Directory UMM :Data Elmu:jurnal:A:Agriculture, Ecosystems and Environment:Vol78.Issue3.May2000:

Short communication

Use of a fractionation procedure to assess the potential for P movement

in a soil profile after 14 years of liquid pig manure fertilization

J.A. Hountin

a

, A. Karam

b,∗

, D. Couillard

c

, M.P. Cescas

b

a_{Agriculture and Agri-Food Canada, Pacific Agri-Food Research Centre, P.O. Box 1000, Agassiz, B.C., Canada V0M 1A0} b_{Équipe de Recherche en Sols Agricoles et Miniers (ERSAM), Département des sols et de génie agroalimentaire, Université Laval,}

Sainte-Foy, Qc., Canada G1K 7P4

c_{INRS-Eau, Université du Québec, Sainte-Foy, Qc., Canada G1V 4C7} Received 2 October 1998; received in revised form 5 July 1999; accepted 23 July 1999

Abstract

The accumulation of P originating∗ _{from animal wastes in the soil profile is of agronomic and environmental concern.} Sequential extraction procedure provides relevant information to mobility, bioavailability and distribution of P in animal manure-amended soils. Such information is needed to assess the potential for biological P leaching, under organic amend-ments, in some Quebec agriculture ecosystems. This study was conducted using soil samples from the long-term experimental plots of the Soil Research Station of the Quebec Ministry of Agriculture, Fisheries and Food at St-Lambert de Lévis, Quebec, Canada. The objective of this work was to examine the distribution of P fractions in the profile of a Le Bras silt loam soil (Gleysol) growing corn (Zea mays L.) which had received liquid pig manure (LPM) for 14 years. The surface soil was fertilized annually with various rates of LPM (0, 30, 60, 90 and 120 cm3_ha−1_{) in a randomized complete block design with four}

repli-cates since 1979. In the fall of 1992, soil samples were collected after the corn harvest from 20 soil plots at 20-cm intervals to a depth of 100 cm. Soil samples were air-dried, crushed, sieved through a 2-mm sieve, and analyzed for soil P fractions using a sequential extraction technique. All treatments resulted in an increase in the labile P pool contents of surface or profile soil compared with the control. Without exception, the amounts of P pools decreased with soil depth and were highly correlated with the LPM rates. The proportion of total P as labile (resin-Pi+NaHCO3-P+NaOH-P) in the soil profile increased from 61

(control) to 79% (highest LPM rate). The amounts of P generally increased in the following order: moderately labile NaOH-Pi

and -Po(254 mg kg−1) > labile P extracted by resin and NaHCO3(209 mg kg−1) > stable P (172 mg kg−1). On average, the

amount of labile Piforms (304 mg kg−1) was larger than the amount of labile Po forms (159 mg kg−1) and represented an

important fraction of the total labile P pools. Compared to the control treatment, the mean concentration of geochemical P (resin-Pi+NaHCO3-Pi+NaOH-Pi+HCl-Pi+residual P) increased in the soil profile by 16, 26, 33 and 50% for 30, 60,

90 and 120 m3_ha−1_{LPM, respectively. On a profile basis, the soil receiving the highest rate of LPM contained 1.8-fold the}

biological P (NaHCO3-Po+NaOH-Po) of the unamended soil. Amounts of labile Popools in the soil profile were highly

correlated with the organic carbon content. Results from this study suggest that heavy application of LPM over the 14-year

∗_{Corresponding author. Tel.:+418-656-2131, ext.: 7420; fax:+418-656-3223.}

E-mail address: antoine.karam@sga.ulaval.ca (A. Karam).

0167-8809/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 ( 9 9 ) 0 0 1 1 2 - 7

period promotes P movement to lower portions of the soil profile. The leaching or translocation of P compounds would be responsible for the distribution of P in the various soil layers. Management of LPM rates is necessary after a long-term fertilization of corn. ©2000 Elsevier Science B.V. All rights reserved.

