Agricultural and Food Science in Finland, Vol. 12 (2003): 165–176 165 © Agricultural and Food Science in Finland Manuscript received April 2003 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. 12 (2003): 165–176. Soil nitrate N as influenced by annually undersown cover crops in spring cereals Hannu Känkänen MTT Agrifood Research Finland, Plant Production Research, FIN-31600 Jokioinen, Finland, e-mail: hannu.kankanen@mtt.fi Christian Eriksson MTT Agrifood Research Finland, Data and Information Services, FIN-31600 Jokioinen, Finland Mauri Räkköläinen MTT Agrifood Research Finland, Laukaa Research and Elite Plant Station, FIN-41330 Vihtavuori, Finland Martti Vuorinen MTT Agrifood Research Finland, Häme Research Station, FIN-36600 Pälkäne, Finland Cover crops can reduce leaching and erosion, introduce variability into crop rotations and fix nitro- gen (N) for use by the main crops. In Finland, undersowing is a suitable method for establishing cover crops in cereals. The effect of annual undersowing on soil nitrate N was studied at two sites. Red clover (Trifolium pratense L.), white clover (Trifolium repens L.), a mixture of red clover and meadow fescue (Festuca pratensis Huds.), and westerwold ryegrass (Lolium multiflorum Lam. var. westerwoldicum) were undersown in spring cereals during six successive seasons, and a pure stand of cereal was grown in two years after that. In all years, the soil nitrate N was measured in late autumn, and in addition in different times of the season in last four years. The effect of undersowing on soil NO 3 -N content was generally low, but in one season when conditions favoured high N leaching, westerwold ryegrass decreased soil NO 3 -N. The negligible increase of N leaching risk in connection with undersowing clovers, associated with late autumn ploughing, supports the use of clovers to increase the cereal grain yield. The highest levels of soil NO 3 -N were recorded at sowing in spring irrespective of whether a crop was undersown or not. NO 3 -N contents were higher in sandy soil than in silt. Undersowing can be done annually in cereal cultivation either to fix or catch N. No cumulative effects on soil nitrate N were associated with undersowing after six years. Key words: cereals, clovers, cover crops, grasses, intercropping, nitrate nitrogen mailto:hannu.kankanen@mtt.fi 166 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 Känkänen, H. et al. Effect of repeated undersowing on soil NO 3 -N Introduction Growing cover crops after harvesting a main crop has positive effects on environment and main crops. Cover crops can reduce soil erosion dur- ing autumn and winter, improve soil fertility (Cart- er and Kunelius 1993) and, when legumes are used, fix nitrogen (N) to the benefit of subsequent crops in a rotation (Marstorp and Kirchmann 1991). Besides this, cover crops can decrease or increase soil mineral N (Breland 1996) and by that means affect positively or negatively N leaching. In Finland, reducing N leaching is needed because of eutrophication of coastal waters (Rekolainen et al. 1995) and high risk for lack of oxygen in lakes, which are shallow and covered by ice dur- ing long winters. Therefore, more knowledge of the effect of both N catching and N fixing cover crop species on soil NO3-N is needed. Undersowing in cereals is a suitable method for establishing a cover crop in northern latitudes (Alvenäs and Marstorp 1993, Jensen 1991). Un- dersowing enables immediate uptake of residu- al N by the cover crop after harvest of the main crop (Breland 1996). It therefore follows that if the establishment of the undersown cover crop is successful, it normally reduces N leaching better than sowing the catch crop after the main crop (Beck-Friis et al. 1993). Similarly, when legumes are undersown instead of sowing after the main crop, more mineralisable N is presum- ably incorporated with the cover crop. The soil nitrate N content before winter is regarded as an important indicator of N leach- ing risk in the Nordic countries (Beck-Friis et al. 1993, Wallgren and Lindén 1994, Breland 1996 and Känkänen et al. 1998). Realised N leaching depends on the permanence of soil freezing during winter. During a mild winter downward movement of water in the soil can occur, but in cold winters water conductivity in soil is low due to frost (Turtola and Kemppai- nen 1998), and large amounts of soil water can drain in the spring when soil and snow melt. Besides N leaching risk on the grounds of soil nitrate N content before winter, timing of the release of cover crop N was studied. For in- stance, the net N mineralization can occur too late to be of use to the succeeding crop (Thorup- Kristensen 1996). When grown annually, the ef- fects of cover crops from different years on soil N can also be mixed, because the release of N from plant material takes longer than one grow- ing season (Jensen 1992). Furthermore, N ferti- lisation and its effect on the growth of both the main and undersown crop are also influencing when the effect of undersown crops on soil N is measured. Soil type can also greatly affect N leaching. Kolenbrander (1969) reported low leaching from clayey soil and high leaching from sandy soils. Egelkraut et al. (2000) reported that soils with greater clay concentrations mineralised less N from added materials. The soils of the experi- ment were chosen in accordance with their pre- dominance