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Published online 11 January 2008
Published in Soil Sci Soc Am J 72:90-97 (2008)
DOI: 10.2136/sssaj2006.0344
© 2008 Soil Science Society of America
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SOIL CHEMISTRY

Distribution of Phosphorus Forms in Soil Following Long-term Continuous and Discontinuous Cattle Manure Applications

Xiying Hao*, Frauke Godlinski and Chi Chang

Lethbridge, AB T1J 4B1 Canada, Agriculture and Agri-Food Canada, Lethbridge Research Center, 5403 1st Ave S., Lethbridge, AB, T1J 4B1 Canada

* Corresponding author (haoxy{at}agr.gc.ca).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Better information on soil P and its distribution in various forms is needed to assess their bioavailability and environmental impact. This study investigates the distribution of P forms following long-term manure application to a calcareous clay loam soil in a semiarid region of Canada. The manure was applied and incorporated annually each fall at 30, 60, and 90 Mg ha–1 yr–1 under rain-fed and at 60, 120, and 180 Mg ha–1 yr–1 under irrigated conditions, continuously for 30 yr and as well as for 14 yr followed by 16 yr of no further application. After 30 yr, soil samples were collected from depths 0 to 15 and 15 to 30 cm for all treatments. Soil P was partitioned into different forms through sequential chemical fractionation. In comparison with the non-manured control, manure application significantly increased the levels of all P forms. The largest increase occurred for soil test P (STP). The increase was greater in irrigated than in rain-fed blocks and greater in the 15- to 30-cm than in the 0- to 15-cm depth. The STP/ sum of all P forms in the soil (TP) ratios of 0.42 to 0.47 in manured soil were significantly higher than in the non-manured control (<0.20), but were not significantly different from the cattle manure applied (0.38). In both soil depths, most P was inorganic P (Pi) with organic P (Po) accounting for ≤5% P. The STP (877 mg kg–1) and TP (2555 mg kg–1) levels in the topsoil of the plots where manure application had been discontinued were higher than in the non-manured control, showing that the effects of manure application in excess of plant use are still evident even 16 yr after the last manure application. Thus, manure application has long lasting and substantial residual effects, still measurable in all soil P forms.

Abbreviations: OC, organic carbon • Pi, inorganic P • Po, organic P • Pt, total P in a particular fraction • STP, soil test P • TC, total carbon • TN, total nitrogen • TP, sum of Pt of all P forms in the soil


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Phosphorus management is important both agronomically and environmentally (Sharpley et al., 1995). An inadequate concentration of P in agricultural soil will hinder crop production, while high P concentrations can lead to eutrophication of nearby surface water (Arai et al., 2005). Applying livestock manure increases P concentration in agricultural soil (Chang et al., 1991; Nair et al., 1995; Zhang and MacKenzie, 1997; Whalen and Chang, 2001; He et al., 2004). In areas with a high density of livestock operations, the majority of farms exceed either manure N- or P-based land application standards (Saam et al., 2005), which vary across states and provinces in North America. This excess application has been linked to decreases in water quality in nearby streams and lakes (Graetz et al., 1999).

Most manure P is in inorganic form (Sharpley and Moyer, 2000; Eghball, 2003) so availability following application to soil should be high (Eghball et al., 2005). Dou et al. (2000) found that most (up to 84%) manure P was in available forms and consequently susceptible to runoff loss after land application. Cooperband et al. (2002) reported that P in manure is commonly associated with Ca and Mg. The high Ca content of some manure may be responsible for modifying the P sorption characteristics of manured soils (Robinson and Sharpley, 1996). This modification along with formation of soluble Ca-P complexes with manure application may contribute to the downward movement and mobility of P in the soil profile (Holford et al., 1997; Siddique and Robinson, 2003). The P from manure may move in soil more readily than P from fertilizer (Eghball et al., 1996), possibly due to a reduction in P bonding strength (Field et al., 1985) and decreased P sorption (Sharpley et al., 1993) following manure application.

