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Forms of organic C and P extracted from tropical soils as assessed by liquid-state [sup.13]C- and [sup.31]P-NMR spectroscopy.

Introduction

Highly weathered soils of the tropics are usually depleted in P-containing primary minerals. Release of plant-available P by rock weathering is therefore limited (Sanchez 1976). In addition, tropical soils are generally rich in, Al and Fe hydrous oxides and consequently they tend to fix polyvalent oxyanions such as phosphate irreversibly due to chemisorption and occlusion (Sollins et al. 1988; Cross and Schlesinger 1995). Without fertilisation, the pool of plant-available P in highly weathered tropical soils is maintained primarily by the microbial-mediated mineralisation of plant and animal debris (Tiessen et al. 1984; Stewart and Tiessen 1987). Only a small part of organically bound P, about 1%, is mineralised in phosphorus-deficient soils per year (Harrison 1982), suggesting that only part of the P bound to soil organic matter (SOM) is plant-available in the short term.

The release of organically bound P is closely linked to the decomposition of SOM (Dalal 1979; Gressel et al. 1996). For temperate forest soils, Gressel et al. (1996) showed that changes in soil organic phosphorus (SOP) structures are closely related to changes in SOM composition. Yet, these studies were restricted to organic forest floor layers and topsoils. No information is available concerning the coupling of soil organic carbon (SOC) and SOP transformations in the mineral soil, especially subsoil horizons. Such information might be of special importance in the tropics where decomposition of SOM and subsequent release of P regulates the P supply to plants.

Liquid-state [sup.13]C- and [sup.31]P- nuclear magnetic resonance (NMR) spectroscopy obtained on alkaline extracts has emerged as a useful and common tool to characterise the chemical structures of SOM (Preston 1996; Conte et al. 1997) and to quantify directly broad classes of organic P and C species in soil (Robinson et al. 1998). [sup.31]P-NMR spectroscopy has recently proved its potential in reflecting the P status of soils and soil separates under different management and climatic conditions (Condron et al. 1990; Guggenberger et al. 1996a, 1996b; Sumann et al. 1998). As both NMR techniques can be run on the same soil extracts, these methods seem to be promising tools for investigating the coupling of SOP and SOC transformation. By means of liquid-state NMR spectroscopy, we are only able to characterise the composition of extractable SOM. For [sup.13]C-NMR, Frund and Ludemann (1989) showed that the alkali-soluble fractions of SOM are representative of the whole soil. In addition, it seems acceptable to compare alkaline-soluble fractions of C and P since these should have similar functions in soil (Gressel et al. 1996).

The objective of our study was to evaluate the correlations between SOC and SOP transformation during the decomposition of SOM under tropical conditions. For this purpose, we compared the [sup.13]C- and [sup.31]P-NMR signature of alkaline soil extracts obtained from a decomposition gradient within a given site (depth profile) and between sites (different management systems).

Material and methods

Site description

This study was carried out in the mountain region of Khun Sathan, Na Noi district, northern Thailand, at an elevation of 1350-1500 m with slopes up to 50% (Fig. 1). The mean annual temperature was 25 [degrees] C, and the average annual rainfall was about 1400 mm with a dry season from December to April. The geological parent materials in this area consisted of shale, sandstone, and siltstone. The site was divided into 4 plots: 20-year-old Pinus kesiya (Royle ex Gordon) reforestation, secondary forest (15 years old) mixed with a 15-year-old Pinus kesiya (Royle ex Gordon) reforestation, primary forest, and cabbage cultivation. The reforestation, the secondary forest, and the cabbage cultivation sites were cleared about 30 years ago and had been used for opium production. About 15-20 years ago, all 3 sites were reforested with Pinus. The cabbage cultivation was cleared again about 10 years ago with a few Pinus trees remaining and intensively used, with high inputs of pesticides and fertilisers (e.g. ammonium sulfate). The soils were derived from shale and were Dystric and Humic Cambisols on lower slopes and Skeletic Umbrisols and Lithic Leptosols on steeper sloping areas. Under the forest vegetation, organic layers covered the mineral soil surface. The forest floor beneath the primary forest was a mull type (Oi horizon only), whereas at the Pinus reforestation and the secondary forest a moder was found (additional Oe horizon). At each plot, one representative soil profile was selected based on a soil survey and sampled by horizons. All samples were air-dried and the mineral samples were sieved to [is less than] 2 mm.

[Figure 1 ILLUSTRATION OMITTED]

