Differences between fractions across locations

Peak area data for the fractions from the other sites was grouped according to fertiliser treatment, with the purpose of establishing whether differences due to geographical origin (encompassing the effects of climate, soil and crop type) were greater or lesser than the general differences resulting from fertiliser treatment. Due to marked difference in composition between FREE and INTRA-AGGREGATE fractions in the FYM compared to inorganic and zero N plots for the UK site, the initial comparison was made only between plots receiving zero N, low N and high N. The distribution of C between functional groups between treatments was very similar, for both the FREE and INTRA-AGGREGATE fractions (Figures 3.4. and 3.5.). Since the differences were small, the data were combined for comparison against equivalent data for the FYM plots.

A considerable difference was found in the O-alkyl and aromatic C content of FREE organic matter from the inorganic N plots, and plots receiving FYM (Figure 3.6.). FREE organic matter from the FYM plots containedl proportionallyl morel aromaticl and

[

Figure 3.6. The mean distribution of C between functional groups inFREE organic matter obtained from long-term plots receiving inorganic or no fertiliser versus those receiving farmyard manure, based on 13

C nuclear magnetic resonance peak areas

Figure 3.7. The mean distribution of C between functional groups inFREE organic matter obtained from long-term plots receiving inorganic or no fertiliser versus those receiving farmyard manure, based on 13

C nuclear magnetic resonance peak areas

%C 0 5 10 15 20 25 30 35 40 45

Alkyl N-alkyl O-alkyl Acetal Aromatic Phenolic Carboxyl Carbonyl

All inorganic N (n=20) Farmyard manure (n=5) 0 5 10 15 20 25 30 35 40 45

Alkyl N-alkyl O-alkyl Acetal Aromatic Phenolic Carboxyl Carbonyl

All inorganic N (n=17) Farmyard manure (n=5)

Figure 3.4. The mean distribution of C between functional groups inFREE organic matter obtained from long-term plots receiving inorganic or no fertiliser, based on

13

C nuclear magnetic resonance peak areas

Figure 3.5. The mean distribution of C between functional groups in INTRA- AGGREGATEorganic matter obtained from long-term plots receiving inorganic or no

fertiliser, based on 13

C nuclear magnetic resonance peak areas

0 5 10 15 20 25 30 35 40 45

Alkyl N-alkyl O-alkyl Acetal Aromatic Phenolic Carboxyl Carbonyl

Unfertilised (n=7) Low iinorganic N (n=7) High inorganic N (n=3) % 0 5 10 15 20 25 30 35 40 45

Alkyl N-alkyl O-alkyl Acetal Aromatic Phenolic Carboxyl Carbonyl

Unfertilised (n=8) Low inorganic N (n=8) High inorganic N (n=4)

alkyl C, and less O-alkyl C. This was consistent with the analysis of spectra from the Broadbalk plots (Section 3.3.1.), and may result from digestion of carbohydrates in rumen (previously seen in FTIR studies; Russell and Fraser 1988). However, this difference was much diminished in the INTRA-AGGREGATE fraction (Figures 3.7.). The general convergence in composition between treatments in the INTRA-AGGREGATE fraction was less pronounced in the Broadbalk plots (Figure 3.3.).

For both FREE and INTRA-AGGREGATE organic matter, the distribution of C between functional groups showed general differences that apply across sites, and confirming that the Broadbalk example was broadly typical. The greatest proportion of C in FREE organic matter was in O-alkyl groups, this peak being much less dominant in INTRA-AGGREGATE organic matter. The alkyl peaks were also greater in the INTRA- AGGREGATE fraction, but the general trend in aromatic C was less pronounced than in the Broadbalk plots. The relative prominence of the alkyl region is centred on a peak representing lipids or waxes, suggesting a higher proportion of transformed organic matter (microbial debris, as well as highly recalcitrant plant components).

A statistical analysis of the peak area data was made using the REML procedure (see Section 3.2.4.), which was suited to the peak area dataset as data was not available for all treatments at all sites. However, since only two of the eight sites had plots receiving green manure, these were excluded. The analysis showed that the alkyl and O- alkyl C content of the FREE and INTRA-AGGREGATE fractions were significantly different (P < 0.05), irrespective of the site or treatment from which they were obtained (Table 3.2.). In addition there were corresponding, significant differences in the quantitatively smaller carboxyl and carbonyl groups, l andl alsol acetall Cl (representing

[TABL 3.2 &

Factor

alkyl N-alkyl O-alkyl acetal aromatic phenolic c'boxyl c'bonyl

Fertilisation NS NS NS NS NS NS NS NS

Fraction * NS * * NS NS * *

Fert x frac NS NS NS NS * NS NS NS

* = significant at 95 % NS = not significant

Figure 3.8. The mean distribution of C between functional groups of FREE a INTRA-AGGREGATE fractions obtained from 26 long-term plots under contrasting management, estimated from 13

C nuclear magnetic resonance peak areas using residual maximum likelihood (bars indicate the standard error of difference)

Functional C group

---% peak area ---

Table 3.2. Significance of the variance components leading to differences between

FREE and INTRA-AGGREGATE organic matter compositon for 26 long-term plots in eight long-term experiments under contrasting management, assessed from 13

nuclear magnetic resonance peak areas using residual maximum likelihood

0 5 10 15 20 25 30 35 40 45

alkyl N-alkyl O-alkyl acetal aromatic phenolic c'boxyl c'bonyl

Free organic matter Intra-aggregate FREE organic matter INTRA-AGGREGATE

hemicellulose). Although the effect of fertilisation was not statistically significant, there was a significant interaction between fertilisation and fraction in the aromatic C content. This may be due to the particularly high aromatic C content of FYM. Mean values for the distribution of C between functional groups estimated by REML (across treatments) is shown for both fractions in Figure 3.8., together with the standard error of difference (s.e.d.) for each functional group.

3.4. Conclusion

In view of the diversity of soils fractionated in this experiment (in terms of soil type, climate, crop and land management), the differences in chemical composition and hence likely reactivity (k) between FREE and INTRA-AGGREGATE fractions were remarkably consistent. The statistical analysis showed that, with the exception of aromatic C derived from FYM, the differences between the fractions were greater than the combined effects of geographical origin or fertilisation strategy. Previously, Mahieu

et al. (1998) have shown a surprising level of consistency in the composition of >300

whole soils characterised by 13C NMR. A detailed examination of NMR spectra for the fractions isolated from a long-term site in the UK suggested that INTRA-AGGREGATE organic matter was more consistent between treatments than that of the FREE fraction, and comprised a greater proportion of more resistant C structures and / or microbial products. These findings support the view that FREE and INTRA-AGGREGATE organic matter occupy a contrasting position in the decomposition sequence, and that their reactivity may be sufficiently distinct and hence enhance model performance by their separate inclusion.