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Influence of long-term irrigation on the distribution and availability of soil phosphorus under permanent pasture.

Introduction

Phosphorus (P) is a major nutrient that influences plant growth and development, and P transformations and mobility in the soil-plant system are controlled by a combination of biological, chemical, and physical processes (Frossard et al. 2000). Soil solution is the primary source of P for plants and micro-organisms, where most P is taken up as inorganic orthophosphate (HP[O.sub.4.sup.2-], [H.sub.2]P[O.sub.4.sup.-]). The concentration of inorganic P present in soil solution at any time is generally very small (<5 [micro]M), and P removed from the soil solution by biological uptake must be replenished by P released from inorganic and organic forms associated with the solid phase (Condron 2004). Inorganic P can be released to solution by desorption and dissolution of mineral P associated with aluminium, iron or calcium, together with mineralisation of organic P. The processes involved in adsorption--desorption and precipitation-dissolution of inorganic P have been studied extensively and are well understood (Frossard et al. 1995). The major management factors that influence the fate and availability of P in pastoral ecosystems relate to the type and intensity of land use, fertiliser inputs, and the associated effects of pasture and livestock management. However, P uptake by plants also relies on sufficient moisture for plant growth, and irrigation is widely used in intensive pastoral farming in New Zealand. It therefore stands to reason that P uptake and utilisation may differ according to soil moisture status and the use of irrigation.

On an annual basis, 60-95% of the P taken up by plants in grazed pasture systems is returned to the soil in litter and root residues or animal excreta (Kemp et al. 2000). In Australia and New Zealand many soils are low in native P and require regular additions of P fertiliser to enable the establishment and maintenance of productive pasture based on white clover (Trifolium repens) and perennial ryegrass (Lolium perenne). Several studies have demonstrated significant accumulation of P in soils under ryegrass-clover pastures that received regular applications of P fertiliser over many years (e.g. Magid et al. 1996). The Winchmore irrigation field trial in Canterbury, New Zealand, had received the same rate of P for 52 years, but with either no irrigation or different rates of flood irrigation. Our hypothesis was that increased pasture productivity and drainage from irrigation over this period would have had a significant impact on the amounts, distribution, and bioavailability of P in the soil profile.

Materials and methods

Soils and soil sampling

The field trial was located at Winchmore Irrigation Research Station (171[degrees]48'E, 43[degrees]47'S) in Canterbury, New Zealand, which is situated at an altitude of 160m with annual rainfall of 740mm. The soil is a Lismore stony silt loam (Orthic Brown [New Zealand], Udic Ustochrept [USDA]) formed from moderately weathered greywacke loess. Flood irrigation (border-dyke) was installed at Winchmore in 1947, and an irrigation trial was established in 1949 which included the following 3 treatments: (i) no irrigation (designated dryland); (ii) irrigation applied every 3 weeks; (iii) irrigation applied when the topsoil (04).075 m) gravimetric moisture level reached 10 or 20% (designated as 10% irrigated and 20% irrigated treatments), received on average with 2.6 and 7.7 irrigations of 100 mm/year, respectively (i.e. 10% = 260 mm/year; 20% = 770 mm/year).

The trial comprised 0.12-ha borders (12 m wide by 100 m long) and included 5 replicates of each irrigation treatment and 4 replicates of the dryland treatment. All treatments received 250 kg/ha of superphosphate in winter annually (c. 25 kg P/ha), and lime (calcium carbonate) was applied to the whole trial in 1948 (5 t/ha) and 1965 (1.9 t/ha) to maintain soil pH at 5.5-6.0. Each treatment has been rotationally grazed by a separate flock of sheep.

The current study was undertaken in 2001. A preliminary experiment was conducted on the dryland and 20% irrigated treatments to determine an appropriate sampling strategy for the main experiment. Consequently, 10 soil samples were taken from the middle of the borders 25-5 m from the headrace, and bulked together for each replicate. Soil samples were taken from replicate borders of the dryland, 10% irrigated, and 20% irrigated treatments at 3 depths (0-0.075, 0.075-0.15, and 0.15-0.25 m) using a 2.5-cm-diameter auger.

