Abstract: This study have been conducted for evaluating agricultural practices and crop type effects on greenhouse gasses (GHG) emissions and soil carbon balance. Due to increase of GHGes in recent decades and their affect on Global warming, study of their emission from agricultural regions, seems to be necessary. The most important GHGes are carbon dioxide and methane. Carbon dioxide is increasing at the rate of 5% a year. Burning fuel and changing land use are two major human activities that result in this increase. Methane has a greenhouse effect as 21 times greater as CO2. At this study carbonic GHGes emissions from 3 fields were measured at Khuzestan Province in Iran. Experiment was conducted at 3 fields including rice-follow rotation (F1), follow-wheat rotation (F2) and melon-wheat rotation (F3); with 8 time sampling and with 3 repeats in completely randomized block design. Chamber method and gas chromatograph technique were used to measure CH4 and CO2 emission. Results showed that the highest amount of emission is related to CO2 and total emission is greatest for F1. This might be due to remaining crop residuals in this field and release of this gas during residuals decomposition. Average of methane emission was greatest for F1 (2.222 mg C m-2 day-1) and greatest emission was measured at near the rice harvest time on October, while the field 2 was a sink for atmospheric methane with mean of -0.106 (mg C m-2 day-1) and it was not significant difference with field 3 (zero). Methane emission from soils is depending on long paddy conditions at soil and these conditions are performing in rice cultivation.
INTRODUCTION
Soils with higher Organic Matter (OM) have higher exchange capacity and buffering ability (Stevenson and Cole, 1999). Organic matter is the main source of greenhouse gas (GHG) emissions from the soil (Post and Kwon, 2000).
Carbon is being transformed in organic and inorganic forms continually at carbon cycle. Therefore, mineralization and organization are two basic processes at carbon cycle. So, atmospheric carbon and soil-plant carbon are important. Carbon interring to plant-soil system is known as carbon fixation and its exiting is known as carbon loss (Rees et al., 2005). Most carbon loss occurs during respiration where soil Organic Carbon (OC) and plant absorbed carbon (by photosynthesis) release into atmosphere in the form of CO2 (Stevenson and Cole, 1999). Usually soil organic matter loss is caused by converting natural lands to agriculture farms; soil respiration includes plant root and microorganisms respiration (Stevenson and Cole, 1999). Soil management plays an important role in carbon loss through respiration (Rees et al., 2005).
Organic carbon loss process is influenced by environmental factors such as ground water (Sanchez et al., 2003), soil temperature and moisture (Parashar et al., 1993; Rochette et al., 2000; Von et al., 2005), soil texture and structure, tillage, Dissolved Organic Carbon (DOC) (Rochette et al., 2000), microbial biomass, nitrogen mineralization (Kaiser et al., 2005), physical properties (Kimble et al., 1998), sun radiation, precipitation (Mattila et al., 2003), plant type (Maljanen et al., 2004), root respiration, discharge (Vose et al., 1995), mineral fertilizers (Rees et al., 2005) and the addition of organic residuals (Khalil et al., 2005).
Smith et al. (2000) had estimated the agriculture lands as the greatest bio-source of carbon loss in Europe.
Lou et al. (2004) used static chamber for measuring CO2 emission in China’s agriculture lands. They found that it had the greatest correlation with soil temperature and then with soil moisture, microbial biomass and DOC. Schimel and Clein (1991) and Raich and Schlesinger (1992) found that soil moisture and temperature are two important factors that have direct effects on plant roots and soil microbial activity and have indirect effects on soil physical and chemical properties.
Because of high potential for agriculture and two crops in a year in Khuzestan, farmers usually burn crop residuals of the first planting and immediately till the soil and plant the second crop. This causes high amount of CO2 emissions during the burning.
Carbonic GHGes includes CO2 and CH4 that have communication with soil sources and agriculture practices.
Lal and Kimbel (1995) stated that atmospheric CO2 is increasing at the rate of 5% a year. They said a significant increase is started from 1850 and believe burning fuel and changing land use are two major human activities that result in this increase.
Methane is one of the most important greenhouse gasses with a 10 year life time and with 21 times as much greenhouse effect as CO2 in 100 years (Neue et al., 1995). It is the most abundant carbonic gas at atmosphere after CO2. Methane concentration at troposphere is 1.6 to 1.8 ppmv and it is increasing with the rate of 0.8 to 1% a year (Khalil et al., 2005).
Different greenhouse gas emissions had been differentially affected by soil moisture and among them, major methane emissions is from paddy and logged lands.
Rice is grown in different agriculture ecosystems, but the biggest distribution (55% of fields) is in the form of paddy fields and the most yields (75% rice world yield) are from these fields (Olszyka et al., 1999). These fields are logged for most of growth period, so, anaerobic conditions emerge at these fields that result in CH4 production and emission (Neue, 1993; Olszyka et al., 1999).
