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Impact of harrowing on the nitrogen dynamics of plants and soil
Soil & Tillage Research 65 (2002) 53–59
Impact of harrowing on the nitrogen dynamics of plants and soil H.-H. Steinmann* Research Center for Agriculture and the Environment, University of Goettingen, Am Vogelsang 6, D 37075 Goettingen, Germany Received 11 April 2001; received in revised form 4 September 2001; accepted 31 October 2001
Abstract Harrowing in cereals is usually carried out to control weeds. In this study, the effects of loosening and aerating the soil with a spring tine harrow on the mineral N content of the soil (NO3 and NH4 þ ) and the N content of the crop were investigated. The experiment was conducted in weed-free summer wheat comparing intensive mechanical treatment with an untreated reference in two consecutive years (1999 and 2000). Each year, during the treatment period, both plant and soil samples were analysed in a dense temporal pattern. When the soil was loosened, its average mineral N content in the 0–30 cm layer was enhanced by only 1.1 and 1.3 kg N ha1 in 1999 and 2000, respectively. The concurrent N uptake of the crop was reduced by 1.2 kg N ha1 in 1999 and 2.2 kg N ha1 in 2000. This small effect was attributed to reduced N uptake and not to enhanced N mineralisation. We concluded that harrowing had only a minor effect on the crop nutrient status. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Harrowing; Soil loosening; N dynamics; Wheat
1. Introduction In cereal crops, harrowing is considered to be one of the most important techniques of mechanical weeding. Harrowing provides high performance, relatively low costs, small labour requirements, and an acceptable efficacy. As well as its effects on weed suppression, farmers appreciate its mechanical effects on soil structure, aeration and water content, since tillage is generally known to affect these soil factors that influence crop growth (Kahn, 1996; Dexter, 1997). Crops growing in soil that had been harrowed, often look better than untreated ones, and even advisors refer to the beneficial side-effects of mechanical treatments. In contrast, the more intensively harrowing is carried out, the more the crop is affected by being uprooted or buried by soil. Rasmussen (1992) described this * Tel.: þ49-551-39-5537; fax: þ49-551-39-2295. E-mail address: [email protected] (H.-H. Steinmann).
demand for a balance between penalties and benefits as selectivity. Nevertheless, mechanical treatments carried out in weed-free crops do not necessarily result in yield depressions even when the implements are intensively utilised (Van der Werff et al., 1991; Steinmann and Heitefuss, 1996). Other non-target effects of mechanical weeding have been presumed, since it was shown that harrowing in winter wheat may lead to an enhanced incidence of leaf diseases (Kakau et al., 1998). Even when there is no direct stimulation of rust and mildew infections, there may be an indirect effect due to an augmented supply of mineral N from the soil, which could lead to an increase in crop susceptibility to diseases. Enhanced N mineralisation due to mechanical weeding is frequently mentioned, but experimental data is too scarce to allow conclusions about the nature and dimensions of this process. Studies on soil nitrogen dynamics have been usually conducted under general tillage management, where tillage is referred
0167-1987/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 2 7 8 - 1
54
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
to as ploughing, rototillage or blade cultivating (Carter, 1994; Franzluebbers et al., 1995; Kandeler et al., 1999). On the other hand, the experimental loosening of soil on a smaller spatial scale such as grinding (Craswell and Waring, 1972) or sieving (Calderon et al., 2000) is often carried out more intensively than those techniques that can be used in the field. As for harrowing, this implement is used more as a tool for mixing soil for special experimental situations (e.g. Nissen et al., 1998) than as a treatment on its own. This work, based on a field experiment was conducted to study the N status of the soil and a crop under an intensive harrowing regime to verify whether mechanical weeding has any effects on N mineralisation and plant N uptake from the soil.
