Phosphorus use efficiency and nitrogen balance of cowpea breeding lines in a low P soil of the derived savanna zone in West Africa

Abstract

Differences in growth, nodulation and arbuscular mycorrhizal fungi (AMF) root infection among recent cowpea breeding lines from IITA were examined at low and high P levels in pot (94 lines) and field experiments (43 lines) at Fashola in the derived savanna zone of Nigeria. Based on their growth performance, these lines were subdivided into 5 groups: (i) poor performance under low and high P conditions; (ii) good performance under low P and poor performance under high P; (iii) intermediate performance under high and low P; (iv) good performance under high and low P conditions; and (v) good performance under high P and poor performance under low P. About 42% of the breeding lines (18 out of 43 lines tested) had the same grouping for the field and pot experiments. Eight cowpea lines (4 P-responders and 4 non-P-responders) were selected from the first experiment for subsequent studies on the effect of P supply (0, 20, 40 and 60 kg P ha-1) on P uptake, P use efficiency, dry matter production, N-fixation, AMF infection and N balance. Dry matter production, shoot/root ratio, total shoot N, and total N-fixed of the non-P-responder line, IT81D-715, were strongly related to P uptake efficiency. The P-responder IT81D-849 had a significant (95%) correlation between AMF and P-use efficiency. The cowpea lines fixed on average 22 kg N ha-1, which was 70% of the plant total N. The N balance based on the difference between the amount of N2 fixed and N exported through the harvest, ranged between −10.6 kg N ha-1 and +7.7 kg N ha-1. Based on its adaptability to grow in low P soils and overall positive N balance, the cowpea line IT81D-715 should be recommended for cultivation when P is the limiting factor.