Sharpley et al. (1984) reported that land application of cattle feedlot manure resulted in increased soil total P (TP), inorganic P (Pi), organic P (Po), and soil test P (STP = H2O-P + NaHCO3–Pt) levels and a decreased soil P sorption index in soil surface soil (0–30 cm). Although increases in soil P concentrations were observed in a calcareous soil to as deep as 210 cm, James et al. (1996) concluded the increases in soil P from manure application do not threaten soil or groundwater quality under deep calcareous soils when limited by acceptable NO3– loading rates. Fertilizer application does not always result in higher P concentrations in the soil leachates (Godlinski et al., 2004) and groundwater. However, Kashem et al. (2004) concluded that as the capacity of the soil to retain added P in stable form is used up, retained P is subsequently held less tightly causing an increase in the ratio of STP to TP. Thus, the risk of P loss from soil will increase as the rate of organic amendment increases.

Soil P bioavailability and mobility depend in part on the amounts of different P forms present in soil (Tiessen et al., 1984). To determine their distribution, a sequential chemical P fractionation procedure was developed by Hedley et al. (1982). This procedure partitions soil P into plant available STP, Fe-oxide- and Al-oxide-associated P (NaOH-extractable), Ca-bound P (HCl-extractable) and residual P (H2SO4 digestible). Information obtained from this chemical soil P fractionation has been useful in determining plant availability of P (Tiessen and Moir, 1993; Cox et al., 1997) in soil receiving poultry manure (Mozaffari and Sims, 1996; Hountin et al., 2000; Sharpley and Moyer, 2000; Adeli et al., 2005), beef manure (Akhtar et al., 2005), pig manure (Sharpley et al., 2004), dairy manure (Nair et al., 1995; Gale et al., 2000; Lehmann et al., 2005), and municipal sewage sludge (Sui et al., 1999). He et al. (2004) showed that single applications of dairy manure and inorganic fertilizer caused similar rates of change in the H2O-P, NaHCO3–Pt and NaOH-Pt forms and they attributed this to the inherent soil properties controlling P transformation in soil.

Understanding the transformation and distribution of various P forms in soil receiving continuous manure application as well as manure's residual effect is essential in developing appropriate P management strategies for sustainable agricultural production and environmental protection. However, there is little information about the distribution of P forms, especially in calcareous soil receiving long-term heavy cattle manure applications. The objectives of our study were to determine how P distributes across different soil P forms under both rain-fed and irrigated conditions in response to (i) three different high rates of annual cattle feedlot manure applications over 30 yr and (ii) the same rates of manure application for 14 yr followed by 16 yr with no further applications.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Site Description and Experimental Design
The experiment was conducted using a calcareous Dark Brown Chernozemic clay loam soil (Typic Haplustolls) in southern Alberta, Canada. Details of the soil and the random split plot experimental design were previously reported by Sommerfeldt and Chang (1985) and Hao et al. (2004). Beginning in 1973, solid cattle feedlot manure was applied each fall after harvest and incorporated immediately after application by one of three methods: plow, rototiller, or cultivator plus disk. Within each tillage treatment (main plot), manure was applied to subplots (7.5 by 15 m) in the rain-fed block at 0 (Treatment Mr0), 30 (Mr30), 60 (Mr60), and 90 (Mr90) Mg ha–1 yr–1 (wet weight) and in the irrigated block at rates of 0 (Mi0), 60 (Mi60), 120 (Mi120), and 180 (Mi180) Mg ha–1 yr–1. Main and subplot treatments were assigned randomly and replicated three times. Application rates of 30 and 60 Mg ha ha–1 yr–1 were the local recommended agronomic rates for rain-fed and irrigated crop production, respectively, at the time the experiment began.

Since 1987, manure has been incorporated in all main plots with a cultivator plus disk due to lack of tillage effect (Chang et al., 1991), which increased the number of replicated manure treatments from three to nine. Also that year, manure applications to three replications originally rototilled were stopped to evaluate the residual effects of 14 annual manure applications on soil properties and crop production. This created three new discontinued manure treatments in the rain-fed (Dr30, Dr60, and Dr90) and irrigated (Di60, Di120, and Di180) blocks (Table 1 ), leaving the continuously manured treatments with six replications. Over the years, some replications were removed and used in other studies so the number of replications for the 7 manure treatments has since varied between 2 and 6.


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  		Table 1. Experimental treatments.

