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Published online 3 October 2006
Published in Agron J 98:1471-1478 (2006)
DOI: 10.2134/agronj2005.0339
© 2006 American Society of Agronomy
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Production Papers

Mixed Paper Mill Sludge Effects on Corn Yield, Nitrogen Efficiency, and Soil Properties

Adrien N'Dayegamiye*

Research and Development Institute for the Agri-Environment (IRDA), 2700 Einstein, Complexe scientifique, D.1.110, Sainte-Foy, QC, Canada G1P 3W8

* Corresponding author (adrien.ndaye{at}irda.qc.ca)

Received for publication December 16, 2005.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Large quantities of mixed paper mill sludges (PMS) are applied annually to agricultural soils in North America. However, little information exists in the literature delineating the impact of land application of PMS on crop N nutrition and soil properties. In a 3-yr field study, (1997–1999), we evaluated PMS effects on corn (Zea mays L.) yields and soil property changes. The study included annual and biennial PMS applications of 20, 40, and 60 Mg ha–1 on wet basis, applied alone or in combination with N fertilizer at reduced rates (90 and 135 kg N ha–1 for 40 and 20 Mg PMS ha–1, respectively), complete N fertilizer for corn (180 kg N ha–1) and a control. Plots were split beginning with the second year for annual and biennial PMS and N fertilizer application. Annual or biennial applications of PMS alone resulted in grain yield increase of 1500 to 3000 kg ha–1 as compared to the unfertilized control. The applications of 20 to 40 Mg ha–1 PMS with N fertilizer at reduced rates (135 and 90 kg ha–1 respectively) achieved higher corn yields compared to PMS applied alone. The PMS applications combined with N fertilizer at reduced rates produced highest corn yields, similar to those obtained with complete N fertilization for corn (180 kg N ha–1). Corn apparent N recoveries (ANR) ranged from 17 to 21% in year of application and from 15 to 22% in residual year, depending of PMS rates. Three PMS applications at 40 to 60 Mg ha–1 yr–1 significantly increased the soil C content by 22 and 26%, and by 18 and 22%, compared to the control and N fertilizer, respectively. Those PMS applications also significantly increased the mean-weight diameter (MWD) of aggregates, and reduced soil bulk density as compared to the control and fertilizer alone treatment. The soil microbial biomass C and the alkaline phosphatase and urease activities were also increased in soils that received PMS. Our results suggest that the applications of PMS with low C/N (19–24) benefit corn growth possibly due to a combination of the higher nutrient availability and the improvement of the soil properties.

Abbreviations: ANR, apparent N recovery • MWD, mean weight diameter of aggregates • PMS, mixed paper mill sludges • SOM, soil organic matter


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
IN RECENT YEARS more than one million tons of PMS were annually applied on agricultural soils in Québec and Ontario, Canada, in the northeastern states of the USA and Norway (Shimek et al., 1988; Simard, 2001; Vagstad et al., 2001; N'Dayegamiye et al., 2003). However, the influence of PMS on plant yield and N nutrition, and their effects on soil properties have not been thoroughly assessed. The PMS generally applied to agricultural land have low C/N (12–25) (Simard, 2001; N'Dayegamiye et al., 2003), which could increase N availability and plant response following their application. Repeated PMS applications to soils could influence soil N supply and modify soil properties to benefit crop growth and nutrition for years after incorporation, as documented in the literature after repeated applications of dairy cattle manure (Estevez et al., 1996).

Before 1995, only primary sludges were produced by paper mills. These wastes were used mainly for degraded site reclamation. Primary sludges have high C/N ratios (>200) and they induced N immobilization, which reduced plant growth (Fierro et al., 2000). Since 1997, new environmental technologies in the paper mill industry have resulted in the production of secondary sludges enriched in N, P and K (N'Dayegamiye et al., 2003). Primary and secondary sludges are then mixed, making this waste more attractive for agricultural use. Currently, PMS are generally comprised of 60% primary sludges and 40% secondary sludges. Few mills produce de-inking paper sludges with higher C/N ratios (30–60) that are also applied to agricultural land (Lalande et al., 2003).

