Chemical Nature and Diversity of Phosphorus in New Zealand Pasture Soils Using 31P Nuclear Magnetic Resonance Spectroscopy and Sequential Fractionation

Abstract

Information on the phosphorus (P) forms of pasture soils is central to agricultural and environmental sustainability. We used a combination of 31 P nuclear magnetic resonance (NMR) spectroscopy of NaOH-EDTA extracts and sequential fractionation to investigate P forms, with an emphasis on organic P in relation to environmental and biophysical variables, in 24 diverse pasture soils taken from around New Zealand (organic C 19-102 g kg À1 , total P 116-2746 mg kg À1 , pH 5.2-7.0). Soils were separated by cluster analysis of soil physicochemical properties and sequentially extracted P pools into those either derived from volcanic-ash materials or not. This separation was also evident for organic P species in NaOH-EDTA extracts, which removed about 75% of total soil organic P. The major organic P compound class was monoesters (24-60% of extracted P), made up of 14 to 91% myo-inositol hexakisphosphate. The next largest organic P class was diesters (0-4% of P extracted), which were enriched in volcanic-ash soils (monoester to diester ratio = 14) compared to non volcanic-ash soils (ratio = 30). Correlation analysis indicated that mean annual temperature had a significant negative and positive effect on monoester and diester concentrations, respectively. This was attributed to better physical protection of monoesters (especially inositol phosphates) from mineralization, and increased diesters from microbial activity and biological turnover. The anomalous behaviour of volcanic-ash soils was attributed to slow mineralization and the sampling of soils at different times of year without full knowledge of the large and highly dynamic microbial biomass P pool which is predominantly diesters.

Phosphorus Distribution in Soils from Australian Dairy and Beef Rearing Pastoral Systems

“…Pasture soils tend to contain a higher proportion of organic P than cultivated soils [9]. Some of the "available" organic P forms play important role in biological cycling of P in soils, however the dynamics of organic P are not completely understood and require further investigation [10,11].…”

“…First identified in 1940 [13,14], phytate (C 6 H 6 Ca 6 O 24 P 6 as a calcium salt) has been considered the abundant organic P form [6,11,15], representing up to 50% of all soil organic P [6,16,17]. However, an investigation by Smernik and Dougherty [18] involving phytate spiking, has indicated that this organic P may have been previously overestimated due to misassignment of resonances in 31 P NMR spectra.…”

“…The use of 31 P NMR spectroscopy enables quantification of many phosphate compounds, including inorganic orthophosphate (PO 4 3´) , polyphosphate (P n O 3n+1 pn`2q´) and pyrophosphate (P 2 O 7 4´) , organic orthophosphate monoesters (RPO 4 2´) and diesters (R 2 PO 4´) , as well as some specific organic P compounds [8,11,25,26]. Of the organic compounds, monoesters are usually the principal form found in soil [6].…”

“…The addition of EDTA enables better characterisation of organic P than possible in NaOH extractions [25]. Due to the relative ease of the method, 31 P NMR analysis of NaOH-EDTA extracts has been widely accepted as the preferred technique for organic P measurement [8,11,18].…”

Matrix based fertilizers reduce nitrogen and phosphorus leaching in three soils

“…Fertilization on the same soils for years will result in N and P sorption on clay and organic matter ionic exchange sites (D'Angelo, 2005;McDowell et al, 2005;Bird et al, 2002Bird et al, , 2003Devevre and Horwath, 2001). Fertilizer N and P will also have become incorporated into organic matter complexes which will result in these nutrients being released as organic matter is decomposed (Bird et al, 2003(Bird et al, , 2002Entry and Emmingham, 1995) .…”