Effects of undersown crops on soil mineral N and grain yield of spring barley

Transcription

1 Europ. J. Agronomy 27 (2007) Effects of undersown crops on soil mineral N and grain yield of spring barley Hannu Känkänen a,, Christian Eriksson b a MTT Agrifood Research Finland, Plant Production Research, FIN Jokioinen, Finland b MTT Agrifood Research Finland, Data and Information Services, FIN Jokioinen, Finland Received 5 July 2005; received in revised form 28 December 2006; accepted 11 January 2007 Abstract Undersowing a cereal crop can reduce nitrogen (N) leaching and increase available N for the successive crop. An undersown crop can also compete with the main crop. Seventeen plant species were undersown in spring barley (Hordeum vulgare L.) to study their suitability regarding establishment, biomass production, competition with the main crop and effects on soil mineral N. Three different seeding rates were evaluated. Italian ryegrass (Lolium multiflorum Lam.) decreased nitrate nitrogen (NO 3 -N) content in late autumn and timothy (Phleum pratense L.) in the succeeding spring. A mixture was optimal to reduce N leaching. Italian ryegrass is a very competitive species that should be undersown at moderate seeding rates to avoid large yield reduction in the main crop. Black medic (Medicago lupulina L.) slightly increased N leaching risk, but red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) did not increase soil NO 3 -N content. As clovers did not compete strongly with the main crop, fairly high seeding rates can be used to maximise N fixation to benefit the successive crop Elsevier B.V. All rights reserved. Keywords: Cereal; Cover crop; Grass; Intercropping; Legume; Soil ammonium nitrogen; Soil nitrate nitrogen 1. Introduction Catch crops are established in northern latitudes by undersowing in a cereal crop. This reduces N leaching (Alvenäs and Marstorp, 1993) and increases available N for the successive crop (Wallgren and Lindén, 1994). Competition between the undersown and the main crop can however greatly reduce the crop growth as a result of deficits of soil moisture, light and plant nutrients (Gist and Mott, 1957). Legumes can supply N to an associated crop, although in the year of crop establishment the supply is modest (Burity et al., 1989). On the other hand, legumes, as with non-leguminous crops, can deplete soil N(Ryle et al., 1979) and thus compete for it with the main crop. Soil mineral N measurements made during autumn and spring represent an important means to evaluate the usefulness of both leguminous and non-leguminous cover crops. The soil nitrate N content before winter is regarded as an indicator of N leaching risk in Nordic countries (Breland, 1996; Känkänen et al., 1998). Corresponding author. Tel.: ; fax: address: hannu.kankanen@mtt.fi (H. Känkänen). During a mild winter, N leaching can occur due to percolation of water in the soil, but in cold winters water conductivity in soil is low due to frost (Turtola and Kemppainen, 1998) and large volumes of soil water can drain in the spring when the soil thaws and snow melts. In both cases mineralisation and movement of N from the top soil to deeper soil horizons occurs before the beginning of the next growing season. Incorporation of cover crops and its timing greatly affect the rate of N release from cover crops and delaying autumn incorporation decreases N leaching risk (Gustafson, 1987; Sanderson and MacLeod, 1994). We however did not till the soil at all, in order to measure the effect of undisturbed cover crop on soil N until the beginning of the succeeding growing season. Westerwold ryegrass (Lolium multiflorum Lam. var westerwoldicum) was used in our earlier studies as an undersown crop (Känkänen et al., 2001). However, it was very competitive with the main crop, as reported by Karlsson-Stresse et al. (1996). They also reported that it unfortunately flowered during the initial year. Perennial species however had many desirable characters. We sought grass species more suitable for undersowing than westerwold ryegrass. White clover and red clover proved to be suitable N-fixing crops for undersowing (Känkänen et al., 2001, 2003). How /$ see front matter 2007 Elsevier B.V. All rights reserved. doi: /j.eja

