Irrigation-Induced Changes in Phosphorus Fractions ... - PubAg - USDA

TECHNICAL ARTICLE

Irrigation-Induced Changes in Phosphorus Fractions of Caribou Sandy Loam Soil Under Different Potato Cropping Systems Zhongqi He, 1 Hailin Zhang, 2 and Mingchu Zhang'

Abstract: Sequential fractionation is a common method used in eval uating the impacts of soil management practices on soil phosphorus (P) distribution. However, to our knowledge, this method has not been used in investigating the effects of irrigation on the changes in soil P fractions. In this work, we measured sequentially extracted P by deionized H 20, 0.5 M sodium bicarbonate (pH 8.5), 0.1 M sodium hydroxide (NaOH), and 1 M hydrochloric acid (HCI) in Caribou sandy loam soil samples from 10 potato fields under different 3-year crop rotations both with and without irrigation. As inorganic fertilizer was applied to these fields, irrigation and rotation management practices mainly affected the dis tribution of inorganic P fractions, but had no significant changes of organic P fractions. The impact of crop rotation was mainly reflected by H 20-extractable P Irrigation had greater influence on stable or re calcitrant P in NaOH, HCI, and residual fractions. Higher levels of NaOH-extractable inorganic P were observed in soil from rainfed fields, whereas higher levels of HCI-extractable P were observed in soils under irrigated management. Our data indicate that irrigation may eventually decrease P availability and runoff potential in these potato soils over the long term because of the partial transfer of P in the sink from the active NaOH fraction to more stable HCI and residual fractions. Whereas in formation and knowledge derived from this study may shed some light on the transformation mechanism of soil P fractions for sustainable agri cultural production, more field data from short- and long-term experiments are needed to confirm our observations. Key words: Soil P fraction, potato, crop rotation, irrigation. (Soil Sci 2011;176: 676-683)

he potato L.) production system in T the northeast United States is characterized by short (2- or 3-year) rotation patterns, extensive tillage, and minimal crop (Solanum tuberosum

residue return, especially during the potato phase of the rotation. The overall productivity of this system has not increased for several decades, despite increased inputs of pesticides, nutrients, and irrigation (New England Agricultural Statistical Service, 2007). Thus, research is needed to investigate what factors constrain the potato production and to determine the optimal combination of practices required to increase productivity and sustainability of potato production in northeast United States. For this purpose, a field experiment was established in Presque Isle, Maine, with an overall goal of enhancing the sustainlUSDA-ARS, Southern Regional Research Center, New Orleans, LA.

Dr. Zhongqi He is corresponding author. E-mail: [email protected]

2 Dept. of Plant and Soil Sciences, Oklahoma State University, Stillwater, OK.

3 Dept. of High Latitude Agriculture, School of Natural Resources and Ag

ricultural Sciences, University of Alaska, Fairbanks, AK.

Received May 24, 2011.

Accepted for publication August 22, 2011.

Financial DisclosureslConflicts of Interest: None reported.