2. Material and methods 2.1. Site conditions and cropping system Field experiments were conducted in 1999 and 2000 on the Marienstein experimental farm of the University of Goettingen (Niedersachsen, Germany, N 51.617; E 9.600). The site is 180 m above sea level with an average annual rainfall of 635 mm and an average annual temperature of 8.5 8C. The actual weather data measured close to the experimental field is given for the period of the experiment in both years (Fig. 1). The soil at the experimental site is a deep brown earth (Cambisol). The average values in the upper 30 cm layer were 13% sand, 72% silt and 15%
clay; pH (CaCl2) of the soil is 7.2; soil organic C content is 0.91% and the amount of total N is 0.12%. The experiment was carried out in a spring wheat (Triticum aestivum, cultivar Tinos, sown on 14 March 1999 and 4 April 2000 with 470 and 450 kernels m2, respectively) following oilseed rape (Brassica napus). The fields had been subjected to a usual rotation of rape, winter wheat (T. aestivum) and winter barley (Hordeum vulgare), for at least 10 years. Deep ploughing (30 cm) is the basic tillage practice. Since winter wheat is usually sown in September and is already far into the tillering stage or even at stem elongation when the conditions for harrowing are suitable, it was decided to replace this crop with a spring sown variety. In this way, the harrowing could be carried out during tillering of the crop. The actual crop was kept free from any N fertiliser. Chemical weed control was applied to prevent effects due to competition and N uptake of weeds (post-emergence application of 52.8 g ha1 Fenoxaprop-P-ethyl). 2.2. Experimental design Experimental layout was a randomised complete block design with two treatments (harrowed, not harrowed/control) and four replications for a total of eight plots. The plot size was 4 m 15 m in 1999 and 2 m 15 m in 2000. The harrowing was carried out with a spring tine harrow (manufactured by Hatzenbichler, Austria) consisting of adjustable flexible tines of 0.6 cm diameter and 40 cm length. Harrowing was undertaken three times in 1999 and
Fig. 1. Daily rainfall and daily average temperature (air temperature at a height of 2 m) at the experimental site during the period of the treatments in two successive years.
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59 Table 1 Date of harrowing and cereal growth stage of the wheat (decimal code of growth stage according to Tottmann (1987)) Treatment number 1 2 3
1999
55
crop was uneven, the available space for sampling was limited and the area of the plots was halved. No combine harvest data could be obtained in this year.
2000
Date
Stage
Date
Stage
26 April 4 May 18 May
13 21 30
8 May 24 May –
21 25
twice in 2000 (Table 1), according to soil and weather conditions each as one pass with moderate velocity (7 km h1) and nearly maximum tine strength. Working depth was about 5 cm.
2.4. Statistical analysis The mean and standard error of mean of the data were calculated. Additionally ANOVA and GLM procedures were used (SAS Institute, 1990). The effect of harrowing was tested for each single date and for the whole data series using Tukey’s honestly significant difference test. Data obtained at harvesting were analysed by using Scheffe’s test as there were some unequal sample numbers. Significance was accepted at a level of p 0:05.
2.3. Soil and plant analyses During the 6 weeks following the first harrowing treatment the above-ground biomass of the crop was harvested approximately every 4 days. Each of these samples was harvested from a previously unharvested area (three subsamples of 0:25 m2 ¼ 0:75 m2 harvest area per plot and per date). In addition, just before the crop was combine-harvested, a last biomass sample was taken. Soil was sampled (depth 0–15 and 15– 30 cm) each time from the same area of the plot where the biomass had just been removed (five cores, 2.0 cm diameter in a 0.25 m2 sample area, three subsamples in each plot). Plant dry matter accumulation was determined drying at 105 8C for 24 h. Part of the dry matter was saved for N analysis. The plant matter taken at the crop harvest was divided into grain and straw to estimate the harvest index. Plant matter samples were milled and sieved for analysis in an elemental analyser (Leco CN 2000) by thermal conductivity. Nitrate ðNO3 Þ N and ammonium ðNH4 þ Þ N were analysed colorimetrically (Skalar Analytical) from the soil samples after extraction with a 0.01 M CaCl2 solution. Whilst the processing of soil cores was carried out immediately after sampling, the extracts were stored at 18 8C and analysed at the end of the sampling period of each year. In 1999 in each plot a subplot of 22 m2 (one-half of each plot) was subjected to crop harvest with a small-plot combine. Therefore, in these subplots no destructive sampling methods were carried out before harvesting to be sure that the sampling procedures did not affect grain yield. In 2000, the emergence of the
3. Results Nitrate was found to represent a greater portion of the mineral N (Nmin) content (roughly 70% in 1999 and 75% in 2000) than ammonium. There were no differences in these proportions between the harrowed samples and the control. In both years, the Nmin content in the upper 30 cm of the soil decreased during the observation time (Fig. 2). In the harrowed plots, Nmin values were usually higher than in the control plots. Despite small differences, on some dates they were statistically significant. The differences between the harrowed and control plots became larger with time during the period of stem elongation of the crop. At the end of the experimental period in each year the Nmin contents in both treatments were again equal. In both years, the first-half of the experiment suffered from drought, whereas in the second phase some rainfall occurred (Fig. 1). When the harrow was applied, within the top 30 cm of the soil, significantly more Nmin was found in the upper-half of this layer. For the two layers, Table 2 shows the average Nmin data for the whole experimental period. In 0–30 cm mineral N was 1.1 kg ha1 greater when harrowed than not harrowed in 1999 and 1.3 kg ha1 greater in 2000. In 1999, the difference was significant. Since harrowing was carried out intensively, the crop suffered from mechanical destruction and the crop growth was delayed during the time of stem elongation (Fig. 3). After repeated harrowing, the
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H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
Fig. 2. Mineral N content (kg Nmin ha1) of the soil in the upper 30 cm during the period of the treatments. Arrows indicate the time of harrowing. The error bars indicate 1 S.E. Each asterisk indicates a significant difference between treatments on a single date (Tukey’s HSD test; p 0:05; n ¼ 4). Table 2 Average mineral N content (kg Nmin ha1, mean S:E:) over the period of the treatments (according to Fig. 2) for two soil layers Treatment
1999 0–15 cm
Harrowed Not harrowed LSD (Tukey)a a
p 0:05.
