Plant diversity (and drought) affect legacy effects on soil fertility

We investigated the effect of grassland diversity, drought and higher nitrogen level on legacy effects. Legacy effects (measured as yield of a follow-on crop, which reflect the influence of the preceding crop) were strongly positively affected by the proportion of legumes. Drought can impact legacy effects, but is modest relative to the effect of plant diversity. Aggregated across both ley and follow-on crop phases, the high-diversity, lower-nitrogen grassland community yielded more than the higher-nitrogen grass monoculture. 

Fig. 1. Overhead shot of the field site with the experimental design and plot management to track the effect of plant diversity, drought and fertiliser level on the legacy effect within plots.

Key messages

• The sown proportion of legume in the grassland most strongly enhanced follow-on crop yield.
• Drought during the grassland ley slightly reduced yield of the follow-on crop.
• Higher fertiliser rate in grassland ley did not enhance yield of the follow-on crop.
• Higher plant diversity in grassland increased rotation yield more than higher fertiliser use.

Approach

In a recent field experiment (Grange et al., 2021), we varied the diversity of six-species grassland communities from monocultures to 6-species mixtures (systematically varying combinations of two species from each of three functional groups of grasses, legumes and herbs). In the grassland phase, all plots received 150 kg/ha/yr of nitrogen fertiliser (150N); we also included a perennial ryegrass monoculture with additional nitrogen fertiliser 300 kg/ha/yr of N (300N). An experimental drought was imposed across all communities, and compared with the rainfed control.  

We grew the grassland communities for two years as part of Guylain Grange’s doctoral research (Fig. 1, left). Guylain then suggested that we investigate the legacy effect of the preceding communities i.e. would the yield of a subsequent crop be affected by the community that used to grow there? To test this, we replaced the grassland communities with a monoculture of Italian ryegrass (Fig. 1, right).

In this way, we investigated how factors that influence the yield of a grassland ley (plant species composition and diversity, drought and fertiliser level) also affect the follow-on crop in a rotation. The specific aims of this study were to:

• Investigate the effect of grassland diversity (from monocultures to six-species combinations) on the performance of a follow-on model crop of Italian ryegrass (L. multiflorum).
• Quantify the effect of an extreme weather event (drought) on legacy effects in a grassland-crop rotation.
• Compare the legacy effects of a range of grassland mixtures at lower nitrogen fertiliser (150N) rate, with the legacy effect from a perennial ryegrass monoculture with higher nitrogen (300N).

Results

 

Fig. 2. Effect of functional group composition (relative proportion of grasses, herbs and legumes) on yield *in the rainfed control* of the Italian ryegrass follow-on crop. The legacy effects can be ascribed to the previous grassland community. Performance of the 300N L. perenne monoculture is indicated in red in the colour bar. 

The effect of plant communities on yield (t/ha) of the follow-on crop was strongly related to the proportion of clover in the preceding grassland community.

The ternary diagram in Fig. indicates how the legacy effect varied across the design space with different proportions of the grass, herb and legume functional groups. The greener the colour, the higher the yield of the Italian ryegrass = the greater the legacy effect.

Drought effect: The experimentally imposed drought in the preceding grassland plot had a modest effect on the legacy effect e.g. the drought effect was smaller than the effect of changing % clover.

Effect of high nitrogen: Legacy effects were *lowest* on the perennial ryegrass with higher nitrogen. (See the inserted values in red in the graph legends of the ternary plots.)

Fig. 3. Effect of functional group composition (relative proportion of grasses, herbs and legumes) on yield *in the experimental drought* of the Italian ryegrass follow-on crop. The legacy effects can be ascribed to the previous grassland community. Performance of the 300N L. perenne monoculture is indicated in red in the colour bar. 

High-diversity, lower-nitrogen grassland out-yielded low-diversity, high-nitrogen grassland across both ley and follow-on crop phases

Our approach allowed us to compare three different management scenarios that vary in terms of their: reliance on legumes (only) with 150N; use of higher plant diversity with 150N, and; reliance on a grass monoculture with 300N. We compare these three scenarios across both the grassland and legacy stages. We use the performance of 150N L. perenne as a reference point for comparison of the three scenarios. 

Relative to the performance of 150N L. perenne, the legume monoculture in grassland had the greatest legacy effect; however, the legume-only yield benefit was considerably lower in the grassland phase (Fig. 5a) than that from the equi-proportional 6-species mixture. Directly comparing the high-diversity, lower-nitrogen plant community (blue bar) with the low-diversity, high-nitrogen community across the combined grassland-crop rotation (dashed bar), plant diversity delivered higher performance (Fig. 5b) than additional fertiliser (300 N L. perenne) (Fig. 5c).

Overall, the high-diversity, low-input grassland community yielded more than low-diversity, high-input grassland (aggregated across both ley and follow-on crop phases).

Fig. 4. Comparison of management scenarios on mean (s.e.) dry matter yield on both the grassland and follow-on crop. Relative to 150N L. perenne monoculture (low-diversity, low-input), we compare the effect on yield across a grassland-crop rotation of a) 150N T. repens clover monoculture, b) 6-species mixture receiving 150N (high-diversity, low-input) and c) 300N L. perenne (low-diversity, high-input). 

References

Grange, G., Brophy, C. and Finn, J.A., 2022. Grassland legacy effects on yield of a follow-on crop in rotation strongly influenced by legume proportion and moderately by drought. European Journal of Agronomy, 138, p.126531. Open Access, with data and statistical code.

 Grange et al. 2021. Plant diversity enhanced yield and mitigated drought impacts in intensively managed grassland communities. Journal of Applied Ecology. (with Open Access data and code)

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