4.1 Evaluation of carboxylate release in different lupin species
In the greenhouse experiment exudation experiment was carried out as a means to evaluate the carboxylate release and consequently the nutrient acquisition efficiency of the cultivars of L. albus (Feodora) and L. angustifolius (Sonate) that were later used in the field experiment for intercropping with barley. Lupins are characterized by an extraordinarily high efficiency to mobilize sparingly available P, Fe and Mn in the rhizosphere through carboxylate release and acidification which is extensively documented in the literature (Cu et al. 2005; Lambers et al. 2013; Pearse et al. 2006; Wiche et al. 2016b), while barley is described as P-inefficient (Marschner 1995). The results successfully demonstrate that the response of the two species was divergent (Table 3) showing a higher release of carboxylates in L. albus under P-deficient conditions, whereas L. angustifolius responded with decreased release of carboxylates (Table 3) and highest exudation rates under high P-supply. For L. albus, this is in congruency with the results from Pearse et al. (2006), Müller et al. (2015) and Neumann and Römheld (2000) who reported increased diffusion of citrate and malate as a consequence of metabolic shifts in carbohydrate allocation from shoot to roots in concert with increased biosynthesis of malate and citrate and decreased citrate turnover in the tricarboxylic acid cycle. Concomitantly, the decreased release of carboxylates in L. angustifolius suggests that this species (or the selected cultivar) lacks the ability to alter carboxylate metabolism following P-deficiency similar to chickpea (Pearse et al. 2006). Based on the above it seems that L. albus should be preferably selected for intercropping aiming at improved plant nutrition in mixed culture systems, especially when plant growth is limited by P-availability. Indeed, the total amounts of carboxylates released per plant were higher in L. albus whereas the exudation rates (per root dry weight) of both lupin species were similar under low P supply (Table 3) which can be explained by thicker roots and consequently a higher total root dry mass relative to the number of active root tip regions of L. albus (Egle et al. 2003) compared to L. angustifolius. However, when the plants were adequately supplied with P, L. angustifolius showed substantially higher carboxylate exudation rates and amounts of citrate released per plant individual compared to L. albus which highlights the high potential applicability of this L. angustifulius cultivar for improvement of nutrient supply for mixed culture cropping with nutrient inefficient species on moderately fertile soils.
4.2 Effect of substrate properties on plant growth and nutrient availability to the plants
Considering the leaf nutrient concentrations which are commonly used as proxies for the nutritional state of plants (Hayes et al. 2014) it was obvious that on both substrates the barley plants suffered from Mn and P deficiency indicated by leaf P concentrations close or even below to the critical value of 2 mg g−1 P and 50 µg g−1 Mn (Marschner 1995). The lowest concentrations of P and Mn (below 1.9 mg g−1 P and 20 µg g−1 Mn) were observed in plants on substrate A treated with low P-doses (Table 5). Surprisingly, comparing leaf, stem and shoot biomass on both substrates we did not observe significant changes in plant yields between the substrates (Tables 4, 5). Compared to substrate A, concentrations in barley leaves as well as bulk shoot contents (Fig. 1, Table 5), were significantly higher on substrate B, indicating an improved nutrient supply on this substrate with its slightly acidic pH. Furthermore, on substrate B leaf P concentration of lupin plants were significantly higher than on substrate A and significantly higher compared to H. vulgare (Table 5, Fig. 3) while on substrate A leaf P concentration in unfertilized plants of L. angustifolius were similar to H. vulgare. Higher nutrient concentrations in lupins compared to H. vulgare can be explained by a higher nutrient acquisition efficiency of lupins (Pearse et al. 2006). Based on P concentrations determined by CAL-extracts both substrates were sufficiently supplied with P (Marschner 1995) but the phosphorus was most likely not present in plant-available forms. Substrate A was slightly alkaline (pH 7.9) (Table 1) which fosters the precipitation of sparingly soluble Ca-phosphates (Mengel et al. 2001) and low solubility of Mn and Fe. In contrast, soil B was slightly acidic (pH 6.8) (Table 1) so that low specific sorption of P (Mengel et al. 2001) as well as higher solubility of Mn and Fe can be expected (Gupta and Chipman 1976). Generally, higher accumulation and concentrations of the nutrients on substrate B was not surprising (Fig. 1, Table 7). However, the higher availability of the elements on substrate B exhibited by higher tissue concentrations and shoot contents was not a priori predictable based on data of the sequential extraction where substrate A showed lower concentrations of P, Ca, Mn, Fe in mobile, exchangeable fractions (Table 2). On the contrary, substrate B was characterized by higher concentrations of P, Fe and Mn bound into organic matter and amorphous Fe-oxyhydroxides (Table 2). This demonstrates that sequential extractions do not sufficiently describe the availability of elements since they do not integrate all soil-associated factors and plant-associated factors overlapping in the rhizosphere in time, space and function (Hinsinger et al. 2009; Vetterlein et al. 2020). This suggests that in this experiment the higher availability of nutrients on substrate B rather depended on the mobility of the elements in the soil (once they are mobilized) as a consequence of pH and thus, a lower reprecipitation/ readsorption of mobilized elements in the rhizosphere of the plants than distribution of elements in operationally defined element fractions. In this light, we emphasize that CAL-extracts (Table 1) exhibited a higher P availability on substrate B which was in agreement with the substrate-induced differences in tissue P concentrations in plants. This suggests that both the CAL-extractant solutions (acidified Ca-lactate), as well as the plants, were able to access moderately stable element pools through acidification and ligand-exchange reactions, especially the lupins with their efficient acquisition strategy.