2 26 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) ever, species with different growth habit were tested to identify alternative or better performing legumes for undersowing. Also species from families other than the Poaceae and Fabaceae were assessed as alternatives for grass catch crops. Few studies have been done to determine the optimal seeding rate of different species when undersown in a cereal crop in Finland (Kauppila, 1983). In spite of the different purposes of cover crops and forage crops, knowledge about establishment of the latter was exploited when the so termed normal seeding rate of an undersown crop was estimated here. When forage grasses and legumes are established with a companion spring cereal crop, the seeding rate of the companion crop is normally reduced to ensure good establishment of the forage crop (Decker et al., 1973). We did not use a reduced seeding rate for the main crop because undersowing should not result in reduced yield of the companion crop. Our aim was to identify undersown crops that effectively catch soil N or fix atmospheric N without increasing N leaching and with minimal negative effects on the main crop. Determination of optimal seeding rates was an additional goal. 2. Materials and methods Six experiments were established in in Jokioinen (60 49 N, E) in different fields. In total, 17 plant species were undersown in spring barley (Hordeum vulgare L.) to study their establishment, biomass production, competition with the main crop and effects on soil mineral N. Ten species were rejected during the first 3 years, and the seven most promising species were studied further in the following years. Three different seeding rates for undersown crops were tested. The standard seeding rate was determined based on knowledge of each species when grown in mixtures and in pure stands. The low seeding rate was half the standard rate and the high seeding rate was double the standard rate. Barley without undersowing was used as a control. The species and seeding rates of undersown crops are given in Table 1. The soils were tentatively classified according to the FAO/Unesco system: 1995 Eutric or Gleyic Cambisol, 1996 Eutric Gleysol, 1997 and 1999 Eutric Cambisol or Eutric Regosol, and 1998 Dystric Regosol (Yli-Halla et al., 2000). Soil type, ph and C content (%) were correspondingly: 1995 silty clay, 6.04, and 3.03, 1996 clay, 6.29 and 3.73, 1997 coarse silt/very fine sand, 6.81 and 2.87, 1998 fine sand, 6.12 and 2.45, and 1999 coarse silt/very fine sand, 6.06 and The experiments were designed as split-plot trials with seeding rates as the main plots (18 37 m 8 m, depending on year), arranged as a randomised complete block with four replicate blocks. The split-plot treatments (undersown crops) were randomised among the subplots (1.5 m 8 m) within each main plot. There was a 70 cm wide area of pure barley between all undersown crops. Sowing was done at such soil moisture as enabled as good a workability of soil as possible and successful establishment of spring cereal growth. In this was between 7 and 20 May. In 1995 sowing was delayed until 31 May 2 June because of an exceptionally rainy May. The seedbed was prepared with an S-tine harrow to 5 cm depth. Spring barley was sown using a combined drill adjusted to this depth. The seeding rate for barley was the standard used in Finland, 450 seeds m 2. Undersowing with an Oyjord experimental sower was done across the cereal rows to 1 2 cm depth, except with rye (Secale cereale L.) and wheat (Triticum aestivum L.) when the depth was 3 4 cm. Undersowing was done after cereal sowing on the same day or on the following day (2 days after cereal sowing in 1995). The plots that were not undersown remained undisturbed during this phase. The row distance for both main crop and undersown crops was 12.5 cm. The trial areas were rolled with a continental Cambridge roller before and after undersowing, except in the wet spring of Table 1 Undersown crop species, seeding rates and year of crop establishment for a series of experiments undertaken at Jokioinen during Undersown crop Seeds (m 2 ) Years Low Standard High White clover (Trifolium repens L.) Red clover (Trifolium pratense L.) Black medic (Medicago lupulina L.) Goat s rue (Galega officinalisl.) Meadow fescue (Festuca pratensis Huds.) Tall fescue (Festuca arundinacea Schreb.) Cocksfoot (Dactylis glomerata L.) Red fescue (Festuca rubra L.) English ryegrass (Lolium perenne L.) Timothy (Phleum pratense L.) Westerwold ryegrass a Italian ryegrass (Lolium multiflorum Lam.) Winter wheat (Triticum aestivum L.) Winter rye (Secale cereale L.) Chicory (Cichorium intybus L.) Caraway (Carum carvi L.) Sainfoin (Onobrychis viciifolia Scop.) a Lolium multiflorum Lam. var westerwoldicum.

