Improved N transfer by growing catch crops – a challenge
Verbesserter N-Transfer durch Zwischenfrüchte – eine Herausforderung
Journal für Kulturpflanzen, 71 (6). S. 145–160, 2019, ISSN 1867-0911, DOI: 10.5073/JfK.2019.06.01, Verlag Eugen Ulmer KG, Stuttgart
Based on the literature amended by some unpublished data and data compilations from the literature, this review identifies the mechanisms of nitrogen (N) losses from arable land and explores the potential of growing catch crops to mitigate N loss risks from the soil-plant system. The nitrate pool in the soil can be regarded as starting point of most of the N losses via gaseous losses and/or leaching from the soil-plant system. Depleting this pool, especially in autumn, lowers the risk of losses and related impairments of the environment. The input into the nitrate pool can be reduced by adjusting the N fertilization to the N demand of the preceding crop, thus decreasing the N surplus. Less intensive soil tillage after the harvest of the preceding crop may lessen N release from the soil organic matter and the crop residues. On the other hand, cover or catch crops and, to a lesser extent, main crops can take up considerable N amounts in autumn and prevent it from being lost. However, in order to reduce N fertilization of the subsequent crop due to an improved N transfer, the big challenge is to harmonize the N demand of the subsequent main crop and the N release from the catch crop residues. Since the latter depends on several factors like accumulated N amount, C:N ratio of the residues, incorporation date and weather conditions, it can hardly be estimated. Another crucial point is the choice of a suitable cover crop because it should not propagate pests or diseases of the main crops.
Key words: Catch crop, N transfer, soil nitrate pool, crop residues
Diese Literaturübersicht in Kombination mit unveröffentlichten Daten zeigt verschiedene Verlustursachen von Stickstoff (N) aus ackerbaulich genutzten Böden auf und diskutiert, inwieweit der Anbau von Zwischenfrüchten helfen kann, N-Verluste aus dem System Boden-Pflanze zu vermindern. Der Pool an Nitratstickstoff im Boden im Herbst kann als Ausgangspunkt für verschiedene Verlustpfade von N (gasförmige Verluste, Auswaschung) aus dem System Boden-Pflanze angesehen werden. Eine Verkleinerung des Bodennitratpools verringert die Verlustrisiken und die damit verbundene Belastung der Umwelt. Der Input zum Nitratpool kann durch eine optimierte N-Düngung der Vorfrucht und eine damit einhergehende Reduzierung der N-Überschüsse vermindert werden. Zudem kann eine reduzierte Bodenbearbeitung einer verstärkten N-Freisetzung nach der Ernte der Vorfrucht entgegenwirken. Zwischenfrüchte und teilweise auch Hauptfrüchte können bereits vor Winter erhebliche N-Mengen in ihrer Biomasse akkumulieren und somit vor einer Verlagerung in tiefere Bodenschichten bewahren. Voraussetzung für eine nachhaltige Verbesserung der N-Ausnutzung ist jedoch, dass der aus den Residuen der Zwischenfrucht freiwerdende Stickstoff von der/den nachfolgenden Hauptfrucht/-früchten für ihre Ertragsbildung genutzt wird; andernfalls wird das Problem nur um ein Jahr verschoben. Da Umfang und Zeitpunkt der N-Mineralisation unter anderem von der N-Menge im Zwischenfruchtbestand, dem C:N-Verhältnis der Residuen, Einarbeitungstermin der Residuen und der nachfolgenden Witterung abhängt, ist eine präzise Voraussage des N-Transfers in die Folgefrucht schwierig. Darüber hinaus muss bei der Wahl der geeigneten Zwischenfrucht (-mischungen) darauf geachtet werden, dass keine Schaderreger vermehrt werden, die auch die Nachfrüchte infizieren können.
Stichwörter: Zwischenfrucht, N-Transfer, Bodennitratpool, Ernterückstände
Nitrogen (N) is an ambivalent plant nutrient. On the one hand, plants require an appropriate N amount for optimal growth and yield. Otherwise, if leaving the plant-soil system, N can impair the environment due to the pollution of the groundwater with nitrate, the eutrophication of natural ecosystems as consequence of ammonia volatilization or the emission of the greenhouse gas N2O.
In order to increase the crop N use efficiency, N losses from the plant-soil system have to be minimized. In humid climates, N losses from the soil mainly occur as nitrate via N leaching or as nitrous N or N2 via denitrification. Gaseous N losses from the canopy and ammonia volatilization from ammonium (NH4+) containing fertilizers (e.g. urea, slurry, biogas digestate) are not considered in this review. Thus, starting point of most of the losses is the nitrate (NO3-) pool in the soil. Nitrate is very mobile in the soil since the anion cannot be adsorbed by clay minerals and, therefore is at risk to be lost from the system via leaching during the percolation period when rainfall exceeds evapotranspiration (mainly during autumn and winter) (Hooker et al., 2008). In addition, under reducing conditions (high soil water content and high amount of water filled pores) soil microorganisms can use nitrate as O2 source in the course of denitrification increasing gaseous losses as N2O or N2. Therefore, the soil nitrate pool should be kept as small as possible during autumn/winter, when crop N requirement is low or non-existent.