Copyright © 2011 by Lippincott Williams & Wilkins

ISSN: 0038-075X

DOl: 10.1097ISS.ObO 13e318233e5cd

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ability of the potato industry in the northeast United States (He et al., 2011, 2010; Larkin et al., 2011; Olanya et al., 2009). In this experiment, five potato cropping systems designed to address specific management goals of soil conservation, soil improvement, disease suppression, and a status quo standard rotation control were evaluated for their effects on soil proper ties, nutrient (N, P, K etc.) utilization, soilborne diseases of po tato, and soil microbial community characteristics. Each system was evaluated under both rainfed and irrigated conditions. Large amounts of phosphorus (P) are usually required for potato pro duction because of poor P take-up efficiency of potato crop (Alvarez-Sanchez et al., 1999). However, strict regulations on nutrient management and rising prices of P fertilizers drive the needs for improving P use efficiency. Because of this, investi gation on P status in soil, especially in potato field receiving high amounts of P application, is important for reducing Ploss from farmland and improvement of P utilization efficiency. The sequential fractionation scheme developed by Hedley et al. (1982) is a common method used in characterizing the biogeochemical P cycling in soils (Cross and Schlesinger, 1995; Takeda et al., 2009). The extracted P fractions typically com prised resin-P, 0.5 M sodium bicarbonate (NaHC0 3)-Pi (in organic P) and NaHC0 3-Po (organic P), and 0.1 M sodium hydroxide (NaOH)-Pi and NaOH-P o' 1 M hydrochloric acid (HCl)-P i, and residual P fractions. In the past decade, the pro cedure has been often modified by replacing the resin P with H20-extractable P to investigate P fractions in organic amend ments and agricultural soils (Codling, 2006; Dou et al., 2003; He and Honeycutt, 2001; He et al., 2003; Sui et al., 1999; Waldrip-Dail et al., 2009). In addition, whereas the original procedure reported Pi only in the HCl fraction, He et al. (2008; 2006c) demonstrated that Po could be present in the HCl frac tion in some manure and soil samples. Thus, it should be ex perimentally determined if the HCl fraction of a sample contains Pi only or both Pi and Po. Generally, resin (or H20)-P and NaHC03-P are considered labile P, NaOH-P moderately labile ~ and HCl-P and residual P-stable P. Recently, Negassa and Leinweber (2009) reviewed the Hedley sequential fractionation method used in studies of the effects of land use and manage ment systems on soil P. Based on the review, more than 100 articles related to the subject have been published in Soil Science and related journals in the past 25 years. Even so, there is no report on evaluating the impacts of irrigation on soil P frac tions using this method. It is important to know the P trans formation under different cropping systems so that farmers can manage them accordingly to sustain their production while minimizing the impact of agriculture on the environment. Pre viously, Falkiner and Polglase (1999) reported that irriga tion, spray-irrigated with secondarily treated municipal effluent, changed the forms and distribution of P in a young Pinus radiata plantation in southeastern Australia. They found that increases in labile Pi (membrane-exchangeable, bicarbonate-extractable, and in soil solution) were confined mostly to the O-to 0.5-m horizon. Soil Science • Volume 176, Number 12, December 2011

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Soil Science • Volume 1 76, Number 12, December 2011

In addition, large amounts of Po were mineralized within the surface 0.7 m, due to stimulation of microbial activity by in creased soil water. In this work, we measured sequentially extracted P fractions in 10 potato fields of Caribou sandy loam soil, which were sub jected to 3-year crop rotations with and without irrigation. Our goals were to (i) evaluate the effect of irrigation on soil P trans formation and (ii) improve our understanding on the mecha nisms of change ofP lability in agricultural soils. Information and knowledge derived from this study may shed light on the dynamics of soil P fractions for sustainable agricultural production.

MATERIALS AND METHODS Soil and Experiment Location The long-term field experiment was carried out at the US Department of Agriculture-Agricultural Research Service field research site in Presque Isle, Maine (latitude 46° 41' N, longitude 68° 2' W) on a Caribou sandy loam (fine-loamy, isotic, frigid Typic Haplorthods). The ratio of sand, silt, and clay in the soil was 51:41 :8. Carbon and N contents were 2.20/0 and 0.17% of dry mass, respectively. The mean annual temperature and pre cipitation were 4.4°C and 93 ern, respectively (Honeycutt et aI., 2005). The detail of the experiment was reported previously (He et aI., 2011, 2010). In brief, there were five cropping sys tems: (i) continuous potato (PP), a nonrotation control; (ii) status quo (SQ), a typical 2-year rotation practice in the area: potato (Year 1) followed by barley (Year 2); (iii) disease suppressive (DS): mustard green manure/winter rapeseed (Year 1}-sorghum sudangrass/winter rye (Year 2}-potato (Year 3); (iv) soil con serving (SC): barley underseeded with timothy (Year 1) timothy sod (Year 2}-potato (Year 3) with mulch after har vest; and (v) soil improving (SI): same as SC, with compost added to each crop. Fertilizer was applied at the annual rate of 2240 kg ha -1 of commercial 10-10-10 (NPK) fertilizer in bands approximately 5 ern to the side and 5 ern below the seed. All rotation entry points were grown each year under both irrigated and rainfed management with five replications arranged in a split-block design, with cropping system as the main plot and water management as the sub-plot. Each cropping system by water management sub-plot combination was 4 m wide by 16 m long, with a 16-m buffer between sub-plots to ensure separa tion of water management treatments applied with a lateral, overhead sprinkler irrigation system. Irrigation water (1.25-cm rainfall equivalent) was applied to all irrigated treatments when 25% of the tensiometers placed in irrigated plots registered 0.5 MPa or higher. Soil samples from all plots were collected in May 2007 at the end of a 3-year crop rotation. Eight cores of field moist soils (7-cm diameter, 20 ern deep) were collected from each plot, thoroughly mixed, and stored in sealed plastic bags at 4°C. A sub-sample of approximately 1 kg was subsequently sieved (2 mm) and stored in a cooler at 4°C until use.