3 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) NPK fertiliser, the composition of which varied during years, was applied at a depth of 7 9 cm at the time of barley sowing. Phosphorus and potassium were applied according to the need of spring barley at the entire experimental area, so that the amount of P was 22 kg ha 1 in 1995 and 1996, 51 kg ha 1 in 1997 and 18 kg ha 1 in 1998 and 1999, and the amount of K 44, 57 and 32 kg ha 1, respectively. Nitrogen was applied at slightly (10 kg ha 1 ) less than the recommended rate to prevent lodging, 80 kg ha 1 in 1995, 1996 and 1997, and 90 kg ha 1 in 1998 and Weeds were controlled with 2.5 l ha 1 Basagran 480 (bentazon 480 g l 1 ) herbicide in all years except The entire experimental area was sprayed with bentazon according to the demands of the most sensitive undersown species, white clover, when the clover had 1 3 leaves and the growth stage of barley was between 13 and 23 (Zadoks et al., 1974). The cereal crop was harvested with a combine harvester at growth stage 92 (Zadoks et al., 1974), between 15 August and 3 September. The stubble was left higher than normally, cm, and straw residues were removed to ensure minimal damage to and good growth potential for the undersown crop. The cover crop was allowed to grow until the end of each trial in the following spring. After the cereals had been dried in an air stream (+40 C) and then sorted, the grain yield was recorded in kg ha 1 at 15% grain moisture. Nitrogen content (%) of grains was measured. Plant samples (0.25 m 2 per plot) were taken at barley harvest and at the end of the growing season. This included aboveground material of the undersown crop at both times and of barley the first time. Plants were cut with scissors at their base. At the second plant sampling root samples from the 0 25 cm soil layer were also taken, but only from plots sown at the normal seeding rate. The root samples were taken mechanically with a tractor borer (12.5 cm 12.5 cm surface area). Roots were washed with a hydropneumatic root washer (Smucker et al., 1982). Organic matter was separated with forceps. Shoot and root samples were dried in an oven (2 h at 105 C and overnight at 60 C), and dry matter yields (kg ha 1 ) were recorded. The root biomass was not sorted on a single plant basis. Nitrogen contents (%) of roots and mixed above-ground biomass of cover crop and weeds were measured. The planned plant sampling procedure was not realized in 1995 because of very uneven plant growth, which resulted from an exceptionally rainy early summer. In 1996 grain yield and samples until harvest were taken, but samples from late autumn to the following spring were not because of a mistake made in ploughing. Because the first two trials were unsuccessful they were repeated for an extra three years in which all planned samplings were successful. The effect of undersowing on soil mineral N (kg ha 1 )was studied by taking soil samples before sowing in spring, at cereal harvest, in late autumn after the first soil frost and at the start of the following growing season. The first sample reflected the original state of mineral N and described the natural variation across the field. The second sample described the period of highest leaching risk, and the last two samples were taken to study the risk of N leaching. Samples were taken from 0 to 30 cm soil layer at all dates and in all plots, but also from 30 to 60 and 60 to 90 cm depths at the last two dates in plots sown at the normal seeding rate for cover crops. Soil samples collected when the ground was not frozen were taken manually from topsoil by mixing 16 cores and mechanically from subsoil by mixing ten cores. When the ground was frozen, all samples were taken mechanically by mixing ten cores. Soil samples were extracted with 2 M KCl. The nitrate (NO 3 ) and ammonium (NH 4 + ) nitrogen contents of the extracts were autoanalysed (air segmented flow analyser, photometric detection) and converted into kg ha 1 (Esala, 1991). Soil mineral N content (SMN) was got by adding up NO 3 -N and NH 4 -N. For soil N data, two different statistical analyses were done. Analysis 1 included data for all seeding rates in the 0 30 cm soil layer and Analysis 2 included data for all soil layers at the normal seeding rate. Examination of soil NO 3 -N was done on the grounds of both analyses at late autumn and following spring, because of those dates best describes the N leaching risk. Similarly, examination of SMN was done on the grounds of analysis 1 at barley harvest and following spring, in order to describe the state of plant available N in topsoil after the growth of the main crop and before the growth of the hypothetical succeeding crop. Statistical analyses of the two different sampling dates for plant samples data, grain yield data and three different sampling dates for soil N data in Analysis 1 were based on the following mixed model: y ijkl = μ + e i + b j(i) + S k + d ik + f j(i)k + C l + g il + h j(i)l + SC kl + k ikl + ε ijkl, (1) where y ijkl is the response for block j in the experiment i, seeding rate k and undersown crop l; μ is the overall mean; S, C, and SC are the fixed effects of seeding rate, undersown crop and their interaction, respectively; e was considered as a random effect of an experiment; b is the random block effect; d, g and k are the random effects representing interactions between the random experiment effect e and the fixed effects of S, C, and SC, respectively; f and h are the random effects representing interactions between the random block effect b and the fixed effects of S and C, respectively; ε is the random error term. The random effects e i, b j(i), d ik, f j(i)k, g il, h j(i)l, k jkl and ε ijkl are all assumed to be mutually independent and normally distributed with zero means and variances σ 2 e,σ2 b,σ2 d,σ2 f,σ2 g,σ2 h,σ2 k,σ2 ε, respectively. Because roots were sampled only at the normal seeding rate, the model for root dry matter yield and N concentration was reduced to the following form: y ijl = μ + e i + b j(i) + C l + g il + h jil (2) The model assumptions were similar to those of model (1) above. For soil Analysis 2, data from the two sampling dates were analysed separately. Statistical analyses of the data were based on the linear mixed model for a series of experiments with a randomized complete block design and repeated measurements. The model equation was otherwise similar to that of model (1), except that the fixed effects were undersown crop, soil layer and their interaction. The covariance structure of the repeated measurements was unstructured allowing the repeated measure-

