40 Liliana Suñer and Juan Galantini
In another cropping systems with no tillage initiated in 1998, where three
of the sequences were exclusively crops, other two were mixed different P
forms were analyzed (Suñer, 2015). The sequence with pasture produced a
reduction in the forms of available P and in both organic and inorganic labile
forms. Also in this sequence a significant increase was observed in the ocluido
organic P in the microaggregates (extractable with NaOH, PoNa), which was
favored possibly by the higher biological activity in the cultivated pasture
(Wick et al., 1998). Although it was expected that P from fertilizers would
affect moderately labiles Pi fractions (PiNa, Buehler et al., 2002). An increase
in the Pe was observed in sequences with crops; this finding would be in
agreement with the fact that at lower pH the divalent form of HPO 42 - , which is
the most assailable form of P for the plants, starts to appear (Holford, 1997).
The fraction of moderately labile organic P (PoNa), was the predominant
fraction in all the treatments and depths. The mixed assays reduced the
proportion of Pi in relation to Po. This decline is much higher in the treatment
with pastures, where the amount of Po significantly increases (moderately
labile Po). In other studies (Suñer et al., 2014) the authors stablished that during
the period with pasture, available P decreased with respect to accumulation of
organic forms. This is possibly associated with the increase of organic matter
that is observed during the cattle breeding period. The labile Po was lower in
cattle breeding systems and higher in agricultural systems.
Regarding the PoNa dynamic, the three sequences: one with pasture and
two with agriculture represent the most important PoNa pool of the
managements analyzed. Taking into account that these three management
sequences presented high values for PoNa and, that from these values,
sequence with pasture showed lower values for Pe, it could be infer that there
is a difference regarding the chemical quality of the molecule, which would
maintain the levels of available P lower in the sequence with pasture. The
extract P-NaOH of this cropping systems were analyzed by Infrared
spectroscopy and monoester phosphate monoester species could be one of the
dominant species in sequence with pasture; this finding is in agreement with
the proposal that the monoester are the most resistant organic species to
mineralization (Guggenberger et al., 1996). This could explain the fact that the
management sequence with pasture does not release P to the soil solution,
maintaining P levels lower.