Agricultural and Food Science in Finland 29 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. © Agricultural and Food Science in Finland Manuscript received December 1998 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. Effects of cattle slurry and cultivation on air exchange in sandy and silty soils from northern Norway Trond Knapp Haraldsen Planteforsk, The Norwegian Crop Research Institute, Holt Research Centre, N-9292 Tromsø, Norway Current address: Jordforsk, Centre for Soil and Environmental Research, N-1432 Ås, Norway, e-mail: trond.haraldsen@jordforsk.nlh.no Gas diffusivity and permeability, and air-filled porosity, were measured in undisturbed soil cores at four water potentials between –1.5 kPa and –60 kPa. Virgin (never ploughed) and cultivated sandy and silty soils from two sites in northern Norway were used in the investigation. The cultivated soils had lower air-filled porosity and gas diffusivity than the virgin ones. Application of slurry (50 Mg ha-1) decreased gas diffusivity and changed the relationship between relative diffusivity and air-filled porosity for both the virgin and cultivated sandy soils and the virgin silty soil. The gas permeability of both the virgin and cultivated silty soil was low, and the relative diffusivity at field capacity less than the limit below which plant growth is affected. Keywords: aeration, cattle slurry, cultivation, gas diffusivity, permeability, porosity Introduction The use of liquid slurry on grassland is common in most districts in western and northern Nor- way. The effective mineral nitrogen in applied slurry is much less than equal amounts of N in inorganic NPK fertilizer (Håland 1988). Loss of N from surface applied slurry by ammonia vola- tilization can be one reason for the decreased fertilizing effect (Van der Meer et al. 1987). Sward damage caused by surface spreading, which concerns smothering, scorching and pos- sibly other unknown effects, often results in a decrease of herbage yield (Prins and Snijders 1987). Slurry application to grassland may re- strict soil aeration and cause evolution of meth- ane, ethane, ethylene and propane. Anaerobio- sis caused by slurry application may also cause denitrification in the soil. However, only a few studies on the effect of slurry on soil aeration have been carried out. In studies with very high applications of slurry on grassland (Stevens and Cornforth 1974, Burford 1976, Egginton and Smith 1986) evolution of methane and ethane, and increased O 2 demand have been found. In the study of Labuda et al. (1976) slurry (100 Mg ha-1) mixed in the plough layer caused mailto:trond.haraldsen@jordforsk.nlh.no 30 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Haraldsen, T.K. Effects of cattle slurry and cultivation on air exchange decrease in O 2 content after application, but did lead to development of anaerobic conditions in the cultivated soil. Soil compaction by heavy tankers during slurry application is a serious problem and re- duces the water infiltration rate (Myhr et al. 1990), and reduces grass yield and nitrogen utili- zation (Douglas and Crawford 1998). Hansen and Bakken (1993) studied the effect of soil com- paction during slurry application in leys on soil aeration. They found that compacted soil had lower O 2 concentrations and higher CO 2 concen- trations than uncompacted soil. Concentrations of O 2 and CO 2 in the soil air were not influenced by fertilizer applications (unfertilized, NPK, and slurry). Only a few papers concerning soil compac- tion and soil aeration in undisturbed soil have been published (Ball 1981, Ball 1987, Ball et al. 1988, McAfee et al. 1989, Rolston et al. 1991). Based on field experiments Ball (1987) conclud- ed that relative diffusivities may be used to iden- tify the water potential and bulk density at which aeration may limit plant growth. Moldrup et al. (1997, 1999) have developed models for predict- ing gas diffusion in undisturbed soils with high accuracy. Gas diffusion was predicted from air- filled porosity, and the introduction of a tortu- ousity parameter and measurements of gas per- meability or gas diffusion at a defined water potential between –10 and –50 kPa improved the accuracy. Cultivation of some soils for grass produc- tion in northern Norway has caused problems with winter damage and poor yields. Some farm- ers reported increased problems after application of cattle slurry, and it was proposed that the use of aerated slurry would be beneficial compared to fresh slurry. In order to identify if the prob- lems were related to the slurry application or the physical properties of the soils, virgin (unculti- vated, never ploughed) and cultivated soils of the same origin were studied. In this paper the effects of cultivation and surface spreading of cattle slurry (fresh and aerated slurry) on air ex- change were studied in soils from northern Nor- way. Paired virgin and cultivated sandy and silty soils were used to assess the effects of anthro- pomorphic activity on air exchange. Undisturbed soil cores from these soils were used in labora- tory experiments. Relative diffusivities were chosen in order to measure the effects of slurry and cultivation on soil aeration. Soils from the same locations have been used in previous