 
Solid beef cattle feedlot manure used throughout this study was obtained from a single commercial cattle feedlot on a clay soil with about 20% sand and 45% clay, compared with the experimental site's clay loam soil containing 38% sand and 31% clay (Gao and Chang, 1996). In the first 20+ yr, the feedlot did not use bedding. Later, straw was added as a bedding material in the winter months. The basic properties of the cattle manure are listed in Table 2 . The low total carbon (TC) content indicates that a large amount of mineral soil particles (66.3% of total manure dry weight) from the feedlot were mixed with the manure. The amount of STP in the manure varied considerably (13 to 62% of TP) with a 30-yr average value at 38% of TP. Manure total dry matter, organic carbon (OC), total nitrogen (TN), and TP applied over 30 yr are given in Table 3 .


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  		Table 2. Characteristics of solid cattle feedlot manure used.

 

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  		Table 3. Total amount of cattle manure dry matter, organic carbon (OC), total nitrogen (TN), total phosphorus (TP), and soil test phosphorus (STP) added over 30 yr.

 
The study area has a semiarid climate. During the 30-yr experimental period (1973 to 2003), the average annual precipitation of 381 mm was lower than the long-term average of 398 mm (>90 yr). Annual precipitation varied between 142 mm (2002) and 695 mm (1978). The annual mean temperature of 6.0°C during this period was higher than the long-term average of 5.3°C. The amount of irrigation water averaged 160 mm yr–1, with a maximum value of 432 mm in 2000 and a minimum value of 0 mm in 1995. Barley was grown on the rain-fed block in all years except 1996 (canola) and 1998 to 2000 (triticale). The same crops were sown on the irrigated block in all years except 1997 to 2000 (corn). All aboveground biomass was removed from the experimental site except about 15 cm of stubble. After harvest, the manure was applied and incorporated into the soil together with the remaining stubble.

Soil Chemical Properties and Phosphorus Fractionation
Soil samples were collected in spring 2003 from all 14 different managed treatments at depths of 0 to 15 cm and 15 to 30 cm (Table 1). All soil samples were air-dried and passed through a 2-mm sieve. Soil TC and TN were determined using fine ground samples (<150 µm) by dry combustion techniques using a Carlo Erba CNS analyzer (Carlo Erba, Milan, Italy). Inorganic C was measured following the method of Amundson et al. (1988). The OC was calculated as the difference between TC and inorganic C.

The P fractionation procedure was similar to Sui et al. (1999), who modified the method of Tiessen and Moir (1993) by (1) not using a resin strip in the water extraction and (2) determining the P concentration in the water extract directly after shaking for 16 h. Briefly, soil was sequentially extracted with H2O, 0.5 M NaHCO3, 0.1 M NaOH, 1 M HCl, and 18 M H2SO4 (+ H2O2 and Li2SO4 + Se powder for digestion). The Pi concentration was measured directly in the extracts. For NaHCO3 and NaOH extracts, the Pt concentration was measured after digesting with ammonium persulfate + 0.9 M H2SO4. The Po in these two extracts was calculated as the difference between Pt and Pi. The TP of the soil is the sum of P over all forms. The P concentration in the extracting solution, either undigested or digested, was determined using an Astoria A2 (Clackamas, OR, USA).

Data Handling and Statistical Analysis
The data were analyzed using the General Linear Model Procedure of SAS Institute Release 8.2 (SAS Institute, 2002). For each P form, the differences among the manure application rates and sampling depths were examined by a standard variance analysis procedure with means separation by Tukey's procedure, performed separately for rain-fed and irrigated conditions. When treatment effects were not significant, data were pooled together and reanalyzed. Contrast analysis was used to examine the irrigation effect for the M60 and D60 treatments and soil depth effect (0 to 15 vs. 15 to 30 cm) for all manure treatments. The STP/TP ratios in soil and applied manure were calculated and effects of manure application on the STP/TP ratio in soil were examined using contrast analysis. The relationships between the amount of manure TP applied and concentrations of various P forms, including the STP/TP ratio, were investigated using regression analysis.


   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Organic Carbon and Total Nitrogen
Organic C and TN increased significantly with the rate of manure applications in both soil depths (0 to 15 and 15 to 30 cm, Table 4 ) after 30 yr of manure application, with the differences between the manured and non-manured control soil most pronounced in the 0- to 15-cm depth. The OC and TN content reached as high as 125.2 g C kg–1 and 13.16 g N kg–1 at the highest application rate (Mi180) (Table 4), but were still significantly lower than the average values for manure (195.6 C and 16.5 g N kg–1, Table 2).