and difference in tendency to leach- ing. Two thirds of soils are clayey in southern and south-western part of Finland, where the cir- cumstances for leaching are otherwise favoura- ble. Sandy soil is the second common soil type in Finland and represents soils with high leach- ing. The aim of the current study was to investi- gate how annually repeated undersowing with legume or grass cover crops affected risk of N leaching through measuring soil nitrate N con- tent before winter. During last four years, soil NO3-N content was measured on different dates in order to determine timing of N mineralisation. Moreover, as N fertilization decreased the yield of clovers and increased the yield of westerwold ryegrass (Känkänen et al. 2001), also effects on soil N was expected, and therefore soil NO3-N content under different N fertilizer regimes was examined, too. Grain yields in this study have been published in an earlier paper (Känkänen et al. 2001). An- nual undersowing with clovers increased, and undersowing with westerwold ryegrass de- creased cereal grain yields. The grain yield was only slightly lower with a mixture of red clover and meadow fescue than with red clover alone. Soil fertility was not notably improved during 167 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. 12 (2003): 165–176. six years of undersowing according to grain yield two years later. Material and methods Experimental sites The experiments were established in 1991 at the Häme Research Station in Pälkäne (61˚20'N, 24˚13'E) and at the Laukaa Research and Elite Plant Station (62˚25'N, 26˚15'E). The soils were classified at two points, which were at the ex- treme ends of the trial areas (the topsoil classifi- cations and the conventional abbreviations of the Finnish names are given in parentheses). At Pälkäne the soil was an Aquic Eutrocryept, (fine sandy loam, KHt) and an Oxyaquic Cryopsam- ment (loamy sand, KHt). At Laukaa the soil was a Typic Cryaquept or Typic Cryaquent (silt loam, Hs) and a Typic Cryaquent or Typic Cryaquept (silt loam, Hs) (Yli-Halla et al. 2000). At Pälkäne, soil in deeper layers varied, changing from silty clay at the extreme end of the first replicate to sandy in the second and third replicates. The greatest variation was in the first replicate, where the content of sand in the 60 to 90 cm soil layer was 14 and 90% in one cor- ner and in the middle of the replicate, respec- tively. Experimental design The experiments were designed as split-plots with N fertiliser application rates as the main plots (size 6 m × 20 m and 7.5 m × 20 m at Pälkäne and Laukaa, respectively), arranged in a randomised complete block design with three replicate blocks. The split-plot treatments (five undersowings) were randomised among the sub- plots (6 m × 4 m and 7.5 m × 4 m at Pälkäne and Laukaa, respectively) within each main plot. Sowing of cereals was done between 17 and 31 May when the soil moisture was adequate for sowing spring cereals. The seedbed was prepared with an S-tine harrow to 4–5 cm and 3–4 cm depth at Pälkäne and Laukaa, respectively. Spring barley (Hordeum vulgare L.) was sown in 1991 and 1994, spring wheat (Triticum aesti- vum L.) in 1992 and 1995 and oats (Avena sati- va L.) in 1993 and 1996, using a combined drill adjusted to those depths. The seed rates for bar- ley, wheat and oats were as normally used in Finland, 450, 500–650 and 500 seeds per m2 re- spectively. Undersowing was done across the cereal rows to about 2 cm depth at Pälkäne and 1 cm at Laukaa using a combined drill at Pälkäne and an Oyjord experimental sower at Laukaa. Undersowing was done after cereal sowing on the same day or on the following day, except at Pälkäne in 1993 and 1995 where it was done five and four days later respectively. The plots that were not undersown remained undisturbed dur- ing this phase. The row distance of cereals and undersown crops was 12.5 cm. The trial areas were rolled with a continental Cambridge roller before and after undersowing. Red clover (Trifolium pratense L.), white clo- ver (Trifolium repens L.), a mixture of red clover and meadow fescue (Festuca pratensis Huds.) (R.C. & M.F.) and westerwold ryegrass (Lolium multiflorum Lam. var. westerwoldicum) were un- dersown in spring cereals in the same plots during six successive seasons, 1991–1996. A treatment without undersowing represented the control. In 1997 and 1998 a pure stand of spring barley was grown on all plots to measure the residual effect of long-term undersowing. The seed rates for red clover, white clover and westerwold ryegrass were 6, 6 and 10 kg ha-1 of viable seed respectively. In the mixture the seed rates for red clover and mead- ow fescue were 3 and 6 kg ha-1, respectively. All cover crop treatments were combined with N treatments at 0, 30, 60 or 90 kg N ha-1. Fertiliser N was applied with a combined drill at a depth of 7–9 cm at the same time with cere- al sowing, and the subplots kept the same N treat- ment across experimental years. Phosphorus and potassium were applied separately to the entire trial area before sowing the cereal, according to recommendations. 