Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/17/2012
Publication Date: 11/26/2012
Citation: Reardon, C.L., Strauss, S.L., Mazzola, M. 2012. Changes in available nitrogen and nematode abundance in response to Brassica seed meal amendment of orchard soil. Soil Biology and Biochemistry. 57:22-29. Interpretive Summary: Organic soil amendments often have been promoted as a means to control soilborne plant diseases and to provide for crop fertility needs. Brassica seed meals are a waster product derived from the oil extraction process. These seed meals have been shown to provide effective control of certain plant diseases and can also serve as a significant source of readily accessible nitrogen. However, the effective use of such materials requires a more comprehensive understanding of their potential non-target effects on overall function of the biology resident to soil systems. This study demonstrated that seed meal derived from certain plant sources can have a negative effect on overall microbiological function in soils. Specifically, oriental mustard (Brassica juncea) seed meal amendment directly suppressed nematode abundance and diversity in soils while canola (Brassica napus) seed meal amendment increased both parameters. The negative effect of mustard seed meal amendment on soil nematode communities resulted in a temporary inhibition of nitrogen mineralization and nitrification. These findings demonstrate that optimizing the use of organic residues for controlling diseases must consider potential non-target effects on overall function of the native soil biology which regulates many different processes that are instrumental in overall plant productivity.
Technical Abstract: Brassica tissues are often promoted as a soil amendment to meet various management objectives, particularly in organic production systems. To predict the efficacy of brassicaceae seed meal amendments as either biofumigants or organic fertilizers, a better understanding of the impacts of seed meal amendment on resident soil microbial communities is warranted. This study analyzed the influence of high (Brassica juncea) and low (Brassica napus) seed meal glucosinolate content in addition to myrosinase inactive derivatives on soil microbial community function with respect to nitrogen cycling. All seed meal amendments examined stimulated NO generation in an orchard soil. N-mineralization and nitrification were temporarily inhibited in response to B. juncea seed meal application but the inhibition was dependent upon myrosinase activity and corresponding generation of allyl isothiocyanate. Microbial communities responded differentially to seed meal amendments: nematodes were enhanced by seed meals with either low glucosinolate or no myrosinase activity, fungi were stimulated by low glucosinolate seed meal, and changes in bacterial abundance were not significantly different from the control. Apart from changes in overall abundance, nematode diversity was also modified in response to seed meal amendment and differed between the stimulatory amendments. Collectively, these results indicate that microbial communities and overall soil function respond differentially to both seed meal type/glucosinolate content and isothiocyanate generation.