Experimental quantification of greenhouse gas (CH₄) emissions from fertilizer amended rice field soils of Kashmir Himalayan Valley

Introduction

Natural wetlands of Kashmir Himalayan valley, India, researched over the years continue to remain the focus of recent attention of workers [1,2]. However, rice fields considered an integral part of wetland ecological research and source of potential green house gas (methane) have been almost ignored.

Methane ranks second major greenhouse gas (GHG) after CO₂ with significant fluxes from agro-ecosystems. Rice fields, in particular, are considered [3,4,5] to be one of the important sources (20%) of CH₄ emission on a global scale. During the last 200 years, its concentration in the atmosphere has more than doubled; the preceding 40 years witnessed about 50% increase. Some recent estimates have been provided [6] of methane from global rice fields; 25.6 Tg CH₄ year^-1, with 95% certainty range of 14.8-41.7 Tg CH₄ year^-1. Sporadic country-wise methane emissions are also available. Very recent study [7] quantified methane emissions from rice paddies in northeast China by integrating remote sensing mapping with a biogeochemical model, and the results showed that total 1.44 Mha (million ha) of rice paddies in the plain emitted 0.43-0.58 Tg CH₄ year^-1 methane. Similarly, studies conducted state-wise for 1994 in India [8] indicated national methane budget estimate of 4.09 Tg CH₄ year^-1 and the trend from 1979 to 2006 was in the range of 3.62 to 4.09 Tg CH₄ year^-1. Four higher emitting or "hotspot" states (West Bengal, Bihar, Madhya Pradesh and Utter Pradesh) in India account for 53.9% of total methane emission.

Data reported [9] show that Indian methane emissions have grown from 18.85 Tg CH₄ in 1985 to 20.56 Tg CH₄ in 2008. The important sources of atmospheric methane documented [10] are microbial mineralization of organic matter under strict anaerobic conditions such as landfills, paddy fields, biomass burning etc. In view of this scenario, methane is rightly implicated in global climate change. Reports [11] indicate that 70% increase in green house gases (GHGs) occurred during 1970-2004, and the global increases in CO₂ are attributed mainly to fossil fuel use while those of methane (and nitrous oxide) are primarily due to agriculture activities. Further, agriculture sector in India contributes 28% of the total GHG emissions. According to Intergovernmental Panel on Climate Change [12] the global average from agriculture is 13.5%. Irrigated rice production, a major food source for a large portion of the world's population is recognized to be a major anthropogenic source of methane. Global emission estimates [13] for this source range from 20 to 100 Tg CH₄ year^-1, which may be 4–30% of the total anthropogenic contribution to the atmosphere. Average estimate of methane 110 Tg CH₄ within a range of 25-170 Tg CH₄ released annually from rice-paddies are reported in Asia [14]. Moreover, Asia claims almost 90% of world's harvested rice-area; 60% production of it in China and India from where sufficient scientific data on this GHG (CH₄) are not available. In order to meet the growing demands of surging trend of population increased rice production is inevitable by adopting modern technologies in all rice ecologies. Therefore, global CH₄ emission from wetland rice agriculture is likely to increase. However, an effective land and water resource management can reduce the emissions substantially.

Throughout the world, attention on various amendments in CH₄ emission is making the research field subject of high importance. Kashmir valley, located in the Western Indian Himalaya is aptly referred to as 'rice bowl' with 2385 thousand quintal production of rice [15], having 12^th rank. The rice being staple food, is the principal Kharif (rainy season) crop cultivated on an area of more than 1.4 lac hectare, and grown between March-April and harvested in autumn. Except for preliminary report [16] on methanogenesis, no work has been carried out from temperate region of Kashmir valley. The impact on CH₄ emission is influenced by combination of nature, rate and application method of minerals as well as organic fertilizers. The research findings [17] show that the influence of organic amendments with inorganic fertilizers cause enhanced CH₄ gas emission as compared to the use of inorganic fertilizer alone. The paucity of information from temperate regions is a constraint in analyzing global scenario of CH₄ flux from wetlands. Adequate experimental data from various agro-climatic regions of the valley are sorely desired to aid in assessing overall role to national/global methane scenario. However, using IPCC coefficients, it is estimated to be 50 Gg (Gg=10⁹).