It seems to be a positive correlation between increasing the rice fields and atmospheric CH4 concentration in recent years. There are about 145 million ha of rice agricultural field all over the world. Khuzestan Province with 50 to 60x103 ha of rice field is one of important rice producing areas in Iran. Most rice fields are in Ahvaz with straight plantation.
The amount of CH4 emission from rice fields depends on plant growth and availability of carbonic material in soil that is affected by plant conditions, irrigation regime, fertilizers type and amount, crop organic material returning to soil and seasonal climate. It is significantly changed by rice type, fertilizers, irrigation time and extra factors depicting the role of management in the emission of CH4 (Neue, 1993; Neue et al., 1996).
Transmission of CH4 from soil to atmosphere is mostly through rice plant. CH4 diffuses from rice rhizosphere to rice stem and moves through rice shoot tube and then go away to atmosphere through leave pores (Olszyka et al., 1999).
At this study for evaluating the effect of crop type and season on carbonic GHGes emissions and carbon balance, the CO2 and CH4 emissions from rice-follow, follow-wheat and melon-wheat rotations have been measured and carbon balance of these fields calculated during one year. So, it is possible to resulting that which fields are source of GHG emissions and which fields are sink for them.
MATERIALS AND METHODS
Study site: This study was conducted in 3 fields 12 km away from Ahvaz around 48° 27’ 50" West to 48° 28’ 59" East and from 31° 13’ 45" South to 31° 14’ 1" North.
Field 1 (F1) with 100 ha rice- follow rotation, field 2 (F2) with 100 ha follow-wheat rotation and field 3 (F3) with 100 ha wheat rotation were selected and 15 ha from each field were studied.
Layout and experiment design: These fields were saline and alkaline lands in the past and were bare or covered by halophyte plants. Plant covers in non-agricultural spaces are Acacia SP, Atriplex SP, Chenopodiaceae family, Alhage SP and ext.
These fields have been planted since 5 to 12 years ago.
F1 was planted from 5 years ago and was used for wheat crop for the first three years and for rice- fallow for two recent years. Rice variety is Red Anbory (local variety).
The 250 to 300 kg ha-1 nitrogen fertilizer (in the form of urea), 50 kg ha-1 triple super phosphate and 100 to 150 kg ha-1 potassium sulfate were used for rice field.
Nitrogen fertilizer was used as 100 kg N ha-1 at planting time and rest at two times during the plant growth. During rice growth period field was continually soaked. Rice residuals were remained at the field and added to the soil with spring tillage. Rice planting at this area starts at the beginning of summer and harvest time is at the middle fall (with 110 to 125 days growth period).
F2 has been planted from 8 years ago and has dominantly been used to planting wheat in the fall and has been fallowed in the summer (at least in 5 recent years). Fertilizers used in this field include 250 kg ha-1 N fertilizer (in 3 periods), 100 kg ha-1 triple super phosphate and 150 kg ha-1 potassium sulfate yearly. This field was irrigated 4 times during wheat growth period and crop residuals were burned before tillage in the middle of fall.
F3 has been planted from 7 years ago and during the past 5 years; it was used for two crops a year. First plantings at this field usually contained melon, water melon, cucumber and grains in summer and second planting was usually wheat in the fall.
| Table 1: | Soil analysis of the fields |
| Table 2: | Average OC loss in the form of GHG (ton/ha/year) |
This field was irrigated by Karoon River 10 to 13 times during summer and 4 to 5 times during fall and crop residuals were burned before fall tillage. Fertilizers used on this field contained 200 kg h-1 N fertilizer and 100 kg h-1 triple super phosphate during the first planting and 200 kg h-1 N fertilizer (3 times during plant growth), 100 kg h-1 triple super phosphate and 100 kg h-1 potassium sulfate during the second planting.
The experiment was a split-plot design with randomized complete block design. Crop type (F1, F2 and F3) were the main plot, time sampling (8 times) were the sub-plots and all treatments were replicated trice.
Soil sampling: Soil sampling was carried out for 30 cm of soil surface in the summer of 2006 and 3 mixed samples for each field were provided. Soil samples air dried and passed through 2 mm sieve and were analyzed for soil texture, Electrical Conductivity (EC), soil saturation percentage, soil saturation acidity (pH), soil lime using contrary titration, Cation Exchange Capacity (CEC) with sodium acetate method, bulk density with core method, gypsum percentage by titration (Table 1).
Furthermore, soil sampling from 5 different depths to 110 cm was carried out using auger 8 times during the year (two times for each season) and organic carbon was determined by walkey black method. Then the average for each season was calculated (Table 2).
Plant sampling: Plant samplings from rice, wheat and melon were performed with a 20x30 cm plot, 8 times along with the soil and air samplings. Plant samples were dried and scaled to determine the field’s biomass.
