Agricultural and Food Science in Finland 553 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. 7 (1998): 553–567. © Agricultural and Food Science in Finland Manuscript received April 1998 A G R I C U L T U R A L A N D F O O D S C I E N C E I N F I N L A N D Vol. 7 (1998): 553–567. Timing incorporation of different green manure crops to minimize the risk of nitrogen leaching Hannu Känkänen Agricultural Research Centre of Finland, Plant Production Research, FIN-31600 Jokioinen, Finland, e-mail: hannu.kankanen@mtt.fi Arjo Kangas Agricultural Research Centre of Finland, South Ostrobothnia Research Station, FIN-61400 Ylistaro, Finland Timo Mela Agricultural Research Centre of Finland, Plant Production Research, FIN-31600 Jokioinen, Finland Unto Nikunen Agricultural Research Centre of Finland, North Ostrobothnia Research Station, Toholampi, FIN-69310 Laitala, Finland Hannu Tuuri Agricultural Research Centre of Finland, Data and Information Services, FIN-31600 Jokioinen, Finland Martti Vuorinen Agricultural Research Centre of Finland, Häme Research Station, FIN-36600 Pälkäne, Finland Seven field trials at four research sites were carried out to study the effect of incorporation time of different plant materials on soil mineral N content during two successive seasons. Annual hairy vetch (Vicia villosa Roth), red clover (Trifolium pratense L.), westerwold ryegrass (Lolium multiflorum Lam. var. westerwoldicum) and straw residues of N-fertilized spring barley (Hordeum vulgare) were incorporated into the soil by ploughing in early September, late October and the following May, and by reduced tillage in May. Delaying incorporation of the green manure crop in autumn lessened the risk of N leaching. The higher the crop N and soil NO 3 -N content, the greater the risk of leaching. Incorporation in the fol- lowing spring, which lessened the risk of N leaching as compared with early autumn ploughing, often had an adverse effect on the growth of the succeeding crop. After spring barley, the NO 3 -N content of the soil tended to be high, but the timing of incorporation did not have a marked effect on soil N. With exceptionally high soil mineral N content, N leaching was best inhibited by growing wester- wold ryegrass in the first experimental year. Key words: Avena sativa, decomposition, Hordeum vulgare, legumes, Lolium multiflorum, minerali- zation, mulching, nitrate nitrogen, Trifolium pratense, Vicia villosa mailto:hannu.kankanen@mtt.fi 554 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. Timing of green manure incorporation: effect on soil nitrogen Introduction The rapid increase in Finland’s set-aside area in the late 1980s and early 1990s caused concern due to results suggesting high rates of leaching from bare fallows (Turtola & Känkänen 1992). Reducing leaching by growing N-absorbing crops on a set-aside field, green fallow, was rec- ommended (Jaakkola 1984). Experiments with several different green fallow crops showed that delayed ploughing in autumn markedly de- creased the risk of nitrate N leaching (Känkänen 1993). The net N mineralization from an incorpo- rated green manure crop is highly dependent on the species grown in the crop (Smith and Sharp- ley 1990), and the chemical composition and phenological stage of the crop (Wivstad 1997). There are considerable differences in N miner- alization rates and amounts of legumes (Marstorp and Kirchmann 1991), and the mineralization rate of non-leguminous crops increases with de- creasing C:N ratios (Vinther 1994). Low temperatures slow down the decompo- sition of plant material and nitrification in soil (Anderson 1960), suggesting low mineralization and leaching during the winter in Finland, when the temperature is near or below 0°C and the soil is usually frozen. On the other hand, minerali- zation at low temparatures is not negligible (Van Schöll et al. 1997), and Müller and Sundman (1988) found a high release of N from buried plant material during the period when the soil was frozen most of the time (5 months of 6.5). DeLuca et al. (1992) found that freeze-thaw treatment of soil resulted in a significant increase in the N mineralization rate and mineral-N flush. In any case, delayed incorporation in autumn has been found to be a promising way of reducing the risk of N leaching (Gustafson 1987, Kyll- ingsbaek 1989). If spring-sown crops are grown after green manure crops, a further benefit may possibly be derived from delaying incorporation until the spring. In some cases, however, incor- poration in spring immediately before the next crop is sown can lead to a net N mineralization that occurs too late for optimum utilization of green manure N by the succeeding crop (Tho- rup-Kristensen 1996). Our aim here was to investigate how the in- corporation time (early or late autumn plough- ing, and spring incorporation by ploughing or reduced tillage) of green manure crops with dif- ferent N contents (hairy vetch, red clover and westerwold ryegrass) affects the risk of N leach- ing. Green manure yields and soil mineral N contents were measured. Incorporation of spring barley straw was compared with that of green manures. Material and methods The experiments were established in 1991 and 1993 at the Institute of Crop and Soil Science in Jokioinen (60°49’N, 23°28’E), the Häme Re- search Station in Pälkäne (61°20’N, 24°13’E) and the South Ostrobothnia Research Station in Ylistaro(52°56’N, 22°30’E). At the Central Os- t ro b o t h n i a Re se a rc h S t a t i o n i n To h o l a m p i (63°49’N, 24°10’E) the experiment was estab- lished in 1991 only. Abbreviations of trials are as follows: Jokioinen 1991–1993 = J1, Jokioi- nen 1993–1995 = J2, Pälkäne 1991–1993 = P1, Pälkäne 1993–1995 = P2, Ylistaro 1991–1993 = Y1, Ylistaro 1993–1995 = Y2 and Toholampi 1991–1993 = T. The experimental soils were tentatively clas- sified according to the Soil Taxonomy (Soil Sur- vey Staff 1998) as follows (the conventional abbreviations of the Finnish names of the soil types are given in parentheses): J1 and J2: very fine Typic Cryaquept (J1: As, J2: Hts) P1 and P2: coarse-loamy Oxyaquic Eutrocryept (HHt) Y1 and Y2: fine-silty Aquic Dystrocryept (LjS) T: coarse-loamy Aquic Dystrocryept (KHt) The Toholampi soil was probably an Aquic Haplocryod but, owing to relatively deep plough- 555 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. 