Schjønning P., Thomsen I.K., Petersen S.O., Kristensen K.

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1 Bibliografiska uppgifter för Pore size distribution, gas diffusivity, and net nitrogen mineralization for soils under shallow tillage and mouldboard ploughing Författare Utgivningsår 2006 Tidskrift/serie NJF Report Nr/avsnitt 4 Ingår i... Utgivare Huvudspråk Målgrupp Schjønning P., Thomsen I.K., Petersen S.O., Kristensen K. NJF-Seminar 378. Tillage system for the benefit of agriculture and the environment Nordiska jordbruksforskares förening (NJF) Engelska Forskare Nummer (ISBN, ISSN) ISSN Denna skrift (rapport, artikel, examensarbete etc.) är hämtad från VäxtEko, databasen som samlar fulltexter om ekologisk odling, växtskydd och växtnäring. Utgivaren har upphovsrätten till verket och svarar för innehållet.

2 Back to Contents Pore size distribution, gas diffusivity, and net nitrogen mineralization for soils under shallow tillage and mouldboard ploughing P. Schjønning 1*, I.K. Thomsen 1, S.O. Petersen 1, K. Kristensen 2 1 Danish Institute of Agricultural Sciences, Department of Agroecology 2 Danish Institute of Agricultural Sciences, Department of Genetics and Biotechnology Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark * Corresponding author. Tel.: ; Fax: ; Per.Schjonning@agrsci.dk Abstract Conversion to shallow (non-inversion) tillage (ST) from moldboard ploughing (MP) alters soil properties as pore size distribution, pore tortuosity and soil organic matter distribution. This in turn influences the conditions for soil aeration and thus for turnover of soil organic matter. Intact soil cores (100 cm 3 ) were taken at 0-4 and cm depth from soil under ST and MP on a loamy sand (18% clay). Six replicate cores per tillage treatment and depth were adjusted to one of seven soil water potentials ranging from - 15 to hpa. Following measurement of gas diffusivity, the samples were incubated at 20 C. Ceramic discs and extraction with KCl were used to determine soil nitrate prior to and after incubation, respectively. Net mineralization was calculated as the difference between pre- and post-incubation soil content of NO 3. At cm soil depth, ST dramatically changed the size distribution of soil pores, and reduced the gas diffusivity as compared to MP soil. For ST at this bottom layer, the level of gas diffusivity at most of the matric potentials studied was below a lower threshold for satisfactory aerobic biotic activity often quoted in the literature. Nitrate accumulation peaked at intermediate water contents, and the results showed that these were determined by gas diffusion constraints. Tillage treatments were not significantly different with regard to the optimum in water content. For the 0-4 cm layer, significantly more N had mineralized in ST than in MP soil, while the opposite was found for the cm layer. Keywords: Shallow tillage; Mouldboard ploughing; Water retention; Pore size distribution; Gas diffusivity; Pore tortuosity; Nitrogen mineralization. 1. Introduction Conventional mouldboard ploughing to ~20 cm depth manipulates 2,500-3,000 tonnes of soil per hectare representing pools of around t C ha -1 and t N ha -1 (Heidmann et al., 2001). A significant distinction between mouldboard ploughing (MP) and shallow tillage (ST) is the (re)distribution of organic inputs such as crop residues and manure, and of soil biota associated with these organic substrates. While MP results in a relatively uniform distribution of organic inputs across ploughing depth, ST will leave these inputs at or near the soil surface (Schjønning & Thomsen, 2006). MP fragments the bulk soil to aggregates throughout the top ~20 cm layer, whereas ST leaves a (variable) part of the former plough layer undisturbed. This in turn changes the density and pore size distribution and hence the conditions for air exchange etc. Consequently, the physical and chemical environment for organic matter turnover is very different depending on tillage practice, and may influence C/N dynamics, including soil N mineralization. Net N mineralization has been reported to decrease in ST as compared to MP systems (e.g. Dowdell et al., 1983; Nyborg & Malhi, 1989), but 110

