Multi-species cover crop mixtures have been increasing in popularity. It has been hypothesized these mixtures produce more biomass, suppress more weeds, scavenge more N, conserve more soil water, stimulate more soil biology, promote higher yields of subsequent crops, and have higher production stability than the best of their monoculture counterparts. This systematic review synthesizes a growing body of cover crop mixture research to assess, for these metrics, whether cover crop mixtures can perform better than their constituent species when planted alone. Searching three databases, we identified 27 studies which compared cover crop mixtures (containing at least three species) to all their constituent species. The studies contained 119 sampled cover crop plantings that met our eligibility criteria. From these, we extracted 243 full comparisons of the bestperforming mixture and best-performing monoculture for the selected metrics. In 88% of these comparisons, the monoculture and mixture performed comparably. In 10% of the comparisons, the monoculture did better, and in 2% of comparisons the mixture performed better. Overall, there are few published studies documenting the superiority of cover crop mixtures over monocultures for our selected metrics. Abbreviations: CC, cover crop; CV, coefficient of variation; LER, land equivalent ratio; RYT, relative yield total. This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.

16 The diversity-productivity, diversity-invasibility, and diversity-stability hypotheses 17 propose that increasing species diversity should lead, respectively, to increased average 18 biomass productivity, increased invasion resistance, and increased stability. We tested 19 these three hypotheses in the context of cover crop mixtures, evaluating the effects of 20 increasing cover crop mixture diversity on aboveground biomass, weed suppression, and 21 biomass stability. Twenty to forty cover crop treatments were replicated three or four 22 times at eleven sites using eighteen species representing three cover crop species each 23 from six pre-defined functional groups: cool-season grasses, cool-season legumes, cool-24 season brassicas, warm-season grasses, warm-season legumes, and warm-season 25 broadleaves. Each species was planted in monoculture, and the most diverse treatment 26 contained all eighteen species. Remaining treatments included treatments representing 27 intermediate levels of cover crop species and functional richness and a no cover crop 28 control. Cover crop planting dates ranged from late July to late September with both 29 cover crop and weed aboveground biomass being sampled prior to winterkill. Stability 30 was assessed by evaluating the variability in cover crop biomass for each treatment 31 across plots within each site. While increasing cover crop mixture diversity was 32 associated with increased average aboveground biomass, this was the result of the 33 average biomass of the monocultures being drawn down by low yielding species rather 34 than due to niche complementarity or increased resource use efficiency. At no site did the 35 highest yielding mixture out-yield the highest yielding monoculture. Furthermore, while 36 increases in cover crop mixture diversity were correlated with increases in weed 37 suppression and increases in biomass stability, we argue that this was largely the result of 3 38 diversity co-varying with aboveground biomass, and that differences in aboveground 39 biomass rather than differences in diversity drove the differences observed in weed 40 suppression and stability. The results of this study contradict popular interpretations of 41 the diversity-productivity, diversity-invasibility, and diversity-stability hypotheses. 43 Introduction44 Increasing species diversity is thought to lead to increased average productivity, 45 increased invasion resistance, and increased stability [1,2]. Respectively named the 46 diversity-productivity, diversity-invasibility, and diversity-stability hypotheses, these 47 three hypotheses, while contested in the field of ecology [3][4][5], have often been treated in 48 the field of agriculture as proven principle with regard to mixed cropping. Increasing crop 49 mixture diversity is often assumed to be associated with increased productivity, increased 50 weed suppression, and increased stability despite a lack of compelling empirical evidence 51 in favor of these assertions [6][7][8][9]. The goal of this study is to test these three...

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.

Soil potassium (K) has traditionally been portrayed as residing in four functional pools: solution K, exchangeable K, interlayer (sometimes referred to as “fixed” or “nonexchangeable”) K, and structural K in primary minerals. However, this four-pool model and associated terminology have created confusion in understanding the dynamics of K supply to plants and the fate of K returned to the soil in fertilizers, residues, or waste products. This chapter presents an alternative framework to depict soil K pools. The framework distinguishes between micas and feldspars as K-bearing primary minerals, based on the presence of K in interlayer positions or three-dimensional framework structures, respectively; identifies a pool of K in neoformed secondary minerals that can include fertilizer reaction products; and replaces the “exchangeable” K pool with a pool defined as “surface-adsorbed” K, identifying where the K is located and the mechanism by which it is held rather than identification based on particular soil testing procedures. In this chapter, we discuss these K pools and their behavior in relation to plant K acquisition and soil K dynamics.

Potassium fixation traps K + ions in the interlayer region of phyllosilicates. This study determined if increased negative interlayer charge caused by structural Fe reduction leads to increased K + fixation. The five reference clays used were illite (IMt-1), kaolinite (KGa-1b), montmorillonite (STx-1b), nontronite (NAu-2), and vermiculite (VTx-1). Soil clays were fractionated from the upper 15 cm of a Belvue loam and a Cherokee silt. Potassium fixation capacities were measured on clay samples of unreduced and reduced forms of each clay. Iron (II) and total Fe contents were determined, and K + fixation was measured by K saturating the clays, followed by five washes of MgCl 2 solution. Iron reduction significantly increased the amounts of K + fixed by NAu-2 and VTx-1. An increase in Fe(II) content caused increases in layer charge and K + fixation. Although NAu-2 exhibited a greater increase in Fe(II) content on reduction than VTx-1, the increase in K + fixation on reduction was greater for VTx-1 because of the tetrahedral location of Fe in VTx-1. For IMt-1, KGa-1b, and STx-1b, Fe reduction did not significantly affect the K + fixation capacities because of their low Fe contents. The Belvue loam released K + in both unreduced and reduced forms. The Cherokee silt did not appreciably release or fix K + in either form. Although much K is removed in the first wash, small amounts of K were removed in subsequent washes, especially for reduced samples of NAu-2 and VTx-1. The washing procedure caused reduced Fe to reoxidize, which resulted in K that was previously fixed to be released.Abbreviations: BEL, Belvue site; CHE, Cherokee site; HDPE, high-density polyethylene. P otassium fixation is the entrapment of K + ions in between collapsed 2:1 phyllosilicate layers (Rich, 1968). In order for layer collapse to occur, interlayer spaces need to be dehydrated. Interlayer cations dehydrate when their attraction to interlayer surfaces exceeds their attraction to their hydration shell (Kittrick, 1966;Eberl, 1980). Potassium ions and other cations such as NH 4 + , Cs + , and Rb + are readily fixed because they have relatively low energies of hydration. Thus, they can easily dehydrate. It should follow then that the greater the attraction of interlayer cations for interlayer surfaces, the greater the likelihood of cation fixation occurring. This study was conducted to test the hypothesis that increased negative interlayer charge caused by structural Fe reduction leads to increased K + fixation. Vermiculites are defined as having greater negative layer charges than smectites, and they also tend to fix more K + ions than smectites (Rühlicke, 1985;Saha and Inoue, 1998). Furthermore, vermiculites with greater layer charge have been found to have greater cation fixation capacities than vermiculites with less layer charge, and smectites with greater layer charge have been found to have greater cation fixation capacities than smectites with less layer charge (Barshad, 1954;Weir, 1965;Inoue, 1983;Eberl et al., 1986;Douglas, 1989 Core Ideas...