Sequential P Fractionation Sequential P fractionation was performed according to a procedure documented by He et aI. (2006b). Briefly, 1.5 g of soil sample was extracted by 25 mL of deionized H 20 at 22°C. After 2-h shaking, the extracts were centrifuged at 14,000g for 30 min at 4°C, and the supernatant was passed through a 0.45-f..Lm filter. The residues were resuspended with 5 mL of deionized water, and the washing supernatant was discarded after centrifuging. Similarly, the residues were then sequentially extracted by 0.5 M NaHC0 3 (pH 8.5), 0.1 M NaOH, and 1 M HCI for 16 h each. Soluble inorganic P (PD in these extracts was determined by a © 2011 Lippincott Williams & Wilkins

Irrigation-Induced Changes in Phosphorus Fractions

modified molybdate blue method (He and Honeycutt, 2005), and total P was determined with an inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Plasma 400 Emission Spectrophotometer; Perkin-Elmer, Norwalk, CT). Organic P (Po) was calculated as the difference between total P and inorganic P

Enzymatic Hydrolysis of Organic P The hydrolyzable portion of organic P in H 20, NaHC0 3 , and NaOH fractions was evaluated according to He et aI. (2004b, 2006a). These fractions were first properly diluted and adjusted to pH 5.0. The incubation mixtures contained the diluted frac tions, enzymes (acid phosphatases from potato and wheat germ at 0.25 activity unit each, plus nuclease PI 2 activity unit mL- 1 mixture), and 100 mM Na acetate (pH 5.0). Enzymatic hydro lysis was carried out at 37°C for 1 h in a temperature-controlled shaker (250 revolutions/min). Controls were included whereby either the enzyme or samples were omitted. Enzymatically hy drolyzable organic P was estimated as the increase in Pi concen tration after enzymatic incubation (He and Honeycutt (2001).

Statistical Analysis The experiment was arranged in a completely randomized design with five replications. The data analysis package in Microsoft Excel 2007 was used for statistical analysis. The Descriptive Statistics Tool Data was used to calculate averages and S.E. Single-factor, two-factor with replication, or two-factor without replication analysis of variance (ANOVA) was used to evaluate the effects of crop rotation and irrigation on changes of soil P fractions. The Correlation Analysis tool was used to an alyze correlation coefficients between soil P fractions and other parameters.

RESULTS Total Soil P The concentration of total soil P in the 10 potato fields was around 1,600 mg kg -1 soil (Table 1). There was no statistically significant difference between the rainfed soil and irrigated soil

TABLE 1. Total Soil P (mg kg- 1 of Dry Soil) in Rainfed and Irrigated Potato Fields With Continuous Potato (PP), Status Quo Potato-Barley (SQ), Disease Suppressive (OS), Soil Conserving (SC), and Soil Improving (51) Cropping Systems

Rainfed A

pp

SQ DS SC SI Means of five cropping systemt Impact by crop rotation

S.E.

Irrigated A

Impact by S.E. Irrigation

1671a* 111 1628a 121 1672a 89 1664a 83 1616a 91 1517a 128 36 1614a 86 1758a 1693a 57 1531a 108 23 1591 29 1682 NS

NS NS NS NS NS t

NS

Data are presented as average (A) offive field replicates with S.E. *A same letter in the same column indicates no significant difference between the cropping systems at P = 0.05. "Jmpacts by crop rotation and irrigation were evaluated using two factor ANOYAwithout replication based on the 10 total soil P data points listed in the table. tStatistically significant at P = 0.05. NS: not statistically significant at P = 0.05.

