Effect of undersown forage or cover crops on the first crops of the crop sequence
At the harvest of the first crop in 2015 (Expt. 1), the different combinations of undersown forage crops (i.e., second crops, functioning as cover crops at this early time point in the sequence) were not found to have any significant effect on the total biomass and grain yields of the main faba bean or oat crops (Table 1). This is consistent with the findings reported by Hauggaard-Nielsen et al. (2009, 2012), who neither observed any effect of an undersown clover-grass cover crop on the dry matter yields of faba bean or oats in a temperate organic cropping system with sandy loam soils. Conversely, other studies have shown that the inclusion of an undersown cover crop mixture improved cereal grain yields in the main crop in organic farming systems (Doltra and Olesen 2013), while a recent review on cover crop practices found that the main crop grain yield may be reduced by an undersown cover crop in comparison with no cover crop (Abdalla et al. 2019). However, the large range of main crops included in the analysis may eliminate the effects of cover crops on individual main crops with potentially different responses to the treatment. Nonetheless, cover crops may provide several other benefits, such as inputs of biologically fixed N, weed reduction, soil carbon sequestration, nutrient retention, and reduced risk of soil erosion, among others (Hauggaard-Nielsen et al. 2009; Hunter et al. 2019). Furthermore, N2-fixing faba bean crop accumulated a higher amount of N than oats in Expt. 1, as expected (Table 2), and there was a high variation depending on the forage crop treatment. Indeed, the effects of undersown cover crop on the grain N yield of the cash crop might vary, being positive with a legume cover crop, negative with non-legumes, and neutral effect for legume-non-legume cover crop mixtures (Abdalla et al. 2019). The weed biomass in the first crops was not influenced by the early stages of the forage crops nor by the cover crops (Fig. 3a). But there was an interaction between the effect of the first crop and the forage crop on weed N accumulation (Table 2), which was higher N in faba bean than in oats and likely reflected a higher soil availability of N due to the symbiotic N2 fixation in faba bean. Furthermore, N accumulation in the weeds under faba bean was significantly higher in the treatment with RC+T than in RC.
Table 1 Dry matter yield of first crops, forage and cover crops, and subsequent cereals in the crop sequences of experiments 1 and 2 during 2015–2017. Data are presented as means (n = 4) with standard errors. The biomass of the forage crops in 2016 is the sum of two cuts in Experiment 1. Means with different letters indicate significant differences within a year at p < 0.05, where lowercase letters indicate significant differences of first crops with the same forage/cover crop treatment, while uppercase letters indicate significant effects of forage/cover crops among treatments with the same first crop. Abbreviations used for the various treatments are as follows: F-No_CC faba bean without cover crop, F-L faba bean and undersown lucerne, F-L+C faba bean and undersown lucerne-cocksfoot grass mixture, F-RC faba bean and undersown red clover, F-RC+T faba bean and undersown red clover-timothy grass mixture, O-No_CC oats without cover crop, O-L oats and undersown lucerne, O-L+C oats and undersown lucerne-cocksfoot grass mixture, O-RC oats and undersown red clover, O-RC+T oats and undersown red clover-timothy grass mixture, F+O intercropped faba bean and oats. Values in bold indicate statistical significance. Results of ANOVA (ns, non-significant; *p < 0.05; ** p < 0.01; ***p < 0.001) Table 2 Nitrogen accumulation of the annual first crops, forage and cover crops, and subsequent cereals in Experiments 1 and 2 during 2015–2017. Data are presented as means (n = 4) with standard errors. The values of N accumulation of forage crops in 2016 are the sum of two cuts in Experiment 1. Values with different letters indicate significant differences within a year at p <0.05, where lowercase letters indicate significant differences of the first crops with the same forage/cover crop treatment, while uppercase letters indicate significant effects of forage/cover crops among treatments with the same first crop. Values in bold indicate statistical significance; 'nd' stands for not determined. Results of ANOVA (ns, non-significant; *p < 0.05; ** p < 0.01; ***p < 0.001) In 2016, the biomass and grain yield of the faba bean and oats was not affected by the undersown cover crops. However, the biomass of the oats in the intercrop with faba bean showed similar values as the oats with undersown cover crops, while the biomass of the faba bean component was considerably lower than the biomass of faba bean when undersown with cover crops (Expt. 2; Table 1). Cereals have been proven to suppress red clover biomass accumulation, while the cereal crop is still present (Gaudin et al. 2013). Early biomass accumulation by the oats (Fradgley et al. 2017) could have resulted in the shading of the undersown forage and cover crop, thus limiting their growth during this phase. However, the competitive pressure is lowered after the cereal harvest, thus allowing for improved growth later in the season (Bergkvist et al. 2011). Similar to Expt. 1, the undersown cover crops did not show any effect on the weed biomass in the first crop of the sequence (Fig. 3d).