4 28 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) ments from the three soil layers to be correlated and to have different variances. The models were fitted by using the residual maximum likelihood (REML) estimation method. The degrees of freedom were computed by the method described by Kenward and Roger (1997). The analyses were performed using the MIXED procedure of SAS/STAT software (Littell et al., 1996). The residual analyses were carried out to check the assumptions of the models. The residuals were checked for normality using box plots (Tukey, 1977). In addition, the residuals were plotted against the fitted values. Comparisons of the treatments were made using two-sided t-type tests. Logarithmic transformations were used to normalize SMN data and square root transformations were applied to above-ground dry matter and N yield data because those response variables were not normally distributed. 3. Results 3.1. Dry matter and N yield of undersown crop Seeding rate affected the dry matter and N yield of the undersown crop (Table 2). Also the undersown crop had an effect on those traits, except on above-ground N yield in late autumn. The growth of legumes under barley, measured as aboveground biomass at barley harvest, was modest (Table 3). There were both low and high yielding grass species. Meadow fescue (Festuca pratensis Huds.), English ryegrass (Lolium perenne L.) and tall fescue (Festuca arundinacea Schreb.) grew poorly (2 25 kg ha 1 at harvest during initial years and kg ha 1 at late autumn in 1997), and were rejected after Timothy (Phleum pratense L.) was studied further because it grew somewhat better than other perennial grasses. Annual and biennial grasses grew best. The above-ground dry matter yield of winter wheat and Italian ryegrass at barley harvest was higher (P < 0.001) than that of legumes and timothy at all seeding rates. The yield of westerwold ryegrass was also higher than that of legumes and timothy (P < 0.05 to P < 0.001). At late autumn, the dry matter yield of Italian ryegrass was higher than that of legumes and timothy (P < 0.05 to P < 0.01), except when compared with black medic at a high seeding rate. The dry matter yield of Italian ryegrass was also higher than the yield of westerwold ryegrass and winter wheat at standard and high seeding rates (P < 0.05). The N yield of the above-ground biomass at harvest varied among grass species (Table 3). Winter wheat and Italian ryegrass yielded more N than westerwold ryegrass and timothy at all seeding rates (P < 0.001, except P < 0.05 between Italian and westerwold ryegrass at low and high seeding rates). N yields of legumes in late autumn were similar or even higher than those of grasses (Table 3). The N yields of white clover and black medic at the high seeding rate exceeded the N yields of Table 2 Statistical significances of seeding rate, crop species and seeding rate crop species interaction for different response variables when white clover, red clover, black medic, westerwold ryegrass, timothy, winter wheat and Italian ryegrass were undersown in spring barley in a series of experiments undertaken at Jokioinen during Response variable Effect Degrees of freedom numerator Degrees of freedom denominator F-Value P-Value Biomass a at harvest Seeding rate <0.001 Undersown crop <0.001 Seeding rate crop Biomass a in late autumn Seeding rate <0.001 Undersown crop Seeding rate crop N yield a at harvest Seeding rate <0.001 Undersown crop <0.001 Seeding rate crop N yield a in late autumn Seeding rate <0.001 Undersown crop Seeding rate crop Root biomass b Undersown crop <0.001 N yield of roots Undersown crop <0.001 Grain yield of spring barley Seeding rate Undersown crop <0.001 Seeding rate crop <0.001 Straw yield of spring barley Seeding rate <0.001 Undersown crop <0.001 Seeding rate crop N yield of grains Seeding rate Undersown crop 7 14 <0.001 Seeding rate crop <0.01 a Above ground dry matter yield. b In late autumn.

5 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) Table 3 Estimated dry matter and N yield (kg ha 1 ) of shoots and roots of crops undersown at different seeding rates in spring barley Seeding rate Undersown crop Biomass (kg ha 1 ) N yield (kg ha 1 ) At harvest Late autumn At harvest Late autumn Shoots Straw difference a Shoots Roots b Shoots Grain N difference c Shoots Roots b Low White clover Red clover Black medic Westerwold ryegrass Timothy Winter wheat d e 15.8 Italian ryegrass f Standard White clover Red clover Black medic Westerwold ryegrass d d Timothy Winter wheat e e Italian ryegrass d e High White clover Red clover Black medic Westerwold ryegrass f e 12.1 Timothy f 7.9 Winter wheat e e 18.3 Italian ryegrass e 22.3 Above-ground (shoots) samples were taken at barley harvest and in late autumn, root samples in late autumn. Difference of air-dry straw yield and grain N yield (kg ha 1 ) between undersown and not undersown barley is shown. All samples were taken in a Higher (+) or lower ( ) straw yield compared to spring barley without undersowing. b Includes barley roots. Without undersowing the biomass of roots was 330 kg ha 1 and N yield 4.9 kg ha 1. c Higher (+) or lower ( ) grain N yield compared to spring barley without undersowing. d Statistical significance of difference of straw yield and grain N yield between undersown and not undersown barley is included in the table as follows: P-value < e Statistical significance of difference of straw yield and grain N yield between undersown and not undersown barley is included in the table as follows: P- value < f Statistical significance of difference of straw yield and grain N yield between undersown and not undersown barley is included in the table as follows: P-value < westerwold ryegrass (P < 0.05) and timothy (P < 0.01). N concentration in clovers in late autumn was also clearly higher than at harvest, and %, respectively. At late autumn, the N concentrations of white and