The present research paper provides the results of field experimentations on CH₄ emission from Kashmir Himalayan valley rice field soils amended with various inorganic and organic fertilizers to augment research base-data from little-explored temperate regions.

Materials and methods

Field set-up and agronomic practices
Field experiments were conducted during Kharif season in the year 2004 at the Research Farm of S-K University of Agricultural Sciences and Technology of Kashmir (SKUAST-K), Shalimar, to study the impact of different nitrogen (N) sources (organic and inorganic) on CH₄ emissions by rice (Oryza sativa) cultivation. In Kashmir, paddy is grown under submerged conditions with water depth of 5-10 cm. The recommended amount of organic fertilizer FYM being 10 tonnes ha^-1; inorganic fertilizer (ha^-1) with respect to different sources being N (80 kg), P₂O₅ (60 kg), K₂O (30 kg), ZnSO₄ (10-15 kg). Seedlings of paddy (SKAU-23) were transplanted on June 17, 2004. Treatments were applied through different sources viz-a-viz control, urea (100% N), Ammonium nitrate (100% N) [100% N refers to application of total recommended N in the form of respective material i.e., Urea or Ammonium nitrate (eg., 100% N from Urea means application of 174 kg urea, Urea has 46% N], Bromethane sulphonic acid (BMSA), Methyl chloride, FYM (100%), wheat straw+cellulolytes, vermicompost (100% N), aquatic weeds, and aquatic weed+FYM. Each treatment was replicated in randomized block design (RBD). The FYM, wheat straw, aquatic weeds, vermicompost and different blends (referring to mixtures of inputs used in the study), as per treatments, were incorporated one week prior to transplantation. Ammonium nitrate, BMSA and Methyl chloride were applied one day after transplanting. A basal dose of Nitrogen (50% of urea) was applied along with 60 kg P ha^-1 and 40 kg K ha^-1 the rest of the Nitrogen was applied in two–split doses i.e., 3 weeks and 6 weeks post transplantation. Basal dose of fertilizers refers to recommended P₂O₅, K₂O and 50% of recommended N ( i.e., 40 kg N); tillering stage (15-18 days after transplanting)=25%N; panicle initiation stage (38-42 days after transplanting)=25%N. The details of treatments and agronomic practices are given in Table 1.

Soil analysis
The soil samples of the experimental sites were analyzed by standard methods of Analysis [18]. The main physical and chemical characteristics of the soil are given in Table 2.

Methane emissions and flux measurements
After transplanting 25-days old seedlings in the rice field the determination of methane emissions was done 100 DAT (days after transplanting). Methane was determined after 100 days after transplanting because before this it was not produced in detectable quantities. Gas samples were collected following closed chamber technique [19]. Gas samples were collected once a day between 11:00 and 11:30 hr at regular intervals of 3-4 days using closed chambers. The chambers were installed at the sampling sites and gas samples were collected at 10 minute interval from chambers involving bottomless plexi chambers (100 x 50 x 30 cm) fitted on a permanently installed base unit (open bottom) and removable top. During gas collection, flood water was used to seal the top of the base unit. A rubber septum was fitted in to chamber through which gas was collected. Water level was never allowed to recede below the base unit. Each chamber was fitted with a thermometer to record the ambient temperature. Soil temperature and redox measurements were done at the time of each gas collection. Gas collection was made through rubber septum with a gastight syringe containing stainless steel hypodermic needles. Collected gas samples were immediately transferred to pre-evacuated vacutainers by hypodermic needles upon gas collection. The samples were analyzed for CH₄ by Gas Chromatograph(Shimadzu 14 BPSTF) fitted with flame ionization detector (FID). The unknown gas samples (0.1 ml) were injected in to stainless steel column Porpaq Q.

