Effect of organic fertilizers on CH4 and N2O production from organic paddy field

Organic fertilizer potentially results in healthy soils and sustainable agriculture. Organic fertilizer is a nutrients source in organic farming. Organic fertilizer application affects CH4 and N2O production. CH4 and N2O production affect increasing greenhouse gas emission concentration, global warming and further climate change. This study aimed to determine organic fertilizers effect on CH4 and N2O production from organic paddy fields. Five types of organic fertilizer namely mixed compost (MC), cow manure (CM), cow manure compost (CMC), straw compost (SC) and solid biogas waste (SBW) were added to paddy soil and determined CH4 and N2O production. The result showed that CH4 and N2O production were affected by organic fertilizer addition. CH4 production was 109.52 mg kg soil-1 day-1 and N2O production was 1.70 mg kg soil-1 day-1. The lowest production of both gasses was in SBW treatment. CH4 production was 177.21 mg kg soil-1 day-1 and N2O production was 2.80 mg kg soil-1 day-1. The highest was in soil with cow manure (CM) treatment. CH4 and N2O production correlated positively with organic carbon and Carbon and Nitrogen ratio (C/N) of organic fertilizers (p < 0.05). Application of solid biogas waste (SBW) with drainage treatment on 10th day and composting are the best effort to mitigate CH4 and N2O production.

Organic matter increases soil productivity and health [13]. However, organic matter may be a substrate for methanogens in CH 4 production [14]. Organic matter stimulates respiration and growth of microorganisms and provides carbon for denitrification [15]. The growth of microorganisms increases oxygen consumption and anaerobic conditions for denitrification [16], which produces N 2 O [17] [18]. Organic paddy field uses organic materials in the production process [19]. Food and Agriculture Organization (FAO) introduced organic agriculture to reduce the contribution of GHGs in agricultural sector [20] and climate change adaptation [21]. Organic fertilizers affect the production of CH 4 and N 2 O. Organic fertilizers are available under various conditions in fertilizer materials, manufacturing processes and fertilizer quality. This study aimed to determine the effect of organic fertilizers on CH 4 and N 2 O production in organic paddy field.  [22] and has characteristics of loamy texture, pH of 6.26, organic carbon of 2.27%, particle density of 1.72 g cm -3 , bulk density of 1.16 g cm -3 , soil permeability of 4.31 cm hour -1 [23] and total nitrogen of 0.11%. Samples of topsoil were taken to determine CH 4 and N 2 O production.

Organic fertilizers
The research used 5 types of organic fertilizers, namely mixed compost (or MC = 1.20% N), cow manure (or CM = 1.21% N), cow manure compost (or CMC = 1.28% N), straw compost (or SC = 1.31% N) and solid biogas waste (or SBW = 1.36% N). Organic paddy soil was added with organic fertilizer win the same nitrogen dose. The difference of C/N value results in a different amount of organic carbon input. The rate of organic fertilizers was determined based on the nitrogen content in the soil and organic fertilizers. The nitrogen rate refers to Sanchez [24] for lowland rice, filled with soil nitrogen and organic fertilizers. In this study, all nitrogen was added from organic fertilizers.

Production of CH 4 and N 2 O
Production of CH 4 and N 2 O was determined under flooding conditions in the laboratory. According to nitrogen content, twenty grams of soil and organic fertilizer were dissolved in 40 ml of distilled water, then incubated. Gas samples were taken with 5 ml syringe for laboratory analysis. The production of CH 4 and N 2 O were measured by gas chromatography, CH 4 using Flame Ionization Detector (FID) and N 2 O using Electron Capture Detector (ECD), and calculated with the formula as follow [25]:

Statistical Analysis.
Correlation analysis was used to determine the relationship between CH 4 and N 2 O production and organic fertilizers' properties [26].