168 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 Känkänen, H. et al. Effect of repeated undersowing on soil NO 3 -N After harvesting the cereal crop with a com- bine harvester in August or September, when the crop was at growth stage 92 (Zadoks et al. 1974), straw residues were collected from the trial area. The cereal crop was harvested, leaving the stub- ble as high as possible, to ensure minimal dam- age to the undersown crop. Soil was ploughed in 1991–1996 in late Oc- tober to ensure a sufficiently long growing sea- son for the undersown crops. In 1992 at Laukaa ploughing was not done until 7 December, after early snow cover had melted. In 1997 ploughing was done in late September, the normal plough- ing time in Finland. Measurements Plant samples (from 0.25 m2 per plot), including above-ground material of the undersown crop, were taken before autumn ploughing. Plants were cut with scissors at the base. Root samples from the 0–25 cm soil layer were taken immediately before autumn ploughing. The root samples were taken manually with a steel box and a spade (25 cm × 25 cm surface area) during the first three years and mechanically with a tractor bor- er (12.5 cm × 12.5 cm surface area) in later years. Roots were washed manually during the first three years and in later years with a hydropneu- matic root washer (Smucker et al. 1982). Organic matter was separated with forceps. Shoot and root samples were dried in an oven (2 hours at 105˚C and overnight at 60˚C), and dry matter, N concentration (%) and yield (kg ha-1) were meas- ured. Measured above-ground material includ- ed only biomass from undersown crops, but root biomass from plants other than cover crops was not separated from root samples. Shoot and root samples were taken in all undersowing years, except at Pälkäne in 1992 because of early snow cover and in 1993 because of almost total fail- ure to establish a cover crop. The effect of undersowing on soil NO3-N concentration before winter was studied by tak- ing annually repeated measurements from each subplot in four experimental phases: 1) the first year of undersowing, 2) additional years of un- dersowing 3) the first year after the last under- sowing year and 4) the second year after the last undersowing year. The experimental phases were chosen in order to bring out the different impli- cation of undersown crop in each phase. In Phase 1 only the cover crop of that year affected, in Phase 2 the effect of current and previous cover crops were mixed, and in Phase 3 only previous cover crops affected. The Phase 4 represents long-term effects after annually undersown cover crops. The soil samples (Table 1) were taken near the beginning of soil frosting, from late October to early December in different years, except ear- lier in autumn in the last year. Until 1995, samples were taken manually by mixing 16 cores from topsoil or six cores from subsoil samples. Because of difficulties in car- rying out complete sampling, the statistical anal- ysis included only a few years in Phase 2 (Ta- ble 1). From 1995, samples were taken mechan- ically by mixing 16 cores from topsoil or ten cores from subsoil samples. Using a machine for sampling after the start of soil frosting assisted in getting the planned samples. From 1995 samples were taken in early spring before soil thowing (Time 1), before sowing in spring (Time 2), at cereal harvest (Time 3), be- fore autumn ploughing (Time 4) and before win- ter, near the start of soil frost (Time 5). These samples were taken from 0 and 90 N treatments from control plots and plots containing white clover and westerwold ryegrass. Because of small amount of observations per each sampling time, there was a disagree between the statisti- cal analysis and the actual situation on field, and results are studied graphically. Soil samples were extracted with 2 M KCl. The nitrate (NO3-) nitrogen contents of the ex- tracts were analysed with a autoanalyser (air seg- mented flow analyser, photometric detection) and converted into kg ha-1 (Esala 1991). Information on weather conditions during the experimental years is given in detail by Känkä- nen et al. (2001). 169 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. 12 (2003): 165–176. Statistical methods Soil nitrate N concentration was analysed using a mixed model: Yijkl = µ + bi + Fj + eij + Uk + FUjk + fijk + Pl + gil + FPjl + hijl + UPkl + FUPjkl + sijkl where Yijkl is the response for block i, fertiliser rate j, undersowing k and period l; µ is the over- all mean; b is the random block effect; F, U and P are the fixed effects of fertiliser, undersowing and period, respectively; FU, FP and UP are the two-factor interactions of the fixed effects and FUP is the three-factor interaction; e, f, g, h and s are the random error terms. The random varia- bles bi, eij, fijk, gil, hijl and sijkl are assumed inde- pendent and normally distributed with zero means and constant variances. Furthermore, the error vectors sijk = (sijk1,…,sijkL) for soil nitrate N were assumed to be independent and multivari- ate normal with zero means and unstructured co- variance matrices Σ. The models were fitted by using the residual maximum likelihood (REML) estimation method. The degrees of freedom were approximated through the method introduced by Kenward and Roger (1997). Accordances of the data with the distributional assumptions of the models were checked graphically. The residuals were checked for normality using box plots (Tuk- ey 1977). In addition, the residuals were plotted against the fitted values. The PROC MIXED pro- cedure (Littell et al. 1996) of SAS/STAT soft- ware was used. Results Biomass of undersown cover crops Growth of