Response

摘要

Erb etal do not challenge my assertion that global NPP may be a constant boundary. Rather, they argue that partitioning the 38% of global NPP appropriated for human use will be complex and that the efficiency of agricultural crop production is variable. The fraction of agricultural NPP consumable by humans as food varies widely by crop type, as Erb et al. have previously reported (7, 2). In addition, more efficient use of current irrigation and fertilizer will allow agricultural food output to be improved in some regions. However, there is much evidence cited in the original Rockstrom analysis (3) that nitrogen and phosphorus cycles may already be saturated, so increasing the use of fertilizer to enhance future NPP to satisfy growing demand may be environmentally