In the context and conditions of this study, adding a forage or cover crop into the first crop did not show any effect on yield in the main crop, which agrees with our first hypothesis. In this early stage of the crop sequence, detectable benefits from facilitation processes cannot be expected. However, having no negative influence from the cover or forage crop on the first crop imply that the crop sequence components complement each other, possibly exploiting different spatial sources of nutrients and water and allowing for an adequate light capture.
Forage crops
Forage crops—cover crop phase
In the first year (2015) of the crop sequence, the total biomass production of the forage crops (which at this stage are functioning as undersown cover crops) was significantly higher for oats than faba bean in all treatments (Expt. 1; Table 1). Similar results were found for the legume component of the forage crops, while the grass component produced more biomass having faba bean as the first crop than in the oats crop. De Notaris et al. (2021) found similar results of less biomass of the undersown cover crop mixtures in faba bean than oats in long-term organic crop rotation in a temperate climate with different fertility management (with and without animal manure; between 80 and 110 kg N ha−1), which was attributed mainly to the longer growth period of the faba bean (faba bean was harvested 2–3 weeks after the cereal). The aboveground growth pattern of oats provides less shading than does faba bean, thus resulting in better light conditions for the early stages of the forage crop. Moreover, the biological N2 fixation is increased under the coexistence with a cereal or grass crop (Peoples et al. 2001), which might have positive implications for the resulting amount of legume forage crop biomass produced when grown a preceding cereal crop compared to when undersown in a grain legume crop. This facilitation process may be explained by the benefits of species mixture with different plant functional traits. Furthermore, the undersown forage crops showed lower total N accumulation in faba bean than in oats, despite that the cereal crop is expected to compete more strongly for soil N than the N2-fixing faba bean crop (Table 2). The relative proportions of legume and grass biomass in the forage crop mixtures indicate that the forage legumes were more competitive against grass when undersown in oats treatments than in faba bean (Hauggaard-Nielsen et al. 2012). In the first year of the crop sequence, when the forage crops were functioning as undersown cover crops, the weed biomass seemed to be higher in the faba bean-preceding crop compared to in the oats, but the difference was not statistically significant (Fig. 3a). This is consistent with the findings of Hill et al. (2016), claiming the higher availability of N from faba bean to promote weed growth.