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  		Table 4. Effect of 30-yr cattle manure treatments on organic carbon (OC), total nitrogen (TN) and total phosphorus (TP) in soil under rain-fed and irrigated conditions.

 
In the discontinued plots, after 16 yr without manure application the OC and TN content were significantly lower than the comparable continuous plots. Soil OC (both rain-fed and irrigated conditions) and TN (rain-fed only) content in surface soil in all discontinued manure treatments are not significantly different from values for the control treatments. Similarly, at 15- to 30-cm depth, soil OC contents (both rain-fed and irrigated conditions) and TN (irrigated only) in all discontinued manure treatments were not significantly different from OC values in the non-manured controls.

Our data suggests that manure application increases soil OC content leading to C sequestration, which is beneficial in reducing the greenhouse effect and global warming. But once the manure application is discontinued, the OC sequestered in soil may eventually re-release back into atmosphere and soil OC will eventually return to premanure treatment levels. The soil OC premanure treatment level reflects the balance of C input–output under cereal crop production under experimental climatic conditions.

Effect of 30 yr Annual Manure Applications on Soil Phosphorus
0- to 15-cm Soil Depth
The 30 annual manure applications resulted in a large input of P (up to 19.78 Mg ha–1 for Mi180) to the soil (Table 3). The TP concentrations in the topsoil (0 to 15 cm) of the non-manured control plots were 1382 (Mr0) and 1375 mg kg–1 (Mi0). The highest rates of manure application over 30 yr resulted in a TP concentration of 3937 mg kg–1 in the rain-fed (Mr90) and 6287 mg kg–1 in the irrigated block (Mi180) (Table 4), almost three (Mr90) and five (Mi180) times greater than the non-manured control. For all P forms, the P concentration in topsoil increased significantly with manure application rates under both rain-fed and irrigated conditions (Tables 5 and 6 ).


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  		Table 5. The concentration of various soil P forms after 30-yr cattle manure treatments under rain-fed conditions.

 

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  		Table 6. The concentration of various soil P forms after 30 yr cattle manure treatments under irrigated conditions.

 
After 30 yr, the STP concentrations increased to 902 mg kg–1 at the lowest manuring rate (Mr30) and reached up to 2940 mg kg–1 at the highest manuring rate (Mi180) compared with <300 mg kg–1 for non-manured controls (Mr0 and Mi0). These correspond to STP increases of three to twelve times (Tables 5 and 6 and Fig. 1 ). In their 6-yr study, Akhtar et al. (2005) found that NH4Cl-extractable P in 0- to 15-cm manured soil was only five times higher than in non-manured soil. The range of results in this study can be attributed to both the number of years and rates of manure applications.


Figure 1
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  		Fig. 1. Increases in various soil P forms in manured soil relative to the non-manured control treatment after 30-yr annual cattle manure application under rain-fed and irrigated conditions.

 
While the mean STP/TP ratio was 0.20 for the rain-fed and irrigated non-manured controls, it increased with the rate of manure application and reached a plateau at about 0.47 (Fig. 2a ). For all manure annual rates > 30 Mg ha–1 yr–1, the STP/TP ratios were not significantly different from the 0.38 ratio in the applied cattle manure (Chang et al., 2005). The similar STP/TP ratios in the manured soil and the manure applied suggests the rate of NaOH-Pt, HCl-P, and H2SO4–P transformation to STP (source) is similar to the rate of crop uptake, downward movement and the transformation of STP to other P forms (sinks) in manured soil.


Figure 2
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  		Fig. 2. Soil test P (STP) to TP ratio in response to manure TP applied over (a) 30 yr and (b) 14 yr with 16 yr no further application (STP = H2O-P + NaHCO3–Pt).

 
The NaOH-Pt and HCl-P forms in the manured plots were 1.7 to 6.0 times higher than in the non-manured controls (Fig. 1). The increases were significantly related to the rate of manure P application (Tables 5 and 6), but were similar to soil TP increases, leading to similar contributions (9 to 15% for NaOH-Pt and 30 to 34% for HCl-P) to TP among all treatments (Fig. 1). The high levels of HCl-P content imply that approximately 30% of P in the manured soil is associated with Ca, consistent with the results of Daroub et al. (2000), who reported the Ca-bound P form accounted for 15 to 42% of TP in three long-term research sites. The H2SO4–P form was less responsive to the manure applications (Table 5), leading to a significant decrease in its contribution to TP (Fig. 1).