2000 15–30 cm
0–15 cm
15–30 cm
7:1 0:2 a 7:3 0:5 6:3 0:3 b 7:0 0:5 0.43 1.13
7:8 0:4 6:9 2:8 1.83
5:7 0:3 5:3 0:3 0.88
biomass production of the treated crop remained up to 4 days behind the control. In 1999, the average shortfall of 136 kg plant dry matter per hectare of the treated variant compared to the control was significant. In 2000, no significant differences were detected. The N accumulation in the above-ground biomass was not affected (Fig. 4). On average, when the wheat was harrowed, the N accumulation was slightly reduced about 1.2 kg N ha1 in 1999 and 2.2 kg N ha1 in 2000, though these differences were not significant. The N content of the biomass of the
Fig. 3. Above-ground biomass (dry matter; t ha1) of the crop stands during the time of the treatments. Arrows indicate the time of harrowing. The error bars indicate 1 S.E. An asterisk indicates a significant difference between treatments on a single date (Tukey’s HSD test; p 0:05; n ¼ 4).
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
57
Fig. 4. N accumulated in the above-ground biomass (kg N ha1 in dry matter) during the period of the treatments. Arrows indicate the time of harrowing. The error bars indicate 1 S.E. ðn ¼ 4Þ. Table 3 Plant and soil data obtained at crop harvest (15 August 1999; 11 August 2000) Treatment
Harrowed Not harrowed LSD (Scheffe)d
Yield (t ha1 DMa)
Percentage of N in grain
Percentage of N in straw
Harvest index
N harvest index
Soil Nmin (kg ha1)b
1999
1999
2000
1999
2000
1999
2000
1999
2000
1999
2000
1.85 a 1.75 b 0.07
2.08 2.04 0.12
0.28 0.27 0.02
0.53 a 0.46 b 0.05
0.47 0.46 0.01
0.52 a 0.51 b 0.01
0.81 0.82 0.01
0.86 0.84 0.01
15.4 16.7 1.7
7.8 7.8 2.4
4.47 4.54 0.41
2000 c
– – –
a
Dry matter. In 0–30 cm. c No data were obtained in 2000. d p 0:05. b
harrowed and the control crops were not remarkably different. The data obtained at the crop harvest is given in Table 3. No significant yield depression occurred due to harrowing. In 1999, a significantly higher N content was found in the grain when the wheat was harrowed. In contrast, in 2000, more N was translocated and accumulated in the straw. Indeed, the determination of the harvest index and N harvest index did not lead to any clear findings about an altered N accumulation within the crop components.
4. Discussion Harrowing by itself—apart from its weed-suppressing effect—has been often described as having little
or no detrimental impact on crop yield (Rasmussen, 1991; Wilson et al., 1993). This was again demonstrated by the grain yield harvested in 1999, despite the crop having undergone three disturbances. When the available experimental area was limited in 2000 hindering combine harvesting, it was considered a minor loss of information as the main focus of this experiment was on the stage of plant growth at the time of harrowing. Intensive loosening of the soil can alter N dynamics within a few weeks (Calderon et al., 2000). In contrast, weed control methods used during the growing season consist of a relatively shallow and slight tillage. However, data cited by Van der Werff et al. (1991), originally published in the 1920s and 1930s, indicated a remarkable increase in the nitrate content of the soil due to hoeing between the crop rows.