Focusing on arable farming in temperate climates, this review identifies the inputs to the soil nitrate pool during autumn/winter (e.g. N fertilization of the preceding crop, mineralization of N from the soil organic matter and/or crop residues) and highlights possibilities to mitigate the risks resulting from a high soil mineral N content at the beginning of/during the percolation period (e.g. autumn N uptake of a main crop or a catch crop, application of organic matter with a wide C:N ratio). Reducing N losses increases the amount of N remaining in the plant-soil system and an improved N transfer into the subsequent crop allows reducing its N fertilizer requirement or, under N limited conditions, improving grain yield and/or protein concentration. However, the amount of successfully transferred N then largely depends on the synchronism of N release and N requirement of the subsequent crop. Therefore, the entire crop rotation has to be considered (Doltra et al., 2014) because a problem caused by the preceding crop, e.g. large N residues may be solved by the subsequent crop due to its large autumnal N uptake (Benincasa et al., 2010) or a high root depth penetration rate (Dresbøll and Thorup-Kristensen, 2014).
The soil mineral N content (SMN; NO3-N plus NH4-N), which can easily be determined, described well the size of the soil nitrate pool (Myrbeck and Stenberg, 2014a), since the ammonium share is generally small compared to the nitrate one, because the nitrification (conversion from ammonium to nitrate) normally proceeds faster than the ammonification (degradation of organic matter to ammonium) (Metzger, 2002).
Initially, the preceding crop and its management affect soil N dynamics via N fertilization and N offtake by the harvest products. N fertilization exceeding the crop requirement as well as inadequate timing increases N surplus (N input minus N offtake by the harvest products), soil mineral N content after harvest and the risk of N losses (Lafond and Pageau, 2008; Constantin et al., 2010; Heumann et al., 2013; Zhou and Butterbach-Bahl, 2014; Durand et al., 2015; Rasmussen et al., 2015; Shepherd and Newell-Price, 2016; De Notaris et al., 2018; Pandey et al., 2018). However, Hansen et al. (2015) were not able to significantly correlate N balances calculated for each growing season and N leaching in the subsequent percolation period. Unrealistic yield expectation by the farmers leads to an overestimated crop N requirement and, in consequence, overestimated N fertilization. On the other hand, unfavorable weather conditions (e.g. reduced radiation during grain filling, drought stress) or diseases may reduce N offtake; however, if these shortcomings occur after all N has been applied, a correction of the N amount is no longer possible (Sieling et al., 2005; Bingham et al., 2007a; Serrago and Miralles, 2014; Martre et al., 2015). As shown by Beaudoin et al. (2005) a reduction in N fertilization below the recommended rate often failed to further reduce N leaching losses.
Nitrogen use efficiency (kg grain per kg N; NUE) of organic fertilizers like slurries or residues from biogas plants is generally lower than that of mineral N fertilizers (Bergström and Kirchmann, 2006). It can be improved if the timing of application clearly depends on crop nutrient demand (and not on soil trafficability), e.g. in spring instead of autumn, and if the applied N amount is taken into account when calculating the additional mineral fertilization. It can be roughly assumed that in cereals and oilseed rape (OSR), NUE of ammoniacal N from organic fertilizers is similar to that of mineral fertilizers (Sieling, 2004; Sieling et al., 2014; Shepherd and Newell-Price, 2016).
The N surplus at field scale or in larger areas e.g. catchment areas is often used to estimate the leaching risk (Doluschitz et al., 1992; Lord et al., 2002; Jansons et al., 2003; Sacco et al., 2003; Sieling and Kage, 2006). It can give an indication of the risk of unwanted effects which are associated with specific farming practices, especially in the wider environment e.g. landscapes and if integrated over a relatively long period of several years or decades (Öborn et al., 2003; Wick et al., 2012; Bechmann et al., 2014; Blicher-Mathiesen et al., 2014). However, in the short-term (e.g. one growth period), fertilizer use (except excessive amounts) and nitrate in the (ground) water seem to be not very directly linked, since a nutrient surplus in itself may not be sufficient to quantitatively determine the amount of nutrient lost via various pathways, due to the interaction with other environmental parameters. For example, the large reserve of organic N in soils and vegetation will inevitably contribute nitrate to leaching once the land is tilled (Macdonald et al., 1989; Sylvester-Bradley and Chambers, 1992; Hansen et al., 2015). If N fertilization meets plant N requirement of the preceding crop in time and rate, only small amounts of unused fertilizer contribute to the autumn nitrate pool (Glendining et al., 1996; Lafond and Pageau, 2008; Engström et al., 2011; Rasmussen et al., 2015). Therefore, N leached from arable soils in northern Europe mainly originates from the mineralization of soil organic matter and/or crop residues in late summer and autumn, when soils being still warm from the summer become moist and plant demand is still low or non-existent (Jenkinson, 1986; Macdonald et al., 1989; Chaney, 1990; Webb et al., 1997).
The weather is the main factor influencing N mineralization. Adequate water availability assumed, increasing soil temperatures enhance N release, whereas water limitation impairs soil microorganism’s activity.
Long-term application of N fertilization increases total soil N amount compared to unfertilized plots (Glendining and Powlson, 1995). Organic fertilizers as slurry or farmyard manure raise soil organic matter per unit of N input more than mineral fertilizers (Ciardi et al., 1988; Uhlen, 1991; Jenkinson et al., 1994; Nardi et al., 2004). Even if timing and amount meet the need of the crop, mineral N fertilizer may increase mineralizable soil N and SMN content by increasing both the amount of crop residues and their N concentration (Glendining and Powlson, 1995; Silgram and Chambers, 2002).
