Phosphorus Dynamic in the Soil-Plant System ... 37content in the Pi labile fraction was lower between tillage systems, a loss of
poorly available forms occurred. Tillage practices may facilitate incorporation
of the most resistant forms into the most dynamic cycle of soil P (Galantini &
Rosell, 1997; Suñer et al., 2014). These results coincide with other research
findings in that surface buildup of residues under conservation systems
increases nutrients with little mobility such as P in comparison with
conventional tillage (Franzluebbers et al., 1995; Holanda et al., 1998; Varsa &
Ebelhar, 1999; Thompson & Whitney, 2000). However, some authors have
reported a P-increase in the 15-22.5 cm depth as well (Rhoton, 2000; Motta et
al., 2002) caused by higher root growth and/or a lower rate of OM
mineralization. In terms of the annual changes of P-forms in the A-horizon
between tillage systems, the differences were about 3.5, 1.7 and 9.4 kg ha-^1
year-^1 for Po, Pi and Pt, respectively. The change in poorly available forms was
4.2 kg ha-^1 year-^1 , which indicates that such amount is being converted from
the most resistant P-forms to the most labile ones; even so, Pi stock decreased
by 1.7 kg ha-^1. When considering the average change in Pi per year and per
soil cm, a sharp decline (from 0.6 to 0.01 kg Pi ha-^1 year-^1 cm-^1 ) was observed
form the upper to the lower part of the A-horizon. Most Pi loss is likely to be
take place in the 0-1 (65%) and 1-5 (22%) cm depths (Suñer, 2015).
The average P-export was 10.9 and 9.4 kg ha-^1 year-^1 under NT and CT,
respectively (Suñer, 2015). The P-residue content incorporated to the organic
fraction and recycling ranged from 3.5 and 3 kg ha-^1 year-^1 under NT and CT,
respectively. Even though the amount of recycled P was higher under NT, the
P-export was also high; thereby increasing fertilizer needs to maintain a
positive input-output balance. However, the negative balance of P by erosion
under CT was much higher than the positive balance by an increase in grain
P-export under NT. If the mean difference of the soil lost by erosion is
compared with the P lost by erosion, then P-concentration in the weathered
soil was 684 μg g-^1 , which is above the soil loss average. Other studies
reported similar results (Hepper, et al. 1996; Michelena, Irurtia, 1995;
Vázquez, 2002). Finally, the most important differences between tillage
systems were found in the amount (kg ha-^1 ) of P-forms, not in their
concentration (μg g-^1 ), as a result of changes in bulk density.
Phosphorus balance under both tillage systems is negative; under NT
because of a higher P-export with harvest, and under CT by erosion of P-rich
particles, particularly in the superficial 5 cm depth of the A-horizon. Losses
are more significant under CT.