Keywords: Pig manure; Inorganic labile P; Organic labile P; Soil-P fractionation

1. Introduction

Phosphorus (P) management is of agronomic and environmental importance (Sharpley et al., 1994). In sustainable agroecosystems, livestock manures should be managed so that they maintain an environment fa-vorable to adequate production and minimize surface and groundwater pollution, especially in agricultural systems in which soils are fertilized at high manure P rates. Although P is considered immobile in agri-cultural soils, a decrease in the P adsorption capacity of soil following manure addition at rates greater than nutritional P requirements of plants may increase the potential for leaching of soluble P (Sutton et al., 1982). Beauchemin et al. (1996) suggested that agricultural soils in Quebec associated with high animal density would be more at risk to leach P into drainage waters than low animal density or forest soils. The potential environmental P hazard of swine manure depends on many factors among which are the application rate and the mobility of bioavailable P. The bioavailability of P is controlled by its chemical pools indicating that it is desirable to determine the amount of P pools in a soil profile rather than in a surface layer. Sharpley et al. (1984) stated that the increases in the amounts of surface soil inorganic P and organic P with cattle feedlot waste (FLW) application over an 8-year pe-riod were due mainly to incorporation of FLW-P into labile soil P fractions. However, the authors observed little movement of inorganic, organic, or available P through the soil profile and concluded that FLW pre-sented little threat to the contamination of groundwa-ter with P. In another study, Simard et al. (1995), ob-served an enrichment in the amount of soil total P of the C horizons from hog (Porcus sp.) farms with a known surplus of on-farm manure N. They concluded that a significant portion of P moving downward in Quebec agricultural soils from the Beaurivage River Watershed accumulated in very labile Pi forms.

Little information, particularly in the eastern Quebec agroecosystem region, is available on the

pattern of P pools distribution in soils receiving high rates of LPM for many years. Data on soil P fractions distribution may be helpful in guiding long-term ap-plications of LPM to control labile P pool losses in the intensive monocultural cropping system and to protect the environment. The aim of this work was, therefore, to obtain information on the distribution of P fractions in the profile of a soil growing corn (Zea

mays L.) which had received LPM for 14 years.

2. Material and methods

2.1. Experimental design and soil samples

Soil samples for this study were collected in the fall of 1992 from long-term liquid pig manure (LPM) application on experimental plots located at the St-Lambert de Lévis Soil Research Station of the Quebec Ministry of Agriculture, Ficheries and Food. The soil is classified as a Le Bras silt loam (Canadian: Humic Gleysol; FAO: Gleysol). The una-mended top soil (0–20 cm) contained 24.5 g organic C kg−1_{, 1118 mg N kg}−1_{, 664 mg total P kg}−1_{, 32 mg} Mehlich-3 extractable-P kg−1_{, and had a pH (H}

2O) of 6.2. The materials and experimental treatments of this study were reported in a previously published pa-per (Hountin et al., 1997); consequently, only a brief outline will be presented here together with the an-alytical methods. Experimental treatments consisted of five application rates of LPM (0, 30, 60, 90 and 120 m3ha−1_{) applied to the soil surface at a growth} stage of 15–25 cm each year beginning in 1979 (Côté and Tran, 1996). The average chemical composi-tion (kg m−3_{) of the manure was as follows: total} N, 3.37; N-NH4, 2.32; P2O5,1.66; K2O, 1.64 (Côté and Tran, 1996); and total C, 29 (N’dayegamiye and Côté, 1989). The experimental design was a random-ized complete block with four replications. Inorganic fertilizer was not applied. In the fall of 1992, three 7-cm diameter soil cores per plot were taken at 20 cm

increments to a depth of 100 cm to assess the move-ment and distribution of C, N and P in the soil profiles (Hountin, 1996). The three cores were composited, air-dried, crushed, and sieved to pass a 2-mm sieve prior to sequential P extraction.

2.2. Hedley soil P pools

The amounts of soil inorganic and organic P (Pi and Po, respectively) in labile (resin-Pi, NaHCO3-Pi and NaHCO3-Po), moderately labile (NaOH-Pi, NaOH-Po), relatively insoluble apatite-P (HCl-Pi) and resistant (in primary minerals) pools was determined in each soil layer samples by the procedure of Hedley et al. (1982) as outlined in O’Halloran et al. (1987). This fractionation technique uses a series of extrac-tants to identify labile inorganic (Pi) and organic (Po) fractions followed by the more stable forms. In brief, soil samples were sequentially extracted by each of the following extractants: HCO3-exchange resin (Dowex-50w-x8), 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH, 1 M HCl, after 16 h shaking with each extrac-tant. The residue containing fairly insoluble Pi and Po forms (residual P) was then digested using 30% H2O2 /concentrated H2SO4. Total P (Pi+Po) in the NaHCO3and NaOH extracts was determined accord-ing to the method of Rowland and Grimshaw (1985). The Po content of a given fraction was calculated by substracting Pi content from total P content of the extract. For all extracts the concentration of Pi was determined colorimetrically by the method of Mur-phy and Riley (1962) using ascorbic acid as reducing agent. Acid or alkali filtered extracts were neutralized prior to P determination.

2.3. Statistical analysis

All statistical analyses were performed using the Statistical Analysis System (SAS Institute, 1990) soft-ware package. Analysis of variance (ANOVA) of the main effect of LPM application rates and soil depths , and their interactions on mean concentrations of P pools were carried out using the general linear model (GLM) statistical program of the SAS statistical pack-age. Linear Pearson correlation procedure was used to show relationships between the various soil P pools.