In their review, Chen et al. (2014) identified four pathways how crop residues may influence the soil N dynamics: biotic immobilization–remineralization, abiotic immobilization, soil organic N mineralization and organic N mineralization of the plant residues. Adequate environmental conditions (mainly soil temperature and soil moisture) for the soil microorganism’s activity provided, N release from soil organic matter and crop residues mostly depend on their C:N ratio and consequently on their N content (Trinsoutrot et al., 2000; Redin et al., 2014).
Crop residues can return large amounts of N back to the soil after harvest. Maidl et al. (1991) found 121 kg N ha–1 in grain pea residues, 148 kg N ha–1 in field bean residues and 44 kg N ha–1 in wheat straw. Due to the narrow C:N ratio of 21 and 27, N release from the grain legume residues happened quicker and to a larger extent than from wheat straw with a C:N ratio of 84; consequently, grain legume residues considerably increase soil mineral N content during autumn. Also incorporation of sugar beet tops in early autumn can enhance N leaching (Thomsen and Christensen, 1998). In addition, plant components dropped before harvest, e.g. petals or leaves, have to be kept in mind. In an experiment with OSR, Malagoli et al. (2005) observed N losses from the crop of up to 45 kg N ha‑1 due to leaf losses between stem elongation and harvest. Mitchell et al. (2001) and Arcand et al. (2013; 2014) highlighted that beside the above-ground residues also the roots have to be taken into account when investigating soil N dynamics.
A wide C:N ratio (> 30) of the residues as occurring in e.g. wheat straw (60–80) decreases the decomposition of and delays N release from the crop residues because the carbon-rich substrates are not able to satisfy the N demand of the microorganisms (Mary et al., 1996; Garnier et al., 2003; Peltre et al., 2016). An improved N availability can enhance N mineralization in such situations. However, in a 4-year field trial, Catt et al. (1998) compared burning and incorporation of wheat straw and observed a significant decrease in N leaching only during the first percolation period, whereas in later winters the effects were smaller or even reversed. Also incorporation of OSR residues which show a wide C:N ratio of about 70 (Kaul, 2004) reduce availability of mineral N in the soil and, in consequence, the risk of nitrate leaching (Justes et al., 1999; Engström and Lindén, 2012). As shown in Fig. 1 OSR straw clearly decreased soil mineral N content during autumn. On the other hand, dry matter (DM) accumulation and N uptake of the following early sown winter wheat crop were impaired by OSR straw, both at the end of autumn growth and at the beginning of spring growth (Table 1; Henke, unpublished data). In addition, further chemical components may potentially cause the nitrogen to become unavailable, e.g. tannic acid or phenolic lignin from the residues (De Neve et al., 2004; Olk et al., 2006).
Fig. 1. Effect of incorporation of oilseed rape straw (26 July) on soil mineral N in 0–30 cm during autumn 2006 (Henke, unpublished data).
Table 1. Effect of incorporation of oilseed rape (OSR) straw (26 Jul 2006) on dry matter accumulation and N uptake of a subsequent winter wheat crop sown on 12 Sep 2006 (Henke, unpublished data)
Without OSR straw
With OSR straw
End of autumn growth (16 Nov 2006)
Dry matter [g m–2]
N uptake [kg N ha–1]
Beginning of spring growth (27 Feb 2007)
Dry matter [g m–2]
N uptake [kg N ha–1]
# – Means with the same letter within a row do not differ significantly at P = 0.05.
Soil tillage stimulates mineralization of crop residues and soil organic matter (Goss et al., 1993; Silgram and Shepherd, 1999) and reduces N uptake by volunteers and weeds, thus increases SMN and the risk of N leaching (Møller Hansen and Djurhuus, 1997; Stenberg et al., 1999; Goulding, 2000; Myrbeck et al., 2014b). Therefore, SMN accumulation in autumn can be lessened by omitting or at least delaying stubble cultivation after harvest and/or soil tillage in autumn (Stenberg et al., 1999).
In intensive cropping systems in high-yielding regions, farmers apply organic and, to a lesser extent, mineral N fertilizers in autumn in order to enhance straw decomposition and to ensure crop growth before the end of autumn growth. In OSR, application of 20–50 kg N ha–1 is usual in minimum tillage systems or if sowing date is delayed, e.g. following wheat as preceding crop. However, the effect on seed yield is not consistent. In most of the cases, seed yield gains were negligible (Ogilvy and Bastiman, 1992; Colnenne et al., 2002; Sieling and Kage, 2010; Engström et al., 2011), whereas in some cases substantial yield increases occurred, even if the unfertilized treatment showed a sufficient crop growth in terms of above-ground DM and N accumulation (Sieling et al., 2017b); however, the underlying pathways remain unclear. Although OSR can take up large amounts of N during autumn growth thus preventing it from leaching in the short-term, the risk of N losses increases in the subsequent year(s) if the applied N will not remove from the system via the harvest products (seed yield). Cereals, e.g. winter barley or winter wheat, receive autumnal N only if following cereals, however, autumn fertilizer N applied in the seed bed seems not to significantly increase yield above applying the entire N after winter (Table 2; Kendall et al., 2017). Due to the later application in cereals compared to OSR, less N will immobilized during straw decomposition thus directly increasing the soil nitrate pool. In consequence, against the background of the small or non-existent yield relevance, autumn N application at least in cereals should be avoided.