Soil characterisation

Texture analysis was conducted after HCl, hot [H.sub.2][O.sub.2], and ultrasonic treatment followed by sieving of sand and determination of silt and clay by the pipette method (Avery and Bascomb 1974). Actual cation exchange capacity ([CEC.sub.a]) was measured with 0.5 M [NH.sub.4]Cl (Truby and Aldinger 1989). The Ca, Mg, Na, K, Fe, Al, and Mn contents in the extracts were determined using atomic absorption spectroscopy (AA400, Varian Associates Inc., Palo Alto, CA). Soil pH was measured in 0.01 M Ca[Cl.sub.2] at a soil:solution ratio of 1:2.5 (w/v). Total C and N were analysed with a CHNS-analyser (Vario EL, Elementar Analysensysteme GmbH, Hanau, Germany). The soils were free of carbonates. Soil P was determined with a slightly modified sequential extraction according to Tiessen and Moir (1993). Resin P was discarded and the samples were extracted with 0.5 M Na[HCO.sub.3] followed by 0.1 M NaOH, 1 M HCl, and concentrated HCl; residual P was extracted with 0.5 M [H.sub.2][SO.sub.4] after heating at 650 [degrees] C for 5 h. Total P was determined in all extracts using an inductively coupled plasma-optical emission spectrometer (ICP-OES, Integra XMP, GBC Scientific Equipment Pty Ltd, Dandenong, Vic., Australia). The alkaline extracts were acidified to pH 1 with HCl and the flocculated organic matter was separated by centrifugation. In the supernatant, inorganic P was measured with the molybdate-blue method (Kuo 1996). The difference between total and inorganic P in the alkaline extracts represents SOP; the sum of all extracts including the residual P represents total P. Extraction of oxalate-soluble Al and Fe ([Al.sub.o] and [Fe.sub.o]) was carried out with acid 0.2 M ammonium oxalate at pH 3 (Schwertmann 1964). Oxalate-soluble Al and Fe were assumed to represent Al and Fe from humus complexes and short-range ordered mineral phases (Parfitt and Childs 1988). Dithionite-soluble Fe ([Fe.sub.d]) was determined by the sodium dithionite-citrate-bicarbonate method (Mehra and Jackson 1960) and includes Fe from more crystalline oxides and hydroxides (Parfitt and Childs 1988). The results of these analyses are shown in Table 1.

Table 1. Description of selected chemical properties of the investigated soil profiles

[CEC.sub.a], actual cation exchange capacity; [Al.sub.o], acid oxalate-extractable Al; [Fe.sub.o], acid oxalate-extractable Fe; 0C, organic cabon; TN, total N; TP, total P; OP, organic P
Horizon Depth pH [CEC.sub.a]
 (cm) (Ca[Cl.sub.2]) ([mmol.sub.c]/kg)

 Cabbage cultivation (Dystric cambisol)

A 0-20 4.6 68.1
AB 20-38 4.3 8.3
Bw1 38-70 4.6 7.1
Bw2 70-120+ 4.5 5.6

 Reforestation (Dystric Cambisol)

Oi 4-2 4.8 n.d.
Oe 2-0 5.0 n.d.
A 0-15 5.4 107.1
Bw 15-38 4.9 33.3
CB 3845 5.1 36.0
C 65+ 5.2 35.3

 Secondary forest (Skeletic Umbrisol)

Oi 1.5-0.5 5.4 n.d.
Oe 0.5-0 5.2 n.d.
A 0-23 5.1 109.8
CA 23-36 4.9 41.5
C 36+ 4.6 21.4

 Primary forest (Humic Cambisol)

Oi 2-0 5.4 n.d.
A 0-23 4.0 8.5
BA 23-40 4.3 2.8
Bw1 40432 4.3 2.1
Bw2 62-90 4.4 1.8
Bw3 90-112 4.3 1.8
Bw4 112-128 4.4 1.7
Bg1 128-155 4.4 1.6
Bg2 155+ 4.4 1.9

Horizon [Al.sub.o] [Fe.sub.o] OC TN
 (%)

 Cabbage cultivation (Dystric cambisol)

A 0.59 0.76 4.5 0.39
AB 0.58 0.78 2.0 0.20
Bw1 0.52 0.66 1.1 0.15
Bw2 0.81 0.73 0.7 0.13

 Reforestation (Dystric Cambisol)

Oi n.d. n.d. 46.5 1.20
Oe n.d. n.d. 36.5 1.32
A 0.30 0.58 4.1 0.38
Bw 0.17 0.33 1.0 0.14
CB 0.17 0.34 0.7 0.12
C 0.14 0.29 0.7 0.12

 Secondary forest (Skeletic Umbrisol)

Oi n.d. n.d. 42.5 1.38
Oe n.d. n.d. 34.6 1.05
A 0.30 0.68 4.8 0.40
CA 0.28 0.68 2.7 0.22
C 0.11 0.24 1.3 0.13

 Primary forest (Humic Cambisol)

Oi n.d. n.d. 45.8 1.73
A 0.56 1.18 6.7 0.50
BA 0.38 0.80 2.1 0.21
Bw1 0.24 0.51 0.9 0.12
Bw2 0.19 0.32 0.5 0.09
Bw3 0.10 0.14 0.4 0.08
Bw4 0.09 0.05 0.3 0.07
Bg1 0.07 0.06 0.3 0.06
Bg2 0.10 0.06 0.5 0.07

Horizon TP OP OC/TP Extraction
 (mg/kg) OC P
 (% of total)

 Cabbage cultivation (Dystric cambisol)

A 2050 710 22 35 17
AB 1220 600 16 35 29
Bw1 1310 590 8 32 20
Bw2 1730 560 4 24 10

 Reforestation (Dystric Cambisol)

Oi 1000 n.d 464 15 32
Oe 1170 n.d 311 20 28
A 1130 470 36 28 27
Bw 470 150 21 26 11
CB 570 120 12 30 12
C 540 120 13 27 11

 Secondary forest (Skeletic Umbrisol)

Oi 840 n.d 506 14 32
Oe 830 n.d 417 19 29
A 650 240 74 36 30
CA 380 170 71 39 32
C 230 90 58 36 27

 Secondary forest (Skeletic Umbrisol)

Oi 790 n.d 580 23 32
A 540 260 123 37 35
BA 290 100 72 4 21
Bw1 240 70 37 30 16
Bw2 250 50 20 28 10
Bw3 230 40 18 21 7
Bw4 190 20 16 25 6
Bg1 180 30 17 29 6
Bg2 140 40 36 22 15


n.d., not determined.

