Nitrogen dynamics following grain legumes and subsequent catch crops and the effects on succeeding cereal crops
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- Hauggaard-Nielsen, H., Mundus, S. & Jensen, E.S. Nutr Cycl Agroecosyst (2009) 84: 281. doi:10.1007/s10705-008-9242-7
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The effects of faba bean, lupin, pea and oat crops, with and without an undersown grass-clover mixture as a nitrogen (N) catch crop, on subsequent spring wheat followed by winter triticale crops were determined by aboveground dry matter (DM) harvests, nitrate (NO3) leaching measurements and soil N balances. A 2½-year lysimeter experiment was carried out on a temperate sandy loam soil. Crops were not fertilized in the experimental period and the natural 15N abundance technique was used to determine grain legume N2 fixation. Faba bean total aboveground DM production was significantly higher (1,300 g m−2) compared to lupin (950 g m−2), pea (850 g m−2) and oat (1,100 g m−2) independent of the catch crop strategy. Faba bean derived more than 90% of its N from N2 fixation, which was unusually high as compared to lupin (70–75%) and pea (50–60%). No effect of preceding crop was observed on the subsequent spring wheat or winter triticale DM production. Nitrate leaching following grain legumes was significantly reduced with catch crops compared to without catch crops during autumn and winter before sowing subsequent spring wheat. Soil N balances were calculated from monitored N leaching from the lysimeters, and measured N-accumulation from the leguminous species, as N-fixation minus N removed in grains including total N accumulation belowground according to Mayer et al. (2003a). Negative soil N balances for pea, lupin and oat indicated soil N depletion, but a positive faba bean soil N balance (11 g N m−2) after harvest indicated that more soil mineral N may have been available for subsequent cereals. However, the plant available N may have been taken up by the grass dominated grass-clover catch crop which together with microbial N immobilization and N losses could leave limited amounts of available N for uptake by the subsequent two cereal crops.
KeywordsCatch croppingPeaFaba beanLupinNitrate leachingNitrogen fixationSubsequent cereals
Legume rotations and organic sources of nitrogen (N) fertility have progressively been replaced with synthetic N fertilizers over the past 3–4 decades (Crews and Peoples 2004). Fertilisers have boosted crop yields, but intensive agricultural systems have negative effects on atmospheric and aquatic environments (Jensen and Hauggaard-Nielsen 2003; Vitousek et al. 1997). In alternative agricultural strategies developed without use of synthetic fertilizers, e.g., organic farming systems, atmospheric N inputs through N2-fixation and recycling of N-rich residues are important to maintain soil fertility (Jensen 1994a; Jensen and Hauggaard-Nielsen 2003). Increasing emphasis on environmentally sustainable development with the use of renewable resources is expected to increase interest in the capacity of legumes to supply N to cropping systems of the future (Crews and Peoples 2004; Peoples et al. 1995; van Kessel and Hartley 2000).
Climatic conditions during autumn and winter, and the turnover of easily decomposable leguminous crop residues may results in N losses and less residual N for subsequent crops (Hauggaard-Nielsen et al. 2003; Jensen 1994b; Mayer et al. 2003b). Nitrate leaching to ground and surface waters is a major environmental problem related to agriculture. The general belief that grain legumes in a rotation increase nitrate leaching may be because: (1) several grain legume crops have weak root systems, (2) legumes are able to fix N symbiotically from the atmosphere and thus have less requirement for soil N and (3) legumes in general have a high N concentration in the tissues (Jensen 1994a; 1996c; Stevenson and van Kessel 1996). However, N is the most limiting nutrient in many agricultural plant production systems (Atkinson et al. 2005) and the key challenge is to match the rate and timing of N supply to crop demand for N (Crews and Peoples 2004; Jensen 1996a; Thomsen and Christensen 1998).
Particularly in Northern Europe one method of reducing nitrate leaching is the use of catch crops to take up plant available soil mineral N, especially in periods with precipitation surpluses (Thomsen 2005). Under temperate growing conditions, undersowing of grass-clover mixtures in cereals is a traditional practice for establishing catch crops (Breland 1996) without reducing the yield of the main crop (Thomsen 2005). The primary purposes of undersowing short duration grass (Lolium perenne L.)–clover (Trifolium repens L.) are limiting N losses through efficient soil mineral N uptake and building soil fertility between the cultivation of primary crops (Breland 1996). Undersowing is less frequent when grain legumes are used as catch crops, possibly because of the weak interspecific competitive ability of grain legumes (Jensen 1991; Liebman and Dyck 1993). Studies on the effect of grass-clover pastures have shown that higher clover content in grass-clover pastures positively affects subsequent spring crop yields (Hauggaard-Nielsen et al. 1998; Kuo and Sainju 1998; Thomsen 2005).