Fig. 1. Distribution of resin-Pi with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

3. Results

3.1. Labile and moderately labile inorganic P pools

The distribution of resin-Pi in the soil pro-files is illustrated in Fig. 1. The resin-Pi repre-sented on average 21% of the total labile forms (resin-Pi+NaHCO3-P+NaOH-P) in the top surface layer. Essentially, resin-Pi represents the labile pool of native adsorbed P (Fitter and Sutton, 1975), even though some of the more soluble precipitated P forms may be extracted by the resin (Tiessen and Moir, 1993). The increase in resin-Piover the control in the 0–20 cm layer varied from 25 mg kg−1_{for the lowest} rate of LPM (30 m3ha−1_{) to 125 mg kg}−1_{for the} high-est rate of LPM (120 m3ha−1_{). The resin-P}

icontent of the surface layer was largely in excess for silage corn P requirement (52 mg kg−1_{as resin-P}

i) when rates of LPM >60 m3ha−1 _{were applied. On a profile basis,} the soil receiving the lower rates (30–60 m3ha−1_{) and} the higher rates (90–120 m3ha−1_{) of LPM contained,} respectively, 1.3- and 2.0-fold the resin-Pi of the un-amended soil. This finding suggests that continued surface application of LPM has a significant effect on resin-Piaccumulation and movement. A similar trend was also observed for a surface loamy soil (Cana-dian: Dark Brown Chernozemic; FAO: Kastanozem) brought about by 20 years of feedlot manure load-ings (0, 30, and 90 t/ha) under non-irrigated condition (Dormaar and Chang, 1995). The resin-extractable Pi constituted from 8 in the control soil to 32% of total P in the amended soil (90 t ha−1_).

Fig. 2. Distribution of (a) NaHCO3-Pi and (b) NaOH-Pi with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

As was observed for resin-Pi, the amounts of NaHCO3-Piand NaOH-Pi (Fig. 2) were significantly (p<0.01) affected by the LPM application rates (Table 1) and decreased with soil depth. These results are in agreement with those of Meek et al. (1979), where it was observed that manure application at high rates to a calcareous soil caused P movement and a resulting increase in NaHCO3-extractable Pi at the 30–60 cm depth. O’Halloran (1993), in a study with dairy liquid manure (DLM) applied on a con-tinuous corn production under reduced tillage, found that DLM application affected the levels of labile Pi fractions. The NaHCO3-Pi levels (Fig. 2) were higher than those for resin-Pi (Fig. 1). On average, the values for NaHCO3-Pi/resin-Pi varied from 1.7 to 2.7 in the soil profiles. The average values for NaHCO3-Pi and NaOH-Pi in the various soil depths (Fig. 2) showed that although most of the Pi was located in the first 0–40 cm of the soil, there were Table 1

Summary of analysis of variance (significance of F values) to determine the effect of LPM rate, soil layer depth and their interactions on the concentrations of inorganic and organic soil P fractions in a Le Bras silt loam soil growing corn which had received LPM for 14 yearsa Source of variation Degrees of freedom Resin-Pi NaHCO3 NaOH HCl-Pi Residual P

Pi Po Pi Po

LPM rate (R) 4 254 106 55 43 95 297 46

Soil depth (D) 4 876 224 40 1180 338 25777 230

Interaction (R×D) 16 45 20 8 17 34 376 41

Model 24 218 69 21 214 95 4596 73

a_{F values significant at p<0.01.}

still substantial amounts in the lower depths. The av-erage level of NaHCO3-Pi+NaOH-Pi located in the 60–100 cm soil layer was ca. 222 mg kg−1 _{where P} had been applied (Fig. 2), and 182 mg kg−1_{where no} P had been applied (control soil profile) indicating that a constant input of P from LPM to surface soil result in an increase in Pi extracted by NaHCO3 and NaOH in deeper horizons.

3.2. Readily labile and moderately labile organic P pools (biological P)

As was observed for easily and moderately labile Pi forms, biological P (NaHCO3-Po and NaOH-Po) also increased with LPM application rate and de-creased with soil depth (Fig. 3). According to Cross and Schlesinger (1995) organically bound P could be a source of labile, plant-available P. Compared to the control, NaHCO3-Po and NaOH-Po contents in

Fig. 3. Distribution of (a) NaHCO3-Po and (b) NaOH-Po with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

the 0–20 cm layer increased, respectively, by 4–47 and 12–31 mg kg−1 _{for 30 to 120 m}3_ha−1 _LPM ap-plication rates. Similar to resin-Pi, NaHCO3-Pi and NaOH-Pi, frequent LPM applications had significant (p<0.01) effect on NaHCO₃-P_oand NaOH-P_o con-tent (Table 1). Although the amounts of NaHCO3-Po were smaller than those of NaOH-Po, these two frac-tions followed the same pattern of distribution in the soil profiles.