3 there are contrasting results depending on crop rotation, time of tillage etc. (Silgram & Shepherd, 1999). Several studies have demonstrated that aerobic microbial activity displays an optimum at a specific water content (e.g. Stanford & Epstein, 1974; Franzluebbers, 1999). Substrate diffusion in soil water becomes limiting at low water contents, whereas oxygen diffusion in air-filled pores is limiting at high water contents (Skopp et al., 1990). Sixty percent water-filled pore space (WFPS) has been proposed as a predictor of maximum microbial activity across soil types (Linn & Doran, 1984; Doran et al., 1988, 1990). Yet, Stanford & Epstein (1974) found the highest nitrification rates at 80-90% WFPS, while Franzluebbers (1999) reported an average optimal WFPS of 42% for a range of soil types. Schjønning et al. (1999) predicted large differences in the optimum WFPS for aerobic activity in soils of varying clay content based on threshold air diffusivity of undisturbed and remoulded soils. Also, Schjønning et al. (2003) found net nitrification to peak at considerably lower WFPS in a loamy sand than in two more structured clay-holding soils (Fig. 1). 14 Net nitrification (µg NO 3 -N g -1 soil) Soil L1 11% clay Soil L3 22% clay Soil L5 34% clay Water-filled pore space (WFPS) Fig. 1. Net nitrification in undisturbed soil cores at different water contents. Data from Schjønning et al. (2003). Profound differences in gas diffusivity have been observed for soils of different texture when in their undisturbed structured condition, whereas the same soils displayed identical gas diffusivity properties when remoulded and repacked (Schjønning et al., 1999). This is an indication of the paramount influence of soil structure on gas diffusivity in soils, and the differences in optimum water content for net nitrification of the three soils in Fig. 1 were probably due to different conditions for oxygen transport (Skopp et al., 1990). Structural differences imposed by different tillage practices have also been shown to influence the conditions for gas diffusivity (e.g. Schjønning, 1989). Thus we anticipated that tillage-derived differences in soil structure would significantly influence the water regime effects on nitrogen turnover in soil. Hence, the aim of the present study was to evaluate the net effect of the physical and biological changes induced by different tillage strategies on i) the optimum water content for microbial activity and ii) the level of net N mineralization. 111

4 2. Materials and methods The study was carried out with soil from a field experiment at Nakskov, Denmark, where MP had been compared continuously with ST for 5 years (Schjønning & Thomsen, 2006). The soil was a sandy loam derived from Weichselian glacial till (~18% clay). The ST practised was stubble tillage to ~10 cm depth followed by a Wäderstad Rapid disc drill. The crop rotation during the experimental period included spring- and winter grown small grain cereals, oats, and sugar beets. In , the crop was winter wheat. In spring 2004 when water content was close to field capacity, undisturbed 100 cm 3 soil cores were collected in steel cylinders (Ø 61 mm, height 34.2 mm) from 0-4 and cm depth of ST and MP treated soil. The tillage treatments were replicated three times in a randomized block design. We sampled 14 cores per depth in each field plot, yielding 42 cores for each tillage treatment and depth. This sampling scheme gave six replicate soil cores per water potential in the laboratory study (see below). The cores were adjusted to each of seven matric potentials between -15 and hpa using either tension tables or pressure plates. In selecting the six replicate cores for each combination of tillage treatment and matric potential (within each depth), we attempted to achieve the same range in wet bulk density for all combinations. We hypothesized that the wet bulk density largely reflected the dry density (and thus total soil porosity), and hence aimed at the same stochastic variability for abiotic conditions influencing physical and biological soil functions. Small ceramic discs (Ø 11 mm, height 3 mm) were placed in capillary contact to the soil on top of each soil core during saturation on tension tables. The discs were removed and analysed for nitrate content when the matric potentials had been achieved. The nitrate concentration in the discs was used for calculation of pre-incubation nitrate content in the soil water of each core (Thomsen & Schjønning, 2003). Following adjustment of water potentials, bulk soil gas diffusivity was determined with a transient state method as described by Schjønning (1985). Samples were then covered with perforated lids and incubated at 20 C in the dark for 31 days. Immediately after incubation, a portion of each core sample was extracted with 100 ml of 1 M KCl for calculation of postincubation content of nitrate. 3. Results and discussion The tillage procedure had only minor influence on the pore size distribution of the top soil that was tilled in both treatments (0-4 cm layer, Fig. 2). In contrast, ST had changed the pore size distribution of the cm layer rather dramatically (Fig. 2); the bulk density was ~1.32 and ~1.61 g cm -3 for the MP and ST soils, respectively. The decrease in pore volume for the ST soil was due to a significant reduction in the volume of large pores, especially those larger than 50 µm (Fig. 2). 112