Nuclear magnetic resonance measurements

For liquid-state [sup.13]C- and [sup.31]P-NMR measurements, the soils were extracted 3 times with an extraction solution containing 0.1 M NaOH and 0.4 M NaF at a ratio of soil to extraction solution of 1: 5 (w/v). The sample size was adjusted to ensure a minimum yield of 200 mg of organic C (OC) in the extracts. The extraction procedure followed the outline of Schnitzer (1982), modified by Sumann et al. (1998). The 0.1 M NaOH was used in order to reduce possible hydrolysis of organic ester P (Condron et al. 1985; Leinweber et al. 1997). Replacement of 0.1 M [Na.sub.4][P.sub.2][O.sub.7] by 0.4 M NaF did not affect the [sup.13]C-NMR spectra of extracts from Mollisols free of carbonates, but greatly improved the extraction yield compared with sole 0.1 M NaOH extraction (Sumann et a/. 1998). The extracts were dialysed (molecular weight cutoff, MWCO, 12 000-14 000 daltons) in order to remove salts and freeze-dried.

Liquid-state [sup.13]C and [sup.31]P-NMR spectra were obtained on an Avance DRX 500 spectrometer (Bruker Analytik GmbH, Karlsruhe, Germany). About 150 mg of the freeze-dried material was dissolved in 3 mL of 0.5 m NaOD in a 10 mm NMR tube. Conditions for [sup.13]C-NMR were: spectrometer frequency, 125.77 MHz; inverse-gated decoupling; acquisition time, 0.33 s; delay time, 1.67 s; line-broadening factor, 100 Hz. This factor was used to enhance the characteristic broad signals of extracted SOM. In order to identify sharp lines resulting from low-molecular-weight substances, a line-broadening of 10 Hz was used. Chemical shifts were given relative to the resonance of tetramethylsilane. The conditions for [sup.31]P-NMR were: spectrometer frequency, 202.46 MHz; no [sup.1]H-decoupling; pulse angle, 90 [degrees] ; pulse delay, 0.2 s; acquisition time, 0.1 s; line-broadening factor, 20 Hz. Chemical shifts were measured relative to 85% orthophosphoric acid.

Signal assignments were made according to literature data (Newman and Tate 1980; Condron et al. 1990; Guggenberger et al. 1995; Kogel-Knabner 1997). The signal areas were calculated by electronic integration. Sharp [sup.13]C resonances at 177 and 135 ppm were identified to be due to mellitic acid using addition of a standard of mellitic acid.

Statistics

Statistical analyses were conducted using the software package STATISTICA 5.0 (StatSoft, Inc., Tulsa, OK). Two-tailed tests were used for evaluating significance of the coefficients of linear correlations between soil parameters and C and P species. Hierarchical cluster analyses of C and P species of the soil horizons were carried out using unweighted pair-group average linkage and Euclidean distances as measurement intervals.

Results and discussion

Chemical soil properties

The soil textures in the profiles of the plots varied from silt loam to clay. Actual cation-exchange capacity ([CEC.sub.a]) in topsoils was relatively high, reflecting soil texture and high contents of organic matter (OM) in the A horizons. The only exception was the primary forest with a low [CEC.sub.a] (Table 1).

The sites showed similar decreases in OC and total N (TN) with soil depth (Table 1). Organic C ranged from 34% to 47% in organic horizons of the forest floor layers and from 0.3% to 6.7% in mineral soil horizons. Total N ranged from 1.7% in organic forest floor layers to 0.1% in the subsoil horizons. The OC:TN ratio was between 27 and 39 in organic layers and decreased to values [is less than] 5 in the subsoil. It is interesting to note that despite fertilisation the OC :TN ratios at the cabbage cultivation site did not differ from those under the Pinus reforestation and the primary forest. Compared with OC: TN ratios reported for mineral soil horizons of acidic temperate forest soils (e.g. Stevenson 1986; Zech et al. 1992), the values found here were small, maybe reflecting the rapid nutrient cycling at the study site.

Total P varied from about 200 mg/kg in the subsoil horizons of the primary forest to about 1200 mg/kg in the organic layers and the upper mineral horizons of the forests and up to 2000 mg/kg under cabbage cultivation (Table 1). These values are high compared with other tropical soils (e.g. Agbenin and Tiessen 19952. The relatively high P status of the cabbage cultivation is assumed to be from addition of inorganic fertilisers. The lowest OC:TP ratios occurred in subsoil horizons; the highest were in the topsoil horizons. Again, the ratios were below those of temperate forest soils (e.g. Stevenson 1986; Zech et al. 1992).

