Legume-grass assemblages are usually more productive than either of these plant functional types in monoculture (Sleugh 2000; Sturludóttir et al. 2014). This is an example of transgressive overyielding in which functional differences between species lead to a strong complementarity (Schmid 2008; Hooper and Dukes 2004). It is generally understood that productivity is enhanced by better spatial and temporal capture of light in the mixed-species canopy, above-ground, and of soil resources below-ground (Gliessman 2015, Homulle et al. 2021). In particular, nitrogen fixation in legumes, spillover of this nutrient to soil and its exploitation by adjacent species has provided an explanation for overyielding in grassland pastures (Høgh-Jensen and Schjoerring 2001, Gylfadóttir et al. 2007; Scott et al. 2018). Little attention appears to have been given to the possibility that grasses may also improve the nutritional status of legumes through modification of soil biogeochemistry, but the findings of the present study suggest this is the case.
Clover and Lotus had higher yields when growing with grasses than when growing alone. No more or less N was measured in foliage of any of the species apart from the native tussock grass that actually acquired less nitrogen when growing with legumes. Otherwise, there were no significant differences in the total foliar content of N. However, as was also indicated by the routine soil analysis, N did not appear to be an important nutrient limiting plant growth in this soil. The experimental conditions provided adequate soil moisture for growth which is likely to have favoured the growth of legumes over grasses, and there may have been little investment in root nodules and reliance on N from this source.
At least six elements (P, S, Ca, Mg, Mn, and B) were deficient in the pot experiment soil. There was evidence from the present study that all these elements, and also Mo, were extracted in higher concentrations, quantities or total amounts from the total soil pool by a combination of legumes and grasses, compared to either plant type alone. The best example was of clover growing with cocksfoot; eight elements were taken up in larger amounts into clover foliage. Field sampling of a more fertile soil similarly showed significantly higher uptake of K, Mn, Cu and B, which suggests compatibility is a ubiquitous phenomenon in grasslands.
Phosphorus, K and S concentrations in the foliage of legumes were higher in the presence of grasses. Foliar concentrations of K and S were also higher in ryegrass when it was growing with legumes. Otherwise, key elements were in lower foliar concentrations in grasses when they were growing with clovers. This was most evident in the native tussock where N, P, K, Ca, B and Zn were in lower foliage concentrations when growing with legumes. Grasses consistently extracted lesser amounts of the total soil pool of nutrients when growing alone. The native tussock grass did not benefit nutritionally from the presence of legumes, but higher P concentration were found in tussock foliage and, as a companion plant, clover had a higher concentration of P when it grown with tussock. It appears that tussock grass has some ability to mobilise P in its rhizosphere.
Possible mechanistic explanations for some of these findings are provided in the scientific literature. Phosphorus is a particularly critical element due to its low solubility and mobility in soil (Scott and Condron 2003; Saleem et al. 2020); P availability in the rhizosphere is increased by secretion of organic exudates and also by the release of protons, the latter particularly in acid soil (Cu et al. 2005). Evidence of partner plants being complementary to one another in the context of exploiting this element has been found previously (Homulle et al. 2021, Lambers et al. 2021). Potassium was not deficient in the experimental soil, but it is known that legumes have a higher K absorption efficiency than grasses (Wang et al. 2014), which would suggest legumes have an advantage in competition for K when growing with grasses. Root exudates containing glutamic acid, tyrosine, and leucine increase K uptake from the soil. In the pot experiment of the present study, K concentrations in Lotus and grass foliage were both higher when they were growing together rather than alone. In the field sampling, clover had higher foliar K concentrations when it was growing with grasses. This may be due to K mobility being increased in the grass rhizosphere soil, but then shared with the legumes.
Of course there are also functional interactions between physico-chemical variable.. The interaction of S with other elements through its modification of rhizosphere conditions generally exceed its own direct nutritional value, but legumes require more S than grasses and microbial activity associated with grass rhizospheres accelerate the oxidation and mineralization of organic S into S2− or SO42− (Wainwright 1984). When soil pH is lower, soil redox potential decreases and solubility of Fe and Mn increases. Grasses also respond to Fe, Zn, Mn and Cu deficiency with enhanced siderophore release from roots (Marschner and Römheld 1994; Erenoglu, et al. 2000). Uptake of Zn, Mn and Cu is poorly controlled in plants (Lambers et al. 2021). In the present study, clover had higher foliar concentrations of these elements when it grew with ryegrass and tussock. Clover-cocksfoot and Lotus-ryegrass assemblages procured more Mn in the present study. Also, in the field sampling of the present study, when clover grew with any of the grass species or multi-species assemblages, foliage concentrations of Mn were higher.
Phytosiderophores may have played a key role in higher Cu and B concentrations in clover foliage when growing with cocksfoot, but mechanistic explanations are likely to be more complex. Understanding Zn deficiency provides a good example (Hafeez et al. 2013). Each essential nutrient has specific physiological and biochemical roles; Zn influences the nitrogen content of legumes (Buerkert et al. 1990; Bolanos et al. 1994) and rhizobia can mobilise Zn in soil to some extent (Chen 2003). The results of the present study indicate that cocksfoot mobilized Zn and its uptake when growing by itself. Clover may then have consumed mobile Zn, limiting availability of this element to cocksfoot, in turn stimulating cocksfoot to synthesize and secrete mugineic acids. Molybdenum is known to be an important trigger for photosynthesis and N metabolism (Imran et al. 2021), and this element was better exploited from the soil pool in the pot experiment of the present study. Similarly, Mg is known to improve N-use efficiency (Tian et al. 2021); foliar concentrations of Mg were lower in grasses in the presence of legumes.
It is difficult to identify the specific plant traits that which lead to species complementarity and transgressive overyielding (Hooper and Dukes 2004; von Felten and Schmid 2008), and clearly nutrient mobility in soil and acquisition by plants is particularly complex. The present study lacks any investigation or analysis of the root systems in the pot experiment or field site, but it is known that there is a substantial rhizophere priming effect on nutrient cycling (Lu 2020). Variations between plant species in root exudation pattern and efficiency of acquiring nutrients has long been discussed (Bardgett et al. 1999), and combinations of clovers and grasses have been shown to increase microbial enzyme activity in soil and release of nutrients from organic matter (Sekaran et al. 2020). Findings in the present study relating to each of the elements that were analysed are hardly definitive, but the body of evidence presented clearly illustrates benefits to legumes through partnership with grasses. Productivity is increased and nutrient acquisition from the soil is enhanced by neighbouring grasses that have a complimentary role. Increases of both productivity and foliar concentrations showed a greater total offtake of P, Zn, Mo and Mn from the soil nutrient pool.
In a broader practical context, increasing the legume component of pasture has been a goal of high-country farming in New Zealand for several decades by attempting to introduce a wide range of exotic clovers and other herbaceous legumes to increase vegetation yields and stock productivity. The findings of the present study indicate that cocksfoot and other grasses, including native tussock, play a mutually beneficial role that previously been largely overlooked. Native tussock grass contained lesser amounts of N and five other nutrients when it was growing with the exotic legumes, suggesting less compatibility, a lack of adaptation to coexistence and perhaps to competition for soil N. Exotic herbaceous legumes appear to offer little benefit to tussock grasses, but sustaining residual assemblages of indigenous species within the productive farming landscape is hugely important to biodiversity conservation in New Zealand. This study suggests it would be worthwhile to elucidate functionality of plant diversity in a wider range of native plants in terms of biodiversity protection, soil biogeochemistry and efficient exploitation of nutrient resources.