Spatial distribution of soil phosphorus fractions in a clayey Oxisol submitted to long-term phosphate fertilization strategies

Due to the strong interaction of phosphorus (P) with soil constituents, P fertilizer placement can significantly affect how crops take up this nutrient. Nonetheless, few studies address the spatial distribution of P at the row-interrow scale according to management strategies. In a 16-yr no-tillage (NT) field experiment involving two different P fertilizer application methods (broadcast or band application) and two P sources (triple superphosphate – TSP or reactive Gafsa phosphate rock-RPR), plus a control treatment, the spatial distribution of P fractions was assessed in two occasions: after the 8th and the 16th crops. This was done vertically to a depth of 30 cm and horizontally from the crop row to the center of the interrows. Broadcast treatments showed total and Mehlich-1 P accumulation at the soil surface while for band application this accumulation was in the crop row region. A small P movement down the soil profile was observed from the 8th to the 16th crop with broadcast application, whereas with band fertilizer this effect was more noticeable, showing increased soil volume under P fertilizer influence even without soil tillage; it is likely that this was partly due to biological P turnover and application at depth. After 16 crops, the soil volume under the influence of P fertilizer was greater under band application while the volume above Mehlich-1 P critical levels for the 0–20 cm layer in regional studies was higher under broadcast application, independent of the P source. Soil organic carbon (SOC) contents were not affected by P placement or source. However, a significant accumulation of SOC was seen from the soil surface downwards after the last eight crops. The spatial distribution of P and SOC was better correlated under broadcast treatments, with high values for both variables at the soil surface; this may explain similar yields to those obtained with band application, where P fertilizer is placed near the main roots in an attempt to reduce P adsorption to the soil solid phase.