7 (1998): 553–567. ing, the E horizon and also the upper part of the Bhs horizon had been mixed with the Ap hori- zon. The fields at Ylistaro differ from the others in the high organic matter content of the plough layer (4–10% and 10–17% in Y1 and Y2, respec- tively). Hairy vetch (Vicia villosa Roth), red clover (Trifolium pratense L.), westerwold ryegrass (Lolium multiflorum Lam. var. westerwoldicum) and straw residues of spring barley (Hordeum vulgare) were incorporated into the soil by ploughing in early autumn (beginning of Sep- tember), late autumn (end of October) and spring (May) and by reduced tillage in spring (Table 1). Vetch, clover and ryegrass were grown as annual, spring-sown green fallows except in 1993 at Pälkäne and Ylistaro where red clover was undersown the previous year in spring bar- ley. The green fallows grew without chemical fertilizer. Spring barley was sown at the same time as green manure crops in the spring of the first experimental year, and with normal rates of NPK fertilizer (60 to 100 kg N ha-1 depending on soil fertility). The dates of spring ploughing and reduced tillage were chosen carefully to ensure that soil moisture was appropriate and that the succeed- ing crop was sown at the same time as autumn ploughed plots. A cultivator was used for reduced tillage at Jokioinen and Ylistaro, and a PTO-dri- ven rotary cultivator (vertically rotating blades) at Pälkäne and Toholampi. Spring barley was sown as the succeeding crop in May in the second experimental year. It was fertilized with 0, 40 and 80 kg N ha-1 after legumes and with 40, 80 and 120 kg N ha-1 after non-leguminous crops. These rates were chosen, as earlier results suggested that the average re- sidual N effect of legume crops in Finland is 40 kg ha-1 (Kauppila & Kurki 1992). Phosphorus and potassium were applied separately, irrespec- tive of N fertilization. The growth of barley was satisfactory except in J1, where the grain yield was < 1000 kg ha-1 because of drought. After barley had been harvested in August in the sec- ond experimental year, the experimental area was ploughed down in September or early October. Oats (Avena sativa) was sown in the following spring and fertilized with N rates 20 kg ha-1 be- low normal. The experiments were designed as split-split-plot trials with preceding crop as the main plot (size 16 m x 24 m), tillage as the sub- plot (4 m x 24 m) and rate of fertilizer N in the second year as the sub-subplot (4 m x 7 m), and with three replicates. Plant samples (0.25 m2 per plot) and root samples from the 0–25 cm soil layer (12.5 cm x 12.5 cm surface area) were taken in the first au- tumn and second spring immediately before the tillage treatment of each plot. Plants were cut with scissors at the base. The root samples were taken manually, washed with a hydropneumatic root washer (Smucker et al. 1982), and separat- ed from other organic matter with tweezers. Plant Table 1. Incorporation and barley sowing dates at the experimental sites. J1 = Jokioinen 1991–1993, J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, P2 = Pälkäne 1993–1995, Y1 = Ylistaro 1991–1993, Y2 = Ylistaro 1993–1995, T = Toholampi 1991–1993. Ploughing Reduced Barley Early autumn Late autumn Spring tillage sowing J1 September 9 October 30 May 12 May 11 May 21 J2 September 3 October 25 May 10 May 10 May 13 P1 September 11 November 5 May 20 May 20 May 20 P2 October 11 October 22 May 19 May 19 May 25 Y1 September 7 October 24 May 19 May 19 May 22 Y2 September 2 October 27 May 17 May 17 May 20 T1 September 5 October 10 May 21 May 21 May 22 556 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. Timing of green manure incorporation: effect on soil nitrogen and root samples were dried in an oven (2 hours at 105°C and overnight at 60°C), and dry matter and nitrogen contents and yields were measured. The soil samples were taken from the 0–30 cm and 30–60 cm layers by mixing 16 cores in topsoil or six cores in subsoil samples. The soil samples were taken four times: 1) in the first year in late autumn as near soil-freezing as possible, 2) in the second spring before sowing of barley, 3) in autumn after harvesting of barley and 4) in the third spring before sowing of oats. The first sampling was carried out in six trials, and the others in all seven trials. After the plots had been divided into different N levels for the succeed- ing barley, samples 3 and 4 were taken from the low-N plots. The soil samples were extracted with 2 M KCl. The nitrate (NO 3 -) and ammoni- um (NH 4 +) nitrogen contents of the extracts were analysed with a scalar autoanalyser (air segment- ed flow analyser, photometric detection) and converted into kg ha-1. September and October were exceptionally cool in the first experimental years, 1991 and 1993: in 1993 the air temperature was 4–5°C below normal. November 1991 was 5°C warm- er than normal. Late autumn 1993 was dry after a rather wet August, and precipitation in both September and November was 35–50 mm below normal. May 1992 was unusually warm and dry. The first date of permanent soil frost in the growing seasons of green manure occurred bet- ween late October and early December. The soil frost lasted from 3 to 6 months, and the snow cover from 3 to 5 months (Table 2). The dates of incorporation, soil freezing and