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H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
Indeed, harrowing has a lesser loosening effect on heavier soils than hoeing and the amount of N mineralised due to the soil disturbance from harrowing is very small, as was demonstrated in this study. The lack of moisture during part of this experiment may have inhibited any broader effects, since rewetting the soil is a key factor for mineralisation (Franzluebbers et al., 1996). However, for the purpose of weed control harrowing is preferentially carried out under dry conditions. Obviously the main effects were measured in the upper layer, where the soil loosening occurred. In contrast, Bo¨ hrnsen (1993) reported higher amounts of nitrate due to harrowing in the 10–20 cm layer, but these findings were not statistically significant. When there are only small effects on the Nmin of the soil, is it possible that there are losses of N from the experimental system, which escape by emission before being measured by the described methods? Mechanical disturbance can certainly increase N2O emissions from the soil for a short time (Granli and Bockman, 1994), but a loss of N via N2O emission seems to be unlikely since tillage causes typically lower emissions than compacted soils (Staley et al., 1990; Granli and Bockman, 1994). Also the amount of gaseous losses would be probably much lower in quantity. In 1999, during a period of 5 months (March–July) only 1.5 kg N2O-N ha1 was emitted from summer wheat at the same site (Lickfett et al., 2000). Indeed, no detectable rates of denitrification could be found in an experiment including an intensive sieving of the soil (Calderon et al., 2000). The reduced N accumulation of the harrowed crop (average of 1.7 kg N ha1, though not significant) supports the small average N surplus of the soil found in the same treatment. Thus, the higher amount of soil N can be considered as an effect of a reduced N accumulation in the crop rather than resulting from an enhanced release of N. The reduced uptake and accumulation of the crop stand, moreover, resulted from the damage caused by harrowing. The transformation of soil N to the crop seems to be unaffected by harrowing. This is emphasised by data taken at crop harvest. The findings of higher N content in grain (1999) are in agreement with Ellis and Howse (1980), who reported a higher N content in grain produced on tilled soils compared to grain harvested under no-tillage.
Usually farmers apply N fertiliser which would mask any effects due to harrowing. In organic farming systems, crops are more likely of benefiting from loosening of the soil, since N-fertilisers are not used. The question, whether the different conditions found in organically farmed fields (e.g. enhanced organic matter content due to livestock manure and green manure) would alter the findings shown here, must be answered by further investigations. To promote proper conditions and to achieve a high level of efficacy a ploughed soil surface is generally considered as a precondition for mechanical weeding (Rasmussen and Ascard, 1995). The experiment described in this paper, therefore, was carried out on a ploughed soil. Mineralising effects due to soil loosening are less expected on soils with a long history of intensive tillage, because their microbial biomass and nutrient pools have been already affected by continual loosening (Calderon et al., 2000). Therefore, it is not likely that soil loosening by a harrow would have any noticeable effect on plant nutrition, though this of course should be confirmed for other site conditions.
Acknowledgements Thanks are expressed to M. Meyer, D. Kopf, U. Kierbaum, G. Dormann for their technical assistance, and to T. Gatesman for helpful comments on the English language. References Bo¨ hrnsen, A., 1993. Several years results about mechanical weeding in cereals. Communications IV. In: Proceedings of the International Conference on IFOAM, pp. 93–99. Calderon, F.J., Jackson, L.E., Scow, K.M., Rolston, D.E., 2000. Microbial responses to simulated tillage in cultivated and uncultivated soils. Soil Biol. Biochem. 32, 1547–1559. Carter, M.R., 1994. A review of conservation tillage strategies for humid temperate regions. Soil Till. Res. 31, 289–301. Craswell, E.T., Waring, S.A., 1972. Effect of grinding on the decomposition of soil organic matter. I. The mineralisation of organic nitrogen in relation to soil type. Soil Biol. Biochem. 4, 427–433. Dexter, A.R., 1997. Physical properties of tilled soils. Soil Till. Res. 43, 41–63. Ellis, F.B., Howse, K.R., 1980. Effects of cultivation on the distribution of nutrients in the soil and the uptake of nitrogen
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59 and phosphorus by spring barley and winter wheat on three soil types. Soil Till. Res. 1, 35–46. Franzluebbers, A.J., Hons, F.M., Zuberer, D.A., 1995. Tillage and crop effects on seasonal soil carbon and nitrogen dynamic. Soil Sci. Soc. Am. J. 59, 1618–1624. Franzluebbers, A.J., Haney, R.L., Hons, F.M., Zuberer, D.A., 1996. Determination of microbial biomass and nitrogen mineralisation following rewetting of dried soil. Soil Sci. Soc. Am. J. 60, 1133–1139. Granli, T., Bockman, O.C., 1994. Nitrous oxide from agriculture. Norw. J. Agric. Sci. Suppl. 12. Kahn, A.R., 1996. Influence of tillage on soil aeration. J. Agron. Crop Sci. 177, 253–259. Kakau, J., Sievert, M., Steinmann, H.-H., 1998. Auswirkungen mechanischer Unkrautbeka¨ mpfung im Winterweizen auf den Befall mit Blattkrankheiten. J. Plant Dis. Prot. Suppl. 16, 705–712. Kandeler, E., Tscherko, D., Spiegel, H., 1999. Long-term monitoring of microbial biomass, N mineralisation and enzyme activities of a chernosem under different tillage management. Biol. Fert. Soils 28, 343–351. Lickfett, T., Schma¨ deke, F., Wolf, I., Flessa, H., Meyer, K., 2000. Lachgasemissionen in einer Rapsfruchtfolge unter dem Einfluss von N-Du¨ ngung, Bodenbearbeitungsintensita¨ t und Fruchtfolgegestaltung. In: Steinmann, H.-H., Gerowitt, B. (Eds.), Ackerbau in der Kulturlandschaft—Funktionen und Leistungen. Mecke, Duderstadt, pp. 213–239. Nissen, H.H., Moldrup, P., Henrikson, K., 1998. Time domain reflectometry measurements of nitrate transport in manure amended soil. Soil Sci. Soc. Am. J. 62, 99–109.
