¹Soils and Crops Research Centre. Agriculture and Agri-Food Canada. Sainte-Foy, Qc.
²Horticultural Research Centre. Laval University, Qc

Paper sludge compost may be an efficient way to improve the nutrient status and biochemical and physical properties of coarse-textured soils devoted to intensive potato production. A study was initiated to assess the impact of addition of a compost of primary papermill sludge and hog manure, used alone or in combination with inorganic fertilizers, on soil quality and potato (Solanurn Tuberosum L. Hilite Russet) yields on a Bevin sandy loam (Orthic Humo-Ferric Podzol) located in St-Ubalde, Québec, Canada. The compost was applied in spring at 0,40,80 and 120 Mg ha^-1 on a wet basis, and was supplemented or not with 0 to 225 kg N ha^-1, 0 to 300 kg P₂0₅ ha^-1 and 200 kg K₂0 ha^-1. The compost was incorporated in the top 10 cm of soil. The inorganic fertilizers were fractionated at planting and hilling.

The addition of 40 Mg compost ha' produced the highest marketable tuber yields independently of the inorganic fertilizer supplement. This amount of compost along with 75 to 150 kg N or 100 to 200 kg P₂0₅ha^-1produced the highest yields. Compost reduced the degree of potato scab coverage on the tubers. Compost addition significantly influenced the soil Kcl extractable N 30 days after planting and Mehlich-3 extractable P and K contents at potato harvest. Enzyme activities were maxima early in the season. Compost addition at 40 Mg ha^-1 resulted in the highest enzymatic activity, Nitrobacter sp. counts and C-CO₂ evolution. Compost addition also influenced the synthesis of particular microbial phospholipid patterns. Addition of inorganic fertilizers to compost resulted in higher enzyme activity than compost alone but had little effect on C mineralization or microbial biomass C. Addition of 120 Mg compost ha^-1 was excessive. Compost, when added at a reasonable rate (40 Mg ha^-1), is a good way to increase the inorganic fertilizer efficiency by increasing potato tuber yields and soil quality.
 
 

Soils used for potato production in Eastern Canada are normally of coarse texture and are subject to degradation when crop rotations are not frequently used. Loss of organic matter is the main form of degradation (Tabi et al. 1990) which results in loss of aggregate stability and increased water (Edwards 1988) or wind erosion. Loss of organic matter and erosion result in diminished water retention capacity, resistance to compaction and cation exchange capacity (Saini and Grant 1980). The N mineralization potential from soil humus will also be decreased since it is closely related to the soil organic matter content (Simard and N'dayegamiye 1993). All of this may result in yield losses and increased fertilizer and irrigation cost (Saini and Lantagne 1974).

Rotations involving crops with large amounts of residues are certainly a good alternative in organic matter management. Large inputs of exogenous organic materials would be preferable if a rapid increase in organic matter content is targeted. The pulp and paper industry generates about 1.5 million tons of residues per year in Quebec (Association des industries forestières du Québec 1997). Although excellent results in term of improvement of soil quality and crop yields have been obtained in crop production with the raw sludge materials (Trépanier et al. 1996; Simard et al. 1998 a, b), these materials have the disadvantages of generating bad odours and composting may be the solution. Also, land spreading of the raw material is only possible in a part of the year. Composting reduces volume and odour and stabilize the organic C (Lampkin 1990). Using of composted materials may have a longer impact on organic matter content than raw materials on these coarse textured soils (Barker 1997). Composted de-inking sludge was previously shown to result in improved soil quality, nutrient efficiencies and crops yields in winter cabbage and dry bean production (Simard et al. 1996, 1997). The objective of this study was to assess the impact of addition of a de-inking paper sludge-hog manure compost on soil quality and potato tuber yields and quality.

 

Materials and Methods

Experimental design

The experiment is carried out in a field of Les Patates Herman Dolbec in St-Ubalde, Québec, Canada. The soil is a Bevin loamy sand (Orthic Humo-Ferric Podzol). The soil had a pH of 5.4 and 45-60 g kg^-1 organic matter. The compost was applied at 0,40, 80 and 120 Mg ha^-1 on a wet basis. Treatments including combinations of 0 to 225 kg N as ammonium nitrate, 0 to 300 kg P ha^-1 as triple superphosphate and 0 to 200 kg K ha^-1 as potassium chloride were added as subplots. Treatments were arranged in a completely randomized split-block design with compost rates as main plots and fertilizer combinations as subplots. All treatments were replicated three times. Compost and inorganic fertilizers were hand-applied on May 28 at pre-seeding and immediately incorporated with disk harrow. Two third of the N fertilizer were applied at seeding and the rest at hilling. Potatoes (cv. Hilite Russet) were planted the next day with a 35 cm in-row spacing and 91.5 cm between rows. Pesticide management was as recommended in Québec (CPVQ 1992). Plots were hand harvested on September23 and tubers were classified according to size in Canada #1 categories.
 