thawing, and barley sowing were considered critical with re- gard to N-leaching risk. The temperature sum between early and late autumn ploughing was at least as high as the temperature sums between late autumn ploughing and soil freezing, and soil thawing and spring incorporation put together (Table 3.). Precipitation, too, was highest dur- ing the first period. The number of days between spring incorporation and barley sowing ranged from 0 to 10, and the temperature sum from 0 to 100. Only in one trial, P2, did it rain (28 mm) between spring incorporation and barley sowing. The number of days between the first soil sampling and soil freezing ranged from 10 to 40. Late autumn ploughing was done at about the same time as soil sampling in four trials, a week earlier in one trial and three and a half weeks earlier in one trial. This might obscure the com- parison of effects of ploughing time on the N leaching risk in differerent trials. However, the temperature from late autumn ploughing to soil freezing was so low (Table 3) that only minor mineralization and changes in soil N would be expected. At Pälkäne early autumn ploughing was mistakenly delayed in 1993, resulting in an 11-day period and a very low temperature sum between early and late autumn ploughing. This Table 2. First and last date of snow cover and soil frost, and maximum depth of soil frost in winter after the growing season of the first experimental year. J1 = Jokioinen 1991–1993, J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, P2 = Pälkäne 1993–1995, Y1 = Ylistaro 1991–1993, Y2 = Ylistaro 1993–1995, T = Toholampi 1991–1993. Snow cover Soil frost First date Last date First date Last date Max. depth J1 December 19 March 23 December 5 March 12 25 cm J2 November 14 April 10 November 10 April 27 101 cm P1 December 16 April 27 December 4 April 27 13 cm P2 November 24 April 7 October 21 May 13 85 cm Y1 December 5 April 19 December 4 May 4 29 cm Y2 November 15 April 6 November 15 May 16 61 cm T December 7 April 27 November 20 May 7 18 cm 557 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. 7 (1998): 553–567. trial also differed from the others in that the soil was frozen at about the same date as the late autumn ploughing was done, and that the first soil samples were not taken. The data from the seven different experimen- tal sites were analysed separately. The main ef- fects of preceding crop and incorporation treat- ment, and their interactions, were tested statisti- cally by analysis of variance according to the split-plot design. If an interaction between pre- ceding crop and incorporation treatment was found, the effects of incorporation treatments were determined separately for the preceding crops. The differences between incorporation treatments were examined in pairs. The means were compared by Tukey’s HSD methods. If there was no interaction, but the main effect of preceding crop or incorporation treatment was significant, it was examined by making paired comparisons. Before the analysis of variance was per- formed, the consistency of data from different examinations with the assumptions of equality of group variances were checked by Box-Cox diagnostic plots, on the basis of which the vari- ables were transformed. Square root, logarithm and reciprocal transformations were used. In addition, the normality of errors was assessed by steam and leaf display and by normal proba- bility plot. All analyses were performed with the SAS statistical package. MIXED (SAS 1992), UNIVARIATE (SAS 1990) and GPLOT (SAS 1991) procedures were used. The soil nitrogen data presented here are medians of the treatments. The differences be- tween treatments are presented as differences between medians. The medians and differences between medians indicate better the practical importance of treatment differences than do means and differences of means based on trans- formed variables. However, the analyses of var- iance were based on transformed variables. Results Crop N The N yields of green manure crops for incor- poration varied, depending on site, year, species and sampling date (Table 4). In autumn, hairy vetch had often produced over 200 kg N ha-1, but after the succeeding winter there was con- siderably less N in material left on the soil Table 3. Air temperature (°C) and precipitation (mm) values between critical days. J1 = Jokioinen 1991–1993, J2 = Jokioi- nen 1993–1995, P1 = Pälkäne 1991–1993, P2 = Pälkäne 1993–1995, Y1 = Ylistaro 1991–1993, Y2 = Ylistaro 1993–1995, T = Toholampi 1991–1993. Mean air temperature Temperature sum, > 0 Precipitation sum period 1 period 2 period 3 period 1 period 2 period 3 period 1 period 2 period 3 J1 6,7 2,2 2,0 354 97 154 128 82 91 J2 4,6 0,1 9,4 249 14 131 55 1 4 P1 6,6 1,9 8,2 365 70 197 112 53 8 P2* 2,7 – 6,9 30 0 48 26 0 4 Y1 6,6 2,1 8,1 322 105 129 117 52 9 Y2 4,0 –0,1 4,7 234 15 9 104 3 0 T 6,4 2,2 7,5 224 118 105 142 55 4 Period 1 is between early and late autumn ploughing, period 2 between late autumn ploughing and the beginning of soil frost and period 3 between the end of soil frost and spring incorporation. *In Pälkäne 1993, early autumn ploughing was delayed by mistake. 