Selection of sugar cane cultivars for associated biological nitrogen fixation using ¹⁵N-labelled soil

Abstract

Ten varieties of sugar cane, and a non-N₂-fixing control crop (Brachiaria radicans), were planted in a concrete tank (20 × 6 × 0.6 m) filled with ¹⁵N-labelled soil, with the objective of selecting those varieties of sugar cane which most benefited from associated biological nitrogen fixation (BNF). The tank contained ca. 80 tonnes of red-yellow podzolic soil (0.080% N) uniformly mixed with ¹⁵N-labelled compost made from sugar cane bagasse, filter cake and ¹⁵N-labelled ammonium sulphate and potassium nitrate. The plant varieties were planted in 4 replicates (blocks) in 3.0 m rows. Temporal changes of ¹⁵N enrichment of the plant available N in the soil were followed by analysing soil and leaf samples at 50 day intervals. At the final harvest (310 days after planting) all plant material was dried, weighed and analysed for total N and ¹⁵N content. Several of the varieties yielded the equivalent of more than 2001 fresh cane per hectare, and all commercial varieties contained more than 250 kg N ha^-1. The,5N enrichment of the soil mineral N decreased from ca. 1.0 to 0.16 atom % ¹⁵N excess during the growth of the sugar cane. This suggests that the different patterns of N uptake by the plants could cause large differences in the enrichment of the plant N making it difficult to quantify BNF contributions accurately. This problem is discussed in detail, but it is apparent that for some of the varieties (Krakatau, SP 70–1143) a minimum of 25%, and perhaps as much as 55% of the plant N was derived from plant-associated BNF.