasing phosphate that can be taken up by microbes and
plants [50]. Microbial activity was higher in the N300 and PS45
plots than the N0 plots in both years, but especially in 2011 when CO2 emissions increased by >60% and phosphatase activity by
>40%. This is consistent with previous studies showing that the
continuous addition of manure had a significant impact on basal
respiration [22] and soil biological activity in general [51]. In
contrast, Aira et al. [52] found that the application of PS reduced
microbial biomass and microbial activity in the short term. It was
found no significant difference between the two fertilizer treatments
for these parameters, perhaps due to the heterogeneity of
replicate soil samples (Table 6). Phosphatase activity can be
repressed by a feedback mechanism induced by the reaction
product phosphate, as shown when microbes are transferred from
phosphate-free to phosphate-supplemented medium [53]. Chunderova
and Zubeta [54] showed that after 4 years of cropping, high
phosphate concentrations at field testing sites inhibited microbial phosphatase activity. However, no such effects were observed in
N300 or PS45 plots even after 8 years of continuous applications,
and others have published similar results [55]. Indeed the treated
plots showed a higher phosphatase activity than control plots,
probably due to the presence of phosphatase in maize roots. High
levels of soluble inorganic phosphate are probably required to
inhibit phosphatase activity in the soil
The mineralizable N content reflects the amount of biologicallyactive N in the soil [48], i.e., the N that can be progressivelymineralized by microorganisms and thus made available togrowing crops. The mineralizable N content increased significantlywhen fertilizer was applied, particularly PS45. This contrasts withthe higher residual soil NO3eN content in the N300 plots, whichincreases the risk of leaching. This combination of abundantmineralizable N but scarce mineral N suggests the N cycle is moretightly coupled to plant requirements in the PS45 plots, withconcomitant positive effects on soil fertility and the environment.Similar results were reported in previous studies comparingorganic and mineral fertilizers for maize production [49].CO2 emissions and phosphatase activity were measured asindicators of general and specific soil microbial activity, respectively.Microbial phosphatases play a central role in the mineralizationof P by catalyzing the hydrolysis of esterified phosphoric acidand releasing phosphate that can be taken up by microbes andplants [50]. Microbial activity was higher in the N300 and PS45plots than the N0 plots in both years, but especially in 2011 when CO2 emissions increased by >60% and phosphatase activity by>40%. This is consistent with previous studies showing that thecontinuous addition of manure had a significant impact on basalrespiration [22] and soil biological activity in general [51]. Incontrast, Aira et al. [52] found that the application of PS reducedmicrobial biomass and microbial activity in the short term. It wasfound no significant difference between the two fertilizer treatmentsfor these parameters, perhaps due to the heterogeneity ofreplicate soil samples (Table 6). Phosphatase activity can berepressed by a feedback mechanism induced by the reactionproduct phosphate, as shown when microbes are transferred fromphosphate-free to phosphate-supplemented medium [53]. Chunderovaand Zubeta [54] showed that after 4 years of cropping, highphosphate concentrations at field testing sites inhibited microbial phosphatase activity. However, no such effects were observed inN300 or PS45 plots even after 8 years of continuous applications,and others have published similar results [55]. Indeed the treatedplots showed a higher phosphatase activity than control plots,probably due to the presence of phosphatase in maize roots. Highlevels of soluble inorganic phosphate are probably required toinhibit phosphatase activity in the soil
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