 

Soil chemical analysis

Samples were taken prior to the beginning of the experiment from the 0-20, 20-40 and 40- 60 cm with a 7 cm auger. Three cores were taken per plot. The samples were air-dried and sieved to 2 mm prior to analysis. Samples were also taken 1 month after planting and at harvest. The soil pH was measured in 0.01 M CaCl₂(1:2 soil : solution); P, K, Ca, Mg and metals were extracted by the Mehlich-3 solution (Tran and Simard 1993). The P content in the extracts was determined by colorimetry using the molybdate reaction (Murphy and Riley 1962) whereas the other elements were determined by flame (K) or atomic absorption spectroscopy. The soil inorganic N was extracted by 2 M KCl (Maynard and Kalra 1993). The NH₄⁺ content of the extracts was determined by the colorimetric reaction with nitroprusside (Nkonge and Ballance 1982) whereas the N0₃^- was determined by liquid chromatography on a Dionex 4000i apparatus equipped with a VDM-II UV Vis-detector (Dionex Corporation, Sunnyvale, CA).
 

Characterization of biochemical properties

Sampling (0-20 cm) was carried out on May 28, June 15, June 25, July 23, August 20, September 17 and October 14. The samples were composed of at least 19 cores taken from the side of the potato hill. Soils were sieved to 4 mm in the field and carried in a refrigerated chest to the laboratory where they were stored at 4⁰C. All analysis were carried out within three weeks of sampling. Carbon mineralization was carried out according to Zibuske (1994). Briefly, 25 g of fresh soil were added to 0.5 L glass Masson^tm jar with a NaOH trap. Pots were closed and incubated at 25⁰C in the dark for up to 42 days. Evolved C0₂was measured, after precipitation with excess of BaCl₂, by titration of the residual NaOH with HCl. Acidic phosphatase and urease activity was measured according to Tabatabai and Bremner (1969), the ß-gluco -and ß-galactosidase by the method of Serra-Wittling et al. (1995), and the activity of fluoresceine diacetate by the method of Inbar et al. (1991). Microbial biomass carbon was estimated following Wu et al. (1990), the nitrobacters were measured from soil suspensions (Rowe et al. 1977) and the phospholipid profiles were done according to Zelles et al. (1992).

Compost properties

The compost had 713 g kg^-1water and was acidic (Table 1). Its C:N ratio was 40 which should normally result in soil N irnrnobilization (Sims 1990). The compost contained 11.3 g kg^-1 total N which is in the low range of what is reported for paper mill residues (AIFQ 1997). Amounts of 745 mg kg^-1 NH₄⁺ and 1028 mg kg^-1 N0₃^- were found in soluble forms which was 15.6 % of total N. The molybdate reactive water-soluble P was 0.24 g kg^-1 which was only 8.5 % of the compost total P content. The C:P ratio was 161 which should result in P immobilization (White 1981). The total K content is very low but well balanced with the total P content. The material had significant contents of Mn, Zn and Cu in total and soluble forms (Table 1). The content of heavy metals was all below the detection limits and that for the quality "A" criteria for compost in Canada (Ministe're de l'Environnement du Québec 1997). The high water-soluble Mn content may be of concern when the compost is added to very acidic soils (pH <5.3).

Soil chemical composition

The inorganic soil N (NH₄⁺ and N0₃^-) content 30 days after planting was significantly increased by compost addition at 80 Mg ha^-1 and more, whereas NH₄NO₃addition had a lesser effect (Figure 2). Gagnon et al. (1998) reported increases in soil profile inorganic N contents of two soils with the addition of commercial composts. Most of this increase was attributed to the inorganic N contribution of the composts. In the present study, 19kg water- soluble N were provided by each amount of 40 Mg ha^-1 compost added.