red clover were also (P < 0.05 to P < 0.005) higher than the N concentration of black medic ( %). The N concentration of winter wheat at barley harvest was higher than that of westerwold ryegrass ( % compared with %, P < 0.05 to P < 0.005). At late autumn, the N concentration of Italian ryegrass ( %) was lower (P < 0.05 to P < 0.001) than that of timothy ( %) and winter wheat ( %). Furthermore, at the low seeding rate the N concentration of westerwold ryegrass (1.9%) was lower than that of winter wheat (2.5%, P < 0.05). Root dry matter yields did not differ among the legumes, but there were large differences among root yields of grasses (Table 3). Root yield of westerwold ryegrass was lower than that of winter wheat (P < 0.01) and Italian ryegrass (P < 0.001), and root yield of timothy was lower than that of Italian ryegrass (P < 0.05). The root yields of legumes were in all cases lower than those of Italian ryegrass and winter wheat (P < 0.05 to P < 0.001). There was root biomass in soil in late autumn in plots without a cover crop; dry matter yield was 330 kg ha 1. Also the root dry matter yield of undersown crops includes barley and weed roots, but their proportion could not be determined. The undersown crop affected the N concentration of roots (F 7, 13.7 = 11.66, P < 0.001). The N concentration of legume roots ( %) was higher than that of grasses (1.50, 1.45, 1.35 and 1.15% in westerwold ryegrass, winter wheat, timothy and Italian ryegrass, respectively). The N concentration of root biomass without undersowing was 1.65%. The N yields of roots of black medic and westerwold ryegrass were lower than those of winter wheat and Italian ryegrass (P < 0.05) Grain and straw yield of spring barley The undersown crop affected the grain and straw yield of spring barley. They were also affected by seeding rate, but for grain yield there was interaction between seeding rate and undersown crop (Table 2). Undersown winter wheat greatly decreased the grain yield (Table 4) and grain N yield (Table 3) of barley at all seeding rates, as did ryegrasses at standard and high seeding rates. The effect of those crops was greater the higher the seeding rate. The effect of timothy on barley was of practical importance only at the high seeding rate. The effect of legumes varied, but on an average the decrease in grain yield of barley caused by

6 30 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) Table 4 Estimated grain yield and yield difference (kg ha 1 ) of spring barley when undersowing with different crops and seeding rates is compared with pure barley stand Seeding rate Undersown crop Seed yield of barley (kg ha 1 ) Comparison to no undersowing Estimate 95% CI for the estimate Estimate 95% CI for the difference P-Value Low No undersowing , 5836 White clover , , Red clover , , Black medic , , Westerwold ryegrass , , Timothy , , Winter wheat , , 254 <0.001 Italian ryegrass , , Standard No undersowing , 5936 White clover , , Red clover , , Black medic , , Westerwold ryegrass , , 163 <0.005 Timothy , , Winter wheat , , 635 <0.001 Italian ryegrass , , 186 <0.005 High No undersowing , 5695 White clover , , Red clover , , Black medic , , Westerwold ryegrass , , 217 <0.001 Timothy , , Winter wheat , , 958 <0.001 Italian ryegrass , , 673 <0.001 Also 95% confidence intervals for estimated yield and 95% confidence intervals and P values for estimated yield differences are presented them was small (Table 4). Also the effect of seeding rate varied among species and there was no clear overall effect of seeding rate among the legumes. On average, the yield decrease caused by white clover at high seeding rate, red clover at low seeding rate and black medic at the standard seeding rate were of practical importance. However, no statistically significant differences were established. The effect of undersown crops on straw yield was similar to that on grain yield, but timothy did not have any effect even at the high seeding rate (Table 3) Soil mineral N in first spring and at harvest Differences in soil NO 3 -N and NH 4 -N contents at the beginning of trials were small, suggesting that the results from other sampling times can be studied separately. The undersown crop affected SMN at cereal harvest (Table 5). Italian ryegrass and winter wheat decreased the SMN as compared with no undersowing at standard and high seeding rates, and westerwold ryegrass and timothy at high seeding rate. The decrease was 7kgha 1 with Italian ryegrass at high seeding rate, and in other cases 3 4 kg ha 1 (Table 6) Soil nitrate N in late autumn The undersown crop affected NO 3 -N content in late autumn, both when data for all seeding rates in soil layer 0 30 cm (Analysis 1) and for all soil layers at the standard seeding rate (Analysis 2) were analysed (Table 5). In Analysis 1, Italian ryegrass decreased soil NO 3 -N content by 4kgha 1 at all seeding rates (Table 6). Also timothy and westerwold ryegrass at normal and high seeding rates had an effect although the decrease was small (2 3 kg ha 1 ). In Analysis 2, there was significant interaction between soil layer and undersown crop (Table 5). Italian ryegrass decreased soil NO 3 -N content by about 4 kg ha 1 in all soil layers (Table 7). In percentage terms the decrease was large, and Italian ryegrass almost emptied the deepest soil layers of NO 3 -N. Westerwold ryegrass and winter wheat tended to slightly decrease soil NO 3 - N content in soil layers and cm, but timothy did not affect it at those depths Soil mineral N in the following spring The undersown crop had a significant effect on the NO 3 - N content at the beginning of the following growing season (Table 5). Timothy decreased soil NO 3 -N content by about 5kgha 1 in all soil layers, but the effect of Italian ryegrass was smaller and was emphasised on top soil (Table 7). However, there was no evidence for decreased soil NO 3 -N content by Italian ryegrass at the low seeding rate and also the effect of timothy was weaker at that seeding rate than at others (Table 6). The undersown crop affected SMN and there was significant interaction between seeding rate and undersown crop (Table 5). White clover and black medic increased SMN at the high seeding rate, and timothy decreased it at the low seeding rate by 5 kg ha 1 (Table 6).