Table 2. Effect of autumn N application on the grain yield of a third wheat (2000 – 2011) (unpublished data)
Amount of autumn N remaining in the system
40 kg N ha–1 on the stubble of the preceding crop
40 kg N ha–1 at sowing
40 kg N ha–1 on the stubble of the preceding crop +
Spring N application:
Crop N uptake reduces SMN content and thereby the risk of N losses via leaching during the subsequent percolation period. Additionally, crops utilize water, thus delaying the beginning of drainage and reducing the amount of percolation (Tribouillois et al., 2018). However, according to Yeo et al. (2014), this effect seems to be negligible. To deplete the soil nitrate pool, main crops can be established in autumn e.g. winter OSR or winter cereals, allowing the N taken up to be used for yield formation without an additional N transfer into another crop. Furthermore, crops can be grown solely aimed to take up as much N as possible before winter, so-called catch crops. In general, if water availability does not limit crop growth, the thermal time in autumn and, therefore the duration of autumn growth, positively correlates with the DM accumulation and amount of crop N uptake (Allison et al., 1998a; De Waele et al., 2017).
Winter OSR is known to be able to take up large N amounts (> 100 kg N ha–1) during autumn (Barraclough, 1989; Reau et al., 1994; Dejoux et al., 2003). In contrast, N uptake of cereals at the end of autumn growth rarely exceeds 30 kg N ha–1 under the growing conditions of northern or central Europe (Myrbeck et al., 2012a; Wahlström et al., 2015).
These ‚crop‘ effects interact with the sowing date. Delaying sowing of winter OSR markedly decreases autumnal growth and N uptake and increases SMN (Henke et al., 2009; Sieling et al., 2017a). In addition, the time span between soil tillage and sowing affects the duration of soil N mineralization and N translocation down the soil profile (Engström and Delin, 2016). Several studies have indicated that earlier sowing of winter wheat may be an alternative tool for reducing nitrate leaching (Thorup-Kristensen et al., 2009; Myrbeck et al., 2012a,b) which is also supported by simulation studies of Peltre et al. (2016) using the Daisy model in continuous winter wheat in Denmark. These studies showed that advancing sowing of winter wheat increased the N uptake during autumn and reduced the amounts of leachable soil N more effectively than normal or late sowing. This is well in line with studies of the effect of catch crop sowing time on N uptake and soil nitrate depletion (Sørensen, 1992; Vos and van der Putten, 1997; Thorup-Kristensen et al., 2003). In addition, Rasmussen and Thorup-Kristensen (2016) concluded that an early sown wheat crop showed an increased rooting depth and increased total root density at the end of autumn growth, which in combination with a higher N uptake resulted in soil N reductions of up to 20 kg nitrate N ha‑1 compared to a late sown one. These differences persisted throughout the growing season in next spring, although late sown wheat continued to increase its rooting depth after anthesis. An improved rooting depth up to 1.8–2.0 m enabled the wheat crop to take up N from deeper soil layers reducing SMN content in the subsoil. Earlier sowing can therefore be an important tool to reduce nitrate leaching loss. Unfortunately, the authors did not test the potential to save N fertilizers in spring. In contrast, despite similar root growth rates, growing spring wheat has only little potential to reduce N leaching. Firstly, maximum N leaching mostly occurs during winter before spring wheat establishment (Herrera et al., 2016). Secondly, compared to spring wheat, winter wheat develops a deeper root system due to a longer growth period allowing taking up nitrate from deeper soil layers (Thorup-Kristensen and Nielsen, 1998; Thorup-Kristensen et al., 2009).
An appropriate time span between harvest of one main crop and sowing of the subsequent main crop (usually established in spring) allows growing autumn-sown cover crops for different ecosystem services (for review see Blanco-Canqui et al., 2015), such as accumulation of soil organic matter (Sapkota et al., 2012; Plaza-Bonilla et al., 2016), weed and pest control (Brust et al., 2014; Björkman et al., 2015; Rueda-Ayala et al., 2015), water retention, affecting soil thermal properties (Haruna et al., 2017), increasing the sunlight reflected back to space by an increased average albedo (Carrer et al., 2018), improving soil structure (Blanco-Canqui et al., 2015), preventing soil erosion (Bodner et al., 2010; Dabney et al., 2010), and reducing losses of other nutrients than N (e.g. P) (Liu et al., 2015; Øgaard, 2015; Aronsson et al., 2016). However, this review focusses on the N dynamic and the above-mentioned aspects will not be further considered. If the time span is too short e.g. due to late harvest of the preceding crop (maize) or to an early end of autumn growth (northern countries), undersowing into the previous crop is an appropriate method allowing catch crops to grow adequately before winter (for review: see Aronsson et al., 2016; but also: Bergkvist et al., 2011; Arlauskienė and Maikštėnienė, 2012; Tuulos et al., 2015; Valkama et al., 2015). Results of Snapp et al. (2008) suggest that seed priming can improve germination and emergence of some cover crops under suboptimal conditions as in compacted soils.
Depending on the purpose of the green manure, different species of crops or their mixtures should be used (Ramírez-García et al., 2015a, b). In low input systems and/or in less fertile environments, introducing additional N into the system might be desirable by growing legumes as pure stands or in combination with grasses due to the biological N fixation (Tonitto et al., 2006; Olesen et al., 2007; Askegaard and Eriksen, 2008; Hansen et al., 2010; Büchi et al., 2015). On the other hand in high intensive cropping systems, the main aim of cover (catch) crops is to take up as much N as possible before the end of autumn growth in order to deplete soil N and thus to minimize N leaching. Non-legume species like grasses, OSR or radish are mainly used to trap N.
