Measurement of decomposition and associated nutrient release from straw (Oryza sativa L.) of different rice varieties using a perfusion system

Maintenance of soil carbon concentrations and synchronisation of nutrient r elease from crop residues to match crop demand is important if soil organic matter levels are to be maintained and nutrients are to be recycled effici ently. Two laboratory perfusion experiments were conducted to study the dec omposition and nutrient release rates from straw of different rice varietie s (Oryza sativa L.) which were collected from the field. Cumulative evoluti on of CO2 and nutrient release from the straw of 20 rice varieties was meas ured over a 6-week period. The CO2 release rate differed between varieties with the peak of CO2 release varying from 2 to 4 days. After 42 days, the p ercentage of C released from the straw ranged from 15.4% in variety PSBRc18 to 38.4% in variety Soc Nau. There was an inverse relationship (r(2)=0.60) between cumulative C release and C:N ratio and a direct relationship (r(2) =0.62) between digestible organic matter (DOM) and cumulative C release. A Straw Quality Index (SQI) was developed to describe the decomposition rate of the rice straw as follows: SQI = -56.85+(11.68 x % N) + (1.25 x % DOM) +(2.59 x % lignin) r(2) = 0.81 These findings indicate that SQI has a potential in assessing the quality o f straw residue materials in predicting their usefulness in crop-residue ma nagement systems. Varieties, Soc Nau, IR67962 and PSBRc18, were used in a s econd perfusion experiment to determine C and nutrient release at 5, 10 and 20 days under aerobic and anaerobic conditions. Despite a two-fold differe nce between varieties in the amount of C evolved over 20 days, the proporti on of nutrient release did not differ significantly between them. When perf usion was performed under anaerobic conditions, there were 45, 32 and 27% r eductions in C evolution compared to aerobic conditions from Soc Nau, IR679 62 and PSBRc18, respectively, but no differences in the proportion of N, P, S, K Mg and Na released between the aeration systems. These results indica te that under flooded conditions, depletion of O-2 decreases decomposition rate of straw but the initial rate of nutrient release is unaffected. This uncoupling of C and nutrient release appears to be related to the more labi le components of the nutrients present in the plants and their physiologica l role.