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SOIL & WATER MANAGEMENT & CONSERVATION

Reclamation of Abandoned Natural Gas Wellsites with Organic Amendments: Effects on Soil Carbon, Nitrogen, and Phosphorus

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

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

	ABSTRACT

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

 
Organic amendments have been used to restore productivity to disturbed soils such as those on abandoned oil and natural gas wellsites. A study was conducted on three abandoned wellsites in southern Alberta, Canada to examine the effects of one-time applications of alfalfa (Medicago sativa L.) hay or beef cattle (Bos taurus) feedlot manure compost on soil properties under continuous wheat (Triticum aestivum L.). The base amendment rate (1x) [dry wt.] was 5.3 Mg ha^–1 for compost and 3.1 Mg ha^–1 for alfalfa. The five amendment rates of 0, 1x, 2x, 4x, and 8x were soil-incorporated at the wellsites. Although approximately twice as much C was applied with alfalfa than with compost, final SOC content was similar for the two amendment treatments, indicating the greater stability of compost-derived C. Nitrate N content in the 0- to 60-cm depth was not affected by compost rate (mean 213 kg ha^–1) but increased by 7.78 kg ha^–1 for each Mg ha^–1 increase in alfalfa rate. This result reflects the greater stability of compost-N compared with alfalfa-N and suggests a lower risk of NO₃–N leaching with compost application. Compost rates >20 Mg ha^–1 resulted in excessive extractable P build-up in the topsoil (up to 95.7 mg kg^–1), which may pose environmental risk to surface water. We recommend amending wellsites with up to 12 Mg ha^–1 of alfalfa or <20 Mg ha^–1 of compost during reclamation to improve C storage and nutrient cycling while minimizing nutrient loss to water systems.

Abbreviations: MKP, modified Kelowna-extractable P • SOC, soil organic carbon

	INTRODUCTION

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

 
Oil and natural gas exploration and production account for much of the estimated 40,000 ha of land disturbed by industrial activity annually in Alberta, Canada (Janz, 1999). Recent estimates indicate that there are more than 117,000 abandoned well licenses in the province, of which at least 33,000 wells are abandoned and unreclaimed (Alberta Energy and Utilities Board, 2005). Provincial regulations require that abandoned wellsites are effectively reclaimed to meet the goal of equivalent land capability (Alberta Environment, 1995). Equivalent land capability is defined as the ability of the land to support various uses after reclamation, at a level similar to that which existed before disturbance by industrial activity. Since many wellsites occur on cultivated land, reclamation of these lands must be a prime consideration if their agronomic value is to be restored. Under current Alberta regulations, oil and gas companies are required to salvage topsoil during wellsite construction for use in reclamation when production ceases (Alberta Environment, 1995).

The efficacy of organic amendments in augmenting the restoration of degraded or disturbed soils is well documented (Brown and Chaney, 2000; Larney and Janzen, 1996; Larney et al., 2000a, 2000b, 2003, 2005). The much higher amendment rates required in reclamation compared to agriculture (Sopper, 1992) has positive implications for the utilization of manure from intensive livestock operations, provided nutrient loss from the soil is minimized. However, although Alberta Environment advocates the use of organic amendments in the reclamation of abandoned oil and gas wellsites (Alberta Environment, 1995) and despite adoption of the practice by industry, there is a limited availability of published research on the subject, particularly in relation to type of amendment and its application rate. Information is available on the use of organic amendments in reclamation after coal strip-mining (Bateman and Chanasyk, 2001; Winter Sydnor and Redente, 2002) but this may not be directly transferable to oil and gas wellsites because of the much smaller vertical and horizontal extent of disturbance during wellsite construction and the different reclamation criteria and procedures involved.

Recently, there have been concerted efforts in southern Alberta to establish the efficacy of different organic amendments in the restoration of productivity to soils disturbed by wellsite construction (Larney et al., 2003, 2005; Zvomuya et al., 2006). These studies demonstrated that organic amendments play an important role in improving soil properties (Larney et al., 2005) and crop yields (Larney et al., 2003) on reclaimed wellsites where topsoil is scarce or absent. Zvomuya et al. (2006) recommended the incorporation of alfalfa (Medicago sativa L.) hay and composted beef cattle manure, which have a low C/N ratio, to safeguard against N deficiency in spring wheat (Triticum aestivum L.) in the first 1 to 2 yr following wellsite reclamation. High P amendments—composted and non-composted manure—were better choices for improving P uptake. Based on these results, compost was the best single amendment for ensuring enhanced uptake of both N and P on reclaimed wellsites in the short term.

