Mineralisation of soil orthophosphate monoesters under pine seedlings and ryegrass.
Continued advances in our understanding of the chemical nature and associated dynamics of soil phosphorus (P) are important to ensure that agricultural and forest ecosystems are managed in a sustainable manner. Soil organic P represents 15-80% of the total soil E mostly derived from plant residues and microbially mediated processes (Stewart and Tiessen 1987).
Plant species vary in their ability to mobilise organic P in soils (Jungk 1996; Taranto et al. 2000; Chen et al. 2002) and thereby influence the nature and distribution of soil P (Nielsen and Dalsgaard 1987; Magid 1993; Chen et al. 2002; Solomon et al. 2002). Recently, a direct link between forest vegetation and soil organic matter composition has been reported (Quideau et al. 2001). In New Zealand the effects of recent widespread conversion from grassland to plantation forestry [predominately radiata pine (Pinus radiata D. Don.)] on soil fertility and nutrient dynamics have been extensively investigated. Results from field studies clearly demonstrated that afforestation of grassland resulted in significant mineralisation of soil organic P and concomitant increases in plant-available inorganic P concentrations (e.g. Condron et al. 1996; Chen et al. 2000). Furthermore, results from a short-term (10 month) glasshouse pot experiment on 15 New Zealand grassland soils confirmed that mineralisation of soil organic P was consistently and significantly greater under radiata pine seedlings compared with ryegrass (Chen et al. 2003). However, in such experiments specific information on the forms of organic P involved is scarce, mainly due to the analytical difficulties involved in soil P speciation. Indeed, more than 50% remains unidentified in many soils (Magid et al. 1996).
Various chemical extraction techniques have been developed for characterising soil P in natural and managed ecosystems (Cross and Schlesinger 1995; Magid et al. 1996; Frossard et al. 2000), but these provide no structural information on soil organic R In the past 2 decades, solution [sup.31]p nuclear magnetic resonance (NMR) spectroscopy has been used to characterise the effects of climate, land-use change, cultivation, and long-term fertilisation on soil P chemistry (e.g. Newman and Tate 1980; Condron et al. 1985, 1990; Cade-Menun and Preston 1996; Turrion et al. 2001; Cade-Menun et al. 2002; Solomon et al. 2002). The quantitative determination of functional P groups, including inorganic orthophosphate, orthophosphate monoesters, orthophosphate diesters, phosphonates, and condensed inorganic and organic phosphates, is well established (Cade-Menun and Preston 1996; Turner et al. 2003b).
The objective of this study was to examine the specific effect of radiata pine seedlings and ryegrass (Lolium perenne L.) on mineralisation of various forms of soil organic E to improve our understanding of the relationship between chemical form and bioavailability. This involved using solution [sup.31]p NMR spectroscopy to identify different forms of organic P in selected soils from the glasshouse pot trial described by Chen et al. (2003).
Materials and methods
Soil samples and glasshouse experiment
Details of soils used and the glasshouse experiment were described by Chen et al. (2003). In brief, 15 surface soil samples (0-7.5 cm) under grassland were collected from around New Zealand, air-dried (25[degrees]C), and sieved (4 mm) before the glasshouse experiment. Two plant species, radiata pine (Breed 'GF12') and perennial ryegrass (cultivar 'Grasslands Nui'), were used in this experiment. Seeds were directly sown into the pots and each species was thinned to 5 per pot after germination. The trial included 3 replicates of each plant species for each soil. All pots were placed on a capillary mat in the glasshouse in a completely randomised design, and soil moisture contents were maintained at c. 70% field capacity. Radiata pine seedlings were inoculated with mycorrhizae (Rhizopogon rubescens Tul.) at a rate of 1 x [10.sup.7] spores per pot, applied in a water suspension, 2 weeks after sowing. No nutrients were added to the pots during the experiment. The average daily temperature during the experiment ranged from 12 to 25[degrees]C.