The extraction yields of OC averaged 18 [+ or -] 4% of total SOC in the organic horizons and 30 [+ or -] 9% of total SOC in the mineral horizons (Table 1). This was about one-half of the yield Gressel et al. (1996) reported for the extraction of organic forest floor material and A horizons of a temperate forest soil. The extraction yields in the forest floor layers were 28-32% of TP and in the mineral soil horizons they varied from 6 to 35% of TP, with the lowest values in the subsoils of the reforestation and the primary forest. The extraction yields for P again were below those reported by Gressel et al. (1996).

[sup.13]C-NMR spectroscopy

The [sup.13]C-NMR spectra of OM extracted from the organic forest layers (Figs 2-4) exhibited a distribution of C species (Table 2) similar to alkaline-soluble material from coniferous and deciduous tree litter from temperate and tropical regions (Gressel et al. 1996; Zech et al. 1996). They also compared well to solid and liquid-state [sup.13]C-NMR spectra of organic matter in the percolation water of forest layers (Vance and David 1991; Guggenberger et al. 1998). The broad signals indicated the heterogeneous composition of the extracted material (Wilson 1987). Signals within the O-alkyl C region (60-110 ppm) accounted for 32-37% of the spectra, next to signals in the aryl C region (110-160 ppm; 26-30%) and in the alkyl C region (0-50 ppm; 22-28%). The signals in the O-alkyl C region were attributed to carbohydrates in the extracted material. The peaks in the alkyl C region at around 30 and 39 ppm are usually assigned to -[CH.sub.2]- units of long and branched chains, respectively (Wilson 1987; Preston et al. 1994). Resonances around 152 ppm (phenolic C) and 129 and 119 ppm (protonated aromatic C), as well as the distinct signal at 56 ppm (methoxyl C), were indicative of lignin-derived compounds (Wilson 1987). Spectra with a line-broadening of 10 Hz showed that low-molecular-weight substances were less abundant in the extracts from the organic forest floor layer (results not shown here).

[Figures 2-4 ILLUSTRATION OMITTED]

Table 2. [sup.13]C- and [sup.31]P-NMR chemical shift ranges and signal response (% of total spectral area)
 [sup.13]C-NMR

Fraction Alkyl Methoxyl O-alkyl Aromatic Phenolic
 0-50 50-60 60-110 110-145 145-160
 ppm ppm ppm ppm ppm

 Cabbage cultivation

A 15 4 29 27 5
AB 13 4 29 31 4
Bw1 13 4 50 14 3
Bw2 13 3 58 10 2

 Reforestation

Oi 24 5 31 20 6
Oe 22 5 29 21 6
A 21 5 30 20 4
Bw 22 5 41 12 2
CB 20 5 43 13 2
C 21 4 43 12 2

 Secondary forest

Oi 17 5 31 23 6
Oe 17 5 27 23 7
A 16 4 27 24 6
CA 16 4 28 27 5
C 18 4 31 22 5

 Primary forest

Oi 22 4 29 21 5
A 19 4 26 24 4
BA 20 4 27 23 3
Bw1 17 4 36 18 4
Bw2 18 4 37 16 3
Bw3 16 4 39 16 3
Bw4 18 4 43 14 3
Bg1 18 4 39 15 3
Bg2 17 4 42 15 3

 [sup.13]C-NMR [sup.31]P-NMR

Fraction Carbonyl Pyro- Unknown Diester P
 160-200 phosphate -2.5 to-1 -1 to 0.5
 ppm -4.8 ppm ppm
 ppm

 Cabbage cultivation

A 20 3 3 7
AB 19 1 3 5
Bw1 16 n.d. 3 3
Bw2 14 n.d. 3 2

 Reforestation

Oi 14 12 n.d. 53
Oe 17 7 n.d. 44
A 20 3 4 9
Bw 18 4 4 13
CB 17 4 3 12
C 18 5 3 13

 Secondary forest

Oi 18 5 n.d. 61
Oe 21 11 n.d. 44
A 23 6 n.d. 21
CA 20 3 4 15
C 20 1 n.d. 19

 Primary forest

Oi 19 8 4 52
A 23 5 3 13
BA 23 1 2 22
Bw1 21 2 2 16
Bw2 22 2 2 7
Bw3 22 1 3 7
Bw4 18 3 3 7
Bg1 21 4 3 8
Bg2 19 1 3 12

 [sup.31]P-NMR

Fraction Teichoic Monoester P Ortho- Phosphonate
 acid P 4.1-5.6 phosphate 19.00
 1.0-2.0 ppm 6.20 ppm
 ppm ppm

 Cabbage cultivation

A 8 65 12 2
AB 5 85 1 n.d.
Bw1 3 91 n.d. n.d.
Bw2 4 91 n.d. n.d.

 Reforestation

Oi 8 24 3 n.d.
Oe 11 31 7 n.d.
A 9 73 n.d. 2
Bw 7 72 n.d. n.d.
CB 7 73 n.d. 1
C 6 73 n.d. n.d.