The amount of inorganic N at harvest was proportional to the amount of NH₄NO₃added N (Figure 3). Compost rates had no significant impact on residual N in the 0-20 cm layer. Gagnon et al. (1998) observed no significant effect of compost on soil N at fall on a Kamouraska clay. The organic N comprised in the compost should be in very stable forms, so it is not surprising that limited impact on residual N was observed.

The amount of Mehlich-3 extractable P was significantly increased by the compost addition whereas P fertilizer had no such effect (Figure 4). Increases in soil-test P were previously reported with the addition of raw de-inking materials (Norrie and Gosselin 1996, Simard et al. 1998). Neilsen et al. (1998) reported a large increase in soil-test P with the addition of composted biosolids and related that to the P added. The increase in Mehlich-3 extractable P of the 0-20 cm layer with the 80 Mg compost ha^-1 was approximately of 40 kg ha^-1. An amount of 64 kg total P of which only 6 kg in inorganic form was added with the compost. If we assume that tuber P uptake was around 20 kg ha^-1 for a total yield of 40 Mg ha^-1, then the efficiency coefficient of the compost P would be close to 100 %. This is very unlikely. A contribution of the soil organic and mineral pools by the action of the compost is more probable. For instance, the observed increase in the Mehlich 3 extractable-P may be related to the impact of compost on enzymes responsible for the mineralization of organic P. Cooper and Warman (1997) had previously shown that compost effect on dehydrogenase activity was related to soil texture. The impact of organic acids/humic substances present in the compost or produced during the decomposition of the compost on the soil P sorption capacity may also have contributed to the increase in soil-test P as suggested by Ohno and Erich (1997) and Iyamuremye et al. (1996).

The amount of Mehlich-3 extractable K was increased by the addition of 200 kg K ha^-1 whereas compost had no residual effects on extractable K (Figure 5). Previous studies with raw materials on turfgrass (Norrie and Gosselin 1996) and barley (Simard et al. 1998) have shown increased Mehlich-3 extractable K contents. The difference between these results and those of the present study may be related to the much higher K uptake by the potatoes than by turfgrass or barley, leaving much less residual Kin the soil at harvest. In the present study, only 11.3 kg K ha^-1 were added to the soil by each increment of 40 Mg compost ha^-1. The K uptake of the potato crop was around 75 kg K ha^-1.   It is therefore not surprising to observe little impact of the compost on soil-test K. The Mehlich-3 extractable Mg content of the 0-20 cm layer was not significantly influenced (p< 0.05) by the compost addition.

The Mehlich-3 extractable Pb and Cd contents were not affected by the treatments (data not shown). The compost used had very low heavy metal content (Table 1), thus it is not surprising to measure no significant increase in the amounts of Mehlich-3 extractable metals.
 

Soil microbial activity

The soil biochemical properties reached their maxima at different time in the growing season. The acidic phosphatase activity was maximum after the fourth week (Figure 6). The peak of activity was recorded early in the growing season, at 2 wk after compost addition for the ß-glucosidase and at 4 wk for the FDA, the 13-galactosidase and the C-CO₂ release in incubation (Figs. 7, 8, 9 and 10). In spite of lower levels, activities of FDA and P glucosidase remained constant until the end of the season whereas the activity of ß-galactosidase reached its lowest point at 8 wk whereas C-CO₂ release was lowest at 12 and 16 wk. Microbial biomass C was highest at 12 wk whereas it was the lowest at 4 and 20 wk (Figure 11). Urease activity was higher in late summer and fall (12 to 20 wk) than early in the season (2 and 4 wk) (Figure 12). Finally, the number of nitrobacters (NO₂^-) tend to peak earlier with low compost rates (Figure 13). Recent addition of fresh C source may promote microbial activity. However, the date at which maximum peak was obtained varied with the enzymes. The various nutrients contained in the soil and c6mpost are released sequentially by the different enzymes, depending upon their complexity. To this end, it seems that the availability of glucose is a primary concern in comparison with galactose early after paper sludge compost addition.

The 40 Mg compost ha^-1 gave the highest values of the selected biochemical properties at every sampling dates. This effect was noted very soon in season (2 wk) for all enzymes with the exception of urease, and for the number of nitrobacters whereas it was later for biomass C (4 wk), urease (8 wk) and C-CO₂release (8 wk). The supplement of inorganic fertilizers to compost resulted in higher enzyme activities. It also enhanced nitrobacters, especially at harvest (16 wk). However, it had no effect on C-CO₂release and microbial biomass C. However, at 4 wk, their addition to 120 Mg compost ha^-1 significantly decreased the microbial biomass C as compared with the unamended treatments. In the present experiment, addition of compost to soil stimulated microbial activity when the rate did not exceed 40 Mg ha^-1. Moreover, supplementation of compost with inorganic fertilizers was beneficial, probably counteracting the high C/N and C/P ratios of the compost.