Seven soils with relatively high proportions of organic P were selected for inclusion in this investigation. These included soils from the North Island (Te Kauwhata, Stratford, Patoka, Mangamahu) and the South Island (Hurunui, Temuka, Pukaki) (Table 1, from Chen et al. 2003). Soil pH values were 5.2 6.5, and concentration of organic carbon (C), total nitrogen (N), and total P was 39.3-93.4, 3.1-8.5, and 0.40-2.75 g/kg, respectively. Organic P comprised 43-81% of the total P. Concentrations of dithionite-extractable iron (Fe) and aluminium (Al) were 5.4-19.3 and 2.0-12.2 g/kg soil, respectively. Oxalate-extractable Fe and Al concentrations were 0.5-7.4 and 1.0-15.8 g/kg soil, respectively. Clay content varied from 40 to 310 g/kg soil, and P sorption index (PSI) values ranged between 3.1 and 59.4 (mg 100/g)/([micro]mol/L).
Initial soils before planting and soils sampled after growth of ryegrass or radiata pine were air-dried (25[degrees]C) and sieved ([less than or equal to] 2 mm) before NaOH EDTA extraction. Solution [sup.31]p NMR spectroscopy was performed on composite soils bulked from 3 replicates of the same treatments. The NaOH-EDTA extraction followed the method described by Cade-Menun and Preston (1996), whereby 5.0 g of air-dried soil was extracted with 100 mL of a solution containing 0.25 M NaOH and 0.05 M EDTA for 16 h at room temperature. Samples were filtered through Whatman 41 papers and subsamples taken for analysis of inorganic P, organic P, and total P by molybdate colourimetry (Tiessen and Moir 1993). The remaining extracts were frozen and lyophilised for NMR analysis.
Solution [sup.31]p NMR spectroscopy
Freeze-dried NaOH-EDTA extracts (approx. 200 mg) were re-dissolved in 0.9 mL of 1 M NaOH and 0.1 mL [D.sub.2]O (for signal lock) and transferred to 5-mm diameter NMR tubes. The addition of NaOH ensures consistent chemical shifts and optimum spectral resolution at a solution pH > 12. Solution [sup.31]p NMR spectra were obtained using a Bruker AMX 600 spectrometer operating at 243 MHz. We used a 30[degrees] pulse width, a total acquisition time of 1.5 s (pulse delay 0.808 s, acquisition time 0.672 s), and broadband proton decoupling. Temperature was regulated at 24[degrees]C, although it is now recommended that temperature should be standardised at 20[degrees]C (Cade-Menun et al. 2002; Turner et al. 2003b). The number of scans collected (2000-10000) depended on the P concentration of the freeze-dried extracts. Compounds were identified by their chemical shift relative to 85% orthophosphoric acid after Lorentzian convolution with a width of 10 Hz. We used the deconvolution process of the Bruker WinNMR program to determine chemical shift and area of individual signals, which were identified from literature reports (Turner et al. 2003b), with general functional classes of P compounds as follows: phosphonates around 19 ppm, inorganic orthophosphate at approximately 6.1 ppm, orthophosphate monoesters at 3-6 ppm, orthophosphate diesters between -0.5 and 2.0 ppm, pyrophosphate at approximately -4 ppm, inorganic polyphosphates around -20 ppm. Within the orthophosphate diesters, signals between 0.6 and 2.0 ppm were assigned to phospholipids and alkali-labile nucleic acids, whereas the signal at approximately 0 ppm was assigned to DNA (Makarov et al. 2002; Turner et al. 2003b). Signals appearing between -0.5 and -2.0 ppm were defined as unknown P (Makarov et al. 1997). Phosphorus functional groups were expressed as a percentage of total signal area and concentrations in the NaOH EDTA extracts were calculated on the basis of the total P in the extracts measured by molybdate colourimetry (see above).
Signals at 5.85, 4.92, 4.55, and 4.43 ppm in the ratio 1:2:2:1 were identified by spectral deconvolution and assigned to myo-inositol hexakisphosphate (I[P.sub.6]) (Turner et al. 2003d). The proportion of the spectral area under these 4 signals was then multiplied by the total P concentration in the NaOH EDTA extract to give the concentration of I[P.sub.6] in the soil (mg P/kg dry soil).