558 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. Timing of green manure incorporation: effect on soil nitrogen surface. The decrease was marked, although the small sample size (0,25 m2 per plot) caused some variation in results. In red clover, the same phe- nomenom was found clearly only in J1. The N yield of red clover in autumn varied more than did that of vetch (40 to 220 kg ha-1). A great pro- portion of the red clover N was in roots, and the amount of N in the roots of red clover grown annually increased sharply between samplings before early and late autumn ploughing. The small size of the root samples caused some un- certaintity in the root yield results. The N of vetch occurred mainly in above-ground plant material. The N yield of westerwold ryegrass ranged from 30 to 90 kg ha-1. The average of the above-ground N content incorporated in plant material varied between trials from 2.1 to 3.1%, 1.2 to 3.4%, 0.9 to 1.8% and 0.8 to 1.4% in vetch, red clover, ryegrass and barley straw, respective- ly. The root N content ranged from 1.5 to 3.0%, 1.3 to 2.8%, 0.8 to 1.7% and 1.1 to 1.9%, re- spectively. Soil mineral N The level of soil mineral N differed greatly both between and within the experimental sites. In the trial with the lowest NO 3 -N levels (J1), the min- imum and maximum values of all samples at 0– 60 cm depth in the first soil sampling after bar- ley were 2 and 14 kg NO 3 -N ha-1 and in the trial with the highest NO 3 -N levels (Y2), 50 and 88 kg NO 3 -N ha-1, respectively. Similarly, in the trial with the lowest NH 4 -N levels (Y1), the values were 8 and 13 kg NH 4 -N ha-1 and in the trial with the highest NH 4 -N levels (Y2), 15 and 50 kg NH 4 -N ha-1. Unless otherwise mentioned, only Table 4. Mean values of above-ground, root and total N yields (kg ha-1) of annually grown crops for incorporation. J1 = Jokioinen 1991–1993, J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, T = Toho- lampi 1991–1993. Above-ground biomass N Root N, 0–20 cm depth V. villosa T. pratense L. multiflorum V. villosa T. pratense L. multiflorum J1 Early autumn 216 157 40 4 38 6 Late autumn 236 83 46 4 135 10 Spring 40 35 21 6 20 8 Reduced tillage 50 32 20 5 55 10 J2 Early autumn 199 82 52 23 38 27 Late autumn 188 115 69 19 87 18 Spring 60 99 31 5 75 14 Reduced tillage 106 70 38 14 70 15 P1 Early autumn 188 45 23 12 13 22 Late autumn 197 53 25 29 78 22 Spring 82 58 32 Reduced tillage 113 49 23 T Early autumn 123 36 45 4 4 8 Late autumn 153 36 12 11 24 15 Spring 48 16 15 6 28 28 Reduced tillage 45 23 17 5 27 32 559 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. 7 (1998): 553–567. statistically significant (P < 0.05) differences between treatments are examined below. Only in some cases did treatments affect the NH 4 -N content in the upper soil layer (0–30 cm). For example, hairy vetch increased the NH 4 -N content in late autumn of the first experimental year in two trials (J2, Y1) by 4–7 kg ha-1 (4–7 kg differences between medians) as compared with barley and westerwold ryegrass. In one trial (Y1), early ploughing of hairy vetch increased the NH 4 -N content by 20 kg ha-1 as compared with other incorporation treatments, but the other trials did not confirm this effect. In the follow- ing spring, red clover increased the NH 4 -N con- tent by 5–9 kg ha-1 as compared with barley in three trials (J1, P2, Y2) and slightly also in oth- er trials. The NH 4 -N content of the deeper soil layer (30–60 cm) was unaffected by the treat- ments. Effect of green manure crop and barley residues on soil NO 3 -N The crops of the first experimental year affected the NO 3 -N content of the upper soil layer in the Table 5. Main effect of preceding crop and incorporation treatment on soil NO3-N (kg ha-1) in late autumn of the first experimental year. Medians of crop and incorporation treatment and statistical significance of F values by analysis of variance (P values). Within columns, medians followed by the same letter are not significantly different at P < 0.05. J1 = Jokioinen 1991–1993, J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, P2 = Pälkäne 1993–1995, Y1 = Ylistaro 1991–1993, Y2 = Ylistaro 1993–1995, T = Toholampi 1991–1993. J1 J21) P11) P22) Y11) Y2 T 0–30 cm crop V. villosa 8b 51b 14b 8 29c 28b 11c T. pratense 3a 5a 2a 4 5ab 14a 4b L. multiflorum 3a 4a 1a 5 3a 16a 2a H. vulgare 4a 6a 3a 5 12b 35b 7b P value 0.011 <0.001 <0.001 – <0.001 0.006 <0.001 incorporation Early autumn 6b 12b 7b 6 19b 26b 9b Late autumn 3a 6a 2a 4 6a 17a 6a Spring 4a 5a 2a 5 7a 23a 5a P value 0.002 <0.001 <0.001 – <0.001 0.013 <0.001 30–60 cm crop V. villosa 2 15b 13b – 11b 29b 5b T. pratense 1 5ab 1a – 8b 12a 1a L. multiflorum 1 2a 23) – 2a 13a 1a H. vulgare 2 6ab 10b – 18b 33b 9b P value – 0.017 <0.001 – 0.004 <0.001 <0.001 incorporation Early autumn 2 8b 11b – 17b 26b 7b Late autumn 1 5a 7a – 9a 17a 5a Spring 1 6a 5a – 10a 18a 3a P value – <0.001 <0.001 – <0.001 0.003 0.003 1) There was an interaction between preceding crop and incorporation treatment in trials J2, P1 and Y1. The interaction is presented in Table 8. 2) No replicates in 0–30 cm and no samples taken from 30–60 cm. 3) The treatment was not included in the analysis because too many observations were lacking. 560 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. Timing of green manure incorporation: effect on soil nitrogen Table 6. Main effect of preceding crop and incorporation treatment on soil NO3-N (kg ha-1) in the spring of the second experimental year. Medians of crop and incorporation treatment and statistical significance of F values by analysis of variance (P values). Within columns, medians followed by the same letter are not significantly different at P < 0.05. J1 = Jokioinen 1991–1993, J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, P2 = Pälkäne 1993–1995, Y1 = Ylistaro 1991–1993, Y2 = Ylistaro 1993–1995, T = Toholampi 1991–1993. J1 J2 P1 P21) Y1 Y2 T1) 0–30 cm crop V. villosa 8 21b 5 32c 30b 47b 16b T. pratense 6 10a 5 6a 10a 33a 5a L. multiflorum 4 7a 7 5a 12a 34a 1a H. vulgare 6 8a 5 9b 12a 38ab 6a P value 0.29 0.016 0.63 <0.001 0.005 0.027 0.002 incorporation Early autumn 6a 10b 5 10c 21c 79c 8c Late autumn 5a 8ab 7 8b 17b 39b 6b Spring 7b 8a 5 7a 10a 34a 5a P value 0.018 0.028 0.17 <0.001 <0.001 <0.001 <0.001 30–60 cm crop V. villosa 3 11 4 17c 27 45b 11c T. pratense 2 11 3 4a 16 28a 3ab L. multiflorum 1 7 4 3a 10 30a 3a H. vulgare 2 8 3 7b 21 44b 6b P value 0.09 0.88 0.83 <0.001 0.19 0.002 <0.001 incorporation Early autumn 