الفهرس 
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Abstract
A field experiment was conducted at The Experimental Farm, Faculty of Agriculture, Al-Azahar University, Assuit, located 375 km south of Cairo, Egypt (27 12- N latitude and 31 09- E longitude) in order to determine how tillage practices affected organic carbon retention and soil CO2 flux. Moreover, to document the changes in soil carbon dioxide flux after tillage and quantify the effects of tillage on the responses of temperature and soil moisture sensitivities of soil carbon dioxide flux.
The experiment was laid out in split split plots design with three replicates and 12 treatments (2 levels of soil moisture content, SMC, 3 tillage manner and 2 nitrogen sources). SMC (65 and 85% FC) occupied the main plots, tillage manner (no, reduced and conventional tillage; NT, RT, CT) served in the split plots and the split split plots were assigned for two nitrogen sources (urea and ureaform). Another treatment in uncultivated soil used as a base line for emitted carbon dioxide. In the summer seasons of year 2011 and 2012, sunflower seeds (sakha 130, variety) were planted in the 14th of June of both seasons. The sunflower plants were harvested after 93 days from planting in each season. Four square meters from each plot were collected as samples for growth and yield measurements. In the winter seasons of 2011/12 and 2012/13, wheat seeds (Triticum aestivum vulgar. CV Sids 1) were sown on December 1st of both seasons. The plants were harvested 145 days after planting in each season. Four square meters from each plot were collected as samples for growth and yield measurements. Soil CO2 flux measurements (Fs) from each treatment were taken weekly in summer and biweekly in winter between 9 and 11 am o’clock. Soil temperature and water content were measured at the same times and locations when CO2 emission was measured. A soil sample was collected at the surface horizon (0–15 cm) to determine the gravimetric soil water content.
The obtained results could be summarized as follows:
1. Effect of soil tillage on soil CO2 flux
Diurnal CO2 patterns were evident during the sunflower growing season as the emitted soil CO2 increased by time proceed for all tillage manners. The emitted soil CO2 ranged between 12.49- 22.65, 14.49- 25.06 and 14.14- 25.95 g/ 100 m2 for no tillage (NT), reduced tillage (RT) and conventional tillage (CT), respectively at 65% SMC when soil fertilized by urea. The trend line inclined is less under NT than that under RT or CT tillage. A slight difference persisted in the emitted soil CO2 as a result of tillage manner at 85 % soil moisture content when soil fertilized by urea. The trend lines of tillage manner are much closed and they increased by time proceed. In general, the emitted soil CO2 confined between 15 and 26 g/ 100 m2 for 65 or 85 %.
No tillage treatment had higher CO2-C flux than RT treatment at the beginning of the evaluation period (till 4th week) and lower flux at the end at 65% soil moisture content when soil fertilized by ureaform. The trend line of emitted soil CO2 from CT treatment shows almost a steady flux through the growing season (10 week) and CO2-C flux values range between 17.71 and 25.40 g/ 100 m2. Weekly soil CO2 flux measured in the NT plot ranged from 13.59 to 24.37 g/ 100 m2, while soil CO2 flux in the RT or CT differed from 13.11 to 26.09 g/ 100 m2 when the soil fertilized by ureaform at 85 % SMC. The amount of emitted soil CO2 from CT treatment was the highest one during the entire growing season.
It is observed that the emitted soil CO2 from NT treatment is less than that from either RT or CT treatment when soil fertilized by urea at 65% SMC through wheat growing seasons. However, among tillage system CO2 emission was numerically in the order: NT< RT< CT. The emitted soil CO2 increased as the time proceeded when soil fertilized by urea at 85% SMC through wheat growing seasons. The emitted soil CO2 ranged between 17.60- 27.00, 17.50- 27.70 and 18.90- 29.90 g/ 100 m2 for NT, RT and CT, respectively. It is observed that the amount of emitted soil CO2 from CT treatment was the highest one during the entire growing season.
The emitted soil CO2 from NT treatment is less than that resulted from RT or CT when soil fertilized by ureaform at 65% SMC through wheat growing seasons. The differences are more obviously as the plants grow up and getting mature. Also, almost similar results are recognized when the soil irrigated at 85 % SMC. After 8th weeks, soil CO2 flux increased by 13.11 % in CT compared to NT treatment. During the rest of the growing season (8 weeks more), the increase in soil CO2 flux started to slow down to be about 2.91% in CT compared to NT treatment.
2. Effect of soil moisture content on soil CO2 flux
There is almost unobvious effect of soil moisture content on soil carbon efflux especially at 65% SMC when soil fertilized by urea or ureaform through sunflower growing seasons. Soil moisture content fluctuated up and down from week to another in almost a regulator manner. Also, soil CO2 emission changed as soil moisture content changed since it slightly increased when soil moisture content increased (85 % SMC) and vice versa.
It is observed that soil CO2 flux and soil moisture had a negative linear relationship when soil fertilized by urea or ureaform at 65% SMC through the wheat growing seasons. The emitted soil CO2 increased as the soil moisture content decrease and this trend started from the 2nd week till the end of the growing seasons. Soil CO2 flux at 85% SMC when soil fertilized by urea or ureaform through the wheat growing seasons showed notable seasonal patterns. The data show that soil CO2 flux and soil moisture had almost a positive linear relationship. The emitted soil CO2 increased as the soil moisture content increased.