Phosphorus in the NaOH-EDTA extracts
Extraction with NaOH-EDTA recovered 46-86% of total soil P, of which organic P (determined by molybdate colourimetry) constituted 34-85% (Table 2). Extracts of pine soils contained more NaOH-EDTA-extractable inorganic P (88-1034 mg/kg), but less NaOH-EDTA-extractable organic P (218-681 mg/kg), than the grass soils (63-931 mg/kg inorganic P and 232-715 mg/kg organic P), despite more P being taken up by pine seedlings (Chen et al. 2003) (Table 2).
Soil phosphorus composition determined by solution [sup.31]p NMR spectroscopy of NaOH-EDTA extracts
Representative solution [sup.31]p NMR spectra of NaOH EDTA extracts of the initial soil, and soils under grass and pine (Pukaki, Mangamahu, and Stratford) are shown in Fig. 1. The relative proportions of inorganic P and organic P in the NaOH-EDTA extracts varied among soils (Tables 2 and 3). Orthophosphate was the dominant inorganic compound, with concentrations between 121 and 1098 mg/kg (27-74% total extractable P). There were smaller concentrations of pyrophosphate (<18 mg/kg), and polyphosphate (4 mg/kg) was detected only in Mangamahu soil after planting with radiata pine (Table 3). In general, concentrations of inorganic P determined by [sup.31]p NMR were greater than by molybdate colourimetry.
[FIGURE 1 OMITTED]
Organic P compounds included orthophosphate monoesters, orthophosphate diesters, and phosphonates (Fig. 1, Table 3). Orthophosphate monoesters were the dominant P species in the NaOH-EDTA extracts, accounting for 24-64% of the total P (Table 3), and 80-97% of the total NaOH-EDTA-extracted organic P (data not shown). Of these, concentrations of I[P.sub.6] ranged between 17 and 142 mg/kg, representing 11-50% of the orthophosphate monoesters (Table 4). Concentrations of I[P.sub.6] increased as the orthophosphate monoesters increased (r = 0.870, P < 0.01). Concentrations of orthophosphate diesters ranged between 1 and 116 mg/kg (<19% total extracted organic P) (Table 3). Of these, the phospholipids constituted 28-100%, and DNA constituted up to 72%. Small concentrations of phosphonates (<13 mg/kg) were detected in some soils, and a trace (1 mg/kg) of an unknown P compound was detected in the initial Manganmahu soil (Table 3).
Influence of plant species on soil phosphorus composition
Concentrations of orthophosphate monoesters consistently decreased following growth of pine (103-480 mg/kg soil) compared with the initial soil (142-598 mg/kg) and soils under grass (122-679 mg/kg) (except the Hurunui soil) (Table 3). In addition, the relative proportion of orthophosphate monoesters was generally smaller in soil under pine than in initial and grass soils (except the Hurunui soil) (Table 3). In soils under ryegrass, concentrations of orthophosphate monoesters decreased in Mangamahu, Hurunui, Te Kauwhata and Patoka soils, but increased in Pukaki, Temuka, and Stratford soils, compared with the initial soils. Clearly, mineralisation of orthophosphate monoesters induced by ryegrass varied among soils.
Concentrations of I[P.sub.6] were 7-48 mg P/kg lower in soils under pine than in most initial soils (except the Patoka soil) and those under grass (except Patoka and Mangamahu soils). These decreases accounted for 18-100% of mineralised orthophosphate monoesters (Table 5). Ryegrass induced limited mineralisation of I[P.sub.6] (3-20 mg P/kg) in 4 soils (Mangamahu, Hurunui, Temuka, Stratford soils) (Tables 4 and 5).
Concentrations of orthophosphate diesters showed no clear trend among plant species within a soil (Tables 3). For example, phospholipid concentrations decreased in the Mangamahu, Pukaki, and Temuka soils after growth of radiata pine compared with the initial soils, but accumulated in the Hurunui, Te Kauwhata, Patoka, and Stratford soils (Table 3).
The ratio of orthophosphate monoesters to orthophosphate diesters has been used as an index of mineralisation capability (Condron et al. 1990; Turrion et al. 2001), but varied widely among soils in the current study (4-286) and showed no clear trend in response to plant growth (Table 3).