stud- ies of the influence of cultivation and slurry ap- plication on infiltration (Haraldsen and Sveis- trup 1994), and the effect of slurry application on the microstructure of the surface layers of the soils (Sveistrup et al. 1995). Material and methods Material Soils from two locations in northern Norway were used; a sand (Typic Cryaquent (Soil Sur- vey Staff 1975), Dystric Arenic Regosol (FAO 1998)) from Pasvik (69°N, 30°E), and a silt loam (Dystric Cryochrept (Soil Survey Staff 1975), Dystric Cambisol (FAO 1998)) from Tana (70°N, 27°E). Soil physical characteristics and organic carbon contents are presented in Table 1. Sveis- trup (1992) gives a complete description of the soils. The climate at both sites is characterised as subarctic continental. The mean annual air temperature is –1.1°C at Pasvik and –0.7°C at Tana (Rustefjelmba). The mean temperature dur- ing summer (June-August) is 11.8°C at Pasvik and 10.5°C at Tana (Aune 1993). The growing seasons (temperatures above 6°C) for the two sites are 124 days and 118 days, respectively. The Tana location was chosen because it of- ten shows restricted plant growth caused by soil compaction during cultivation, and winter dam- age caused by ponding and ice-cover (Lorentzen 1984). The virgin soil supported birch forest (Betula pubescens) with grasses and mosses. A thin mor humus layer covered the soil. The cul- tivated soil has been ploughed periodically since the 1920’s, and the present sward age was seven years. The vegetation cover consisted mostly of 31 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. weed species with meadow grass (Poa annua) as the dominant species (Sveistrup 1992). The grass was cut for silage once a season, and the meadow was used for grazing for two weeks in late August during the previous four years (7.3 cows ha-1). During the same periods as for the meadow, cows (approx. 3.6 cows ha-1) grazed the forest area. The soils of the Pasvik location had no par- ticular agronomic problems. The studied site was situated on the edge of a peat bog, which was drained by open ditches of approximately 1 m depth and cultivated seven years prior to the in- vestigations. A shallow peat layer (20–30 cm) covered a layer of outwashed sand more than 1 m thick, over sedimentary clay. The cultivated and the virgin sites were situated on either side of an open drainage ditch. The vegetation at the virgin site, was mostly peat moss (Sphagnum spp.), with heather (Ericaceae spp.) and scattered pine trees (Pinus sylvestris). The cultivated site had a five-year-old ley, mainly consisting of smooth meadow grass (Poa pratensis) and tim- othy (Phleum pratense). The ley was harvested once a year for silage and not grazed later in the season. The weight of the tractors, which had been used in recent years, was approximately 3 Mg at both Tana and Pasvik (Sveistrup 1992). According to Myhr and Njøs (1983) slurry ap- plication and one harvest will represent a wheel track cover of 133%. Undisturbed soil monoliths were collected at each site in rigid plastic cylinders. The cylin- ders were designed to be used as lysimeters. The inner diameter was 23.5 cm and the soil mono- liths had a height of 25 ± 1 cm. Twelve mono- liths were collected at each site, within a radius of 1.5 m, and transported to Holt Research Sta- tion. The distance between the virgin and culti- vated soils was approximately 15 m. In order to avoid influence of different vege- tation and differences in humus content between the virgin and cultivated soils, the vegetation on the sites and the humus/peat layer were careful- ly removed before digging. Samples were taken from the same depth in the mineral soils both at the virgin and the cultivated soils. The soil was removed around the plastic cylinders, as they Table 1. Physical properties of the soils (according to Sveistrup, 1992). Depth, Particle size distribution, Bulk Total Air-filled Organic g 100 g-1 density, porosity, porosity, carbon, cm Sand, Silt, Clay, Mg m-3 m3 m-3 m3 m-3 g 100 g-1 2–0.06 0.06– <0.002 mm 0.002 mm mm Pasvik, sandy virgin 0–14 90 8 2 1.47 0.47 0.37 0.5 14–36 96 4 0 1.53 0.46 0.41 0.3 Pasvik, sandy cultivated 0–7 81 15 4 – – – 3.1 7–21 94 5 1 1.44 0.47 0.25 1.1 21–44 96 3 1 1.50 0.46 0.35 0.5 Tana, silty virgin 0–7 21 70 9 – – – 6.3 7–25 26 67 7 1.14 0.58 0.26 0.8 Tana, silty cultivated 0–5 51 44 5 1.53 0.43 0.06 2.5 5–23 48 46 6 1.44 0.46 0.07 2.0 23–36 65 31 3 1.49 0.47 0.18 0.4 32 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Haraldsen, T.K. Effects of cattle slurry and cultivation on air exchange were pressed down to enclose the soil monoliths. A layer of grease was applied to the inside of the cylinder walls before sampling to ensure firm contact between wall and soil. The soil in the cylinders was collected from the plough layer and the top of the horizon below at the cultivat- ed sites, and from the upper part of the mineral soil at the virgin sites. The depth of ploughing was approximately 20 cm both at Pasvik and Tana. Because of vibration during transportation the virgin soil from Pasvik had compacted and this caused somewhat higher bulk density than in its natural state. The soil monoliths were used in a laboratory infiltration experiment at Holt Research Station (Haraldsen