While these studies identified the most suitable amendments for use in wellsite reclamation, they did not establish the optimum amendment rates needed for soil restoration. It is important to determine the level of organic amendment that is needed on sites where amounts of salvaged topsoil are inadequate to meet the required replacement depth. This may increase flexibility in reclamation efforts and eliminate the need for the unsustainable practice of importing topsoil. With the increased interest in composting fresh cattle manure, it is important to establish quantitative, research-based guidelines for use of compost in oil wellsite reclamation. Composted or non-composted manure for reclamation offers an opportunity for increasing the C content of soils while utilizing a product (manure) that is increasingly perceived as waste in areas of intensive livestock production.

Quantitative management information is needed to assist reclamation specialists make informed decisions on the use of compost or alfalfa in their reclamation programs. Our overall objective was to determine the rates of compost and alfalfa hay necessary to optimize the agricultural productivity of reclaimed wellsites. We hypothesized that application of these amendments at optimum rates could significantly improve productivity of reclaimed wellsites. In this paper, we report on the effects of differential rates of the organic amendments on soil C, N, and P.

	MATERIALS AND METHODS

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

 
Study Sites
Three abandoned wellsites (0.48–1.0 ha) leased from agricultural landowners in southern Alberta were selected for this study. Two of the sites (Turin and Sundial) were located near Turin (49° 58' N, 112° 31.6' W) and the third (Hussar) near Hussar (51° 02' N, 112° 41' W). The soils at all sites were Orthic Dark Brown Chernozems (Typic Haploborolls) with a loam (Hussar) or clay loam texture (Turin and Sundial). The Hussar and Turin sites were constructed in February 1999 and September 2000, respectively, as sump sites for disposal of drilling wastes (drilling fluids, wellbore casing cement). Reclamation was initiated in August 1999 at the Hussar site and November 2000 at the Turin site and involved topsoil replacement according to provincial regulations (Alberta Environment, 1995). The Sundial site, which was located near (2 km) the Turin site, was added to the study in 2003. This site had been initially constructed in August 2001. Topsoil replacement at the site was completed in June 2003. Before topsoil replacement, all sites were ripped to a depth of 60 cm using a Caterpillar D6 tractor with a three-shank ripper (Caterpillar, Inc., Peoria, IL). The sites were rotospiked (0–15 cm) using a Howard HB120 3 pt. hitch pto-driven rotospike (Kongskilde Ltd, Strathroy, ON) to incorporate wheat straw (1.4 Mg ha^–1) following topsoil replacement.

Experimental Layout
The experiment at each site was laid out in a randomized complete block design with a factorial treatment layout and four replications. Each individual plot was 8 x 3 m in area. The factors were amendment (alfalfa and beef cattle feedlot manure compost) and amendment rate (0, 1x, 2x, 4x, and 8x). The base rate (1x) was 5.3 Mg ha^–1 for compost and 3.1 Mg ha^–1 for alfalfa. The target rates, together with the actual rates incorporated, are presented in Table 1. The highest rate (8x) reflects compost and manure application rates normally used on-farm (Gagnon and Simard, 1999) and in the reclamation of abandoned oil and natural gas wellsites in agricultural areas in Alberta (Larney et al., 2003, 2005). The base rate (1x) for each amendment was calculated to supply 75 kg ha^–1 total N. With the planned shift from N-based to P-based organic amendment application in Alberta, it is important to evaluate lower rates than the current 8x to reduce the risk of surface water pollution.

Amendment Application
For each plot, compost and alfalfa hay were weighed (to the nearest 0.1 kg) in garbage pails in the field using a Sartorius Combics 60-kg scale (Sartorius Inc., Mississauga, ON) to give application rates described above. Subsamples were collected from the amendments destined for each individual plot to determine water content (oven-drying to a constant weight at 60°C), and total N, P, and C concentrations. The amendments were spread evenly across the soil surface with garden rakes and immediately incorporated to 10 cm with a 2-m wide 3 pt. hitch pto-driven rototiller (model XB, Maletti S.P.A., Modena, Italy). Amendments were incorporated 25 June 2002 at Turin, 2 July 2002 at Hussar, and 10 July 2003 at Sundial.

