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a Université Laval. Pavillon Paul Comtois. Département des Sols et Génie Agroalimentaire. Québec, QC. G1K 7P4. Canada
b Agriculture et Agroalimentaire Canada. 2560 Boul. Hochelaga. Québec, QC, G1V 2J3 Canada
c Institut de Recherche et de Développement en Agroenvironnement (IRDA). 2700, Rue Einstein. Québec, QC, G1P 3W8 Canada
* Corresponding author (judith.nyiraneza.1{at}ulaval.ca).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: FA, fulvic acids • HA, humic acids • MWD, mean weight diameter • NHF, nonhumified fraction • PMN, potentially mineralizable nitrogen • SOC, soil organic carbon • SOM, soil organic matter
Received for publication April 4, 2008.
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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The decline in SOM and related properties by long-term mineral fertilization is more pronounced in cropping systems where fewer residues are returned to the soil (Khan et al., 2007). According to Layese et al. (2002) and Alvarez (2005), mineral N fertilization alone is insufficient to maintain SOM levels unless it is combined with a high return of crop residues. Worldwide, manure is used as a source of organic matter to improve soil bulk density (Arriaga and Lowery, 2003), water-stable aggregation (Estevez et al.,1996; Whalen et al., 2003), microbial biomass and activity (Gunapala and Scow, 1998), and crop yield (Arriaga and Lowery, 2003; Nyiraneza and Snapp, 2007). Manure can also increase soil C and N reserve by increasing protected SOM within aggregates (Aoyama et al., 1999).
The quality of organic matter and its role in maintaining soil functions is related to the quality of organic amendment. Some studies have demonstrated that soil aggregation is affected by labile (Bipfubusa et al., 2008) and humic material (Piccolo and Mbagwu, 1990; Bipfubusa et al., 2008). Comparing fresh and composted amendments, Bipfubusa et al. (2008) showed that humic substances play a significant role in stabilizing aggregates in compost-amended soils, whereas fungi are involved in maintaining aggregation in the presence of fresh amendments. Soil organic matter is composed of different compartments which differ in biochemical composition, biological stability, and turnover rate (Paustian et al., 1992). It is then important not only to consider the quantity of SOM but also its fractions to better understand how different managements can affect SOM and related properties in short- and long-term basis.
The long-term effects of mineral fertilization on SOM and physical properties are still divergent in literature. Even though the positive effects of manure on soil properties have been widely recognized, the long-term effects of mineral fertilization with or without manure are poorly documented in intensive cropping systems such as silage corn–cereal rotation with removal of residues. Under temperate climates, the response of SOM to changes in management practices is slow (Schjønning et al., 1994). For this reason, long-term studies are required to provide valuable information about changes in SOM and related soil properties (Hati et al., 2007). The objective of the present study was to assess the long-term effects of mineral fertilization and dairy cattle (Bos taurus) manure applications on soil C and N contents, C content in humic substances, macroaggregate stability, soil N availability, and crop yield and N uptake in a silage corn–cereal rotation with removal of crop residues.
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MATERIALS AND METHODS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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A factorial experiment was arranged in a split-plot design with annual dairy cattle manure application (0 and 20 Mg ha–1 on a wet weight basis) as the main factor, and mineral fertilization (PK, NP NK, NPK, NPKMg, and a control without fertilizer) as the subfactor. For this study, only three treatments were considered (Control, PK, and NPK) to assess N fertilization effect on soil properties, corn yield, and N uptake. The size of each experimental unit (subplot) is 8 by 10 m. All experimental treatments are replicated three times and the soil is cropped to a 4-yr rotation. The crop sequence in manured and unmanured plots was corn–corn (Zea mays L.)–wheat (Triticum eastivum L.)–beetroot (Beta vulgaris L.) from 1977 to 1985, corn–corn–wheat–canola (Brassica campestris L.) from 1986 to 1989, and corn–corn–wheat–barley (Hordeum vulgaris L.) from 1990 to present. All crop residues are removed from the field at harvest. Similary, the tillage regimes are similar in the main plots: the soil was plowed to a depth of 20 cm with a conventional moldboard each fall and disk-harrowed (0–10 cm) in the spring to prepare seed bed.