 Secondary forest

Oi 6 22 6 n.d.
Oe 11 27 7 n.d.
A 11 59 n.d. 3
CA 8 67 n.d. 3
C 8 72 n.d. n.d.

 Primary forest

Oi 9 16 11 n.d.
A 13 58 n.d. 8
BA 5 66 n.d. 4
Bw1 5 73 n.d. 2
Bw2 6 83 n.d. n.d.
Bw3 5 84 n.d. n.d.
Bw4 8 79 n.d. n.d.
Bg1 6 79 n.d. n.d.
Bg2 6 78 n.d. n.d.


n.d., not detectable.

The composition of the alkaline-extractable OM in the Oi and in the Oe horizon under the reforestation (Fig. 2) as well as under the secondary forest (Fig. 3) did not differ (Table 2). There was no clear decrease of O-alkyl C and increase in carbonyl C resonances as found for forest humus profiles in temperate regions (Gressel et al. 1996; Zech et al. 1996).

The extractable SOM of the mineral horizons (Figs 2-5) tended to be enriched in O-alkyl C and depleted in aryl C relative to the extractable SOM of the organic horizons (Table 2). Lower proportions of methoxyl, aromatic, and phenolic C, especially in the deeper subsoil horizons, reflected advanced decomposition of lignin compounds with soil depth. This agreed with the assumption of Oades et al. (1988), who stated that in aerobic soils the O-aromatic phenol bond of lignin is readily oxidised and is not a recalcitrant grouping which accumulates in soil. The decay of the lignin-derived structural moieties followed an increase in O-alkyl C resonances, indicating a relative accumulation of soluble carbohydrates. Their enrichment in preference to the chemically more stable lignin has been assumed to be partly the result of microbial resynthesis (Zech et al. 1996; Amelung et al. 1997). The abundances of O-alkyl C resonances, however, were significantly negatively correlated with the extraction yields of OC (R = -0.74, P [is less than] 0.01). This result agrees with the finding that O-alkyl C is more easily desorbable from mineral surfaces by alkaline extractants than aromatic C (Kaiser,and Zech 1999). The highest abundances of O-alkyl C occurred in the spectra of extractable SOM of the Bw 1 and Bw2 horizon from the cabbage cultivation. This might result from input of dissolved organic matter, which is usually rich in O-alkyl C (Kaiser et al. 1997), with the percolation or interflow water. The cabbage cultivation site had only little to no plant cover at the beginning of the rain season in April, and so large transfer of soluble substances from the topsoil into the subsoil may happen.

[Figure 5 ILLUSTRATION OMITTED]

The proportions of alkyl C remained fairly constant with soil depth (Table 2). According to Zech et al. (1992), aliphatic structures such as long chain fatty acids may be selectively preserved during SOM decomposition. Also, Baldock et al. (1992), using CPMAS [sup.13]C-NMR, assumed that advanced decomposition of SOM results in a relative preservation of alkyl C. This relative increase of alkyl C could not be supported by the results of the present study. The differing results may be caused by the overestimation of alkyl C and subsequent underestimation of aromatic C using CPMAS [sup.13]C-NMR spectroscopy (Ernst et al. 1990; Golchin et al. 1997).

The oxidation degree of the extractable SOM remained almost constant with soil depth, as indicated by the constant abundance of signals in the carbonyl region (20[+ or -]3%). Exceptions were again the extractable SOM from the cabbage cultivation Bw1 (16%) and Bw2 horizon (14%). The contribution of amide structures to the resonances in the carbonyl region was assumed to be little because the N content of the extractable SOM was small (2-3.5%).

Comparing the topsoils of the 3 forest plots, no differences in the composition of extractable SOM could be observed (Table 2). This was surprising as there was a difference in input quantity and quality of fresh organic matter with the different forest systems, which might be expected to result in a different composition of extractable SOM (Condron and Newman 1998). We assume that rapid decomposition of plant debris and rapid resynthesis of microbe-derived matter under the tropical climate may blur the differences of different organic matter sources (Zech et al. 1996).

At the cabbage cultivation site, the abundance of substituted aromatic C was somewhat larger than at the forest sites. A possible reason for this finding could be that weeds were burned annually. Almendros et al. (1992) attested that heating organic matter results in an increase in aromaticity of the remains, at the expense of carboxyl C and O-alkyl structures. Haumaier and Zech (1995) compared the compositions of oxidation products of charred plant residues and alkaline-extractable SOM. Based on their results they suggested that black carbon is a possible source of highly aromatic soil humic acids. These suggestions were confirmed by Skjemstad et al. (1996), who provided evidence that black carbon largely contributed to SOC. Golchin et al. (1997) observed an increase in carboxyl groups and non-oxygenated aromatic C in soils with annual vegetation burning using different [sup.13]C-NMR techniques. We therefore attribute the relatively large proportions of C-substituted aromatic C in the extractable SOM of the upper horizons of the cabbage cultivation site to pyrogenic carbon (Table 2).

