31P NMR spectroscopy and structural models of soil organic phosphorus under Eucalyptus

Organic phosphorus (P o ) plays an important role in tropical forest nutrition on strongly-weathered soils. However, of its role in Eucalyptus nutrition little is known. For this purpose, soil P forms were determined by NaOH-Na 2 EDTA extraction and 31 P nuclear magnetic resonance spectroscopy ( 31 P NMR) at 10 sites of Eucalyptus plantations in Brazil. The objectives were (1) to determine the amounts and forms of soil P o ; (2) to build a structural model of the soil P cycle to provide quantitative estimates of the transformation processes of soil P. Soil P was found to consist of large amounts of Monoesters-P (on average 43.7 mg kg −1 ) followed by Diesters-P in the form of DNA (on average 4.2 mg kg −1 ), as well as containing high reserves of P i species (on average 101.6 mg kg −1 of the ortho -P; 2.7 mg kg −1 of the pyrophosphate). Thestructural model indicated that the P cycle is regulated by clay, soil organic carbon, total P and pH. High total P o concentrations (on average 48 mg kg −1 ) reveal a high potential of P supply to Eucalyptus trees, in which Diester-P (DNA) would be the main source P for the available P pool. The close interconnection between ortho -P, pyrophosphate, DNA and Monoester-P compounds provides evidence of a strong influence of soil biological activity on the distribution and accumulation of these P compounds and therefore on the availability of P. Thus, these results are relevant to establish soil fertility management strategies for low-fertilizer input Eucalyptus systems in soils with low fertility.