Managing crop residues, fertilizers and leaf litters to improve soil C, nutrient balances, and the grain yield of rice and wheat cropping systems in Thailand and Australia

This paper reports on the influence that residue and fertilizer management have on nutrient balances, soil organic matter (SOM) dynamics, and crop yields of a flooded rice system in northeast Thailand (1992-1997) and a wheat-forage legume rotation in eastern Australia (1992-1998). Both soils had been subject to at least 18 years of cultivation and had lost up to 90% of the original labile (C-L) and 85% of the total carbon (C-T). For the rainfed rice cropping systems of northeast Thailand, a system is described in which small applications of leaf litter from locally grown trees are applied annually to rice paddy soils prior to transplanting. Annual applications of 1500 kg ha(-1) of leaf litter from different locally grown shrubs for five seasons resulted in increases in rice grain yield in 1997 of between 20 and 26% above the no-leaf litter control. Nutrient balances, determined by the difference between the inputs (fertilizer and added leaf litters) and outputs (grain and straw), indicated net positive balances of up to 457 kg N ha(-1), and 60 kg N ha(-1), after five seasons of leaf litter applications. Sulfur and potassium balances resulted in net deficits of up to -13 kg S ha(-1) and -52 kg P ha(-1), where no leaf litter was applied and rice straw was removed following harvest. Soil carbon (C) concentrations increased significantly only where higher fertilizer rate and rice stubble retention were combined. The poor management of fertilizers and crop residues, and excessive cultivation has also resulted in large soil fertility losses in the grain growing areas of Eastern Australia. After five wheat and two legume/fallow crops, negative N balances of up to -303 kg ha(-1) were calculated for the treatments where wheat stubble was not retained and bare fallow leys were used. The balance of nutrients such as K, which are contained in larger proportions in stubble, were found to be up to -362 kg ha(-1) on the straw-removed treatments and up to +29 kg ha(-1) on the straw-retained treatments. Forage legume leys resulted in short term increases in CL and the carbon management index (CMI). Sustainable farming systems require that crop yields are stable through the maintenance of soil fertility and the balance of nutrients in the system. Increases in soil C levels require sustained periods of balanced fertilizatibn and residue retention. Crown Copyright (C) 2003 Published by Elsevier B.V. All rights reserved.