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Global warming as affected by incorporation of variably aged biomass of hairy vetch for rice cultivation.

Introduction

The monocrop culture in temperate countries such as Korea and Japan is mainly rice-based, and cover crops are mostly grown in winter as animal feed or as a green manure. In general, Chinese milkvetch (Astragalus sinicus) or hairy vetch (Vicia villosa), as leguminous cover crops, and rye (Secale cerealis) or barley (Hordeum vulgare) as non-leguminous cover crops, are cultivated in paddy soils (Kim et al. 2007; Zhang et al. 2007). The incorporation of cover crops as green manure improves soil organic matter content and minimises the chemical fertiliser requirement (Elfstrand et al. 2007). This practice also improves rice growth and yield along with increasing soil carbon (C) content and microbial activities. Nonetheless, green manuring is responsible for nutrient enrichment and greenhouse gases (GHGs) emissions (Pramanik and Kim 2012; Thangarajan et al. 2013; Kim et al. 2013).

The incorporation of cover crop biomass into paddy soil enhances microbial processes, which in turn increase methane (C[H.sub.4]), carbon dioxide (C[O.sub.2]), and nitrous oxide ([N.sub.2]0) emissions. Among GHGs, C[H.sub.4] and [N.sub.2]0 have 25 and 298 times more global warming potential (GWP) than C[O.sub.2] (IPCC, 2007). Generally, [N.sub.2]0 emission is affected by the amount of available N in the soil (Kim et al. 2013), indicating that incorporation of high N-containing biomass facilitates [N.sub.2]O emission from paddy soils (Kim et al. 2013) depending on its decomposition rate. However, soil C sequestration can play a vital role in minimising GHGs emission.

The sequestered soil C should be considered for evaluating the impact of GHG emissions from paddy soils. It could be determined by soil organic C changes over the long term (Pan et al. 2004; Shang et al. 2011), although this method is not sensitive enough to detect seasonal or annual changes (Zheng et al. 2008). However, net ecosystem C budget (NECB) can provide a scientific basis for determination of soil C sequestration (Chapin et al. 2006; Smith et al. 2010). The daily net ecosystem C[O.sub.2] exchange can be evaluated by a chamber-based technique developed by Burkart et al. (2007) and Zheng et al. (2008); the amount measured depends on microbial decomposition rates.

The microbial decomposition of green manure is affected by the C : nitrogen (N) ratio and forms of C (cellulose and lignin content) in plant tissue (Vigil and Kissel 1991). Because lignin and cellulose are more recalcitrant forms of C in plant tissue than arc soluble C compounds (sugars, organic acids, proteins), a high content of lignin and cellulose slows down decomposition process (Gunnarsson and Marstorp 2002). Because hairy vetch is generally allowed to grow for ~7 months before incorporation, its cellulose and lignin contents, and hence decomposition rates followed by GHG emission, might vary depending on its growth duration. However, previous studies showed only the influence of cover crop biomass incorporation on GHGs emissions (Haquc et al. 2013, 20156; Kim et al. 2012, 2013). Therefore, the impacts of growth duration of hairy vetch on total GWP needs to be evaluated by using C[H.sub.4] and [N.sub.2]0 fluxes and soil C changes in a paddy field.

Materials and methods

Experimental site, hairy vetch harvesting, and rice cultivation

The field experiment was conducted at the agricultural farm of Gyeongsang National University (36[degrees]50'N, 128[degrees]26'E), Jinju, South Korea. The soil was silt loam in texture and classified as typic Haplaquents (Pramanik et al. 2013), with somewhat impeded drainage. Initial soil properties were: pH (1 : 5 in [H.sub.2]O) 6.2, organic matter 20.4g [kg.sup.-1], available [P.sub.2][O.sub.5] 79mg [kg.sup.-1] and bulk density 1.39 g [cm.sup.-3]. Recommended seeding rate (90 kg [ha.sup.-1]) of hairy vetch was used (Jeon et al. 2011; Haque et al. 2013, 20156). Seeds were spread on the field on 1 November 2011. Aboveground parts were harvested at the 183-, 190-, 197-and 204-day-old stage and incorporated into soil at 3 Mg [ha.sup.-1] before rice was transplanted. The soil was flooded immediately after incorporation of the hairy vetch biomass.

