38 Liliana Suñer and Juan Galantini
Crop Rotation
Agriculture significantly affects P availability due to the extraction caused
by crop harvesting, transforming the low-solubility labile forms (Bra et al.,
2006, Papiernik et al., 2008) and modifying the equilibrium among the
different P forms (Suñer et al., 2014). A sustainable agriculture should
consider the nutrient balance in each specific site. However, in the case of P
the intensive interaction with the soil matrix does not facilitate prediction of P
balance in the availability of this nutrient in the agroecosystem. In this sense,
long term field trials constitute a useful tool to study the behavior and
transformation of P soil under contrasting conditions of agricultural rotations
and contribution of phosphate fertilizers (Suñer et al., 2014). Galantini et al.,
2007 found that tillage significantly increases Po mineralization and the
soluble P, which is directly related to microbial activity increase (Ohno et al.,
2005 ). McKenzie et al. (1992) found that the wheat (Triticum aestivum L.)-
fallow sequence was the most effective for Po mineralization. When a legume
crop was added to the cropping rotation, soil Pi values decreased (Echeverría
et al., 1993). The P compounds strongly bound to soil fine fraction have been
shown to be unaffected by cropping systems (Galantini & Rosell. 1997, Rosell
et al., 2000). Phosphorus in the coarse fraction, however, sharply decreases
due to cultivation. Results of these studies suggested that the greater
decomposition of the SOM in the coarse fraction facilitated availability of Po
and Pi for subsequent crops.
In soils of the Province of Buenos Aires, Argentina, three cropping
sequences after 12 years were analyzed: WG, one year wheat and one year
cattle grazing on natural grasses, WW, continuous wheat, WL, 3 years of
mixes grazed, and 3 years of winter crops (Suñer et al., 2014).
In that cropping systems soil organic and inorganic P concentrations
decreased under the different cropping sequences with a sharp decrease in the
Po:(Po+Pi) ratio. Inorganic P was the most abundant form in both reference and
cultivated soils. Soil Pi content was significantly lower in WW than in WG
and WL. The Pt concentration (extractable with perchloric acid) showed lower
concentrations in cultivated than reference soils and high variability among
treatments and years. The main differences in P fraction were observed
between reference and cultivated soils. The highest variability of Po than SOC
suggests quality variations during crop sequences (Duval et al., 2013).
However, total SOC, as well as total Po, include fraction with different quality
and dynamics, which individually will be considered. The effect of continuous
wheat with fertilizer application on SOC content was a consequence of annual