Temporal changes in shrinkage behavior of two paddy soils under alternative flooding and drying cycles and its consequence on percolation

Paddy field is generally subjected to many cycles of alternative flooding and drying (AFD) during rice growing. The AFD cycles can create a large variation in soil structure that subsequently affects soil water and nutrient retention and migration. This study aimed to investigate the temporal change in soil shrinkage behaviour and to evaluate its consequences on water percolation in two paddy fields under AFD. One paddy field cultivated for 20 years (YPF) and the other one for over 100 years (OPF) were subjected to either AFD or continuous flooding (CF) as a control. During every AFD cycle, soil cores were sampled to determine soil shrinkage curve in the laboratory while cracks and water percolation were determined in the field. Our results showed that both paddy soils presented a temporal change in soil bulk density, soil shrinkage and cracks with cycles of AFD. The two paddy soils showed a high shrinkage capacity (COLE>0.06), in which the OPF was significantly greater than the YPF (P<0.001). The cracks area density, however, was lower in the OPF (3.66%) than in the YPF (5.13%). This discrepancy can be explained by more swelling clay content and higher soil organic matter in the OPF but greater AFD intensity in the YPF. As a result, the YPF showed higher water percolation as compared with the OPF. Our work demonstrates that the temporal changes in soil structure in paddy soils depend not only on intrinsic soil properties but also on external hydraulic stress. (C) 2012 Elsevier B.V. All rights reserved.