Thirty-day-old seedlings of the Dongjinbyeo rice cultivar (Japonica type) were transplanted at three per hill. They were placed at 15 cm by 30 cm spacing on 6 June 2012. The recommended dose of chemical fertilisers (N, [P.sub.2][O.sub.5], [K.sub.2]O at 110, 45, 58 kg [ha.sup.-1]) was used. Basal fertiliser (N, [P.sub.2][O.sub.5], [K.sub.2]O at 55, 45, 40.6 kg [ha.sup.-1]) was applied just before transplanting, 22 kg N [ha.sup.-1] at active tillering stage and 33 kg N [ha.sup.-1] and 17.4 kg [K.sub.2]O [ha.sub.-1] at one week before panicle initiation stage. The field water depth was maintained 5-7 cm above the soil surface throughout the crop-growing season. Rice was harvested on 15 October 2012, and total grain and straw yields were recorded after air-drying. Rice growth and yield characteristics were investigated at the maturity stage.

Characterisation of hairy vetch

Hairy vetch was harvested at the 204-, 197-, 190-, and 183-day-old stages and biomass accumulations were measured after oven drying at 70[degrees]C for 72 h. Total C and N were estimated by using a CHNS analyser (LECO Corp., St. Joseph, Ml, USA). Cellulose content was determined following a colourimetric method with an anthronc reagent at 620 nm (Updegraff 1969), and lignin content was determined using the APPITA P11s-78 method (Appita 1978).

C[H.sub.4], [N.sub.2]O, and C[O.sub.2] gas sampling and analysis

Transparent glass chambers (62 cm by 62 cm by 112 cm) were placed permanently in the flooded soil after rice transplanting to monitor C[H.sub.4] and [N.sub.2]O emission rates. Eight rice plants were enclosed in a chamber. There were four holes in the bottom of each chamber to maintain a water depth 5-7 cm above the soil surface. Closed chambers made out of acrylic columns (each 20 cm by 20 cm) were placed at a soil depth of 20 cm between rice plants for measuring C[O.sup.2] emission rates (Lou et al. 2004; Xiao et al. 2005; Iqbal et al. 2008; Haque et al. 2015a, 2015b). All chambers were kept open in the field throughout the rice-cultivation period, except during gas sampling. Each chamber was equipped with a circulating fan for gas mixing and a thermometer to monitor temperature during sampling time. Air gas samples were collected at 0 and 30 min after closing a chamber by using 50-mL gas-tight syringes. Gas samplings were carried out three times each day (at 08:00, 12:00 and 16:00) to get average C[H.sub.4], [N.sub.2]O, and C[O.sub.2] emission rates. Collected gas samples were immediately transferred to 30-mL air-evacuated glass vials sealed with a butyl rubber septum for analysis.

Gas concentrations in the collected air samples were measured by gas chromatography (GC-2010; Shimadzu, Tokyo) with a Porapak NQ column (Q 80-100 mesh). A flame ionisation detector, thermal conductivity detector, and [sup.63]Ni electron capture detector were used for quantifying C[H.sub.4], C[O.sub.2], and [N.sub.2]O concentrations, respectively. The temperatures of the column, injector, and detector were adjusted to 100[degrees]C, 200[degrees]C, and 200[degrees]C for C[H.sub.4], 45, 75, and 270[degrees]C for C[O.sub.2], and 70, 80, and 320[degrees]C for [N.sub.2]O, respectively. Helium and [H.sub.2] gases were used as the carrier and burning gases, respectively.

Estimation of C[H.sub.4], C[O.sub.2], and [N.sub.2]O emissions

Methane, C[O.sub.2], and [N.sub.2]O emission rates were calculated from the increase in C[H.sub.4], C[O.sub.2], and [N.sub.2]0 concentrations per unit surface area of the chamber for a specific time interval. A closed chamber equation was used to estimate C[H.sub.4], C[O.sub.2], and [N.sub.2]O fluxes from each treatment (Haque et al. 2013; Pramanik et al. 2013).

F = [rho] x (V/A) x ([DELTA]c/[DELTA]t) x (273/T)

where F is flux of C[H.sub.4] and C[O.sub.2] (mg [m.sup.-2] [h.sup.-1]) and [N.sub.2]o ([micro]g [N.sub.2]O [m.sup.-2] [h.sup.-1]); [rho] is the gas density of C[H.sub.4], C[O.sub.2], and [N.sub.2]O under a standardised state (mg [cm.sup.-3]); V is the volume of the chamber ([m.sup.3]); A is the surface area of the chamber ([m.sup.2]); [DELTA]c/[DELTA]t is the rate of increase of C[H.sup.4], C[O.sup.2], and [N.sub.2]O gas concentrations in the chamber (mg [m.sup.-3] [h.sup.-1]); and T is temperature (273 + mean temperature, [degrees]C) of the chamber.