The predominance of orthophosphate monoesters in these soils is consistent with reports of the organic P composition of world soils (e.g. Newman and Tate 1980; Condron et al. 1985; Turri6n et al. 2001; Turner et al. 2003e). Orthophosphate monoesters include inositol phosphates, sugar phosphates, and mononucleotides (Condron et al. 1990). Of these, the inositol phosphates are most stable, being protected from enzymatic attack by strong complexation with soil minerals, clays, and humic compounds (Turner et al. 2002). This suggests that soil I[P.sub.6] is relatively unavailable for uptake by plants, and although some studies have reported that plants could use I[P.sub.6] amended in solution or soil culture, there are few reports of uptake of I[P.sub.6] from soil (e.g. Martin and Cartwright 1971; Shibata and Yano 2003). However, we detected consistent decreases in orthophosphate monoesters and I[P.sub.6] in soil under pine seedlings, which clearly indicated the utilisation of such compounds by these plants. Mineralisation of orthophosphate monoesters and I[P.sub.6] by ryegrass was observed in some soils (e.g. Mangamahu and Hurunui soils), but was more limited than under pine (Tables 4 and 5).
Variability in the utilisation of I[P.sub.6] by pine seedlings and ryegrass among soils, is almost certainly explained by differences in soil properties, which are known to exert marked controls on the biological availability of inositol phosphates. For example, Martin and Cartwright (1971) demonstrated that I[P.sub.6] was more available to plants grown in a weak P-fixing soil compared with a high P-fixing soil. Organic P derived from microbial and plant sources is chemically and biochemically modified and stabilised differently in different soil matrices. For example, I[P.sub.6] may have been less biologically available in the more acidic soils due to the formation of insoluble Fe or Al complexes (Jackman and Black 1951). Indeed decreases in I[P.sub.6] concentrations following growth of radiata pine were significantly correlated with the original level of the dithionite and oxalate-extractable Al in soil (r = -0.709 and -0.660, P < 0.05), and no mineralisation of I[P.sub.6] occurred in the Patoka soil (for both pine seedlings and ryegrass) that contained a high concentration of oxalate and dithionite-extractable A1. It is unlikely that ryegrass synthesises significant quantities of I[P.sub.6] so accumulation of orthophosphate monoesters (e.g. Temuka and Stratford soils under ryegrass) and I[P.sub.6] under ryegrass (e.g. in the Pukaki and Te Kauwhata soils) probably reflects either differences in extraction efficiency, or possibly the synthesis of I[P.sub.6] by microbes (Caldwell and Black 1958).
The mechanisms involved in mineralisation of soil orthophosphate and I[P.sub.6] by pine seedlings remain unknown. Differences in utilisation of organic P by ryegrass and pine seedlings may be linked to different types of mycorrhizal associations, because ryegrass is associated with vesicular arbuscular mycorrhizae (VAM) (Powell 1977), whereas roots of radiata pine seedlings are associated with ectomycorrhizae (ECM) (Chu-Chou and Grace 1979). In addition to more efficient P uptake by ECM-associated roots compared with VAM-associated roots (Marschner and Dell 1994), ECM hyphae have been found to produce phosphatase and phytase enzymes and low molecular weight organic acids, which may enhance the solubility and enzymatic hydrolysis of recalcitrant organic P in soil (e.g. Jones et al. 1998; Yadav and Tarafdar 2003). Chen et al. (2003) showed that phosphatase activity of ectomycorrhizal roots of radiata pine seedlings were consistently and significantly higher than that of ryegrass roots, and concentrations of soluble organic C in pine soils was also significantly greater than in grass soil. We hypothesise that these account for the greater utilisation of orthophosphate monoesters and I[P.sub.6] by pine seedlings compared with ryegrass.
Although the results from this study provide direct evidence that radiata pine seedlings are better able to utilise orthophosphate monoesters (including I[P.sub.6]) than ryegrass, it is acknowledged that the results from this 10-month glasshouse experiment can only reflect the difference in utilisation of orthophosphate monoesters and I[P.sub.6] among plant species at the early stages of growth and development, particularly for radiata pine. Data from field studies have also demonstrated that concentrations of orthophosphate monoesters were lower in soil under pine plantations (up to 30 years old) compared with adjacent grassland (Condron et al. 1996), but it is difficult to extrapolate the results from this study to long-term effects of radiata pine trees on soil in the field. In fact, it is nearly impossible to investigate the effects of pure plant species on soil in the field due to the different management and existence or invasion of other plant species (e.g. understorey).