5 Pore volume (m 3 m -3 ) >200 µm µm µm µm µm 6-15 µm 2-6 µm µm <0.2 µm MP ST MP ST 0-4 cm layer cm layer Fig. 2. Pore size distribution for mouldboard ploughed (MP) and shallowly tilled (ST) soil for soil layers above (0-4 cm) and below (14-18 cm) the ST tillage depth. The tubeequivalent pore diameters (see legend box) were calculated from the matric potentials, assuming standard values for water surface tension etc. Fig. 3 shows soil gas diffusivity normalized to the level of diffusion in free air and related to the soil air-filled pore space. For the 0-4 cm layer, the air-filled pore space was approximately identical for the two tillage treatments at each of the seven matric potentials (compare the data-points in the x-direction ), which reflects the nearly identical pore size distributions (Fig. 2). Further, for that layer the relation between airfilled pore space and gas diffusivity seemed to be nearly identical for ST and MP soils. The physical conditions for gas exchange in these cores were thus about the same. In contrast, the consistent change in the pore size distribution of ST compared to MP soil for the cm layer caused much poorer conditions for gas diffusivity in ST soil (Fig. 3). The four most wet matric potentials displayed average gas diffusivity values below the range normally considered critical to aerobic biotic activity in soil (e.g. Stepniewski, 1981). Relative gas diffusivity (-) cm MP ST cm Threshold for Threshold for critical aeration critical aeration MP ST Air-filled pore space, ε a (m 3 m -3 ) Fig. 3. Relation between relative gas diffusivity and soil air-filled pore space for ST and MP soil. Bars indicate +/- SE error. Soil matric potentials (-hpa) are shown by figures close to the MP data points. Fig. 4 shows net nitrification for each individual soil core as related to its volumetric water content. Although no consistent difference in the physical conditions for soil 113

6 microbial activity could be detected from the gas diffusivity measurements in the 0-4 cm layer (Figs. 2 and 3), a significantly higher net nitrification was found in ST than in MP soil (Fig. 4 and Table 1). This was most probably due to a higher content of soil organic matter in the upper cm of ST as compared to MP soil (Schjønning & Thomsen, 2006). At cm depth, the highest net nitrification rates were found in MP soil although the estimated maximum rate at the optimum water content was not significantly higher at the P=0.05 level (Fig. 4, Table 1). Net nitrate mineralization (µg NO 3 -N g -1 soil) cm cm MP ST MP ST Volumetric water content (m 3 m -3 ) Fig. 4. Net nitrification for all individual soil cores of ST and MP soil related to the volumetric soil water content. Lines indicate best fit with a second degree polynomium. Model estimates of maximum nitrification and the corresponding water content are given in Table 1. Table 1. Second degree polynomium model estimates of maximum net nitrification and the corresponding water contents for the data shown in Fig. 2. Means followed by the same letter in a row are not significantly different (P=0.05). Soil layer cm Max. nitrification Water content at max. nitrification µg NO 3 g -1 soil m 3 m -3 MP ST MP ST a 24.9 b a a a(1) 5.5 a(1) (0.448) a a 1 P~0.1 For neither of the soil layers could the estimated optimum in water content for maximum net nitrification be differentiated among tillage treatments (Table 1). However, the estimated optima were in accordance with the expected water contents at a threshold relative gas diffusivity of that could be calculated from combining data in Fig. 2 and Fig. 3. This supports the Skopp concept of diffusion constraints regulating soil organic matter turnover. The results also implied that for the cm layer having great differences in pore size distribution and conditions for gas diffusivity, the maximum net nitrification occurred at lower matric potentials (more dry soil) for the ST than for the MP soil. The impact of these differences on the net nitrification and hence crop nutrition under field conditions will depend on the water regime of the specific growing season. Thomsen & Christensen (2006) found no significant ST effects on N availability and crop yield for five Danish soils studied in a single year. 114