Forage crops—cash crop phase
In the second year of the crop sequence, the total biomass of the forage crop did not show any difference between the treatments. However, the biomass of both legume and grass was affected by the first crop and the forage crop composition. In particular, lucerne biomass was strongly reduced in the L+C mixture compared to the other cover crop treatments after faba bean and compared to L+C after oats as the first crop (Table 1). Furthermore, the grass biomass was higher after faba bean than after oats. The forage grass may have benefitted more when undersown in faba bean than in oats due to less intense competition for plant-available soil N from faba bean. In addition, the more N-rich plant residues of faba bean are likely to contribute to further increases in soil N availability for grasses undersown in faba bean than in oats. This is in accordance with previous findings (Hauggaard-Nielsen et al. 2012; Neugschwandtner et al. 2015), where the increase in soil N availability due to the grain legume improves the growth of the grass compared to the forage legume. This may be due to the ability of grasses to better compete for soil mineral N (Blanco-Canqui et al. 2015). Indeed, the grass forage crops showed a higher amount of N accumulated after faba bean than after oats, and higher N accumulation was seen in cocksfoot than in timothy after both faba bean and oats (Table 2). The only significant difference in total forage crop N accumulation during 2016 (sum of the two biomass harvests) was that it was higher in the lucerne and cocksfoot mixture after oats than after faba bean. This may be supported by the higher Ndfa in lucerne grown together with cocksfoot after oats than after faba bean (211 vs 94 kg N ha−1) (Fig. 4). The results on Ndfa indicate that the forage legumes acquired a large part, often more than 50%, of their N from soil, with lucerne in mixture with cocksfoot after oats being the main exception with a relatively high proportion of N from N2 fixation (Fig. 4). Forage legumes undersown in faba bean likely had access to relatively high levels of soil N, due both to the inputs of fixed N from faba bean and the lack of competition for soil N from cereals or grasses. When interpreting these results, it is important to be aware of the limited precision of the 15N natural abundance method. We observed that the difference in 15N between reference plants and the β values of lucerne and red clover was sometimes less than 5 δ15N units, which limits the precision when calculating Ndfa (Unkovich et al. 2008; Carlsson and Huss-Danell 2014). Despite this methodological uncertainty, our results showed a clear positive effect of the presence of oats and cocksfoot on symbiotic N2 fixation in lucerne (Fig. 4), which follows the logic that the cereal and grass competed efficiently for soil N and forced lucerne to rely relatively more on N2 fixation.
Perennial legumes may deposit N that can be recovered by grasses in cover crop mixtures (Schipanski and Drinkwater 2012). Reiss and Drinkwater (2020) reported no difference in the amount or the proportion of legume N derived from N2 fixation in a cover crop mixture compared to the same legumes as sole cover crop. Frankow-Lindberg et al. (2009) showed that mixtures of forage crops have a higher ability to suppress weeds but also pointed out red clover as a weaker competitor. However, our results did not show any difference among the forage legumes and legume grass mixtures in their weed suppression during the second year of the forage crops (Fig. 3b). In addition, there was no difference in the weeds N accumulation either between crops or between preceding crops in the second year (Table 2). We argue that well-established and dense forage stands developed all treatments with undersown forage crops, thus restricting the weeds through limited access to light and/or belowground growth resources (Ringselle et al. 2017). The treatments without undersown forage crops correspond to including a 1-year fallow in the crop sequence, which is a very uncommon practice in arable cropping systems with similar conditions as the present study but used in some semiarid temperate areas. It can therefore be argued that the comparisons between the presence and absence of a forage crop during the second year have little agronomic interest. Nevertheless, it was necessary to include the treatments without forage crops to assess if and how the undersown forage crops affect the first crop in the sequence.
Cover crops
The total biomass production of the cover crops in Expt. 2 was significantly higher only for red clover undersown in oats than in faba bean (Table 1). Furthermore, the cover crops biomass was slightly higher than the biomass obtained in the forage sole legumes in 2015 (Expt. 1). Several studies have shown higher aboveground biomass produced by undersown cover crops (e.g., red clover) at winter wheat harvest in stockless organic farming systems (Amossé et al. 2014) and conventional based cereal systems in similar climate conditions to our field experiment (Bergkvist et al. 2011). Furthermore, results showed no significant effect between the legume cover crops. Contrary to our results, Amossé et al. (2014) found greater biomass accumulation by the red clover than the lucerne cover crop due to a higher N accumulation by the red clover, thus indicating better competition by the red clover than lucerne against the winter wheat.
