Solomon Acquaye

Effects of controlled-release coated urea (CRCU) on soil microbial biomass N in paddy fields examined by the 15N tracer technique

Experiments were conducted in paddy fields at Shiga and Chiba Prefectures to study the effects of controlled-release coated urea (N-LP100) on soil microbial biomass and N uptake of rice plants by the 15N-tracer technique, during one cropping season. Three field fertilizer treatments (Zero N: 0 kgN ha−1, 15N-LP100: 64 kg N ha−1 and 15NH4Cl: 100 kg N ha−1) were set-up in the Shiga field experiment. After transplanting in the paddy fields at Shiga and Kashiwa (Chiba), a number of rice hills with standard growth were selected randomly and enclosed by polyacryl-plastic frames designated as microplots. 15N-LP100 (64 kg N ha−1) was applied in the Shiga and Kashiwa microplot experiments and the Shiga field experiment as deep-side placement (5 cm away from rice hill and 5 cm soil depth). Total N uptake of rice plants was analyzed in the course of plant growth. In addition, soils from the field fertilizer treatment plots and microplots (divided into 11 blocks) were taken and analyzed for microbial biomass N (BN) and biomass 15N (B15N). The results indicated that; (1) Plant N uptake from basal-applied fertilizers at the end of the study in the Shiga field experiment was 71.9 and 26.0% for 15N-LP100 and 15NH4Cl, respectively. In the Kashiwa microplot experiment, plant N uptake from applied 15N-LP100 was 51.2% at 67 days after transplanting (DAT) (2) Throughout the cropping season, BN was the highest, intermediate and the lowest for 15NH4Cl, 15N-LP100 and Zero N field experimental plots in the Shiga experiment, respectively. (3) In the micro-plot experiments, BN and B15N were generally higher in the soil block with deep-side application of 15N-LP100 compared with the other soil blocks. The deep-side placement of 15N-LP100 ensured a high efficiency of utilization of its N by rice plants. The method of 15N-LP100-placement also affected the spatial heterogeneity of microbial biomass N in the microplots.

Comparative effects of app1ication of coated and non-coated urea in clayey and sandy paddy soi1 microcosms examined by the 15N tracer technique

Abstract Nitrogen fertilizer and soil types exert an impact on plant growth. An experiment was conducted to compare the effects of application of controlled-release coated urea (CRCU) and urea on the growth of rice plants cultivated in gley (clayey) and sandy paddy soils. The 15N labeled fertilizers were applied at the rate of 8 g N m−2 for CRCU and 10 g N m−2 for urea. Both soil and fertilizer types significantly influenced the plant N and 15N uptake at 30 and 60 days after transplanting (DAT), with only the fertilizer type influencing the 15N uptake at 90 DAT. Sandy soil and urea increased the plant N and 15N uptake compared to the gley soil and CRCU at 30 DAT while the opposite was observed at 60 DAT, with CRCU inducing a higher 15N uptake than urea at 90 DA T. Plant 15N uptake at harvest was similar for the two types of fertilizers. Also at harvest, the average fertilizer use efficiency and proportion of unaccounted-for N for CRCU were about 74 and 25%, compared to 39 and 60% for urea, respectively. At harvest time, the total N and 15N contents in the plant parts were similar between gley and sandy soils, and between CRCU and urea, respectively, Except for the harvest index and the percentage of filled grains that were significantly influenced by the soil type, there were no significant differences in the dry weight of straw, panicle, grains, and 1,000-grains between the two soil and fertilizer types at harvest, Statistically non-significant results were obtained for the two soil and fertilizer types despite the fact that the application of CRCU in gley and sandy soils increased the values of the agronomic traits by >40% compared to urea application in the two soil types, The soil type affected the agronomic traits of rice plants much more significantly than the fertilizer type applied. Even though analysis in the absence of 15N tracer showed only minimal variations in the effects of CRCU and urea application on the agronomic traits of rice plants, the 15N analysis revealed a significant influence of the fertilizer type on the plant 15N uptake, Higher fertilizer use efficiency and a lower proportion of unaccounted-for N achieved with CRCU application resulted in a similar or in some aspects superior performance of the rice crop to that obtained with urea application, although the amount of total N applied in the CRCU treatment was 20% less than the total N amount applied in the urea treatment.