undersown crops varied between years because of variable weather conditions, and among sites and treatments. In 1993 at Laukaa the dry matter yield of red clover, white clover and the mixture of red clover and meadow fes- cue was only 100–200 kg ha-1 before autumn ploughing, when high N fertilisation was used. However, generally undersowing was a success- ful means of establishing a cover crop, which yield was normally > 1000 kg ha-1. The yield of red clover, white clover and the mixture of red Table 1. Soil samples for NO 3 -N analysis taken before winter during four different phases when red clover, white clover, a mixture of red clover and meadow fescue and westerwold ryegras were undersown in spring cereals during six successive seasons at Pälkäne and Laukaa experimental sites. First year of Additional years of First year after Second year after undersowing undersowing discontinuation of discontinuation of undersowing undersowing Soil N fertili- N fertili- N fertili- N fertili- layer sation sation sation sation Site (cm) Cropsa) (kg ha-1) Cropsa) (kg ha-1) Years Cropsa) (kg ha-1) Cropsa) (kg ha-1) Pälkäne 0–30 1–5 0,30,60,90 95,96 1–5 0, 90 1–5 0,30,60,90 30–60 1,3,5 0, 90 95,96 1,3,5 0, 90 1,3,5 0, 90 60–90 1,3,5 0, 90 95,96 1,3,5 0, 90 1–5 0, 90 Laukaa 0–30 1–5 0,30,60,90 1–5 0,30,60,90 92,93,96 1–5 0, 90 1–5 0,30,60,90 30–60 1–5 0, 90 96 1–5 0, 90 1,3,5 0, 90 60–90 1–5 0, 90 96 1–5 0, 90 1,3,5 0, 90 a) 1 = no undersowing, 2–5 = spring cereals undersown in six years with 2 = red clover, 3 = white clover, 4 = mixture of red clover and meadow fescue, 5 = westerwold ryegrass. 170 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 Känkänen, H. et al. Effect of repeated undersowing on soil NO 3 -N Table 2. Average yield and N-% of above ground (shoots) and root dry matter of crops undersown annually in spring cereals during 1991–1996 at Pälkäne and Laukaa experimental sites. Pälkäne Laukaa N fertili- sation shoots roots shoots roots shoots roots shoots roots Undersown crop (kg ha-1) (kg ha-1) (kg ha-1) (N-%) (N-%) (kg ha-1) (kg ha-1) (N-%) (N-%) Red clover 0 1000 1860 3.05 2.21 920 1370 3.18 2.62 30 580 1400 3.09 1.80 820 1150 3.10 2.84 60 600 1250 3.12 1.69 530 780 3.29 2.83 90 460 1100 3.42 1.96 380 600 3.35 2.70 White clover 0 1780 2170 3.19 1.97 1230 590 3.58 2.48 30 1460 1850 3.14 1.77 860 420 3.42 2.61 60 1500 1350 3.31 1.88 660 290 3.52 2.78 90 1240 1620 3.45 1.86 540 400 3.58 2.58 Red clover & 0 700 2090 2.85 1.97 780 1110 3.20 2.47 meadow fescue 30 560 1350 2.97 1.27 670 840 3.02 2.70 60 760 1700 2.80 1.57 430 500 2.95 2.47 90 310 1410 3.00 1.44 400 700 2.81 2.14 Westerwold 0 800 1610 2.09 0.91 310 320 1.78 1.10 ryegrass 30 1190 2030 1.99 0.91 320 210 1.78 1.08 60 1340 1940 2.23 0.94 390 200 1.89 1.06 90 1190 2060 2.54 1.09 470 420 1.87 1.13 clover and meadow fescue decreased, and that of westerwold ryegrass slightly increased, with increasing N fertilisation. The above ground and root biomass N contents were highest for red and white clover and lowest for ryegrass. The root N concentrations were somewhat higher at Laukaa than at Pälkäne (Table 2). Soil NO 3 -N before winter At Pälkäne, the undersown crop did not have a significant main effect on soil NO3-N content before winter at 0–30 cm, but the effect varied between experimental phases (Table 3). In the 30–60 cm soil layer, differences associated with undersown crops were established (F2, 8.73 = 4.14, P = 0.054), and there was also a statistically sig- nificant interaction between phase and under- sown crop (Table 3). No statistically significant differences were found among samples from the 60–90 cm layer. There was no statistically sig- nificant interaction between N fertilisation and undersown crop at any soil layer or in any phase. At Laukaa, the undersown crop affected (a statistically significant main effect, F4, 26.7 = 5.61, P = 0.002) the NO3-N content in the 0–30 cm soil layer, but the effect depended on the exper- imental phase (Table 3). No significant effects were found at deeper soil layers. There was no statistically significant interaction between N fertilisation and undersown crop at any layer or for phase. For each case of a main effect of the under- sown crop there was an interaction between ex- perimental phase and undersown crop, too. Con- sequently, the effect of the undersown crop is not detailed at main effect level, but differences in each phase are expressed. In experimental Phase 1 samples were taken only at Laukaa from the 0–30 cm soil layer. Soil NO3-N content was quite low, 4.5 kg ha -1 (stand- ard error of means, SEM 0.71) without under- sowing. Westerwold ryegrass decreased soil 171 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. 