The seasonal C[H.sub.4], C[O.sup.2], and [N.sub.2]O fluxes during entire rice cultivation period were computed according to Singh et al. (1999) as:

Seasonal C[H.sub.4], C[O.sup.2], and [N.sub.2]O flux = [[summation].sup.n.sub.i] ([R.sub.i] x [D.sub.i])

where [R.sub.i] is the rate of C[H.sub.4], C[O.sup.2], and [N.sub.2]O flux (g [m.sup.-2] [day.sup.-1]) in the ith sampling interval, [D.sub.i] is the number of days in the ith sampling interval, and n is the number of sampling intervals.

Estimation of net ecosystem carbon budget (NECB)

Carbon dioxide emission budget and soil organic carbon (SOC) changes were estimated using NECB. We summarised the findings of Ciais et al. (2010), Smith et al. (2010), Jia et al. (2012), Ma et al. (2013), and Haque et al. (2015a) for determination of NECB as follows:

NECB = GPP - [R.sub.e] - harvest - C[H.sub.4] + green manure (1)

GPP = NPP + [R.sub.a] (2)

[R.sub.e] = [R.sub.a] + [R.sub.h] (3)

where GPP, NPP, [R.sub.e], [R.sub.a], and [R.sub.h] represent gross primary production, net primary production, ecosystem respiration, autotrophic respiration, and heterotrophic respiration, respectively. Harvest includes rice straw and grains, and green manure C inputs were calculated from incorporated, variably aged biomass (3 Mg [ha.sup.-1]).

Equation 1 can be converted to Eqn 4 by using Eqns 2 and 3 as:

NECB = NPP - [R.sub.h] - harvest - C[H.sub.4] + green manure (4)

The NPP was estimated according to Smith et al. (2010).

Net global warming potential (GWP)

The net GWP of the cropland ecosystem was estimated according to Ma et al. (2013) and Haque et al. (2015a).

Soil sampling and analyses

Soil redox potential (Eh) and temperature were measured from each plot during gas sampling by using an Eh meter (PRN-41; DKK-TOA Corp., Tokyo) and a portable thermometer, respectively, each placed in the soil at 3-5 cm depth. Analysis of other soil chemical properties was performed after rice harvest on 15 October 2012. Soil samples were collected at 0-15 cm depth from five points, air-dried, and sieved (<2 mm). Soil pH (1:5 in [H.sub.2]O), available phosphate (Rural Development Administration Korea 1988), dissolved organic C (DOC), dissolved organic N (DON) (Lu et al. 2011), and total C and N concentrations (CHNS-932 analyzer; LECO Corp.) were determined. A core sampler (10 cm length, inner diameter 5 cm) was used for measuring bulk density by drying the soil at 105[degrees]C for 24h (Blake and Hartge 1986).

Statistical analyses

Analysis of variance (ANOVA) was performed by using the GLM procedure to determine the effects of different treatments. A one-way ANOVA was carried out to compare the means of the different treatments by using Tukey's honestly significant difference test; P values, t-statistics, and 95% confidence intervals were used to compare observed values.

Results

Characterisation of hairy vetch

The amount of aboveground biomass (6.5 Mg [ha.sup.-1]) of hairy vetch significantly (P [less than or equal to] 0.05) increased after 204 days of growth (Table 1). Biomass accumulation was less in 183-day-old plants than in 204-day-old plants. The C, N, cellulose and lignin contents increased with age of the plants. Cellulose contents significantly increased with age of plants and were highest in the 204-day-old plants. Lignin contents significantly varied between 183-and 204-day-old plants. The lowest and highest C : N ratios of 9.62 and 15.58 were observed with 197- and 183-day-old plants, respectively (Table 1).

Changes in soil chemical properties

Soil Eh values

Although soil was flooded immediately after vetch biomass incorporation, little variation due to treatments was observed in Eh values. Soil Eh values sharply decreased below -100 mV after 7 days of rice transplanting (Fig. 1). Irrespective of treatments, Eh was reduced greatly (to around -200 mV) throughout the rice-growing period and it increased slightly at harvest time.

