The experimental details are as follows: subsamples of 10 g were treated with (a) (15NH4)2SO4 (60 atom%) and extracted immediately; (b) (15NH4)2SO4 (60 atom%) and extracted after 24 h; (c) (15NH4)2SO4 (60 atom%) + unlabeled glycine and extracted after 24 h; (d) unlabeled (NH4)2SO4 + 15N-glycine (98 atom%) and extracted after 24 h