The form of N present in organic residues determines its availability to crop. Manure N availability to crop in the first yr after application is mostly determined by its inorganic N content (Beauchamp, 1986). However, the inorganic N (NH4–N and NO3–N) content in PMS represents between only 5 to 21% of total N (Simard, 2001; Arfaoui et al., 2001; N'Dayegamiye et al., 2003). Thus, most of N in PMS is in organic form and must be mineralized to complete crop N needs. High N mineralization and N availability occurs from decomposing organic residues with N content above 2% as for PMS, and immobilization occurs below that N concentration (Palm and Sanchez, 1991).

Crop response to organic residues (manure, green manure or PMS) cannot be attributed solely to nutrient supply. When added to the soil, organic residues such as cattle manure (Magdoff and Amadon, 1980) and green manure (Wagger et al., 1998; N'Dayegamiye and Tran, 2001) improved crop yields beyond what could be attributed to the sole effect of supplied N. Other beneficial effects may be related to the improvement of physical, chemical and biological properties. For example, improving soil structure promotes better root growth and plant nutrient absorption, increases soil moisture and improves soil microbial activities for better nutrient mineralization (Kirchner et al., 1993; Wagger et al., 1998).

It has been demonstrated that the nature of organic residues determines its influence on microbial activity and soil aggregation (Lynch and Bragg, 1985). Microbial biomass and activity in soil is an indicator of C turnover and availability from organic residues (Tian et al., 1992). De-inking paper sludges have higher C/N ratios than PMS (Fierro et al., 2000), and organic residues with high C/N ratios and high lignin content decompose slowly in soil, and their influence on soil microbial activity and aggregation is low (N'Dayegamiye and Angers, 1993; Fierro et al., 2000). The PMS that have high labile C content and low C/N ratios (N'Dayegamiye and Watt, 2000) could stimulate microbial growth, enzymatic activity, nutrient availability and promote soil aggregation.

Knowing the agronomic benefits of PMS will facilitate their use as source of nutrients and soil conditioner. The objectives of this 3-yr study were to: (i) determine corn yields, apparent N recoveries in year of application and in subsequent year; and (ii) assess changes in soil properties following 3 yr of annual PMS applications.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The PMS used in this 3-yr study (1997–1999) were collected from the Abitibi Consolidated paper mill (Belgo division, Shawinigan, Québec, Canada). Selected characteristics of these sludges are reported in Table 1. Overall, the PMS were made of approximately 60% primary sludges and 40% secondary sludges. Organic C content ranged from 385 to 456 g C kg–1, C/N ratios from 19 to 24 and pH from 6.8 to 8.0 (Table 1). The PMS total N content ranged from 16.5 to 21.9 g kg–1 and the NH4–N content from 957.6 to 1996 mg kg–1, representing 5 to 10% of total N. All PMS used in the study contained high levels of major and minor elements, K being an exception.


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  		Table 1. Physical and chemical properties of paper mill sludges applied from 1997 to 1999.

 
During the three cropping seasons (May–September), mean air temperatures were 21.5, 23.3 and 24°C in 1997, 1998, and 1999, respectively. Total precipitation was 203.5, 395.7 and 606.9 mm, respectively. The experiment was conducted in central Quebec, Canada (46°35' N; 72°12' W). The soil was a Godfroy silt loam (sandy over clayey, mixed, nonacid, frigid, Typic Humaquept). The particle-size distribution of the A horizon (0–20 cm) was 270 g kg–1 sand, 490 g kg–1 silt and 240 g kg–1clay. The initial soil pH value was 6.8 and the soil C and N contents were 38 g kg–1 and 2.1 g kg–1, respectively. The soil available P and K contents were 0.12 g kg–1 and 0.13 g kg–1, respectively.

A completely randomized block experiment included three PMS rates (20, 40, and 60 Mg ha–1 on a wet basis) applied alone or with N fertilizer (90 and 135 kg–1, for PMS rates of 40 and 20 Mg ha–1 respectively). Treatments also included a control without PMS or N fertilizer, and a mineral N fertilizer (180 kg ha–1) as recommended for corn cropping in the area of experiment. No N fertilizer was applied to 60 Mg ha–1 plots. To ensure that P and K availability were not a limiting factor to crop growth, all treatments including the control received P and K fertilizers at the rates recommended for corn, depending on soil analysis (15 kg P and 85 kg K ha–1). The experimental plots were 10 m wide by 12 m long with 12 rows of corn planted on 0.76-m row spacing.