Phospholipid signature markers indicated the establishment of specific microbial communities in soil receiving organic amendments when compared to inorganic fertilizers (data not shown). On the basis of our data, community fatty acid profiles can be used to assess the relative similarities and differences of microbial communities. However, detailed interpretation in terms of biomass or taxonomic composition must be taken with caution until our knowledge of the quantitative and qualitative distribution of fatty acid profiles over a wide variety of taxa and the effects of growth conditions on fatty acid profiles is more advanced.
 

Tuber marketable yields

There was also a significant interaction between compost and fertilizer P. In absence of fertilizer P, 80 Mg compost ha^-1 produced the highest marketable yield (Figure 15). For the 100 and 200 kg P ha^-1, an input of 40 Mg compost ha^-1 produced the highest yields. The 120 Mg compost ha^-1 reduced yields compared to the no compost treatment for all P rates except the 100 kg P ha-'. The positive response of tuber yields to compost addition is reduced as fertilizer P rate is increased. These results confirm those of the soil analysis that U clearly showed that compost increased soil test P and soil enzyme P activity. This resulted in increased P availability for the potato crop.
 

Tuber quality

There was a significant compost-NH₄NO₃interaction on tuber specific gravity (data not shown). In absence of NH₄NO₃, compost decreased the tuber specific weight. The increase in the amount of compost added in presence of NH₄NO₃had the inverse effect, reducing the negative impact of N fertilizer on specific gravity. The compost addition did not reduced the soil soluble mineral content of the high inorganic N fertilizer treatments 30 days after planting (Figure 2). The increase in tuber specific gravity may therefore be related to a better supply of soil P or to a factor unidentified by the present experiment.

There was a significant decrease in potato scab index with compost addition (Figure l6). Although this observation is difficult to explain, it may be related to the increase in microbial activity from compost addition which may have suppressed the activity of the scab species. There was no significant effect of the compost .addition on the proportion of tuber showing the "hollow hearth" symptom.
 
 

This study indicated that application of compost to this sandy soil, although it had a high organic matter content, may be beneficial to potato tuber yields. The soil inorganic N content was increased 30 days after compost application and so was the amount of extractable P. This indicated that the hypothesis of soil N and P immobilization that was suggested by the C/N and C/P ratio of the compost was not valid in this studied soil. In that sense, excessive compost addition should be avoided since they was not beneficial to tuber yields and soil quality. Compost beneficial effects are normally of short term. As for any other soil amendments, care should be taken to apply the right amount. A larger amount may have been appropriate in a soil with a lower organic matter content. A study conducted in the adjacent part of the field indicated that the beneficial effects of compost are inversely proportional to the soil organic matter content (Cambouris et al. 1997).

We suggested that mixing the raw material with hog liquid manure may help to reduce the immobilization of soil N and the need of complementary inorganic N fertilizer addition to non-leguminous crops (Simard et al. 1998). This may also help to reduce the surplus of manure in some watersheds if the compost is exported to the potato producing areas. The results of the present study suggest that the compost production is a very viable alternative in the management of these two by-products of the forest and agricultural industries. Potatoes are produced on very sandy soils in Québec, and are highly prone to wind and water erosion. Timely addition of compost is probably one of the best ways to maintain or improve their productivity through the input of stable C compounds. The compost is richer in P and K than the raw material and is so less likely to produce nutrient imbalances for the crop. In addition, it meets the "A" criteria for compost; its use on potato production would not be restricted.

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This study is supported by Les Composts du Québec and Agriculture and Agri-Food Canada through the Matching Industrial Initiative Grant Program. The assistance of Antoine d'Avignon and Gilles Fortin in the field, of the personnel of the Joseph Rhéaume farm for evaluating the tuber size and quality, and of Alain Larouche and Sylvie Michaud in the laboratory is appreciated. We wish to thank Les Patates Herman Dolbec for providing the site and helping in the soil preparation and in the spraying of the pesticides.
 
 
 
 
 

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