2.2 Study methods
To estimate the input and output of nutrient content, the nutrient
system is regarded as a black box. Annual statistics for Meixian and
Wugong have been published since 1990 by the Agriculture Bureaus of both
counties. Data from the yearbooks from 1992 to 2017 were used, including
different crops in these land uses (e.g., kiwi, wheat, maize, beans,
rapeseed, vegetables, melon, and apple), synthetic N, P, and K
fertilizer inputs, numbers of livestock, and population.