3.3. Stable P forms (HCl-Piand residual P)

In general, the HCl-Pi contents were very low in all the soil layers (Fig. 4), indicating that only a small proportion of manure P was added to this fraction. The HCl-Pi pool would correspond to the P bound to Ca at the surface of apatite and other minerals (Simard

Fig. 4. Distribution of HCl-Piwith depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

et al., 1995). Sharpley et al. (1991), studying the im-pact of long-term swine manure application on soil and water resources in Eastern Oklahoma reported that only small amounts of HCl-Piaccumulated in the soil profile. The content of HCl-Pi was shown to be not affected by repeated applications of barnyard manure (Campbell et al., 1986). As seen from Fig. 5, the ap-plication rates had little influence on the distribution of HCl-Piform in the soil profile when compared with each other. Large amounts of residual P were found in the soil profiles (Fig. 5). As the amount of resid-ual P in the first 0–40 cm layer ranged from 83.6% of the total stable P fraction for the control treatment to 93.0% for the highest rate of LPM. In all treatments,

Fig. 5. Distribution of residual P with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

the residual P distribution pattern followed the same trend as the labile and moderately labile P pools. With-out exception, concentrations of residual P decreased with depth.

4. Discussion

After 14 years of organic fertilization of corn, the amounts of soil P pools were significantly affected (p<0.01) by LPM treatments compared to the check (Table 1). The average soil P pool contents in the var-ious depths showed that both Piand Poconcentrations were highest in the manured soil (Figs. 1–5). This is in agreement with the result of Tran and N’dayegamiye (1995), who found that long-term application of dairy cattle manure to the same Le Bras soil increased both surface soil Pi and Po forms in each pool measured by the Hedley et al. (1982) fractionation procedure. Similar findings were also reported by Dormaar and Chang (1995), where after 20 years of cattle feed-lot manure (CFM) loading, the amounts of all labile Pi and Po forms in the Ap horizon of a Lethbridge loam soil generally increased with all CFM applica-tions compared with the check. On a profile basis, the soil receiving the highest rate of LPM (120 m3/ha) over 14 years contained 1.8-fold the total labile Poof the unamended soil, suggesting that long-term appli-cation of LPM and returning plant residues to the soil would increase the labile organic fractions in soil lay-ers. The increase in biological P (Po) was generally greater for soil samples with a high organic carbon (Co) content. A regression analysis was made between Po and the Co. The resulting equation was written as Po=92.29+5.56 Co, with a highly significant corre-lation coefficient of [r=0.85 (p<0.01)], suggesting a possible role of the soil organic fraction in the accumu-lation of Poin deeper depths due to P movement with microbial cells and cellular debris (Hannapel et al., 1964). On the other hand, the subsoil transport of or-ganic contamination is recognized to be related to the movement of soluble organic C (Donald et al., 1993). The role of dissolved organic carbon in the transport of P in agricultural and forested ecosystems has been observed by Qualls and Haines (1991) and Schoenau and Bettany (1987). The fact that bioavailable P lev-els in the lower soil depths were higher under LPM conditions would imply a downward movement of P

compounds. The leaching or translocation of P com-pounds would be responsible for the distribution of P in the various soil layers. Frequent application of animal manures to soils has been shown to promote downward movement of P in soil profiles (Mozaffari and Sims, 1994). The amounts of P in all fractions were significantly correlated with each other (data not shown). These relationships are in agreement with the known fact that available soil P pools are constantly replenished through reactions of dissolution or des-orption of more stable inorganic P and through the mineralization of organic P (Tiessen and Moir, 1993). Similar relationships were reported by McKenzie et al. (McKenzie et al., 1992a, b) in a long-term crop ro-tation and fertilizer effects study on P transformations in a Chernozemic and a Luvisolic soil. On a profile basis, increasingly higher average amounts of P pools were obtained in the following order (Figs. 1–5): P ex-tracted by NaOH (254 mg kg−1_{) > P extracted by resin} and NaHCO3(209 mg kg−1) > stable P (172 mg kg−1). Compared to the NaHCO3-Pi content, greater amounts of NaOH-Pi were found in the soil profiles. Tiessen and Moir (1993) reported that NaOH-extractable Piis thought to be associated with amorphous and some crystalline Al and Fe phos-phates while NaHCO3-extractable Pi is thought to consist of Piadsorbed on surfaces of more crystalline P compounds, sesquioxides or carbonates. The high proportion of NaOH-Piin the soil profile could proba-bly be due to amorphous Al and Fe retention. In many Quebec soils, P sorption capacity has been related to Fe and Al amorphous sesquioxides (Laverdière and Karam, 1984). For the manure Pisystem, moderately labile Pi would constitute the major soil P sink de-rived directly from added inorganic P and indirectly from amorphous Fe and Al-bound Pi.