Role of microbial biomass in biogeochemical processes in paddy soil environments

Abstract Soil microbial biomass (hereafter referred to as microbial biomass), defined holistically as the living component of soil organic matter (i.e., all organisms with a volume less than 5,000 μm3, e.g., bacteria, fungi, protozoa), is actively involved in biogeochemical processes that occur in soil microniches of paddy soils. These processes include organic matter decomposition, microbial oxidoreduction, and cycling of N, C, and plant nutrients. The nature and extent of these biogeochemical processes cannot be approximated without understanding the involvement of microorganisms in these processes. Microbially-mediated biogeochemical processes, such as photosynthesis, N2-fixation, organic matter decomposition, subsoil oxidoreduction, and nutrient immobilization, lead to increases in the content of microbial biomass, while processes, such as biomass turnover and mineralization, lead to its decrease. In addition, the level of microbial biomass in the paddy soil ecosystem is affected by many biotic and abiotic factors, such as N fertilization, organic matter applications, soil type, flooded-upland soil rotation, and soil depth. Methods to measure soil microbial biomass as a single pool of organic matter include substrate-induced respiration, chloroform fumigation-incubation, chloroform fumigation-extraction, and adenosine triphosphate. However, these holistic methods provide little information about the community composition and physiological state of the soil microbial biomass and reasons why the soil microbial biomass changes over time and under different conditions. However, these methods are useful in understanding of cycling and dynamics of soil organic matter, especially where whole suites of organisms are involved. Culturing and isolation of microorganism provide answers to the shortcomings of the single pool biomass methods. Newer methods that can provide valuable information about the physiological state of soil microbial communities and provide more sensitivity to detect changes in these communities include: gene-based analytical methods, microbial activity, tracer isotopes, and analysis of biomarkers. Depending upon the objectives of a study or the problem to be addressed, any of the analytical methods described offer a best and efficient approach to analyze soil microbial biomass.

Comparative Effects of Application of Coated and Non-Coated Urea in Clayey and Sandy Paddy Soil Microcosms Examined by the ^ N Tracer Technique : II. Effects on Soil Microbial Biomass N and Microbial ^ N Immobilization(Fertilizers and Soil Amendments)

Abstract Nitrogen Fertilizer and soil types exert an impact on plant and soil microbial biomass (SMB). A 15N tracer experiment was conducted to compare the effects of the application of controlled-release coated urea (CRCU) and urea on SMB in gley (clayey) and sandy paddy soils. The fertilizers were applied at the rate of 8 g N m−2 for CRCU as deep-side placement and 10 g N m−2 for urea mixed into soil or applied into floodwater. The soil type and soil layer (surface: few millimeter depth of surface soil to include benthic algae; subsurface: 1 to 20 em depth), but not the fertilizer type, affected the amount of microbial biomass N (BN). On an area basis, subsurface soil layers contained about 2-3 times the amount of BN in the surface layers. The seasonal average BN amount i.e. at 1 to 20 em depth, in the gley soil was 1.67 g N m−2 , compared to 1.20 g N m−2 for the sandy soil. The proportion of BN in total soil N was significantly influenced by the soil type and soil layer, and was higher for the surface layers of both soils and subsurface layer of the sandy soil than for the subsurface layer of gley soil. Soil type, soil layer, and fertilizer type significantly influenced the amount of microbial biomass 15N (B 15N)Unlike BN, the amount of B 15N was significantly higher in the surface (11,9–177,3 mg N m−2) than in the subsurface soil layers (4.8–83.6 mg N m−2), especially with urea application between 60 and 120 DAT (days after transplant ing). At 30 DAT, the subsurface layer of the sandy soil showed a higher B 15N (218 mg N m−2) amount than the surface layer (133.4 mg N m−2). Sandy soil (4,8-–218 mg N m−2) and urea (6.2–218 mg N m−2) induced a larger increase of the amount of B 15N than the gley soil (6.2–83.6 mg N m−2) and CRCU (4.8–40 mg N m−2). Again, the sandy soil, surface soil layers, and urea induced a higher proportion (% ) of B 15N in BN than the gley soil, subsurface soil layers, and CRCU, respectively. The soil type affected BN more than the fertilizer type, which showed only minimal differences. However, 15N analysis revealed the existence of greater differences in the effects of CRCU and urea on the B 15N amount. Sandy soil and the application of urea led to a higher microbial N immobilization than the gley soil and CRCU application, respectively.