A nearby 'wilderness' site was also included for comparison. This area was on the same soil type and had not been used for agriculture since the establishment of the station in 1947 (i.e. no fertiliser inputs, grazing, or irrigation) and was under a mix of native grasses and herbs (Williams and Haynes 1992). The wilderness area was divided up in 4 sections to obtain 4 pseudo-replicates, and soil samples were taken at 0-0.075, 0.075-0.15, and 0.15-0.25 m as described above.

Soil analyses

Soil samples were air-dried and sieved (<2 mm) before analysis, and all analyses were expressed on an air-dried basis. Total carbon (C) and total nitrogen (N) were determined using a LECO 2000 CNS Analyser; total soil P was determined using the perchloric acid digestion method described by Kuo (1996). Organic P was determined using the ignition method described by Saunders and Williams (1955), and inorganic P determined as the difference between total P and organic P. Plant-available inorganic P was assessed by sodium bicarbonate extraction (Olsen P) using a soil : solution ratio of 1 : 20 (Olsen et al. 1954).

Soil pH (soil : water ratio 1 : 2.5) and particle size distribution (sand >20 [micro]m, silt 2-20[micro]m, clay <2 [micro]m) were determined on composite samples of all replicates using a Malvern Mastersizer-S particle size analyser after soil organic matter was oxidised by treatment with hydrogen peroxide and samples dispersed using Calgon.

Isotopic exchange kinetics (IEK) was used to determine soil inorganic P availability (Frossard et al. 1995; Frossard and Sinaj 1997; McDowell et al. 2001; Chen et al. 2003a). This involved shaking 10 g of soil with 99 mL of deionised water for 16 h before the addition of I mL of solution containing approximately 0.1 MBq of carrier-free [sup.33]P (Product BF 1003, Amersham Pharmacia Biotech UK Ltd) with continuous agitation on a magnetic stirrer. Solution samples were removed from the soil/water suspension after 1, 10, 30, and 60 min and filtered immediately to <0.201[micro]m before determination of 31p by malachite green colourimetry and [sup.33]P by scintillation counting (1 mL + 5 mL scintillation cocktail; Packard Ultima Gold). Isotopic exchange kinetics provided data on intensity (Cp, water-soluble inorganic P, mg P/L), capacity (R/[r.sub.1]), and quantity (E values, isotopically exchangeable soil inorganic P pools) factors that characterise soil P availability. The R/[r.sub.1] is the ratio of the total introduced radioactivity to the radioactivity remaining in the solution after 1 min, which provides an estimate of soil P adsorption capacity (the greater the value of R/[r.sub.1], the greater the P adsorption capacity of the soil). The pluricompartmental model was used to determine quantities of inorganic P in the following pools (mg P/kg):

(i) Pool of inorganic P isotopically exchangeable within 1 min(E.sub.1min]): ions present in this pool are composed of inorganic P in the soil solution and ions that are adsorbed on the solid phase of the soil but have the same kinetic properties as those in solution; inorganic P in this compartment is immediately plant available.

(ii) Pool of inorganic P isotopically exchangeable between 1 min and 3 months ([E.sub.1min-3m]): this pool corresponds to the concentration of inorganic P exchangeable during a period equivalent to the time of active P uptake by the entire root system of an annual crop.

(iii) Pool of inorganic P which cannot be exchanged within 3 months ([E.sub.>3m]): this pool represents recalcitrant forms of inorganic P which are not readily available to plants. It is determined as the difference between total soil inorganic P and the sum of the other inorganic P pools described above.

Statistical analysis

An analysis of variance was performed for each variable at each depth using GENSTA (2004) (7th edn, VSN International Ltd, Oxford, UK) and a least significant difference (I.s.d. at P = 0.05) was determined to compare treatment means (Saville 1990). Means of the variables from replicated samples taken from the wilderness site were not analysed as part of the main trial data set and are therefore presented separately for comparative purposes.