3b 10 3 8b 25b 60b 5b Late autumn 2a 9 7 5a 18a 39a 4a Spring 2a 9 4 5a 16a 31a 5a P value 0.014 0.15 0.45 <0.001 <0.001 <0.001 0.014 1) There was an interaction between preceding crop and incorporation treatment in trials P2 and T. The interaction is presented in Table 9. first autumn in all sampled trials, in the follow- ing spring in five trials and in the following au- tumn in three trials. In the deeper soil layer, the crops affected NO 3 -N content in the first autumn in five trials, in the following spring in three tri- als and in the following autumn in five trials. In the second spring after incorporations, the crops of the first experimental year no longer affected soil NO 3 -N. In late autumn of the first experimental year there was clearly more soil NO 3 -N in both soil layers after vetch than after the other green ma- nure crops (Table 5). The NO 3 -N contents after barley were similar to those after ryegrass and red clover in the upper soil layer, except in three trials (Y1, Y2, T) in which they were higher. In the deeper soil layer, the NO 3 -N contents after barley were similar to those after vetch. In the following spring, soil NO 3 -N was high- er after vetch than after other crops in five trials out of seven (Table 6): the difference between the medians of vetch and other crops was 9–26 kg ha-1 in the upper layer. In the deeper soil lay- er, the NO 3 -N content was higher after vetch in two trials (P2, T) and lower after ryegrass and clover in three trials (P2, Y2, T) than after bar- ley. In the third sampling, the differences between crops were small in the NO 3 -N content of the upper soil layer (Table 7). In spite of statistical- ly significant differences in the deeper layer in five trials, the differences between the crops were 561 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. 7 (1998): 553–567. small except in one trial (Y2), in which the amount of NO 3 -N after barley was exceptional- ly high (100 kg ha-1). At the fourth sampling, the crop of the first experimental year had no sig- nificant effects on soil NO 3 -N. Effect of incorporation treatments on soil NO 3 -N The effect of incorporation treatment on the NO 3 - N content of the upper soil layer in the first au- tumn was statistically significant in all sampled trials, in the following spring in six trials and in the next autumn in three trials. In the deeper soil layer, incorporation treatment affected NO 3 -N content in the first autumn and following spring in five trials and in the following autumn in four trials. In the spring of the third experimental year, the incorporation treatment no longer affected soil NO 3 -N. The timing and method of incorpo- ration generally had a similar effect on soil NO 3 - N, irrespective of the preceding crop, but in some cases there was an interaction between incorpo- ration treatment and the crop incorporated. These interactions are mentioned in the context of the Table 7. Main effect of preceding crop and incorporation treatment on soil NO3-N (kg ha-1) after harvest of spring barley in the second experimental year. Medians of crop and incorporation treatment and statistical significance of F values by analysis of variance (P values). Within columns, medians followed by the same letter are not significantly different at P < 0.05. J1 = Jokioinen 1991–1993, J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, P2 = Pälkäne 1993–1995, Y1 = Ylistaro 1991–1993, Y2 = Ylistaro 1993–1995, T = Toholampi 1991–1993. J1 J2 P1 P21) Y1 Y2 T 0–30 cm crop V. villosa 5ab 7 4 7b 9 14 3b T. pratense 8b 8 2 4ab 8 9 1a L. multiflorum 4a 8 2 4a 9 16 1ab H. vulgare 6ab 8 4 6ab 9 17 2ab P value 0.053 – – 0.039 0.91 0.09 0.02 incorporation Early autumn 6ab 8 3 8c 9ab 15 2 Late autumn 6a 7 3 7bc 11b 14 2 Spring ploughing 8b 9 4 5ab 10ab 9 2 Reduced tillage 4a 7 3 4a 7a 14 1 P value 0.01 – – <0.001 0.025 0.08 0.25 30–60 cm crop V. villosa 4ab 2 5b 3b 11b 43ab 1 T. pratense 5b 2 3a 2ab 9ab 31a <1 L. multiflorum 3a 3 2a 2a 7a 37a 1 H. vulgare 5ab 3 6b 3ab 10ab 100b 1 P value 0.019 – 0.013 0.031 0.043 0.014 – incorporation Early autumn 5a 3 4 3b 15b 54b 1 Late autumn 4a 2 4 3ab 9ab 63b 1 Spring ploughing 7b 3 4 3ab 10ab 38ab 1 Reduced tillage 3a 2 4 2a 7a 33a <1 P value <0.001 – 0.54 0.031 0.006 0.005 – 1) There was an interaction between preceding crop and incorporation treatment in trial P2. The interaction is presented in Table 10. 562 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. Timing of green manure incorporation: effect on soil nitrogen ment: early ploughing after barley did not cause a clear increase in the NO 3 -N content (table 8). In the following spring, the soil NO 3 -N con- tent was highest in five trials out of seven in both layers when ploughing was done in early autumn although, from a practical point of view, the dif- ference was small except in Y1 and Y2 (Table 6). In Y1 and Y2, the soil NO 3 -N content was higher in both soil layers after late autumn ploughing than after incorporation in spring, al- though the difference was statistically signifi- cant only in the upper layer. In P2 and T, from a practical point of view, significant differences were found in values after hairy vetch and red clover, but those after westerwold ryegrass and barley incorporation treatments did not differ clearly (interaction, Table 9). In the third sampling, the timing and method of incorporation had only a minor effect on the NO 3 -N content of the upper soil layer (Table 7). In the deeper soil layer, marked differences in NO 3 -N contents were found only in trials at Ylis- Table 8. Effects of incorporation treatments of different preceding crops on soil NO3-N (kg ha-1) in late autumn of the first experimental year in trials with interactions. Medians of incorporation treatment in different crops, and statistical signifi- cances of interaction between crop and incorporation treatment by analysis of variance (P value). Within columns, medians followed by the same letter are not significantly different at P < 0.05. J2 = Jokioinen 1993–1995, P1 = Pälkäne 1991–1993, Y1 = Ylistaro 1991–1993. 