The concentration of Po (extracted by NaHCO3 and NaOH) was much smaller than Pi. The Po concentration increased with the rate of manure application under rain-fed conditions while no significant differences were observed under irrigation (Tables 5 and 6). However, under both rain-fed and irrigated conditions, Po accounted for ≤5% TP and this percentage was not affected by the manure treatment. Other studies have also shown greater increases in Pi than Po in soils that have received long-term applications (>10 yr) of different types of manures (Gale et al., 2000; Motavalli and Miles, 2002; Sharpley et al., 1998; Sharpley et al., 2004). The Po level may be less important in soils with a long history of high manure applications than following medium- and short-term manure applications (Lehmann et al., 2005).

The effect of irrigation on P form distribution was also assessed using the M60 treatment. Significantly lower STP (H2O-P + NaHCO3–Pt), HCl-P, H2SO4–P, and TP levels, but a significantly higher NaOH-Pt level, were associated with irrigation when manure was applied at 60 Mg ha–1 annually for 30 yr (Mr60 vs. Mi60) (Tables 5 and 6). The lower STP levels with irrigation may reflect greater crop uptake and possibly downward leaching to lower soil depths. The lower H2SO4–P and HCl-P levels are probably due to greater transformation from H2SO4–P and HCl-P to NaOH-Pi under better moisture conditions with irrigation.

15- to 30-cm Soil Depth
The TP concentration in the 15- to 30-cm soil increased from ≤1000 mg kg–1 in the controls to as high as 3226 mg kg–1 under rain-fed (Mr90) and 5577 mg kg–1 under irrigated conditions (Mi180) (Table 4). The increases of various P forms were directly related to the rate of cattle manure application and significant for every P form, except NaHCO3–Po and NaOH-Po, under both rain-fed and irrigated conditions (Tables 5 and 6).

Similar to topsoil, the increase was greatest in STP while H2SO4–P increased least in response to the manure application (Fig. 1). The STP in the rain-fed and irrigated manure plots was 7.1 to 38.9 times greater than the control. In contrast, results from a 6-yr study with four beef manure applications on a silt loam soil indicated no increase in the 15- to 30-cm soil depth (Akhtar et al., 2005). These differences can be explained by the much longer time and higher manure application rates in our experiment. The mean STP/TP ratio was <0.1 for the rain-fed and irrigated controls and increased with the rate of manure application to 0.34–0.43 (Fig. 2b). For all manure annual rates >30 Mg ha–1 yr–1, the STP/TP values were not significantly different from the ratio (0.38) reported for the applied cattle manure (Chang et al., 2005).

The NaOH-Pt and HCl-P in the 15- to 30-cm layer increased 1.9 to 9.2 times in manured treatments compared with the non-manured control (Table 5). These rates of increase were similar to that of TP, so their contributions to TP (9 to 14% for NaOH-Pt and 28 to 33% for HCl-P) were similar for all manure application rates. A much higher contribution (56%) of HCl-P to TP has been reported in a calcareous soil from Manitoba, Canada where different rates of mineral P fertilizer were used (Yang et al., 2002). The higher STP levels in the manure used were probably responsible for the high STP and low HCl-P observed in our study.

As observed in the topsoil, the 40% increase in H2SO4–P at the highest rate of manure application (Table 5) is far less than the increases in TP (up to 6 times), so the H2SO4–P contribution to TP decreased with rate of manure application from 53% in M0 to 16% in Mr90 and 12% in Mi180. Irrigation had no significant effect on P levels in any of the P forms for the 15- to 30-cm depth at manure application rate 60 Mg ha–1 yr–1 (Mr60 vs. Mi60). The Po concentrations (NaHCO3–Po and NaOH-Po) were low and similar to the values observed in 0- to 15-cm soil (Table 5). For both soil depths, Po accounted for <5% TP and was not affected by the manure treatment.

Contrast analysis revealed that there was significantly lower STP, NaOH-Pt, and HCl-P in the 15- to 30-cm soil than in the 0- to 15-cm soil for manure treatments under rain-fed conditions (Table 5), but similar STP and NaOH-Pt for both soil depths under irrigated conditions (Table 6). This suggests that downward transport of P in irrigated soil may have narrowed differences between the two depths. This is consistent with the lower STP levels in surface soil under irrigated than rain-fed conditions when the same manure application rate was used (Mr60 vs. Mi60). High manure application rates lead to increased numbers of macro- and mesopores (Miller et al., 2002), which allow P to be preferentially transported to deeper soil layers after high intensity rainfall or irrigation.