7 4. Conclusions Our results indicate that diffusion constraints determined optimum water contents for net nitrification in soil, but the differences between tillage treatments were not statistically significant. However, due to differences in pore size distribution of the cm layer ST and MP soils, net nitrification occurred at lower matric potentials (more dry soil) for the ST than for the MP soil. At 0-4 cm depth a significantly higher net nitrification at optimum water content was found in ST compared to MP soil. The opposite trend was found for the cm layer, but the difference was not statistically significant. 5. Acknowledgements We thank Olav Høegh from the local office of the Danish Agricultural Advisory Service for giving us access to the field trial. Dr. B.T. Christensen provided valuable inputs in the planning of these studies. Bodil B. Christensen, Karin Dyrberg, Michael Koppelgaard and Stig T. Rasmussen are acknowledged for their technical assistance in carrying out the measurements. This study was financed by the Danish Institute of Agricultural Sciences. 6. References Doran, J.W., Mielke, L.N., Stamatiadis, S Microbial activity and N cycling as regulated by soil water-filled pore space. Proceedings of the 11th ISTRO Conference, Edinburgh, United Kingdom, pp Doran, J.W., Mielke, L.N., Power, J.F Microbial activity as regulated by soil water-filled pore space. Proceedings of the 14th ISSS Congress, Kyoto, Japan, Vol. III, Dowdell, R. J., Crees, R., Cannell, R. Q A field study of effects of contrasting methods of cultivation on soil nitrate content during autumn, winter and spring. J Soil Sci. 34, Franzluebbers, A.J Microbial activity in response to water-filled pore space of variably eroded southern Piedmont soils. Appl. Soil Ecol. 11, Heidmann, T., Nielsen, J., Olesen, S.E., Christensen, B.T., Østergaard, H.S Ændringer i indhold af kulstof og kvælstof i dyrket jord: Resultater fra kvadratnettet DJF rapport nr. 54, Markbrug, 73 pp. Linn, D.M., Doran, J.W Effect of water-filled pore space on carbon dioxide and nitrous oxide production in tilled and non-tilled soils. Soil Sci. Soc. Am. J. 48, Nyborg, M., Malhi, S.S Effect of zero and conventional tillage on barley yield and nitrate nitrogen content, moisture and temperature of soil in north-central Alberta. Soil & Tillage Research 15, 1-9. Schjønning, P A laboratory method for determination of gas diffusion in soil (En laboratoriemetode til måling af luftdiffusion i jord). Tidsskrift for Planteavl, 89, 132. Beretning nr. S1773, 19pp. Schjønning, P Long-term reduced tillage. II. Soil pore characteristics as shown by gas diffusivities and permeabilities and air-filled porosities. Soil Tillage Res. 15, Schjønning, P., Thomsen, I.K Screening of reduced tillage effects on soil properties for a range of Danish soils. Proceedings of NJF-seminar No. 378, Odense, May