Results

Mean pH, organic C, total N, total P, inorganic P, organic P, and particle size analysis data for soils taken from the various irrigation treatments and the wilderness area are presented in Table 1. Soil pH values were similar for the dryland and irrigated treatments (5.7-5.8), whereas the pH of the wilderness site soils was less than soils from other treatments (5.2-5.3). Organic C and total N concentrations in equivalent soil depths were similar across all treatments (including the wilderness site), although organic C was slightly less in the 0-0.075 m soil from the 20% irrigated treatment (3.68%) than the dryland and 10% irrigated treatments (4.28-4.31%). As expected, concentrations of total P and inorganic P were greater in soil at all depths taken from the dryland and irrigated plots than from the wilderness area. Concentrations of total and inorganic P were also consistently greater in soil from the dryland treatment than from the 10% and 20% irrigated treatments. Organic P concentrations were greater in soil (especially 0-0.075 m) taken from the 10% and 20% irrigated treatments than the dryland treatment and wilderness area. The relative proportions of sand, silt, and clay in all soil depths from the irrigation treatments and wilderness area were similar.

Trends in Cp were similar to those obtained for total inorganic P (Table 2). In the topsoil (0-0.075 m), there was no significant difference between Cp concentrations of the dryland (0.78 mgP/L) and 10% irrigation (0.58 mgP/L) treatments, although levels were significantly lower in the 20% irrigated treatment (0.28 mgP/L). In the 0.075-0.15 m layer, the dryland treatment Cp (0.28 mg P/L) was significantly greater than both irrigated treatments (0.07-0.12 mg P/L), whereas Cp concentrations for the deepest layer were very low for all treatments (0.03-0.05mgP/L) and differences between treatments were not significant (Table 2). The R/[r.sub.1] ratio in the top layer (0-0.075 m) showed no significant difference between treatments, whereas in the deeper layer (0.075-0.15 m), R/[r.sub.1] for the dryland treatment was significantly less than for the 20% irrigated treatment. In the deepest layer (0.15-0.25 m), R/[r.sub.1] ratios were markedly greater in the 10% and 20% irrigated treatments than the dryland treatment.

Differences in the immediately available [E.sub.1min] inorganic P pool were similar to those forthe Cp concentrations (Table 2). In the first layer (0-0.075 m), the dryland treatment had the greatest [E.sub.1min] concentration (17.1 mg P/kg) compared with the two irrigated treatments (8.6-13.5 mgP/kg). In the 0.075-0.15 m soil, [E.sub.1min] showed the same trend as observed for the Cp concentrations. Surprisingly, there were no significant differences in the concentrations of inorganic P in the [E.sub.1min] and [E.sub.24h-3m] pools between the dryland and irrigated treatments, except in the 0.075-0.15m soils where levels were greatest for the dryland treatment. Olsen P concentrations were significantly greater in soils from the dryland treatment at all depths than in the irrigated treatments; Olsen P levels were also significantly lower in the 0-0.15m soils from the 20% irrigated treatment than the 10% irrigated treatment (Table 1).

Data presented in Table 2 also revealed that differences between concentrations of what can be considered 'recalcitrant' inorganic P (i.e. [E.sub.3m-1y] and [E.sub.> 1y] pools)were broadly similar to those observed for total inorganic P. Thus, concentrations of inorganic P in the [E.sub.3m-1y] and [E.sub.>1y] pools were markedly and significantly greater in soils from the dryland treatment than from the 20% irrigated treatment at all depths. Similar differences were evident between soils from the dryland and 10% irrigated treatments for the [E.sub.>1y] pool. Furthermore, concentrations of [E.sub.3m-1y] + [E.sub.> 1y] inorganic P were significantly lower in soils from the 20% irrigated treatment than the 10% irrigated treatment at all depths, except for the [E.sub.>1y] pool in the 0.075-0).15m soil. In the 0-0.075m soil, significant differences in concentrations of inorganic P in the combined [E.sub.>3m] pool were determined between the dryland (479mg P/kg), 10% irrigated (346 mg P/kg) and 20% irrigated (159 mg P/kg) treatments. The same trend was evident in the deeper soil layers, except that the difference between the irrigation treatments in the 0.075-0.15 m soil layer was not significant.