Although the STP values in the 15- to 30-cm depth were lower than for the 0- to 15-cm depth, the 7 to 39 times greater STP/TP ratios in manured than control soil were significantly higher than those obtained for 0- to 15-cm soil (3 to 12). Possible P downward movement from the upper soil layer might have occurred under irrigation. Heavy rainfall in May to July 1995 (309 mm) may also have leached some STP in both rain-fed and irrigated plots from the 0- to 15-cm soil to the 15- to 30-cm depth. Less P uptake by plants occurs in the 15- to 30-cm depth as most roots are located near the surface. In addition, the transformation from NaOH-Pi, HCl-P, and H2SO4–P to STP might be greater under better moisture conditions in the 15 to 30 cm than in 0- to 15-cm soil. All of above may have contributed to the observed higher STP/TP ratio in manured treatments than the control in 15- to 30-cm soil than 0- to 15-cm soil (Fig. 1).

The results of this study clearly demonstrate that the recommended local manure application rates (30 Mg ha–1 yr–1 for rain-fed and 60 Mg ha–1 yr–1 for irrigated fields), causes P to accumulate over time, especially in STP form. While increases in STP are beneficial to crop production, excess accumulation in soil could increase the risk of environment pollution through surface runoff, P leaching, or wind erosion. Recently, more cattle feedlot operators are composting manure so that the weight and volume of the final product are reduced significantly (Larney and Hao, 2007). This product can be shipped further away from the feedlot operations, reducing the amount of manure (and thus P) applied to land near feedlots. In North America, most provinces and states are considering more strict guidelines or regulations on the amount of manure that can be applied to land to limit the environmental impact.

Effect of 14 Annual Manure Applications Followed by 16 Years with No Manure Application on Soil Phosphorus
0- to 15-cm Soil Depth
Manure applications in the first 14 yr supplied from 1.50 (Dr30) to 8.49 Mg TP ha–1 (Di180) to soil (Table 3). Both STP and TP concentration was still elevated 16 yr later after no further manure applications (Table 4). Because irrigated cereal production removes about 30 to 35 kg P ha1 yr–1 (Alberta Agriculture, 1993) and about half that for rain-fed crops, the amount of P removed in 30 yr is only about 10 to 35% of TP applied. Unlike soil OC and TN contents, which had returned to levels similar to those at the start of the experiment, the STP and TP reductions occurred at a much slower rate.

For both rain-fed and irrigated conditions, the TP values are lower than the comparable plots with continuous annual manure applications, but still higher than the non-manured control (Table 4). The H2O-P and NaHCO3–Pt were significantly higher (Dr60 and Dr90 under rain-fed and Di120 and Di180 under irrigated) than in the non-manured control (Tables 5 and 6 and Fig. 3 ), while no significant differences were observed in NaOH-Pt, HCl-P, and H2SO4–P among all treatments.


Figure 3
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  		Fig. 3. Increases in various soil P forms in manured soil relative to the non-manured control treatment after 14 yr annual cattle manure application and 16 yr with no further application under rain-fed and irrigated conditions.

 
While the STP/TP ratios also increased with the amount of manure TP applied, the ratios were much lower than for the continuous manure treatments (Fig. 2a). The STP/TP ratio for Dr30 was significantly lower than the ratio in the manure applied but was not significantly different from the ratio in the control (Mr0). After ceasing manure applications, STP in the soil was reduced through P uptake by annual cereal crops, which have most of their roots near the surface. Also, transformation to more stable forms and downward movement of STP may have exceeded the transformation from more stable P to STP. Similar to the continuous plots, there was very little Po (NaHCO3–Po and NaOH-Po accounted for <4% of TP) and Po concentration was not affected by the manure treatment (Tables 5 and 6).