Crop Establishment
All sites were seeded (100 kg ha^–1) to Katepwa spring wheat within a day of amendment incorporation using a John Deere 9450 hoe drill (Deere and Co., Moline, IL) with 17.5-cm row spacing. The Sundial site received 45 kg N ha^–1 (broadcast) and 9 kg P ha^–1 (with seed). A second crop (100 kg ha^–1 seeding rate) was established in late May 2003, at the Hussar and Turin sites. At Turin, 45 kg N ha^–1 (broadcast) and 11 kg P ha^–1 (with seed) were applied. The Hussar site was high in soil N and P levels and, therefore, received no N or P fertilizer.

Due to the extremely dry conditions during July and August 2003, the spring wheat at Sundial performed poorly and the crop was abandoned. Therefore, the site was seeded (83 kg ha^–1) to winter wheat (cv. AC Readymade) on 24 Sept. 2003 and received 37 kg ha^–1 N (broadcast) and 9 kg ha^–1 P (with seed). Snowfall on 16 Sept. 2003 ensured adequate soil moisture for germination and good stand establishment in spring 2004.

A third spring wheat crop (100 kg ha^–1 seeding rate) was established at the Hussar and Turin sites in 2004. The Turin site received 35 kg ha^–1 of fertilizer N and no P fertilizer. Soil available P (MKP) levels in fall 2003 were 72 kg ha^–1 in the 0- to 15-cm depth of the unamended check plots and were considered adequate. At Hussar, soil tests in fall 2003 indicated high NO₃–N (228 kg ha^–1 in the 0- to 60-cm depth) and available P (71 kg ha^–1 in the 0–15 cm depth) levels in unamended plots, therefore the plots did not receive N or P fertilizer.

Post-Treatment Soil Sampling
Soil samples (composite of three samples per depth in each plot) were taken from the 0- to 7.5-, 7.5- to 15-, 15- to 30-, and 30- to 60-cm depths 5 mo post-treatment at Hussar and Turin according to procedures described above for baseline sampling. Additional samples were taken in September/October 2003 at all sites, representing the second post-treatment sampling (15 mo post-treatment) at the Hussar and Turin sites and the first (3 mo post-treatment) at the Sundial site. The final samples were taken in September 2004 at all sites. These samples represented the third set taken after addition of amendments at Turin and Hussar (some 27 mo later) and the second set at Sundial (15 mo after incorporation). The samples were collected as previously described from the 0- to 7.5-, 7.5- to 15-, 15- to 30-, 30- to 60-, 60- to 90-, and 90- to 120-cm depths. All samples were air dried, followed by coarse- (<2 mm) and fine-grinding (<0.15 mm) before laboratory analysis.

Laboratory Analysis
Amendment Carbon, Nitrogen, and Phosphorus
Total C and total N concentrations in the alfalfa hay and compost were determined in fine-ground (<0.15 mm) material using a CNS analyzer (model 1500, Carlo Erba Instruments, Milan, Italy). Total P was determined colorimetrically after digestion with H₂SO₄ and H₂O₂ (Thomas et al., 1967).

Soil Carbon, Nitrogen, and Phosphorus
Nitrate-N was extracted with 2 M KCl and determined using a colorimeter (AutoAnalyzer II SC, Technicon Industrial Systems, Tarrytown, NY). Available P was measured colorimetrically (Murphy and Riley, 1962) following extraction by the modified Kelowna method (Ashworth and Mrazek, 1995) in which 5 g of air-dried soil were extracted with 50 mL of solution (0.015 M NH₄F, 1.0 M NH₄OAc, and 0.5 M HOAc). Total C and N were determined as described earlier for the amendments. Inorganic C was measured by the method of Amundson et al. (1988). Soil organic carbon (SOC) was estimated as the difference between total C and inorganic C.

Statistical Analysis
Data were analyzed by linear mixed-effects models for repeated measures using the MIXED procedure of SAS (Littell et al., 1996; SAS Institute, 2005) with year as the repeated measures factor. In the combined analysis, year, site, organic amendment, rate, and their interactions were considered fixed effects in the model, while replicates nested within site were modeled as random effects along with their interactions with the fixed effects. Mixed-model F-tests based on the Kenward-Roger (Kenward and Roger, 1997) adjusted denominator degrees of freedom approximation were used to assess significance of fixed effects and interactions in the model. The heterogeneous Toeplitz (TOEPH) covariance matrix was chosen as the best-fitting covariance structure since it had the smallest Akaike's Information Criterion (AIC) of all structures tested (Littell et al., 1996). In the absence of a significant repeated measures main effect or interaction, only data from the final sampling was subjected to analysis of variance using the MIXED procedure. A Tukey multiple pairwise comparison procedure (P = 0.05) was used for all pairwise comparisons. When rate main or interaction effects were significant, regressions were developed and tested for significance using actual rather than relative amendment rates. Regressions were also developed relating SOC, NO₃–N, and MKP content to amendment nutrient additions. Effects were considered significant if P 0.05.