Many of the [sup.13]C-NMR spectra of extractable SOM from the subsoils showed 2 distinct [sup.13]C resonances at 177 and 135 ppm (e.g. Figs 4 and 5). Spectra with 10 Hz line-broadening proved that these sharp signals resulted from a specific compound (Fig. 6). In all spectra where they appeared, the signals occurred at a signal intensity ratio of 1: 1, indicating that the compound consisted of equal numbers of aromatic carbons and carboxyl groups. The absence of other sharp resonances provided evidence that the compound in question was highly symmetrical and contained only one type each of aromatic and carboxyl C. The only aromatic carboxylic acid that exhibits such structural features is mellitic (benzenehexacarboxylic) acid. Addition of mellitic acid to the extractable SOM from the AB horizon of the cabbage cultivation indeed resulted in sharp signals at 135 and 177 ppm (Fig. 6). We therefore conclude that the distinct signals in the [sup.13]C-NMR spectra of extractable subsoil SOM were caused by mellitic acid. Other benzenepolycarboxylic acids also gave signals in the range of 135-140 ppm for the carboxyl-substituted aromatic carbons, but additional signals at higher field (126-130 ppm) for unsubstituted ring carbons. The presence of other carboxyl-substituted aromatics may be indicated by broad resonances in the range of 135-140 ppm, underlying the sharp mellitic acid signal at 135 ppm (cf. Fig. 6, PF-Bw3). Glaser et al. (1998) pointed out that mellitic acid was not produced by oxidation of materials obtained by common humification reactions but was the product of the oxidation of charred plant material. The presence of mellitic acid in subsoils gives, therefore, additional evidence that black carbon contributed to SOC in these soils, and that charred plant materials are not stable in the tropical environment.

[Figure 6 ILLUSTRATION OMITTED]

[sup.31]P-NMR spectroscopy

The [sup.31]P-NMR spectra of soil extracts from the 4 plots (Figs 2-5) revealed the presence of 7 different [sup.31]P spin environments, namely phosphonates (~19 ppm), inorganic orthophosphate P (~6.2 ppm), orthophosphate monoester-P (4.1-5.6 ppm), teichoic acid-P (1.0-2.0 ppm), orthophosphate diester-P (~0.2 ppm), pyrophosphate-P (~-4.8 ppm), and unknown P species (-2.5 to -1 ppm). The monoester-P fraction probably indicated hydrolysis products of plant-derived phospholipids such as inositol phosphate and choline phosphate (Anderson 1980). At the cabbage cultivation site, monoester-P might additionally result from the input of inorganic P due to fertilisation and annual burning. Increasing soil content of orthophosphate is assumed to increase monoester-P in soil (Guggenberger et al. 1996b). The diester-P fraction contains, among other compounds, nucleic acids and phospholipids. The fraction is considered to represent a labile SOP fraction (Stevenson 1986; Tate and Salcedo 1988). Phosphonates and teichoic acids may be derived from lysis products of microorganisms (Condron et al. 1990; Bedrock et al. 1994).

The [sup.31]P-NMR spectra obtained on alkaline extracts of the organic horizons of the forest floor clearly differed from those of the mineral soil horizons (Figs 2-5; Table 2). In the organic forest floor, diester P was the major P form accounting for 43-61% of total intensity, with little contribution of monoester-P (16-31%). In contrast, the spectra of extracts of the mineral horizons were dominated by monoester-P (59-91%) with only a minor proportion of diester-P (2-21%; Table 2). The ratios between monoester-P and diester-P in the mineral soil generally corresponded to those reported by Tate and Newman (1982), Hawkes et al. (1984), Condron et al. (1990), Gressel et al. (1996), and Guggenberger et al. (1996a) for temperate and tropical uncultivated grassland and forest soils. A smaller range of monoester-P values was found in this study.

In previous studies dealing with the identification of P forms, monoester-P was usually the dominant P form in surface soils (e.g. Hawkes et al. 1984; Dai et al. 1996; Gressel et al. 1996). We found that monoester-P was also the dominant P structure in the subsoil (Table 2). In the soil profiles from the secondary and primary forest, the proportions of monoester-P increased with increasing soil depth (Table 2). A similar result was reported for a north American spodosol subsoil (Dai et al. 1996). The relative accumulation of monoester-P with increasing soil depth might reflect its higher stability compared with the more labile diester-P and teichoic acid-P structures (Guggenberger et al. 1996b; Magid et al. 1996) that are more easily degraded (Bedrock et al. 1994). Highest abundances of monoester-P were detected in the extracts from the Bw1 and Bw2 horizon of the cabbage cultivation (Table 2), i.e. those horizons in which the SOM might be influenced by input of dissolved organic material with percolation or interflow water (see above).

Compared with the forested sites, the proportions of diester-P were smaller in the extracts of the cabbage cultivation (Table 2). This probably reflected an enhanced mineralisation of the more labile diester-P form due to cultivation practices, resulting in higher proportions of monoester-P (Table 2; Condron et al. 1990).

In the forest Oe and A horizons, we observed high teichoic acid-P contents. With soil depth, teichoic acid-P relatively decreased compared with monoester-P (Table 2). This may be due to reduced microbial biomass in the subsoil. Small signals of phosphonates, a third labile P species of mainly microbial origin, occurred only in the surface mineral horizons. The largest abundance of phosphonate resonances was in extracts from the upper mineral soil under the primary forest (Table 2). They were present in neither the extracts of the forest floor nor those of the subsoil horizons.