12 (2003): 165–176. NO3-N content by 1.8 kg ha -1 as compared with no undersowing (Table 4). In experimental Phase 2, soil NO3-N content without undersowing at Pälkäne was 5.2, 7.4 and 5.5 kg ha-1 and at Laukaa 2.4, 2.5 and 1.5 kg ha-1 in soil layers 0–30, 30–60 and 60–90 cm, respec- tively. At Pälkäne in two upper soil layers west- erwold ryegrass decreased soil NO3-N content (Table 4). In the 60–90 cm layer at Pälkäne and the 30–90 cm layer at Laukaa there were no dif- ferences associated with different crops. In experimental Phase 3, soil NO3-N content without undersowing at Pälkäne was 8.7, 8.8 and 7.8 kg ha-1 and at Laukaa 10.0, 5.0 and 2.1 kg ha-1 in soil layers 0–30, 30–60 and 60–90 cm, respectively. At both sites there were signs of increased soil NO3-N contents after white clo- ver, but all differences were small and only once statistically significant (Table 4). In experimental Phase 4, soil NO3-N content without undersowing at Pälkäne was 7.6, 4.3 and 2.4 kg ha-1 and at Laukaa 7.2, 6.1 and 2.5 kg ha-1 in soil layers 0–30, 30–60 and 60–90 cm, respec- tively. At Laukaa a mixture of red clover and meadow fescue increased soil NO3-N content by 3.1 kg ha-1 at 0–30 cm (Table 4). Otherwise there were no differences associated with different crops. Soil NO 3 -N in different dates during final years Sampling time greatly affected the NO3-N con- tent of soil. The content was often clearly high- est before sowing in spring (Time 2), especially in 1996 at both sites and in 1997 at Pälkäne (Fig. 1). This was regardless of soil layer or N fertili- sation. Almost without exception there was more NO3-N with white clover than with westerwold ryegrass. However, when compared with no un- dersowing, the effect of an undersown crop var- ied among sampling dates. N fertilisation did not greatly affect the soil NO3-N. Only at Pälkäne there was 1–5 kg NO3-N ha -1 higher content per soil layer at 90 N than at 0 N on dates other than at sowing. However, the effect of an undersown crop was similar at both N rates. Table 3. Statistical significances of N fertiliser level x crop, crop x experimental phase and N level x crop x experimental phase interactions on soil nitrate N content (kg ha-1) before winter, when red clover, white clover, a mixture of red clover and meadow fescue and westerwold ryegras were undersown in spring cereals during six successive seasons at Pälkäne and Laukaa experimental sites. Layer (cm) N level x crop Crop x phase N level x crop x phase F-value P- F-value P- F-value P- and DF value and DF value and DF value Pälkäne 0–30 F 12, 32.3 = 0.83 0.620 F 8, 40 = 2.95 0.011 F 24, 40.1 = 1.02 0.464 30–60a, b) F 2, 7.0 = 1.13 0.377 F 4, 8.9 = 5.58 0.016 F 4, 4.9 = 1.90 0.250 60–90a, b) F 2, 8.5 = 1.27 0.330 F 4, 8.2 = 1.43 0.307 F 4, 8.2 = 2.20 0.157 Laukaa 0–30 F 12, 25.3 = 0.52 0.884 F 12, 33 = 2.14 0.042 F 36, 44.3 = 1.21 0.269 30–60b) F 4, 17.1 = 0.37 0.826 F 4, 16.7 = 0.74 0.581 F 4, 16.7 = 0.29 0.882 60–90b) F 4, 16.8 = 0.73 0.584 F 4, 12.4 = 1.10 0.398 F 4, 9.9 = 1.61 0.248 a) No undersowing, white clover and westerwold ryegrass included. b) 0 N and 90 N included. DF = degrees of freedom At Pälkäne phases 2, 3 and 4, and at Laukaa in 0–30 cm all phases and in 30–60 cm and 60–90 cm phases 2 and 3 are included. Phase 1 = the first year of undersowing, Phase 2 = additional years of undersowing, Phase 3 = the first year after the last undersowing year, Phase 4 = the second year after the last undersowing year. 172 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 Känkänen, H. et al. Effect of repeated undersowing on soil NO 3 -N Table 4. The differences and P values between estimated means of soil nitrate N (kg ha-1) before winter, when cereal cropping without undersowing is compared with using different undersown crops, in cases where the crop x experimental phase interaction had a statistical significance (P<0.05). Standard error of difference (SED) is presented for each comparison. Phase Undersown crop Pälkäne, 0–30 cm Pälkäne, 30–60 cm Laukaa, 0–30 cm estimate P value estimate P value estimate P value 1 red clover –0.6 0.203 white clover –0.3 0.434 R.C. & M.F. –0.8 0.093 westerwold ryegrass –1.8 <0.001 SED ––0.43 2 red clover –0.5 0.231 –0.4 0.197 white clover –0.2 0.518 –0.2 0.745 –1.1 0.001 R.C. & M.F. –0.6 0.141 –0.7 0.016 westerwold ryegrass –1.8 <0.001 –3.3 <0.001 –0.4 0.139 SED –0.37 –0.5 ––0.27 3 red clover –0.5 0.347 –1–– 0.133 white clover –0.8 0.148 –1.8 0.367 –1.5 0.027 R.C. & M.F. –0.5 0.309 –1.3 0.052 westerwold ryegrass –0.2 0.693 –0.7 0.706 –0.1 0.832 SED –0.53 –1.84 ––0.62 4 red clover –0.1 0.885 –1.7 0.115 white clover –0.8 0.208 –0.3 0.615 –1.6 0.152 R.C. & M.F. –0.7 0.287 –3.1 0.006 westerwold ryegrass –0.1 0.819 –0.7 0.219 –0.2 0.844 SED –0.65 –0.5 ––1.06 Phase 1 = the first year of undersowing, Phase 2 = additional years of undersowing, Phase 3 = the first year after the last undersowing year, Phase 4 = the second year after the last undersowing year. R.C. & M.F. = mixture of red clover and meadow fescue. In cases with no values, no soil sampling was done. At Pälkäne, the westerwold ryegrass de- creased soil NO3-N content cleary in autumn and winter 1995–1996, after a high ryegrass yield in 1995. The effect was greater at 90 N than at 0 N. The above ground dry matter yield of wester- wold ryegrass was 2220 kg ha-1 at 0 N and 3640 kg ha-1 at 90 N in 1995, but only 120 kg ha-1 in 1996 at both N levels. White clover increased the NO3-N content clearly in 1997 at spring sow- ing, although the growth of clover was weak in 1996. The above ground dry matter yield of white clover before ploughing was 4020 kg ha-1 and 1050 kg ha-1 at 0 N, and 2850 kg ha-1 and 50 kg ha-1 at 90 N, in 1995 and 1996, respectively. At Laukaa, white clover increased the mean NO3-N content in 1996 before sowing in spring (Time 2). However, only two replicates were sampled, and the difference was mainly attrib- utable to one of them. The effect of westerwold ryegrass on decreasing soil NO3-N was record- ed in the last year of the experiment when no N fertilisation was given. This was the only marked long term or N fertilisation effect established in this study. Discussion Risk for N leaching can be decreased with un- dersowing with grasses, although effect of west- 173 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. 