A lot of literature is available describing the positive effects of growing catch crops on soil mineral N (SMN) content in autumn and N leaching during the following percolation period in autumn and winter as well as on yield and N uptake of the subsequent crop (examples given in Table 3; for reviews see also: Thorup-Kristensen et al., 2003; Valkama et al., 2015; Aronsson et al., 2016).
Table 3. Examples for DM yield and N uptake of cover crops and their effects of soil mineral N before winter and N leaching
Cover crop DM yield
Cover crop N uptake
Soil mineral N reduction before winter [% of the control before winter]
N leaching reduction
1989/90 – 1992/93
cereals, oilseed rape
undersown clover, ryegrass and mustard
0.75 – 2.41
29.2 – 60.4
3 – 8
2001/02 + 2002/03
0.8 – 1.8
16 – 56
65 – 86
2003/04 – 2004/05
under-sown clover +/- ryegrass
15 – 75
1990 – 2007
white mustard, radish,
1.07 – 2.32
29 – 37
9 – 32
2012/13 – 2013/14
0.5 – 3.4
11 – 97
1989/90 – 1994/95
0.6 – 1.4
15 – 34
5 – 70
2 – 58
2002/03 + 2004/05
undersown fodder radish
33 – 40
2004 – 2006
1.0 – 3.5
3 – 119
20 – 67
Komainda et al.,. 2016
2012/13 + 2013/14
0.1 – 1.2
2 – 52
2008/09 + 2009/10
crimson clover, ryegrass
2008 – 2010
0.9 – 1.8
27 – 65
60 – 67
2004 – 2010
0.6 – 3.1
16 – 83
52 – 62
7 – 34
2006/07 – 2008/09
19.4 – 52.3
1988 – 1994
rye, undersown ryegrass
36 – 46
35 – 100
forage rape, winter rye, white turnip
0.3 – 2.4
10 – 53
1990/91 + 1991/92
2.0 – 5.7
75 – 167
41 – 85
legumes, non-legumes and their mixtures
30 – 200
16 – 78
16 – 29
2009/10 + 2010/11
winter turnip rape
2.4 – 3.2
57 – 74
28 – 52
1991/92 + 1995/96
0.1 – 4.0
3 – 141
# nr – not reported
Briefly, the reduction of the nitrate pool positively correlates with the DM accumulation and N uptake of the catch crop. Since the below-ground N uptake often is neglected (due to difficulties in correctly determining root biomass), the positive effects are frequently underestimated (Kristensen and Thorup-Kristensen, 2004; Munkholm and Hansen, 2012; Li et al., 2015a; Komainda et al., 2016). Komainda et al. (2016) suggested using allometric relationships to overcome this shortcoming. Furthermore, fine roots are decomposed and released faster than coarse roots (Jani et al., 2015).
Overall, aside the cover crop species, the length of the growth period (preferably expressed as thermal time) or, more precisely, the amount of intercepted photosynthetically active radiation (PAR) determines the potential DM accumulation and N uptake before the end of autumn growth (Brant et al., 2011; Hashemi et al., 2013; Agneessens et al., 2014; Yeo et al., 2014; Büchi et al., 2015; Stobart et al., 2015; White et al., 2016b; Koch et al., 2017; De Notaris et al., 2018). Vos and van der Putten (1997) derived the potential of DM accumulation and N uptake from the number of days after sowing. They identified the leaf expansion as key process and estimated a light use efficiency of 1.12 g dry matter accumulated per MJ intercepted global radiation being lower than that for main crops as e.g. wheat (Sieling et al., 2016), presumably due to the decreasing temperature during autumn growth.
Several authors discussed the impact of the characteristics of the catch crop root system (depth, architecture) on soil N depletion (Thorup-Kristensen, 2001; Dunbabin et al., 2003; Wahlström et al., 2015) and the factors which it depends on like soil tillage (Munkholm and Hansen, 2012). For instance, Wendling et al. (2016) found five nutrient acquisition strategies depending on the root system of the different species: 1. ‘biomass’ group (high root and shoot biomass, dense tissue, large diameter and high root area; 2. ‘length’ group (high root length density (RLD) and high root and leaf area); 3. ‘intermediate’ group with characteristics of group 1 and 2 (high root and shoot mass, high RLD and high root and leaf area); 4. ‘diameter’ group (large root diameter, high leaf dry matter content); 5. ‘SLA’ group (high specific root length, high specific leaf area (SLA), low root tissue density, low root mass, low RLD, low root area). Species specific parameters of the rooting system as a high rooting depth and a high root growth rate (rapid root establishment), which depends mainly on temperature sum, positively correlate with the ability to deplete soil nitrate (Thorup-Kristensen, 1994, 2001; Kristensen and Thorup-Kristensen, 2004; Herrera et al., 2010). In addition, kinetic parameters of nitrate uptake, e.g. Michaelis-Menten constant and maximum uptake rate, may differ between the catch crop species (Lainé et al., 1993).