3. Effect of soil temperature on soil CO2 flux
It is observed that there is almost no relation between soil CO2 flux and temperature sensitivity (Q 10) through sunflower growing season at 65 % SMC when the soil fertilizered by urea or ureaform. The data clearly showed that the trend line of soil CO2 emission increased as the time proceed with R2 of 0.47 and 0.51 when soil treated by urea and ureaform, respectively. On the other hand, the trend line of Q10 goes almost steady as the time goes on with R2 of almost 0.01 when soil treated by urea or ureaform.
At high soil moisture content (85 % SMC), there is a gentle relationship between soil CO2 emission and temperature sensitivity (Q10) through the sunflower growing seasons regardless the fertilizer type. The trend line of soil CO2 emission increased as the time proceed with R2 of 0.59 when soil treated by urea or ureaform. On the other hand, the trend line of Q10 goes slightly up as the time goes on with R2 of almost 0.15 when soil treated by urea or ureaform.
The average Q10 values is 2.13 at 65 % SMC regardless the fertilizer type. At 85 % SMC, the average Q10 values is 2.29 and 2.54 when soil treated by urea and ureaform, respectively. A maximum CO2 flux of about 24.99 g/ 100 m2/ week was found at a Q10 value of about 2.29 and 41 % soil moisture content. A minimum CO2 flux of about 12.22 g/ 100 m2/ week was found at a Q10 value of about 1.81 and 33 % soil moisture content.
It is observed that there is no relationship between soil CO2 flux and temperature sensitivity (Q 10) at 65% SMC when soil fertilized by urea or ureaform through the wheat growing seasons. The trend line of Q10 goes almost parallel to the horizontal axis as the time goes on. Soil CO2 flux and Q10 showed notable seasonal patterns at 85% SMC when soil fertilized by urea or ureaform through the wheat growing seasons. Soil CO2 flux and Q10 had almost a positive linear relationship.
4. Effect of nitrogen fertilization on soil CO2 flux
Soil CO2 emission increased as the time proceeded under both fertilizers through sunflower growing seasons at different SMC (65 or 85 %). Soil CO2 emission was slightly higher under ureaform fertilization than that under urea fertilization till fluctuation point occurred at the 6th week of the growing season. There is almost no effect of using urea or ureaform fertilizer on soil CO2 emission at different SMC (65 & 85 %) through wheat growing seasons. The effect of N fertilization on soil CO2–C emission is inconsistent and it is highly site-specific, which can be attributed to differences in soil conditions.
5. Effect of management practices on soil aggregates and soil CO2 flux
Total aggregate percent (TA) and mean weight diameter (MWD) increased as SMC increases during sunflower growing season of 2011. Also, tillage manner affected both TA and MWD and it might be arranged in descending order of under NT > RT > CT regardless the fertilizer effect. On average basis, the highest and lowest TA values of 50.00 and 39.60 were recorded in NT and RT treatment, respectively regardless the SMC or fertilizer types. The highest and lowest MWD values of 60.45 and 50.43 were recorded in RT and CT treatment, respectively regardless the SMC or fertilizer types.
Total aggregate percent and MWD increased as SMC decreases during wheat growing seasons. Also, tillage manner affected both TA and MWD and it might be arranged in descending order of under NT > RT > CT regardless the fertilizer effect. On average basis, the highest and lowest TA values of 49.60 and 34.91 were recorded in NT and CT treatment, respectively regardless the SMC or fertilizer types. The highest and lowest MWD values of 54.15 and 43.86 were recorded in NT and CT treatment, respectively regardless the SMC or fertilizer types.
6. Yield as a factor of soil management practices
Sunflower grain yield was significantly affected by soil moisture content and fertilizer type as well as it was high significantly affected by tillage manner. Sunflower grain yield was higher in the plots treated by ureaform than that treated by urea at 65 or 85 % SMC. Sunflower grain yield was superior under conventional tillage followed by reduced then no tillage. On average basis of both seasons, sunflower grain yield was 1.97, 2.01 and 2.22 ton/ fed for NT, RT and CT, respectively at 65% SMC. It was 1.05, 1.76 and 2.22 ton/ fed for the corresponding treatments at 85 % SMC. The yield component of sunflower (head diameter, plant height, and seed index and oil seed) followed the same trend of sunflower yield. The interaction effects of SMC, fertilizer types and tillage manner were high significantly affected sunflower yield and its components.
Wheat yield and its component were influenced by management practices such as SMC, fertilizer types and tillage manner. Wheat grain yield in the plots treated by ureaform was higher than that treated by urea at 65 or 85 % SMC. In the 2nd season, wheat grain yield at 85 % SMC was higher than that at 65 % SMC. Wheat grain yield was significantly affected by soil moisture content and fertilizer types. On the average basis of both seasons, wheat grain yield was 3.36 and 3.23 ton/ fed at 65 and 85% SMC, respectively. Wheat grain yield was 3.28, 3.37 and 3.42 ton/ fed for NT, RT and CT, respectively at 65% SMC. It was 3.06, 3.16 and 3.48 ton/ fed for the corresponding treatments at 85 % SMC. The yield component of wheat (straw yield, plant height, and seed index and seed N %) followed the same trend of wheat yield. The interaction effects of SMC, fertilizer types and tillage manner were high significantly affected wheat yield and its components especially in 2nd season.