0–30 cm 30–60 cm V. vil- T. pra- L. multi- H. vul- V. vil- T. pra- L. multi- H. vul- losa tense florum gare losa tense florum gare J2 P=0.003 P=0.034 Early autumn 56c 16b 9b 10b 22b 8b 7b 5a Late autumn 30a 4a 3a 6ab 14a 4a 2a 7a Spring 51b 4a 4a 5a 12a 5a 2a 6a P1 P<0.001 P=0.009 Early autumn 13a 7b 3b 6b 22b 9b 41) 14a Late autumn 17b 2a 1a 3a 8a 1a – 9a Spring 14ab 2a 1a 2a 9a 1a <1 10a Y1 P=0.016 P=0.15 Early autumn 48b 18b 12b 19b No interaction Late autumn 21a 4a 2a 10a Spring 24a 5a 2a 12a 1) The treatment was not included in the analysis because too many observations were lacking. main effects of each sampling, and the values are shown in Tables. Early ploughing in autumn increased the up- per soil layer NO 3 -N content in the late autumn of the first experimental year in all trials. Com- pared with other incorporation treatments, which differed only slightly from each other, the in- crease varied mainly from 2 to 13 kg ha-1 (Table 5). In trials J2, P1 and Y1, an interaction was also found between preceding crop and incor- poration treatment (Table 8). All exceptions re- fer to cases in which hairy vetch was the pre- ceding crop: in Y1, the increase in NO 3 -N con- tent due to early autumn ploughing was greater after vetch than after other crops. In J2, late ploughing after vetch resulted in by far the low- est NO 3 -N content, contrary to P1. In the deeper soil layer, early ploughing distinctly increased the NO 3 -N content in five trials out of six, and slightly so also in the sixth trial (J1) (Table 5). In J2 and P1 an interaction was found between the preceding crop and the incorporation treat- 563 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. 7 (1998): 553–567. taro (Y1, Y2). In Y1, the NO 3 -N content was low- er after reduced tillage than after early autumn ploughing. In Y2, both early and late autumn ploughing resulted in a higher NO 3 -N content than did reduced tillage, the medians being 54, 63 and 33 kg ha-1, respectively. After spring ploughing the NO 3 -N content (median 38 kg ha-1) was lower than after autumn ploughing treatments, although the difference was not sta- tistically significant. In P2, the effect of incor- poration varied, depending on the preceding crop: with early autumn ploughing after hairy vetch the NO 3 -N content was 4–6 kg ha-1 higher than with other treatments (Table 10). Discussion The N content of incorporated plant material had a crucial effect on mineral soil N content after incorporation, as shown also in earlier experi- ments (Smith and Sharpley 1990, Marstorp and Kirchmann 1991, Vinther 1994). After hairy vetch, the NO 3 -N content of the soil was clearly higher than after other green manure crops with lower N contents. Likewise the effect of timing of the incorporation was greatest after vetch. After N-fertilized spring barley, the NO 3 -N con- tent of the soil was often high in spite of a low N Table 10. Effects of incorporation treatments of different preceding crops on soil NO3-N (kg ha-1) in autumn of the second experimental year in trial P2 with interactions. Medians of incorporation treatment in different crops, and statistical significances of interaction between crop and incorporation treatment by analysis of variance (P value). Within columns, medians followed by the same letter are not significantly different at P < 0.05. P2 = Pälkäne 1993–1995. 0–30 cm 30–60 cm V. villosa T. pratense L. multiflorum H. vulgare P2 P=0.39 P=0.004 Early autumn No interaction 8b 2a 2a 3a Late autumn 3a 3a 2a 3a Spring 4a 3a 2a 3a Reduced tillage 2a 1a 1a 3a Table 9. Effects of incorporation treatments of different preceding crops on soil NO3-N (kg ha-1) in spring of the second experimental year in trials with interactions. Medians of incorporation treatment in different crops, and statistical signifi- cances of interaction between crop and incorporation treatment by analysis of variance (P value). Within columns, medians followed by the same letter are not significantly different at P < 0.05. P2 = Pälkäne 1993–1995, T = Toholampi 1991–1993. 0–30 cm 30–60 cm V. vil- T. pra- L. multi- H. vul- V. vil- T. pra- L. multi- H. vul- losa tense florum gare losa tense florum gare P2 P=0.003 P=0.007 Early autumn 59b 9c 6b 10a 22b 8b 4b 7ab Late autumn 38a 6b 7b 7a 15a 3a 2a 6a Spring 30a 4a 4a 9a 15a 3a 2a 9b T P=0.021 P=0.012 Early autumn 17a 10b 6b 8a 18b 4a 4b 6ab Late autumn 18a 6b 3b 6a 10a 3a 3ab 4a Spring 14a 2a 1a 6a 10a 2a 2a 8b 564 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. Timing of green manure incorporation: effect on soil nitrogen content in the straw residues incorporated, but the timing of incorporation did not have a marked effect on soil N. The N content of incorporated plant material was most important to the effect of timing of the incorporation. The higher the N content of in- corporated material, the greater the increase in the NO 3 -N content in soil after early autumn ploughing as compared with later incorporations. Lindén and Wallgren (1993) also found that ear- ly ploughing caused a greater accumulation of mineral N in the soil during the autumn after clo- ver and grass-clover leys than after cereals. In addition to the higher quantity of N, this finding may be explained by the more rapid mineraliza- tion of N-rich materials (Jensen 1992). As shown by the soil NO 3 -N content before winter, delaying incorporation of the green ma- nure crop in autumn as much as possible, while taking into consideration the appropriate soil moisture, lessens the risk of N leaching. The same conclusion was reached in Canada by