Manure Application
Dairy cattle manure (20 Mg ha–1 on a wet weight basis) was applied each fall from 1977 to 2003. In fall of 2004 to 2006, manure was not applied to assess its residual effects on yield, N uptake, and soil nutrient contents. Manure was supplied by the same farm since the establishment of this experiment and contained on average 200 g dry matter kg–1, 470 g C kg–1, 22 g N kg–1, 8.1 g P kg–1, 29.2 g K kg–1, 15.7 g Ca kg–1, and 6.0 g Mg kg–1 (dry matter basis).
Corn Fertilization
The present study was performed in 2005 and 2006, when all plots were cropped to the corn phase of rotation. Corn (cultivar Hyland, HL-S009; 2250 Heat Units) was planted at 78,000 seeds ha–1, with 0.75 m row spacing.
From 1977 to 2006 inclusively, mineral fertilizers were surface applied in spring before crop seeding. During corn phase of rotation, N was applied at a rate of 160 kg N ha–1 as NH4NO3 and the fertilizer rates for P2O5 and K2O differed in plots with and without manure application based on soil test analysis. With manure, P was applied as superphosphate at a rate of 40 kg P2O5 ha–1, and K was applied at a rate of 60 kg K2O ha–1 as potassium chloride. Without manure, the rates of P2O5 and K2O were 60 and 120 kg ha–1, respectively. Every 5 yr, lime was applied at 3 Mg ha–1 to all plots.
Soil Sampling and Preparation
Soil physical properties (bulk density, total porosity, water-stable aggregates), nutrient contents (SOC, TN, P, K, Ca, Mg, Fe, Mn, Zn), and humic substance fractions were measured only in 2005, assuming that their interannual variability was low. On the other side, preseeding NO3–N, PMN, yield, and N uptake were analyzed each year (2005 and 2006) because of their high variability among years.
Soil samples were collected to a depth of 20 cm in spring 2005 before fertilizer application. In each experimental unit, six soil cores were taken randomly with a 2-cm diam. stainless auger (Oakfield model B, Oakfield Apparatus Co., Oakfield, WI), bulked to make a composite sample, and sieved at 2 mm in the field. These soil samples were used to measure KCl-exchangeable NO3, PMN, soil macro- and micronutrient contents (P, K, Ca, Mg, Fe, Mn, Zn), and for the fractionation of humic substances.
Bulk density and porosity were measured on soil samples collected in late summer of 2005. Two intact soil cores (6.5 cm height by 6.5 cm diam.) were collected per plot. Finally, one soil block was taken in each plot with a spade to a depth of 20 cm, taking care to avoid soil compression, to assess water-stable aggregation. The soil blocks were sieved at 8 mm in the field and kept at 4°C until analysis.
Soil Analyses
Bulk density (BD) was measured by the core method using soil cores described above. The same cores were used to assess total porosity macro- and microporosity following Klute's method (Klute, 1986) with minor modifications. The equivalent pore radius was calculated from the measured soil pressure applying the following simplified Jurin's law d = 3/h. Where d is the pore diameter, h is the height of the tension table. Briefly, the cores were saturated by total immersion in water for 24 h. The cores were weighed and then fitted to a tension table placed at 50 cm (draining pores > 60µm) for 24 h. The cores were weighed and returned to the tension table placed at 100 cm (draining pores > 30µm) during 24 h. The cores were weighed, dried at 105°C for 24 h and weighed again. Calculations were made as follows:
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Water-stable macroaggregates (>0.25 mm) were determined by wet-sieving (Angers and Mehuys, 1993). Briefly, 40 g of field-moist soil sieved at 8 mm were put on the top of a series of sieves of decreasing openings (5, 2, 1, and 0.25 mm). The sieves were immersed in water and agitated for 10 min. The soil remaining on the four different sieves was collected and oven dried at 65°C until constant weight. The weight of aggregates remaining on each sieve was corrected for sand content and expressed as a percentage of total dry soil (Elliott et al., 1991). The MWD was calculated according to Kemper and Rosenau (1986). The proportion of initial soil recovered as macroaggregates was calculated.
