Azolla decomposition and nitrogen availability to lowland rice
1989
Ventura, W. | Watanabe, I. | Padre, B. Jr. | Buenaventura, G. | Ramirez, C.
Most azolla grown in the field are deficient in P and hence low in N. The effect of N content of various azolla species on their decomposability in soil was determined. Mineralization of Azolla N of the four azolla species (A. microphylla 4018, A. caroliniana 3010, A. pinnata var pinnata 7001 and A. pinnata var imbricata 5) was determined by incubating fresh azolla in flooded Maahas clay soil. In the field N availability of these four species and rice yield response were studied. High and low N % contents were obtained by changing P content, because N content decreases as P decreases. 15N labelled materials were used in two rice crops. Mineralization rates (mineralized N/ total N in azolla) after 2-week incubation were plotted against N contents of azolla. Decomposability generally decreased as N content decreased. At 3.5% N, decomposability was about 50%. Mineralization was slow after two weeks. Azolla of all four species grown for 9 and 12 days in the field propagation plot had higher N mineralization rates than ferns grown for 24 days. Despite wide range of N contents (1.8 to 3.6%), azolla was effective for rice production and N nutrition. With similar rates, azolla N of the four species were equally effective as with urea N. When applied basally, from 40 to 100 % of the total absorbed labelled N was already taken up at 42 days after transplanting. Correlation coefficients of azolla N% and 15N recovery by rice were significant. Different azolla species with similar N contents, however, had different 15N recoveries. Recoveries of residual N were low and similar among azolla species and urea during the second rice crop, and they ranged from 3.5 to 4.6% of the amount of N applied before the main rice crop.
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