7 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) Table 5 Statistical significances of seeding rate, crop species and seeding rate crop species interaction (Analysis 1, soil layer 0 30 cm), and crop species, soil layer and crop species x soil layer interaction (Analysis 2, soil layers 0 30, and cm) on soil NO 3 -N and SMN (kg ha 1 ) Response variable Effect Degrees of freedom numerator Degrees of freedom denominator F-Value P-Value Analyse 1 SMN 0 30 cm at harvest Seeding rate Undersown crop <0.01 Seeding rate crop NO 3 -N 0 30 cm late autumn a Seeding rate Undersown crop <0.001 Seeding rate crop NO 3 -N 0 30 cm at spring b Seeding rate Undersown crop Seeding rate crop SMN 0 30 cm at spring b Seeding rate Undersown crop Seeding rate crop <0.01 Analyse 2 NO 3 -N 0 90 cm late autumn a Soil layer Undersown crop <0.001 Crop soil layer <0.01 NO 3 -N 0 90 cm at spring b Soil layer Undersown crop <0.001 Crop soil layer Undersown crops in spring barley were white clover, red clover, black medic, westerwold ryegrass, timothy, winter wheat and Italian ryegrass. a After soil frost, sampling varying between 15 November and 10 December. b At time when spring tillage preceding sowing is normally done, varying between 22 April and 11 May. 4. Discussion Cover crops for fixing or catching N established by undersowing have advantages and disadvantages. Increase in availability of N for the following crop or reduced N leaching is often accompanied by a reduced yield of the main crop. Optimal conditions should result in minimal competition with the main crop and maximum growth capacity after harvest. In this study, cover crops clearly differed from each other in this respect. Undersown crops with intense early growth, including winter wheat, greatly decreased the grain yield of barley. This is in agreement with the findings of Akanvou et al. (2001), who showed that productivity and competitive ability of crops are strongly linked in intercropping. Large differences in N content and yield of undersown crops were found also in late autumn. The differences were not only between legumes and grasses, but also within each group, suggesting differing capacities for green manuring and leaching prevention. The complexity of factors (trials including N fixing and catching crops, overwintering and not overwintering crops, and differently N releasing crops) means, that the effect on N leaching risk cannot be concluded solely from N yield of cover crops. Because of the different competitive character of cover crops, N storage of the main crop varies, as does the N content of remains. Further, varying N mineralization occurs during spring, between soil thaw and the normal sowing time (Känkänen et al., 2003), that is before the last soil sampling date here. N release occurs simultaneously with potential N catchment by a cover crop. Thus, although precise changes in soil N between two sampling dates inevitably remains unknown, soil nitrate N after and at the beginning of the growing season is a feasible tool for estimation of N leaching risk, together with total N storage of plant biomass. Winter wheat did not have the characteristics of a suitable undersown crop. It greatly decreased the grain yield of barley but did not influence the soil nitrate N content during late autumn and the following spring. Its growth up to barley harvest was strong, but declined afterwards. The poor growth and low N content of barley with winter wheat contributes to a smaller effect on soil N based on N yield of winter wheat than could be expected. Thus, the only possible reason for sowing winter wheat under spring barley would be for relay intercropping (Roslon, 2003), in order to save time and avoid difficulties at sowing in autumn. As with earlier findings (Känkänen et al., 2001, 2003), westerwold ryegrass decreased the grain yield of barley, but had no marked effect on soil NO 3 -N. This indicates that a more suitable grass cover crop would be needed. Italian ryegrass effectively decreased NO 3 -N content in late autumn in all soil layers assayed, suggesting that its use in areas with high leaching risk would be beneficial. This is in agreement with Pietola and Alakukku (2005), who found that the root growth rate and density of ryegrass was high in the late season. Further, Lemola et al. (2000) reported Italian ryegrass to reduce markedly total annual N leaching. According to our results, however, the effect of Italian ryegrass on soil NO 3 -N was much weaker in the following spring because it grew poorly or did not grow at all after winter. In addition, it greatly decreased the grain yield of barley, especially when standard or high seed-

