WETLANDS, Vo[. 15, No. 4, December 1995, pp. 386-391 © 1995, The Society of Wetland Scientists

WATERLOGGING EFFECTS ON ORGANIC PHOSPHORUS FRACTIONS IN A TOPOSEQUENCE OF SOILS

Gerardo Rubio, ~ Ra61 S. Lavado? Alicia Rendina,'~ Marta Bargiela, z Claudia Porcelli, z and Alicia E de Iorio-'

~ Departamento de Suetos ~- Departamento de Qu{rnica

Facultad de Agronomfa Universidad de Buenos Aires, Avenida San Mart[n 4453

1417 Buenos Aires, Argentina

Abstract: Organic P plays a key role as a source and sink of P in hydromorphic soils. Our objective was to study the influence of hydromorphism on the distribution of soil organic P fractions. The research was performed at 4 sites from a toposequence of the Flooding Pampa (Argentina), which varied from a neve~ flooded upland site to a lowland site that remains waterlogged most of the year. All the soils were of the same age, parent material, vegetation type (native grassland), and land use. Total P content was higher in the soil of the lowland site than in that of the upland site. In all sites, at the 0-10 cm depth, most soil P was found in the organic fraction (ranging from 68 to 80%). The sum of the moderately resistant and resistant to mineralization fi'actions was greater than the labile and moderately labile fractions. The differences in the relative distribution of organic P fractions were minimal among sites. As a consequence, in these soils developed from the same parent material and with the same vegetation type (native grassland) and land use, the duration of waterlogging had little influence on the fractionation of soil organic R

Key Words: Flooding Pampa, soil organic phosphorus, waterlogging, grasslands, organic matter

INTRODUCTION

Phosphorus (P) is one of the most common factors limiting productivity in natural ecosystems. The role of soil organic fractions as a source and a sink of P has been widely recognized (Tate 1984). Soil organic P is composed of several fractions, ranging from labile to resistant to mineralization (Bowman and Cole 1978, Tate 1984). The release of inorganic P (the main plant- available P form) from organic sources occurs through mineralization and is related to several factors, among them the chemical composition of the organic P (Bow- man and Cole 1978, Tiessen et al. 1983).

Several approaches have been used to fractionate organic P according to its lability (Martin 1964, Bow- man and Cole 1978, Hedley et al. 1982). These meth- ods have been generally successful in reflecting several processes that affect soils. Thus, changes in lability of the organic P have been assessed under the influence of vegetation characteristics, fertilization, cultivation, and manure application (Sharpley et al. 1984, Sharpley and Smith 1985, Schoenau et al. 1989, Trasar-Cepeda c t a l . 1990, Mttgid ttnd Nielsen 1992, Mu~id 1993). Although organic P tends to accumulate in wet soils (Condron et al. 1990), there are few studies investi- gating the distribution of organic P fractions in soils under differing hydromorphic regimes.

The "Flooding Pampa" is a large area (90,000 km ~) of the coastal plain of Argentina that experiences fre- quent floods covering much of the area. There are tem- porary and permanent wetlands and toposequences of soils varying with soil hydromorphism (Lavado and Segat 1989). In all these soils, P is mainly found in organic forms (Culot and Bolafio 1967). P has been identified as the most deficient nutrient in the native grasslands (Soriano 1991). In a pot experiment, native grass species showed positive responses to P fertiliza- tion, under both waterlogged and non-waterlogged conditions (Rubio et al. 1995). In addition, the grazed grasslands produce a low, but permanent output of P (Chaneton et al. 1995). Thus, it is important to know the relationship between the distribution and avail- ability of organic P and soil hydromorphism The main objective of this study was to evaluate the influence of the duration of soil waterlogging on the distribution of organic P fractions.

MATERIALS AND METHODS

Description of Study Sites and Soil Characterization

The work was carried out in the center of the Flood- ing Pampa, Argentina (36 ° 30'S, 58 ° 30'W). This area

386

Rubio et al., W A T E R L O G G I N G EFFECTS ON ORGANIC P FRACTIONS 387

Table 1. Relief, dominant species, and soil classification at four Flooding Pampa sites.