	RESULTS AND DISCUSSION

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

 
Baseline Soil Properties
Bulk densities determined before treatment application were uniform across the plots at each site, indicating uniform topsoil replacement at each site. Mean bulk densities across the sites ranged from 1.25 to 1.61 Mg m^–3 in the 0- to 15-cm depth, and 1.48 to 1.76 Mg m^–3 in the 15- to 30-cm depth (Table 2). Soil organic C, total N, and total P contents determined in the 0- to 30-cm depth at the outset of the study were also uniform across the plots at each site. Mean nutrient concentrations in this depth ranged from 29.5 to 44.4 Mg ha^–1 for SOC, 3.74 to 5.95 Mg ha^–1 for total N, and 1.74 to 2.24 Mg ha^–1 for total P (Table 2).

For a given rate, total C and nutrient (N and P) additions varied less among sites for alfalfa hay compared with compost (Table 3). Coefficients of variation (CV) across the sites ranged from 5.3 to 7.0% for total C with alfalfa application compared with 18.6 to 21.7% with compost application. For total N and total P, the CVs were 6.7 to 25.2 and 12.1 to 31.7%, respectively, for alfalfa, compared with 17.4 to 19.3 and 34.5 to 38.1% for compost. At all rates, C and N additions from compost were higher at Hussar than at Turin and Sundial, indicating the higher concentration of these nutrients in the compost applied at Hussar. Although we endeavored to apply the same rates of N as both alfalfa and compost, on average (3 sites x 4 rates) the compost N rate (316 kg ha^–1) was about 10% higher than the alfalfa N rate (287 kg ha^–1).

Soil Organic Carbon
On average, SOC content in the 0- to 30-cm depth was higher at the Hussar site (46.3 Mg ha^–1) than the Sundial and Turin sites (mean 33.3 Mg ha^–1), indicating the inherently higher level of SOC associated with the wetter and cooler climate at Hussar (Table 4). Soil organic C was significantly affected by rate but not type of amendment (Table 4). Across the two amendments at all sites, SOC increased by 0.13 Mg ha^–1 (r² = 0.45; P = 0.004) for each Mg ha^–1 increase in the amendment rate.

View larger version (31K): [in this window] [in a new window]   Fig. 1. Effect of (a) alfalfa (Medicago sativa L.)- and (b) compost-C rates on soil organic carbon (SOC) content in the 0- to 30-cm depth 15 mo (Sundial) and 27 mo (Hussar and Turin) after application. Vertical bars represent standard errors.

The vertical distribution of SOC varied little below the 0.3-m depth. Within the 0- to 0.3-m depth, SOC concentration decreased with soil depth (Fig. 2). Significant increases in SOC over the unamended control, which were confined to the top 15-cm profile, occurred at the highest alfalfa rate (8x) at all sites and also at the second highest rate (4x) at the Hussar and Sundial sites. The two lower alfalfa rates had no significant effect on the SOC at all sites. The pattern was similar for compost, except that the 2x compost rate also gave significantly higher SOC concentrations than the unamended control at the Hussar and Turin sites.

View larger version (25K): [in this window] [in a new window]   Fig. 2. Vertical distribution of soil organic C (SOC) in the 0- to 30-cm depth as affected by alfalfa (Medicago sativa L.) [(a), (c), and (d)] and compost [(b), (d), and (f)] rates 27 mo after application at Hussar [(a) and (b)] and Turin [(e) and (f)], and 15 mo after application at Sundial [(c) and (d)]. Horizontal bars represent standard errors.

Soil organic matter, hence SOC content, is a particularly important attribute of soil quality and the inherent productivity of a soil (Gregorich et al., 1994; Hammermeister et al., 2003). Loss of SOC and nutrients has been blamed for lower productivity on disturbed Haploborolls (Campbell and Souster, 1982).