Proportions of total soil inorganic P in immediately plant-available forms (i.e. [E.sub.1min], [E.sub.1min-24h], [E.sub.24h-3m]) were consistently greater in the 20% irrigated treatment than the 10% irrigated and dryland treatments. Thus, in the 20% irrigated treatment, plant available inorganic P accounted for 57, 64 and 69% of total inorganic P in the 0-0.075, 0.075-0.15, and 0.15-0.25 m soils, respectively. These were much greater than corresponding data for the 10% irrigated (39, 51, and 52%) and dryland (33, 43, and 42%) treatments. The relative proportions of plant-available inorganic P in the wilderness site soils were 59, 48, and 45% for the 0-0.075, 0.075-0.15, and 0.15-0.25m depths, respectively.

Discussion

The dryland treatment had the greatest concentrations of total P and inorganic P compared with treatments irrigated at 10 and 20% soil moisture, despite the fact that these treatments received the same amount of fertiliser P for 52 years. This may be attributed to a combination of factors including decreased pasture growth and output in animal products, together with increased P loss in overland flow and by leaching from irrigated treatments compared with dryland.

The apparent accumulation of inorganic P in soil under dryland compared with irrigation can be mainly attributed to decreased pasture growth. Unpublished agronomic data for the irrigation trial for the period 1958-99 shows that dry matter yield for the dryland treatment was on average 43% less than that for the 20% irrigated treatment, while corresponding mean yields for the 10% irrigated treatment were 17% less than the 20% irrigated treatment. Similarly, the fact that concentrations of soil organic P were consistently greater in the irrigated treatments than the dryland reflects increased inputs and turnover of organic P (Magid et al. 1996; Frossard et al. 2000).

The finding that concentrations of organic P in soil from the wilderness site were similar to those found in the dryland site was surprising, especially since the wilderness site was not grazed and did not receive any fertiliser or lime over 52 years. Although not statistically comparable, the similar soil organic P concentrations in the dryland and wilderness sites is of interest because it reflects similar outcomes as a result of different soil processes. The dryland treatment received inorganic P inputs as fertiliser and increased P cycling resulting from livestock grazing. These 2 factors did not occur on the wilderness site. Inorganic P appears to be lower in the wilderness site than the dryland treatment, and the organic P to inorganic P ratio was higher in the wilderness site. This suggests that biological activity in the wilderness site was lower than the dryland resulting in reduced relative mineralisation of organic P. This in turn is consistent with findings reported by Fraser et al. (1994) and Fraser and Piercy (1996) who showed that earthworm numbers and biomass in the wilderness area were lower than in the dryland treatment. The comparatively high levels of organic P in the wilderness soil might also reflect differences in the botanical composition of the sward compared with the dryland treatment.

According to the model developed by Williams and Haynes (1992), P removed annually in animal products amounted to 0.42, 0.84, and 1.08 kgP/ha from the dryland, 10% irrigated, and 20% irrigated treatments, respectively. Recent data on border-dyke irrigated pastures in South Canterbury, New Zealand, estimated that annually up to 4 kgP/ha could be lost in irrigation outwash (Carey et al. 2004). This value can be much greater if irrigation follows soon after P fertiliser is applied or stock have been grazing. For example, Austin et al. (1996) found losses of up to 12.9 kgP/ha from irrigated pastures in response to irrigation on the day of P fertiliser application. If we assume equal outwash then the 20% irrigated treatment (irrigated 7.7 times on average a year) should lose about 3 times more P than the 10% irrigated treatment (irrigated on average 2.6 times a year). During the last 52 years of operation it is also likely that significant quantities of P had moved down the soil profile especially in irrigated treatments where flooding markedly increased drainage compared with regular low intensity rainfall. A recent detailed lysimeter study measured annual leaching losses of 0.6-2.5 kgP/ha from a Lismore soil at 0.70m under irrigated pasture, while most P was lost via preferential flow immediately following irrigation and was found to be mainly present as particulate organic P (Toor et al. 2003, 2004, 2005). The latter may partly explain the greater concentrations of organic P evident in the 0.075-0.25 m soils under the irrigated treatments than the dryland treatment (Table 1).