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Rasmussen, J., 1991. Optimising the intensity of harrowing for mechanical weed control in winter wheat. In: Proceedings of the Brighton Crop Protection Conference, pp. 177–184. Rasmussen, J., 1992. Testing harrows for mechanical control of annual weeds in agricultural crops. Weed Res. 32, 267– 274. Rasmussen, J., Ascard, J., 1995. Weed control in organic farming. In: Glen, D.M., Greaves, M.P., Anderson, H.M. (Eds.), Ecology and Integrated Farming Systems. Wiley, Chichester, pp. 49– 67. SAS Institute, 1990. SAS Users’ Guide, Version 6, 4th Edition. SAS Institute, Cary, NC. Staley, T.E., Caskey, W.H., Boyer, D.G., 1990. Soil denitrification and nitrification potentials during the growing season relative to tillage. Soil Sci. Soc. Am. J. 54, 1602–1608. Steinmann, H.-H., Heitefuss, R., 1996. Mechanische Beka¨ mpfung von Galium aparine L. in Winterweizen. J. Plant Dis. Prot. Suppl. 15, 431–439. Tottmann, D.R., 1987. The decimal code for the growth stages of cereals, with illustrations. Ann. Appl. Biol. 110, 441–454. Van der Werff, H.M.G., Kloster, J.J., van der Schans, D.A., Boone, F.R., Veen, B.W., 1991. The effect of inter-row cultivation on yield of weed-free maize. J. Agron. Crop Sci. 166, 249–258. Wilson, B.J., Wright, K.J., Butler, R.C., 1993. The effect of different frequencies of harrowing in the autumn or spring on winter wheat, and on the control of Stellaria media (L.) vill., Galium aparine L. and Brassica napus L. Weed Res. 33, 501– 506.
Impact of harrowing on the nitrogen dynamics of plants and soil H.-H. Steinmann* Research Center for Agriculture and the Environment, University of Goettingen, Am Vogelsang 6, D 37075 Goettingen, Germany Received 11 April 2001; received in revised form 4 September 2001; accepted 31 October 2001
Abstract Harrowing in cereals is usually carried out to control weeds. In this study, the effects of loosening and aerating the soil with a spring tine harrow on the mineral N content of the soil (NO3 and NH4 þ ) and the N content of the crop were investigated. The experiment was conducted in weed-free summer wheat comparing intensive mechanical treatment with an untreated reference in two consecutive years (1999 and 2000). Each year, during the treatment period, both plant and soil samples were analysed in a dense temporal pattern. When the soil was loosened, its average mineral N content in the 0–30 cm layer was enhanced by only 1.1 and 1.3 kg N ha1 in 1999 and 2000, respectively. The concurrent N uptake of the crop was reduced by 1.2 kg N ha1 in 1999 and 2.2 kg N ha1 in 2000. This small effect was attributed to reduced N uptake and not to enhanced N mineralisation. We concluded that harrowing had only a minor effect on the crop nutrient status. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Harrowing; Soil loosening; N dynamics; Wheat
1. Introduction In cereal crops, harrowing is considered to be one of the most important techniques of mechanical weeding. Harrowing provides high performance, relatively low costs, small labour requirements, and an acceptable efficacy. As well as its effects on weed suppression, farmers appreciate its mechanical effects on soil structure, aeration and water content, since tillage is generally known to affect these soil factors that influence crop growth (Kahn, 1996; Dexter, 1997). Crops growing in soil that had been harrowed, often look better than untreated ones, and even advisors refer to the beneficial side-effects of mechanical treatments. In contrast, the more intensively harrowing is carried out, the more the crop is affected by being uprooted or buried by soil. Rasmussen (1992) described this * Tel.: þ49-551-39-5537; fax: þ49-551-39-2295. E-mail address: [email protected] (H.-H. Steinmann).
