Crop yield and the fate of nitrogen and phosphorus following application of plant material and feces to soil

Organic materials are the most important sources of nutrients for agricultural production in farming systems of semi-arid West Africa. However, reliance on locally available organic nutrient sources for both crop and livestock production is rapidly becoming unsustainable. A series of feeding and agronomic trials have been conducted to address the role of livestock in sustainable nutrient cycling. This paper reports results of a greenhouse study that evaluated the effects of applying crop residue and browse leaves, or feces derived from these feeds, at equal organic-N application rates (150 kg ha(-1)), alone or with fertilizer-N (60 kg ha(-1)), on pearl millet (Pennisetum glaucum [L.] R.Br.) dry matter (DM) yield, nitrogen (N) and phosphorus (P) uptake, on soil nutrients, and on total, labile and recalcitrant fractions of soil organic matter (SOM). Millet DM and cumulative N uptake were most affected by fertilizer-N, followed by plant species and amendment type, although various interactions among these treatments were noted due to variations in the composition of the applied amendments. Fertilizer-N increased total millet DM by 39%, N uptake by 58% and P uptake by 17%, and enhanced N mineralization from most organic amendments, but was applied insufficiently to totally offset N and P immobilization in pots containing leaves of low initial N and P content. Feces alone appeared to supply sufficient N to meet millet-N demands. Nitrogen use efficiency was, in most cases, higher in pots amended with feces than with leaves. Nitrogen in feces apparently mineralized more in synchrony with millet-N demands. Also, the relatively high cell wall content of feces may have provided an effective, temporary sink for fertilizer-N, which upon remineralization provided more N to millet than pots amended with leaves. Whereas most of the P contained in feces mineralized and was taken up by millet, most leaves immobilized P. Assessing the costs and benefits associated with the direct land application of biomass as a soil fertility amendment versus feeding biomass first to livestock then using feces (and urine) to fertilize the soil requires information on both crop and livestock production and associated impacts on nutrient cycling.