Responses of soil nematodes to combined bio-organo-mineral fertilizers on upland rice cropping in the highlands of Madagascar

Upland rainfed rice cropping in the highlands of Madagascar is strongly limited by poor Ferralsol mineral fertility. There is an urgent need to identify efficient and sustainable fertilization practices that improve soil fertility without inducing pest proliferation. For that purpose, using a field trial for 2 successive years, we tested the effect of 16 fertilization practices on the abundance and taxonomic diversity of soil active nematodes, which are known to be biological indicators of soil fertility. We tested both fertilization practices traditionally used by farmers and innovative ones based on the assemblage of organic, mineral and biological (earthworms and mycorrhiza) fertilizers. We identified eight types of practices: (1) no fertilization; (2) fertilization with NPK and urea; (3) low input rates (3 t dry matter ha−1) of organic fertilizers without NPK; (4) low input rates of organic fertilizers with NPK; (5) high input rates (6 t dry matter ha−1) of organic fertilizers; (6) high input rates of organic fertilizers with mineral fertilizers; (7) high input rates of a mixture of organic fertilizers and (8) high input rates of a mixture of organic fertilizer with mineral fertilizers. After 2 years, we identified 41 soil nematode taxa. The taxonomic composition of the nematode communities revealed that Ferralsols are a stressful environment for the soil biota. The low abundance of opportunistic bacterivores indicated that the different fertilization practices did not significantly and deeply increase the amount of plant-available nutrients, and thus soil fertility. However, organic fertilizers significantly increased the abundance of omni-predators, indicating a moderately mature food web. Some practices induced worrying increases in the endoparasitic Meloidogyne and Pratylenchus, which requires monitoring of root symptoms to avoid the establishment of their populations. Monitor soil nematode communities in upland rice growing on Ferralsols could be used as agronomic indicators to promote sustainable rice production systems in Madagascar.