In the first (1997) and third yr (1999) of the experiment, PMS and N fertilizer were applied to the entire surface of plots. In the second yr (1998) of the experiment, the plots were split to study annual vs. biennial PMS and N fertilizer applications on corn yields and N nutrition. The subplots were 5 m wide by 12 m long. The same treatments described above were applied to half of each plot and no N fertilizer or PMS were applied on the second half of the plot to evaluate residual PMS and N fertilizer effects.

The PMS and fertilizers were applied in late April of each year and immediately incorporated by disking in the 0- to 10-cm soil layer. Corn hybrid (Dekalb 343–2550 corn heat units, CHU) was seeded at 76 100 plants ha–1 every year. Corn grain yields were determined by harvesting 12-m of the center two rows of each plot with a combine. To determine aboveground corn N content, whole plants were cut at the soil surface, chopped, weighed, and a 600-g to 800-g subsample was dried at 65°C and weighed again for dry matter calculations. Grain specific gravity was determined on a 500-g grain sample.

Analytical Methods
The PMS subsamples were collected the day of their application, and were kept at 4°C until analyzed. Samples were dried at 70°C, weighed to determine dry matter content, and ground to pass through a 2-mm sieve before analysis. The PMS pH was measured on the dried sample in a PMS/water mixture (1:1). Organic C and N content was determined by dry combustion using a LECO C-N-S 1000 analyzer (LECO Corp., St. Joseph, MI). Mineral N as NH4–N and NO3–N was analyzed on wet samples, extracted using 2 M KCl (Bremner and Mulvaney, 1982) and measured with a Technicon Autonalyzer. Total N content in plant samples, and P, K, Ca, and Mg contents of PMS were extracted by digestion in H2SO4 and H2SeO3 with the addition of H2SO4 for 1 h at 400°C (Isaac and Johnson 1980). The PMS contents of B, Cu, Fe, Mn, and Zn were obtained after dry-ashing at 450°C for 3 h, and extraction in 2 M HCl (Richards, 1993). Total N content in plant samples was determined on an Automated Technicon Autoanalyzer, and mineral elements in PMS by inductively coupled plasma optical emission spectrometer (PerkinElmer 4300 DV, Boston, MA).

Composite soil samples made from 12 cores were collected from the top (0–20 cm) soil layer in April 1997 at the initiation of the study. Samples were air-dried and sieved to pass a 2-mm sieve. A portion was kept to determine soil pH, soil texture and mineral element contents. Another portion was ground to pass through a 0.25-mm screen for organic C and total N analysis. Organic C was determined by the Walkley–Black method (Allison et al., 1965), and total N by Kjeldahl digestion (Bremner and Mulvaney, 1982). As usually recommended for Quebec soils, extractable P, K, Ca and Mg were extracted in a Mehlich III solution (Mehlich, 1984). Total N levels were measured on an Automated Technicon Autoanalyzer and mineral elements (P, K, Ca, and Mg) on inductively coupled plasma optical emission spectrometer (PerkinElmer 4300 DV, Boston, MA). Particle size analysis was performed on air-dried soils by the pipette method after the destruction of organic matter with H2O2 and dispersion with sodium hexametaphosphate (Gee and Bauder, 1986). Soil pH was measured in 1:1 soil/water solution.

At the end of the experiment in October 1999, six soil cores (5-cm diam.) were collected to a depth of 20 cm to determine changes in physical and biological properties and the content of soil C and N. Soil samples were obtained only from plots receiving annual PMS and N fertilizer treatments, and the control. The soil cores were pooled to make one composite soil per plot, sieved in the field to pass a 6-mm sieve and then stored at 4°C until analysis. A portion of the moist soil samples was used to determine soil structure and aggregation, microbial biomass C, urease and phosphatase activities, and N mineralization. Subsamples of the moist soil were air-dried and ground at <0.5 mm to determine the soil C and N contents and soil organic matter (SOM) density fractions.

