Seed-applied micronutrient toxicity to rhizobia and impaired legume nodulation

Micronutrients are sometimes mixed with rhizobial inoculants in liquid solutions to promote the growth and development of legume crops. The compatibility of rhizobia with micronutrients is poorly documented. The objective of this study was to assess the effect of some frequently used micronutrients on rhizobial inoculant survival, nodulation, and nutrition of chickpea and field pea.

Four Australian commercial rhizobia (CC1192, SARDI969, WSM1455, and WU425) were assessed in vitro for their compatibility with plant micronutrients as used in a liquid mixture (4% MnSO4, 2% ZnSO4, 0.2% CuSO4, and 0.04% Na2MoO4). The impact of this mixture on nodulation and plant nutrition was tested on chickpea and field pea under glasshouse conditions. The micronutrient mixture was toxic to all tested rhizobia. While MnSO4 and Na2MoO4 were not inhibitory to rhizobia, individual CuSO4 (>20 µmol l-1 or 0.0005%) and ZnSO4 (>250 µmol l-1 or 0.007%) were lethal to rhizobia. Rhizobial survival was relatively low in a combination of 20 µmol l-1 CuSO4 and 250 µmol l-1 ZnSO4 due to their interactive effect. When rhizobial peat inoculants were suspended in the micronutrient mix, only 35% of rhizobia were recovered at sowing time (1 h after preparation), resulting in poor nodulation of both legumes. Separation of rhizobia (inoculants on seeds) from the micronutrient mixture (applied in-furrow) reduced the risk of inadequate nodulation, while meeting plant nutritional requirements and symbiosis.