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FIG. 1. Water-extractable inorganic (I) and organic (0) P in rainfed and irrigated potato fields with different cropping managements. Data are the average of five field replicates. Error bars represent the S.E. "ns" and *** are for not statistically significant at P = 0.05 and statistically significant at P = 0.001, respectively. For cropping management abbreviations, see Table 1.

in the same cropping system. Single-factor ANOYA analysis in dicated no significant difference (P > 0.05) impacted by crop rotation or irrigation. Two-factor ANOYA with replication anal ysis showed similar observations that irrigation did not affect soil total P content. In addition, there was no interaction of cropping systems and irrigation as the P values for F > F erit were 0.729, 0.136, and 0.916 for cropping system, irrigation, and of the in teraction of the two factors, respectively. However, overall, the apparent total soil P level in the rainfed soil was consistently higher than that in the corresponding irrigated soils. The a,:,e~age total soil P for five cropping systems between the two rrnga tion treatments was 1,682 and 1,591 mg kg -1. The difference of 91 mg P kg -1 of soil suggested that 3-year irrigation lowered the surface (~20 ern) soil P by an average of 5.4°;{, in these fields. Two-factor ANOYA analysis without replication indicated that the change was statistically significant at P = 0.05 (Table 1).

H20-Extractable P The levels of H 20 Pi ranged from 5.7 to 10.1 mg kg -1 of soil (Fig. 1). TI1e H 20 Pi in the PP and SQ fields was similar. Compared with the PP soil, crop rotations with DS, SC, and SI

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FIG. 2. Sodium bicarbonate-extractable inorganic (I) and organic (0) P in rainfed and irrigated potato fields with different cropping managements. Data are the average of five field replicates. Error bars represent the S.E. Symbol "ns" is for not statistically significant at P = 0.05. For cropping management abbreviations, see Table 1.

increased the level of H 20 Pi' The impact of crop rotations on H 20 Pi in both rainfed and irrigated soils was statistically significant (P = 0.001 and 0.01, respectively). The levels of H 20 P were about 40% to 50% less than those of H 20 Pi in these 10 soils. The impact of crop rotations on H 20 Po in both rainfed and irrigated soils was also statistically significant (P = 0.001 and 0.05, respectively). Irrigation increased H 20 Pi in both rotation systems. However, only the increase in the PP field was statistically sig nificant (P = 0.05). In contrast to the first PP soils, irrigation decreased the level of H 20 Pi in the DS, SC, and SI soils, al though the decrease was not statistically significant at P = 0.05. The distribution pattern and the impact of irrigation on H 20 Po were similar to those of H 20 Pi. The averages of total H 20-extractable P (organic + inorganic) in the five rainfed and five irrigated cropping systems were rather similar (11.0 vs. 10.9 mg kg -1 soil) (Table 2). The results indicated that irrigation did not affect the total H 20-extractable P pool in these fields.

NaHC0 3-Extractable P The second extractant, 0.5 M NaHC03 (pH 8.5), extracted about 15 to 20 times more P than H 20 from these soils (Fig. 2).

