Microbial inoculants : global reliability and specific application in a mixed cropping system on marginal land in India

Abstract

The application of microbial inoculants (biofertilizers) is a promising technology for future sustainable farming systems in view of rapidly decreasing phosphate stocks and the need to more efficiently use available nitrogen (N). Various microbial taxa are currently used as biofertilizers, based on their capacity to access nutrients from fertilizers and soil stocks, to fix atmospheric nitrogen, to improve water uptake or to act as biocontrol agents. Since the results of biofertilization in the field are inconsistent we conducted a meta-analysis to quantify benefits of biofertilizers in terms of yield increase, nitrogen and phosphorus use efficiency, based on 171 peer reviewed publications that met the eligibility criteria. Major findings are: i) the superiority of biofertilizer performance in dry climates over other climatic regions; ii) yield response due to biofertilizer application was generally small at low soil P levels; efficacy increased along higher soil P levels in the order arbuscular mycorrhizal fungi (AMF), P-solubilizers and N-fixers; iii) success of inoculation with AMF was greater at low organic matter content and at neutral pH. Our comprehensive analysis provides a basis and guidance for proper choice and application of biofertilizers. Rainfed farms on marginal lands will be most affected by scarcity of non-renewable resources such as fertilizers. Mutualistic root organisms like AMF can substantially contribute to a more resilient, sustainably intensified dryland farming system. We are interested to study the possibility to use AMF as “biofertilizers” in an intercropping system in Indian agriculture, planting pigeon pea (Cajanus cajan) seedlings pre-inoculated with AMF into a field sown with finger millet (Eleusine coracana). By destructive sampling over five weeks we estimated a hyphal growth of 4.1mm d-1 by C. etunicatum which is 1mm faster per day than all other estimates. To study the potential of Rhizophagus fasciculatus, Claroideoglomus etunicatum and Rhizophagus intraradices to spread from AMF-inoculated pigeon pea to un-inoculated finger millet seedlings, we established experimental microcosms in the greenhouse, in which the pigeon pea and two finger millet plantlets were kept in separate pots, connected by soil bridges of 5 or 12 cm length inaccessible to roots but accessible to fungal hyphae. We found that depending on the distance different AMF were promoting the growth of finger millet better. We also detected transport of fertilized nitrogen along the hyphae via stable isotope analysis over a distance of up to 12 cm. However these results also depended on the AMF species. We conclude that the row distance between the crops and the choice of AMF species play a crucial role for the application of AMF as biofertilizer and their growth promotion. To understand the effects the biofertilizers Pseudomonas fluorescens and two AMF species on the microbial community in the soil, both the bacterial community and the community of AMF were studied. Samples were collected at harvest from mono- and intercropped pigeon pea and finger millet at two field sites in South India at the University of Agricultural Sciences, GKVK campus, Bangalore and Kolli hills, Tamil Nadu state, India. DNA was extracted from rhizosphere soil surrounding the roots. To detect changes in the bacterial community automated ribosomal intergenic spacer analysis (ARISA) was conducted and treatments were compared using principal component analysis. The strongest effect was found to be exerted by the plant species; biofertilization had no effect on the bacterial community. To detect changes in the AMF community we amplified the whole ITS ribosomal unit and sequenced the barcoded samples with the PacBio platform. Although OTUs from Glomeromycota were found, the sequencing depth remained too little to make firm conclusions about the changes in the AMF community. Our second goal was to trace the applied inoculum at harvest and, although only few sequences were recovered, the inoculum of Rhizophagus fasciculatus could be traced in some treatments.