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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Soil Carbon and Nitrogen Dynamics Following Application of Pig Slurry for the 19th Consecutive Year

I. Carbon Dioxide Fluxes and Microbial Biomass Carbon

^a Soils and Crops Research and Development Centre, Agriculture and Agri-Food Canada, 2560 Hochelaga Blvd., Sainte-Foy, QC, Canada, G1V 2J3
^b Institut de recherche et de développement en agroenvironnement, Complexe Scientifique, 2700 Einstein St., Sainte-Foy, QC, Canada, G1P 3W8

rochettep{at}em.agr.ca

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Agricultural soils often receive annual applications of manure for long periods. The objective of this study was to quantify the effects of 19 consecutive years of pig (Sus scrofa) slurry (PS) application on CO₂ emissions and soil microbial biomass. Soil temperature, soil moisture, and extractable soil C were also determined to explain the variations in CO₂ emissions and soil microbial biomass. Long-term (19 yr) treatments were 60 (PS60) and 120 Mg ha^-1 yr^-1 (PS120) of pig slurry and a control receiving mineral fertilizers at a dose of 150 kg ha^-1 yr^-1 each of N, P₂O₅, and K₂O. Very high CO₂ emissions (up to 1.5 mg CO₂ m^-2 s^-1) occurred during the first 2 d after PS application. Following that peak, decomposition of PS was rapid, with one-half the total emissions occurring during the first week after slurry application. The rapid initial decomposition was exponential and was attributed to the decomposition of the labile fraction of the slurry C. The second phase was linear and much slower and probably involved more recalcitrant C material. Cumulative annual decomposition was proportional to the application rate, with 769 and 1658 kg C ha^-1 lost from the 60 and 120 Mg ha^-1 doses, respectively. Pig slurry application caused a rapid increase in soil microbial biomass (from 100 to up to 370 mg C kg^-1 soil), which coincided with a peak in the concentration of extractable C and in CO₂ emissions. Field estimates of the microbial specific respiratory activity suggested that the difference in soil respiration between the two slurry treatments was due to differences in the size of the induced microbial biomass rather than to differences in specific activity.

Abbreviations: MBC, microbial biomass C • PS, pig slurry • PS60, application of pig slurry at 60 Mg ha^-1 yr^-1 • PS120, application of pig slurry at 120 Mg ha^-1 yr^-1 • SOM, soil organic matter • SRA, specific respiratory activity

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
DECOMPOSITION OF SOIL ORGANIC MATTER (SOM) affects the dynamics of various nutrients such as N and P, influences soil physical and chemical properties, and directly impacts the atmospheric concentration of CO₂, a potent greenhouse gas (Intergovernmental Panel on Climate Change, 1996; Baldock and Nelson, 2000). Animal wastes are a significant source of organic C for agricultural soils. Hog industry produces 8.8 million m³ of slurry annually in the province of Québec, Canada (Trudelle, 1996). Most of this slurry is applied to agricultural soils either in spring or fall, where it is subjected to decomposition by the soil biota. Application of animal slurry usually increases the size of the soil microflora both in the short (Opperman et al., 1989) and long term (N'dayegamiye and Côté, 1989; Rochette and Gregorich, 1998).

Decomposition of pig slurry in soils has been studied in laboratory incubations. Large variations in decomposition rates have been observed between slurries (Dendooven et al., 1998a, 1998b; Morvan and Leterme, 1999) (Table 1) . These differences have been related to variations in slurry composition resulting, among other factors, from animal type (Morvan and Leterme, 1999) and from storage and handling conditions (Kirchmann and Lunvall, 1993). It is not clear whether information obtained from laboratory incubations can be directly applied to field conditions where many factors simultaneously affect decomposition. We are not aware of experimental data describing decomposition rates of pig slurry under field conditions.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
The experiment was conducted from 30 May 1997 to 27 May 1998 at the St-Lambert Research Farm of the Québec Ministry of Fisheries, Agriculture, and Food near Québec City, Canada (46°05' N, 71°02' W, altitude 110 m). The soil was a Le Bras loam (frigid Aeric Haplaquept) with 0.31 g sand, 0.42 g silt, and 0.27 g clay g^-1 soil. Soil C and N contents averaged across treatments were 19.9 and 1.2 g kg^-1, respectively, prior to the 1997 slurry application. Measurements were made on plots that were initiated in 1979 to assess the impact of long-term application of PS on silage maize (Zea mays L.) yields and soil properties (N'dayegamiye and Côté, 1989). Tillage operations were one pass of chisel plow in the fall and two passes of field cultivator in the spring prior to planting. The treatments consisted of annual applications of either mineral fertilizer (150 kg ha^-1 NH₄NO₃–N, 150 kg ha^-1 P₂O₅, 150 kg ha^-1 K₂O) (control) broadcasted prior to planting (27 May 1997), or pig slurry at rates of 60 (PS60) or 120 Mg ha^-1 (PS120) banded at the six- to eight-leaf stage (30 June 1997) repeated three times in randomized blocks. In 1997, the slurry was from a commercial hog and sow operation and contained 2.1 kg m^-3 total N, 0.99 kg m^-3 NH⁺₄–N, 1.15 kg m^-3 P, and 1.29 kg m^-3 Ca. Maize was planted on 28 May 1997 at 76000 seeds ha^-1 and 0.75-m interrows. Slurry was applied in 0.6-m-wide bands and shallow-incorporated (0.075 m) in each interrow using a liquid manure spreader equipped with drop tubes and tines mounted on a boom at the rear of the spreader (Jokela and Côté, 1994). Average maize aboveground dry matter yields between 1990 and 1995 were 7.1 Mg ha^-1 for the control, 7.9 Mg ha^-1 for PS60, and 9.3 Mg ha^-1 for PS120.