TABLE 2. Impacts of Irrigation on Soil P Fraction Distribution

Inorganic P

H 2O NaHC0 3 NaOH HCI Residual

Organic P

Total P

Rainfed

Irrigated

Rainfed

Irrigated

Rainfed

Irrigated

7.1±0.8 176.2 ± 4.5 770.7 ± 21.6 174.9 ± 3.6 NA

7.4 ± 0.7 ll S 153.2 ± 4.8* 572.0 ± 10.7 t 234.6 ± 5.9 t NA

3.9 ± 0.5 86.0 ± 3.7 302.7± 18.9 ND NA

3.4 ± 0.4 ll S 82.5 ± 6.6 ll S 350.4 ± 14.5 ll s ND NA

11.0 ± 1.3 262.2 ± 6.7 1061.0±17.8 174.9 ± 3.6 170.4 ± 3.7

10.9 ± 1.1 ns 235.6 922.4 234.6 188.8

± ± ± ±

7.2 t 20.1 t 5.9 t 2.0*

Data are presented as means and S.E. of P concentrations (mg kg 1 of dry soil) in the five cropping systems under rainfed or irrigation conditions shown in Figs. 1 to 5. *Statistically significant at P = 0.01, for difference in inorganic, organic, or total P in each fraction between rainfed and irrigated soils. tStatistically significant at P = 0.05, for difference in inorganic, organic, or total P in each fraction between rainfed and i~igated s~il~. . +Statistically significant at P = 0.001, respectively, for difference in inorganic, organic, or total P in each fraction between rainfed and irrigated soils. NA: not applicable; ND: not detected; NS: not statistically significant at P = 0.05.

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However, the relative changes in NaHC0 3 P between the crop ping systems were smaller than those in H 20 P and not statis tically significant (P = 0.05). The ratios of NaHC0 3 Pi:P0 were similar to the ratios of H 20 Pi:P0' although quantity ofNaHC0 3 Pi and Po was higher as compared with P from the water ex traction. The average of Pi in the NaHC0 3 fraction was 176 ± 4.5 mg kg -1 for the five rainfed fields and 153 ± 4.8 mg kg- 1 for the five irrigated field samples (Table 2). In addition, around 80 mg Po kg " ! soil was also extracted into the NaHC0 3 frac tions from both rainfed or irrigated fields. However, the differ ences were less than the variances among the field replicates so that no statistically significant impacts of irrigation on the NaHC0 3 P were observed in any of the five cropping systems (Fig. 2). However, the statistically significant impact of irriga tion was observed on NaHC0 3 Pi and total P when the general trends of changes ofNaHC0 3 P were analyzed using means of the five rainfed and five irrigated cropping systems (Table 2).

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NaOH-Extractable P Among the four sequential extractants, 0.1 M NaOH ex tracted most P from the soil samples (Fig. 3). Sodium hydroxide P accounted for about 63% and 58% of total soil P of rainfed and irrigated samples, respectively. Even so, there were no significant differences in either NaOH-P i or NaOH-P 0 among the five crop ping systems. The average of NaOH-P i was 770 ± 21.6 mg kg- 1 soil for the five rainfed samples and 572 ± 10.7 mg kg -1 soil for the five irrigated soil samples (Table 2). In all five soils, irriga tion consistently decreased the concentrations of Pi' The dif ferences in Pi between rainfed and irrigated plots were 203, 230, 150, 185, and 224 mg kg -1 soil for P~ SQ, DS, SC, and SI, respectively. These differences represented 26.6%, 27.8%, 21.5%,23.7%, and 28.6% of changes of NaOH-P i in these soils. The average of Po in the NaOH fractions was 290 ± 14.6 mg kg -1 soil for the five rainfed soils and 350 ± 14.5 mg kg- 1 for the five irrigated field samples (Table 2). The change trend of NaOH-extractable Po by irrigation was also consistent among the five cropping systems. Unlike NaOH-P i, irrigation increased NaOH-extractable Po by 41,133,34,17, and 13 mg kg -1 soil for PP, SQ, DS, SC, and SI, respectively.

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FIG. 3. Sodium hydroxide-extractable inorganic (I) and organic (0) P in rainfed and irrigated potato fields with different cropping managements. Data are the average of five field replicates. Error bars represent the S.E. Symbol "ns," ", "", and *** are for not statistically significant at P = 0.05 and statistically significant at P = 0.05, 0.01, and 0.001, respectively. For cropping management abbreviations, see Table 1.


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However, only the 56% increase with SQ was statistically sig nificant (P < 0.05). Relative increases with other four cropping systems were small (40/0-12%) and not statistically significant (P > 0.05) (Fig. 3). Thus, the overall impact of irrigation on NaOH-Po was also not statistically significant (Table 2).