Production of CH 4 and N 2 O.
The pattern of CH 4 production was the same (Figure 1), as in Mujiyo et al [27]. The maximum production occurred on 15 th day. The pattern of CH 4 production in soil plus CM was slightly different, still high on 22 th day. Organic carbon input was of CM (higher C/N) so that CH 4 production was still high on 22 th day. Composting increases carbon loss as CO 2 [28][29] so CM (without composting) has higher carbon content that leads to higher CH 4 production. The additions of compost to soil enhance the quality and quantity of soil organic matter [30], also as source of nutrients [31]. Production of N 2 O showed a similar pattern (Figure 1). N 2 O production was low on day 1, increased and reached a maximum on 15 th day, then decreased. Production of CH 4 and N 2 O reached a maximum on 15 th day. CH 4 and N 2 O production did not reach the maximum as a mitigation option. The 10 th day of drainage will reduce CH 4 production because before 15 th day, CO 2 and CH 4 mainly lose carbon through organic matter mineralization, but we must pay attention to N 2 O production. Nitrogen is lost through NH 3 volatilization and N 2 O emission from nitrification and denitrification [32].  4 production in soil with CM treatment was 177.21 mg CH 4 kg soil -1 day -1 . CH 4 production in soil with MC treatment was 21.33% lower than soil plus CM. Meanwhile, soil with CMC treatment reduced CH 4 production by 25.82%, soil with SC treatment by 32.45% and soil with SBW treatment 38.20% compared to soil with CM treatment (Figure 2). CH 4 production in the soil without organic fertilizer of 79.53 mg CH 4 kg soil -1 day -1 [33] was lower than plus organic fertilizers. N 2 O production on CM treatment was 2.80 mg N 2 O kg soil -1 day -1 . The addition of SBW reduced N 2 O production up to 39.24% compared to soil with CM treatment. Meanwhile, soil with SC treatment reduced N 2 O production by 28.48%, CMC 24.18% and MC 12.58% compared to soil with CM treatment (Figure 2). Referring to the results obtained by Anshori et al [33], organic fertilizers increased N 2 O production. The addition of SBW to organic paddy soil gave the lowest CH 4 and N 2 O production. Amendments with fine organic matter, like SBW, negligibly increase methane production and emission. Application of composted material, which has a higher degree of humification, only slightly increases methane formation and fluxes [34]. We proved that SBW is the best alternative for mitigation of CH 4 and N 2 O production.

The correlation of CH 4 and N 2 O production with organic fertilizer properties.
The difference in CH 4 and N 2 O production occurred due to differences in organic fertilizers' properties, such as carbon (C) and C/N fertilizers. There was positive correlation between C, C/N, phosphorus (P) and potassium (K) organic fertilizers with CH 4 and N 2 O production. The negative correlations occurred in sulfur (S), iron (Fe), manganese (Mn) and copper (Cu) with CH 4 and N 2 O production (  [37]. Composting reduces fertilizer C/N. Fertilizer C/N has a positive correlation with CH 4 production ( Table 1). Methanogenic bacteria are thought to be more active at high fertilizers C/N. According to Kim et al [38] composting can reduce CH 4 emissions. P and K of organic fertilizers positively correlated with CH 4 production, while negatively correlated to S, Fe and Mn. Wihardjaka et al [39] also found a positive relationship between soil P 2 O 5 and K 2 O with CH 4 production and negative to SO 4 , Fe and Mn. These soil properties parameters affect the reduction-oxidation and soil pH [40]. Organic fertilizers application with high C/N tends to increase the production of CH 4 . Efforts to mitigate CH 4 emissions can be done by reducing C/N of organic fertilizers through composting. Soil organic carbon has a positive correlation with N 2 O production because it is denitrification substrate [41]. Nitrification-denitrification is a source of N 2 O emission [42]. Organic carbon increases the nitrification-denitrification, resulting in higher N 2 O production. However, C/N of organic fertilizer has a positive correlation with N 2 O production. C/N ratio is the main indicator of composting stability and final product maturity [43]. Organic materials which high C/N contain high organic carbon, nitrification-denitrification is more active, so that the production of N 2 O is higher. According to Addo et al [44], organic carbon is a source of energy for denitrification, positively correlating with N 2 O  [45]. N 2 O production is positively correlated with P and K of organic fertilizer and negatively with S, Fe and Mn. Ions play a role in the production of N 2 O through the nitrificationdenitrification process [46] [47][48] [49]. Application high C/N of organic fertilizers tends to increase the production of N 2 O. Similar to CH 4 mitigation above, mitigation of N 2 O emissions can be done also by composting to reduce C/N of organic fertilizers.

Conclusion
Organic fertilizers application increased CH 4 and N 2 O production in organic paddy field. CH 4 and N 2 O production with organic fertilizers application and flooded condition reached a maximum on the 15 th day, and the lowest was on soil with soil biogas waste (SBW) treatment. SBW application and treatment drainage on 10 th day was the best treatment to mitigate CH 4 and N 2 O production in this study. Positive correlation between CH 4 and N 2 O production from soil treated with organic carbon and C/N of organic fertilizers also provides an alternative to mitigate CH 4 and N 2 O through composting.