2.2.3 Wet, dry atmospheric deposition and symbiotic N2 fixation
Atmospheric N deposition from agricultural land was estimated by multiplying the sown area by the atmospheric N deposition rate of the corresponding area (Hong et al., 2013). Two main data sets were used to calculate N deposition over the study period 1992-2017: (1) the linear relationship between N deposition rate and year in Northwest China from 1992 to 2010 (Liu et al., 2013); (2) we calculated the atmospheric deposition of N in Northwest China from 2010-2017 following the long-term trends identified by (Fu et al., 2020) and (Wen et al., 2020). Symbiotic N fixation in cropland was estimated based on the relationship between N fixation and grain yield. (Anglade et al., 2015).
Atmospheric P and K deposition was based on Li and Jin. (2011). We assumed a mean distribution across geographic regions and used the same P and K deposition rates throughout the study period.