Crop residue management effects on organic matter in paddy soils - The lignin component

Intensified rice cropping is not only increasing rice yields but also the amount of crop residue. In particular the lignin component of the residues may accumulate in submerged systems such as paddy soils due to incomplete decomposition under oxygen-limited conditions. Here, we examined the short and long-term effects of crop residue management on soil organic matter (SOM) in paddy soils of two experimental sites in China (Nanjing, Changsha) and one in the Philippines (Los Banos, Laguna). We determined organic C and total N, characterized the composition of alkaline-extractable SOM by C-13 NMR spectroscopy, and the lignin component by CuO oxidation. The results revealed no significant changes in organic C, total N and total lignin-derived phenols upon incorporation of crop residue at the recently established experiments at Nanjing and Los Banos. In contrast, all tested variables increased at the long-term experiment at Changsha. Within 16 years (1990 to 2005), the organic C increased by 41% in the 'residue incorporated' plots while at the 'residue removed' plots the increase was 16%. Similar trends were found for total N. Lignin-derived phenols in 'residue incorporation' plots increased about 45% (12% for 'residue removed' plots). Parameters addressing alteration of lignin changed at the initial stages of the experiments. These changes are comparable to those observed in aerated cropping systems. No further changes in the lignin composition occurred at later stages of the experiments. The C-13 NMR spectra revealed a largely unchanged SOM composition, thus supporting the results of the CuO oxidation. In summary, the additional input of crop residue in paddy soils seems to result in the mere accumulation of OM; the larger proportion of lignin-derived phenols reflects the input of crop residue. The comparable degree in oxidative alteration of lignin between 'residue removed' and 'residue incorporated' treatments indicates that the soil systems can cope with the increased residue-derived lignin input. (c) 2008 Elsevier B.V. All rights reserved.