Changes in available nitrogen and nematode abundance in response to Brassica seed meal amendment of orchard soil

A better understanding of the effects of Brassicaceae seed meals on resident soil microbial communities is necessary to predict the efficacy of these plant residues as either a biofumigant or organic fertilizer. This study analyzed the influence of high (Brassica juncea) and low (Brassica napus) glucosinolate content seed meals in addition to myrosinase-inactive derivatives on soil microbial community function with respect to nitrogen (N) cycling. All of the seed meal amendments stimulated nitric oxide (NO) generation in an orchard soil. N-mineralization occurred in response to B. juncea seed meal application but the amount of mineralization was reduced by the presence of active myrosinase and corresponding generation of allyl isothiocyanate. Microbial communities responded differentially to seed meal amendments: nematode abundance was enhanced by seed meals with either low glucosinolate or no myrosinase activity whereas fungal and bacterial abundance in soil did not exhibit significant changes in response to any seed meal amendment. In addition to changes in overall abundance, nematode diversity was also modified in response to seed meal amendment and differed among the amendments that enhanced nematode abundance. Collectively, these results indicate that microbial communities and overall soil function respond differentially to both seed meal type/glucosinolate content and isothiocyanate generation. These findings have significance for the efficient use of Brassicaceae residues as a source of plant available nitrogen.