The soil nitrate pool is the starting point not only for nitrate leaching losses, but also a main source for N2O emissions due to denitrification. Therefore, N once taken up by a catch crop is saved to get lost until catch crop residues are decomposed and bounded N is remineralized (Delgado et al., 2010). Results on denitrification losses during or following catch cropping are not consistent. Some authors reported of no or only small changes in N2O emissions due to catch crops (Sanz-Cobena et al., 2014; Peyrard et al., 2016). Simulating scenarios of cover crop introduction in southern France revealed a reduction of greenhouse gas emissions compared to bare soils, mainly due to an increase in carbon storage in the soil, although N2O emissions slightly increased (Tribouillois et al., 2018). On the other hand, the risk of N2O emissions during winter increases if residues are already incorporated in autumn. Additionally, higher emissions can occur during the growth of the succeeding main crop, especially if residues show a narrow C:N ratio (Baggs et al., 2000; Rosecrance et al., 2000; Li et al., 2015b, 2016; Pimentel et al., 2015), which, however, can be compensated by adapting N fertilization of the subsequent crop to the estimated N release (Guardia et al., 2016).
As mentioned above, incorporation of crop residues with a wide C:N ratio (e.g. wheat straw) can reduce the soil mineral N pool in autumn as a result of microbial immobilization (Catt et al., 1998; Silgram and Chambers, 2002; Garnier et al., 2003; Beaudoin et al., 2005). Accordingly, the application of substrates with large C:N ratios e.g. biochar or hydrochar (products of the carbonization of biomass in different conversion processes) is discussed in order to induce and/or to intensify N immobilization beside of other effects like improving soil structure, soil field capacity or bioavailability of key nutrients (Atkinson et al., 2010).
Results from pot, lysimeter and field experiments suggest that application of biochar has the potential to mitigate N leaching losses (Bargmann et al., 2014; Hüppi et al., 2016, Libutti et al., 2016). However, the (short-term) effects on crop productivity are not consistent. In a meta-analysis of literature data, Liu et al. (2013) estimated an increase of 11% across a wide range of experimental conditions with greater responses in pot experiments, for dry land crops and in acid, poor-structured soils. In contrast, Gajić and Koch (2012), Nelissen et al. (2015) and Hüppi et al. (2016) did not identify any increases in yield, nutrient uptake or N use efficiency. The main problem is an adequate remineralization of immobilized N by the start of the following spring (Chaves et al., 2007; Gajić and Koch, 2012). From a practical point of view, the availability (application of 10–30 t ha–1 are assumed) and the associated high costs of biochar currently oppose to a broad use (Liu et al., 2013; BUND, 2015).
It is evident that growing catch crops is suitable to reduce the soil nitrate pool in autumn and, in consequence, the risk of nitrate leaching losses, resulting in larger N amount retained in the plant-soil system (Tonitto et al., 2006; Pedersen et al., 2009; Plaza-Bonilla et al., 2015;). However, the challenge is to transfer as much as possible of the nitrogen bounded in the catch crop biomass into the subsequent crop(s) reducing its N fertilizer demand. This requires a timed coincidence of the N release from the catch crop residues and the N demand of the following main crop. If the N mineralization occurs before the main crop is able to utilize this nitrogen (e.g. due to incorporation in autumn or early freezing off, narrow C:N ratio), large nitrate leaching losses can occur during a mild and wet winter and spring (Bergkvist et al., 2011). Delayed incorporation of green manure residues has the potential to reduce the susceptibility of mineral N to leaching and yields more N available to a subsequent crop (Lahti and Kuikman, 2003). On the other hand, N released too late to meet the N requirement of the subsequent crop (Cicek et al., 2015) shifts the nitrate problem into the next percolation period.
The amount of the N fixed or captured by the cover crop can be utilized by the subsequent main crop largely varies and depends on several factors (Table 4 and White et al., 2016a for some examples). When assessing NUE of cover crop N by the subsequent crop, it has to be distinguished between an N limited system (mostly in organic agriculture) and an intensive (conventional) system. If the succeeding main crop remains unfertilized or receives only small N fertilizer amounts, NUE is generally higher compared to adequate N supply (Berntsen et al., 2006; Olesen et al., 2007; Askegaard and Eriksen, 2008; Doltra and Olesen, 2013). However, the challenge is to quantify the seasonal soil N availability patterns in order to adjust the actual N fertilization on the basis of the N requirements of the main crop following cover cropping (Vaughan et al., 2000; Cherr et al., 2006; Tonitto et al., 2006; Heumann et al., 2013; Dresbøll and Thorup-Kristensen, 2014).
Table 4. Examples for the effects of cover crops on the N uptake of the succeeding crop.