On average, the amount of total labile Pi forms (304 mg kg−1_{) was larger than the amount of labile} Po forms (159 mg kg−1) and represented an impor-tant fraction of the total labile P (Pi+Po) pools. The proportion of total P as labile (resin-Pi+NaHCO3 -P+NaOH-P) in the soil profile increased from 61% (control treatment) to 79% (highest LPM rate). Com-pared to the control treatment, average geochemical P (resin-Pi+NaHCO3-Pi+NaOH-Pi+HCl-Pi+residual P) concentration increased in the soil profile by 16, 26, 33 and 50% for 30, 60, 90 and 120 m3ha−1_LPM, respectively. Moreover, average geochemical P

con-tent in the lower depths of the soil was higher under LPM conditions. Such results would indicate that high inputs of LPM would favour the movement and build up of P in lower depths.

When compared to the control treatment, the lower (30–60 m3ha−1_{) and higher (90–120 m}3_ha−1₎ ap-plication rates over 14 years produced, respectively, a 1.1- and 1.3-fold increase in HCl-Pi. A similar trend could be observed with the data of Tran and N’dayegamiye (1995) where manured soil contained 1.2-fold the HCl-Pi of the unamended soil. This was probably due to the low P input from mineralization of manure Pounder the prevalent acidic conditions in the soil profiles. The pH(H2O) values of the soil sam-ples ranged from 5.2 to 6.4, depending on treatment and soil depth. On a profile basis, the soil receiving the highest rate of P (1.70 Mg P ha−1_{) over 14 years} produced a 1.5-fold increase in residual P, suggest-ing that primary minerals present in the Le Bras soil could be important sinks for the soluble P originat-ing from LPM. The results are consistent with the findings of Sharpley et al. (1991) which established a relationship between levels of added P (from manure) and soil residual P. From an agronomic point of view, this increase would appear to have little practical sig-nificance considering the fact that this fraction would contribute little to actual P uptake by plants.

5. Conclusion

The results showed that 14-years of LPM applica-tion has increased soil P pools in the soil profile of Le Bras silt loam. There was a moderate accumulation of labile P pools in subsurface soil horizons compared with untreated soil. On average, 1.7-, 1.4-, 1.2- and 1.3-fold increases were found in resin-Pi, NaHCO3-P, NaOH-P and total labile P pools contents, respectively, of treated compared with untreated 0–100 cm profiles. Soil profiles which had received up to 60 m3ha−1 LPM for 14 years have a lower proportion of labile P pools than plots treated with 90–120 m3_ha−1_LPM. Although the downward movement of P in the soil pro-file has been demonstrated in this study by using the P fractionation procedure, further studies are needed to assess the impact of soil labile P pools accumulated through years of application of LPM on groundwater quality.

Acknowledgements

The financial support of this study by a grant from the National Science and Engineering Research Coun-cil of Canada and by a dedicated financial assistance provided to the senior author by the Institut National de la Recherche Scientifique, Université du Québec, are greatly appreciated. The authors acknowledge the Soil Research Service Staff of the Ministry of Agri-culture, Fisheries and Food of Quebec for allowing the use of one of their long term experimental trials to conduct the present study. Special thanks are extented to Mr. Denis Côté for his assistance in selecting the site for collection of soil samples. Mr. D. Côté kindly permitted access to his research plots and provided the chemical composition of liquid pig manure.

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increments to a depth of 100 cm to assess the move-ment and distribution of C, N and P in the soil profiles (Hountin, 1996). The three cores were composited, air-dried, crushed, and sieved to pass a 2-mm sieve prior to sequential P extraction.