Soil Surface Carbon Dioxide Fluxes
In situ CO₂ fluxes were measured by the dynamic closed chamber method detailed by Rochette et al. (1997) and briefly described as follows. Three acrylic frames (0.60 by 0.60 m; 0.14-m height; 6.35-mm wall thickness) were inserted to a depth of 0.10 m in each plot on 29 May 1997. The frames completely covered the width of the slurry application band between maize rows and were left at the same locations for the duration of the experiment. When slurry was applied, the tines were lifted to clear the frame and the slurry was deposited at the surface and immediately mixed into the top 7.5 cm of soil with hand tools. The frames height was measured at regular intervals during the experiment (48 measuring points per frame) to account for variations in headspace due to soil settling.

The CO₂ flux measuring system was equipped with an LI-6200 CO₂ analyzer (LI-COR, Lincoln, NE) and a 0.15-m-high square plexiglass chamber covering the same area as the frames. During each CO₂ flux measurement, the chamber was fixed to a frame and the CO₂ concentration inside the chamber was measured once every second during four 20-s successive periods. The mean CO₂ flux (F_CO2) on each frame was calculated as follows:

(1)

where C_i is the mean CO₂ concentration during period i (L L^-1); t_i is the mean time of period i (s); m_m and m_v are the molecular mass and volume of CO₂, respectively; V is the total system volume including chamber, frame, and gas analyzer tubings (68 L depending on frame height); and A is the area covered by the chamber (0.36 m²).

Soil temperature at a depth of 10 cm inside each frame and volumetric water content at depths of 5, 10, 20, and 40 cm (one profile per plot) were measured at the time of the CO₂ flux measurements. Soil temperature and soil moisture were measured using copper-constantan thermocouples and time domain reflectometry probes, respectively. The effects of treatments were analyzed at each date using a two-way analysis of variance (SAS Institute, 1989).

Soil Analyses
Soil samples were collected from the 15-cm surface soil layer 19 times during the experiment. Samples were sieved in the field at 6 mm and stored immediately at 3°C. Soil microbial biomass C (MBC) measurements were carried out within 24 h of sampling using the chloroform fumigation–extraction technique (Wu et al., 1990). Two 50-g subsamples of field-moist soils were placed in 100-mL beakers. One subsample was fumigated for 24 h at room temperature in a vacuum desiccator containing 25 mL of CHCl₃. The other subsample was kept in the dark at 3°C for 24 h. Both fumigated and nonfumigated soils were extracted with 100 mL of 0.25 M K₂SO₄. After shaking for 1 h on an oscillating shaker, the suspensions were centrifuged at 1000 g and filtered. The organic C content of the extracts was determined by UV-persulfate oxidation on a DC-180 Carbon Analyzer (Dorhman Co., Santa Clara, CA). An extraction efficiency (K_ec factor) of 0.45 was used to calculate the MBC (Wu et al., 1990). Specific respiratory activity of soil microbial biomass was calculated as the quotient of respired CO₂ to MBC. For that purpose, MBC was expressed in grams of C per square meter using bulk densities measured in 1994 (1.24 g cm^-3 in the control, 1.24 g cm^-3 in the PS60, and 1.30 g cm^-3 in the PS120 plots [M.A. Bolinder, 1995, personal communication]).