Soil nitrates were extracted by 2 M KCl on field-moist samples as described by Bremner (1965). The NO3 content in the KCl extracts were determined on an automated colorimeter (Model AAII, Technicon Instruments, Tarrytown, NY). The PMN was assessed as described by Keeney (1982). Briefly, 10 g of air-dried soil were mixed with 25 mL of distilled water and incubated in plastic bottles fitted with airtight lids (Desi-Vac, www.vwrsp.com/catalog) at 40°C for 7 d. A particular attention was paid to ensure that air was pumped out before closing the bottles. The difference in the amount of KCl-extractable NH4–N present in the initial soil and at the end of incubation was used to estimate PMN. The NH4 concentration in the KCl extracts was determined with the colorimeter described for NO3 analysis.
Soil P, K, Ca, Mg, Fe, Mn, and Zn contents were analyzed at the beginning of the study (2005) on air-dried soil passed through a 2-mm sieve to assess soil fertility status in relation to the cropping history. These nutrients were extracted using Mehlich III extractant (Mehlich, 1984) and the concentration was assessed with an Inductively Coupled Plasma Optical Emission Spectrometer (Optima 4300 DV, PerkinElmer Corp., Norwalk, CT).
Humic substances were extracted according to the method described by Schnitzer et al. (1981). Ten grams of air-dried soil ground to 0.15 mm was weighed in 250-mL polyethylene centrifuge tubes with 100 mL of 0.1 M NaOH and 0.1 M Na4P2O710H2O. The tubes were shaken for 24 h on a reciprocal shaker and centrifuged for 20 min at 3000 x g. The supernatant was decanted and centrifuged for 15 min at 1500 x g. A 25-mL aliquot of the supernatant was acidified to pH 2 with H2SO4 50% (v/v) and humic acids (HA) were allowed to precipitate for 24 h at 4°C. The aliquot was centrifuged for 15 min at 1500 x g, and the supernatant (fulvic acids) was decanted and recovered in an empty plastic tube. The precipitated HA fraction was oven-dried at 45°C and resuspended in 25 mL of 0.5 M NaOH.
The fulvic acids (FA) were then separated from the nonhumified fraction (NHF) by adsorption of FA onto polyvinylpyrolidone (Sequi et al., 1986). A 25-mL aliquot of FA was passed through a column containing 12 g of polyvinylpyrolidone resin (Aldrich, Germany), previously purified and equilibrated with 0.005 M H2SO4. The column was then rinsed with 25 mL of 0.005 M H2SO4 to remove all NHF. The FA sorbed onto the column were eluted with 25 mL of 0.5 M NaOH. The three fractions obtained (NHF, FA, HA) were stored at 4°C until analysis.
The organic C content in whole soil, water-stable aggregates, and HA and FA fractions was determined using the Walkley–Black method (Allison et al., 1965), whereas the C content of the NHF fraction (liquid samples) was analyzed on a total C analyzer (Model TOC 5000, Shimadzu Corp., Kyoto, Japan). The total N content in whole soil, was determined by the micro-Kjeldhal method (Bremner, 1965). The humification index of SOM was calculated as the ratio of C in NHF to the sum of C present in HA and FA (Sequi et al., 1986). The polymerization index of SOM was calculated as the ratio of C in HA to C in FA (Orlov, 1995).