and Sveistrup 1994) before the start of the air exchange studies. In the experiment (Haraldsen and Sveistrup 1994) the following treatments were applied; (i) no cattle slurry (NCS), (ii) fresh cattle slurry (FCS), equivalent to 50 Mg ha-1, (iii) cattle slurry aerated for 4 weeks at 37°C (ACS), equivalent to 50 Mg ha-1. The fresh slurry had 7.3% dry matter and the aerated slurry had 7.0% dry matter. The cattle slurry was from the same source as the slurry used at Holt in the experiment of Myhr et al. (1990). The slurry was applied evenly on the soil surface. The temperature in the laboratory was kept constant at 10 ± 2°C. At the start of the ex- periment there were four replicates. After the infiltration measurements, one week after slur- ry application, soil cores (height 5 cm, volume 203.5 cm3) for soil physical analyses were col- lected from replicate 1. The sampling depth was 0–5 cm. Because of the slurry layer at the top, the volume of the soil in each cylinder was not exactly 203.5 cm3. The height of the soil in the cylinders was measured with accuracy of 0.5 mm, and all soil physical data were corrected for the real volume of the soil. Three soil cores for each treatment were taken. A thin layer of grease was applied to the inside of the cylinders. The soil cores were packed in transport cases and transported by car to the Swedish University of Agricultural Sciences, Uppsala, where the sam- ples arrived in good conditions. After the infiltration measurements, one month after slurry application, soil cores were taken from replicate 2 in the same way. These soil cores were also packed in transport cases, and sent to Uppsala in Sweden by mail. Unfor- tunately, the virgin soil from Pasvik had subsid- ed during the transport. With exception of two cores this soil could not be used in further ana- lyses. The soil in two cores with virgin soil from Tana had also subsided and was not analysed. Laboratory methods The following parameters were determined for all samples; dry bulk density (γ t ), density of sol- ids (γ s ) (Andersson 1955), diffusivity at differ- ent water potentials (D), air permeability coef- ficient at different water potentials (K a ) and loss on ignition, (g 100 g-1). The temperature in the laboratory was 20±2°C. Moisture retention curves were determined by using porous ceramic plates operated by suc- tion (Andersson 1971). The samples were first saturated from below, then the water release characteristic and air-filled porosities were de- termined at water potentials equivalent to –0.5, –1.5, –3.0, –5.0, –10 and –60 kPa. Water poten- tials between 0 and including –5.0 kPa were ob- tained by a suspended, hanging column of wa- ter, those at –10 and –60 kPa by a vacuum pump. After equilibrium was attained at each tension, the samples were weighed and transferred to equipment for determining D and K a . The air- filled porosity (E g , m3 m-3) at a given water po- tential is here defined as the difference between volumetric water content at saturation (E) and volumetric water content (E w ) at that water po- tential. When all measurements had been carried out at all water potentials, the samples were dried at 105°C for 72 hours. Gas diffusivities of the soils were determined in an apparatus described by Edling (1986) which uses a transient state principle. The soil samples were left in the soil cores and a steel chamber of equal dimension was fitted above each, with an airtight seal. The chamber was flushed with N 2 gas for 120 seconds via a valve, which was the 33 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. sealed. After a diffusion period of 3–10 minutes, depending on the water potential, gas samples (50 µl) were taken from the chamber using a Hamilton gas-tight syringe at a sampling sep- tum on the upper surface. N 2 concentrations were analysed by a Gas Chromatograph (Hewlett Packard 5880A with Molecular Sieve 13 X col- umn). 12 soil cores could be analysed in a se- ries. Gaseous diffusion is usually expressed as relative diffusivity (D/D 0 ), the ratio between dif- fusion of a gas through bulk soil (D) and in air (D 0 ). The gas used in this experiment, N 2 , has a D 0 of 20.1 mm2/s at 20°C (Armstrong 1979). Diffusion coefficient for the soil sample was calculated from the rate of decrease in the N 2 concentration in the chamber. The time interval for diffusion ranged from 48 hours for the silty soil at –1.5 kPa water potential to 10 minutes for the sandy soil at –60 kPa water potential. After the diffusion measurements, the sam- ples were transferred to an apparatus for deter- mination of air permeability, described by An- dersson 1969), in which the volume of air drawn through the soil sample as a result of pressure differential of between 1 and 400 Pa is meas- ured for a 120 seconds period. Air permeability is expressed as K a , µm2. Because zero air per- meability was measured in some samples, log- transformation of air permeability was made by the equation (1). logK a = log10(K a + 0.01) (1) In this experiment 3 to 6 parallel samples per treatment have been used. Green and Fordham (1975) recommend eight to ten air permeability measurements per site, although good correla- tion with soil moisture content and air content has been detected with as few as