Phosphorus dynamics and characterization under long-term rotation trial

The direct addition of organic matter (OM) to soils generally has a favorable influence on phosphorus (P) dynamics and availability. However, the influence of indirect accumulation of OM through cropping systems on soil properties has been rarely considered. Crop rotations, especially those involving legumes with cereals, can have a differential effect on soil OM over the long term. In a study in a semi-arid rainfed area of northern Syria, which is typical of the Mediterranean area of West Asia and North Africa (WANA) region, we assessed the indirect effect on soil P dynamics of common cereal-based long-term rotations. As the soil at the site (Chromic Calcixerert) was uniformly treated with the same P rate, treatment differences were assumed to be due to rotation/management effects. The treatments effects were evaluated by determining the total and organic P distribution patterns in the soil profile, by measuring NaHCO3-available P after different incubation periods (3, 6, and 9 weeks) and temperatures (0, 15, and 25°C), and by estimating adsorption and desorption parameters from isotherms. Crop sequences affected total P distribution only in the wheat/medic rotation. Application of 90kg nitrogen (N) ha-1 increased organic P compared to plots receiving no N. Only in the wheat/medic rotation did heavy grazing intensity cause significant differences in total and organic P compared to medium or no grazing. Mineralization of P was significantly decreased by wettingdrying, but not by temperature and mineralization time. Langmuir adsorption maxima varied significantly with rotation sequence. However, fallow, which had lowest percentage OM (1.09%) had consistently lower values, while medic with highest OM levels (1.32%) generally had the highest adsorption maxima. Desorption data (Elovitch b values) significantly varied with rotations, but tended to be the inverse of the adsorption maxima data. Some observed trends could be explained by changes in the OM status caused by the rotation treatments, but the type of OM and its P content from the roots and residues of the crops may have had an influence. The high clay and calcium carbonate contents of soils of the region probably mask any possible effects of the normally small amounts of OM on P dynamics.