Corn Yield and Nitrogen Uptake
The two innermost rows of corn of each experimental plot were harvested in late autumn. Entire corn plants were passed in a one-furrow chopper and the fresh material weighed. Eight subsamples were taken to make one composite sample per plot. The samples were dried at 60°C until constant weight to determine dry matter content. The N content of the samples was determined by micro-Kjeldhal digestion (Bremner, 1965) and the NH4 concentration of the digests was determined on an automated colorimeter described for soil NO3. Corn N uptake was calculated as the product of tissue N concentration and dry matter yield.
Statistical Analysis
An analysis of variance was performed on the data with the MIXED procedure of SAS (SAS Institute, 1999) where the block and block x manure interaction were considered as random effects, and fertilization, manure, and manure x fertilization as fixed effects. A priori contrasts were used to test the differences among least square means (LSMEANS) of the experimental treatments.
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RESULTS AND DISCUSSION |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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Long-term addition of mineral fertilizers, with and without manure, significantly reduced the proportion of water-stable macroaggregates and their MWD, compared to the unfertilized control (Table 1). In addition, the reduction in water-stable macroaggregates and MWD tended to be greater where N was applied (PK vs. NPK). This could be attributed to enhanced SOM mineralization caused by long-term application of fertilizers, and especially mineral N (Fox, 2004; Khan et al., 2007), thereby decreasing the stability of soil macroaggregates.
Carbon, Macro- and Micronutrient Contents of the Whole Soil
After 28 yr of the experiment, the organic C content of whole soil was 21 g kg–1 without manure and from 25 to 29 g kg–1 with manure (Table 2
). Though not statistically significant, the plots receiving manure had a SOC concentration on average 27% higher than the unmanured ones. However, after 28 yr of annual plowing, all treatments except the control with manure had SOC concentration lower than the initial level (28 g kg–1). Reddy et al. (2003) and Hati et al.(2007) reported significant increases in soil organic C with long-term (28–30 yr) application of manure at rates as low as 15 Mg ha–1 in a sandy loam and a clayey soil, respectively. This is surprising because both studies were located in a warmer area (India) than our study, where we would expect a faster SOM mineralization. However, their soils had a lower initial SOC level (6.6–11.4 g kg–1) than in our study, and may have responded more strongly to low C inputs. Moreover, contrary to our study, crop residues were returned to the soil in the previous cited studies which likely further stimulated soil C storage.
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The soil total N content ranged from 1.46 to 1.55 g kg–1 in unmanured soils, and from 1.89 to 2.10 g kg–1 in manured soils (Table 2). Manure application significantly increased total N content compared to unmanured soils. With respect to the initial value (2.2 g total N kg–1), an average loss of 0.025 g N kg–1 yr–1 was observed. These values are lower than the rate of 0.036 g N kg–1 yr–1 reported by N'Dayegamiye et al. (1997) after 14 yr in the same experiment, suggesting that, contrary to SOC, N was lost at a slower rate in the second part of this long-term experiment. On average, all treatments showed a decline in total N content over 28 yr, but manured treatments showed annual decline three times lower than unmanured ones. The general decline in soil total C and N could be attributed to a combination of residues removal with the fall moldboard plowing. In the NPK treatment, this decline was further increased because N fertilizers also promote SOM mineralization (Fox, 2004, Khan et al.,2007). As discussed for total C, the NPK treatment could not maintain total N at initial level.