four cores per site. Statistical methods The statistical analyses were conducted in the SAS-programme package. Stepwise multiple regressions were calculated by use of the proce- dure REG and a significance level of 0.05 was used. To test if the slurry treatments gave differ- ent regression equations, the homogeneity-of- slopes model in the procedure GLM was used (SAS Institute Inc. 1987). Outliers, which were due to clear errors in laboratory measurements, were removed before the statistical analyses. Results and discussion Air-filled porosity According to Cassel and Nielsen (1986) an air- filled porosity of 0.1 m3 m-3 at field capacity is often quoted as a limit below which aeration is inadequate. A water potential of -10 kPa is often used as an approximation of “field capacity” (Cassel and Nielsen 1986). The sandy soil from Pasvik in this investigation is permeable to wa- ter and has a field capacity near to –10 kPa . The silty soil from Tana has a field capacity at a wa- ter potential of – 5 kPa. This is due to the layers of the soil with approximately 50 cm of silt loam over the sandy layer. The measurements of air-filled porosity at different water potentials (Table 2) showed a high variability both between samples from the same replicates and between the two replicates. There were no systematic trends in air-filled porosities between the two replicates (data not shown). The air-filled porosity was lower both in the cultivated sandy Pasvik soil and the silty Tana soil than in the virgin soils. The values of air- filled porosity of the cultivated Pasvik soil were above the critical limit for aeration at –10 kPa water potential. The virgin and cultivated Tana soils had too low air-filled porosity for adequate plant growth at –5 kPa water potential. In the virgin state the Tana soil had higher air-filled porosity than the critical limit at –10 kPa water potential. The slurry treatments did not influence air- filled porosity at any tension in the investigated soils. However, at –60 kPa water potential the 34 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Haraldsen, T.K. Effects of cattle slurry and cultivation on air exchange virgin Tana soil with fresh slurry had significant- ly lower air-filled porosity than without slurry. Diffusion measurements Gas movement by diffusion in response to par- tial pressure gradients, rather than mass flow in response to total pressure gradients, is the main mechanism by which gas exchange occurs in soils (Buckingham 1904, Romell 1922). At –1.5 kPa water potential there was a high variability between replicate samples of all treat- ments (Table 3). Because of the high variability at this water potential, no effects of cultivation or slurry treatment on diffusion were detected. The variability probably reflects differences in pore continuity, caused by water films making “bottlenecks” in coarse continuous pores or mak- ing the pores discontinuous. At lower water po- tentials the relative variability was lower. This was also found by Lindström and McAfee (1987). Both the sandy soil from Pasvik and the silty soil from Tana generally had higher relative dif- fusivities in virgin state than when cultivated. Table 2. Air-filled porosity (m3 m-3) at different water potentials for virgin and cultivated soils from Pasvik and Tana. Sampling depth 0–5 cm (SE=standard error). Air filled porosity (m3 m-3) Applied potential, kPa Location Cultivation Slurry –1.5 –3.0 –5.0 –10 –60 Pasvik Virgin No 0.035 0.074 0.172 0.285 0.355 SE (n=3) 0.005 0.012 0.008 0.003 0.003 Fresh 0.046 0.097 0.212 0.300 0.335 SE (n=3) 0.008 0.004 0.005 0.005 0.009 Aerated 0.052 0.080 0.185 0.266 0.353 SE (n=3) 0.010 0.008 0.008 0.016 0.011 Pasvik Cultivated No 0.038 0.040 0.066 0.165 0.265 SE (n=7) 0.006 0.004 0.005 0.010 0.005 Fresh 0.053 0.056 0.086 0.142 0.260 SE (n=6) 0.006 0.004 0.013 0.019 0.026 Aerated 0.023 0.037 0.077 0.126 0.226 SE (n=4) 0.006 0.009 0.006 0.009 0.010 Tana Virgin No 0.047 0.045 0.053 0.105 0.302 SE (n=5) 0.009 0.007 0.010 0.023 0.013 (n=3) Fresh 0.054 0.053 0.071 0.119 0.168 SE (n=6) 0.009 0.007 0.011 0.014 0.015 (n=5) Aerated 0.052 0.060 0.074 0.109 0.242 SE (n=6) 0.009 0.013 0.018 0.018 0.026 Tana Cultivated No 0.026 0.044 0.044 0.059 0.101 SE (n=6) 0.003 0.005 0.005 0.003 0.008 Fresh 0.027 0.041 0.043 0.059 0.119 SE (n=6) 0.005 0.005 0.006 0.004 0.009 Aerated 0.033 0.046 0.049 0.060 0.113 SE (n=6) 0.006 0.007 0.007 0.006 0.018 (n=5) 35 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. Diffusion is highly related to air-filled porosity (E g ). As already mentioned, air-filled porosity showed a high variability both between samples from the same replicates and between replicates. The relationship between relative diffusivity (D/ D 0 ) and porosity/air-filled porosity is described in the literature through various types of equa- tions (Buckingham 1904, Penman 1940, Mar- shall 1959, Millington 1959, Currie 1960, 1961, Bakker and Hidding 1970, Stepniewski 1981, Ball et al. 1988). Many of these authors have described the relationship between D/D 0 and E g by equation (2). D/D 0 = a E g b (2) According to Stepniewski (1981) the value of coefficient a is associated with the total po- rosity of the soil, whereas b is associated with air content and continuity