Effects of Incorporated Rye and Hairy Vetch Cover Crop Residue on the Persistence of Weed Seeds in the Soil

Abstract

Incorporation of cover crop residue into the soil has been suggested as a means for reducing weed seedbanks. To explore this hypothesis, we buried mesh bags of seeds mixed with sand at 15-cm depth in late fall in plots that had been planted with rye (Secale cerealeL.) or hairy vetch (Vicia villosaRoth.) or left unplanted. Separate bags contained either velvetleaf (Abutilon theophrastiMedik.), giant foxtail (Setaria faberiHerrm.), Powell amaranth (Amaranthus powelliiS. Watson), or common lambsquarters (Chenopodium albumL.). The experiment used a randomized complete block design with five replications, and enough bags were buried to allow a final recovery in each of the following three springs. Each spring, bags were exhumed, and seeds were either counted and tested for viability or mixed with chopped cover crop material or simply stirred for control bags, and the material was reburied. The experiment was completed twice with initial burials in fall of 2011 and 2013. Rye had no consistent effect on persistence of seeds of any of the species. For two observation intervals, rye increased persistence of a species; for another two intervals, it decreased persistence relative to the control; but mostly rye did not affect persistence. Hairy vetch decreased persistence ofC. albumandA. powelliiin both runs of the experiment but had no effect on persistence ofA. theophrastiandS. faberi. Germination of the first two species is promoted by nitrate, whereasA. theophrastigermination is not sensitive to nitrate, andS. faberiis only rarely nitrate sensitive. We suggest that nitrate released during decomposition of hairy vetch may have promoted fatal germination ofC. albumandA. powellii. Incorporation of legume cover crops like hairy vetch may provide a means for decreasing the seedbanks of the many weed species whose germination is promoted by nitrate. The lack of any reduction ofA. theophrastiandS. faberiseed persistence in response to hairy vetch and the inconsistent and mostly negligible effect of rye indicate that a general increase in readily decomposable organic matter through incorporation of cover crops may be ineffective at reducing weed seedbanks.

Tillage and Cover Crop Effects on Weed Seed Persistence: Do Light Exposure and Fungal Pathogens Play a Role?

“…With respect to cover crop effects on seed persistence, our results are consistent with several recent studies suggesting that residues with high C:N ratios may increase seed persistence relative to residues with low C:N ratios or bare soil (Hill et al 2016; Mohler et al 2018). We found no evidence that cover crop–mediated changes in fungal pathogens were responsible for the observed increase in persistence of D. sanguinalis , but the mechanism and significance of this effect remains unclear.…”

“…Our results are generally consistent with those of several other studies that also evaluated the impact of cereal rye on persistence of various summer annual weeds. Mohler et al (2018) also found that the persistence of A. powellii was unaffected by rye cover crop residue. However, they found that in the grass species S. faberi , seed persistence (in one of two burial studies) was twice as high in the presence of rye compared with a bare-soil treatment.…”

“…Rye is also a well-known source of allelochemicals (Barnes et al 1987) that may be toxic to decay agents of weed seeds. Another potential mechanism, suggested by both Hill et al (2016) and Mohler et al (2018), is that cover crops with high C:N ratios, like cereal rye, reduce fatal germination of seeds by lowering their exposure to soil nitrate. Because nitrate is well known to stimulate germination of many weed species, including both A. powellii (Brainard et al 2006) and D. sanguinalis (Gallart et al 2008), nitrogen immobilization following incorporation of high C:N cover crops may reduce fatal germination and hence increase persistence of these species.…”

Cover crops as a weed seed bank management tool: A soil down review

“…This same study concluded that although the biomass accumulation of hairy vetch was very climate-dependent, the total biomass accumulation increased due to delayed termination. Therefore, cover crop termination timing is an important management decision for weed suppression when considering the effect of legume biomass accumulation and resulting effects on plant available N. Mohler et al (2018) evaluated the seed fate of two N-sensitive weeds (Chenopodium album and Amaranthus powellii) and two primarily N-insensitive weeds (Setaria faberi and Abutilon theophrasti) and found that hairy vetch reduced the persistence of the N-sensitive weeds by causing fatal germination, but had no effect on persistence of the other weeds.…”

Weed Seedbank and Weed Biomass Dynamics in a Long-Term Organic Vegetable Cropping Systems Experiment

“…In the other systems, seedlings emerging after cover crop incorporation in June were killed by bed preparation for planting the cabbage in July. The release of nitrogen from the legumes may have triggered substantial fatal germination of these nitrate-sensitive Amaranthus species (Mohler et al 2018), and the fresh residue would have promoted attack on seedlings by pathogenic fungi that the cabbage transplants were robust enough to resist (Mohler et al 2012). The cabbage crop itself was cultivated, hand hoed, and then cultivated late (September 1, 2006).…”