Agriculture Reference
In-Depth Information
Figure 2. 1-D liquid-state
13
C NMR spectra of humic acids in NaOD extracted from unamended soil
(C
0
) at the start of the experiment and after 30 yrs. (C
30
) and amended consecutively for 30 yrs with
cow manure (CM) and crop-residues (CR). Tentative chemical shift assignements for these samples
correspond to representative major functional groups of humic substances.
The amendments, on the contrary, did not favour any accumulation of alkyl-C in the HA,
thus modifying the positive effect of the crop rotation alone. In CR
30
, in particular, the
observed decrease in alkyl-C may be due to: i) the relatively low
alkyl-C content in crop
residues (wheat/corn), ii) the modest microbial production of
alkyl-C during soil crop-residue
decomposition, and iii) the small amount of transformation
of mobile alkyl-C into rigid alkyl-
C by cross-linking or association
with soil mineral particles [43]. Both CR
30
and CM
30
treatments showed, after 30-years, a C increase in the aromatic region (165-130 ppm), thus
with the amendments, the increase in humic C content (Table 1) seems to be mainly due to
