Soil Phosphorus Forms and Contents As Determined By 31P-NMR after 28 Years of Different Mineral P Fertilization Under Continuous Corn Production.

Soil phosphorus forms and contents as determined by 31P-NMR after 28 years of different mineral P fertilization under continuous corn production Abdi, D.a, Cade-Menun, B. J.b, Ziadi, Na., and Morel, C.c,d aAgriculture and Agri-Food Canada, Québec, QC, Canada; bAgriculture and Agri-Food Canada, Swift Current, SK, Canada; cINRA, Villenave d’Ornon, France; dBordeaux Sciences Agro, Gradignan, France. Phosphorus (P) fertilization is required to sustain P availability to plants and productivity. The quantitative roles of soil inorganic and organic P forms are still unclear. The reference technique to characterize soil P forms is 31P nuclear magnetic resonance (31P-NMR) spectroscopy. Our objective was to assess the effects of long-term (28 yr) mineral P fertilization on soil P forms as determined by 31P-NMR. The study was carried out on a long term-term experimental site (1972-2000) located on an acidic, sandy soil in south-west France. Three P rates of triple superphosphate (averaging 0, 44, and 96 kg P ha-1 yr-1) were applied to continuous cultivated corn (Zea mays L.) since 1972, with four replicate plots. Soil samples were collected in spring 2000 at the 0-25 cm profile, analysed for phosphate ions concentration in soil solution (Cp), total P (TP) and total organic P (TOP) contents, and extracted in NaOH-EDTA solution before 31P-NMR spectroscopy. Results indicate that both TP and Cp greatly varied. The Cp ranged from 2.60 (±0.26) mg L-1 for 96 kg P ha-1 yr-1 to 0.44 (±0.13) mg L-1 without applying P, and varied strictly proportionally to cumulative difference between applied P and exported P in corn grains (y= 0.95×10-3x + 1.11, r²=0.96, 12 observations). In contrast, TOP did not vary significantly between P treatments. Indeed, the mean of soil TOP content was 112 (±18) mg P kg-1, which represented 39% of TP in the 0 kg P ha-1 yr-1 treatment, and only 19% of TP with the addition of 96 kg P ha-1 yr-1. The most abundant P forms were orthophosphate, myo-IHP and scyllo-IHP. Other results will be presented and discussed.