of pores filled with air. In the literature values of a in the range of 0.15–40 (E g , m3 m-3) have been presented, while the reported values of b are in the range of 0.8– Table 3. Relative diffusivity (D/D 0 ) at different water potentials for virgin and cultivated soils from Pasvik and Tana. Sampling depth 0–5 cm. SE=standard error. Relative diffusivity (D/D 0 ) Applied potential, kPa Location Cultivation Treatment –1.5 –3.0 –5.0 –10 –60 Pasvik Virgin No 0.00038 0.0033 0.0238 0.0945 0.167 SE (n=3) 0.00017 0.0009 0.0028 0.0046 0.010 Fresh 0.00015 0.0051 0.0137 0.0448 0.063 SE (n=3) 0.00008 0.0016 0.0021 0.0019 0.005 Aerated 0.00028 0.0062 0.0173 0.0425 0.104 SE (n=3) 0.00014 0.0021 0.0027 0.0051 0.004 Pasvik Cultivated No 0.00116 – 0.0058 0.0194 0.068 SE (n=7) 0.00064 – 0.0010 0.0011 0.002 Fresh 0.00041 – 0.0040 0.0110 0.042 SE (n=6) 0.00011 – 0.0016 0.0028 0.009 Aerated 0.00049 – 0.0048 0.0116 0.036 SE (n=4) 0.00032 – 0.0012 0.0021 0.003 (n=3) Tana Virgin NCS 0.00045 – 0.0023 0.0092 0.072 SE (n=5) 0.00022 – 0.0010 0.0030 0.008 (n=4) (n=4) (n=3) Fresh 0.00045 – 0.0045 0.0100 0.010 SE (n=6) 0.00024 – 0.0016 0.0022 0.003 (n=4) (n=5) (n=3) Aerated 0.00237 – 0.0069 0.0106 0.026 SE (n=6) 0.00073 – 0.0006 0.0016 0.005 (n=5) (n=5) Tana Cultivated No 0.00039 – 0.0016 0.0055 0.013 SE (n=6) 0.00027 – 0.0006 0.0001 0.002 Fresh 0.00031 – 0.0005 0.0043 0.012 SE (n=6) 0.00026 – 0.0001 0.0013 0.002 Aerated 0.00035 – 0.0012 0.0050 0.014 SE (n=6) 0.00016 – 0.0005 0.0017 0.004 (n=5) (n=4) 36 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Haraldsen, T.K. Effects of cattle slurry and cultivation on air exchange 4 (Glinski and Stepniewski 1985). Lindström and McAfee (1987) gave values of a in the range of 0.1–0.7, using the same method as in the present investigation on relatively impermeable soils. Their b values were 1.4 to 3.3. Different regression models were tested, both exponential (like equation (2)), quadratic, poly- nomial and linear. A regression equation for each slurry treatment was made in order to see if the relationship between relative diffusivity (D/D 0 ) and air-filled porosity (E g ) had changed. The re- lationship was fitted with quadratic equations on the Pasvik soils, and on the virgin Tana soil. Best fit for the cultivated Tana soil was obtained by linear equations (Table 4). For the virgin sandy soil from Pasvik there was a significantly steeper slope on the regres- sion line for the treatment without slurry than for the slurry treatments (P<0.001), and there was a significant difference between the slurry treatments (P<0.01) (Fig. 1). Infiltration meas- urements (Haraldsen and Sveistrup 1994) for replicate 1 showed higher infiltration for aerat- ed slurry than for fresh slurry. However, the in- filtration rate for replicate 1 was considerably higher after application of aerated slurry than for the other three replicates. After application of fresh slurry replicate 1 showed a slightly higher infiltration rate than the other three replicates, and the difference in infiltration rate between the FCS and ACS treatments was not significant (Haraldsen and Sveistrup 1994). Therefore, the real difference in diffusivity between the ACS and FCS treatments may be less than measured for replicate 1. Unfortunately the samples from replicate 2 for the virgin sandy soil from Pasvik was disturbed by transport. In the micromopho- logical investigations (Sveistrup et al. 1995) the virgin Pasvik soil was described as fairly heter- ogeneous, and results from different replicates would be important when interpreting the results of different soil physical measurements. For the cultivated sandy soil from Pasvik there was a significantly steeper slope on the regression line for the treatment without slurry than for the slurry treatments (P<0.001) and no significant difference between the slurry treat- ments (P>0.05) (Table 4, Fig. 2). Micromorphological studies of soil samples from the Pasvik soil showed that the slurry did not affect the microstructure (Sveistrup et al. 1995). The slurry was present as a more or less continuous layer of variable thickness on the soil surface. The slurry layer had two phases: (i) a layer of raw, undecomposed but fragmented straw on top of (ii) a layer of homogeneous, col- loidal, pale yellowish brown material. Except for a few thin coatings in the uppermost 50 µm, there Table 4. Influence of slurry application on the relationship between relative diffusivity (D/D 0 ) and air– filled porosity (Eg). Location Cultivation Slurry Regression equation n R2 Pasvik Virgin No D/D 0 = –0.0074 + 1.34 Eg2 15 0.98 Aerated D/D 0 = –0.0042 + 0.80 E g 2 15 0.94 Fresh D/D 0 = –0.0035 + 0.55 E g 2 14 0.95 Pasvik Cultivated No D/D 0 = –0.00042 + 0.92 E g 2 28 0.94 Aerated D/D 0 = 0.0009 + 0.61E g 2 18 0.88 Fresh D/D 0 = –0.0016 + 0.62 E g 2 24 0.95 Tana Virgin No D/D 0 = –0.0013 + 0.79E g 2 16 0.98 Aerated D/D 0 = 0.003 + 0.37 E g 2 22 0.93 Fresh D/D 0 = 0.0015 + 0.35 E g 2 17 0.57 Tana Cultivated No D/D 0 = –0.0039 + 0.16E g 24 0.84 Aerated D/D 0 = –0.0029 + 0.11E g 21 0.58 Fresh D/D 0 = –0.0027 + 0.11E g 24 0.59 37 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. was no evidence of penetration of organic mat- ter into the mineral soil. No clear differences between aerated and