Pea (Pisum sativum L.), faba bean (Vicia faba L.) and lupin (Lupinus angustifolius L.) grain legumes were selected for this study because they are the most adapted species to temperate growing conditions. In Denmark, pea is grown more frequently than faba bean and lupin because of their late maturity (Knudsen et al. 2004).
The aim of this study was to determine nitrate leaching after grain legumes (faba bean, lupin or pea) as compared to a non-fixing crop (oat), with and without undersown grass-clover mixture. Leaching was measured using lysimeters and yield effects were determined through two cropping cycles with subsequent spring wheat (Triticum aestivum), followed by winter triticale (Triticale hexaploide). Crops were grown without the addition of chemical N fertilizer in order to mimic organic farming practices and highlight the effect of organic N sources on subsequent crops.
Materials and methods
Site and soil
The lysimeter experiment was carried out from April 2004 until August 2006 at Risø DTU, Technical University of Denmark (55°41′N, 12°05′E). The 25-year mean annual rainfall at Risø DTU is 550 mm and mean annual air temperature is 8°C with monthly mean maximum and minimum daily air temperature of 16°C (July) and −1°C (February). To secure sufficient establishment of crops during the 1 month of growth, the lysimeters were irrigated with 40 and 50 mm water in 2004 and 2005, respectively.
For this experiment 16 concrete lysimeters (diameter 1.5 m, depth 1.0 m) were used, described in detail by Mortensen and Engvild (1995). From 1991 to 1995, the lysimeters were cropped with sole cropped winter wheat fertilized with 150 kg N ha−1 year−1 (NPK 13-6-9). All aboveground plant biomass produced in that period was removed. During 1995–1998 the lysimeters were left unmanaged. From 1998 to 2000 the lysimeters were used for a study including pea, barley and pea-barley, and with winter oat (Avena sativa L.) as subsequent crop followed by a fallow until spring 2000 (Hauggaard-Nielsen et al. 2003). Subsequently, the lysimeters were left unmanaged, but water was pumped out to avoid water logging. In autumn 2003, all aboveground material was incorporated, followed by application of 50 kg urea-N ha−1.
Before sowing in spring 2004, soil samples (0–90 cm) were collected from four randomly selected lysimeters with a 2.5 cm diameter soil auger. The soil cores were thoroughly mixed and a 50 g subsample was taken. The remaining soil was returned to the core hole. Soil samples were stored at below 5°C overnight until extraction in 2 M KCl (1:10 soil:extractant) on the following day, after which the samples were frozen prior to analyses of NO3/NO2 and NH4 by standard colorimetric methods, using a segmented flow injection autoanalyser (Technicon Autoanalyser II). The soil initially contained 13.6 ± 0.4 g mineral N m−2 (mean ± SE, n = 4). For other soil characteristics see Hauggaard-Nielsen et al. (2003).
The cultivars used were Javlo (field pea), Marcel (faba bean), Rose (lupin), and Gundhild (oat). The lysimeters were laid out in a complete one-factorial randomised design with pea, faba bean, lupin and oat sole crops, and four replicates per treatment, giving a total of 16 lysimeters. Seeds were sown by hand on April 16th, 2004. In each treatment two of the lysimeters were undersown with a catch crop, consisting of a mixture of ryegrass and white clover, 2 weeks after sowing the main crop. 2 weeks after seedling emergence actual plant densities were recorded at 90, 50, 130 and 350 plants m−2 for pea, faba bean, lupin and oat, respectively. On August 2nd, crops were hand harvested, air dried, threshed and the straw was chopped (1.5–3.5 cm length). The crop residues were applied to the respective lysimeters on April 9th, 2005 followed by the incorporation of the catch crop into the soil by tilling. Proportions of clover and grass in the incorporated sward were estimated from visual evaluations of the surface cover.