2.2. Hedley soil P pools

The amounts of soil inorganic and organic P (Pi and Po, respectively) in labile (resin-Pi, NaHCO3-Pi and NaHCO3-Po), moderately labile (NaOH-Pi, NaOH-Po), relatively insoluble apatite-P (HCl-Pi) and resistant (in primary minerals) pools was determined in each soil layer samples by the procedure of Hedley et al. (1982) as outlined in O’Halloran et al. (1987). This fractionation technique uses a series of extrac-tants to identify labile inorganic (Pi) and organic (Po) fractions followed by the more stable forms. In brief, soil samples were sequentially extracted by each of the following extractants: HCO3-exchange resin (Dowex-50w-x8), 0.5 M NaHCO3 (pH 8.5), 0.1 M NaOH, 1 M HCl, after 16 h shaking with each extrac-tant. The residue containing fairly insoluble Pi and Po forms (residual P) was then digested using 30% H2O2 /concentrated H2SO4. Total P (Pi+Po) in the NaHCO3and NaOH extracts was determined accord-ing to the method of Rowland and Grimshaw (1985). The Po content of a given fraction was calculated by substracting Pi content from total P content of the extract. For all extracts the concentration of Pi was determined colorimetrically by the method of Mur-phy and Riley (1962) using ascorbic acid as reducing agent. Acid or alkali filtered extracts were neutralized prior to P determination.

2.3. Statistical analysis

All statistical analyses were performed using the Statistical Analysis System (SAS Institute, 1990) soft-ware package. Analysis of variance (ANOVA) of the main effect of LPM application rates and soil depths , and their interactions on mean concentrations of P pools were carried out using the general linear model (GLM) statistical program of the SAS statistical pack-age. Linear Pearson correlation procedure was used to show relationships between the various soil P pools.

Fig. 1. Distribution of resin-Pi with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

3. Results

3.1. Labile and moderately labile inorganic P pools

The distribution of resin-Pi in the soil pro-files is illustrated in Fig. 1. The resin-Pi repre-sented on average 21% of the total labile forms (resin-Pi+NaHCO3-P+NaOH-P) in the top surface layer. Essentially, resin-Pi represents the labile pool of native adsorbed P (Fitter and Sutton, 1975), even though some of the more soluble precipitated P forms may be extracted by the resin (Tiessen and Moir, 1993). The increase in resin-Piover the control in the 0–20 cm layer varied from 25 mg kg−1_{for the lowest} rate of LPM (30 m3ha−1_{) to 125 mg kg}−1_{for the} high-est rate of LPM (120 m3ha−1_{). The resin-P}

icontent of the surface layer was largely in excess for silage corn P requirement (52 mg kg−1_{as resin-P}

i) when rates of LPM >60 m3ha−1 _{were applied. On a profile basis,} the soil receiving the lower rates (30–60 m3ha−1_{) and} the higher rates (90–120 m3ha−1_{) of LPM contained,} respectively, 1.3- and 2.0-fold the resin-Pi of the un-amended soil. This finding suggests that continued surface application of LPM has a significant effect on resin-Piaccumulation and movement. A similar trend was also observed for a surface loamy soil (Cana-dian: Dark Brown Chernozemic; FAO: Kastanozem) brought about by 20 years of feedlot manure load-ings (0, 30, and 90 t/ha) under non-irrigated condition (Dormaar and Chang, 1995). The resin-extractable Pi constituted from 8 in the control soil to 32% of total P in the amended soil (90 t ha−1_).

Fig. 2. Distribution of (a) NaHCO3-Pi and (b) NaOH-Pi with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

As was observed for resin-Pi, the amounts of NaHCO3-Piand NaOH-Pi (Fig. 2) were significantly (p<0.01) affected by the LPM application rates

(Table 1) and decreased with soil depth. These results are in agreement with those of Meek et al. (1979), where it was observed that manure application at high rates to a calcareous soil caused P movement and a resulting increase in NaHCO3-extractable Pi at the 30–60 cm depth. O’Halloran (1993), in a study with dairy liquid manure (DLM) applied on a con-tinuous corn production under reduced tillage, found that DLM application affected the levels of labile Pi fractions. The NaHCO3-Pi levels (Fig. 2) were higher than those for resin-Pi (Fig. 1). On average, the values for NaHCO3-Pi/resin-Pi varied from 1.7 to 2.7 in the soil profiles. The average values for NaHCO3-Pi and NaOH-Pi in the various soil depths (Fig. 2) showed that although most of the Pi was located in the first 0–40 cm of the soil, there were Table 1

Summary of analysis of variance (significance of F values) to determine the effect of LPM rate, soil layer depth and their interactions on the concentrations of inorganic and organic soil P fractions in a Le Bras silt loam soil growing corn which had received LPM for 14 yearsa

Source of variation Degrees of freedom Resin-Pi NaHCO3 NaOH HCl-Pi Residual P

Pi Po Pi Po

LPM rate (R) 4 254 106 55 43 95 297 46

Soil depth (D) 4 876 224 40 1180 338 25777 230

Interaction (R×D) 16 45 20 8 17 34 376 41

Model 24 218 69 21 214 95 4596 73

a_{F values significant at p<0.01.}

still substantial amounts in the lower depths. The av-erage level of NaHCO3-Pi+NaOH-Pi located in the 60–100 cm soil layer was ca. 222 mg kg−1 _{where P} had been applied (Fig. 2), and 182 mg kg−1_{where no} P had been applied (control soil profile) indicating that a constant input of P from LPM to surface soil result in an increase in Pi extracted by NaHCO3 and NaOH in deeper horizons.

