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Roth, Phillip J.(2012): The cycling and ageing of organic nitrogen during 2,000 years of rice paddy- and 700 years of non-paddy soil development
Bonner Bodenkundl. Abh. 53 218 S., 28 Abb., 19 Tab. Bonn 2012

 

Summary

Paddy soils accumulate organic matter but exhibit low nitrogen (N) availability for plants. This study was carried out to improve the understanding of the origin and fate of soil organic nitrogen (SON) in paddy soils. In particular, I hypothesized that prolonged paddy management (i) enhances the accumulation of N in bulk soil organic matter as well as in the form of fungal- and bacterial residues, (ii) reduces the turnover of peptide-bound amino acids so that protein ageing may occur, (iii) leads to a long-term sequestration of N in charred organic matter forms and (iv) decelerates the short-term cycling of rhizodeposits through the SON pool. For this purpose I sampled a chronosequence of soils, which have been under rice-upland crop rotation for 0 to 2,000 years with adjacent continuous non-paddy sites (700 years) in the Bay of Hangzhou, Zheijang Province, China. The samples were analyzed for total SON and for several biomarkers: amino sugars for microbial N residues, amino acid enantiomers for aged N, benzene-polycarboxylic acids for charred organic matter and stable isotopes for different carbon (C) and N cycling. In order to assess the short-term turnover of rice rhizodeposits, a microcosm experiment was conducted where rice was grown on 50 and 2,000 year-old paddy soils and pulse labeled with 13CO2. I then traced the 13C label in bulk soil organic matter (SOM) and in amino acid enantiomers.
The results showed that (i) after land embankment, paddy- and non-paddy soils accumulated SON at a rate of 61 and 77 kg ha–1a–1. The systems reached steady-state conditions after 172 (10.7 t N ha–1) and 110 (8.1 t N ha–1) years, respectively. Amino sugar analyses indicated that in paddy soils, a significant fraction of SON (0.3 t amino sugar-N ha–1) had been immobilized in microbial cell wall residues. Yet, this N accumulation was limited to the initial stages of paddy soil development and it occurred at similar rates for both, fungal- and bacterial residues. Only below the plough pan, higher proportions of N were found in bacterial residues.
(ii) Between 30 and 46% of topsoil N was found in peptide-bound amino acids, 20–30% was non-hydrolyzable N and another 10–20% of total N (mainly peptides) was hydrolyzed but not broken down to monomers. Amino acid stocks initially dropped after land reclamation, but under rice cultivation they re-increased by 16 kg N ha–1a–1 until steady-state conditions were reached after 203 years (2.7 t amino acid-N ha–1). In the non-paddy soils, this steady-state was reached faster (65 years) and at a lower level (1.5 t N ha–1). Similar portions of D-alanine and muramic acid suggested that microbial cell walls were preserved as intact macromolecules. Further, D-enantiomer contents of lysine and aspartic acid increased with soil depth, hinting at an ageing of the respective SON forms. Yet, racemization rates of these amino acids were unaffected by cropping management. Surprisingly, the paddy soils exhibited a lower δ15N ratio (4.6‰) when compared to the non-paddy soils (5.6‰). This reflects that more N was re-introduced to the paddy soils by fixation of atmospheric N and that these soils exhibited overall lower net N losses and supposedly a closer N recycling than non-paddy soils.
Rice paddy management usually goes along with the burning of rice straw. Indeed, (iii) charred organic matter (i.e. black carbon, BC) accumulated in paddy soils, also approaching a steady-state, but not until 307 years of cultivation. Nevertheless, BC contributed only some 14% to soil organic carbon and, since the C/N ratio of rice char was high, I resumed that charred N forms (black N) were not responsible for a long-term increase in stable SON stocks. In consequence, I concluded that it was rather the short-term microbial turnover than the long-term stabilization of N in specific pools that determined overall SON dynamics.
A key driver of short-term microbial SON turnover is rhizodeposition. The microcosm experiment (iv) revealed that rhizodeposits were rapidly included in the SON cycle especially through the peptide pool, being an intermediate (1–7 days) host for rhizodeposit-C. However, the isotopic label was preserved in bulk SOM for more than two weeks after labeling with an elevated residence time in the older paddy soil.
In sum, SON is accumulated in paddy soils, but the data suggest that these systems are no endless sink for total SOM. This accumulation, driven rather by a lower bioaccessability than by an intrinsic stability of SON, is restricted to the anthraquic horizon, whereas a substantial part of subsoil N most likely does not participate in overall N cycling. This increased decoupling of N dynamics between top- and subsoil is induced by the hindered vertical transport through the dense paddy plough pan and most likely contributes to the low overall N availability in these agro-ecosystems.

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