Orthophosphate diesters originate from plant and microbial sources (Makarov et al. 2002) and are considered relatively labile in temperate soils. The low charge density and shielded phosphate groups mean that they sorb only weakly in the soil and are, therefore, accessible to microbial and enzymatic attack (Magid et al. 1996). Solomon et al. (2002) found that concentrations of orthophosphate diesters were greater in soils under natural forest (24-27% of the total extracted P) compared with adjacent plantation forest (13-15%) and a cultivated field (9-10%). Orthophosphate diesters can accumulate in the clay fraction of soil (Amelung et al. 2001) and under waterlogged conditions (Mahieu et al. 2000). Makarov et al. (2002) reported that DNA accumulated in acid soils under cool conditions, whereas phospholipids and teichoic acids accumulated in soils with higher microbial activity. Concentrations of DNA are also strongly correlated with soil microbial biomass in temperate pasture soils (Turner et al. 2003c). The inconsistent effects of plant species on the levels of orthophosphate diesters among soils in the present study probably reflect the interaction of plant species and soil type, and might be related to a combination of factors including various soil mineral matrix and the diverse microbial activities induced by plant species.
This study showed a much wider range of orthophosphate monoester to diester ratios than those typically reported in the literature. Turrion et al. (2001) suggested that soil organic matter quality and rainfall affect microbial activity and the orthophosphate monoester to diester ratio, and Turner et al. (2003a) used temporal changes in this ratio to detect seasonal mineralisation of orthophosphate diesters in upland soils of northern England. Solomon et al. (2002) indicated an increase in the orthophosphate monoester to diester ratio after land-use change from the natural forest to plantation forest and crop land, showing that more orthophosphate diesters had been depleted in response to land-use change. However, we detected no significant relationships between the orthophosphate monoester to diester ratio and soil pH, soil C:N ratio, and microbial activity (as C[O.sub.2] respiration) across soils (data not shown), suggesting that the ratio may not be a good indicator of changes in quality of soil organic P in the short term.
Our results confirm that radiata pine enhances the mineralisation of soil organic P compared with ryegrass (Chen et al. 2003). In particular, orthophosphate monoesters, including I[P.sub.6], were mineralised by radiata pine, whereas the capability for ryegrass to utilise them was limited. Mineralisation varied among soils, almost certainly due to differences in soil properties that influence the degree of stabilisation of inositol phosphates. Clearly, inositol phosphates cannot be regarded as 'unavailable' or 'recalcitrant' organic P in all soils, although further studies are required to elucidate the precise mechanisms involved in the stabilisation and turnover of inositol phosphates in soil.