3.2. Readily labile and moderately labile organic P pools (biological P)

As was observed for easily and moderately labile Pi forms, biological P (NaHCO3-Po and NaOH-Po) also increased with LPM application rate and de-creased with soil depth (Fig. 3). According to Cross and Schlesinger (1995) organically bound P could be a source of labile, plant-available P. Compared to the control, NaHCO3-Po and NaOH-Po contents in

Fig. 3. Distribution of (a) NaHCO3-Po and (b) NaOH-Po with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

the 0–20 cm layer increased, respectively, by 4–47 and 12–31 mg kg−1 _{for 30 to 120 m}3_ha−1 _LPM ap-plication rates. Similar to resin-Pi, NaHCO3-Pi and NaOH-Pi, frequent LPM applications had significant (p<0.01) effect on NaHCO₃-P_oand NaOH-P_o

con-tent (Table 1). Although the amounts of NaHCO3-Po were smaller than those of NaOH-Po, these two frac-tions followed the same pattern of distribution in the soil profiles.

3.3. Stable P forms (HCl-Piand residual P)

In general, the HCl-Pi contents were very low in all the soil layers (Fig. 4), indicating that only a small proportion of manure P was added to this fraction. The HCl-Pi pool would correspond to the P bound to Ca at the surface of apatite and other minerals (Simard

Fig. 4. Distribution of HCl-Piwith depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

et al., 1995). Sharpley et al. (1991), studying the im-pact of long-term swine manure application on soil and water resources in Eastern Oklahoma reported that only small amounts of HCl-Piaccumulated in the soil profile. The content of HCl-Pi was shown to be not affected by repeated applications of barnyard manure (Campbell et al., 1986). As seen from Fig. 5, the ap-plication rates had little influence on the distribution of HCl-Piform in the soil profile when compared with each other. Large amounts of residual P were found in the soil profiles (Fig. 5). As the amount of resid-ual P in the first 0–40 cm layer ranged from 83.6% of the total stable P fraction for the control treatment to 93.0% for the highest rate of LPM. In all treatments,

Fig. 5. Distribution of residual P with depth in Le Bras silt loam soil amended with liquid pig manure over 14 years. Horizontal bars indicate standard deviation.

4. Discussion

After 14 years of organic fertilization of corn, the amounts of soil P pools were significantly affected (p<0.01) by LPM treatments compared to the check

(Table 1). The average soil P pool contents in the var-ious depths showed that both Piand Poconcentrations were highest in the manured soil (Figs. 1–5). This is in agreement with the result of Tran and N’dayegamiye (1995), who found that long-term application of dairy cattle manure to the same Le Bras soil increased both surface soil Pi and Po forms in each pool measured by the Hedley et al. (1982) fractionation procedure. Similar findings were also reported by Dormaar and Chang (1995), where after 20 years of cattle feed-lot manure (CFM) loading, the amounts of all labile Pi and Po forms in the Ap horizon of a Lethbridge loam soil generally increased with all CFM applica-tions compared with the check. On a profile basis, the soil receiving the highest rate of LPM (120 m3/ha) over 14 years contained 1.8-fold the total labile Poof the unamended soil, suggesting that long-term appli-cation of LPM and returning plant residues to the soil would increase the labile organic fractions in soil lay-ers. The increase in biological P (Po) was generally greater for soil samples with a high organic carbon (Co) content. A regression analysis was made between Po and the Co. The resulting equation was written as Po=92.29+5.56 Co, with a highly significant corre-lation coefficient of [r=0.85 (p<0.01)], suggesting a

possible role of the soil organic fraction in the accumu-lation of Poin deeper depths due to P movement with microbial cells and cellular debris (Hannapel et al., 1964). On the other hand, the subsoil transport of or-ganic contamination is recognized to be related to the movement of soluble organic C (Donald et al., 1993). The role of dissolved organic carbon in the transport of P in agricultural and forested ecosystems has been observed by Qualls and Haines (1991) and Schoenau and Bettany (1987). The fact that bioavailable P lev-els in the lower soil depths were higher under LPM conditions would imply a downward movement of P