To separate SOM density fractions, 20 g of finely ground soil (<0.5 mm) was suspended in 100 mL of NaI solution with a specific gravity of 1.59 g cm–3 (Janzen, 1987; N'Dayegamiye et al., 1997). This suspension was stirred and centrifuged at 5600 x g for 30 min. The supernatant solution was decanted onto a 0.45-µm filter under suction; this procedure was repeated three times to ensure complete recovery of the light fraction (LF). The material remaining on the filter was considered as the LF and the remaining soil as the heavy fraction (HF). The LF and HF fractions of SOM were dried at 65°C, weighed and determined for their organic C and total N contents. Total N and organic C content of soil and density fractions of SOM was determined on a LECO C-N-S 1000 analyzer (LECO Corp., St. Joseph, MI).

Water-stable aggregation was determined by the wet sieving method. Forty grams of air-dried soil (>6 mm) were put on the top of a series of sieves (5, 2, 1, and <0.25 mm) and the sieves were immersed in water and shaken for 10 min. The soil fractions recovered on each sieve were dried at 65°C for 24 h, weighed, corrected for sand and expressed as a percentage of total dry soil (Kemper and Rosenau, 1986). Aggregate MWD was calculated according to Haynes and Beare (1997).

Acid and alkaline phosphatase activities were determined using p-nitrophenol phosphate at pH 6.5 and pH 11 respectively, according to Tabatabai (1982). Duplicate field-moist soil samples (1 g) were incubated at 37°C for 1 h, and the p-nitrophenol released by the enzymes was measured by spectrophotometry (Hitachi U-1000) at 420 nm. Urease activity was determined on duplicate field-moist soil samples (2.5 g) incubated at 37°C for 2 h (Tabatabai, 1982). The NH4–N produced was determined by spectrophotometry at 636 µm using indophenol blue.

Soil microbial biomass C was determined using the chloroform fumigation extraction method (Vance et al., 1987). Duplicate field-moist soil subsamples (50 g) were fumigated in closed vessels with chloroform (CHCl3) for 24 h. The soluble C content of fumigated and unfumigated (control) soils were exctracted with 0.5 M K2SO4. The C in the extracts was analyzed using persulfate oxidation on a carbon autoanalyzer (Dohrmann DC-180, Santa Clara, CA). An extraction efficiency coefficient of 0.38 was used to convert soluble C into biomass C (Vance et al., 1987).

The soil mineralization potential was measured by incubating field-moist soil samples (150 g) at 25°C for 20 d at field capacity. The nitrates were extracted with 2 M KCl (Bremner and Mulvaney, 1982) before and after the period of incubation and determined on Technicon analysis system. The N mineralization potential represents the difference between the nitrate contents at the beginning and the end of the incubation.

Calculations
Data obtained on corn N uptake in 1997 and 1998 were used to determine apparent N recoveries and efficiency for N fertilizer and PMS. Apparent N recovery was calculated by the difference method using mineral fertilizer or PMS treatments and the unfertilized and unamended control. Apparent N recovery in the year of PMS application (1997) and ANR in residual year (1998), was calculated as follows:

Formula 1[1]

Statistical Methods
For corn yields, grain specific gravity and N uptake in the first yr (1997), data were analyzed with an analysis of variance with single-degree of freedom contrasts to determine treatment differences. In the second and third yr, a two-way analysis of variance for split-plot design and contrasts were used with PMS and N fertilizer treatments as main plot and application frequency (annual vs. biennial) as subplot. The soil physical and biological properties measured in 1999 in plots with annual PMS and N fertilizer applications only were subjected to an analysis of variance and means of treatments were compared using the least significant difference after a significant ANOVA test at P ≤ 0.05. Analyses were conducted using the SAS GLM procedure (SAS Institute, 1989).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Corn Yields and Nitrogen Uptake
Data on corn grain yields are presented in Tables 2 and 3. In the 3 yr of the study, PMS applied alone at rates varying from 20 to 60 Mg ha–1 significantly increased corn grain yields by 1500 to 3000 kg ha–1, compared to the unfertilized control. However, grain yields were generally lower in plots that received PMS alone at lower rates (20–40 Mg ha–1), than in treatments with complete N fertilizer for corn (180 kg ha–1) or with PMS applications combined with mineral N fertilizer (Tables 2 and 3). On average, annual PMS application at 20 to 40 Mg ha–1 combined with 135 and 90 kg ha–1 respectively, increased corn yields by 2000 kg ha–1, compared to PMS applied alone. However, the incorporation of PMS without N fertilizer at high PMS rate (60 Mg ha–1) produced high corn yields similar to N fertilizer (180 kg ha–1). Results indicate that PMS additions at lower rates (20–40 Mg ha–1) should be combined with N fertilizer in the first years of land PMS applications to achieve highest corn yields similar to recommended N fertilizer for this crop.


