A decline in productivity in wetland rice has been detected in some intensively-cultivated experimental farms in Asia since the early 1980s. Increased doses of fertilizer N are being used in both experimental and farmers' fields to maintain the original yield levels. Little attention has been paid to judicious management of native soil N, which is the principal N source for rice, and to biological N2 fixation (BNF), which largely replenishes the soil N concentration. We review here various effects of long-term flooding and puddling associated with intensive cultivation of wetland rice on soil N availability and BNF. Some strategies are suggested to efficiently manage these two N sources to sustain high productivity of the ricelands.

Efficient management of soil and biologically fixed N2 in intensively-cultivated rice fields

The influence of wheat straw cover, tillage, and irrigation on metribuzin and alachlor movement and dissipation in an Appling coarse sandy loam (Typic Hapludult) and the influence of soil depth on their rate of dissipation were measured in field and laboratory experiments conducted in 1987 and 1988. Overall, metribuzin moved more than alachlor in both years. Alachlor movement was greater in tilled plots compared to no-till plots in 1988. Metribuzin movement was greater in no-till plots in 1987. Straw cover had little effect on the movement of alachlor, but the presence of 2800 kg ha−1 of straw on the soil surface increased the downward movement of metribuzin in both years compared to soil with no straw cover. The rate of alachlor dissipation in the soil was faster in straw-covered and no-till plots in both years and in the high irrigation level in 1988. Metribuzin dissipation was not affected by any of these factors. Alachlor rate of dissipation did not differ among depths in the field; however, dissipation differed in the laboratory in the order 0 to 20 cm > 20 to 45 cm = 45 to 68 cm. Metribuzin dissipated faster at the 8-cm depth in the field compared to the 58-cm depth. In the laboratory, metribuzin dissipation rates followed the order 0 to 20 cm > 20 to 45 cm > 45 to 68 cm. At the greatest depth, the dissipation of metribuzin was faster in the field than in the laboratory.

https://www.jstor.org/stable/info/4045080

Alachlor and Metribuzin Movement and Dissipation in a Soil Profile as Influenced by Soil Surface Condition

In this report we studied the effects of mechanical grinding of paddy soils on nitrogen mineralization and bacterial number when soils were incubated under a submerged condition after grinding. Nitrogen mineralization was increased by grinding soil samples as compared with those without grinding. The value of (Nd-Nu)/Nu, where Nd is the amount of ammonia-nitrogen formed by incubation of disrupted samples and Nu those of the undisrupted soils, was in good correlation with clay/humus ratio. Although significant difference was not observed between the number of aerobic bacteria with undisrupted and disrupted soil samples, higher anaerobic bacterial numbers were found with disrupted than with undisrupted ones. A significant correlation was also obtained between the amounts of nitrogen mineralization increased by the grinding of soil and the numbers of anaerobic bacteria.

BACTERIOLOGICAL STUDIES ON THE MINERALIZATION OF ORGANIC NITROGEN IN PADDY SOILS : I. Effects of Mechanical Disruption of Soils on Ammonification and Bacterial Number

The effects of soil management on seasonal changes in soil microbial communities were examined in a continuously cropped field of cabbage (Brassica oleracea var. capitata) using phospholipid fatty acid (PLFA) analysis. Soil samples from each plot treated with cattle manure compost (CMC), grass compost (GRC) and chemical fertilizers (CHF) were taken at monthly intervals from April 2003 to March 2004. The results showed that microbial biomass was smaller in the CHF-treated plot than in the other plots. The mole-percentage contributions of biomarkers for Gram-negative bacteria slightly decreased from April to September and increased from September to March in all plots. The mole-percentage contributions of biomarkers for fungi and actinomyces were greater in the CHF-treated plot compared with the other plots. The mole-percentage contributions of biomarkers for Gram-positive bacteria increased in the order of CMC > GRC > CHF. Higher ratios of cyclopropyl fatty acids to their precursors and lower rati...

https://www.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1747-0765.2008.00242.x

Season and soil management affect soil microbial communities estimated using phospholipid fatty acid analysis in a continuous cabbage (Brassica oleracea var. capitata) cropping system

Inocula of 103, 104, and 105 Bacillus thuringiensis spores/g non-sterilised soil apparently failed to germinate and viability declined by up to 55% during 90 days incubation at 25°C and -0.10 MPa. In autoclaved (121°C, 20 min) soil, no net mortality was detected in any of the 3 levels of inocula and detection of germination was erratic. No germination was detected yet sporadic growth occurred with inocula of 103 spores/g soil, and both (sporadic) germination and growth were detected at 104 spores/g soil. Final levels of these inocula were 500–2000% greater than their initial levels. Germination and growth were not detected in the inoculum of 105 spores/g soil. By contrast, inocula of 104 Bacillus cereus spores/g germinated in both non-sterilised and autoclaved soil, but then exhibited bacteriostasis. No net changes in inoculum size were detected in either soil treatment. We suggest that the mortality of B. thuringiensis spores in non-sterilised soil resulted from an inherent inability of the spor...

Effect of incubation in non-sterilised and autoclaved arable soil on survival of Bacillus thuringiensis and Bacillus cereus spore inocula

It is well known that tillage of soil produces greater aeration and incorporates crop residues and fertilizers into inner soil layers, thus stimulating the microbial activities and increasing the rate of decomposition of soil organic matter. Tillage operation has been, therefore, one of the important practices to increase soil productivity.

Yoshiharu Tokunaga

Papers

Effect of the difference of tillage operations on microbial properties of soil layers