8 32 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) Table 6 The differences and P values between estimated means for soil mineral N (SMN, kg ha 1 ) at barley harvest and in following spring, and soil nitrate N (NO 3 -N, kg ha 1 ) in late autumn and in following spring in 0 30 cm soil layer, when cereal cropping without undersowing is compared with using different undersown crops Seeding rate Undersown crop Barley harvest Late autumn a Following spring b SMN NO 3 -N NO 3 -N SMN Difference P-Value Difference P-Value Difference P-Value Difference P-Value Low No undersowing c White clover Red clover Black medic Westerwold ryegrass Timothy <0.01 Winter wheat Italian ryegrass < Standard No undersowing c White clover Red clover Black medic Westerwold ryegrass Timothy Winter wheat Italian ryegrass 4.0 < < High No undersowing c White clover Red clover Black medic Westerwold ryegrass Timothy < Winter wheat Italian ryegrass 6.9 < < a After soil frost, sampling varying between 15 November and 10 December. b At time when spring tillage preceding sowing is normally done, varying between 22 April and 11 May. c Estimated mean. ing rates were used. Therefore, the use of Italian ryegrass in areas characterised by leaching can be considered, but it is generally too competitive to be a useful undersown crop. Timothy competed weakly with spring barley, especially when the highest seeding rate was not used. Spaner and Todd (2003) reported no decrease of barley grain yield, although they used a seeding rate for timothy higher than the highest used in this study. In addition, they used red and alsike clover (Trifolium hybridum L.) in a mixture with timothy at seeding rates similar to that used for red clover in this study. Using a low seeding rate decreased the already weak effect of timothy on soil NO 3 -N in late autumn, but timothy was the most effective crop to catch N in the succeeding spring, suggesting its use on soils that are not tilled until spring. Timothy could catch N in spring, because it as a perennial crop overwintering well in Finland did grow after winter, contrary to ryegrasses. This is in agreement with Pietola et al. (manuscript), who found the root system of timothy to develop in spring more intensively than that of red clover and winter wheat. In our study the growth of winter wheat was observed to be especially poor in the second spring, obviously because of sowing at an unnatural time in previous spring. It seems that a mixture of Italian ryegrass and timothy would be a suitable undersown N catch crop. Italian ryegrass would mainly catch N in autumn and timothy in spring when spring cultivation or no-till is used. In order to avoid substantial yield decrease of the main crop, the seeding rate of each species in the undersown mixture must be lower than when undersown singly. Because Italian ryegrass effectively caught soil N in autumn, even at a low seeding rate (200 seeds m 2 ), this rate should not be exceeded in a mixture. Also the effect of timothy on soil N in spring was adequate at the lowest seeding rate, and thus 400 seeds m 2 should suffice in the mixture. Undersown white clover, red clover and black medic did not compete strongly with the main crop. Only a modest effect on barley yield was recorded, in agreement with earlier findings (Garand et al., 2001; Känkänen et al., 2001; Spaner and Todd, 2003). Undersown legumes have however also been reported both to decrease (Brandt et al., 1989) and increase (Hartl, 1989) the yield of main crops. We did not find any distinct effect of seeding rate of legumes on barley yield, contrary to the findings of Sheaffer et al. (2002), who showed a negative relationship between soybean (Glycine max (L.) Merr.) grain yields and seeding rate of intercropped medic. Although N content of legumes in late autumn was so high that rapid mineralisation of N can be expected, as reported by Smith and Sharpley (1990), only black medic had a slight tendency to increase leaching risk of NO 3 -N. Marstorp and Kirchmann (1991) found that both white clover and black medic mineralised much larger amounts of N than red clover from autumn to early spring. However, they ploughed the soil in