Site Relief Dominant Species Soil

| Ridge Bromus unioloides L. Thaptoargic Carduus acanthoides L. Hapludolls Botriochloa htguroides (D.C.) Pilgcr

2 Danthonia montevidensis Hack et Arech Eleocharis viridans Ktikent Sporobolus indicus (L.) R.Br.

3 Paspalidium paludivagum Hitch et Case Mentha pulegiurn L. Danthonia montevidensis Hack et Arech

4 Swale Sofanum glaucophyflum Desf. Atternantera philoxeroides (Mart.) Griseb Leersia hexandra Sw.

Midslope

Lowland

Typic Natraquolls

Typic Natraquolls

Typic Argiaquolls

is characterized by little slope (ranging from 1% to 0 . 0 0 1 % ) and low relief (Lavado and Segat 1989). It has a temperate and subhumid climate (mean temper- atures 8.5°C in July and 21.5°C in January; mean an nual rainfall 950 rain). Rainfall is evenly distributed through the year. Four sites on a 4-kin long topose- quence corresponding to representative soils and na tire grassland communities were studied. In Table 1, the main species and soil classification of each site are described, Site 1 represents the upland ridges not al L fected by waterlogging. These areas are usually de- voted to agriculture, although the samples for this study were from an area under native grassland. In site 2 (mid slope), the soils are subjected to waterlogging during late winter and early spring. Although the wa- terlogging period is variable, an average duration of two months can be estimated. Site 3 represents the typical lowland and possesses more intense hydro- morphic features because of longer flooding periods, In site 4 (a swale), soils remain waterlogged for most of the year, The difference in elevation between sites l and 2 was 0.68 m and that between sites 2 and site 4 was 0.13 m.

In all the sites, the native grassland has been grazed

by cattle for the last century. No P fertilization was recorded in any place. Four soil samples at two depths (0-10 cm and 10-20 cm) were taken in each site in autumn when none of the sites were waterlogged.

Typical soil hydromorphism indications, such as mottles and concretions, are absent in topsoil of site 1 but increase from site 2 to site 4. Particle size distri- bution was determined by the pipet method (Gee and Bauder 1986). Soil samples were analyzed for organic C by Walkley and Black method (Nelson and Som- mers 1982), pH by paste method (Rhoades 1982), and available P by Olsen method (Olsen and Sommers 1982).

Organic P Fractionation

Soil organic P was fractionated by the sequential method of Bowman and Cole (1978). This method is specific for organic P and characterizes organic P frac- tions according to their solubilities in different extrac- lants, obtaining a gradation from a labile to a highly- resistant P traction (Table 2). In each step, organic P was determined by difference with inorganic P after a HNO:HCIO4 digestion. Total organic P content was

Table 2. Summary of the technique used to determine organic soil P fractions (Bowman and Cole 1978).

Organic P Fractions Extraction

Labile Moderately labile

Nlodcr/~tcl¥ rcMatant (fulvic acid fraction)

Resistant (hun-tic acid fraction)

5 g soil. Add 100 ml 0.5M NaHCO,, extraction time 0.5 h 2 g soil. Add 100 ml 1.0M H_,SO~ soluble, extraction time 3 h.

Determine organic P in the extract (*). Residue: Add 100 ml 0.5M NaOH. extraction time 6 h. Collect extract and determine inorganic E This inorgtmic P is summed to organic P (*) to obtain the fraction

At:idify th~ alkali ~ntra~m from p~Yioua at*p with IlCl to pll 1.0-1.8. After centrifugation, determine concentration of organ- ic P in the acid solution

Fraction determined by difference between the organic P of the alkali extract and the moderately resistant P

388 W E T L A N D S , V o l u m e 15, No. 4, 1995

Table 3. Some propcrties of the soils from 4 Flooding Pam- pa sites (0-10 and 10-20 cm depth).

Site 1 Site 2 Site 3 Site 4

0 - t 0 cm

Clay (%) 29.1 27.6 32.0 32.0 Silt (%) 22.0 46.9 45.1 46.7 Sand (%) 48.9 25.5 22.9 21.3 Org C (mgtg) 19.0 26.0 27.(I 35.0 pH (paste) 5.5 6.3 6.4 5.2 Org C:org P 83.8 98.0 126.3 104.7

10-20 cm

Clay (%) 29.7 30.4 33.1 24.1 Silt (%) 23.0 41.8 44.9 49. I Sand (%) 47.3 27.8 22.0 26.8 Org C (mg/g) 7.0 12.0 11.0 14.0 pH (paste) 7.7 7.1 7.1 6.0 Org C:org P 57.8 94.7 88.3 69,6

es t imated by the sum o f the individual fractions. Total P was ext rac ted with a mix ture o f ni tr ic-sulfuric acids and a final t rea tment with perchlor ic acid to g ive a c lear solution (AI Abbas and Barber 1964), For the P de terminat ion , the Murphy and Ri ley (1962) me thod was fo l lowed,

Statistical Ana lyses

The data were ana lyzed by A N O V A . When s i g n i l icant d i f fe rences (0.05 level) were found, var ia t ions among soils were examined by L S D test.