The amount of P in 1 M HCI extracts of these soil samples measured by the molybdate blue method was higher than that measured by ICP-AES (data not shown). Because of the nega tive differences, we were unable to calculate the Po concentration in these HCI fractions; thus, we assumed that there was only Pi detected in the HCI fractions of these soil samples, and the concentrations measured by ICP-AES were used as the Pi con centrations in these HCI fractions. The concentrations of Pi in the HCI fractions of the five rainfed samples were about the same at around 170 mg kg -1 soil (Fig. 4). Although crop rota tions did not impact the Pi level in the HCI fraction, irrigation consistently increased HCI-extractable Pi in all five fields. The average of Pi in the HCI fractions was 175 mg kg -1 soil for the five rainfed samples, significantly lower than 235 mg kg -1 soil for the five irrigated field samples (Table 2). Irrigation statistically impacted the HCl-extractablc P levels in four of the five cropping systems (Fig. 4).

1000

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and irrigated potato fields with different cropping managements. No organic (0) P was detected in this fraction. Data are the average of five field replicates. Error bars represent the S.E. Symbol "ns," ", and ** are for not statistically significant at P= 0.05 and statistically significant at P = 0.05 and 0.01, respectively. For cropping management abbreviations, see Table 1.

After the four-step extraction, 160 to 200 mg P kg -1 soil remained in the residues (Fig. 5). Similar to the NaOH and HCI fractions, there were small differences in the residual P among soil samples with different cropping systems as the averages of the residual P were 170 and 189 mg P kg -1 soil for the five rainfed cropping systems and the five irrigated cropping systems, respectively (Table 2). However, compared with the rainfed soil samples, the residual P concentrations in the irri gated soil samples tended to be higher. The largest difference (30 mg P kg- 1 soil) was found in the soils from the SQ crop ping system, and the smallest difference (1 mg P kg - 1 soil) was with the soils from the SI cropping system. The high variance of residual P from the replicated plots resulted in only the largest www.soilscLcom


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FIG. 5. Residual P in rainfed and irrigated potato fields with

different cropping managements. Data are the average of five field replicates. Error bars represent the S.E. Symbol "ns" and ** are for not statistically significant at P = 0.05 and statistically significant at P = 0.01, respectively. For cropping management abbreviations, see Table 1.

impact of irrigation on residual ~ with the SQ soils being sig nificantly (P < 0.05) affected by irrigation. However, irrigation was found to increase in the residual P as shown by the statis tically significant difference between the means of the rainfed and irrigated soils (Table 2).

Enzymatically Hydrolyzable P There were no consistent findings for the hydrolysis ofP0 in the pH-adjusted H20, 0.5 M NaHC0 3 (pH 8.5), and 0.1 M NaOH extracts. In some extracts, the Pi concentrations were only slightly higher in the enzymatic incubation mixtures than the controls. In other samples, the Pi concentrations were even lower in the enzymatic incubation mixtures than the controls, appar ently because of experimental errors greater than the hydrolyz able P in those samples (He et aI., 2004b). Therefore, we concluded that there were no substantial amounts of enzymati cally hydrolyzable organic P in the H20, NaHC0 3 , and NaOH fractions; even distinguishable amounts of organic P were present in these fractions.

DISCUSSION P Distribution in Different Pools Phosphorus in the Caribou sandy loam soil with different potato-centered rotation systems distributed in the five pools were separated by the sequential fractionation in the order of H20-P NaHC0 3> H20 fractions. The impacts were mainly on the Pi portion. Irrigation had no impacts on the Po portion of these fractions. Perhaps this was due to little or no external Po input, but inorganic fertilizer was applied annually to these fields. Vu et aI. (2008) also reported that sequentially extracted Po fractions were either unaffected or decreased with the increasing inorganic fertilizer application rates. Otani and Ae (1999) found that Po in Andosols did not differ between treatments following 28 years of manure application, although the contents of total ~ total organic C, and Pi were significantly increased. Vu et aI. (2008) attributed this phenomenon to the increase in the rate of Po mineralization that resulted in stable levels of Po. As NaOH-P was the most abundant pool in most soils in this experiment, it is regarded as a primary P sink of soil. Similar finding was reported by field and laboratory P in cubation with animal manure (He et aI., 2004b, 2006b; Zheng et aI., 2002, 2004). Moreover, this study showed that P in the NaOH pool seemed to be mobilized and redistributed to other © 2011 Lippincott Williams & Wilkins