A fraction of the soil samples was air dried and sieved at 2 mm for the determination of soil pH (CaCl₂, 0.01 M). Total C and N contents were determined on a Leco CNS 1000 (LECO, St. Joseph, MI) on finely ground (0.05 mm) soil samples.

	Results and discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Soil temperature at 10 cm was not affected by the treatments (Fig. 1a) . In 1997, temperature increased in June, peaked in July, was slightly lower in August (probably as a result of the shading by the maize canopy), and gradually decreased in September and October. In 1998, soil temperature increased rapidly in May due to a warm and dry spring. Soil moisture was similar between treatments and was averaged across treatments for each depth (Fig. 1b). Soil moisture below 5 cm remained between 0.20 and 0.35 m³ m^-3 during most of the study. Water contents above 0.35 m³ m^-3 were measured in early July 1997 following slurry application and abundant rainfalls in the fall of 1997 and in the spring of 1998 (Fig. 1b).

View larger version (26K): [in this window] [in a new window]   Fig. 1 Temporal variations of (a) soil temperature at a depth of 10 cm for maize soils amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha-1 (PS120) and (b) soil moisture at depths of 5, 10, 20, and 40 cm averaged across treatments

View larger version (14K): [in this window] [in a new window]   Fig. 2 Temporal variations of CO2 flux from maize soils amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha-1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997

Carbon Dioxide Emissions in the Manured Plots
Values of F_CO2 on the manured and control plots were similar early in the season when significant differences (P < 0.05) were measured only on the last day prior to application of PS (Fig. 2). Thus, addition of PS in the 18 previous years had little impact on the oxidation of soil C early in 1997. In contrast, significant differences (P < 0.05) were observed on 14 of the 18 sampling dates following the application of PS in 1997. The increase in F_CO2 resulting from the addition of PS could be split into three periods. First, the fluxes were increased by a factor of 10 to 30 compared with background values during the first 2 d following application. These values are very high, and CO₂ release from displacement of physicochemical equilibrium probably dominated the emissions during these first 2 d. Carbon dioxide is produced in slurry during storage by the hydrolysis of urea and the anaerobic decomposition of volatile fatty acids (Sommer and Husted, 1995). The CO₂ in solution forms pH-dependent chemical equilibriums (carbonates) with other species such as NH₃ (Sommer and Husted, 1995). The pH of the slurry was 8, while that of the surface soil was 5.69 ± 0.13 in PS60 and 5.79 ± 0.15 in the PS120 plots following PS application (1 July). These values are lower than the threshold (7.2) below which ammonium carbonates are dissociated and CO₂ is released. Génermont (1996) also reported very large carbonate-induced CO₂ emissions (1.4 mg CO₂ m^-2 s^-1) following application of 132 m³ ha^-1 of cattle (Bos taurus) slurry under field conditions.

Following this initial burst, an adjustment phase of 20 d was observed during which the differences between manured and control plots decreased gradually. We postulate that this period reflected the time during which the soil heterotrophs were using up the amount of readily decomposable organic substrates of the PS. Finally, the adjustment phase was followed by a period of small but fairly constant differences between the manured plots and the control plots. During this period, more recalcitrant organic substrates were probably decomposed. The temporal pattern of F_CO2 from late July to the end of 1997 was the same on manured and control plots, indicating that the temporal variations of the flux were governed by factors other than the addition of slurry, probably the root–rhizosphere respiration of the maize crop.

Slurry-Induced Carbon Dioxide-Carbon Losses
Slurry-induced CO₂–C emissions were estimated by subtracting F_CO2 on the control plots from F_CO2 on the manured plots. For this purpose, only the CO₂ emissions from within the slurry application band were considered. Cumulative slurry-induced CO₂–C losses were then calculated by linearly interpolating CO₂ emissions between sampling dates, assuming that measurements made between 0900 and 1200 h were a good estimator of average daily F_CO2. The early PS-induced CO₂ kinetics were similar for both slurry doses, with 50, 60, and 70% of the total emissions lost after 11, 21, and 40 d, respectively. After 70 d, 87% of the total PS-induced CO₂ was lost in the PS60 plots and 82% in the PS120 plots. Approximately one-half of the losses occurred during the first week after slurry addition (Fig. 3) . Total losses in 1997 from the PS60 plots (769 kg C ha^-1) were 46% of those measured from the PS120 plots (1658 kg C ha^-1), indicating a linear response of C oxidation to the amount of substrate added and suggesting that there were no physical or chemical limitations to increased microbial activity with increased amount of PS added. Gregorich et al. (1998) reported a decreasing response of CO₂ emissions with increasing dose of solid cattle manure under field conditions. Differences between the CO₂–C emissions induced by solid and liquid manures can be explained at least partly by the easier access to the highly diluted substrates in PS than in solid manure, which usually forms clods in which the inner portions are physically protected from the attack by decomposers.