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  		Table 2. Effects of paper mill sludges and N fertilizer on plant dry matter, corn grain yields, specific gravity and N uptake (Year 1 results, 1997).

 

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  		Table 3. Effects of annual and biennial paper mill sludges and N fertilizer application on corn grain yields, specific gravity and N uptake (1998 and 1999).

 
Corn grain yields were lower in the subplots with PMS application in 1997 only compared to those with PMS applications in 1997 and 1998 (Table 3). Contrast analysis show that PMS application frequency (annual vs. biennial) was significant at P < 0.01 (Table 3). Yield response was linearly related to PMS rates in 1997 and 1998, and quadratic in 1999 suggesting that residual effects after two PMS applications reduced yield response to following PMS incorporations.

Corn grain specific gravity was not influenced by N fertilizer or PMS applications, compared to the unfertilized control. Corn N uptake was increased by PMS applications in all years. This PMS effect was quadratic suggesting important soil contribution on corn N nutrition. Corn N uptake was lower with biennial than annual incorporation of PMS (Table 3).

The data obtained indicate that corn yields and N uptake were strongly increased by PMS applications. Most of N in PMS is in organic form and must be mineralized to complete crop N needs. For that, synchronization between N mineralization and N uptake is an important factor controlling N availability to subsequent crops. Our results are consistent with other published studies that demonstrated that crop response to PMS applications depends on the length of growing season for a particular crop; corn, being a long season crop utilizes PMS nutrients well. This is in agreement with previous studies reporting that paper mill sludges, applied on different soil types and under various climatic conditions, significantly increased the yield of winter cabbage (Brassica oleracea var capitata L. ‘Bartolo’), sweet and grain corn (Simard, 2001; N'Dayegamiye et al., 2003), and mixed grass alfalfa stand (Medicago sativa L.) and timothy (Phleum pratense L.) (Arfaoui et al., 2001).

Results on corn N uptake indicate that PMS applications provided significant amounts of available N in the first yr of application and in subsequent year resulting in high corn yields. Corn N recovery from PMS and N fertilizer were calculated and are presented in Table 4. For N fertilizer treatment (180 kg ha–1), the ANR was 49% in 1997 and 13% in the residual year (1998). The ANR from PMS rates providing 163 to 489 kg N ha–1 ranged from 17 to 21% in the year of application and from 15 to 21% in the residual year. The ANR in the year of application decreased with increasing PMS rates and total inputs (Table 4). On a Baudette soil series (fine-silty over clayey, mixed, non acid, mesic, Typic Humaquept), N'Dayegamiye et al. (2003) also obtained lower ANR for corn with applied PMS at high rates (60–90 Mg ha–1). The ANR values measured in PMS-treated soils in the year of application were generally half of those obtained with mineral N fertilizer (Table 4). This is likely because PMS N was mostly in the organic form (90–95%. Table 1), and therefore must be mineralized before being available to crop. The PMS ANR in the residual year (1998) were generally higher than those of mineral N fertilizer.


