Can intensification reduce emission intensity of biofuel through optimized fertilizer use? Theory and the case of oil palm in Indonesia

Closing yield gaps through higher fertilizer use increases direct greenhouse gas emissions but shares the burden
over a larger production volume. Net greenhouse gas (GHG) footprints per unit product under agricultural
intensification vary depending on the context, scale and accounting method. Life cycle analysis of footprints
includes attributable emissions due to (i) land conversion ('fixed costâ'); (ii) external inputs used ('variable costâ');
(iii) crop production ('agronomic efficiencyâ'); and (iv) postharvest transport and processing ('proportionalâ' cost).
The interplay between fixed and variable costs results in a nuanced opportunity for intermediate levels of intensification
to minimize footprints. The fertilizer level that minimizes the footprint may differ from the economic
optimum. The optimization problem can be solved algebraically for quadratic crop fertilizer response equations.
We applied this theory to data of palm oil production and fertilizer use from 23 plantations across the Indonesian
production range. The current EU threshold requiring at least 35% emission saving for biofuel use can
never be achieved by palm oil if produced: (i) on peat soils, or (ii) on mineral soils where the C debt due to conversion
is larger than 20 Mg C ha^-1, if the footprint is calculated using an emission ratio of N₂O-N/N fertilizer
of 4%. At current fertilizer price levels in Indonesia, the economically optimized N fertilizer rate is
344-394 kg N ha^-1, while the reported mean N fertilizer rate is 141 kg N ha^-1 yr^-1 and rates of
74-277 kg N ha^-1 would minimize footprints, for a N₂O-N/N fertilizer ratio of 4-1%, respectively. At a C debt
of 30 Mg C ha^-1, these values are 200-310 kg N ha^-1. Sustainable weighting of ecology and economics would
require a higher fertilizer/yield price ratio, depending on C debt. Increasing production by higher fertilizer use
from current 67% to 80% of attainable yields would not decrease footprints in current production conditions.