Long-term application of manure significantly increased soil K, Mg, and Zn contents. Although not significant at the 0.05 probability level, manure also increased P, Ca, Fe, and Mn contents by 65, 32, 1.7, and 17%, respectively (Table 2). Soil build-up in various nutrients following repeated manure application was reported in previous studies (Berry et al., 2003; Mikhailova et al., 2003; Prakash et al., 2007). Long-term application of mineral fertilizers significantly increased soil P, K, Mg, and Mn contents, had little effect on Fe and Mn, and decreased Ca content (Table 2). Nitrogen application (PK vs. NPK) significantly decreased soil P, K, Mg, Fe, and Mn contents. This was attributed to the significant increase in crop yield in the presence of mineral N fertilizer which likely increased soil nutrient uptake. Increased removal of soil nutrients following N application was also reported by Singh et al. (2001) and Ketterings et al. (2006). Application of N fertilizer (PK vs. NPK) decreased soil pH although lime was applied every 3 yr (Table 2). This was likely due to the acidifying effect of nitrification.
Carbon Content in Humic Substances, in Macroaggregates, Humification, and Polymerization Indices
Long-term manure additions significantly increased the C content of HA but did not significantly influence the C content of FA and NHF (Table 3
). On average, the proportion of whole soil C accounted for by C present in HA, FA, and NHF was 17, 9, and 17%, respectively, in the absence of manure, and 22, 8, and 16% in the presence of manure. Our values differ from those reported by Yang et al. (2004), who reported that 28% of whole soil C was present in FA and 18% in HA. The differences may be ascribed to different soil type, climatic conditions, and management practices. Stout et al. (1981) reported that NHF may account for 15 to 25% of whole soil C, and includes living and dead roots, decomposing organic residues, and microbial metabolites. The proportions found in our study (16–17%) are thus in the lower range of values reported by Stout et al. (1981).
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Manure and mineral fertilization did not influence the polymerization index, which ranged from 1.7 to 2.1 in unmanured treatments, and from 2.5 to 2.8 in manured treatments (Table 3). According to Orlov (1995), soil humus is dominated by HA when the polymerization index is >1, whereas FA dominates when the index is <1. In our study, HA were therefore prevailing in all cases, and even though the differences were not significant, long-term application of manure tended to further increase the HA content compared to the soil receiving only mineral fertilizers.
There was a trend toward increased C content in macroaggregates size by manure application. On average, increases ranged from 15 to 31%, depending on the size fraction (Table 4 ), with the largest increases found in the two largest size fractions (>5 mm and 2–5 mm fractions; P = 0.06). Though the same trend was observed with size fractions <2 mm, the effect of manure was not statistically significant at the 0.05 probability level. This is indicating that, long-term manure application even at moderate rate, can increase protected SOM within aggregates. Although mineral fertilization decreased the proportion of water-stable macroaggregates and MWD, relative to the control (Table 1), it did not decrease the concentration of C in the various aggregate size classes compared to the control (Table 4). When mineral fertilization is not accompanied with C inputs, such as in the present cropping system, the C-rich aggregate binding agents are decomposed by soil microorganisms and may not be replaced, which would explain the decrease in aggregate stability (Table 1). However, this decrease in binding agents was not reflected in the total C content of aggregates.
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Corn yield was significantly higher in soils receiving manure in both years (Table 5). Although not statistically significant, previous manure applications tended to increase N uptake by an average of 110 and 79% in 2005 and 2006, respectively, compared to the unmanured plots. This is in agreement with long-term studies conducted in North America where increases in corn yield due to manure application were reported (Ma et al., 1999, 2003; Arriaga and Lowery, 2003). As manure was ceased 2 yr before this experiment, higher corn yield in manured plots may not be only attributed to improved soil nutrient status with manure, but also to improved soil structure and to the stimulation of SOM mineralization (Kaur et al., 2005; Nyiraneza and Snapp, 2007).
Long-term mineral N fertilization (NPK) significantly increased corn yield and N uptake, compared to PK and the control (Table 5), although soil aggregation deteriorated in NPK treatment (Table 1). This effect of mineral N fertilizer on yield and N uptake is explained by the high N requirements of corn and the fact that N present in mineral fertilizer is readily available for plant nutrition. In addition, mineral N fertilizer stimulates SOM mineralization (Fox, 2004; Khan et al., 2007).
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONSREFERENCES |
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