8 Schjønning, P., Thomsen, I.K., Møberg, J.P., de Jonge, H., Kristensen, K., Christensen, B.T Turnover of organic matter in differently textured soils. I. Physical characteristics of structurally disturbed and intact soils. Geoderma, 89, Schjønning, P., Thomsen, I.K., Moldrup, P., Christensen, B.T Linking soil microbial activity to water- and air-phase contents and diffusivities. Soil Science Society of America Journal 67, Silgram, M., Shepherd, M.A The effects of cultivation on soil nitrogen mineralization. Adv. Agron. 65, Skopp, J., Jawson, M.D., Doran, J.W Steady-state aerobic microbial activity as a function of soil water content. Soil Sci. Soc. Am. J. 54, Stanford, G., Epstein, E Nitrogen mineralization - water relations in soils. Soil Sci. Soc. Am. Proc. 38, Stepniewski, W Oxygen diffusion and strength as related to soil compaction. II. Oxygen diffusion coefficient. Polish Journal of Soil Science 14, Thomsen, I.K. & Christensen, B.T Turnover and crop uptake of N under reduced tillage and moldboard ploughing. Proceedings of NJF-seminar No. 378, Odense, May Thomsen, I.K. & Schjønning, P Evaluation of a non-destructive technique for inorganic soil N measurement. Geoderma 113,

9 Back to Contents On-the-go measurements of soil penetration resistance on a Swedish Eutric Cambisol Elisabeth Bölenius 1, Gustav Rogstrand 2, Johan Arvidsson 1, Bo Stenberg 3 and Lars Thylén 4 1 SLU, Dep. of Soil Sciences, Div. of Soil Management, P.O. Box 7014, SE Uppsala, Sweden 2 BC Ministry of Agriculture and Lands, 1767 Angus Campbell Rd, Abbotsford, Canada 3 SLU, Dep. of Soil Sciences, Div. of Precision Agriculture, P.O. Box 234, SE Skara, Sweden 4 Väderstad-Verken AB, Box 85, SE Väderstad, Sweden Corresponding author: Elisabeth.Bolenius@mv.slu.se (tel) (fax) Abstract In order to better explain spatial variations within fields, soil physical properties need to be studied in more depth. Newer, more efficient methods of studying properties such as penetration resistance are therefore required. A horizontal, on-line penetrometer capable of measuring at three different depths simultaneously was developed. Measurements were conducted in 2004 and 2005 in Sweden and a correlation was found between areas with higher penetration resistance and lower yield. Areas with lower penetration resistance also had a higher root growth early in the season. Keywords: Horizontal penetrometer, penetration resistance, on-line measurement, spatial variation 1. Introduction Yield variations within fields can be very high, several tons of grain per hectare (Thylén, 1997). If the reasons for these variations are known, site-specific inputs can save resources, the negative effects on the environment can be reduced and yield levels can be maintained or even increased (Robert, 1999). The main focus within precision agriculture so far has been on variations in the topsoil, concentrating mostly on chemical properties. A large number of studies were conducted in the early 1990s in attempts to explain yield variations within fields by variations in plant nutrient status. The relationships obtained turned out to be quite weak (Mallarino et al., 1999). Relationships between soil physical parameters and yield, especially in the subsoil, are seldom studied, often due to labour intensive, and therefore expensive, traditional methods of measuring such parameters. However, soil physical properties may be able to explain a significant proportion of the variations in yield. The spatial variation in cone index and other physical parameters has been studied by a number of researchers but few have studied the relationship with yield (Isaac et al, 2002; To & Kay, 2005). A large Swedish study on yield variations in sugarbeet showed that factors influencing root development and water transport in soil had the largest effect on yield (Berglund et al., 2002). The objective of the present study was to develop a site-specific, horizontal, on-line, soil penetrometer for studying soil penetration resistance and its correlation with yield. The device was equipped with a NIR-instrument to allow measurement of soil texture 106