demand for a balance between penalties and benefits as selectivity. Nevertheless, mechanical treatments carried out in weed-free crops do not necessarily result in yield depressions even when the implements are intensively utilised (Van der Werff et al., 1991; Steinmann and Heitefuss, 1996). Other non-target effects of mechanical weeding have been presumed, since it was shown that harrowing in winter wheat may lead to an enhanced incidence of leaf diseases (Kakau et al., 1998). Even when there is no direct stimulation of rust and mildew infections, there may be an indirect effect due to an augmented supply of mineral N from the soil, which could lead to an increase in crop susceptibility to diseases. Enhanced N mineralisation due to mechanical weeding is frequently mentioned, but experimental data is too scarce to allow conclusions about the nature and dimensions of this process. Studies on soil nitrogen dynamics have been usually conducted under general tillage management, where tillage is referred
0167-1987/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 1 9 8 7 ( 0 1 ) 0 0 2 7 8 - 1
54
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
to as ploughing, rototillage or blade cultivating (Carter, 1994; Franzluebbers et al., 1995; Kandeler et al., 1999). On the other hand, the experimental loosening of soil on a smaller spatial scale such as grinding (Craswell and Waring, 1972) or sieving (Calderon et al., 2000) is often carried out more intensively than those techniques that can be used in the field. As for harrowing, this implement is used more as a tool for mixing soil for special experimental situations (e.g. Nissen et al., 1998) than as a treatment on its own. This work, based on a field experiment was conducted to study the N status of the soil and a crop under an intensive harrowing regime to verify whether mechanical weeding has any effects on N mineralisation and plant N uptake from the soil.
2. Material and methods 2.1. Site conditions and cropping system Field experiments were conducted in 1999 and 2000 on the Marienstein experimental farm of the University of Goettingen (Niedersachsen, Germany, N 51.617; E 9.600). The site is 180 m above sea level with an average annual rainfall of 635 mm and an average annual temperature of 8.5 8C. The actual weather data measured close to the experimental field is given for the period of the experiment in both years (Fig. 1). The soil at the experimental site is a deep brown earth (Cambisol). The average values in the upper 30 cm layer were 13% sand, 72% silt and 15%
clay; pH (CaCl2) of the soil is 7.2; soil organic C content is 0.91% and the amount of total N is 0.12%. The experiment was carried out in a spring wheat (Triticum aestivum, cultivar Tinos, sown on 14 March 1999 and 4 April 2000 with 470 and 450 kernels m2, respectively) following oilseed rape (Brassica napus). The fields had been subjected to a usual rotation of rape, winter wheat (T. aestivum) and winter barley (Hordeum vulgare), for at least 10 years. Deep ploughing (30 cm) is the basic tillage practice. Since winter wheat is usually sown in September and is already far into the tillering stage or even at stem elongation when the conditions for harrowing are suitable, it was decided to replace this crop with a spring sown variety. In this way, the harrowing could be carried out during tillering of the crop. The actual crop was kept free from any N fertiliser. Chemical weed control was applied to prevent effects due to competition and N uptake of weeds (post-emergence application of 52.8 g ha1 Fenoxaprop-P-ethyl). 2.2. Experimental design Experimental layout was a randomised complete block design with two treatments (harrowed, not harrowed/control) and four replications for a total of eight plots. The plot size was 4 m 15 m in 1999 and 2 m 15 m in 2000. The harrowing was carried out with a spring tine harrow (manufactured by Hatzenbichler, Austria) consisting of adjustable flexible tines of 0.6 cm diameter and 40 cm length. Harrowing was undertaken three times in 1999 and
Fig. 1. Daily rainfall and daily average temperature (air temperature at a height of 2 m) at the experimental site during the period of the treatments in two successive years.
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59 Table 1 Date of harrowing and cereal growth stage of the wheat (decimal code of growth stage according to Tottmann (1987)) Treatment number 1 2 3
1999
55
crop was uneven, the available space for sampling was limited and the area of the plots was halved. No combine harvest data could be obtained in this year.
2000
Date
Stage
Date
Stage
26 April 4 May 18 May
13 21 30
8 May 24 May –
21 25
twice in 2000 (Table 1), according to soil and weather conditions each as one pass with moderate velocity (7 km h1) and nearly maximum tine strength. Working depth was about 5 cm.
2.4. Statistical analysis The mean and standard error of mean of the data were calculated. Additionally ANOVA and GLM procedures were used (SAS Institute, 1990). The effect of harrowing was tested for each single date and for the whole data series using Tukey’s honestly significant difference test. Data obtained at harvesting were analysed by using Scheffe’s test as there were some unequal sample numbers. Significance was accepted at a level of p 0:05.
