31 P NMR Spectroscopy and Structural Models of Soil Organic Phosphorus under Eucalyptus

Organic phosphorus (Po) 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-Na2EDTA extraction and 31P nuclear magnetic resonance spectroscopy (31P NMR) at 10 sites of Eucalyptus plantations in Brazil. The objectives were (1) to determine the amounts and forms of soil Po; (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 Pi 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 Po 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.