Cover crop N uptake
Cover crop N use by the succeeding crop
1968 – 1992
70, 110 or 150
2001 – 2004
92 – 142
+27 – +41
1997 – 2008
24 – 59
+1 – +39
1998 – 2008
25 – 59
+10 – +27
1989/90 – 1994/95
15 – 34
optimum N rates
–9 – +10
2008/09 – 2011/12
crimson clover, Italian ryegrass
51 – 135
–60 – +17
red clover, winter vetch, clover-grass mixture
153 – 226
+20 – +23
1997 – 2004
21 – 37
+4 – +6
2010 – 2011
0 – 225
2005 – 2010
vetch, vetch + oats, mustard
23 – 79
durum wheat, sorghum, sunflower
optimum N rates
2002/03 – 2004/05
different legumes and non-legumes
17 – 141
potatoes, winter rye, spring barley
–5 – +28
1997/98 – 1998/99
169 – 189
+21 – +39
1990/91 – 1991–92
different legumes and non-legumes
75 – 167
+13 – +67
1993/94 – 1995/96
56 – 140
–130 – +60
2005/06 + 2006/07
barley, hairy vetch
30 – 200
–5 – +105
1994/95 + 1995/96
rye, wheat, hairy vetch
30 – 245
0 – 300
–40 – +72
42 – 123
–3 – +36
# nr – not reported
Decomposition of catch crops residues in order to provide the nitrogen bounded with the subsequent main crop mainly depends on the soil temperature, concomitant the date of incorporation or freezing off of the catch crop biomass (Lahti and Kuikman, 2003; Thorup-Kristensen and Dresbøll, 2010). The higher the temperature sum or the earlier the incorporation, the higher is the rate of N release. Thönnissen et al. (2000) and Mansoer et al. (1997) suggested that incorporation of plant residues may speed up decomposition and N release by buffering temperature and water regimes relative to the surface. In addition, residues with a narrow C:N ratio and/or high N content e.g. from legumes decompose faster than e.g. grass residues (Ranells and Wagger, 1996, 1997; Vos and van der Putten, 2001; Justes et al., 2009; Pantoja et al., 2015, 2016; Sievers and Cook, 2018). In an incubation experiment with radish residues of different age as well as with those of white mustard and perennial ryegrass, Thomsen et al. (2016) found a linear correlation between net N release and residue C:N ratio respectively N concentration. Consequently, green manure consisting of legumes as pure stands or as mixtures are able to provide more N at early growth stages than non-legumes (Brennan et al., 2013; Kramberger et al., 2014; Li et al., 2015a; Finney et al., 2016; Tribouillois et al., 2016a; St. Luce et al., 2016). In contrast, high lignin and polyphenol contents impair N mineralization (Seneviratne, 2000).
Wagger (1989) and Benincasa et al. (2010) pointed out a contradiction: If the incorporation of catch crop biomass is delayed, the catch crop can accumulate more biomass and take up more soil mineral N; however, the time span for the soil microorganisms to decompose the incorporated residues is reduced before the establishment of the subsequent crop (Garwood et al., 1999). A longer growth period increases residue C:N ratio which may additionally delay N release. On the other hand, main crops like OSR and cereals require high N amounts early in spring when soil temperatures are low impairing decomposition of catch crop residues. Due to a harvest in summer, those crops are not able to utilize N released during summer and early autumn. Therefore, in crops like sugar beet or maize with a long growing period N demand coincides better with N mineralization.
So far, it was assumed that N not take up by a (catch) crop will be lost mainly due to nitrate leaching during the percolation period over winter. This is more or less the case in environments where the water balance exceeds the retention capacity of the soil. However, under some conditions where no or low N leaching losses (low rainfall and/or high field capacity of the soil), depletion of the soil nitrate pool due to catch crop N uptake can exceed N release from the catch crop residues thus reducing the N supply for the succeeding crop. Thorup-Kristensen et al. (2003) termed this occurrence ‘pre-emptive competition’. Removal of catch crop above-ground biomass e.g. for biogas feedstock production, delayed incorporation or growing catch crops with a wide C:N ratio additionally decrease N availability (Thorup-Kristensen and Dresbøll, 2010; Alonso-Ayuso et al., 2014; Li et al., 2015a; Gabriel et al., 2016).
Repeated cover cropping in the crop rotation increases in the capacity of the soil to provide N presumably due to a positive N input to the system (Constantin et al., 2011; Gabriel et al., 2016). While Constantin et al. (2010, 2011) did not observe any reduction in the potential to reduce N leaching, Garwood et al. (1999) reported on elevated nitrate concentrations in drainage water with the number of years under cover cropping. Based on observations and FASSET simulations, Berntsen et al. (2006) also estimated an increase in nitrate leaching if cover crops are grown each year.
Knowledge of the amount of cover crop N uptake and the subsequent N release is required to adjust N fertilization of the succeeding crop(s). Since environmental conditions (e.g. weather, soil) can vary largely, the use of crop growth model(s) and of decomposition model(s) can provide useful information. Cover crops normally remain in vegetative stages, thus DM accumulation and N uptake mainly depend on radiation, temperature sum, water and nutrient availability, whereas photoperiod and vernalization are negligible (Habekotté, 1997; Vos and van der Putten, 1997; Qi et al., 1999; Constantin et al., 2015; Tribouillois et al., 2016b). No translocation processes into harvest organs have to be considered. Under adequate soil temperature and soil moisture conditions (Ruffo and Bollero, 2003), N mineralization from cover crop residues is mainly affected by N concentration and C:N ratio (Justes et al., 2009; Finney et al., 2016; Thomsen et al., 2016). Other approaches subdivide crop residues (and soil organic matter) into 2–6 pools, defined by their respective composition and rate of C turnover (Jenkinson et al., 1990; Mueller et al., 1997; Henke et al., 2008). However, the proportions of the respective pools as well as the estimates of their decay rates remain difficult to determine.
In temperate climates, growing catch crops in autumn is a very efficient management measure to prevent nitrate from leaching or denitrification; however, it is not per se sustainable. This nitrogen has to be transferred into the succeeding crop(s) allowing a reduction of its fertilizer N demand, and to be exported from the system by the harvest products. Otherwise, leaching and denitrification risks may be postponed but not solved.