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  		Table 4. Corn apparent N recoveries (ANR) in year of PMS application (1997) and in residual year (1998).{dagger}

 
Results obtained in this study indicate that a large portion of PMS organic N was mineralized both in the year of application and in residual year. In the 2 yr (1997 and 1998), N use by corn represented 34 to 42% of the added PMS-N (Table 4), depending on PMS rates, which suggests that at moderate application rates (40–60 Mg ha–1) PMS supplied 110.80 to 190.70 kg N ha–1 to corn, respectively. These results were similar to those obtained by Simard (2001) for PMS applied at 11.5 to 46 Mg ha–1 to winter cabbage and sweet corn. The ANR values obtained with PMS are close to those that are reported for dairy cattle manure (Beauchamp, 1986; Klausner et al., 1994) and green manure (N'Dayegamiye and Tran, 2001), which generally have similar C/N ratios. Contrary to PMS in this study, de-inking paper mill sludges, which have higher C/N (>60) and lower N contents slowly mineralize in soil and that induces a significant N immobilization and low N availability to crops (Fierro et al., 2000). Primary paper mill sludges and hog manure compost with high C/N (40) significantly increased marketable potato (Potato tuberinsis L.) tuber yield only when combined with complete N fertilizer rate as recommended for the crop (Lalande et al., 2003).

The yield response obtained with PMS in this study may have been due to more then N additions. Enhanced corn yields may also be attributed to more favorable physical, chemical and biological conditions in soils amended with PMS, as was also observed for green manures applied to soils under similar climatic conditions (Abdallahi and N'Dayegamiye, 2000). For example, yield increases ranging from 5 to 25% have been attributed to manure improvement of soil properties (Magdoff and Amadon, 1980).

Soil Properties
Changes in soil physical and biological properties and in C content of soil and density fractions of SOM after three PMS applications are presented in Tables 5Go to 7. The PMS applications alone or with N fertilizer significantly increased the proportion of water-stable aggregates >5 mm and the MWD of aggregates, as compared to N fertilizer treatment and the control (Table 5). Moreover, the proportion of soil macro-aggregates >5 mm were increased three to four times in soil that received PMS, in comparison with N fertilizer and the control. Results indicate that repeated PMS applications even at low rates (20 Mg ha–1) have influenced soil macro-aggregation. Three PMS applications at 40 to 60 Mg ha–1 yr–1 significantly reduced the soil bulk density in 0- to 10-cm soil layer (Table 5), due probably to their significant effect on the improvement of soil structure.


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  		Table 5. Effects of 3-yr applications of paper mill sludges and N fertilizer on soil water-stable aggregates and bulk density.

 

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  		Table 6. Effects of 3-yr applications of paper mill sludges and N fertilizer on soil biomass, enzyme activities and mineralizable N content.

 

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  		Table 7. Effects of paper mill sludges and N fertilizer on soil C content and SOM fractions (1999).

 
The influence of PMS on soil aggregation could be related to the nature of the waste. Paper mill sludges generally contain high cellulose levels (Table 1) and therefore have a high mineralization rate (N'Dayegamiye and Watt, 2000). This suggests that PMS are rapidly mineralized and therefore stimulate the soil microbial biomass and their impact on subsequent aggregation. Organic residues with high C/N and high lignin content slowly decompose in soil and their influence on soil aggregation is low (N'Dayegamiye and Angers, 1993).

The effects of PMS on the soil aggregation also could be related to the transitory components in decomposing PMS (polysaccharides) and to the growth of soil microorganisms (fungi, bacteria), that are known to participate in soil aggregation (Oades and Waters, 1991). During aggregate formation, micro-aggregates are bound together by polysaccharides and hyphae to form stable macro-aggregates (Oades, 1984). However, they are considered only temporary binding agents as they persist for about a year. Unless they are replaced annually, the binding effects disappear and the amount of water-stable aggregates declines (Oades, 1984). This suggests that labile components (polysaccharides) in decomposing PMS and the soil microbial biomass can rapidly induce soil aggregation, but this effect may be transient and disappear after a short time as measured in two cycles of a wheat (Triticum aestivum L.) and field pea (Poa pratensis L.) rotation (Chan and Heenan, 1999). Results of this study indicate that repeated PMS applications even at a low rate (20 Mg ha–1) have sustained soil aggregation and increased water-stable aggregates.