10 soil texture, water content and soil organic matter content on-line simultaneously with penetration resistance. 2. Materials and methods Study site and conducted measurements The area used in the study, approx. 10 ha, was situated in a field with clay soil at Kvarnbo farm, 5 km west of Uppsala, Sweden. Measurements were conducted in 2004 and 2005 at 10, 30 and 50 cm depth in lines which were 10 m apart, parallel to the direction of tillage and harvest. Soil water content and vertical penetration resistance were measured at 10, 30 and 50 cm depth at 20 places spread across the area. At the same 20 places root depth was measured during the season in pits down to 1 m depth. The field has been yield-mapped since Equipment A four-share parallel-plough was modified to support three parallel, horizontally mounted, soil penetrating cones (Fig. 1). The cone-angle was 30, with a base diameter of 6.3 cm. The cone was connected to a Bosch draught sensor (Fig. 2) capable of registering forces between 25 kn and +25 kn. The instrument is capable of measuring soil penetration resistance at three depths (10, 30 and 50 cm), speed and position which are recorded with a Trimble SweeEight GPS every second. The instrument was used at a speed of approximately 1.5 m s -1 and was equipped with a stone release mechanism and a system to record actual working depth every second. The 1000 Hz signal from the draught sensor was reduced to a 1 Hz signal to lessen the noise. The NIR-instrument was placed behind the draught sensor of the cone working at 30 cm depth. Fig. 1. Soil-penetrating cone on horizontal penetrometer. Fig. 2. Soil-penetrating and draught sensor. 3. Results Fig. 3 shows a yield map for the area while Fig. 4-6 show data from penetration measurements performed after harvest and gravimetric water contents for the three different soil depths investigated. The penetration resistance at 30 cm (Fig. 5) and 50 cm (Fig. 6) showed a similar pattern whilst the resistance at 10 cm (Fig. 4) seemed to be inversely correlated to that at the other two depths. There was also an inverse correlation between penetration resistance at the two lower depths and the yield to a certain degree (Figs. 3, 5 and 6) with higher yields obtained at areas of lower resistance. 107

11 Fig. 3. Yield of spring barley (ton ha -1 ). Fig. 4. Penetration resistance and water content at 10 cm depth after harvest. Fig. 5. Penetration resistance and water content at 30 cm depth after harvest. Fig. 6. Penetration resistance and water content at 50 cm depth after harvest. The maximum root depth of spring barley at 20 places is shown in Fig. 7. Places with lower penetration resistance (pits 1-5, 9-10 and 14-15) showed deeper root depth in May (average 44 cm) compared to places with higher penetration resistance (average 30 cm) (pits and 16-20). 108

12 105 Maximum root depth, cm Pits May June July Fig. 7. Maximum root depth of spring barley at twenty places in May, June and July. 4. Conclusions The new horizontal penetrometer tested worked well in the field. The signals were stable and the construction was sufficiently robust to tolerate stones in the soil. The initial measurements at Kvarnbo show that variations in penetration resistance can explain part of the variation in crop yield. This horizontal, on-line penetrometer can measure a large area considerably faster than a traditional penetrometer, which will make it easier to incorporate soil physical parameters into precision agriculture. 5. References Isaac, N.E., Taylor, R.K., Staggenborg, S.A., Schrock, M.D. and Leikam, D.F., Using cone index data to explain yield variation within field, Agricultural engineering. 4, Mallarino, A.P., Oyarzabal, E.S. and Hinz, P.N., Interpreting Within-Field Relationships Between Crop Yields and Plant Variables Using Factor Analysis. Precision Agriculture. 1, Robert, P Precision Agriculture: research needs and status in the USA. In: Precision Agriculture 99, J.V. Stafford (Ed), UK, BIOS Scientific Publishers, pp Thylén, L., Consistency in yield variation and optimal nitrogen rate. In: Stafford J.V. (Ed), Precision Agriculture 97, UK, BIOS Scientific Publishers, pp To, J. and Kay B.D., Variation in penetrometer resistance with soil properties: The contribution of effective stress and implications for pedotransfer functions. Geoderma 126,