R E S U L T S A N D D I S C U S S I O N

Smal l d i f ferences in clay content were obse rved in the 0 - 1 0 cm depth among soils (Table 3). Clay min- erals have the greatest capaci ty to physical ly and chemica l ly bind organic P in soils (Tiessen et ai. 1983). C lay-organ ic P associa t ions are genera l ly resis- tant to minera l iza t ion forms (Tiessen et al. 1983). Soils o f upland r idges (site 1) showed the highest proport ion o f sand, which or ig ina ted in during eol ian processes (Tricart 1973). Part icle size distr ibut ion is s imi lar for the o ther three soils, with more silt and less sand com- pared to the upland r idges (Table 3).

Organic carbon contents increased f rom the non-hy- d romorph ic to the more hydromorph ic sites for both soil depths (Table 3). Be tween ex t r eme sites (1 vs 4), the organic C content a lmost doubled. Total P was also h ighes t at site 4, the mos t hyd romorph ic soil (Table 4). These results can be attr ibuted to two main factors: 1) in the lower site, the organic matter decompos i t i on rate wou ld be restr ic ted during part o f the year due to

Table 4. Total R available R organic R fractions of organic R and humic acid P/fulvic acid P ratio from the 4 Flooding Pampa sites (0-10 and 10-20 cm depth).

Site 1 Site 2 Site 3 Site 4

0-10 cm

Total P (mg/kg) 283 a 343 a 314 a 420 b Available P (Olsen)(mg/kg) 7_6 a 8.3 a 9.2 a 9.4 a Organic P (mg/kg) 226 a 265 a 213 a 334 a O r g a n i c P ( % of totalP) 79 a 75 a 68 a 80 a

Organic P fractions Labile (rag&g) 4.5 a 7.3 a 9.3 a 8.1 a Mod. labile (mg/kg) 80 a 91.7 a 80.7 a 120.6 a Mod. resistant (mg/kg) 92,5 a 88.1 a 76.4 a 97.8 a Resistant (mg/kg) 49.7 a 78.2 a 47.3 a 107.8 b Humic acid P:Fulvic acid P 0.5 a 0 .9a 0.6 a 1.1 a

10-20 cm Total P (mg/kg) 197 a 233 b 2t5 a 302 c Available P (Olsen)(mg/kg) 3.5 a 6.6 a 5.5 a 4 a Organic P (mg/kg) 121 a 126 a 124 a 201 b O r g a n i c P ( % of total P) 61 a 54 a 57 a 66 a

Organic P fractions Labile (mg&g) 7.1 a 5.2 a 4.7 a 4.2 a Mod. labile (mgtkg) 41.8 a 51.1 a 34.B a 89,9 a Mod. resistant (mg&g) 46.5 a 35 a 42 a 47.8 a Resistanl (mg&g) 25.8 a 35.4 a 2[).3 a 59.1 b Humic acid P/Fulvic acid P 0.6 a 1 a 0.5 a 1.2 b

Means within the same row followetl by the same letter are no~ different (P < 0.05) according to LSD method.

Rubio e t aI . . W A T E R L O G G I N G EFFECTS ON ORGANIC P FRACTIONS 389

walerlogging (Condron et al. 1990, Novak and Bertsch 1991), favoring organic C and P accumulations; and 2) the lower site would act as a collecting area, re- ceiving C and P in soluble and colloidal forms (Shar- pley 1985, Taboada et al+ 1987). At all sites, there was greater C and P content at the 0 -10 cm depth than at the 10-20 cm depth (Tables 3 and 4). The variations in organic C and organic P were parallel: therefore, differences in the C:P ratios in soil organic mattcr were minimal among sites (Table 3). A similar tendency can be noticed in data presented by Roberts et al. (1985), who worked in soils located at different positions in the landscape. Site conditions, as a consequence, would have affected soil organic C and P in a similar way. Since the organic C:organ ic P ratio is affected by the relative rates of C and P mineralization, this parameter has been used as an index of P immobili- zation or mineralization (Smeck 1985, Harrison 1987). Our study (Table 3) showed values ranging from 84 to 126 (at the 0 -10 cm depth); thus, P mineralization would be slow in all sites. An important factor con- trolling P mineralization is soil pH (Harrison 1982). In our case, there is an order of 1 pH unit difference among soils, which could affect mineralization, but no definite trends in soil pH were apparent along the to- posequence (Table 3).

As was previously observed in the area (Culot and Botafio 1967), most soil P is found in the organic frac- tion. At the 0 -10 cm depth, organic P accounted for 68 to 80 % of the total P (Table 4). This proportion fell at the 10-20 cm depth (ranging from 54 to 66%). The duration of waterlogging did not affect this pro- portion since there were no statistically significant dif- ferences among sites (Tablc 4).