12 (2003): 165–176. erwold ryegrass on soil NO3-N content was of- ten small, probably due to poor growth after ce- real harvest. However, when the N leaching risk was high in autumn 1995 – spring 1996 at the 90 N level in Pälkäne, the ryegrass grew vigor- ously in autumn and markedly reduced the amount of NO3-N in soil. Italian ryegrass, which grows more reliably in autumn than westerwold ryegrass, was shown by Lemola et al. (2000) to reduce N leaching by 27 to 68% depending on soil type. However, the undersown ryegrass can reduce soil mineral N already during main crop growth, as reported by Breland (1996). Conse- quently, the competitive effect of the undersown crop should be taken into account. In these ex- periments cereal grain yield was decreased sub- Pälkäne NO 3 -N, kg ha -1 0 N Crop Time Year Depth 0-30 cm 30-60 cm 60-90 cm 0 10 20 30 40 50 60 70 80 90 3 1995 4 5 1 1996 2 3 4 5 1 1997 2 3 4 5 1 1998 2 1 3 5 135 1 3 5 1 3 5 1 3 5 1 35 1 35 135 13 5 1 3 5 1 3 5 1 3 5 1 3 5 1 35 1 3 5 Laukaa NO 3 -N, kg ha -1 0 N Crop Time Year Depth 0-30 cm 30-60 cm 60-90 cm 0 10 20 30 40 50 60 70 80 90 3 1995 4 5 1 1996 2 3 4 5 1 1997 2 3 4 5 1 1998 2 1 3 5 135 1 3 5 1 3 5 1 3 5 1 35 1 35 135 13 5 1 3 5 1 3 5 1 3 5 1 3 5 1 35 1 3 5 NO 3 -N, kg ha -1 90 N Crop Time Year Depth 0-30 cm 30-60 cm 60-90 cm 0 10 20 30 40 50 60 70 80 90 3 1995 4 5 1 1996 2 3 4 5 1 1997 2 3 4 5 1 1998 2 1 35 13 5 1 3 5 135 1 3 5 135 1 3 5 1 3 5 13 5 1 3 5 13 5 1 3 5 135 1 3 5 1 35 NO 3 -N, kg ha -1 90 N Crop Time Year Depth 0-30 cm 30-60 cm 60-90 cm 0 10 20 30 40 50 60 70 80 90 3 1995 4 5 1 1996 2 3 4 5 1 1997 2 3 4 5 1 1998 2 1 35 13 5 1 3 5 135 1 3 5 135 1 3 5 1 3 5 13 5 1 3 5 13 5 1 3 5 135 1 3 5 1 35 Fig. 1. Soil NO 3 -N (kg ha-1) at different sampling times from early autumn in 1995 to spring in 1998 when spring cereals were annually undersown in 1991–1996 at Pälkäne and Laukaa. Three soil layers (0–30, 30–60 and 60–90 cm) and two fertiliser N levels (0 and 90 kg ha-1). Crop: 1 = no undersowing, 3 = white clover undersown and 5 = westerwold ryegrass undersown. Time: 1 = in March before soil thawing, 2 = in May before sowing, 3 = between September 13 and 25 after cereal harvest, 4 = in October before autumn ploughing and 5 = in November near soil frosting. 174 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 Känkänen, H. et al. Effect of repeated undersowing on soil NO 3 -N stantially due to westerwold ryegrass (Känkänen et al. 2001). Drainage runoff and nitrate N leaching in Finland are normally highest in November, De- cember and April (unpublished data from two leaching experiment fields at MTT). It suggests that sampling time in late autumn was accepta- ble for define N leaching risk, although leach- ing occurs between cereal harvest and late Oc- tober, too. In addition, soil N analyses in four last years with increased sampling dates showed, that the amount of nitrate N was often largest in spring. Obviously the mineralisation was high in spring, and nitrate N was found also in deep- er layers because of water flow through soil af- ter snow and soil thawing. This suggests, that a cover crop should have capability to catch N also in spring, contrary to westerwold ryegrass, in case of the succeeding crop is late to catch the mineralised N. When clovers are used for fixing N for sub- sequent crops, there is a risk for increased N leaching. Also in this study white clover tended to increase the soil NO3-N content, but did not greatly increase N leaching risk, according to soil NO3-N measurements taken near soil frosting. Undersown red clover did not increase N leach- ing risk. The soil NO3-N contents were similar in a mixture of meadow fescue and red clover as in a pure stand of red clover, although non-legu- minous residues in a mixture normally decrease net N mineralisation as compared with pure le- guminous residues (Kuo and Sainju 1998). Sim- ilar soil NO3-N content as in pure red clover was however to some extent anticipated, because clo- ver dominated in the mixture. The negligible in- crease of N leaching risk in connection with un- dersowing clovers supports the use of clovers to increase the cereal grain yield, as they did here (Känkänen et al. 2001). Late autumn ploughing probably resulted in fairly low soil NO3-N after legumes. Känkänen et al. (1998) reported that delaying incorpora- tion of N-rich crops decreased the N leaching risk under Finnish conditions, as also reported by Gustafson (1987) in Sweden and Sanderson and MacLeod (1994) in Canada. In the present study the incorporation took place very late, when the mean air temperature between plough- ing and soil frost was near 0˚C, and ploughing was often done after the first soil frost. A low mineralisation rate, resulting from late incorpo- ration, is also an obvious reason why higher N yield of legumes at low fertiliser N rates did not lead to more soil NO3-N. Because high amounts of inorganic N on soils with continuos cereals are found only sometimes (Esala 2002), like here in 1995–1996 in Pälkäne, also the decreasing effect of a grass cover crop on N content of soil is only occasionally sub- stantial. Problem is how to identify the situation, when N cathing