Air sampling: Nine static chambers were built to collect the gasses. Chambers framework was built from poly-ethylene tube with 1 cm thickness and 20 cm diameter. One side of tubes was locked with transparent polyethylene with 1 cm thickness and two orifices were shebanged for a specific pipe (for air sampling) and a thermometer in the middle of framework.
Chambers with 1 m height were considered for rice field. Although rice plant do not produce methane, but because of air tube; it is a major way for methane to escape from soil to atmosphere (about 50 to 80% of total methane emission). It is necessary that plant place in chamber. On the other hand, to measure net soil carbonic gas emission, a few chambers with 40 cm heights were put at spaces without plant with waterlogged soil to measure CO2 and CO in F1. Chambers with 40 cm heights for F2 and F3 were put in the soil near the plants. To collect soil air, chambers placed at fields and 5-7 cm inserted into soil and packed with mud, then after 4 h air sampling done with a 60 mL syringe and thermometer was read. Air sampling has been done 8 times during this study. One hour after sampling air samples were translocated to the laboratory and were read by a Gas Chromatograph (GC) system with FID (Flame Ionization Detector) and TCD (Thermal Conductivity Detector) detectors. To calculate the initial gas in chamber, air sampling from 2 m free air was also done for the blank. To get net emission blank readings was subtracted from samples readings, so; some what that gas absorbed through soil emission is negative. Reading data then revised for standard temperature and calculated on the base of carbon emission mass from surface at time unit. Calculations were done with Excel software and results analyzed with SPSS (version 13) software.
RESULTS AND DISCUSSION
Gasses emission amounts are near as reported by Jrecki and Lal (2006) (Table 2). Results show that average carbon losses in the form of CO2 and CH4 are significantly different among the fields. Total emission is greatest for F1 and the highest amount of emission is related to CO2. This might be due to decomposition of remaining crop residuals in this field. But in two other fields, residuals were burned and large amounts of CO2 were released to atmosphere. In F3, because of two crops in a year, more residuals added to soil and more CO2 emission was observed compared to F2 (Fig. 1, 2).
Increase in CO2 emission after October in F1 is because of rice harvest at the end of October and start of aerobic condition which increase residuals degradation that produce CO2. In F2 and F3 because of soil plow and wheat plant at November CO2 emission increase.
| Table 3: | OC % process during time |
| Table 4: | OC balance at 30 cm surface soil of fields (ton/ha/year) |
Carbon dioxide emission is greater for F1 and F3 rather than F2, probably because of higher OC in F1 and F3. Organic carbon changes with time are shown in Table 3.
Methane emission in F1 is greatest too and emission is highest in October. This time is end of rice growth and start of harvest, so highest reduction condition and highest CH4 emission have been exist. Due to aerobic condition in F2 and F3, CH4 emission is very low and zero, respectively.
Organic carbon amount changes: Many parameters such as soil formation factors including parent material, climate, topography, plant cover and time affect OC amount. Organic carbon changes is very small, but some factors such as land use type, land management, soil humidity condition and irrigation and specific plant covering existence can strongly change it. Due to same soil series and same soil properties in the selected fields (Table 1), OC changes is related to environmental factors.
Organic matter measurement from different depths of these lands was carried out 18 years ago. This has been done 8 times during the year (two times a season) for the purpose of this study. Average amounts for each season appear in Table 3. Significant increase in OC amounts because of agricultural operation was observed and this increase is greatest in F1. Results show carbon balance is positive and soil OC has increased during the time. Despite high temperature in the summer time, rice cultivation brings humidity and temperature to that level which microbial activity could take place and decomposition of residuals goes on due to this process and increased at the end of dry season.
| Fig. 1: | CO2 emission from fields |
| Fig. 2: | CH4 emission from fields |
Soil carbon balance: Carbon balance is calculated from the differences between carbon entrance and losses in the year (Table 4).
Major part of OC entrance is from crop residuals and carbon losses include erosion, residual burning and gas emission. Organic materials were not added to these fields due to the field’s history, so OC entrance is only from crop residuals. Organic carbon loss by surface erosion is zero or very little because of very low gradient, so OC exit is due to residuals burning and gas emission. With regard to soil bulk density, yearly OC increasing amounts calculate for ton at hectare at year scale. It is possible to calculate additional OC to soil with measuring the dry residuals biomass, considering that 35% of residuals dry weight is OC approximately. Burned biomass regarding that the upper biomass after harvest is 50 to 60% of total biomass (upper plus inner biomass) was calculated. Gas emission amounts were measured during this study. Biomass burning was not performed in F1, so major amount of additional residuals loss is in gas form. In F3, two plantings was done during the year and more crop residuals added to soil but major amount of this lost with burning, so remained OC and also gas emission is less than F1. F2 with one planting a year and burning the biomass has minor OC yearly increase as well as gas emission.