Table 1. Chemical and physical properties determined for the selected New Zealand grassland soils before planting (Chen et al. 2003) PSI, P sorption index Soil Series USDA Soil pH Organic C Total N Classification (g/kg) (g/kg) Mangamahu Dystrochrept 5.3 39.3 3.8 Pukaki Dystrochrept 5.2 49.1 3.1 Hurunui Dystrochrept 5.6 78.7 6.7 Te Kauwhata Humult 5.7 50.0 4.8 Temuka Aquept 6.5 39.6 3.8 Patoka Udand 5.7 93.4 8.5 Stratford Udand 5.4 67.8 7.0 Soil Series Total P Organic P C/P (mg/kg) (mg/kg) (A) Mangamahu 402 315 (78) 98 Pukaki 663 534 (81) 74 Hurunui 904 670 (74) 87 Te Kauwhata 938 606 (66) 53 Temuka 1056 509 (48) 38 Patoka 1585 1051 (66) 59 Stratford 2746 1166 (43) 25 Dithionite Oxalate Clay Silt (g/kg) (g/kg) (%) (%) Fe Al Fe Al Mangamahu 9.8 3.4 6.3 2.5 18 40 Pukaki 5.4 2.4 2.6 2.8 12 28 Hurunui 12.1 2.7 2.5 1.9 23 42 Te Kauwhata 19.3 4.1 5.9 2.9 26 38 Temuka 4.1 2.0 2.8 1.2 21 40 Patoka 8.5 12.2 4.7 15.8 12 29 Stratford 9.0 7.9 7.4 14.0 11 27 Sand PSI (%) (mg 100/g)/ ([micro]mol/L) Mangamahu 42 32.0 Pukaki 60 20.8 Hurunui 35 14.2 Te Kauwhata 36 18.8 Temuka 39 3.7 Patoka 59 59.4 Stratford 62 48.2 (A) Data in parentheses are percentage of soil organic P over total P Table 2. Soil total P contents (mg/kg) and NaOH-EDTA-extractable P (mg/kg) in the selected New Zealand grassland soils before (I) and after planting with ryegrass (G) and radiata pine seedlings (P) NaOH--EDTA extractable P Soil series Sample Total Total P (A) Inorganic (B) soil P Mangamahu I 402 298 (74) 60 (20) G 377 295 (78) 63 (21) P 357 306 (86) 88 (29) Pukaki I 663 422 (64) 62 (15) G 652 441 (68) 67 (15) P 596 397 (67) 121 (30) Hurunui I 904 623 (69) 198 (32) G 814 498 (61) 136 (27) P 795 516 (65) 190 (37) Te Kauwhata I 938 633 (68) 275 (43) G 893 559 (63) 218 (39) P 853 522 (61) 272 (52) Temuka I 1056 626 (59) 306 (49) G 919 593 (65) 272 (46) P 958 565 (59) 371 (66) Patoka I 1585 749 (47) 409 (55) G 1482 677 (46) 328 (48) P 1422 687 (48) 384 (56) Stratford I 2746 1655 (60) 946 (57) G 2607 1645 (63) 931 (57) P 2616 1715 (66) 1034 (60) NaOH--EDTA extractable P Soil series Sample Organic P (B) Mangamahu I 238 (80) G 232 (79) P 218 (71) Pukaki I 360 (85) G 375 (85) P 276 (70) Hurunui I 425 (68) G 362 (73) P 327 (63) Te Kauwhata I 359 (57) G 342 (61) P 250 (48) Temuka I 320 (51) G 322 (54) P 194 (34) Patoka I 340 (45) G 348 (52) P 303 (44) Stratford I 708 (43) G 715 (43) P 681 (40) (A) Data in parentheses are the proportion (%) of the total soil P. (B) Data in parentheses are the proportion (%) of the NaOH-EDTA extractable P. Table 3. Concentrations of P compounds (mg/kg) in the NaOH-EDTA-extracts as determined by [sup.31]P NMR spectroscopy Data in parentheses are the proportion (%) of the total extracted P calculated from total signal area. Trace amounts (1 mg/kg) of unknown P compounds were found in initial Manganmahu soil only; polyphosphate (4 mg/kg) was found only in Mangamahu soil after planting with radiata pine Soil series Sample (A) Inorganic P Orthophosphate Pyrophosphate Mangamahu I 133 (45) 6 (2) G 146 (50) 9 (3) P 187 (61) 2 (1) Pukaki I 131 (31) 7 (2) G 121 (27) 10 (2) P 174 (44) 9 (2) Hurunui I 285 (46) 14 (2) G 222 (45) 13 (3) P 201 (39) 11 (2) Te Kauwhata I 402 (64) 13 (<1) G 329 (59) 14 (3) P 347 (66) 9 (2) Temuka I 448 (72) n.d. G 362 (61) 12 (2) P 416 (74) 8 (1) Patoka I 340 (45) 9 (1) G 301 (45) 11 (2) P 319 (47) l0 (2) Strafford I 970 (59) 16 (1) G 880 (54) 18 (1) P 1098 (64) 18 (1) Soil series Sample (A) Organic P Orthophosphate Phospholipids monoester Mangamahu I 142 (48) 9 (3) G 122 (41) 4 (1) P 103 (34) 4 (l) Pukaki I 270 (64) 9 (2) G 283 (64) 19 (4) P 204 (51) 2 (0.5) Hurunui I 297 (48) 3 (1) G 241 (48) 6 (1) P 251 (49) 30 (6) Te Kauwhata I 209 (33) 2 (<1) G 199 (36) 6 (1) P 153 (29) 5 (1) Temuka I 169 (27) 7 (<1) G 200 (34) 13 (2) P 136 (24) 1 (<1) Patoka I 358 (48) 13 (2) G 327 (48) 9 (1) P 318 (46) 16 (2) Strafford I 598 (36) 31 (2) G 679 (41) 40 (2) P 480 (28) 68 (4) Soil series Sample (A) Organic P Monoester-P/ DNA Phosphonate diester-P ratio (B) Mangamahu I 3 (1) 4 (1) 12 G 3 (1) 12 (4) 19 P 3 (1) 3 (1) 15 Pukaki I 3 (1) 3 (1) 24 G 8 (2) n.d. 11 P 2 (<1) 6 (2) 51 Hurunui I 15 (2) 7 (1) 16 G 8 (2) 9 (2) 18 P 17 (3) 6 (1) 5 Te Kauwhata I 4 (<1) 3 (1) 35 G 8 (1) 4 (1) 15 P 3 (1) 4 (1) 18 Temuka I 2 (0.4) n.d. 19 G 6 (1) n.d. 11 P n.d. 5 (<1) 286 Patoka I 23 (3) 6 (1) 10 G 23 (3) 6 (1) 10 P 20 (3) 4 (1) 9 Strafford I 28 (2) 13 (1) 10 G 28 (2) n.d. 10 P 48 (2.8) 3 (0.2) 4 n.d., Not detected. (A) I, initial soil before planting; G, ryegrass; R radiata pine. (B) The ratio of orthophosphate monoesters to the sum of phospholipids and DNA. Table 4. Concentrations (mg P/kg soil) of myo-inositol hexakisphosphate determined by spectral deconvolution and solution [sup.31]P NMR spectroscopy Values in parentheses are the proportion (%) of the total extracted P Soil series Initial soil Ryegrass Pine seedlings Mangamahu 38 (13) 20 (7) 31 (10) Pukaki 131 (31) 142 (32) 83 (21) Hurunui 107 (17) 87 (18) 60 (12) Te Kauwhata 27 (4) 39 (7) 17 (3) Temuka 53 (8) 50 (9) 37 (7) Patoka 72 (10) 78 (11) 88 (13) Stratford 124 (8) 106 (7) 77 (5) Table 5. Mineralisation (-) or accumulation (+) of myo-inositol hexakisphosphate (I[P.sub.6]) in soil over the 10-month growth of ryegrass or radiata pine Soil series Sample (A) I[P.sub.6] mineralised (-) or RP (B) accumulated (+) (mg P/kg) % Mangamahu G -19 93 P -7 19 Pukaki G 11 n.a. P -48 73 Hurunui G -20 36 P -47 104 Te Kauwhata G 12 n.a. P -10 18 Temuka G -3 n.a. P -16 47 Patoka G 6 n.a. P 16 n.a. Stratford G -18 n.a. P -47 40 n.a., Not applicable. (A) G, soils after planting ryegrass; P, radiata pine. (B) Relative proportion of the amount of I[P.sub.6] mineralised over orthophosphate monoester mineralised in the 10 months of growth.
The New Zealand Forest Research Institute and Lincoln University provided funding for this study. The Bruker AMX 600 NMR spectrometer at Queen Mary College, University of London, was provided by the University of London Intercollegiate Research Scheme.
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Manuscript received 7 February 2003, accepted 12 December 2003
C. R. Chen (A,G), L. M. Condron (B), B. L. Turner(C), N. Mahieu (D), M. R. Davis (E), Z. H. Xu (A,F), and R. R. Sherlock (B)
(A) Co-operative Research Centre for Sustainable Production Forestry, Faculty of Environmental Sciences, Griffith University, Nathan, Qld 4111, Australia.
(B) Soil, Plant and Ecological Sciences, PO Box 84, Lincoln University, Canterbury 8150, New Zealand.
(C) Soil and Water Science Department, University of Florida, 106 Newell Hall, PO Box 110510, Gainesville, FL 32611, USA.
(D) Department of Chemistry, Queen Mary College, University of London, London E1 4NS, UK.
(E) New Zealand Forest Research Institute, PO Box 29237, Fendalton, Christchurch, New Zealand.
(F) Queensland Forestry Research Institute, PO Box 631, Indooroopilly, Qld 4068, Australia.
(G) Corresponding author; email: email@example.com