15- to 30-cm Soil Depth
As in the topsoil, the TP concentration in the discontinued plots at this depth was still higher than in the non-manured control (Table 4). The P concentration was still elevated for all P forms but only significantly for H2O-P under both rain-fed and irrigated conditions (Tables 5 and 6). In contrast to surface soil (0–15 cm), where STP/TP ratios in discontinued plots were much lower than in the continuous plots (Fig. 2a), STP/TP ratios in the 15- to 30-cm soil were similar to ratios in the continuously manured treatment and were not significantly different from ratios in the manure applied (except Mr30 and Mr60, Fig. 2b). In other words, depletion of STP in this depth occurred at the same rate as TP, and resulted in little change to the STP/TP ratio over time. The STP level reflected a balance between crop uptake, transformation of STP to NaOH-Pt and HCl-P, and STP leaching below 30 cm soil (which reduce STP levels) and transformation of NaOH-Pt and HCl-P to STP and STP leaching from surface soil (which increases 15- to 30-cm STP levels).

Results from the discontinued plots demonstrate that even after 16 yr without manure application the P concentration in the soil remains high. The decreases in soil P levels after manure application ceased were much slower in subsurface (15 to 30 cm) than surface soil.

Regression Analysis
Linear regression analysis indicates that concentrations of all P forms, except for H2SO4–P, were linearly related to the cumulative amount of manure TP added (Table 7 ). In the linear models, the slope of each equation reflects the rate of increase in soil P concentration in response to the manure TP applied. For the continuously manured treatments, the order of increases in P concentration was NaHCO3–Pt > HCl-P > H2O-P > NaOH-Pt in the 0- to 15-cm soil depth (Table 7). A similar pattern was observed in the 15- to 30-cm depth with similar or slightly lower rates of increase. Schwartz and Dao (2005) also reported a linear relationship between P extracted from soil and the amount of stockpiled or composted cattle manure TP applied. They suggest that Ca in manures and fertilizers depresses P solubility and extractability as Ca-P (HCl-P) is less soluble and tends to accumulate.


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  		Table 7. Relationships between soil P concentration of various P forms and total cattle manure TP applied.

 
For the discontinued manure treatment, the greatest rate of increase was for NaHCO3–Pt followed by HCl-P, NaOH-Pt, and H2O-P. No significant relationship exists for H2SO4–P and manure TP (Table 7). For the same total amount of manure TP application, the rates of increases across all P forms were lower in the discontinued than in the annually manured soil. In other words, a low rate of manure application over a longer period of time (30 yr) will lead to a higher soil P than the same amount of manure applied at a higher rate over a shorter time (14 yr manuring followed by 16 yr no manuring). The lower rate of soil P increase in response to manure TP in the discontinued manure treatment may reflect a longer time for P to react with the soil matrix, leach, or be taken up by crops. This lower rate of P increase is consistent with P distribution in soil aggregate (Hao et al., 2004), also based on this study. As they point out, if the purpose of manure application is to maximize the P for crop production, the manure should be applied at a lower rate over a longer period of time. However, if the purpose of manure application is to dispose of it, then higher application rates over a shorter period of time should result in lower P content as manure has a longer time to react with soil matrices.

Both the ANOVA and regression analysis imply that manure application had less effect on stable P in soil, while significantly impacting STP. Furthermore, the increase in STP is beneficial for crop production, but excess amounts could negatively impact the environment through leaching and runoff. Results from the discontinued plots shows that the effect of manure application on soil P is long-lasting and that treatment effects are still present 16 yr after manure application has ceased.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
This study assesses the effect of long-term manure applications on the distribution of P forms in calcareous soil. Compared with non-manured control plots, 30 annual cattle manure applications increased P concentration in the 0- to 15- and 15- to 30-cm soil depths for most inorganic P forms. The increases were linearly correlated to the cumulative amount of P applied in manure, except for H2SO4–P. The amounts and rates of increase in STP were higher than for TP. These results indicate that continuous application of manure P in excess of crop needs will lead to a large accumulation of STP in soil, which poses a potential threat to surface and groundwater quality.

The effect of manure applications (except Dr30 treatment) on most soil P forms were still significant even 16 yr after manure application has ceased, indicating that the effect of excessive manure P application is long lasting. The high amount of P remaining in soil poses a continuous threat to the environment. For the rain-fed treatment with lowest rate of manure application (Dr30), soil P and its forms had returned to their initial levels. This indicates that all P in manure eventually becomes available to crops. Thus, P limits for manure application should be developed to prevent excessive buildup of P in soil and avoid potential threats to the environment.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
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Received for publication October 4, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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