The injector and detector temperatures were maintained at 100 and 120°C respectively. Standard CH₄ gas was used for comparison. CH₄ gas appeared after retention time of 1.1 minute. The chromatograms were processed by Omega software. The methane flux (F) was calculated using the equation: F=(V/A) (ΔC/Δt) whereas F=CH₄ gas flux, V=volume of head space chamber, A=Soil surface under chamber and ΔC/Δt is change in concentration per unit of time.

Results and discussion

Physico-chemical characteristics of soil
The soil properties of the experimental sites indicate silty clay loam texture having bulk density of 1.25 g cm^-3 and non-acidic (pH=6.8) character. The fertility status of the soil is medium in relation to available nitrogen, phosphorus and potassium. The physical properties of soil are good enough to support the growth of rice crop (Table 2).

Seasonal methane flux
The methane fluxes from the experimental sites were highly variable. The seasonal changes in daily CH₄ emissions as influenced by urea, Ammonium nitrate, BMSA, Methyl chloride, FYM, wheat straw+urea, wheat straw+cellulolytes, vermicompost, aquatic weeds and aquatic weeds+FYM related to growth stages of rice crop are graphically given in Figure 1. The observations revealed that in most of the treatments, CH₄ emissions were at their lowest and beyond detectable levels within first three days after transplantation. However, treatments of urea, FYM, wheat straw+urea, wheat straw+cellulolytes, vermicompost, aquatic weeds, and aquatic weeds+FYM caused CH₄ emission from 0.19 to 0.47 kg ha^-1 d^-1. On 9^th day, CH₄ flux fluctuated between 0.08-0.48 kg ha^-1 d^-1 with peak recorded in relation to aquatic weed+FYM. Thereafter, steady decline in emissions were observed over next 20 days. The second maximum peak (SM) was recorded on 45 DAT touching 0.83 kg ha^-1 d^-1 for FYM and 0.77 kg ha^-1 d^-1 for aquatic weed+FYM. However, in relation to urea, first maximum 0.83 kg ha^-1 d^-1 occurred on 45 DAT while that of vermicompost (0.62 kg ha^-1 d^-1) was on 35 DAT. Methane fluxes registered declining trend over the next 3 weeks. Methane emissions increased substantially during following weeks and reached third maxima on 65 DAT for FYM treatment (0.90 kg ha^-1 d^-1) and vermicompost (0.96 kg ha^-1 d^-1). A mean of 1.13 kg ha^-1 d^-1 was recorded for urea treated site on 70 DAT. Marked decline in CH₄ emission occurred after 70 days, and practically no methane could be recorded on 100 DAT. Studies conducted [20] on IARI rice fields during kharif (rainy season) reported CH₄ emissions at their lowest value during early submerged and before harvesting period. The authors observed first maximum peak (FM) at 15 DAT, second (SM) at 46 DAT, and third one (TM) at 69 DAT. Higher CH₄ emission levels in FYMtreatment site observed during the present study seem consistent with results [20] which is attributable to more readily decomposable organic matter as compared to other treatments. The production of increased CH₄ from organic matter treatment in comparison to the plots containing no exogenic organic matter is well documented [21]. The noticeable increase in emissions recorded after 3-4 days continued over the next 15 days reaching a first maximum peak (FM) at 15 DAT in relation to FYM (0.58 kg ha^-1 d^-1) and aquatic weed+FYM (0.61 kg ha^-1 d^-1). In relation to Ammonium nitrate, BMSA and Methyl chloride treatments, depressed CH₄ fluxes were recorded in the range of 0.19-0.29, 0.04-0.14, and 0.10-0.27 kg ha^-1 d^-1 respectively. CH₄ emission from treated site was higher than the control. The results obtained during the present study indicate that the findings are in agreement with observations [22] that the fertilized plots emit the higher levels of methane. Recent report [5] indicate that methane emissions in rice fields can be as much as 90% higher in continuously flooded rice fields compared to other water management practices, independent from straw addition.