fresh slurry were found. Stevens and Cornforth (1974) found that large amounts of slurry could completely seal the soil surface. The sealing effect was obtained with fresh slurry, sieved slurry, and with fine solids, but not with coarse solids, supernatant or water. The results from Stevens and Cornforth (1974) indicated that the colloidal organic matter in the slurry, which overlied the mineral soil, represent- ed a barrier for gas diffusion. The diffusivity of this thin layer was less than in the sandy soil. Burford (1976) concluded that the high water content in the slurry and particulate organic matter restricted aeration in the soil. In the present experiment a smaller air-filled porosity and relative diffusivity in the cultivat- ed soils, as compared to the virgin soils, were found. Sveistrup et al. (1995) found that the cul- tivated Pasvik soil was richer in fine mineral particles than the virgin one. Differences in soil structure were also observed, single grained in the virgin soil and microaggregates, pellicular or bridged-grain microstructures in the cultivat- ed soil. Table 1 also shows higher silt and clay contents in the top layer of the cultivated Pasvik soil compared to the virgin soil. Because of the textural differences between the cultivated and virgin soils, this probably partly explains the lower air-filled porosity and smaller diffusivity in the cultivated Pasvik soil, which is not a re- sult of compaction by agricultural machines but could be a result of mixing soil layers during cultivation. For the virgin silty soil from Tana there were significant differences between the equations of the NCS treatment and the slurry treatments (P<0.001). The equations for the FCS and ACS treatments were not significantly different (P>0.05) (Fig. 3, Table 4). The poorer R2 for the FCS treatment was partly due to a high varia- tion between replicate samples, and partly due to lower air-filled porosity at –60 kPa water po- tential (Table 2). Sveistrup et al. (1995) report- ed some variations in structure between the treat- ments; micromorphological studies showed higher density in the ACS and FCS treatments than at the NCS treatment. The samples investi- gated by Sveistrup et al. (1995) were from repli- cates 1 and 3 in the infiltration experiment (Har- aldsen and Sveistrup 1994). As mentioned earli- er, samples from replicates 1 and 2 were used in the present experiment. For the cultivated silty soil from Tana there were no significant differences between the re- gression equations for the different treatments (P>0.05) (Table 4, Fig. 4). Since the slurry treat- Fig. 1. Relative diffusivity (D/D 0 ) at different air-filled po- rosities (E g ) for the virgin sandy soil from Pasvik (NCS=no slurry, FCS=fresh slurry, ACS=aerated slurry). Fig. 2. Relative diffusivity (D/D 0 ) at different air-filled po- rosities (E g ) for cultivated sandy soil from Pasvik (NCS=no slurry, FCS=fresh slurry, ACS=aerated slurry). 38 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Haraldsen, T.K. Effects of cattle slurry and cultivation on air exchange ments did not influence the relationship between D/D 0 and E g , it seems safe to conclude that the gas diffusivity was not limited by the slurry lay- er at the top but rather by the soil itself. The study of micromorphological features in the virgin and cultivated Tana soils showed a clear difference in structure and porosity due to cultivation. The virgin Tana soil had a well-de- veloped granular microstructure in the upper 1 cm and a partly compacted platy structure with lens-shaped aggregates in deeper layers. The cultivated soil was massive or had a weakly de- veloped blocky microstructure (Sveistrup et al. 1995). A comparison of figures 3 and 4 clearly shows that the cultivated Tana soil had less air- filled porosity and lower relative diffusivity than the virgin silty soil from Tana. The critical limit for relative diffusivity (D/D 0 ) below which crop growth is affected is in the range of 0.005–0.02 (Grable and Siemer 1968). Boone et al. (1986) showed that the oxy- gen consumption in the soil greatly influenced the critical limits for the oxygen diffusion coef- ficient, and soil compaction was found to in- crease the oxygen consumption in the soil. At field capacity (–5 kPa) both the uncultivated and cultivated Tana soils had lower relative diffu- sivity than the lower limit of Grable & Siemer (1968). This result is not surprising due to the great problems with grass growth at this site (Lorentzen 1984), and the large amount of weed grasses (Poa annua) observed. Application of slurry to the Tana soil will increase the oxygen demand in the soil and would make the soil more anaerobic (Egginton and Smith 1986). Applica- tion of slurry to this type of soil will therefore have negative impacts on grass growth due to poor aeration. At field capacity (–10 kPa) the uncultivated sandy Pasvik soil had a higher rel- ative diffusivity than the limit of Grable and Sie- mer (1968). The cultivated Pasvik soil without slurry application had a relative diffusivity close to 0.02, while slurry application reduced the D/ D 0 relationship to 0.011–0.012. However, slurry is normally applied in practical farming on this soil without any problems for grass growth. Air permeability measurements The variability in air permeability between par- allel samples was high, especially for the sam- ples from the Tana soil. However, this high vari- ability probably caused that slurry application did not significantly influence the effects on the relationship between air permeability (K a ) and air-filled porosity (E g ), in neither the Pasvik or Tana soils. Fig. 3. Relative diffusivity (D/D 0 ) at different air-filled po- rosities (E g ) for virgin silty soil from Tana (NCS=no slurry, FCS=fresh slurry, ACS=aerated slurry). Fig. 4. Relative diffusivity (D/D 0 ) at different air-filled po- rosities (E g ) for cultivated silty soil from Tana (NCS=no slurry, FCS=fresh slurry, ACS=aerated slurry). 