2.3. Soil and plant analyses During the 6 weeks following the first harrowing treatment the above-ground biomass of the crop was harvested approximately every 4 days. Each of these samples was harvested from a previously unharvested area (three subsamples of 0:25 m2 ¼ 0:75 m2 harvest area per plot and per date). In addition, just before the crop was combine-harvested, a last biomass sample was taken. Soil was sampled (depth 0–15 and 15– 30 cm) each time from the same area of the plot where the biomass had just been removed (five cores, 2.0 cm diameter in a 0.25 m2 sample area, three subsamples in each plot). Plant dry matter accumulation was determined drying at 105 8C for 24 h. Part of the dry matter was saved for N analysis. The plant matter taken at the crop harvest was divided into grain and straw to estimate the harvest index. Plant matter samples were milled and sieved for analysis in an elemental analyser (Leco CN 2000) by thermal conductivity. Nitrate ðNO3 Þ N and ammonium ðNH4 þ Þ N were analysed colorimetrically (Skalar Analytical) from the soil samples after extraction with a 0.01 M CaCl2 solution. Whilst the processing of soil cores was carried out immediately after sampling, the extracts were stored at 18 8C and analysed at the end of the sampling period of each year. In 1999 in each plot a subplot of 22 m2 (one-half of each plot) was subjected to crop harvest with a small-plot combine. Therefore, in these subplots no destructive sampling methods were carried out before harvesting to be sure that the sampling procedures did not affect grain yield. In 2000, the emergence of the
3. Results Nitrate was found to represent a greater portion of the mineral N (Nmin) content (roughly 70% in 1999 and 75% in 2000) than ammonium. There were no differences in these proportions between the harrowed samples and the control. In both years, the Nmin content in the upper 30 cm of the soil decreased during the observation time (Fig. 2). In the harrowed plots, Nmin values were usually higher than in the control plots. Despite small differences, on some dates they were statistically significant. The differences between the harrowed and control plots became larger with time during the period of stem elongation of the crop. At the end of the experimental period in each year the Nmin contents in both treatments were again equal. In both years, the first-half of the experiment suffered from drought, whereas in the second phase some rainfall occurred (Fig. 1). When the harrow was applied, within the top 30 cm of the soil, significantly more Nmin was found in the upper-half of this layer. For the two layers, Table 2 shows the average Nmin data for the whole experimental period. In 0–30 cm mineral N was 1.1 kg ha1 greater when harrowed than not harrowed in 1999 and 1.3 kg ha1 greater in 2000. In 1999, the difference was significant. Since harrowing was carried out intensively, the crop suffered from mechanical destruction and the crop growth was delayed during the time of stem elongation (Fig. 3). After repeated harrowing, the
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H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
Fig. 2. Mineral N content (kg Nmin ha1) of the soil in the upper 30 cm during the period of the treatments. Arrows indicate the time of harrowing. The error bars indicate 1 S.E. Each asterisk indicates a significant difference between treatments on a single date (Tukey’s HSD test; p 0:05; n ¼ 4). Table 2 Average mineral N content (kg Nmin ha1, mean S:E:) over the period of the treatments (according to Fig. 2) for two soil layers Treatment
1999 0–15 cm
Harrowed Not harrowed LSD (Tukey)a a
p 0:05.
2000 15–30 cm
0–15 cm
15–30 cm
7:1 0:2 a 7:3 0:5 6:3 0:3 b 7:0 0:5 0.43 1.13
7:8 0:4 6:9 2:8 1.83
5:7 0:3 5:3 0:3 0.88
biomass production of the treated crop remained up to 4 days behind the control. In 1999, the average shortfall of 136 kg plant dry matter per hectare of the treated variant compared to the control was significant. In 2000, no significant differences were detected. The N accumulation in the above-ground biomass was not affected (Fig. 4). On average, when the wheat was harrowed, the N accumulation was slightly reduced about 1.2 kg N ha1 in 1999 and 2.2 kg N ha1 in 2000, though these differences were not significant. The N content of the biomass of the
Fig. 3. Above-ground biomass (dry matter; t ha1) of the crop stands during the time of the treatments. Arrows indicate the time of harrowing. The error bars indicate 1 S.E. An asterisk indicates a significant difference between treatments on a single date (Tukey’s HSD test; p 0:05; n ¼ 4).