4.3 Relationships between the substrate, P-fertilization and lupins on plant growth and nutrient availability in mixed cultures
In this experiment, we used a replacement model, where within the mixed cultures, barley was replaced with 11% of L. albus and L. angustifolius (Wiche et al. 2016a). Although there were slight reductions in yields following a replacement, growth substrate, different levels of P-supply and intercropping did not affect plant yields of barley. With the exception of substrate A and on plots with low P-fertilizer amendment, intercropping with L. angustifolius slightly increased the leaf biomass of barley (Table 5). Of course, plant growth and yield predominantly depend on the nutritional state of the plants which was experimentally controlled by substrate properties, the addition of P-fertilizer and intercropping with lupins. Moreover, the efficiency of intercropping strongly depends on the nutritional status of the lupin plants because under conditions of adequate nutrient availability below ground traits of intercropping plants may not deliver additional benefits. Thus, positive effects of intercropping can be especially expected under conditions of moderate to low nutrient availability. However, under conditions of growth limitation neighbouring plants with more efficient nutrient acquisition efficiency initially satisfy their own nutrient demands. This may lead to root competition for resources and only sharing elements with neighbouring plants after own nutritional demands are satisfied (Cu et al. 2005; Gunes and Inal 2009; Wiche et al. 2016b)
In our experiment, the addition of the P-fertilizer did not influence the P concentrations and contents of barley plants neither on substrate A nor on substrate B (Tables 4 and 5). Possibly, the doses were not high enough (1 g m−2 or 3 g m−2 P) to improve the plants’ nutrient supply due to a fast P fixation e.g. as Ca-phosphates on substrate A so that it was not available for barley and/or the lupin plants strongly competed with barley for phosphate. P concentrations in lupins significantly increased when P was added (Fig. 3) indicating a strong root competition for essential elements between lupins and barley. Finally, resource facilitation in mixed cultures strongly depends on the nutrient status of the lupin plants, their responses through the release of carboxylates influencing the solubility of the elements in the rhizosphere and migration of elements between the intermingling root systems (Cu et al. 2005; Wiche et al. 2016a; Wiche et al. 2017a). The availability of P and micronutrients was higher in substrate B than on substrate A (Table 1, Fig. 1). Therefore, the low performance of L. angustifolius and L. albus in mixed cultures with barley on substrate B (Fig. 1, Table 5) could be explained by the synergetic effects of reduced carboxylate release by the lupins, especially of L. albus (Table 3), and higher substrate-induced solubility of the elements fostering element uptake by the barley plants. Nevertheless, increased Mn concentrations and accumulation (Fig. 1, Table 5) in mixed cultured on substrate B indicate that cluster roots of L. albus were still active even when P-fertilizer was added. It has to be noticed that even on substrate B the plants were still undersupplied with Mn (Table 5, Section 4.2) which is an additional factor triggering carboxylate release by lupins (Lambers et al. 2013; Lambers et al. 2015; Marschner and Römheld 1994). Concomitantly, carboxylates of L. albus are known to strongly affect the availability of Mn as this species is considered a hyperaccumulator of Mn (Lambers et al. 2015). In this regard, lacking effects in mixed cultures with L. angustifolius might indicate a lower ability of L. angustifolius to respond to deficiency of Mn, while decreased accumulation of P and Mn in presence of L. angustifolius could be due to competition of barley and lupins for these nutrients.
On substrate A, intercropping with L. angustifolius slightly increased leaf P concentrations of low P-dosed plants above the critical level of 2 mg g−1 suggesting that the improved nutritional state of the barley plants was responsible for the increase in leaf biomass (Table 5). On this alkaline substrate, leaf and shoot nutrient concentrations and contents of barley were exclusively positively affected (Table 5, Fig. 1) on experimental plots with low P-addition although the leaf P concentrations of lupins suggested a lower P-supply in L. angustifolius (Fig. 3) which should lead to decreased root activity of this lupin species (Table 3). However, in plots with a higher P-supply, we observed a better plant growth of lupins (data not shown here) so that it is reasonable that the mobilized nutrients were initially taken up by the lupins without any positive effects on barley. Concomitantly, increased concentrations and accumulation of Ca, Mn and Fe in mixed cultures with low P supply (Table 5, Fig. 1) most likely originated from resource facilitation under the growth limiting conditions of substrate A, where neighbouring lupins improved the nutritional status of barley plants.