Sand- erson and Mac Leod (1994), who incorporated lupin in autumn at dates very close to those in this study. Because of the high soil NO 3 -N con- tent after hairy vetch, as was also observed by Goffart et al. (1992), early autumn ploughing of vetch should be avoided. Even with red clover, delayed ploughing was clearly beneficial. When cereal stubble was tilled, the timing of autumn ploughing had less effect on the soil NO 3 -N, even though the NO 3 -N content in the 30–60 cm soil layer was about the same in late autumn after barley as it was after N-rich crops. The benefi- cial effect of delaying the incorporation of a growing green manure crop is pronounced due to the prolonged N uptake. In addition, the grow- ing green manure crop keeps the soil drier than does stubble, resulting in less downward water flow and a better capacity to carry machines. Tilling in late autumn did not generally in- crease the soil NO 3 -N content in the spring of the second experimental year as compared with spring incorporations, except in two trials at the Ylistaro experimental site, where the soil had a high mineral N content. There, late autumn ploughing made for a slight increase in the NO 3 - N flowing down to the deeper soil layer as com- pared with spring tillage; the effect became even more pronounced in Y2 after the succeeding spring barley. This can be interpreted as an in- creased leaching risk, although the following crops may be able to use N from soil layers deep- er than those sampled here (Thorup-Kristensen 1996). At other sites with moderate soil mineral N contents, the period between late autumn in- corporation and winter was short enough to in- hibit N mineralization. The decomposition rate could be influenced by soil mineral N content, as shown in the experiment by Mary et al. (1996), where low soil mineral N slowed down the de- composition rate of wheat straw residues. Because of the higher soil NO 3 -N content in the spring of the second experimental year, in most of the trials more N was potentially avail- able for spring barley after early autumn plough- ing than after late autumn ploughing. In addi- tion, at Ylistaro more N was available after late autumn ploughing than after ploughing or re- duced tillage in spring. However, the grain yield of spring barley did not correlate with these soil N figures, as shown in an article to be published later. Postponing the tillage of barley stubble to the spring on light soils caused a slight increase in the NO 3 -N content in the deeper soil layer just before the tillage as compared with late autumn ploughing. A possible explanation is the immo- bilization associated with straw decomposition (Christensen 1986 and Mary et al. 1996) after autumn ploughing, which inhibits the leaching of N on sandy soils (Kolenbrander 1969). When barley stubble is incorporated in spring, immo- bilization by straw decomposition cannot occur before the leaching period caused by snow melt. We may therefore conclude that although late autumn ploughing may increase N leaching as compared with spring tillage in some cases of high soil NO 3 -N content, it may, after N poor crops, even lessen the leaching risk as compared with spring tillage. However, both late autumn and spring incorporations lessen the risk of N leaching as compared with early autumn plough- ing. This is in accordance with the finding of 565 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. 7 (1998): 553–567. Breland (1994) that clover decomposed rapidly even at 5°C, and that N losses from plant resi- dues rich in N could be reduced by ploughing in the residues late in the autumn or by leaving them on the soil surface until the spring. Christensen (1986) noted similar decompo- sition rates for straw buried at depths of 5, 10 and 15 cm. The present study provides weak evidence that reduced tillage decreased the soil NO 3 -N content in the autumn following spring tillages, mainly in the presence of N-rich soil. Soil type has a bearing on the effect of till- age timing on the growth of the succeeding crop and must therefore be taken into account when considering spring instead of autumn tillage. Spring ploughing is not suitable for heavy clay soil (Mikkola 1989), but, as we found here, re- duced tillage with a cultivator in spring leads to good growth of spring barley. On other soil types, reduced tillage in spring often causes poor growth of spring barley. As shown by the present study, reduced tillage of red clover, a perennial that strongly competes with cereal crops by re- growth, is particularly unsuitable. Conclusions Owing to its beneficial effect on the risk of ni- trogen leaching and its suitability for cereal sow- ing in the succeeding spring, ploughing in late autumn is a recommended procedure for incor- porating green manure crops. The importance of delaying incorporation increases with the in- crease in the crop nitrogen content and soil NO 3 - N content. Spring ploughing can be used on all but clay soils. Reduced tillage in spring also re- duces N leaching, but its suitability is impaired by the adverse effects on the growth of the suc- ceeding crop. In fields with an exceptionally high soil mineral N content, the simplest way to in- hibit N leaching is to avoid N fertilization and crops with a high N content. References Anderson, O.E. 1960. The effect of low temperatures on nitrification of ammonia in Cecil sandy loam. Soil Science Society of America Proceedings 24: 286– 289. Breland, T.A. 1994. Measured and predicted mineraliza- tion of clover green manure at low temperatures at different depths in two soils. Plant and Soil 166: 13– 20. Christensen, B.T. 1986. Barley straw decomposition un- der field conditions: effect of placement and initial nitrogen content on weight loss and nitrogen dynam- ics. Soil Biology & Biochemistry 18: 523–529. DeLuca, T.H., Keeney, D.R. & McCarty, G.W. 1992. Ef- fect of freeze-thaw events on mineralization of soil nitrogen. Biology and Fertility of Soils 14: 116–120. Goffart, J.P., Ninane, V. & Guiot, J. 1992. Green manures and nitrate leaching in winter in the loam region of Belgium. In: Francois, E. et al. (eds.) Nitrogen Cy- cling and Leaching in Cool and Wet Regions of Eu- rope. COST 814 workshop, Gembloux, Belgium, October 22–23, 1992. p. 132–133. Gustafson, A. 1987. Nitrate leaching from arable land in Sweden under four cropping systems. 