View larger version (12K): [in this window] [in a new window]   Fig. 3 Slurry-induced CO2–C losses from maize soils amended with pig slurry at 60 (PS60) and 120 Mg ha-1 (PS120). Pig slurry was applied on 30 June 1997

The 1997 annual PS application caused a rapid increase in MBC in early July, which was linear with the rate of application (Fig. 4) . Laboratory studies have shown this rapid increase in MBC immediately following PS application (Saviozzi et al., 1997). The peak in MBC lasted only for a few days but the difference between the two amended treatments lasted for 2 mo. The peak in early July, right after PS application, coincided with the peak of respiration activity (Fig. 2). Jensen et al. (1997) studied the dynamic response of MBC and field respiration to rape (Brassica napus L. var. napus) straw application and found that MBC did not contribute significantly in explaining the variability of soil surface CO₂ flux. A similar conclusion was reached by Rochette and Gregorich (1998) using solid cattle manure. Our results are in agreement with those observations, except for the period immediately following PS application when both CO₂ flux and MBC increased for a short time. The difference between our study and the two others may be that PS contains high levels of soluble C and this was reflected in an increase in extractable C in the soil following PS application as well (Fig. 5) . Differences in extractable C between treatments were only significant for a few days after PS application (P < 0.01 on 2 and 8 July). It is also possible that part of the increase in MBC measured a few days following PS application be attributable to the presence of microbes in the applied manure as proposed for compost by Perucci (1990).

View larger version (17K): [in this window] [in a new window]   Fig. 4 Temporal variations of soil microbial biomass C for maize soils amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha-1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997

View larger version (17K): [in this window] [in a new window]   Fig. 5 Temporal variations of extractable C for maize soils amended with mineral fertilizer (control), and pig slurry at 60 (PS60) and 120 Mg ha-1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997

Higher respiration following addition of PS can be the result of an increase in microbial biomass or its activity. Specific respiratory activity (SRA) of the soil microbial biomass, which is the ratio of the respiratory activity to microbial biomass, was calculated to determine the relative contribution of these two factors. Specific respiratory activity has been determined under laboratory incubation conditions (Santruckova and Straskraba, 1991) but rarely under field conditions (Rochette et al., 1999a). In our study, CO₂ fluxes and MBC were influenced by three C sources: native soil C, plant activity, and PS. In order to isolate the effect of PS from the contribution of plant roots and native soil C to respiration and microbial biomass, we subtracted the values of the control from the slurry treatments. The values of SRA induced by PS ranged from zero or slightly negative to 0.37 g CO₂–C g MBC d^-1 (Fig. 6) , which is in the range of values found under laboratory conditions (Santruckova and Straskraba, 1991).

View larger version (12K): [in this window] [in a new window]   Fig. 6 Temporal variations of specific activity of soil microbial biomass for maize soils amended with mineral fertilizer (control) and pig slurry at 60 (PS60) and 120 Mg ha-1 (PS120). Mineral fertilizer was applied on 29 May and pig slurry on 30 June 1997. MBC is microbial biomass C

	Conclusions

TOP ABSTRACT INTRODUCTION Materials and methods Results and discussion Conclusions REFERENCES

 
Our approach allowed the characterization of the mineralization of PS under field conditions. The decomposition of PS was rapid, with one-half the total emissions occurring during the first week after slurry application. The early decomposition was proportional to the application rate. The rapid initial decomposition was exponential and was attributed to the decomposition of the labile fraction of the slurry C. The second phase was linear and much slower, and probably involved more recalcitrant C material. Pig slurry application caused a rapid increase in soil microbial biomass that coincided with an increase in the concentration of extractable C and in CO₂ emissions. Field estimates of the microbial specific respiratory activity suggested that the difference in soil respiration between the two slurry treatments was due to differences in the size of the induced microbial biomass and not to its specific activity.

	ACKNOWLEDGMENTS

 
This work was supported by the PERD-Climate Change program of Agriculture and Agri-Food Canada. We thank F. Gagné, P. Jolicoeur, R. Baillargeon, N. Bissonnette, J. Lizotte, and P. Drouin for assistance in soil respiration and microbial biomass measurements and plot maintenance. The numerous discussions and the review provided by Dr. M.H. Chantigny are also gratefully acknowledged.

Received for publication July 1, 1999.

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