Increasing the abundance of soil macro-aggregates and reducing bulk density provides a microenvironment that supports increased biological activity and soil mineralization potential, a key component of soil fertility. Reducing bulk density positively impacts many soil properties including increased soil water holding capacity, greater pore space, and higher water infiltration rates. The application of PMS increased soil microbial biomass C and alkaline phosphatase activity compared to the control and N fertilizer treatments (Table 6). Urease activity and mineralized N were increased only for the two higher PMS rates (40 and 60 Mg ha–1). The increase of soil microbial biomass and enzymatic activity is a reflection of PMS C turnover and availability to microflora as was also demonstrated by Tian et al. (1992). The PMS application significantly increased alkaline phosphatase and urease activities (Table 6), which are related to soil microbial biomass (Frankenberger and Dick, 1983). These data indicated that PMS applications increased soil microbial biomass and consequently influenced extracellular polysaccharide production (Tisdall and Oades, 1982) implied in the binding of soil micro-aggregates into macro-aggregates, as indicated earlier by Tardif (1996) and Golchin et al. (1997).

Changes in C and N contents of the soil and the density fractions of SOM are shown in Table 7. The PMS additions at 40 to 60 Mg ha–1 increased the soil C content by 22 and 26%, and 18 and 22%, compared to the control and N fertilizer, respectively. The PMS applications also increased the C and N concentrations in the LF of SOM, but had no effects on the HF. The highest C and N concentrations occurred were about 10 times as great in the LF fraction, as compared to the whole soil and the HF. The increases in C and N contents of the LF of SOM is probably due to the abundance of labile C in these wastes, and also due to the stimulation of soil biomass growth (Table 6). The LF of SOM is strongly related to soil microbial biomass, soil enzyme and carbohydrate contents (Janzen et al., 1992). Golchin et al. (1994) indicated that the soil LF is constituted of microbial biomass and metabolites, and decomposing OM originating from plant debris and organic wastes. Organic matter included in LF would also be involved in soil aggregation (Oades, 1984; Golchin et al., 1994), which could explain the significant increases of soil macro-aggregates (>5 mm) following PMS application (Table 5), although significant correlations were not obtained among those parameters.

When further decomposed, the SOM present in LF is adsorbed on clay minerals, becomes HF, and is then stabilized against further decomposition (Golchin et al., 1994). Surprisingly three consecutive applications of PMS had no significant effect on the C content of HF as also observed by N'Dayegamiye and Tran (2001), following two incorporations of five species of green manures into soil. Increases in C content of HF could be measured only after long-term application of PMS as was demonstrated after 20 yr of annual application of dairy manure at 20 Mg ha–1 (N'Dayegamiye et al., 1997). The significant increase of soil organic C after only three PMS applications was unexpected. This increase is probably due to amorphous SOM from decomposing PMS and microbial products, which constitute the LF of SOM (Golchin et al., 1994). As this labile SOM could gradually be decomposed, this suggests that periodic PMS applications are necessary to maintain optimal SOM levels and to sustain microbial activity and aggregation. Results obtained in this study indicate that three applications of PMS with low C/N ratios stimulated soil microorganism growth and activity, increased the SOM as well as soil macro-aggregates (Tables 5Go–7), and therefore could have influenced corn growth, yields and N nutrition.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Our results indicate a high corn response to the applications of PMS and this could be attributed to nutrient availability and to the improvement of soil properties. Three yearly applications of PMS significantly enhanced soil microbial activities and soil aggregation, increasing water stable macro-aggregates. These PMS effects on soil properties were significant with PMS rates (40–60 Mg ha–1) suggesting that they were related to a high input of labile C. Despite the high PMS ANR in year of application and in residual year, supplementary applications of N fertilizer at a reduced rate were required to achieve high corn grain yields similar to those produced with recommended mineral N fertilizer alone. Introducing PMS in intensive cropping systems is recommended to increase soil OM content, soil macro-aggregates and microbial activities, and therefore to sustain the soil fertility and quality.


   ACKNOWLEDGMENTS
 
The author thanks Dr. Chantal Hamel and Martin Chantigny of Agri-Food Canada for reviewing the early draft of this manuscript. This research was supported by Abitibi Consolidated Inc. and the Research Institute for Agri-Environment (IRDA). Special thanks are extended to Anne Drapeau and Michel Noël for their assistance in plot maintenance, data collection, and soil analysis.


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




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Z. Hafida, J. Caron, and D. A. Angers
Pore Occlusion by Sugars and Lipids as a Possible Mechanism of Aggregate Stability in Amended Soils
Soil Sci. Soc. Am. J., October 29, 2007; 71(6): 1831 - 1839.
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