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Summary: Leaching of nitrogen from soil grown with ryegrass incorporated at different times. Tidsskrift for Planteavl 93: 337–342. Känkänen, H. 1993. Nitrogen in soil and yields of cere- als after green fallows. Soil Tillage and Environment. Proceedings of NJF seminar no. 228, Jokioinen, Fin- land, 8–10 June 1993. p. 260–264. 566 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. Timing of green manure incorporation: effect on soil nitrogen Lindén, B. & Wallgren, B. 1993. Nitrogen mineralization after leys ploughed in early or late autumn. Swedish Journal of Agricultural Research 23: 77–89. Marstorp, H. & Kirchmann, H. 1991. Carbon and nitro- gen mineralization and crop uptake of nitrogen from six green manure legumes decomposing in soil. Acta Agriculturae Scandinavica 41: 243–252. Mary, B., Recous, S., Darwis, D. & Robin, D. 1996. Inter- actions between decomposition of plant residues and nitrogen cycling in soil. Plant and Soil 181: 71–82. 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Soil nitrogen mineral- ization in the presence of surface and incorporated crop residues. Agronomy Journal 82: 112–116. Smucker, A.J.M., McBurney, S.L. & Srivastava, A.K. 1982. Quantitative separation of roots from compacted soil profiles by the hydropneumatic elutriation system. Agronomy Journal 74: 500–503. Soil Survey Staff 1998. Keys to soil taxonomy. U.S. De- partment of Agriculture, Natural resources Conser- vation Service. 8th edition. U.S. Government Print- ing Office, Washington D.C., USA. 326 p. 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 Ni- trate Leaching by Cover Crops. First Progress Re- port of EU Concerted Action (AIR3) 2108. p. 49–54. Turtola, E. & Känkänen, H. 1992. Nitrogen leaching from set-aside fields. In: Francois, E. et al. (eds.) Nitro- gen Cycling and Leaching in Cool and Wet Regions of Europe. COST 814 workshop, Gembloux, Belgium, October 22–23, 1992. p. 136–137. Van Schöll, L., Van Dam, A.M. & Leffelaar, P.A. 1997. Mineralisation of nitrogen from an incorporated catch crop at low temperatures: experiment and simulation. Plant and Soil 188: 211–219. Vinther, F.P. 1994. N-mineralization and denitrification after incorporation of catch crops and plant residues. The use of catch or cover crops to reduce leaching and erosion. Proceedings of NJF seminar no. 245, Knivsta, Sweden, 3–4 October 1994. p. 105–114. Wivstad, M. 1997. Plant morphology and content of ni- trogen, cell wall and lignin at different phenological stages of red clover and yellow sweetclover. Swed- ish Journal of Agricultural Research 27: 3–14. 567 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. 7 (1998): 553–567. Kun viherlannoituskasvusto muokataan maahan, voi kasvusto sisältää suuria määriä typpeä. Etenkin pal- kokasveja sisältävien kasvustojen typpipitoisuus on korkea, jolloin typen vapautuminen on edullisissa olosuhteissa nopeaa. Viherlannoituksen typen saami- nen tehokkaasti seuraavan kasvin käyttöön on paitsi taloudellisesti kannattavaa, suotavaa myös typen huuhtoutumisen vähentämiseksi. Tutkimus sai alkunsa viherkesantoalan voimak- kaasti kasvaessa 1990-luvun alussa, ja perustuukin erilaisiin viherkesannon toteuttamisvaihtoehtoihin tavanomaisessa viljanviljelyssä. Aiempien tutkimus- tulosten mukaan viherlannoitteen muokkausajankohta vaikuttaa typen vapautumiseen ja huuhtoutumiseen. Tässä tutkimuksessa haettiin parhaita ajankohtia typ- pipitoisuuksiltaan erilaisten kasvustojen muokkaami- seen. Seitsemässä kokeessa MTT:n neljällä tutkimus- asemalla yksivuotisena viherkesantona kasvaneet ruisvirna, puna-apila ja westerwoldin raiheinä sekä tuleentuneena korjatun ohran sänki ja puintijäte kyn- nettiin maahan aikaisin tai myöhään syksyllä (vaih- dellen syyskuun alusta marraskuun alkuun) tai ke- SELOSTUS Viherlannoituskasvuston kynnön viivyttäminen vähentää typen huuhtoutumista Hannu Känkänen, Arjo Kangas, Timo Mela, Unto Nikunen, Hannu Tuuri ja Martti Vuorinen Maatalouden tutkimuskeskus väällä ennen kylvömuokkausta. Kyntöjen vaihtoeh- tona kasvustot muokattiin maahan kyntämättä kevääl- lä ennen varsinaista kylvömuokkausta. Viherkesannon syyskynnön viivyttäminen vähensi typen huuhtoutumisriskiä. Kynnön viivyttämisestä oli sitä enemmän hyötyä, mitä typpipitoisempaa kasvusto oli. Etenkin virnan aikainen kyntäminen aiheutti suu- ren huuhtoutumisriskin, sillä kasvimassan korkean typpipitoisuuden lisäksi virnakasvuston kokonaistyp- pimäärä oli suuri. Ohran sängen kyntöajankohta vai- kutti melko vähän typen huuhtotumisriskiin. Niinpä maatilan syyskynnöt kannattaa aloittaa sängiltä ja päättää typpipitoisimpiin viherkesantoihin. Kevätkyntöä voidaan käyttää myöhäisen syyskyn- nön ohella huuhtoutumisen hillitsemiseen savimaita lukuunottamatta. Myös kyntämätön perusmuokkaus keväällä vähensi typen huuhtoutumista, mutta hait- tasi usein seuraavan viljan kasvua. Niissä kokeissa, joissa maan typpipitoisuus oli poikkeuksellisen kor- kea, typen huuhtoutumista hillittiin parhaiten kasvat- tamalla kesantovuonna typpilannoittamatonta wester- woldin raiheinää. 568 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. Timing of green manure incorporation: effect on soil nitrogen Title Introduction Material and methods Results Discussion Conclusions References SELOSTUS