Role of native soil biology in Brassicaceous seed meal-induced weed suppression
Introduction
Growth in organic and sustainable agricultural production systems has generated demand for compatible weed control strategies. Brassicaceous seed meal (BSM) residue, a waste product of the oil extraction process, can provide a local resource for supplemental nutrients (Hoagland et al., 2007), disease control (Lazzeri and Manici, 2001, Mazzola et al., 2001, Zasada and Ferris, 2004, Mazzola and Mullinix, 2005), and/or weed suppression (Brown and Morra, 1997). However, the mechanisms contributing to the observed BSM weed control remain unclear (Boydston and Hang, 1995, Brown and Morra, 1997).
Decreased weed emergence has been repeatedly documented following soil incorporation of Brassicaceous crop and BSM residues (Boydston and Hang, 1995, Al Khatib et al., 1997, Brown and Morra, 1997). The mechanism of weed suppression has been attributed to allelopathy, which is defined as the inhibitory effect of one plant or microorganism on another through chemical release from the donor to the environment (Kobayashi, 2004). Glucosinolate hydrolysis products are thought to be responsible for the weed suppression induced by Brassicaceous residues (Brown and Morra, 1997). The hydrolytic enzyme, myrosinase, and water are required for glucosinolate hydrolysis. The type, concentration, and functionality of glucoinolate hydrolysis products vary among Brassicaceous species. Glucosinolates are present in all Brassicaceous plant parts, but are most concentrated in seed (Borek and Morra, 2005). If cold pressed, residual BSM retains glucosinolate content and viable myrosinase after seed oil extraction (Borek and Morra, 2005). Therefore, it is reasonable to hypothesize that glucosinolate hydrolysis products have a role in the weed suppression resulting from application of BSM.
Although weed suppression by Brassicaceous residues has long been attributed to glucosinolate induced allelopathy, there has not been a consistent relationship between observed weed suppression and measured glucosinolate content. For example, significant plant suppression has been observed with low glucosinolate content Brassica napus residues (Boydston and Hang, 1995, Brown and Morra, 1996, Al Khatib et al., 1997). These authors suggested either effective action by a relatively small amount of a specific but unidentified glucosinolate hydrolysis product, or that microbial degradation resulted in production of other inhibitory compounds. Some glucosinolate hydrolysis products such as ionic thiocyanate have biocidal effects, and are used as the active ingredient in several commercial herbicides (Borek and Morra, 2005). However, these products control weeds at effective ionic thiocyanate (SCN−) concentrations of 137–1366 kg SCN− ha−1, much higher than that found in BSM amendment rates that have been found to be phytotoxic (Borek and Morra, 2005). Phytotoxicity has been observed at BSM amendment rates of 1000–4000 kg SM ha−1, with only 8.8–35.3 kg SCN− ha−1, assuming complete conversion to toxic hydrolysis products (Borek and Morra, 2005). In addition, soil physical, chemical, and biological characteristics influence expression and longevity of allelochemicals under field conditions (Inderjit et al., 2001).
Incorporation of plant residue, including Brassica spp., is also commonly associated with rapid increases in total microbial activity, which can include plant pathogenic soil fungi and oomycetes (Grünwald et al., 2000, Manici et al., 2004, Cohen et al., 2005), with many capable of inciting root, stem, or seed rots (Pitty et al., 1987) that can be fatal to both crop and weed species. Many members of the genus Pythium incite both pre- and post-emergent damping-off of plants. Populations of Pythium spp. in soil are amplified in response to organic matter addition, survive in competition with other microorganisms (Chen et al., 1988) and withstand frequent cultivation (Grünwald et al., 2000, Mazzola and Gu, 2000).
Application of Brassicaceous amendments may provide an alternative weed control strategy, but the mechanism of action must be better understood to generate guidelines and recommendations for use of this practice as a management tool. These studies were performed in or with multiple orchard soils to test the hypothesis that induced amplification of resident Pythium spp. contributes to the weed suppression observed in response to BSM amendments.
Section snippets
Soils and soil treatments
Studies were conducted at or in soils collected from three experimental orchards: the Columbia View Experimental (CV) orchard, Orondo, WA; the Wenatchee Valley College-Auvil Teaching and Demonstration (WVC) orchard, East Wenatchee, WA; and the Tukey Horticulture Research and Experimental (TU) orchard, Pullman, WA. Soils at these sites are characterized as Adkins very fine sandy loam (coarse–loamy, mixed, mesic Xeric Haplocalcid) with 1.3% organic matter (OM) and pH 7.6, Pogue sandy loam
Weed emergence and biomass in the greenhouse
Seed meal treatments resulted in significant (P < 0.05) reductions or increases in plant emergence, with the response being seed meal or plant dependent. T. aestivum emergence and survival were reduced by amendment of soil with B. napus, G. max or S. alba SM, relative to the control (Table 1a). In contrast, pasteurization, B. juncea amendment, and mefenoxam treatments typically increased plant emergence (Table 1a). Emergence of V. villosa was low overall and consistent treatment effects were not
Relationship between SM amendment and Pythium on weed suppression
Application of Brassicaceous plant residues has been promoted as a viable strategy for the control of diverse yield-limiting pests (Lazzeri et al., 2003, Pascual et al., 2004). However, as has been the case for a variety of bio-based amendments, use of Brassicaceous residues for control of weeds and soil-borne diseases has not been widely adopted due to the inconsistency in performance realized across production systems. The ability to determine the underlying factor(s) limiting efficacy of
Acknowledgments
We would like to thank the USDA-CSREES Organic and Integrated Grant Program and Washington Tree Fruit Research Commission for support of this research. Thanks also to Sheila Ivanov and Kevin Hansen for technical assistance in lab and field experiments, Kurt Schroeder for assistance with real-time PCR, Tim Paulitz and Steve Jones for critical review of the manuscript, and Kent Mullinix at WVC orchard and Deb Pehrson at TU for allowing us to collect soil for greenhouse experiments. We acknowledge
References (39)
- et al.
Control of soil-borne plant pests using glucosinolate-containing plants
Advances in Agronomy
(1997) - et al.
Brassica napus seed meal soil amendment modifies microbial community structure, nitric oxide production and incidence of Rhizoctonia root rot
Soil Biology and Biochemistry
(2005) - et al.
Plant phenolic acids in soils: sorption of ferulic acid by soil and soil components sterilized by different techniques
Soil Biology and Biochemistry
(1989) - et al.
Nematode suppression with brassicaceous amendments: application based upon glucosinolate profiles
Soil Biology and Biochemistry
(2004) - et al.
Weed suppression with Brassica green manure crops in green pea
Weed Science
(1997) - et al.
Ionic thiocyanate (SCN−) production from 4-hydroxybenzyl glucosinolate contained in Sinapis alba seed meal
Journal of Agricultural and Food Chemistry
(2005) - et al.
Rapeseed (Brassica napus) green manure crop suppresses weeds in potato (Solanum tuberosum)
Weed Technology
(1995) - et al.
Hydrolysis products of glucosinolates in Brassica napus tissues as inhibitors of seed germination
Plant and Soil
(1996) - et al.
Registration of ‘IdaGold’ mustard
Crop Science
(1997) - et al.
Registration of ‘Pacific Gold’ condiment yellow mustard
Crop Science
(2004)