Although main crops like winter OSR or, to a lesser extent, early sown winter cereals, are able to accumulate some N before winter, an additional N uptake in autumn may lead to excessive crop growth hindering optimal canopy management in spring. This does not inevitably ensure improved yield formation or higher N offtake with the harvest products (Sieling and Kage, 2010). In addition, an early sowing date improving root growth and N uptake likely goes along with a higher pressure in weeds, pests and diseases. On the other hand, a delay in soil tillage can reduce N mineralization in autumn. However, if leading also to a postponed sowing date, yield may be impaired due to a lower ability to compete with weeds (Stenberg et al., 1999).
Catch crops have to be wisely integrated into a crop rotation, which is difficult to realize in intensive production systems with only winter crops like winter OSR, winter wheat and winter barley, since no adequate growing period can be provided to accumulate appreciable N amounts. Especially growing winter wheat after OSR rape or grain legumes regularly results in high N leaching losses due to a high N release between OSR harvest (end of July) and wheat establishment (mid to end of September) and to the poor N uptake before winter (Sieling and Kage, 2006). In order to avoid secondary dormancy, it is currently recommended to delay tillage, thus to allow OSR seeds lost during harvest to germinate (Pekrun et al., 2006; Thöle and Dietz-Pfeilstetter, 2012; Huang et al., 2018). After 3–4 weeks, growth of rapeseed volunteers has to be ceased by herbicides or soil tillage to prevent pathogens e.g. club root (caused by Plasmodiophora brassicae) or nematodes from propagation. However, since the use of herbicides comes more and more under criticism, the mandatory soil tillage additionally enhances N release from soil organic matter and crop residues, which, in turn, cannot completely be utilized by the succeeding wheat crop, thus enhancing the risk of N leaching. Although some species (e.g. Avena strigosa) may be able to overgrow and thus to suppress rapeseed volunteers, the risk remains that some rapeseed volunteers survive being a host for club root allowing it to form a new generation. Additionally, the germination rate of OSR seeds decreases due to unfavorable conditions in a fast growing catch crop canopy, thus increasing the OSR seed bank in the soil (Soltani et al., 2017). An establishment of a catch crop after the destroying rapeseed volunteers requires a delay of winter wheat sowing in order to ensure sufficient N uptake by the catch crop. From a practical point of view, farmers will not like late sowing dates of wheat since this decreases the likelihood of suitable sowing conditions (e.g. due to increasing rainfall in late autumn).
Another crucial point is the choice of a suitable catch crop species in order to avoid a further increase of pathogens. Generally, catch crops can act as a ‘green bridge’ providing a permanent food source for e.g. aphids or free-living nematodes or representing a pool for viruses which can be easily broadcasted by aphids into adjacently growing new crops. The latter becomes more and more a problem as increasing temperatures in autumn due to the climate change extend the period of aphid activity, thus increasing the risk of virus infections (e.g. Barley Yellow Dwarf Virus). In addition, the catch crop itself can be a host for pathogens, thus increasing the inoculum in the succeeding main crop. For example, no crucifers should be grown in OSR based rotations due to the club root disease. Phacelia can contribute to the spread of Verticillium spp.. Cereals such as rye or barley are hosts for take all (caused by Gaeumannomyces graminis) (De Cara et al., 2011).
A successful improvement of the N dynamic requires that the succeeding crop utilizes the N recovered by the catch crop as much as possible, thus allowing to reduce its N fertilizer demand adequately. Therefore, crop N demand has to coincide with the N release from the catch crop residues (White et al., 2016a). Often, the crop rotation has to be adapted accordingly, which may reduce the economic return. Winter crops are less suitable because their optimal sowing dates interfere with the catch crop N uptake in autumn. Since in temperate climates the slow increase of soil temperature delays residue decomposition during spring, spring cereals can only partly use N mineralized from catch crops (Cicek et al., 2015) while the early harvest in July/August hampers the utilization of N released during summer and autumn. Therefore, crops with a late harvest in autumn like sugar beet or maize are more suitable; however, farmers have to have the possibility to utilize them, e.g. as substrates for biogas plants.
An early incorporation of catch crop residues, especially with a narrow C:N ratio can lead to a precipitate N release thus increasing N leaching (Thomsen, 2005; Thorup-Kristensen and Dresbøll, 2010). On the other hand, residues with a wide C:N ratio or a high lignin concentration (grasses) may not be completely decomposed during the growing period after incorporation (Pantoja et al., 2015), thus again increasing the risk of N losses in the subsequent percolation period. Therefore, the effects of catch crop growing have to be considered over more than one cropping period (Thomsen and Christensen, 1998; Baggs et al., 2000). In their simulation study of genotype × environment × management interactions, Dresbøll and Thorup-Kristensen (2014) highlighted the need to consider the fate of N within the whole cropping system in order to optimize its N use efficiency instead of regarding only at a single crop. For example, an N surplus of a crop or N rich residues (e.g. of grain legumes) must not inevitably increase N leaching losses if the succeeding crop (OSR or catch crops) is able to take up this N before winter. As long as the nutrients are not lost from the field, they may be used by later crops. However, Doltra et al. (2014) believed that the currently used catch crop management does not sufficiently control the risk of N losses and reminded to develop more efficient strategies in the future. In intensive cropping systems with a high proportion of OSR and winter wheat, this will demand a change of the crop rotation, e.g. to reduce the occurrence of OSR or grain legumes as preceding crop before winter wheat since this sequence does not offer any wise possibility to handle the N problem. Crop growth and N mineralization models would be useful to estimate autumn N uptake and N release in the succeeding crop(s) allowing the farmer to adjust the N fertilization.
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