Identification of a reference crop to estimate nitrogen fixed biologically by pigeonpeas
Mburu, M.W. K.
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Knowledge of quantifying N fixation by pigeonpeas is useful in assessing its nitrogen contribution to sustainable crop production. An experiment was conducted in the greenhouse at Kabete, University of Nairobi to identify the most suitable reference crop that could be used to quantify N fixed by medium duration (ICEAP-00557) and long duration pigeonpeas (ICEAP-00040 and 1CEAP-00053). Maize (Hybrid-511 and Katumani-composite), sorghum (MB30 and IS25545) and cotton (HART-89M and UKA-591l46) were evaluated as reference crops using completely randomized design replicated three times. Plant dry mass (roots and shoots) and root length density were determined at 120 days after planting for two seasons. The proportion of nitrogen fixed by pigeonpeas was calculated using N-difference method. The results showed that over 50% of the root biomass of all the crops was at the top 30 em and it was more vigorous in cereals than in pigeonpea. Cereals also showed higher root length density and accumulated more shoot dry mass than pigeonpeas and ifused as the reference crops they would underestimate N fixed as a result of temporal and spatial variation in N uptake from the soil. Cotton had similar rooting characteristics and growth pattern as pigeonpea an indication of its suitability as a reference crop. The amount of N fixed was significantly different among pigeonpea duration types, and the values depended on the cotton variety used as a reference crop. Long duration pigeonpeas fixed more N than the medium duration, when both cotton varieties were used as the reference crops, suggesting any of the two varieties could be used to assess N fixed by the long duration pigeonpeas. However, the medium duration showed negative values when cotton was used as a reference crop in the first season. This implies a cotton variety or an alternative crop that closely matches the growth pattern ofthe medium duration pigeonpea is needed.