Better integration of animal manure in crop production systems, and recycling of N in human excreta and compost are further ways to reduce N fertilizer use. In practice only a fraction of this potential reduction in N fertilizer use is realized, in part because of the segregation of crop and livestock production systems and the lack of economic incentives for recycling. The concept of ecological sanitation aiming at closure of local material flow cycles (Langergraber and Muelleggera, 2005) could be a way to recycle human excreta, but is only applied in a few countries. Janssen and Oenema (2008)
suggested that globally a large part (34-68 per cent) of synthetic N fertilizer could be substituted with manure-N. Replacing fertilizer-N with manure is not a 1:1 exchange, since they differ in plant N availability. Also, the P-to-N ratio in manure is higher than the optimum ratio for crops, and may therefore result in the accumulation of P in soil. Compound animal feed often contains additions of copper and zinc; as a result, accumulation of these metals has occurred in some countries (see references in Janssen and Oenema, 2008).
Here, we have calculated the global potential for recycling of animal manure, by replacing N fertilizer with stored manure in the inventory discussed above. Since the N in animal manure is partly present in organic forms, we assume that 60 per cent is effectively available for plant uptake. The remainder is lost through NH3 volatilization or becomes part of the soil organic matter pool. Soil organic matter decomposes gradually and the N mineralized is lost through leaching and denitrification (Janssen and Oenema, 2008). Replacing synthetic N fertilizer with manure-N results in a reduction of global N fertilizer use by 33 per cent (from 82 to 56Tg yr-1), and a reduction of N2O emissions by 11 per cent (Table 5.3). Although N fertilizer use is reduced and replaced by animal manure-N, the emissions are not reduced proportionally. This is related to the background emissions, which are not reduced.
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