On April 15th, 2005, spring wheat (Amaretto) was sown in all plots and harvested on August 2nd, 2005. The straw was incorporated into the lysimeter as per the previous year. Early November 2005, winter triticale (Dinaro) was sown (450 plants m−2) in all lysimeters and harvested August 3rd, 2006.
In the lysimeters, the most problematic perennial weeds (primarily Asteraceae spp.) were removed by hand and neither fertilizers nor herbicides were applied. Scarecrows and fencing were established to avoid damage by birds and hares.
Sampling and analytical methods
The use of N resources and N2 fixation were studied by the 15N natural abundance technique, using oat as reference plant for calculating N2 fixation in faba bean, lupin and pea (Shearer and Kohl 1986).
Crops were harvested at physiological maturity each year, by cutting as close as possible to the soil surface. All plant samples were dried at 70°C to constant weight to determine total DM production. After threshing, the samples were separated into straw and grain. Total N and natural abundance of 15N were determined on 5–10 mg subsamples of finely ground material using an elemental analyser (CE Instruments EA 1110) coupled in continuous flow mode to an isotope ratio mass spectrometer (Finnigan MAT DeltaPlus).
In March 2004, leachate water was pumped (Grundfos JP4-45) from the plastic wells and discarded. The plastic wells were then cleaned and leachate water was collected from the 17th of November 2004 until the 27th of April 2006, when climatic events (precipitation) were anticipated to induce N leaching. From each sample, a 200 ml subsample was filtered through a Whatman-42 filter and kept frozen prior to analyses for NO3 using a segmented flow injection autoanalyser (Technicon Autoanalyser II).
Calculations and statistics
Natural δ15N abundance of faba bean, lupin, pea and oat with and without undersown grass-clover catch crop
The calculations assume that δ15N of reference plants (oat) provide a measure of the δ15N of soil mineral N available to the legume. According to Unkovich et al. (1994) levels of δ15N in the reference plants should preferably be above 2.0‰ using natural abundance technique with a δ15N difference between the reference crop (oat) and the B value which should be above 1.0‰ (Table 1). The B values used were −0.72‰ for pea (Hauggaard-Nielsen et al. 2003) and −0.60 and −0.55‰ for faba bean and lupin, respectively (Knudsen et al. 2004).
Soil N balances were calculated from monitored N leaching from the lysimeters, and measured N accumulation from the leguminous species, as N2 fixation minus N removed in grains. It was assumed that 14.6, 17.2, and 15.2% of total N accumulation in faba bean, lupin and pea, respectively, was present as belowground (roots and rhizodeposits) plant N (Mayer et al. 2003a).
Analysis of variance was carried out on data using the GLM procedure of the SAS software (SAS 1990). The significance of differences between treatments was estimated using F-tests and probabilities equal to or <0.05 considered significant. Assumptions of normal distribution and variance homogeneity were tested graphically using residual plots. The significance of difference between treatments was estimated using Tukey’s studentized range test, with α = 0.05 if a main effect or interaction was significant.
Aboveground dry matter production
There were no significant differences in the effects of preceding crops or catch crops on subsequent spring wheat DM production (Fig. 2b). Although wheat DM production was about 30% higher when grown after faba bean than after oat a rather high variability (Least Significant Difference with probabilities equal to or less than 0.05 = 305) blurred potential differences. Catch crops tended (P = 0.33) to positively affect DM production of wheat following grain legumes (670 vs. 560 g DM m−2 with and without catch crops, respectively), an affect not observed following oat. There was no effect from the previous crops or the presence of grass-clover on winter triticale (Fig. 2c).
Aboveground nitrogen accumulation
The grain legumes did not result in significantly higher N accumulation in subsequent spring wheat than oat (Fig. 3b) and no effect of catch cropping could be identified.
Since no artificial tension was applied to the lysimeters to maintain equilibrium between lysimeter suction and soil matrix potential, it is assumed that the soil water dynamics were slightly different compared to a field situation. In certain periods of the year leaching may have been slower and in others faster due to changes of the capillary forces in the soil profile.
