“Fixed” NH4
+ varied significantly with the soil depth, with a
progressive reduction down the profile (Table 2). In the soil used
for the study, the clay content increased slightly with soil depth
(534–564 g kg1 soil). Differences in recent non-exchangeable
NH4
+ down the soil profile were probably due to the surface
addition of fertilizer N (Figueiredo et al., 2013b). The interaction of
years of study and soil depth was not significant (Table 3), but a
tendency for higher values of “newly
fixed” NH4
+ was observed in
year 1 at 0–40 cm and 0–20 cm in year 2 (250 mg NH4
+–N kg1),
compared with 20–40 cm in year 2 and 40–60 cm in both years
(229 mg NH4
+–N kg1) (Fig. 2). The increase of NH4
+ “fixation” in
the surface layers is explained by the surface incorporation of N
fertilizers at basal dressing.