Effects of amending an Ultisol with bambara groundnut seed residue on soil fertility, microbial nutrient cycling and yield of cucumber

ABSTRACT Residue from processing bambara seeds is abundant in West Africa, but scientific knowledge on the suitability of this residue for improving soil fertility is limited. Bambara seed residue (BSR), at 0, 10, 20 and 30 t ha−1, was applied annually to an Ultisol in a two-year field experiment with cucumber. BSR input at 30 t ha−1 produced the highest yield of 256.80 g (= 0.46 t ha−1) compared with 25.70 g (= 0.05 t ha−1) in the control. While BSR had no effect on pH, the input at 20 and 30 t ha−1 significantly increased organic C, total N and K in the soil and increased macro-aggregation by 153% and 193% after the first and second year, respectively. Basal soil respiration at day 3–14 showed increased and rapid labile C utilisation with increased BSR input rates, followed by more persistent CO2-C evolution rates during days 14–42, and significantly higher rates in the control than in the BSR treatments during days 35–42. BSR at 30 t ha−1 increased microbial C by 159%, microbial N by 205% and improved activities of β-glucosidase and tyrosine-aminopeptidase 7.5- and 3.0-fold, respectively. However, reduced activities of S- and many C-cycle enzymes limited the suitability of BSR as a nutrient source for crop growth. Nonetheless, its potential to enhance soil structure and organic C storage in the Ultisol was beneficial and the C pool in soil treated with 30 t ha−1 BSR was sufficiently large to increase N and K availability.