Total nitrate leaching (g N m−2) for the whole sampling period differed significantly (P = 0.006) between crops. However, no effect of the catch cropping strategy or interactions between cover crop and catch crops (P = 0.87) were found. From mid November until mid December 2004 cumulative leaching was significantly lower below oat (0.2 g N m−2) than below the grain legumes (0.8 g N m−2). The pattern was similar in spring 2005, with significantly (P = 0.032) higher N-leaching in the treatments with than in those without catch crops. Due to the rather low precipitation in autumn 2005 (Fig. 1b) only one water sample could be collected late December, showing no significant effect of either cropping or undersowing strategy. The highest leaching rates were recorded in spring 2006, with significantly higher values than for soils previously planted with pea than for those planted with faba bean and oats. Leaching values for lupin, faba bean and pea were not significantly different, while leaching was higher below lupin than below oats. Furthermore, leaching during the spring growing season of winter triticale was generally higher in the treatments without catch crops during autumn 2005 than in the treatments with catch crops. This effect was especially significant in pea and lupin.
Soil nitrogen balance
Nitrogen balances were calculated as inputs (N2-fixation + seed N) − N output (grain N + N leaching) from the two 2004–2005 cropping sequences: grain legumes (faba bean, lupin, pea) and oat (Apr 04–Aug 04); spring wheat (Apr 05–Aug 05) with (+) and without (−) undersown grass-clover in spring 2004
Apr 04–Aug 04
Seed N sown
N2 fix straw
N2 fix grain
N2 fix roota
Sep 04–Aug 05
Seed N sown
Apr 04–Aug 05
Grain legume nitrogen accumulation
Grain legume N2 fixation estimated by the natural abundance technique is uncertain in the plots without grass-clover, because the difference between δ15N of oat as reference plant and the δ15N signature of the grain legumes was not >2.0‰ (Table 1) as recommended by Unkovich et al. (1994). However, similar %Ndfa values were found for the three grain legume species with and without catch crops (Fig. 3). Furthermore, the %Ndfa estimates were comparable to values reported in other studies (Knudsen et al. 2004; Peoples and Craswell 1992; Unkovich et al. 1997) except for the extraordinarily high faba bean values.
Faba bean DM production (Fig. 2a) and N2-fixation capacity (Fig. 3a) were higher than those of lupin and pea, which did not differ. The results were comparable to those obtained in a pot experiment by Mayer et al. (2003a), except that in that experiment pea N accumulation was 50% lower than that of lupin. The high faba bean yield indicates that this species is well adapted to temperate growing conditions, making it suitable for expansion of the grain legume area in temperate regions. In addition, the timing of faba bean maturity was similar to that of pea, contrary to its reputation of late maturity (Knudsen et al. 2004).
Total aboveground oat N accumulation was significantly lower than the grain legumes total aboveground N accumulation (Hauggaard-Nielsen et al. 2003). However, oat DM production was higher than that of pea and lupin, indicating favourable soil N fertility conditions in the lysimeters, as also shown by the similar amounts of soil mineral N taken up by oat, pea and lupin, possibly reducing pea and lupin %Ndfa levels (Jensen 1986). Andersen and Olsen (1993) and Karlsson-Strese et al. (1998) report reduced cover crop yield when undersowing pastures causing a loss of income for the farmer (Hansen et al. 2007). This was not found for the present study (Fig. 2a, 3a) indicating strong cover crop interspecific competitive ability.
Subsequent cereals growth
It is commonly observed that cereal yield is enhanced after a grain legume (Armstrong et al. 1997; Jensen 1996c; Peoples et al. 1998). Additional N below the grain legumes, which may have been taken up by the grass-clover catch crop, may have been released after catch crop soil incorporation (Helander 2004). Furthermore, the inclusion of catch crops in the rotation sequence may influence other growth factors than soil mineral N. This could be factors such as soil moisture status, reduced soil erosion, improved soil physical properties, increased nutrient retention, suppression of weeds and reduction in diseases and insects (Fageria et al. 2005). These factors could all affect the yield of subsequent cereals. However, the present lysimeter study found no significant effect of preceding crop or catch crops on either spring wheat DM production (Fig. 2b) or N accumulation (Fig. 3b).