Total Nitrogen
Total N content in the 0- to 30-cm depth remained highest at the Hussar site (Table 4) and changed little with amendment application at all sites compared with baseline samples. The total N content in this depth was similar for alfalfa and compost (mean 4.53 Mg ha^–1), reflecting the almost equal N additions from these sources at the outset of the study. Larney et al. (2005) also reported similar N contents for alfalfa and compost 40 mo. after amendment incorporation, while Cox et al. (2001) found no significant amendment (compost vs. wheat straw vs. coal ash) difference in one of 2 yr. The highest amendment rate (8x) in our study resulted in significantly higher total N content than the lower rates, but no significant differences were detected among the lower rates, including the control. The total N contents measured for alfalfa and compost in this study (equivalent to a mean concentration of 1.27 g kg^–1 at the 8x rate) are slightly lower than those (1.7–2.1 g kg^–1) reported by Larney et al. (2005) on similar soils at the same rate of 8x. The apparently higher concentrations in their study were expected since their sampling depth (0–15 cm) was shallower than that used in our study (0–30 cm), which extended beyond the depth of amendment incorporation. Our study also provided additional information on responses at lower rates, indicating no significant increase in total N as amendment rate increased from 0 to 4x, while the 8x rate significantly increased total N compared with all the other rates tested.

Regressions of total N on actual amendment rates indicated that, for both alfalfa and compost, total N accumulation in the soil increased by 16 kg ha^–1 for each Mg ha^–1 increase in amendment rate (r² = 0.8; P < 0.001). Since amendment N concentration was higher for alfalfa than compost, this result indicates the greater stability of compost N.

Nitrogen supply is usually the major limiting factor in most reclamation schemes (Lanning and Williams, 1981). Although inorganic fertilizers are an important N source, they provide a flush of nutrients which may be quickly lost from soils through leaching or runoff. Nitrogen cycling in agricultural systems is dependent on the accumulation of soil organic matter from which N may be released slowly by microbial decomposition (Lanning and Williams, 1981). Therefore, amendments, such as compost and alfalfa, which supply N and other nutrients in addition to organic matter, may be a good fit for reclamation programs aimed at reverting abandoned wellsites to agricultural use.

Nitrate-Nitrogen
Soil NO₃–N content in the 0- to 60-cm depth differed significantly among the sites at the end of the study (Table 4). On average, the NO₃–N content was highest (P < 0.001) for the Hussar site (468 kg ha^–1) and similar for the Sundial and Turin sites (mean 137 kg ha^–1). Significant amendment-by-rate interactions were detected for NO₃–N content. Across the amendments, the NO₃–N content was not affected by compost rate (mean 213 kg ha^–1) but increased by 7.78 kg ha^–1 (r² = 0.89; P = 0.006) for each Mg ha^–1 increase in alfalfa rate (Fig. 3a). This result reflects the lower stability of alfalfa N compared with compost N and highlights the potential risk of NO₃–N leaching at high alfalfa rates. Also, despite the higher NO₃–N content of alfalfa-amended soils, cumulative N uptake by the wheat crop at all sites was similar for the two amendments (data not presented).

View larger version (27K): [in this window] [in a new window]   Fig. 3. Effect of alfalfa (Medicago sativa L.) and compost rates on (a) soil nitrate N (NO3–N, 0- to 60-cm), and (b) modified Kelowna-extractable P (MKP, 0- to 30-cm) content at the end of the study in 2004 (averaged across all three sites). Vertical bars represent standard errors.

View larger version (33K): [in this window] [in a new window]   Fig. 4. Effect of (a) alfalfa (Medicago sativa L.)- and (b) compost-N rates on soil nitrate N (NO3–N) content in the 0- to 60-cm depth 15 mo (Sundial) and 27 mo (Hussar and Turin) after application. Vertical bars represent standard errors.

View larger version (30K): [in this window] [in a new window]   Fig. 5. Vertical distribution of soil nitrate N (NO3–N) in the 0- to 60-cm depth as affected by alfalfa (Medicago sativa L.) [(a), (c), and (d)] and compost [(b), (d), and (f)] rates 27 mo after application at Hussar [(a) and (b)] and Turin [(e) and (f)], and 15 mo after application at Sundial [(c) and (d)]. Horizontal bars represent standard errors.

Available Phosphorus
Available P (MKP) content in the 0- to 30-cm depth was similar (mean 116 kg ha^–1) for the Hussar and Turin sites and significantly lower (P < 0.001) for the Sundial site (Table 4). The lower MKP at the Sundial site was despite the larger overall amendment P rates applied at the site (Table 3) and reflects the lower inherent MKP status of the soil. Final MKP content in the unamended plots averaged 29 kg ha^–1 at the Sundial site compared with 92 and 95 kg ha^–1 at the Hussar and Turin sites, respectively.