Soil properties

The DOC content in the soil gradually decreased from 369 to 298 mg [kg.sup.-1] depending on the age of the incorporated biomass, and it was lowest in the 204-day-old plants (Table 2). A similar trend was observed with DON content. DOC and DON contents in soil varied significantly among treatments after transplanting (Fig. 2). DOC was highest at 30 days after transplanting (DAT), when 190-day-old hairy vetch was incorporated at 3 weeks before transplanting. However, peak DON levels were observed at 65 DAT irrespective of treatments, although the greatest amount was recorded in 183-day-old hairy vetch that was incorporated 4 weeks before transplanting. Soil at rice harvest had higher total C and N and available [P.sub.2][O.sub.5], lower DOC and DON contents, and the lowest bulk density in the plots with 204-day-old incorporated hairy vetch (Table 2). However, there was no significant difference in DON due to incorporation of 190-204-day-old hairy vetch during rice cultivation.

Rice grain yield and growth characters

The lowest grain and straw yields were recorded when 183-day-old hairy vetch was incorporated in the soil, even though tiller production was similar irrespective of treatments (Table 3). The highest grain yield was obtained with incorporation of 197-day old vetch (6.85 Mg [ha.sup.-1]). The highest straw yield and 1000-grain weight were found with incorporation 197-day-old vetch.

Greenhouse gas emissions

C[H.sub.4] fluxes from soil

The C[H.sub.4] emission fluxes increased gradually up to 21 DAT and decreased thereafter. Moreover, pre-transplanting C[H.sub.4] emission rate was much lower than post-transplanting rate, irrespective of treatments (Fig. 3). However, C[H.sub.4] emission rate was the lowest (491.6 mg [m.sup.-2] [day.sup.-1]) with incorporation of 204-day-old hairy vetch (Tabic 3). The cumulative C[H.sub.4] emission from the plot with 197-day-old-vetch incorporated was 701 [+ or -] 6.23 kg [ha.sup.-1], which was 9%, 11%, and 19% higher than plots with 183-, 190-, and 204-day-old-vetch incorporated, respectively (Table 3).

C[O.sub.2] fluxes from soil

The C[O.sup.2] emission fluxes increased gradually up to 60 DAT and decreased thereafter in all treatments (Fig. 3). By contrast, C[O.sup.2] emission rates and seasonal fluxes were highest in the plots with 204-day-old-vetch incorporated (Table 3). However, pre-transplanting C[O.sup.2] emission was much lower than post-transplanting emission, irrespective of treatments (Fig. 3).

[N.sub.2]O fluxes from soil

The [N.sub.2]O emission flux was similar among the plots with variably aged vetch biomass incorporated (Fig. 3). Irrespective of treatments, the highest [N.sub.2]O emission fluxes were observed ~60-65 DAT and there were no significant differences in emission rates (Table 3).

Net ecosystem carbon budget (NECB)

Rice cultivation significantly increased the NPP and TOC input under similar amount of biomass (3 Mg [ha.sup.-1]) incorporated soil. Rice biomass production contributed ~83-84% of TOC input, with 16-17% contributed by fertiliser and vetch biomass (Table 4). Among the treatments, TOC output was ~6.06-6.82 Mg C [ha.sup.-1]. Rice harvest contributed ~83-84% removal of TOC, and ~16-17% of the TOC removal was due to mineralised C loss. Interestingly, C[O.sup.2]-C loss was 1.5-2.0 times greater than C[H.sub.4]-C loss under flooded soil conditions. As a result, NECB was 1301-1364 kg C [ha.sup.-1] during the rice-growing season.

Measurement of global warming potentials

Total GWP value was highest (18.0 Mg [ha.sup.-1]) in the plots with 197-day-old hairy vetch incorporated and least (15.9 Mg [ha.sup.-1]) in the plots with 204-day-old hairy vetch incorporated (Table 5). Seasonal C[H.sub.4] and C[O.sup.2] flux during rice cultivation season contributed ~81% and 18% of GWP, respectively, and the seasonal [N.sub.2]O flux contributed ~1%.