Previous research has shown contrasting effects of cover crops on weed suppression, from less successful (Teasdale 1996; Hiltbrunner et al. 2007) to greater weed suppression with increasing cover crop biomass (Finney et al. 2016; Hill et al. 2016). In the present study, no weed suppression by the cover crops was confirmed (Fig. 3d), which further highlights the context specificity of cover crop-weed interactions. However, the biomass of weeds was lower in oats and the faba bean-oats intercrop compared to the faba bean sole crop (Fig. 3d). The improved weed suppression when adding a cereal as an intercrop with a grain legume is one of the key benefits reported in previous intercropping research (Bedoussac et al. 2015).
Subsequent crops
The subsequent winter wheat crop biomass or grain yield (Expt. 1; 2017) did not show any consistent response to the first crops or forage crops (Table 1). However, the total biomass production of winter wheat after faba bean undersown with red clover was higher than when oats were undersown with red clover. Hauggaard-Nielsen et al. (2009, 2012) found similar results in that subsequent spring wheat production did not vary with different preceding crops. However, they observed a significant reduction in the wheat yield when a clover-grass cover crop was undersown in the previous crop. This response was accompanied by a lower availability of N in the soil (Hauggaard-Nielsen et al. 2009), suggesting an enhanced competition for soil N by the cover crop, short-term N immobilization when incorporating cover crop biomass, and, consequently, a reduced N availability for the subsequent wheat crop compared to the treatment without undersown grass clover. In the present study, the subsequent spring barley (Expt. 2) was not influenced by the first crop (faba bean or oats) as observed by Hauggaard-Nielsen et al. (2009, 2012). This might be because of the relatively poor growth of faba bean in 2016, limiting the amount of N inputs in faba bean residues, and/or that the faba bean residue was slowly mineralized. The grain yield was higher when lucerne was included as an undersown cover crop in faba bean and when red clover was included in oats than in the treatments without a cover crop (Table 1). In particular, the results underline that legume cover crops can provide benefits in terms of increased crop yields in cereal-based cropping systems and also that a legume cover crop can partly compensate for low N inputs from the first crop (main crop). Doltra and Olesen (2013) demonstrated that spring barley was less responsive to the effect of previous cover crops due to lower N recovery efficiency. However, the magnitude of this effect depended strongly on the cereal species, cover crop species, and on soil and crop rotation type. The treatments without a forage crop in Expt. 1 (No_CC) were included mainly to assess the competition resulting from undersowing forage crops on first crop performance. We found that the winter wheat yield was as high after no forage crop as after undersown of forage legumes. However, as discussed above (end of Section 3.2), the practical relevance of the fallow (No_CC) may be questioned, since for many farmers, it would mean a reduction in income.
The subsequent winter wheat crop consistently accumulated larger amounts of N after faba bean than after oats, although the difference was only significant in the treatment with red clover as a forage crop. Usually, the N availability for the subsequent crops depends on the N accumulation provided by the preceding legume crops, which is most often linked to their production of biomass (Bergkvist et al. 2011). However, there may be other factors affecting the N availability such as C/N and N/lignin ratio as well as the placement of the cover crop, i.e., whether incorporated into the soil or left on the soil surface. The lack of significant differences in winter wheat yield and N accumulation after forage crops may be explained by the fact that all the aboveground forage biomass had been removed, and thus a large part of the fixed N has left the cropping system. However, residues from roots and stubbles are expected to leave a legacy of N in the rotation, where the C/N ratio of forage crop residues influences the synchrony between N mineralization and N demand of the following crop (Blesh et al. 2019). According to Høgh-Jensen et al. (2004), roots and stubble contain the equivalent of 25% of the total amount of fixed N in harvested aboveground biomass of lucerne and red clover, which in our case corresponds to 10–50 kg fixed N per ha left in the field after the second harvest of the forage crops in Expt. 1. Asynchrony, through the potential short-term immobilization of N, may have limited the use of the residual N by the subsequent winter wheat in the present study, and some of this N might become available for crops later in the rotation. In Expt. 2, the combination of diversified crop rotations with legumes, cover crops, and intercrops showed no significant differences for the N accumulation in the spring barley. Long-term experiments have shown yield improvements in crop rotations, including grass-legume leys (Persson et al. 2008), and the crop diversification of our experiments might, thus, be expected to benefit cereal grain yields in a longer perspective (Bowles et al. 2020).