Diel cycle of methane flux
Data on diel variations of CH₄ flux recorded at different intervals for 24 hr duration are set in Figure 2. First observation for overnight emission was done at 07 hr. Collection during the day were made at regular intervals of four hour (11, 15, 19 hrs). The results clearly show that higher emission rates were recorded in the afternoon. Published report [23] shows high methane emissions rates during 12-15 hr. The diel-cycle sequence of CH₄ flux was in the order of 15 hr>11hr>19>07hr. The diel emissions in relation to various treatments depicted marked fluctuations. The qualitative trend, though similar displayed significant quantitative variability with FYM, vermicompost, and urea treatments showing higher CH₄ emissions as compared to other treatments during afternoon period. The wheat straw+urea, and wheat straw+cellulolytes treatment had almost similar diel pattern of methane flux.

Total methane flux
Total estimated CH₄ emission obtained by integrating the seasonal variations is representative of annual values as the rice fields, after crop harvest are not flooded until next spring season. Total emission in the control ranged from a minimum of 0.08 to a maximum of 0.81 kg ha^-1 and seasonal emission was estimated to be 39.32 kg ha^-1. Higher values of 65.78 kg ha^-1 season^-1 was obtained for aquatic weed+FYM followed by 64.24 for FYM-treated soil. Vermicompost treatment gave a value of 60.96 kg ha^-1 season^-1 (Figure 3). Markedly low values of 4.85, 9.98 and 13.67 kg ha^-1 season^-1 were recorded for 2BMSA, Methyl chloride and Ammonium nitrate treated soils respectively. Except for these three, in all other treatments, the seasonal emissions were higher than the control depending on the type of fertilizer amendments. The results reveal that the impact on CH₄ emission is influenced by type of fertilizer. The inhibition of methanogenesis due to nitrate-addition may be due to toxic effects or the presence of intermediate nitrites. Inhibition effect of sulphate could be due to high redox conditions as well as competition amongst diverse microorganisms for methanogenic substrate. Field experiment conducted [24] at research farm of the IARI, New Delhi, during wet season (July-October) with various rice cultivars reported maximum seasonal emission (27.2 kg ha^-1) for 'PUSA 933' and minimum (15.6 kg ha^-1) for 'PUSA 169'.

The present study shows that methane emission was higher in treatments (aquatic weed+FYM), FYM, Urea, and vermicompost indicating readily decomposable organic materials stimulate the emission of methane in rice-fields. In order to meet the demand of an increasing population, global rice production has been estimated to double by the year 2020, which may increase CH₄ production by up to 50% [13] . Such a scenario calls for improved rice cultivation techniques and manure management to retard methane emissions as an amelioration strategy.

Conclusion

The present study concludes that methane emissions from natural wetlands including rice fields are deeply implicated in global warming and their experimental quantification is a research topic of high priority today. Rice in some circles is condemned as the major culprit for the global warming, climate change and environmental degradation. The application of biodigested manures over the fresh organic manures like farmyard manure, green manure etc constitute one of the amelioration mechanisms to retard methane emission in rice cultivation. As such rice (and livestock) deserves adequate attention to target CH₄ emission reduction. Recent report [25] shows highest (>70%) methane emission contribution from Indian livestock as compared to various other subsectors from agriculture. Deficient data on this greenhouse gas, especially from temperate regions, limits the interpretation of fall-out of climate change. There is a need to strike a balance between food security through rice protection and sustainability of resource-base and global climate.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

The authors contributed actively in implementation, data generation and interpretation of results.

Acknowledgement

Support by research grant from Indian Council of Agricultural Research (ICAR), New Delhi, India is gratefully acknowledged. SKUAST-K provided working facilities.

Publication history

Editor: Naresh Singhal, University of Auckland, New Zealand.
Received: 13-Jul-2013 Revised: 12-Aug-2013
Accepted: 19-Aug-2013 Published: 02-Sep-2013