Available P content in the 0- to 30-cm depth increased by 3.24 kg ha^–1 (r² = 0.96; P = 0.001) for each Mg ha^–1 increase in compost rate (Fig. 3b). Alfalfa rate, on the other hand, had no significant effect on the MKP content, indicating the much lower P additions from the alfalfa hay compared with the compost.

While the much higher P additions from compost help explain the observed increases in MKP accumulation, further analysis also indicated higher linear increases in MKP accumulation associated with compost P compared with alfalfa-derived P (Fig. 6). With alfalfa application, significant MKP response to P addition (r² = 0.97; P = 0.02) occurred only at the Sundial site where the MKP content increased at low P rates but was depressed at higher rates (Fig. 6a). By comparison, the MKP content increased by 0.58 kg ha^–1 (r² = 0.94; P < 0.001) at the Hussar and Turin sites and by 0.47 kg ha^–1 (r² = 0.93; P = 0.01) at the Sundial site for each kg ha^–1 of compost P applied (Fig. 6b).

View larger version (30K): [in this window] [in a new window]   Fig. 6. Effect of (a) alfalfa(Medicago sativa L.)- and (b) compost-P rates on modified Kelowna-extractable (MKP) content in the 0- to 30-cm depth 15 mo (Sundial) and 27 mo (Hussar and Turin) after application. Vertical bars represent standard errors.

View larger version (25K): [in this window] [in a new window]   Fig. 7. Vertical distribution of modified Kelowna-extractable P (MKP) in the 0- to 30-cm depth as affected by alfalfa (Medicago sativa L.) [(a), (c), and (d)] and compost [(b), (d), and (f)] rates 27 mo after application at Hussar [(a) and (b)] and Turin [(e) and (f)], and 15 mo after application at Sundial [(c) and (d)]. Horizontal bars represent standard errors.

The high MKP levels associated with compost application emphasize the need for caution when using compost to boost agricultural productivity in reclaimed soils. Alberta fertilizer P recommendations for soils such as those used in this study suggest that MKP levels > 50 mg kg^–1 (0- to 15-cm depth) are excessive for spring or winter wheat production (Alberta Agriculture, Food and Rural Development, 2003; McKenzie et al., 1995). In our study, this threshold was exceeded at compost rates >20 Mg ha^–1 (rate 4x). Available P levels (0- to 15-cm depth) at the 21.3 Mg ha^–1 (4x) and 42.4 Mg ha^–1 (8x) compost rates averaged 52.2 and 95.7 mg kg^–1, respectively. This P enrichment can present problems of surface water eutrophication if the P is entrained in surface water (Sims et al., 2000).

	CONCLUSIONS

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

 
Based on actual organic amendment rates, both alfalfa hay and compost significantly improved the SOC status of all reclaimed wellsites compared with the unamended control. However, compost was the more effective amendment for increasing SOC content per unit of organic C applied. Because of the greater stability of its C, compost becomes the preferred amendment for longer term soil C storage. Alfalfa C is readily decomposable and therefore may not provide long-lasting measurable increases in SOC. Nitrate-N accumulation in the 0- to 60-cm depth increased linearly with incremental alfalfa rates and alfalfa-derived N but was not significantly affected by compost or N derived from the compost. On the other hand, MKP content increased linearly with increasing rates of compost and compost-P but changed little with alfalfa and alfalfa-P rates. While SOC and MKP accumulations were confined primarily within the vicinity of the depth of amendment incorporation, greater vertical distribution was evident for the more leachable NO₃–N. Our results indicate potential risk of surface soil enrichment with P and subsequent loss of the nutrient to surface water at compost rates in excess of 20 Mg ha^–1. Combining lower rates of compost and alfalfa could potentially reduce the environmental risks associated with P and NO₃–N while providing ample supplies of both nutrients for plant uptake.

	ACKNOWLEDGMENTS

 
Funding for this project from EnCana Corporation, Calgary, Alberta and the Matching Investment Initiative of Agriculture and Agri-Food Canada is gratefully acknowledged. The contributions of Brian Handerek, Clarence Gilbertson, Bonnie Tovell, Wayne McKean, Harvey Dyck, Frank Megella, and Rodney Volk toward field and laboratory work are much appreciated. Particular thanks are due to landowners, The Hutterian Brethren of Turin, Glen Muller and Larry Machacek for their cooperation.

	NOTES

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

 
Lethbridge Research Centre contribution no. 38706055.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication October 25, 2006.

	REFERENCES

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

 

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