Nitrogen fixation of groundnut and its residual nitrogen benefits to succeeding cassava

Experiment was conducted at the Northeast Regional Agricultural Centre farm, Tha Phra with the aims of quantifying nitrogen fixation in two recommended groundnut cultivars using the 15N isotope dilution technique and measuring the nitrogen benefits to succeeding cassava. The groundnut cultivars were KK 60-1, KK 60-3 and Non-nod (a reference plant) while the cassava cultivar was Rayong 3. A randomized complete block design (RCBD) with four replications was used for the first crop while a 3*2 factorial in RCBD was used for the succeeding crop. There were three treatments (three groundnut cultivars) in the first crop. The succeeding crop consisted of 6 treatments i.e. 6 combinations of 2 stover management practises and 3 groundnut cultivars. The results indicated that the groundnut cultivar KK 60-3 was superior to KK 60-1 in terms of dry matter production, pod yield and nitrogen fixation. KK 60-3 fixed 194 kg N/ha (72 percent of its total N) while KK 60-1 fixed 129 kg N/ha (64 percent of its total N). Where the stover was removed there was a negative net input from N2 fixation in all cultivars ranging from -65 kg N/ha in Non-nod to -80 kg N/ha in KK 60-3. Where the stover was returned there was a positive net input from N2 fixation in the N2 fixing cultivars (38 and 97 kg N/ha in KK 60-1, KK 60-3, respectively) and the loss of soil nitrogen was reduced to only 12 kg N/ha in Non-nod, Cassava grown after the two N2 fixing cultivars produced significantly more total dry matter than that grown after Non-nod. The increased total dry weight was due to an increase in stover (stems and leaves) dry weight rather than an increase in root dry matter yields. Cassava grown in where stover had been returned grew better tnan that grown after the stover had been removed, however the cassava stover (stems and leaves) was more affected by returning the stover than cassava roots. The total residual benefits of groundnut to succeeding cassava were found to be 24.4 to 70.6 kg N/ha (35-60 percent of the cassava total N) using the N-balance method and were 6.9-34.8 kg N/ha (10-31 percent of the total N) using the direct 15N labeled stover method.