At the O-lO cm depth, the organic P fraction distri- bution was not significantly diftbrent among sites (Fig- ure 1). This means that the relative proportion among the organic P fractions was similar, although the soils showed different contents of organic E Similarly, at the 10-20 cm depth, the soils showed only small dif- ferences in the distribution of organic P fractions (Fig- ure 1). While the soils of the toposequence have dif- ferent levels of waterlogging, they have the same age and parent material (quaternary loess), similar clay contents, climatic conditions (temperate, humid weath- er), vegetation type (natural grasslands), and land use (slight cattle ~razing). In the current study, soil hydro- morphism seems to have little effect on the soil or- ganic P dislribution. Analyzing the data published by Schoenau and co-workers (1989) obtained from anoth- er P fractionation method, we detected no differences in the organic P composit ion between hydromorphic and non-hydromorphic soils. They only fractionated the organic P in two forms, but the relative proportion between them was fairly constant. In our work, soil

50

+o

"" 30 0. . 9 r- a 20

o

o 10

0 labi le

0-I0 cm

moder , moder , res is tant lab i le res is tan t

50

W 40

" 30 a . . o

g 20 0

"6 10

l ab i l e

10-20

b b

m o d e r , moder , resistant labi le resistant

I ~ Sile t l ~ Site 2 ~ ] Site 3 [ ] Site 4

Figure 1. Organic P fractions at two depths in a topose- quence of soils. Bars in the same fraction with the same letter are not significantly different (P<0.05) according to LSD method. Groups of bars in the same fraction without letters denote no significant differences (P<0.05) between sites (ANOVA).

depth did not affect the distribution of organic P forms; percentages of each fraction were quite similar between both layers of each soil (Figure ! ).

Labile organic P accounted for a small proportion of the total organic P (< 6%, Figure 1). This fraction, which is extracted by sodium bicarbonate, is recog- nized as the most likely to undergo mineralization (Harrison 1982, Tiessen et al. 1983). The fraction con- sists of low molecular weight compounds with high solubility (Frossard et al. 1989). Condron et al. (1990) found that waterlogging, by restricting organic P de- composit ion rates, leads toward the persistence and ac- cumulation of labile compounds. Our results do not support Condron's observation since percentages of la- bile fractions were not affected by site location.

390 WETLANDS, Volume 15, No. 4, 1995

Schoenau et at. (1989) found labile organic P accu- mulation at the subsurface level (below 0.10 m) as a consequence of mobilization processes. In our study, the percentages of each fraction were not affected in the first 0.20 m, suggesting no mobilization of soil P in the upper soil profile. No statistically significant dif- ferences among sites were found in available (Olsen) P values (Table 4). These values can be considered low to supply the P needs of the grassland species (Lavado et al. 1993).

The moderately labile fraction v as the organic P form with the higher relative proportion in sites 2, 3, and 4 (Table 4; Figure 1). However, in all soils, the sum of the moderately resistant and resistant t~ min- eralization fractions was greater than the sum of the labile and moderately labile fractions. Some differ- ences were found among sites in the resistant fractions (Figure I). These differences can be analyzed through the humic acid P/fulvic acid P ratio (Ph/Pf) (Table 4). In the more hydromorphic soil, the Ph/Pf ratio ex- ceeded 1 (Table 4), indicating that the proportion of P present in humic acids prevailed over that present in fulvic acids. This observation suggests some effect of soil hydromorphism in the organic P stabilization pro- cess. In several soils of the area under study, the soil humic acid/fulvic acid ratio has been determined in terms of C (C humic acid/C fulvic acid Ch/Cf). At our site 2, Taboada et al. (1988) tound a Ch/Cf ratio of 1.68. In a nearby toposequence, soils equivalent to those at our sites I, 2, and 4 showed a Ch/Cf ratio of 2.93, 1.93, and 2.15, respectively (Rosell et al. 1978). The comparison of these Ch/Cf ratios with our Ph/Pf ratios (Table 4) indicates a higher concentration of P in the fulvic fraction. This result agrees with those reported by Batsula and Kuvonosova (1973) and Schoenau and Bettany (1987). In other soils, they found a higher P concentration in the fulvic acid frac- lion than in the humic acid fraction.

C O N C L U S I O N S

A higher degree of waterlogging led to higher con- tents of total and organic P in the lower sites, as com- monly found. However, waterlogging had little effect on the distribution of organic P fractions. These results suggest that the homogeneity in parent material (age and type), clay content, and vegetation type predomi- nate in determining organic P fractionation over the effect of the duration of waterlogging.

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Manuscript received 23 January 1995; revision received 21 June 1995; accepted 6 September 1995.