cover crop should be used. Soil type is a significant factor in decision making. It was found by Lemola et al. (2000), that silt, similar to Laukaa in this study, has very low ten- dency to NO3-N leaching compared with sand and peat soils. Also the regional need for inhibit eutrophication of waters (Rekolainen et al. 1995) or protect drinking water should be taken into account. On the other hand the results suggest, that undersowing clovers is safe, if only avoided in situations mentioned above. Moreover, the significance of soil NO3-N content varies accord- ing to the risk of leaching and the changing re- quirements of main crops during the year. The highest amounts of soil NO3-N were found at sowing, indicating establishment of conditions for high use of mineralised N by the succeeding crop (Esala 2002). Hiitola and Eltun (1996) found greater de- crease of soil mineral N at 120 N as compared with at zero N fertilisation for undersown Ital- ian ryegrass, but here the effect of westerwold ryegrass was normally similar independent of N fertilisation rate. However, the highest N fertili- sation level (90 N) in our study did not exceed the recommended amount for a cereal crop (Esala 1991), which decreased residual N in the soil after harvest and the need for N catchment in autumn. Hansen et al. (2000) reported increased leach- ing after long-term (24 years) undersowing with perennial ryegrass, but no increase in soil NO3- N contents was found in the present study. The 175 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. 12 (2003): 165–176. six-year undersowing period was probably too short for increased mineralisation. On the con- trary, when no N fertilisation was given, the soil NO3-N at Laukaa in the last year of the experi- ment was lower after westerwold ryegrass than after no undersowing. This is in agreement with the findings of Schröder et al. (1997), who re- ported that ploughing unfertilised Italian rye- grass immobilized soil mineral N. Acknowledgements. We wish to thank Prof. T. Mela, who essentially contributed to the start and contents of this study. We express our gratitude towards all those people who as- sisted with this research in the field and in the laboratory. The research was funded by the Ministry of Agriculture and Forestry. References Alvenäs, G. & Marstorp, H. 1993. Effect of a ryegrass catch crop on soil inorganic-N content and simulated nitrate leaching. Swedish Journal of Agricultural Re- search 23: 3–14. Beck-Friis, B., Lindén, B. & Marstorp, H. 1993. Nitrogen in crops and soil in cultivation systems with autumn or spring ploughing and with and without catch crops. In: Elonen, P. & Pitkänen, J. (eds.). Soil till- age and environment. Proceedings of NJF seminar no. 228. Jokioinen, Finland, 8–10 June 1993. p. 133– 144. Breland, T.A. 1996. Green manuring with clover and rye- grass catch crops undersown in small grains: effects on soil mineral nitrogen in field and laboratory ex- periments. Acta Agriculturae Scandinavica, Section B, Soil and Plant Science 46: 178–185. Carter, M.R. & Kunelius H.T. 1993. Effect of undersowing barley with annual ryegrasses or red clover on soil structure in a barley-soybean rotation. Agriculture, Ecosystems and Environment 43: 245–254. Egelkraut, T.M., Kissel, D.E. & Cabrera, M.L. 2000. Effect of soil texture on nitrogen mineralized from cotton residues and compost. Journal of Environmental Quality 29: 1518–1522. Esala, M. 1991. Split application of nitrogen: effects on the protein in spring wheat and fate of 15N-labelled nitrogen in the soil-plant system. Annales Agricultur- ae Fenniae 30: 219–309. Esala, M. 2002. Inorganic nitrogen content in soil in spring as a tool for predicting optimal fertilization. In: Opti- mal nitrogen fertilization – tools for recommendation. Proceeding from NJF seminar 322, Ås, March 29– 30, 2001. p. 113–117. Gustafson, A. 1987. Nitrate leaching from arable land in Sweden under four cropping systems. Swedish Jour- nal of Agricultural Research 17: 169–177. Hansen, E.M., Djurhuus, J. & Kristensen, K. 2000. Ni- trate leaching as affected by introduction or discon- tinuation of cover crop use. Journal of Environmen- tal Quality 29: 1110–1116. Hiitola, K. & Eltun, R. 1996. The effect of undersown cov- er crops on grain yield and the nitrogen content of plants and soil. Norsk landbruksforskning 10: 211– 220. Jensen, E.S. 1991. Nitrogen accumulation and residual effects of nitrogen catch crops. Acta Agriculturae Scandinavica 41: 333–344. Jensen, E.S. 1992. The release and fate of nitrogen from catch-crop materials decomposing under field con- ditions. Journal of Soil Science 43: 335–345. Känkänen, H., Eriksson, E., Räkköläinen, M. & Vuorinen, M. 2001. Effect of annually repeated undersowing on cereal grain yields. Agricultural and Food Science in Finland 10: 197–208. Känkänen, H., Kangas, A., Mela, T., Nikunen, U., Tuuri, H. & Vuorinen, M. 1998. Timing incorporation of dif- ferent green manure crops to minimize the risk of ni- trogen leaching. Agricultural and Food Science in Finland 7: 553–567. Kenward, M.G. & Roger, J.H. 1997. Small sample infer- ence for fixed effects from restricted maximum likeli- hood. Biometrics 53: 983–997. Kolenbrander, G.J. 1969. Nitrate content and nitrogen loss in drainwater. Netherlands Journal of Agricultural Science 17: 246–255. Kuo, S. & Sainju, U.M. 1998. Nitrogen mineralization and availability of