Applicability of phosphate buffer extractable organic nitrogen as an indicator of available nitrogen in the sandy soils of the Sahel zone of Niger, West Africa

As phosphate buffer extractable organic nitrogen (PEON) has been successfully used to evaluate the available nitrogen in Japanese soils, we tested whether it could also be used with sandy soils of the Sahel zone of Niger. Soils were collected from 33 farm fields in the Fakara region of Niger. Pearl millet (Pennisetum glaucum L. cv. Haini-Kirei) was grown in these soils for 4 weeks with all nutrients except nitrogen supplied, and plant nitrogen uptake was measured. We analyzed PEON, the inorganic and total nitrogen present at the start of the experiment, and the nitrogen mineralized during incubation in the same soils. Both PEON and mineralizable nitrogen were significantly correlated with nitrogen uptake by pearl millet. The PEON + inorganic nitrogen and mineralizable nitrogen + inorganic nitrogen were also significantly correlated with nitrogen uptake. Unlike in Japanese soils, the correlation between PEON and mineralizable nitrogen was not high in the sandy Sahelian soils. The absorbance at 280 nm and the content of Coomassie Blue reactive substances detected using the Bradford method for soil extracts treated with 0.067 mol L-1 phosphate buffer were highly correlated with nitrogen uptake by pearl millet. These results suggest that extraction with 0.067 mol L-1 phosphate buffer is a convenient method for evaluating the available nitrogen in sandy Sahelian soils.