Nitrogen cycling and fixation | [Ciclo y fijacion del nitrogeno]

The section's activities were in three areas: (1) Rhizobium inoculant producc tion and rhizobium ecology; (2) N cycling in pastures, and (3) N cycling in rice-pasture system (1) The request and supply of inoculants for internal and external sources continued at approximately the same level as previously even though commercial scale requests are now managed by ICA. Screening trials were discontinued except for experiments on further characterization of some strains. Using a newly developed indirect-ELISA technique, a serological study of 91 strains isolated from Centrosema or Arachis revealed that these strains could be classified into 19 serogroups. In general strains categorized as V, VX, and X were less diverse than strains categorized as Y, YZ and Z. The results indicate that the serogroups are neither limited to a single host nor to a geographic zone. The technique developed will be useful to identify strains further, to check on inoculant quality, and also will aid the study of rhizobial persistence in soil. (2) The decomposition of litter from a range of 11 grass and legume species was studied in the field. Decomposition rates of legumes but not grasses were best correlated with lignin:N ratios but not percentage N or C:N ratios. Exponential decay curves were fitted to the data to produce decay constants and litter half-lives. Of the legumes S. capitata decomposed fastest (half-life 36 days) and D. ovalifolium the slowest (half-life 328 days). There were no differences between grass species (average half-life of 121 days). Litter accumulating during the wet season will be used to estimate rates of cycling of nutrients in ungrazed plots and will be extended to grazed plots next year. Simulations of the N cycle in grazed grass-legume pastures indicate that for a sustain pasture, in terms of N, a legume content of between 20-31 percent of the herbage DM is required over a pasture utilization range of 10-40 percent. Inclusion of roots in these simulations reduces the legume requirement by about 5 percent (i.e. to 15-26 percent). The simulations also indicated that variations in the percentages of internal remobilization of N are of greater importance than variations in the recovery of N from excreta or plant litter. Between 6-27 percent of the herbage biomass can be expected to enter stable soil OM thereby contributing to enhanced soil fertility under pastures. 3. The mineralisation of N from soils under 11 different grass, grass-legume pastures or savanna was studied and will be related to the subsequent yields of a rice crop sown on the same sites. To date analyses are incomplete. A study of the nodulation of C. acutifolium grown with a rice crop and given 80 kg N/ha and P and S fertilizer revealed that N did not inhibitor nodulation, P fertilizer enhanced nodulation and S fertilizer had no effect on nodulation. Soil mineral N levels after application of 30 kg N/ha were not increased to a level likely to inhibit nodulation or N2 fixation. Therefore levels of N fertilizer used for rice are unlikely to have a detrimental effect on the symbiosis of a forage legume grown in a rice-pasture system.