pools by irrigation. The mobilized P portion was partly con verted to Po in the same fraction, and the majority was transferred to more stable or recalcitrant HCI and residual fractions. How ever, the combined increase in Pi in the HCI fractions and Po in the NaOH fractions due to irrigation was 116 mg kg -1 soil, less than 199 mg Pi kg -1 soil or 58% of the average reduced amount in the NaOH fractions by irrigation. In other words, another 42 % of Pi in the NaOH fractions reduced by irrigation might have been transferred to NaHC0 3 and H20 fractions. As P in these two fractions was more labile, this portion ofP from NaOH-P i did not accumulate in the two fractions but was rather reduced by plant uptake, runoff, or leaching from the irrigated soils. A comple mentary pattern of changes of P in H20, NaHC0 3 , and NaOH fractions observed in laboratory incubation experiments could support this hypothesis (He et al., 2004b, 2006b; Waldrip-Dail et al., 2009). Another possibility was that the downward move ment of P in the irrigated surface soils was faster than that in the rainfed surface soils. This downward movement has been ob served with long-term P fertilization (He et al., 2009; Wang et al., 2007), although we have not seen any reports on the effect of irrigation.

Possible Mechanisms of P Transfer Between Fractions To improve understanding of the P transfer mechanisms between different fractions, using all data from the 10 soils (five rainfed and five irrigated soils), we analyzed the correlation

coefficients between soil P fractions, soil total P, and relevant parameters published previously (He et al., 2010). The H20-P i did not correlate statistically with P in other fractions, soil pH, and phosphatase activities (Table 3A), perhaps due to its high lability, which made this portion of P not retained in these soils to be correlated with P transferring parameters. The positive significant correlation coefficients between the NaHC0 3-Pi, NaOH-P i, and soil total P implied that fertilizer P applied to the soils could be mainly deposited into the two fractions. The HCI Pi and residual P fractions were negatively correlated with the NaHC0 3-Pi and NaOH-P i fractions, which confirms the transfer of some P from the NaHC0 3 andiorNaOH fractions to the stable HCI and residual fractions. Soil pH seems to be the major factor causing the transfer of NaOH-Pi to HCI-Pi, as soil pH was negatively correlated with NaOH-P i but positively with HCI-Pi. None of the three soil phosphatase activities showed significant correlation with the soil Pi fractions. The correlations between these P fractions in terms of total extracted P (Pt ) were the same as those in Pi fractions, although the correlation coefficients were not exactly the same (Table 3B). Unlike NaHC0 3-P i , the NaHC0 3-Pt was negatively correlated with soil pH, alkaline phosphatase, and phosphodiesterase. Apparently, the contribution of NaHC0 3-P0 made these corre lations statistically significant. In other words, the portion of NaHC0 3-Po might have been actively involved in the inter change of P pools, even though no significant changes were observed in NaHC0 3-Po levels among the 10 soils (Fig. 2). Our observation was supported by the findings of Chen et al, (2002),

TABLE 3. Correlation Coefficients Among Soil P Fractions, SoilTotal P, Soil pH, Acid Phosphatase (acPase), Alkaline Phosphatase

(aIPase), and Phosphodiesterase (diPase) A. With Pi (Inorganic P) in Fractions NaOH-P i NaHC0 3-Pi NaOH-P i HCI-Pi Residual P Total P Soil pH acPase alPase diPase