The effect of irrigation on the fate and utilisation of applied fertiliser P is also evident from IEK data (Table 2). Thus, concentrations of immediately available inorganic P (Cp, [E.sub.1min]) were greater in the dryland soils than both irrigated treatments. This trend is also evident in corresponding data for the [E.sub.1min-24h] and [E.sub.24h-3m] inorganic P pools for the 0-0.075 and 0.075-0.15 m soils, and also in the Olsen P data for all soil depths (Table 1). However, it is also clear that a significant proportion of the inorganic P accumulated in the dryland soils was converted to recalcitrant forms as shown by the markedly greater concentration of inorganic P in the [E.sub.>3m] pool compared with the soils from the irrigated treatments. This can be attributed to changes in the physico-chemical nature of inorganic P adsorbed on mineral surfaces with time, including solid-state diffusion (Frossard et al. 1995). As expected, the overall amounts and availability of inorganic P was markedly lower in the undeveloped wilderness soils than the fertilised dryland and irrigated soils, especially in the 0-0.075 and 04). 15 m depths.

It is also interesting to note that concentrations of total and [E.sub.>3m] inorganic P were consistently lower in the 0.075-0.15 and 0.15-0.25 m soil depths from the 20% irrigated treatment than the 10% irrigated and dryland treatments (Table 2). This clearly suggested that significant P transfer by leaching occurred below 0.25m in the 20% irrigated treatment with time.

The P retention capacity of the Lismore 0-0.075m soil as indicated by the R/[r.sub.1] values in Table 2 (2.2-3.6) is within the low to medium range for New Zealand soils. Chen et al. (2003b) reported that the mean R/[r.sub.1] value for 15 New Zealand grassland soils was 4.5 (1.1-17.7), with the highest values found for soils derived from andesitic tephra (8.9-17.7). The R/[r.sub.1] values in the 0.15-0.25 m soils were markedly and significantly greater in the 10% and 20% irrigated soils (10.2 and 12.8, respectively) than the dryland soil (6.6) (Table 2). This indicated an increase in the capacity of the irrigated soils to retain P at depth, which is consistent with data presented by Sinaj et al. (2002) for a Lismore soil under the same long-term irrigation regime. However, this increased retention capacity is not consistent with the decreased concentrations of inorganic P in the 0.15-0.25 m soil as discussed above, and is not reflected in the concentrations of oxalate Fe and AI, which were similar under all treatments (data not presented). This may be at least partly attributed to the predominance of particulate organic P in drainage, which would be less readily adsorbed onto soil colloid surfaces (Toor et al. 2003). It is also possible that other factors might explain this finding such as the increased biotic uptake at depth or the leaching of P sorptive organic matter at depth. Sinaj et al. (2002) demonstrated that in a Lismore soil under border-dyke irrigation, concentrations of organic C in preferential flow areas at 0.25-0.60 m were significantly greater than in corresponding matrix soils. Furthermore, Borggaard et al. (1990) found that organic matter inhibited aluminium oxide crystallisation resulting in poorly crystalline oxides with high sorption capacity, whereas increased N inputs to support pasture growth and N and C inputs via dung and urine returns can increase the requirement and uptake of P by biota (McDowell and Monaghan 2002). However, a clear explanation for the increase in P retention capacity at depth in the irrigated soils is unclear, and warrants further investigation.