9 H. Känkänen, C. Eriksson / Europ. J. Agronomy 27 (2007) Table 7 The estimated means for soil nitrate N (kg ha 1 ) in 0 30, and cm soil layer in late autumn and in following spring, and differences and P values between the estimated means when cereal cropping without undersowing is compared with using different undersown crops Soil layer (cm) Undersown crop Late autumn Succeeding spring Estimate a Difference P-Value Estimate a Difference P-Value 0 30 No undersowing 7.0 (2.4; 20.6) 13.7 (8.1; 23.4) White clover 5.1 (1.8; 14.7) (8.4; 24.4) Red clover 6.1 (2.1; 18.1) (6.8; 19.7) Black medic 5.3 (1.8; 15.6) (7.8; 22.4) Westerwold ryegrass 5.0 (1.7; 14.7) (6.5; 18.9) Timothy 4.6 (1.6; 13.4) (5.1; 14.8) Winter wheat 6.5 (2.2; 19.2) (8.6; 24.9) Italian ryegrass 2.8 (1.0; 8.3) 4.2 < (5.1; 14.7) No undersowing 5.5 (1.9; 15.7) 10.5 (6.2; 17.9) White clover 5.1 (1.8; 14.7) (5.0; 14.4) Red clover 5.8 (2.0; 16.7) (4.7; 13.7) Black medic 4.2 (1.5; 11.9) (6.9; 19.9) Westerwold ryegrass 3.5 (1.2; 10.1) (5.6; 16.1) Timothy 4.7 (1.6; 13.5) (3.1; 9.1) 5.2 <0.01 Winter wheat 3.8 (1.3; 10.9) (5.9; 17.1) Italian ryegrass 1.4 (0.5; 4.0) 4.1 < (4.2; 12.2) No undersowing 4.6 (1.6; 13.2) 10.5 (6.2; 17.9) White clover 5.0 (1.8; 14.2) (6.2; 18.0) Red clover 5.6 (2.0; 16.0) (6.1; 17.6) Black medic 4.0 (1.4; 11.2) (8.6; 24.7) Westerwold ryegrass 3.1 (1.1; 8.8) (6.9; 19.9) Timothy 4.2 (1.5; 12.0) (2.7; 7.7) 6.0 <0.001 Winter wheat 2.9 (1.0; 8.3) (6.3; 18.4) Italian ryegrass 1.0 (0.4; 2.9) 3.6 < (4.5; 13.0) a 95% confidence limits for the estimate are in parentheses. autumn and we did not. Black medic differs from white clover because as an annual crop it did not survive through winter and thus transferred the fixed N into the soil without reusing it in spring. There was more soil N available to plants during the beginning of the following growing season when legumes were sown at high seeding rates. Because the grain yield of the main crop was not affected by the seeding rate of undersown legumes, using a relatively high seeding rate seems to be feasible. However, this represents an additional cost for the farmer that has to be taken into account. Moreover, mineral N content in spring only partly describes the effect of the previous crop because N mineralised during the summer stimulates yield of the following crop. This effect is dependent not only on N content of the crop (Francis et al., 1994) but also the method by which the cover crop is incorporated (Maillard and Vez, 1991). In this study we did not incorporate the cover crops, but made the last measurement by sampling soil N at the time when spring tillage, preceding spring sowing, is normally done. In addition, direct drilling could be done soon afterwards. However, Lemon et al. (1990) did not recommend direct drilling in such a case; they reported that availability of clover N was not synchronised with the needs of the successive crop. 5. Conclusions A mixture of Italian ryegrass and timothy undersown in spring barley is advantageous on occasions when diminishing N leaching risk is sought. Italian ryegrass catches nitrate N especially in autumn and timothy in spring. Only moderate seeding rates are needed to avoid substantial yield decrease of the main crop. Red and white clover undersown in spring barley do not markedly increase N leaching risk or decrease grain yield of the main crop. Fairly high seeding rates can be used to ensure high N yield of a clover cover crop. References Akanvou, R., Bastiaans, L., Kropff, M.J., Becker, M., Characterization of growth, nitrogen accumulation and competitive ability of six tropical legumes for potential use in intercropping systems. J. Agron. Crop Sci. 187 (2), Alvenäs, G., Marstorp, H., Effect of a ryegrass catch crop on soil inorganic- N content and simulated nitrate leaching. Swedish J. Agric. Res. 23, Brandt, J.E., Hons, F.M., Haby, V.A., Effects of subterranean clover interseeding on grain yield, yield components, and nitrogen content of soft red winter wheat. J. Prod. Agric. 2, Breland, T.A., Green manuring with clover and ryegrass catch crops undersown in small grains: effects on soil mineral nitrogen in field and laboratory experiments. Acta Agric. Scand. Section B Soil Plant Sci. 46, Burity, H.A., Ta, T.C., Faris, M.A., Coulman, B.E., Estimation of nitrogen fixation and transfer from alfalfa to associated grasses in mixed swards under field conditions. Plant Soil 114, Decker, A.M., Taylor, T.H., Willard, C.J., Establishment of new seedings. In: Heath, M.E., et al. (Eds.), Forages. The Science of Grassland Agriculture, third ed. The Iowa State University Press, Ames, Iowa, U.S.A., pp Esala, M., Split application of nitrogen: effects on the protein in spring wheat and fate of 15 N-labelled nitrogen in the soil-plant system. Ann. Agric. Fenn. 30,

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