39 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. Air permeability and air-filled porosities were fitted to the equation (3): logK a = logM + NlogE g (3) where M and N are empirical constants (Ball et al. 1988). Values of M, N and R2 are given in Table 5. The poor correlation between K a and E g for the Tana soils was influenced by variability in continuity and size distribution of air-filled pores (Fig. 5 and 6). The wide range of E g -val- ues which give an air permeability of zero (in Figs. 5 and 6 is 0 represented by 0.01) is a sig- nificant indication of this (Ball 1987, Ball et al. 1988). Edling (1986) found a similar variability of results as in the present investigation. Sam- ples having identical air content could have air permeability values which differed by 1000 times. In some of Edlings samples a relatively Table 5. Regression parameters from air permeabilities and air porosities for sandy soils from Pasvik and silty soils from Tana. Soil Cultivation log M N R2 n Pasvik Virgin 2.54 2.02 0.76 43 Pasvik Cultivated 2.19 1.47 0.56 70 Tana Virgin 3.16 2.11 0.31 55 Tana Cultivated 4.25 3.37 0.45 70 high air permeability was found at air-filled po- rosities <0.05, while it was zero at this air con- tent in other cases. In thin sections Sveistrup et al. (1995) observed that there was less pores in the upper 1 cm of the virgin Pasvik soil com- pared to at 3 cm depth. The lower percentage of pores close to the surface was probably a result of traffic by agricultural machinery (Sveistrup et al. 1995). The relatively poor relationship be- tween K a and E g for the cultivated Pasvik soil may also indicate that this soil was influenced by continuity and size distribution of air-filled pores. Blackwell et al. (1990) introduced the term pore organization, O, which is dependent of the arrangement and shape of the macropore space, and defined by (4): O=K a /Eg (4) Fig. 5. Air permeability (K a , µm2) at different air-filled po- rosities (E g ) for virgin silty soil from Tana (NCS=no slurry, FCS=fresh slurry, ACS=aerated slurry). Fig. 6. Air permeability (K a , µm2) at different air-filled po- rosities (E g ) for cultivated silty soil from Tana (NCS=no slurry, FCS=fresh slurry, ACS=aerated slurry). 40 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Haraldsen, T.K. Effects of cattle slurry and cultivation on air exchange Pore organization was calculated for the Pas- vik and Tana soils (K a measurements of 0 was replaced by 0.01 µm2) and plotted in graphs (Figs. 7 and 8) similar to the presentation of O by Blackwell et al. (1990). From Fig. 7 no clear evidence of different pore organization between the virgin and cultivated Pasvik soils can be ob- served. The variation in pore organization was largest for air-filled porosities <0.1 m3 m-3. The pore organization of the Tana soils differed sub- stantially from that of the Pasvik soils. At air- filled porosities <0.1 m3 m-3 the virgin Tana soil was influenced by a massive matrix with blocked pores and some macropores involved in gas transmission, causing O to vary from 0.1 to more than 100 µm2, while most of the O-values ranged from 10 to more than 1000 µm2 for the virgin Tana soil, which was better structured (Fig. 8). Gas transport by mass flow is less important in the soil than transport by diffusion (Romell 1922). Although soil air permeability per se is not a particularly important aeration parameter, it does reflect the soil’s water permeability (McAfee et al. 1989). Theoretically the correla- tion between relative diffusivity (D/D 0 ) and air permeability (K a ) may vary greatly because con- tinuity and pore size distribution of air-filled pores influence D/D 0 and K a in different ways. McAfee et al. (1989) found the relationship fit- ted by the equation D/D 0 =aK a b. In this study the relationship was found to be log-log linear (Table 6). The regression analysis showed that cultiva- tion did not significantly affect the relationship between D/D 0 and K a for the Pasvik and Tana soils, but the relationship was significantly dif- Fig. 7. Pore organization (O, µm2) at different air-filles porosities (E g ) for cultivated sandy soil from Pasvik (PC) and virgin sandy soil from Pasvik (PV). Fig. 8. Pore organization (O, µm2) at different air-filles porosities (E g ) for cultivated silty soil from Tana (TC) and virgin silty soil from Tana (TV). Table 6. Regression equations for the relationship relative diffusivity (D/D 0 ) to air permeability (K a , µm2). Soil Cultivation Regression equation R2 n Pasvik Virgin