Conclusions

The findings of this study clearly demonstrated that border-dyke irrigation had a major impact on the nature, distribution, and availability of P in soil under grazed pasture receiving identical P inputs for 52 years. Phosphorus accumulation in soil under dryland compared with irrigation conditions reflected differences in pasture production and P removal in drainage and animal products, although the proportion of inorganic P in recalcitrant forms was markedly greater in the dryland soils. Similarly, increased pasture production accounted for higher concentrations of organic P in irrigated soils than dryland. Despite the fact that irrigation resulted in an apparent increase in P retention capacity at 0.15-0.25 m, concentrations of inorganic P were lower in the irrigated soils at this depth, which suggests that significant transfer of P occurred below 0.25m under irrigation. Thus, while irrigation improved pasture productivity and P utilisation it also increased P loss by leaching and the associated long-term risk of adverse impacts on aquatic environments.

Acknowledgments

The authors thank Dr Parmjit Randhawa and Neil Smith for assisting with soil sampling, and Dr David Saville of AgResearch for advice on statistical analysis.

Manuscript received 16 May 2005, accepted 9 December 2005

References

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L. M. Condron (A,F), S. Sinaj (B), R. W. McDowell (C), J Dudler-Guela (B), J T. Scott (D), and A. K. Metherell (E)

(A) Agriculture and Life Sciences, PO Box 84, Lincoln University, Canterbury 8150, New Zealand.

(B) institute of Plant Sciences, Swiss Federal Institute of Technology Zurich (ETHZ), Postfach 185, CH-8315 Eschikon-Lindau, Switzerland.

(C) AgResearch Limited, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand.

(D) AgResearch Limited, c/o Agriculture and Life Sciences, PO Box 84, Lincoln University, Canterbury 8150, New Zealand.

(E) Ravensdown Fertiliser Co-operative Limited, PO Box 1049, Christchurch, New Zealand.

(F) Corresponding author. Email: condronl@lincoln.ac.nz
Table 1. Mean values for selected physical and chemical properties
determined for dryland, irrigated, and wilderness soils

Treatment pH Total C Total N Total P

 (%)

 0-0.075m

Dryland 5.7 4.31 0.38 1301
10% Irrigated 5.8 4.28 0.39 1249
20% Irrigated 5.7 3.68 0.35 1081
l.s.d. (P=0.05) -- 0.30 0.026 146
Significance -- *** ** *
Wilderness 5.2 4.21 0.32 873

 0.075-0.15m

Dryland 5.8 3.31 0.29 1033
10% Irrigated 5.8 3.21 0.30 935
20% Irrigated 5.7 2.69 0.26 810
l.s.d. (P=0.05) -- 0.220 0.021 100.4
Significance -- *** *** ***
Wilderness 5.4 3.06 0.24 745

 0.15-0.25 m

Dryland 5.8 2.09 0.18 748
10% Irrigated 5.7 2.01 0.18 681
20% Irrigated 5.8 1.74 0.16 606
l.s.d. (P=0.05) -- 0.31 0.023 87.5
Significance -- n.s. n.s. *
Wilderness 5.3 2.48 0.19 696

Treatment Inorganic Organic P Olsen P Sand

 P(mg P/kg)

 0-0.075m

Dryland 727 574 54 230
10% Irrigated 577 672 41 240
20% Irrigated 372 709 27 210
l.s.d. (P=0.05) 167 80 8 --
Significance ** ** *** --
Wilderness 300 573 20 210

 0.075-0.15m

Dryland 551 482 34 230
10% Irrigated 367 568 19 250
20% Irrigated 242 568 12 240
l.s.d. (P=0.05) 112.5 74.4 4 --
Significance *** * *** --
Wilderness 265 479 12 230

 0.15-0.25 m

Dryland 355 393 15 270
10% Irrigated 266 414 7 280
20% Irrigated 195 411 5 280
l.s.d. (P=0.05) 68.7 50.0 2.6 --
Significance *** n.s. *** --
Wilderness 300 396 8 240

