Cover crop mixture diversity, biomass productivity, weed suppression, and stability.

A M Florence,C A Francis,J L Lindquist,R A Drijber,L G Higley,Upendra M Sainju

doi:10.1371/journal.pone.0206195

Abstract

The diversity-productivity, diversity-invasibility, and diversity-stability hypotheses propose that increasing species diversity should lead, respectively, to increased average biomass productivity, invasion resistance, and stability. We tested these three hypotheses in the context of cover crop mixtures, evaluating the effects of increasing cover crop mixture diversity on aboveground biomass, weed suppression, and biomass stability. Twenty to forty cover crop treatments were replicated three or four times at eleven sites using eighteen species representing three cover crop species each from six pre-defined functional groups: cool-season grasses, cool-season legumes, cool-season brassicas, warm-season grasses, warm-season legumes, and warm-season broadleaves. Each species was seeded as a pure stand, and the most diverse treatment contained all eighteen species. Remaining treatments included treatments representing intermediate levels of cover crop species and functional richness and a no cover crop control. Cover crop seeding dates ranged from late July to late September with both cover crop and weed aboveground biomass being sampled prior to winterkill. Stability was assessed by evaluating the variability in cover crop biomass for each treatment across plots within each site. While increasing cover crop mixture diversity was associated with increased average aboveground biomass, we assert that this was the result of the average biomass of the pure stands being drawn down by low biomass species rather than due to niche complementarity or increased resource use efficiency. At no site did the highest biomass mixture produce more than the highest biomass pure stand. Furthermore, while increases in cover crop mixture diversity were correlated with increases in weed suppression and biomass stability, we argue that this was largely the result of diversity co-varying with aboveground biomass, and that differences in aboveground biomass rather than differences in diversity drove the differences observed in weed suppression and stability.

Highlights

Increasing species diversity is thought to lead to increased average productivity, invasion resistance, and stability [1,2]
While increasing cover crop mixture diversity was associated with increased average cover crop biomass productivity, we contest the traditional interpretation of this result as evidence of increased niche complementarity or resource use efficiency of diverse mixtures
We argue that increased niche complementarity or resource use efficiency of mixtures should be demonstrated by increased absolute productivity rather than average productivity, which we did not observe

Summary

Introduction

Increasing species diversity is thought to lead to increased average productivity, invasion resistance, and stability [1,2]. Cover crops are used to provide a variety of functions, many of which are positively related to cover crop productivity. These functions include weed suppression, soil nutrient retention, soil erosion control, and organic matter addition. It has been suggested that by increasing cover crop mixture diversity, the various functions of cover crops will be enhanced and stabilized. It has been proposed in both the popular press and the scientific literature that increasing cover crop mixture diversity should be associated with increased productivity, weed suppression, and biomass stability—claims that parallel the assertions made by the diversity-productivity, diversity-invasibility, and diversity-stability hypotheses The majority of previous plant mixture studies in agriculture and ecology have found that while the average productivity of mixtures often exceeds the average productivity of the constituent species in pure stands, the most productive mixture is not necessarily more productive than the most productive single species [7,14,15,16,17]

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Results

Conclusion