mixed leguminous and non-leguminous cover crop residues in soil. Biology and Fertility of Soils 26: 346–353. Lemola, R., Turtola, E. & Eriksson, C. 2000. Undersow- ing Italian ryegrass diminishes nitrogen leaching from spring barley. Agricultural and Food Science in Fin- land 9: 201–215. Littell, R.C., Milliken, G.A., Stroup, W.W. & Wolfinger, R.D. 1996. SAS system for mixed models. Cary, NC, USA: SAS Institute Inc. 633 p. Marstorp, H. & Kirchmann, H. 1991. Carbon and nitrogen mineralization and crop uptake of nitrogen from six green manure legumes decomposing in soil. Acta Agriculturae Scandinavica 41: 243–252. Rekolainen, S., Pitkänen, H., Bleeker, A. & Felix, S. 1995. Nitrogen and phosphorus fluxes from Finnish agri- cultural areas to the Baltic Sea. Nordic Hydrology 26: 55–72. Sanderson, J.B. & MacLeod, J.A. 1994. Soil nitrate pro- file and response of potatoes to fertilizer N in rela- tion to time of incorporation of lupin (Lupinus albus). Canadian Journal of Soil Science 74: 2, 241–246. Schröder, J.J., Ten Holte, L. & Janssen, B.H. 1997. Non- overwintering cover crops: a significant source of N. Netherlands Journal of Agricultural Science 45: 231– 248. Smucker, A.J.M., Mc Burney, S.L. & Srivastava, A.K. 1982. Quantitative separation of roots from compacted soil profiles by the hydropneumatic elutriation system. Agronomy Journal 74: 500–503. 176 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 Känkänen, H. et al. Effect of repeated undersowing on soil NO 3 -N Thorup-Kristensen, K. 1996. Effect of catch crop incor- poration time on N availability for a succeeding crop. In: Schröder, J.J. (ed.). Long term reduction of nitrate leaching by cover crops. First Progress Report of EU Concerted Action (AIR3) 2108. p. 49–54. Tukey, J.W. 1977. Exploratory data analysis. Reading: MA, Addison-Wesley. 688 p. Turtola, E. & Kemppainen, E. 1998. Nitrogen and phos- phorus losses in surface runoff and drainage water after application of slurry and mineral fertilizer to perennial grass ley. Agricultural and Food Science in Finland 7: 569–581. Viljan aluskasveja käytetään sekä typen huuhtoutu- misen estämiseen että typen tuottamiseen. Ensin mai- nittuun tarkoitukseen sopivia ovat heinät ja jälkim- mäiseen ilmakehästä typpeä sitovat apilat. Tutkimuk- sessa selvitettiin, miten toistuva aluskasvien käyttö vaikuttaa herkästi huuhtoutuvan nitraattitypen mää- rään maassa kahdella huuhtoutumisherkkyydeltään erilaisella maalajilla. Aluskasvien merkitystä typen huuhtoutumisriskin kannalta arvioitiin myöhäissyk- syllä otettujen maanäytteiden avulla. Lisäksi tutki- muksen loppuvuosina maan nitraattitypen muutoksia seurattiin ottamalla näytteitä typen huuhtoutumisen ja käytön kannalta kriittisinä ajankohtina. MTT:n Laukaan ja Pälkäneen tutkimusasemilla viljeltiin puna-apilaa, valkoapilaa, puna-apilan ja nur- minadan seosta sekä westerwoldin raiheinää kuusi vuotta toistuvasti kevätviljan aluskasvina. Nitraatti- typen määrä maassa loppusyksyllä ei yleensä ollut suuri, eikä myöskään aluskasvien vaikutus siihen. Vaikka valkoapila usein lisäsi ja westerwoldin raihei- nä vähensi nitraattitypen määrää, niiden merkitys ty- pen huuhtoutumisriskin kannalta oli normaalisti vä- häinen. Raiheinä kuitenkin vähensi nitraattitypen määrää selvästi silloin, kun olot sekä heinän kasvun että huuhtoutumisen kannalta olivat edulliset. Wallgren, B. & Lindén, B. 1994. Effects of catch crops and ploughing times on soil mineral nitrogen. Swed- ish Journal of Agricultural Research 24: 67–75. Yli-Halla, M., Mokma, D.L., Peltovuori, T. & Sippola, J. 2000. Suomalaisia maaprofiileja. Abstract: Agricultur- al soil profiles in Finland and their classification. Pub- lications of Agricultural Research Centre of Finland. Serie A 78. 104 p. Zadoks, J.C., Chang, T.T. & Konzak, C.F. 1974. A deci- mal code for the growth stages of cereals. Weed Research 14: 415–421. SELOSTUS Aluskasvien toistuvan käytön vaikutus maan nitraattityppeen viljan viljelyssä Hannu Känkänen, Christian Eriksson, Mauri Räkköläinen ja Martti Vuorinen MTT (Maa- ja elintarviketalouden tutkimuskeskus) Aluskasvimenetelmään liittyvä myöhäinen syys- kyntö oli ilmeisesti osasyy siihen, että apiloiden typpi ei ehtinyt vapautua huuhtoutumiselle alttiiseen muo- toon ennen talven tuloa. Yleensäkin nitraattitypen määrä oli suurimmillaan kevätkylvön aikaan, eli typpi mineralisoitui lähellä optimaalista ajankohtaa viljan käytön kannalta. Toisaalta nitraattitypen määrän voi- makas kasvu roudan sulamisen ja kylvön välillä voi merkitä myös huuhtoutumisriskiä. Typen keräämisen kannalta voisikin herkästi huuhtoutuvilla mailla olla enemmän hyötyä talvehtivasta heinäkasvista kuin yk- sivuotisesta raiheinästä. Pitkäaikaisvaikutuksia ei kuusi vuotta kestäneestä aluskasvien käytöstä havait- tu. Vaikka typen huuhtoutuminen Suomen oloissa on usein pientä verrattuna eteläisempiin viljelyalueisiin, on täälläkin tilanteita, jolloin huuhtoutuminen on merkittävää. Huuhtoutumisen riskiä voidaan pienen- tää viljelemällä heinää viljan aluskavina. Apiloita voidaan toistuvastikin viljellä aluskasveina sitomaan viljojen käyttöön typpeä ilman että typen huuhtou- tumisen riski oleellisesti kasvaa aluskasvittomaan vil- jelyyn nähden. Syyskyntö on silloin tehtävä niin myö- hään kuin maan rakennetta vaarantamatta on mahdol- lista. Soil nitrate N as influenced by annually undersown Introduction Material and methods Results Discussion References SELOSTUS