Discussion

Different GHG emission patterns were observed with incorporation of variably aged cover crop biomass for rice cultivation. Moreover, flooding of the paddy field immediately after transplanting resulted in reduction of Eh, which was catalysed by the available source of C from the added green biomass, and the C increased C[H.sub.4] emission rates (Figs 1, 3). Methane gas is emitted from paddy soil under anaerobic conditions mediated by methanogens (Pramanik and Kim 2013). This group of anaerobes requires low redox potentials, uses a limited range of C[O.sup.2], [H.sub.2], and acetate, and relies on a common set of enzymes (Schimel and Gulledge 1998; Thauer et al. 2008). In our study, higher C[H.sub.4] emissions were observed at 21 DAT irrespective of treatments. These results indicated that higher levels of C[H.sub.4] could be emitted by direct diffusion-ebullition pathways at the initial rice-growing stage (Kruger et al. 2002). A similar pattern of C[H.sub.4] emission was reported by Kim et al. (2013).

The C source in soil mainly depends on organic materials of varying C : N ratios. The C : N ratio of added biomass has been shown to affect decomposition rates when N is limiting in the soil. Lower C : N ratios indicate higher N in plant tissue, resulting in less N limitation to microbial activity and, therefore, faster decomposition (Vigil and Kissel 1991). In our study, 197-day-old hairy vetch had a slightly lower C : N ratio (Table 2), which might have favoured a faster decomposition than other treatments and generated higher DOC as a labile C source for C[H.sub.4] production (Fig. 3). Although the C : N ratio in 204-day-old vetch biomass was similar to that of 197-day-old biomass, CFI4 emission was low compared with other treatments. This might have been due to the greater content of recalcitrant cellulose and lignin (Gunnarsson and Marstorp 2002). Donnelly et al. (1990) reported that the high lignin content in plant tissue is a major factor controlling organic matter degradation rates in forest ecosystems.

Soil also acts as an important sink for C[H.sub.4] because of the presence of methanotrophs (Thauer et al. 2008). Methanotrophs use C[H.sub.4] as an electron acceptor and C[O.sup.2] is generated as a byproduct. The activity of these microbes is stimulated by N fertilisation (Bodelier and Laanbroek 2004). Therefore, higher C[O.sup.2] emission was observed in plots with 204-day-old-vetch incorporated than in other treatments, but the GWP value was not affected. However, these increased C[O.sup.2] fluxes measured from the closed chamber during the unit time scale used cannot represent net exchanges of C[O.sup.2] from soil, because of the contribution of plants and microorganisms to respiration. The net exchanges of C[O.sup.2] from soil can be due to soil C sequestration changes, which could be measured by SOC changes over a long time scale (Pan et al. 2004; Shang et al. 2011), but this method is not sensitive enough to detect seasonal changes (Zheng et al. 2008). The NECB can provide a scientific basis to determine the net exchanges of C[O.sup.2] (Chapin et al. 2006; Smith et al. 2010). However, negative and positive values of NECB were observed, because organic matter was either not added or added (Haque et al. 2015a). The NECB represents ecosystem C gain after harvest on a seasonal scale. In our investigation, cover-crop biomass incorporation sequestrated -1.30-1.36 Mg [ha.sup.-1] of C[O.sub.2]-C (NECB) in rice soil (Table 4).

Emission of [N.sub.2]O from flooded rice soils is usually not considered (Granli and Bockman 1994). In our study, [N.sub.2]O emission was low for all treatments, but plots with 183-day-old-vetch incorporated had somewhat higher [N.sub.2]O emission than other plots (Fig. 3). The cover-crop biomass incorporation might have contributed to [N.sub.2]O emission through both nitrification and denitrification processes under anaerobic conditions (Kim et al. 2013). Organic amendments provide energy for soil microbes, which eventually increases soil microbial biomass and denitrification rates because of decreased soil redox potential (Koster et al. 2011). There is a marked relationship between organic amendment, soil microbial properties, and [N.sub.2]O emission. However, [N.sub.2]O emission under anaerobic condition was negligible because of the fast conversion of [N.sub.2]O to [N.sub.2] (Granli and Bockman 1994). Comparatively high [N.sub.2]O emissions were observed at 60-65 DAT in all treatments (Fig. 3), probably because of N side-dressing and soil temperature increase (Kurganova and Lopes de Gerenyu 2010). The higher [N.sub.2]O emission rates in soil with 183-day-old hairy vetch incorporated (Fig. 3) was directly proportional to the increased DON content of soil (Fig. 2). The cumulative [N.sub.2]O emissions from paddy soils showed a highly positive correlation (r=0.91*) with DON contents. Incorporation of a leguminous cover crop creates a pool of readily available N and, therefore, stimulates [N.sub.2]O emissions (Kim et al. 2013), but available N content may vary with ageing of vetch during incorporation. Accordingly, we measured different [N.sub.2]O emissions patterns depending on age of the vetch plants incorporated (Table 3). Therefore, the age of incorporated leguminous cover crops such as hairy vetch is an important factor to consider when trying to minimise emission of GHGs (C[H.sub.4], C[O.sub.2] and [N.sub.2]O) during rice cultivation.

