Long‐term nitrogen balance for pearl millet (Pennisetum glaucum L.) in an acid sandy soil of Niger

Abstract

AbstractOn acid sandy soils of Niger (West Africa) fertilizer N recovery by pearl millet (Pennisetum glaucum L.) is often more than 100 per cent in years with normal or above average rainfall. Biological nitrogen fixation (BNF) by N2‐fixing bacteria may contribute to the N supply in pearl millet cropping systems. For a long‐term field experiment comprising treatments with and without mineral fertilizer (F) and with and without crop residue application (CR) a N balance sheet was calculated over a period of six years (1983‐1988).After six years of successive millet cropping total N uptake (36‐77 kg N ha−1 yr−1) was distinctly higher than the amount of fertilizer N applied (30 kg N ha−1 yr−1). The atmospheric input of NH4‐N and NO3‐N in the rainwater was about 2 kg N ha−1 yr−1, 70 % in the form of NH4‐N. Gaseous NH3 losses from urea (broadcast, incorporated) were estimated from other experiments to amount to 36 % of the fertilizer N applied. Nitrogen losses by leaching (15 to > 25 kg N ha−1 yr−1) were dependent on the treatment and on the quantity and distribution of single rainfall events (>50 mm). Decline in total soil N content (0‐60 cm) ranged from 15 to 48 kg N ha−1 yr−1. The long‐term N balance (1983‐1988) indicated an annual net gain between 6 (+CR‐F) and 13 (+CR+F) kg N ha−1 yr−1. For the control (‐CR‐F) the long‐term N balance was negative (10 kg N ha−1 yr−1). In the treatment with crop residues only, the N balance was mainly determined by leaching losses, whereas in treatments with mineral fertilizer application the N balance depended primarily on N removal by the millet crop. The annual net gain in the N balance increased from 7 kg ha−1 with mineral fertilizer to 13 kg ha−1 in the combination mineral fertilizer plus crop residues.In both the rhizosphere and the bulk soil (0‐15 cm), between 9 and 45% of the total bacterial population were N2‐fixing (diazotrophic) bacteria. The increased N gain upon crop residue application was positively correlated with an increase in the number of diazotrophic and total bacteria. The data on bacterial numbers suggest that the gain of N in the longterm N balance is most likely due to an N input by biological nitrogen fixation. In addition, evidence exists from related studies that the proliferation of diazotrophs and total bacteria in the rhizosphere due to crop residue application stimulated root growth of pearl millet, and thus improved the phosphorus (P) acquisition in the P deficient soil.