Resonances of inorganic P forms were not prominent in the [sup.31]P-NMR spectra of the alkaline extracts. Signals of orthophosphate were restricted to forest floor horizons (up to 11% of the total signal area) and the mineral topsoils of the cabbage cultivation which received inorganic P input due to fertilisation and annual burning of weeds. Signals due to pyrophosphate mainly occurred in forest floor extracts (up to 12%); in the extracts of the mineral horizons they comprised between 0 and 6% of the total signal area (Table 2). The low extractability of inorganic P forms by the NaOH/NaF mixture may be due to the strong binding of orthophosphate and pyrophosphate to Al and Fe hydrous oxides. Another probable reason is a partial loss of inorganic P upon dialysis (Sumann et al. 1998).

Linkages between carbon and phosphorus transformation

Gressel et al. (1996) reported significant correlations between organic C and P structures in the surface soil horizons of a Californian mixed-conifer forest soil, using [sup.13]C- and [sup.31]P-NMR spectroscopy. We used hierarchical cluster analysis to classify our samples with respect to their [sup.13]C- and [sup.31]P-NMR signature of alkaline-extractable matter. Compared with conventional correlation analysis, a hierarchical cluster analysis has the advantage that the whole distribution of C and P species can be compared in a single run. The cluster analysis clearly separated the organic forest floor horizons (Oi, Oe) from the mineral horizons (Fig. 7). Apparently, organic forest floor layers and forest mineral soil horizons comprised 2 compartments of the ecosystem, differing in SOM chemistry and probably dynamics. We assume this to reflect the influence of soil minerals in the decomposition of SOM.

[Figure 7 ILLUSTRATION OMITTED]

Next to the organic forest floor layers, the Bw1, Bw2, and less closely the AB horizons of the cabbage cultivation site were separated from the rest of the mineral horizons. In case of the Bw1 and Bw2 horizon, and possibly also in case of the AB horizon, the composition of extractable SOM could be influenced by input of dissolved organic matter rich in O-alkyl C (Kaiser et al. 1997). As mentioned above, the A and AB horizon of the cabbage cultivation site may contain organic matter derived from annual weed burning. In addition, the whole profile was rich in P due to fertilisation, which may also affect the dynamics and thus the composition of organic P forms (Guggenberger et al. 1996b). We therefore decided to exclude the organic forest floor horizons and the horizons from the cabbage cultivation site from further analysis of the linkage between organic C and P.

When only considering the mineral horizons of the forested sites we found significant correlations between chemical SOC and SOP structures (Table 3). Diester-P, phosphonates, and teichoic acids were positively correlated to aromatic and phenolic C (all decreasing with depth) and negatively to O-alkyl C (increasing with depth). This result is opposite to the findings of Gressel et al. (1996) obtained for a decomposition gradient from the upper forest floor horizon down to the mineral topsoil (A horizon). The 3 P forms are thought to be of microbial origin (Tate and Salcedo 1988; Condron et al. 1990; Bedrock et al. 1994). If this is true, the contrary course of these P forms and O-alkyl C within the soil profile may question the assumption that the increase of O-alkyl C with depth (Table 2) is the result of microbial recycling of carbohydrates (Zech et al. 1996).

Table 3. Significant correlations for solution [sup.13]C- and [sup.31]P-NMR results and relevant chemical soil properties within the mineral horizons of the three forested plots
C and P species O-alkyl C Aromatic C Phenolic C Carbonyl C

Alkyl C -0.56(*)
Methoxyl C
O-alkyl C
Aromatic C -0.95(***)
Phenolic C -0.77(**) 0.85(***)
Carbonyl C -0.74(**) 0.63(**) 0.53(*)
Pyrophosphate
Diester-P -0.59(*) 0.56(*) 0.50(*)
Teichoic acid-P -0.56(*) 0.59(*) 0.58(*)
Phosphonate -0.76(**) 0.74(**) 0.57(*)
Monoester-P 0.69(**) -0.68(**) -0.51(*)
Unknown -0.54(*)

C and P species [Al.sub.o] [Fe.sub.o] OC

Alkyl C
Methoxyl C
O-alkyl C -0.78(**) -0.81(***) -0.80(***)
Aromatic C 0.69(**) 0.74(**) 0.75(**)
Phenolic C 0.59(*)
Carbonyl C 0.58(*) 0.57(*) 0.52(*)
Pyrophosphate
Diester-P 0.51(*)
Teichoic acid-P 0.62(*) 0.61(*) 0.87(***)
Phosphonate 0.96(***) 0.95(***) 0.87(***)
Monoester-P -0.78(**) -0.83(***) -0.81(***)
Unknown


(*) P < 0.05; (**) P < 0.01; (***) P < 0.001; n = 15.