downward movement of P in soil profiles (Mozaffari and Sims, 1994). The amounts of P in all fractions were significantly correlated with each other (data not shown). These relationships are in agreement with the known fact that available soil P pools are constantly replenished through reactions of dissolution or des-orption of more stable inorganic P and through the mineralization of organic P (Tiessen and Moir, 1993). Similar relationships were reported by McKenzie et al. (McKenzie et al., 1992a, b) in a long-term crop ro-tation and fertilizer effects study on P transformations in a Chernozemic and a Luvisolic soil. On a profile basis, increasingly higher average amounts of P pools were obtained in the following order (Figs. 1–5): P ex-tracted by NaOH (254 mg kg−1_{) > P extracted by resin} and NaHCO3(209 mg kg−1) > stable P (172 mg kg−1). Compared to the NaHCO3-Pi content, greater amounts of NaOH-Pi were found in the soil profiles. Tiessen and Moir (1993) reported that NaOH-extractable Piis thought to be associated with amorphous and some crystalline Al and Fe phos-phates while NaHCO3-extractable Pi is thought to consist of Piadsorbed on surfaces of more crystalline P compounds, sesquioxides or carbonates. The high proportion of NaOH-Piin the soil profile could proba-bly be due to amorphous Al and Fe retention. In many Quebec soils, P sorption capacity has been related to Fe and Al amorphous sesquioxides (Laverdière and Karam, 1984). For the manure Pisystem, moderately labile Pi would constitute the major soil P sink de-rived directly from added inorganic P and indirectly from amorphous Fe and Al-bound Pi.

On average, the amount of total labile Pi forms (304 mg kg−1_{) was larger than the amount of labile} Po forms (159 mg kg−1_{) and represented an} impor-tant fraction of the total labile P (Pi+Po) pools. The proportion of total P as labile (resin-Pi+NaHCO3 -P+NaOH-P) in the soil profile increased from 61% (control treatment) to 79% (highest LPM rate). Com-pared to the control treatment, average geochemical P (resin-Pi+NaHCO3-Pi+NaOH-Pi+HCl-Pi+residual P) concentration increased in the soil profile by 16, 26, 33 and 50% for 30, 60, 90 and 120 m3ha−1_LPM, respectively. Moreover, average geochemical P

con-tent in the lower depths of the soil was higher under LPM conditions. Such results would indicate that high inputs of LPM would favour the movement and build up of P in lower depths.

When compared to the control treatment, the lower (30–60 m3ha−1_{) and higher (90–120 m}3_ha−1₎ ap-plication rates over 14 years produced, respectively, a 1.1- and 1.3-fold increase in HCl-Pi. A similar trend could be observed with the data of Tran and N’dayegamiye (1995) where manured soil contained 1.2-fold the HCl-Pi of the unamended soil. This was probably due to the low P input from mineralization of manure Pounder the prevalent acidic conditions in the soil profiles. The pH(H2O) values of the soil sam-ples ranged from 5.2 to 6.4, depending on treatment and soil depth. On a profile basis, the soil receiving the highest rate of P (1.70 Mg P ha−1_{) over 14 years} produced a 1.5-fold increase in residual P, suggest-ing that primary minerals present in the Le Bras soil could be important sinks for the soluble P originat-ing from LPM. The results are consistent with the findings of Sharpley et al. (1991) which established a relationship between levels of added P (from manure) and soil residual P. From an agronomic point of view, this increase would appear to have little practical sig-nificance considering the fact that this fraction would contribute little to actual P uptake by plants.

5. Conclusion

The results showed that 14-years of LPM applica-tion has increased soil P pools in the soil profile of Le Bras silt loam. There was a moderate accumulation of labile P pools in subsurface soil horizons compared with untreated soil. On average, 1.7-, 1.4-, 1.2- and 1.3-fold increases were found in resin-Pi, NaHCO3-P, NaOH-P and total labile P pools contents, respectively, of treated compared with untreated 0–100 cm profiles. Soil profiles which had received up to 60 m3ha−1 LPM for 14 years have a lower proportion of labile P pools than plots treated with 90–120 m3_ha−1_LPM. Although the downward movement of P in the soil pro-file has been demonstrated in this study by using the P fractionation procedure, further studies are needed to assess the impact of soil labile P pools accumulated through years of application of LPM on groundwater quality.

Acknowledgements

The financial support of this study by a grant from the National Science and Engineering Research Coun-cil of Canada and by a dedicated financial assistance provided to the senior author by the Institut National de la Recherche Scientifique, Université du Québec, are greatly appreciated. The authors acknowledge the Soil Research Service Staff of the Ministry of Agri-culture, Fisheries and Food of Quebec for allowing the use of one of their long term experimental trials to conduct the present study. Special thanks are extented to Mr. Denis Côté for his assistance in selecting the site for collection of soil samples. Mr. D. Côté kindly permitted access to his research plots and provided the chemical composition of liquid pig manure.

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