Treatment Silt Clay
 (g/kg)

 0-0.075m

Dryland 520 250
10% Irrigated 490 270
20% Irrigated 520 270
l.s.d. (P=0.05) -- --
Significance -- --
Wilderness 540 250

 0.075-0.15m

Dryland 520 250
10% Irrigated 510 240
20% Irrigated 520 240
l.s.d. (P=0.05) -- --
Significance -- --
Wilderness 530 240

 0.15-0.25 m

Dryland 480 250
10% Irrigated 490 230
20% Irrigated 480 240
l.s.d. (P=0.05) -- --
Significance -- --
Wilderness 510 250

n.s., Not significant; * P < 0.05; ** P < 0.01; *** P < 0.001.

Table 2. Mean values for kinetic parameters and isotopically
exchangeable inorganic P pools determined for dryland, irrigated,
and wilderness soils

Treatment CP R/[r.sub.1] [E.sub.1min]
 (mg P/L)

 0-0.075 m

Dryland 0.78 2.2 17.1
10% Irrigated 0.58 2.5 13.8
20% Irrigated 0.28 3.2 8.6
l.s.d. (P=0.05) 0.270 0.40 4.54
Significance ** *** **
Wilderness 0.12 3.6 4.2

 0.075-0.15 m

Dryland 0.28 3.1 8.8
10% Irrigated 0.12 4.1 4.5
20% Irrigated 0.07 5.3 3.7
l.s.d. (P=0.05) 0.058 0.9 1.88
Significance *** *** ***
Wilderness 0.04 5.7 2.3

 0.15-0.25 m

Dryland 0.05 6.6 3.3
10% Irrigated 0.03 10.2 2.8
20% Irrigated 0.04 12.8 4.5
l.s.d. (P=0.05) 0.029 2.52 2.39
Significance n.s. *** n.s.
Wilderness 0.03 7.8 2.4

Treatment [E.sub.1min-24h] [E.sub.24h-3m] [E.sub.3m-1y]
 (mg P/kg)

 0-0.075 m

Dryland 82.6 149 63
10% Irrigated 77.2 140 56
20% Irrigated 74.0 130 41
l.s.d. (P=0.05) 12.05 28.0 13.6
Significance n.s. n.s. **
Wilderness 54.7 116 37

 0.075-0.15 m

Dryland 72.5 152 61
10% Irrigated 54.2 129 47
20% Irrigated 52.1 101 28
l.s.d. (P=0.05) 16.4 29.2 14.5
Significance * ** ***
Wilderness 34.2 91 34

 0.15-0.25 m

Dryland 40.1 108 46
10% Irrigated 38.8 95 35
20% Irrigated 54.1 77 19
l.s.d. (P=0.05) 23.23 30.2 12.5
Significance n.s. n.s. **
Wilderness 40.1 92 31

Treatment [E.sub.>1y] [E.sub.>3m] (A)

 0-0.075 m

Dryland 416 479
10% Irrigated 290 346
20% Irrigated 118 159
l.s.d. (P=0.05) 122.7 133.4
Significance *** ***
Wilderness 88 88

 0.075-0.15 m

Dryland 257 318
10% Irrigated 133 180
20% Irrigated 58 86
l.s.d. (P=0.05) 84.3 97.1
Significance *** ***
Wilderness 104 138

 0.15-0.25 m

Dryland 158 203
10% Irrigated 94 129
20% Irrigated 40 60
l.s.d. (P=0.05) 35.7 46.7
Significance *** ***
Wilderness 135 166

n.s., Not significant; * P < 0.05; ** P < 0.01; *** P < 0.001.

(A) [E.sub.3m-1y] + [E.sub.>1y] (recalcitrant P).
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Author:Condron, L.M.; Sinaj, S.; McDowell, R.W.; Dudler-Guela, J.; Scott, J.T.; Metherell, A.K.
Publication:Australian Journal of Soil Research
Geographic Code:8AUST
Date:Mar 15, 2006
Words:5491
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