Measurement of Carbon and Nutrient Dynamics in Soil-Plant Systems and the Role of Plant Residues in the Development of Sustainable Cropping Systems

Abstract

The decline in soil organic matter of many agricultural soils in many areas, which has resulted from cropping, has increased interest in using plant residues and other organic materials to improve soil productivity. To develop the effective use and management of residues, a detailed understanding of decomposition rate and nutrient release pattern is needed, and this requires techniques that can be used to monitor changes in decomposition and nutrient release of residues. The UNE perfusion apparatus, which utilises a hospital drip bag and administration set, has been developed by modification of the apparatus of Nyamai (1992) which utilised a more elaborate glass device. Residues were perfused with 0.005M CaCl₂ solution, a concentration aimed to simulate a soil solution, the CO₂ released during breakdown trapped in KOH and the net CO₂ analysed by titration of KOH against HCI. A control treatment, with no residue, was included. A comparative study of the UNE apparatus and Nyamai apparatus indicated close agreement between the two techniques. The advantages of the UNE system are that it is easy and cheap to construct and operate, which means that sufficient units can be assembled to conduct valid comparisons. The UNE perfusion apparatus was used to measure the rate of decomposition and nutrient release of 5 plant residues: wheat ('Triticum aestivum') straw, residues of two legume crops - chickpea ('Cicer arietinum') straw and medic ('Medicago truncatula') hay, and leaf litter from two tree legumes - 'Albizia chinensis' and 'Flemingia macrophylla'. The decomposition study was carried out at 25°C for 84 days in a controlled temperature laboratory. To verify the results from the perfusion study, a glasshouse experiment was conducted to measure the growth and nutrient uptake of wheat grown in an Alfisol soil amended with the five plant residues used in the perfusion study, applied at two rates, 3 and 15 t ha⁻¹ . A no residue control was also included. Following the application of residues and a basal application of all nutrients except N, two consecutive wheat crops were grown for 10 and 12 weeks, respectively. Yields were expressed relative to the control and the apparent nutrient recoveries as [Nutrient content of treatment] - [Nutrient content of control] x 100 / [Nutrient content of added residue]. ... It was concluded that the residues which breakdown more slowly, such as Flemingia leaf, are more suitable for building up soil C and nutrients to benefit the following crops than rapid breakdown residue. The response of crops to different types of applied legume residues suggests that management of residue breakdown, by choice of species and fertiliser application, can have significant effects on the short, medium and perhaps even long term availability of nutrients. Therefore, choice and management of residues can affect the input of carbon and other nutrients into the system and the rate at which soil organic matter turns over. This in turn will affect the size of soil organic matter pool and the fertility of soil, both of which contribute to the sustainability of the system.