-0.223 -0.134 0.227 0.283 0.064 -0.033 0.187 0.464 00411

0.810* -0.625 -0.901 t 0.653 t -0.558 0.326 -0.324 -0.417

Residual P

Total P

-0.890 t -0.866* 0.766* -0.768* 0.531 -0.269 -0.267

0.751 t -0.504 0.636 t -0.416 0.279 0.260

-0.571 0.571 -0.417 0.259 0.355

-0.853* 0.238 -0.500 -0.457

NaOH-P t

HCI-P t §

Residual P

Total P

-0.782* -0.732* 0.929 t -0.868* 0.375 -0.430 -0.393

0.751 t -0.504 0.636 t -0.416 0.279 0.260

1.000 -0.571 0.571 -0.417 0.259 0.355

1.000 -0.853* 0.238 -0.500 -0.457

B. With P t (Soil Total P) in Fractions

NaHC0 3-Pt NaOH-P t HCI-Pt Residual P Total P Soil pH acPase alPase diPase

-0.274 0.076 0.104 0.217 0.124 -0.125 0.222 00412 0.366

0.833* -0.568 -0.788* 0.852* -0.781 * 0.161 -0.651 t -0.662 t

*Coefficients are statistically significant at P = 0.01. "Coefficients are statistically significant at P = 0.001. tCoefficients are statistically significant at P = 0.05. §HCl-Pt is equal to HCl-P i .


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He et al.

who reported that the NaHC0 3-P0 in the rhizosphere sandy loam soil with perennial ryegrass and radiata pine was nega tively related to increases in organic C, microbial biomass, and alkaline phosphatase and phosphodiesterase activities.

CONCLUSIONS In this work, we evaluated sequentially extracted P in Caribou sandy loam soil samples from five potato production systems in a 3-year crop rotation with and without irrigation. As only inorganic fertilizer was applied to these fields, these man agement practices mainly impacted the distribution of inor ganic P fractions, with little significant changes in organic P fractions. Crop rotation and irrigation impacted soil P distribu tion in two different patterns. The most labile P fraction, that is, H20-extractable P, was significantly impacted by crop rotation, with the highest H20-extractable P found in the continuous potato and the SI cropping managements (i.e., barley under seeded with timothy (Year 1}--timothy sod (Year 2}--potato (Year 3) with mulch after harvest, compost application instead of commercial fertilizer). Irrigation had greater influence on stable and recalcitrant P fractions. Higher levels of NaOR-extractable inorganic P were observed in soil from rainfed fields, whereas higher levels of HCI-extractable P and residual P were observed in soils under irrigated management. Our data suggested that irrigation seemed to mobilize and redistribute part of the NaOH P The mobilized P portion was partly becoming labile P for plant uptake and also subjected to runoff loss or was converted to Po in the same fraction, or perhaps was transferred to more stable or recalcitrant HCI and residual fractions. Correlation analysis suggested that soil pH was the major factor contributing to the conversion of P between different fractions with functions of alkaline phosphatase and phosphodiesterase in NaRC0 3 P To our knowledge, this work is the first report on using the se quential fractionation to characterize the impact of irrigation on soil P distribution, which improves our understanding on the mechanisms of P lability changes. More short- and long-term field experiments are needed to further confirm our observations. ACKNOWLEDGMENT Trade names mentioned in the article are for information only and do not constitute endorsement, recommendation, or exclusion by the US Department of Agriculture-Agricultural Research Service. REFERENCES Alvarez-Sanchez, E., 1. D. Etchevers, 1. Ortiz, R. Nunez, V Volke, L. Tijerina, and A. Martinez. 1999. Biomass production and phosphorus accumula tion of potato as affected by phosphorus nutrition. 1. Plant Nutr. 22: 205-217. Chen, C. R., L. M. Condron, M. R. Davis, and R. R. Sherlock. 2002. Phos phorus dynamics in the rhizosphere of perennial ryegrass (Lolium perenne L.) and radiata pine (Pinus radiata D. Don.). Soil Biol. Biochem. 34: 487-499. Codling, E. E. 2006. Laboratory characterization of extractable phosphorus in poultry litter and poultry litter ash. Soil Sci. 171:858-864. Cross, A. E, and W H. Schlesinger. 1995. A literature review and evaluation of the Hedley fractionation: Applications to the biogeochemical cycle of soil phosphorus in natural ecosystems. Geoderma 64: 197-214. Dou, Z., G. Y. Zhang, W L. Stout, 1. D. Toth, and 1. D. Ferguson. 2003. Efficacy of alum and coal combustion by-products in stabilizing manure phosphorus. 1. Environ. Qual. 32:1490-1497. Falkiner, R. A., and P. 1. Polglase. 1999. Fate of applied phosphorus in

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