In the present study, the inclusion of a forage crop did not result in any effect on weed biomass in the subsequent winter wheat (Fig. 3c). Several studies highlighted the benefits of implementing green manures (perennial legumes as a sole crop or in plant mixtures) in the crop rotation for weed suppression, by limiting the input of new seeds in the weed seed bank (Melander et al. 2020) or by leaving the green manure for longer periods (Sjursen et al. 2012). Indeed, weed biomass is a less suitable indicator for understanding the mechanism of plant-plant competition than plant density or ground cover by weeds. From the present study, we cannot discern whether forage crops also have had a weed-suppressing effect in the subsequent winter wheat, and that this effect was counteracted by cover crop plants re-emerging as weeds in the subsequent crop. Conversely, in organic crop rotation, green manures (plant material was removed) presented a residual effect on weed seed banks 3 years after termination (Melander et al. 2020). These results may be influenced by the duration of the green manure, lack of soil disturbance, and the ability of the green manure to re-grow. Furthermore, in Expt. 1, red clover was encountered among the weed samples collected from the subsequent winter wheat, indicating a poor effect of the tillage and herbicide (or a strong effect of the clover to survive). There is evidence that cover crops may return as volunteer weeds in the subsequent crop, thus limiting the effect of the cover crop on weed suppression and the yield of the subsequent crop (MacLaren et al. 2019). Likewise, cover crops in Expt. 2 did not show any effect on weed suppression in the following crop (Fig. 3e). Several factors such as cover crop growth and biomass, decomposition rate, and method of termination (Blanco-Canqui et al. 2015) or increase in N supply by legume cover crops (Sjursen et al. 2012) may have influenced the weed biomass in the spring barley. Salonen and Ketoja (2020) demonstrated a limited effect of cover crops on weed suppression in organic cereal-based rotations under reduced tillage. Furthermore, no significant differences were observed in the N accumulation in weeds in the subsequent cereal crops in either of the two experiments (Table 2). The total N accumulated by the weeds was higher after oats in both experiments, although this was not statistically significant.
The present study hypothesized that the yield and total N accumulation of the subsequent cereal would be influenced by preceding crops in the sequence. This was not fully achieved by the different combinations of crop diversification practices. However, adding a cover crop in the rotation increased the grain yield of the spring barley, and adding a forage crop in combination with faba bean in the crop rotation tended to increase the yield of the winter wheat for both the biomass (p = 0.2) and grain (p = 0.5). Furthermore, we hypothesized that forage and cover crops could increase weed suppression. However, this effect might be dependent on other management factors and plant species interactions. It is also possible that the relatively small plot size used in our experiments (2 × 12 m) was not sufficient for capturing potential patchiness in the weed distribution, which might have caused large variations and thus reduced the possibility to obtain statistically significant differences in weed biomass. Since the results of this study did not show any penalty on the subsequent crop by the weed biomass, there may be other services provided by either the crops in the rotation or weed diversity. There are still challenges associated with the implementation of crop diversification practices, such as better synchrony of the crops in the rotation.
The novelty of this study lies in the combination of different crop diversification practices in cereal-based crop sequences in a temperate climate. We intended to give insights on how diversified crop rotations relate to resource acquisition by understating plant species interactions. The results demonstrated that replacing cereal with a grain legume earlier in the sequence and the addition of forage or cover crops had either neutral or positive effects and could partly compensate for the low preceding crop effects from cereals in less diverse cereal-based cropping systems in terms of crop productivity and N acquisition. Our findings thus show promising effects of combining several crop diversification practices in crop rotations and highlight the need for continued research for identifying and optimizing the positive long-term effects of combined crop diversification practices.