In this study, diester-P, teichoic acids, and phosphonates were positively correlated with oxalate-soluble Al and Fe ([Al.sub.o] and [Fe.sub.o]; decrease with depth; Table 3) that are indicators of short-range-ordered Al and Fe hydrous oxides (Parfitt and Childs 1988). Aromatic C was also related positively to [Al.sub.o] and [Fe.sub.o] contents, whereas the proportions of O-alkyl C were negatively correlated with [Al.sub.o] contents. This result agrees with the observation that there is little if any sorption of O-alkyl C to hydrous oxides, in contrast to a strong sorption of aromatic C to Al and Fe hydrous oxides (Kaiser and Zech 1997; Kaiser et al. 1997). An experiment where beech litter was incubated in the presence of various oxidic mineral phases suggested that labile P species are protected against decomposition through stabilisation on mineral surfaces (Miltner et al. 1998). Also Sumann et al. (1998) assumed that diester-P may be stabilised by minerals. Sorption to mineral surfaces is considered to be an important geochemical process in the accumulation and preservation of organic matter in soils and sediments (Haider 1992; Hedges and Oades 1997). We conclude that the parallel depth distribution of diester-P, teichoic acids, and phosphonates and aromatic C could be the result of selective sorption of these structures. With increasing depths, the contents of Al and Fe oxides and consecutively their effect on the sorptive stabilisation of these labile P forms decrease.

Nevertheless, it should be noted that the extraction yields of C exceeded that of P in the subsoils but not in the topsoils (Table 1). This makes comparisons between C and P dynamics difficult due to increased risk that alkaline extracts are not representative for C and P forms.

In contrast to diester-P, teichoic acids, and phosphonates, the proportions of monoester-P resonances (increasing with depth) were positively related to those of O-alkyl C structures and negatively to those of aromatic and phenolic C (Table 3). This result is also different to that of Gressel et al. (1996), who found a positive relationship between monoester-P and alkyl-C signal intensities in liquid-state NMR spectra obtained from extracts of forest floor and topsoil horizons. This underlines the influence of minerals on SOM dynamics. Assuming that the increase of O-alkyl C proportions with depth resulted from microbial resynthesis of carbohydrates (Zech et al. 1996; Amelung et al. 1997) and that the decrease of aromatic and phenolic C resonances reflected the breakdown of structural components of plant litter (Oades et al. 1988), it is surprising that there were increasing proportions of resonances of monoester-P with increasing soil depth. Monoester-P is often assumed to be mainly plant-derived. We suggest that the accumulation of monoester-P relative to diester-P, phosphonates, and teichoic acids reflected preferential mineralisation of these labile P forms. Several studies have shown that phosphodiesters are rapidly hydrolysed and therefore preferentially mineralised compared with orthophosphate monoesters (e.g. Hawkes et al. 1984; Hinedi et al. 1989).

Conclusions

In this study we elucidated the vertical distribution of soluble SOC and SOP structures in differently managed tropical soils of Thailand. Our results indicated the following.

(1) Charred organic material releases aromatic C into the soil. The charred residues seem to not be stable; subsequent oxidation results in the release of free mellitic acid and other benzenepolycarboxylic acids in subsoil SOM, resonating at 135 and 177 ppm in the [sup.13C]-NMR spectra.

(2) The relationships between distinct C and P forms in organic forest floor horizons are different from those in the mineral soil horizons. Therefore, the dynamics of the decay of plant debris change upon contact with soil minerals.

(3) Also the relationships of C and P forms in the mineral soil of intensively cultivated tropical sites may be different from those in forest mineral soils. This could be due to fertilisation, movement of dissolved organic matter, and input of charred material from annual burning.

(4) In mineral soil horizons, changes in SOC species distribution are related to changes in SOP composition. As soil depth increases, labile SOP forms such as diester-P and teichoic acid-P seem to be preferentially mineralised. Consequently, the more stable monoester-P is relatively enriched.

Acknowledgments

This study was partly supported by the International Board of Soil Research and Management (IBSRAM). We thank all the people of the IBSRAM for help concerning the organisation of field work, especially Suraphol Chandrapatya. We also thank the Royal Forest Department (RFD) of Thailand for allowing us to carry out research at the Khun Sathan RFD Station. Especially we are grateful to Warin Jirasuktaveekul, Head of the Watershed Research Division at the RFD, Bangkok. For help during sampling we thank Suntorn Ratchadawong, Department of Land Development (DLD), Nan, and the staff of the RFD-Station at Khun Sathan, Mr Mai, Ed, and all the others. We also thank Nualsri Kanchanakool, Head of the Soil Analyses Division at the DLD in Bangkok, for access to the DLD laboratory facilities.

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Manuscript received 6 September 1999, accepted 10 March 2000

A. Moller(AE), K. Kaiser(A), W. Amelung(A), C. Niamskul(B), S. Udomsri(C), M. Puthawong(D), L. Haumaier(A), and W. Zech(A)

(A) Institute of Soil Science and Soil Geography, University of Bayreuth, 95440 Bayreuth, Germany.

(B) International Board for Soil Research and Management, Bangkok 10900, Thailand.

(C) Department of Land Development, Bangkok 10900, Thailand.

(D) Royal Forest Department, Nan 55110, Thailand.

(E) Corresponding author; email: andreas.@uni-bayreuth.de
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Author:Moller, A.; Kaiser, K.; Amelung, W.; Niamskul, C.; Udomsri, S.; Puthawong, M.; Haumaier, L.; Zech, W
Publication:Australian Journal of Soil Research
Geographic Code:9THAI
Date:Sep 1, 2000
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