4.4 Effect of substrates, P-fertilization and lupins on the availability of REEs in mixed cultures
In soils, REEs share many chemical similarities with essential plant nutrients, especially calcium (Brioschi et al. 2013; Censi et al. 2014; Censi et al. 2017; Martinez et al. 2018; Wyttenbach et al. 1998). Thus, nutrient bearing soil phases such as phosphates, organic matter and Fe-oxyhydroxides are important hosts for these elements (Diatloff et al. 1993; Zhimang et al. 2000; Cao et al. 2001; Wiche and Heilmeier 2016; Wiche et al. 2016b). Accordingly, in the soil used for the field experiment, REEs were mostly present in fractions 3–5 and with slight enrichment in fraction 3 of substrate B (Table 2). Low soil pH and the presence of dissolved organic matter strongly impact mobility and plant availability of REEs (Cao et al. 2001; Diatloff et al. 1993; Pourret et al. 2007; Tyler and Olsson 2001; Zhimang et al. 2000). As such, the higher concentrations (Table 6) and accumulation (Fig. 2) of REEs on substrate B in comparison to substrate A can be attributed to a higher solubility of the elements in this soil. Higher accumulation of LREEs relative to HREEs observed in this study (Table 6, Fig. 2) closely follow the natural abundance of the elements in the substrates (Table 3). Furthermore, the literature indicates a preferential uptake of LREEs compared to HREEs (Censi et al. 2017, Martinez et al. 2018) due to higher stability of HREE-organic complexes and stronger adsorption of HREEs at ion exchange sites in the soil. These, in turn, may have contributed to these results. Surprisingly, in this study leaf concentrations of REEs were constantly higher than in the stems and the plants mostly responded by changes in leaf REE concentrations (Table 6) which contradicts the findings of Brioschi et al. (2013) and Tyler et al. (2004) who demonstrated decreasing concentrations in the order roots > stems > leaves. A similar order demonstrating REE concentrated in plants as root > leaf > stem > grain was observed by Li et al. (2001), Wen et al. (2001) Yuan et al. (2018) and Xu et al (2003). This suggests that either species-specific or cultivar-specific traits that we did not consider in this study influenced the REE distribution in our plants. Nonetheless, we emphasize that differences in substrates as well as intercropping with lupins impacted both leaf and bulk shoot contents of barley (Fig. 2) although in barley the predominant portion of the shoot biomass consisted of stems (Table 5). Leaves only accounted for one-third of the total shoot biomass (Table 5) and changes in foliar REE absorption due to treatment measures was impactful enough to compensate the lower biomass of this plant part when total shoot contents are considered (Fig. 2). Similar to the findings for nutrients (see Section 4.2), REE concentrations on substrate B were predominantly influenced by substrate without significant effects of P-fertilizer addition or presence of lupins in mixed cultures. However, on substrate B presence of L. albus significantly decreased both shoot REE concentrations and contents, especially when the plants were fertilized with P which highlights an immobilization or uptake of the elements by the lupins under conditions where mobility of the elements is high. Unfortunately, our experimental design did not allow to explore the processes beyond these effects. On the alkaline substrate A, the addition of P-fertilizer significantly reduced both LREE and HREE concentrations in monocultured barley plants (Table 6). This can be attributed to a precipitation of the elements as hardly soluble REE-phosphates at alkaline conditions (Saatz et al. 2016, Han 2020) or a “dilution” effect originating from slightly higher shoot biomass (Table 5) which is frequently reported for non-essential elements (Chien and Menon 1995). Compared to the monocultures, the presence of L. angustifolius significantly increased tissue concentrations and shoot contents of both LREEs and HREEs in mixed cultured barley. Increased REE availability in mixed cultures with lupins was already described by Wiche et al. (2016b) but without considering differences in substrates or nutrient availability. In the present study positive effects of mixed cultures were only visible on the alkaline, P-fertilizer amended soil and in presence of L. angustifolius which releases higher carboxylates under sufficient P-supply (Table 3). Most probably, under these conditions the carboxylates released by lupins mobilized the REEs through formation of soluble REE-carboxylate complexes (Wiche et al. 2017a) in the rhizosphere of the lupins. Since REEs are not essential for plant growth and complexes of REEs are discriminated relative to their ionic forms during plant uptake (Han et al. 2005; Wiche et al. 2017a) the complexes were obviously not adsorbed by the lupins itself enabling the movement to the intermingling barley roots where different chemical properties might have fostered the decay of complexes and thus root uptake and transport of REEs to the shoots.