H.-H. Steinmann / Soil & Tillage Research 65 (2002) 53–59
57
Fig. 4. N accumulated in the above-ground biomass (kg N ha1 in dry matter) during the period of the treatments. Arrows indicate the time of harrowing. The error bars indicate 1 S.E. ðn ¼ 4Þ. Table 3 Plant and soil data obtained at crop harvest (15 August 1999; 11 August 2000) Treatment
Harrowed Not harrowed LSD (Scheffe)d
Yield (t ha1 DMa)
Percentage of N in grain
Percentage of N in straw
Harvest index
N harvest index
Soil Nmin (kg ha1)b
1999
1999
2000
1999
2000
1999
2000
1999
2000
1999
2000
1.85 a 1.75 b 0.07
2.08 2.04 0.12
0.28 0.27 0.02
0.53 a 0.46 b 0.05
0.47 0.46 0.01
0.52 a 0.51 b 0.01
0.81 0.82 0.01
0.86 0.84 0.01
15.4 16.7 1.7
7.8 7.8 2.4
4.47 4.54 0.41
2000 c
– – –
a
Dry matter. In 0–30 cm. c No data were obtained in 2000. d p 0:05. b
harrowed and the control crops were not remarkably different. The data obtained at the crop harvest is given in Table 3. No significant yield depression occurred due to harrowing. In 1999, a significantly higher N content was found in the grain when the wheat was harrowed. In contrast, in 2000, more N was translocated and accumulated in the straw. Indeed, the determination of the harvest index and N harvest index did not lead to any clear findings about an altered N accumulation within the crop components.
4. Discussion Harrowing by itself—apart from its weed-suppressing effect—has been often described as having little
or no detrimental impact on crop yield (Rasmussen, 1991; Wilson et al., 1993). This was again demonstrated by the grain yield harvested in 1999, despite the crop having undergone three disturbances. When the available experimental area was limited in 2000 hindering combine harvesting, it was considered a minor loss of information as the main focus of this experiment was on the stage of plant growth at the time of harrowing. Intensive loosening of the soil can alter N dynamics within a few weeks (Calderon et al., 2000). In contrast, weed control methods used during the growing season consist of a relatively shallow and slight tillage. However, data cited by Van der Werff et al. (1991), originally published in the 1920s and 1930s, indicated a remarkable increase in the nitrate content of the soil due to hoeing between the crop rows.
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Indeed, harrowing has a lesser loosening effect on heavier soils than hoeing and the amount of N mineralised due to the soil disturbance from harrowing is very small, as was demonstrated in this study. The lack of moisture during part of this experiment may have inhibited any broader effects, since rewetting the soil is a key factor for mineralisation (Franzluebbers et al., 1996). However, for the purpose of weed control harrowing is preferentially carried out under dry conditions. Obviously the main effects were measured in the upper layer, where the soil loosening occurred. In contrast, Bo¨ hrnsen (1993) reported higher amounts of nitrate due to harrowing in the 10–20 cm layer, but these findings were not statistically significant. When there are only small effects on the Nmin of the soil, is it possible that there are losses of N from the experimental system, which escape by emission before being measured by the described methods? Mechanical disturbance can certainly increase N2O emissions from the soil for a short time (Granli and Bockman, 1994), but a loss of N via N2O emission seems to be unlikely since tillage causes typically lower emissions than compacted soils (Staley et al., 1990; Granli and Bockman, 1994). Also the amount of gaseous losses would be probably much lower in quantity. In 1999, during a period of 5 months (March–July) only 1.5 kg N2O-N ha1 was emitted from summer wheat at the same site (Lickfett et al., 2000). Indeed, no detectable rates of denitrification could be found in an experiment including an intensive sieving of the soil (Calderon et al., 2000). The reduced N accumulation of the harrowed crop (average of 1.7 kg N ha1, though not significant) supports the small average N surplus of the soil found in the same treatment. Thus, the higher amount of soil N can be considered as an effect of a reduced N accumulation in the crop rather than resulting from an enhanced release of N. The reduced uptake and accumulation of the crop stand, moreover, resulted from the damage caused by harrowing. The transformation of soil N to the crop seems to be unaffected by harrowing. This is emphasised by data taken at crop harvest. The findings of higher N content in grain (1999) are in agreement with Ellis and Howse (1980), who reported a higher N content in grain produced on tilled soils compared to grain harvested under no-tillage.
Usually farmers apply N fertiliser which would mask any effects due to harrowing. In organic farming systems, crops are more likely of benefiting from loosening of the soil, since N-fertilisers are not used. The question, whether the different conditions found in organically farmed fields (e.g. enhanced organic matter content due to livestock manure and green manure) would alter the findings shown here, must be answered by further investigations. To promote proper conditions and to achieve a high level of efficacy a ploughed soil surface is generally considered as a precondition for mechanical weeding (Rasmussen and Ascard, 1995). The experiment described in this paper, therefore, was carried out on a ploughed soil. Mineralising effects due to soil loosening are less expected on soils with a long history of intensive tillage, because their microbial biomass and nutrient pools have been already affected by continual loosening (Calderon et al., 2000). Therefore, it is not likely that soil loosening by a harrow would have any noticeable effect on plant nutrition, though this of course should be confirmed for other site conditions.
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