logD/D 0 = –2.95 + 1.09logK a 0.75 43 Pasvik Cultivated logD/D 0 = –2.98 + 1.03logK a 0.76 70 Tana Virgin logD/D 0 = –2.85 + 0.50logK a 0.65 53 Tana Cultivated logD/D 0 = –2.80 + 0.54logK a 0.77 69 41 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 8 (1999): 29–44. ferent for the two soils. This difference between the two soils may be due to differences in pore continuity, tortuosity, constriction and size. Because air permeability measurements are much less time consuming and do not require expensive equipment like a gas chromatograph, it would be of interest to predict relative diffu- sivity from air permeability. Because of the var- iability of air permeability results, the models are not particularly predictive. In this investiga- tion all information about the sealing effect of slurry would have been lost if the relative diffu- sivity had been predicted from air permeability instead of being measured. Methodological problems One problem with this experiment is that there had to be differences in time between the meas- urements at the different water potentials. There was a time difference of about three months be- tween the measurements at –1.5 kPa water po- tential and at –60 kPa water potential. During the time in the laboratory, growth of mycelium was observed in the slurry layer. The negative effect of slurry on gas diffusion, which was sig- nificant at –10 kPa and –60 kPa, was obtained after at least two months in the laboratory at 20°C. Therefore, the negative effects of slurry on gas movement in the soil seems to be a rela- tively persistent. A significant decrease in infil- tration rate due to slurry application was found in the same soils after three month (Haraldsen and Sveistrup 1994). The amounts of slurry used in this experi- ment gave a continuous cover of slurry on the soil surface. This experiment was carried out on soils without plant cover, and the slurry was probably more evenly applied than in practical farming. The methods used in this experiment did not enable a quantification of the real effects of slurry on gas movement in field. In field growth of grass, tunnelling of earthworms and dung fly larvae may perforate the continuous cover of slurry, and improve the aeration (Bur- ford 1976, Haraldsen and Sveistrup 1996). Conclusions In a study from northern Norway lower air-filled porosities and relative diffusivities were found of cultivated sandy and silty soils than in virgin (uncultivated) soils. Application of slurry (50 Mg ha-1) reduced the relative diffusivity and changed the relationship between relative diffusivity and air-filled porosity for the virgin and the culti- vated sandy soil and the virgin silty soil. There was no significant difference between aerated and fresh slurry on gas diffusion in the cultivat- ed sandy soil and the virgin silty soil. However, there was a significant difference in the effects of aerated and fresh slurry on gas diffusion on the virgin sandy soil. The air permeability meas- urements showed a high variability, and no in- fluence of slurry application on air permeability was found. The relationship between air-perme- ability and air-filled porosity was influenced by the variability in non-continuous air-filled po- rosity between replicate samples. This was es- pecially pronounced in the silty soils. The vir- gin and cultivated silty soils used in this investi- gation had lower relative diffusivity at field ca- pacity than the lower value of limit below which crop growth is affected. The dominance of weed grass (Poa annua) at the cultivated silty soil in- dicates a high risk for stress on sown grass spe- cies and winter damage. Acknowledgements. Dr. Currie is thanked for valuable com- ments on the experimental methods and presentation of the results. Professors Njøs and Håkansson, and Dr. Riley are thanked for useful comments on the manuscript. 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Effects of cattle slurry and cultivation on air exchange SELOSTUS Lietelannan levityksen ja viljelyn vaikutus maan ilmanvaihtoon Pohjois-Norjan hiekka- ja hiesumailla Trond Knapp Haraldsen Planteforsk, Norja Tutkimuksessa verrattiin viljelemättömän (ei koskaan kynnetty) ja viljellyn hiekka- ja hiesumaan huokoi- suutta, diffuusionopeutta kaasuille ja ilmanläpäisyky- kyä. Maanäytteet kerättiin kahdelta paikkakunnalta Pohjois-Norjasta. Näytteistä, joissa maan luontainen rakenne oli säilytetty, määritettiin kaasun diffuusio- kerroin ja ilmanläpäisykyky sekä ilman täyttämä huo- kostila neljässä eri veden potentiaalissa, joka vaih- teli välillä –1,5 kPa ja –60 kPa. Viljeltyjen maiden ilman täyttämä huokostila ja kaasun diffuusiokyky olivat pienempiä kuin viljelemättömien. Lietelannan levitys (50 Mg ha-1) vähensi kaasun diffuusiokykyä ja muutti suhteellisen diffuusion riippuvuutta ilman täyttämästä huokostilasta sekä viljelemättömällä että viljellyllä hiekkamaalla ja viljelemättömällä hiesu- maalla. Kaasunläpäisykyky sekä viljelemättömällä että viljellyllä hiesumaalla oli pieni ja suhteellinen diffuusiokyky jäi maan kosteuden vastatessa kenttä- kapasiteettia pienemmäksi kuin raja, jonka alapuolel- la kaasun vaihto rajoittaa kasvin kehitystä. Title Introduction Material and methods Results and discussion Conclusions References SELOSTUS