Similarly, winter triticale DM production was not affected by the previous crop or catch cropping strategy (Fig. 3c). For both cereal crops in the rotation the possible beneficial effects of grass-clover and grain legumes may have been suppressed as a result of a high initial soil N content, reducing N2 fixation of grain legumes (Jensen 1986; Hauggaard-Nielsen and Jensen 2001). Other factors than N may also influence subsequent crops as reported by Stevenson and van Kessel (1996) from a field-scale study showing that 91% of the yield advantage of wheat following pea (compared to wheat–wheat rotation) was associated with non-N effects of pea (mainly reduced leaf disease).
Nitrogen leaching and catch cropping
Synchronization of microbial net N mineralization and crop demand is central to any improvements in nutrient conservation in agroecosystems. At the end of the sampling period, after more than 17 months of experimentation, leaching in the oat lysimeters was 5.93 (n = 4) ± 0.34 (SE) g N m−2 compared to 6.75 ± 1.23, 7.57 ± 0.06 and 8.30 ± 1.05 g N m−2 in the faba bean, lupin and pea lysimeters, respectively, with significantly lower leaching with catch cropping than without.
Ryegrass catch crops have been shown to reduce N leaching significantly (Jensen 1994b). Under temperate growing conditions, ryegrass undersown in spring barley can accumulate between 1.2 and 2.0 g N m−2, when harvested in November (Thomsen and Jensen 1994). Grass-clover N accumulation was unfortunately not measured in the present study. Substantial N leaching losses from cultivation of grain legumes without the use of catch crops has been reported (Hauggaard-Nielsen et al. 2003; Maidl et al. 1996; Thomsen et al. 2001) supported by an in general higher residual soil N following grain legumes than following cereals (Francis et al. 1994; Maidl et al. 1996). This should favour the growth of grass in an undersown grass-clover catch crop (Ghaley et al. 2005), because ryegrass is able to accumulate the excess N supplied by the grain legumes (Jensen 1994a, 1996c; Stevenson and van Kessel 1996). Before incorporation of the sward in April a higher proportion of grass than clover, when undersown below grain legumes as compared to cereals, was estimated using visual evaluations of the surface cover (data not shown). Below oats, less soil mineral N is available and therefore the N2 fixation ability of clover improves its interspecific competitive ability and raises the proportion of clover in the sward (De Neergaard et al. 2002; Hauggaard-Nielsen et al. 1998). Using visual evaluations of the surface cover this was verified in the present study (data not shown). The dynamic response of the grass-clover proportion to preceding crops might outbalance the often reported subsequent cereal yield effects from grain legumes (Armstrong et al. 1997; Jensen 1996c; Peoples et al. 1998).
Soil N balances
Negative soil N balances over the 1½ year period indicate soil N depletion, except for the crop sequence including faba beans (Table 2). This is in contrast to results of Jensen (1996a), where a 4 year field study showed a positive N balance for sole cropped pea, whereas, N balance was negative for barley. This was later supported by a lysimeter study including the same species and similar climatic conditions (Hauggaard-Nielsen et al. 2003). The grain legumes were expected to show positive partial N balances due to the fixation of N2 (Hauggaard-Nielsen et al. 2003; Jensen 1996b). Positive differences between total N2 fixed and grain N yield of 45–113, 65–97, and 27–47 kg N ha−1 have been reported for faba bean, lupin and pea, respectively (Evans et al. 2001). Faba bean accumulated more N from the atmosphere and left more residual N in the soil than the other two grain legumes, possibly increasing the risk for N leaching (Thomsen et al. 2001). This is corroborated by the significantly higher cumulative N leaching in the faba bean treatment without undersown grass-clover during winter and early spring 2006 (Fig. 4a). The extraordinarily high faba bean N2-fixation estimates could not be traced back to errors in the use of natural abundance technique (Shearer and Kohl 1986; Unkovich et al. 1994).
Faba bean showed a substantial positive soil N balance indicating available soil mineral N for subsequent cereals. Nitrate leaching during autumn and winter following grain legume cultivation, before a subsequent spring wheat crop, was significantly lower when undersowing a catch crop of grass-clover. However, aboveground DM production of either spring wheat or the following winter triticale was not significantly affected by preceding crop or undersowing a catch crop. Soil mineral N might be taken up by the grass-dominated grass-clover sward, which together with microbial N immobilization would leave no extra grain legume-derived mineral N for either the first or the second subsequent cereal.
The European Commission Contract No. FOOD-CT-2004-506223 New Strategies to Improve Grain Legumes for Food and Feed (GLIP) funded this study.