Effect of P fertilizer application on N balance of soybean crop in the guinea savanna of Nigeria

Soybean (Glycine max (L.) Merr.) is becoming increasingly important in the cereal-based cropping system of the Nigerian Guinea savanna zone and this justifies research on its effects on soil N. Although soybean can obtain 50% or more of its N requirement from the atmosphere, the N contribution of the crop to the system depends on the amount of N contained in roots, haulms, and fallen leaves after grain harvest. At four sites in the northern Guinea savanna, the effects on N balance of P fertilizer and soybean varieties of different duration were tested. The varieties received P fertilizer at the rates of 0, 30, and 60 kg P ha⁻¹. The total N accumulated aboveground at harvest averaged 104 kg N ha⁻¹ in the early and medium varieties, and 135 kg N ha⁻¹ in the late varieties. Across all varieties and sites, total N content was increased by 40–47% when P was applied. Apparent N harvest index averaged 85% but was not significantly affected by variety or P rate. When only grain was exported, the calculated N balance of the early and the medium varieties was −2.6 to −12.2 kg N ha⁻¹ while the longer duration varieties had positive N balances ranging from 2 to 10.9 kg N ha⁻¹. The N accrual was negative when P was not applied and ranged from 2.4 to 5.2 kg N ha⁻¹ with P application. The interaction of variety and site on the N balance was significant at P<0.05. N balance at the southernmost site was −14.2 kg N ha⁻¹ compared with 2.6–10 kg N ha⁻¹ at the northern sites where N₂ fixation was higher. The estimate of N balance is reduced when soybean haulms are exported. A positive N contribution by soybean is, therefore, possible in a soybean–cereal rotation when: (i) P is applied, (ii) the soybean variety is late maturing, and (iii) only grain is exported.