The highly recalcitrant cellulose and lignin content in vetch due to its delayed harvesting slows the decomposition rate and subsequently reduces C[H.sub.4] and [N.sub.2]O emissions from paddy soils (Saini et al. 1984; Donnelly et al. 1990; Gunnarsson and Marstorp 2002; Singh et al. 2010; Feng et al. 2012). A similar trend was observed in our study with C[H.sub.4] and [N.sub.2]O emissions. It is suggested that incorporation of aged hairy vetch biomass decreases both the methanogenesis and nitrification-denitrification processes under anaerobic conditions. The reduced C[H.sub.4] and [N.sub.2]O emission rates in the plots with 204-day-old hairy vetch compared with the plots with 190-and 183-day-old hairy vetch might be because of a slower decomposition rate. Cellulose is the most abundant carbohydrate present in plant residues and organic matter in nature, but its decomposition rate is slow when it is in association with lignin (Vigil and Kissel 1991; Johnson et al. 2006). It seems that high cellulose and lignin contents can be obtained with older plants, but it was not possible to go beyond a 204-day time span because rice crop has to be established by the first week of June. Net GWP changed because of variable missions of C[H.sub.4], [N.sub.2]O, and SOC (NECB) with the varying age of hairy vetch (Table 5). We included SOC in the GWP determination along with C[H.sub.4] and [N.sub.2]O to capture respiratory C[O.sub.2] from heterotrophs and autotrophs. Ma et al. (2013) also included SOC for determination of net GWP. The GWP value for C[H.sub.4], [N.sub.2]O, and C[O.sub.2] (NECB) was 18.0 Mg [ha.sup.-1] for 197-day-old vetch, which was significantly (P [less than or equal to] 0.05) higher (4.83-11.83%) than for 204-, 190-, and 183-day-old vetch. There was no significant grain yield reduction in rice through 204-day-old hairy vetch biomass incorporation. We saw that more cover crop biomass can be obtained from older plants, which are then incorporated into the soil, and this minimises GWP.

Conclusion

The cellulose and lignin contents increased with ageing of hairy vetch plants, and in turn were responsible for decreased DOC and DON contents in soil. The decreased C[H.sub.4] and [N.sub.2]O emission from paddy soil with 204-day-old vetch biomass incorporated indicated that age of cover crops is an important factor for determination of GWP during rice cultivation. We conclude that the use of 204-day-old hairy vetch, or any green biomass having a similar composition, at 3 Mg [ha.sup.-1] can be the best practice to enhance biomass productivity of rice and to reduce GWP in rice cultivation.

Acknowledgements

This study was carried out with the support of the 'Development of greenhouse gas emission factors and its verification for estimating the mitigation activity of agricultural greenhouse gas emissions (project no. 314081021SB010)', Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries, Republic of Korea.

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http://dx.doi.org/10.1071/SR15061

Md Mozammel Haque (A,C,D), Jatish Chandra Biswas (C), Tatoba R. Waghmode (A), and Pil Joo Kim (A,B,D)

(A) Division of Applied Life Science (BK 21 Plus), Gyeongsang National University, Jinju, 660-701, South Korea.

(B) Institute of Agriculture and Life Science, Gyeongsang National University, Jinju, 660-701, South Korea.

(C) Soil Science Division, Bangladesh Rice research Institute, Bangladesh.

Corresponding authors. Email: mhaquesoil@yahoo.com; pjkim@gnu.ac.kr

Table 1. Biomass production, nitrogen, carbon, cellulose, and lignin
content of hairy vetch at different harvesting period

Within columns, means followed by the same letter are not
significantly different (at P=0.05, Tukey's honestly significant
difference test)

Age of vetch    Biomass production     N       C
plants (days)    (Mg [ha.sup.-1]
                dry weight basis)

183                    3.2c          2.72d   42.33a
190                    3.8c          3.97c   42.30a
197                    5.3b          4.39b   42.25a
204                    6.5a          4.08a   42.94a

Age of vetch    Content (%)   Lignin    C : N
plants (days)    Cellulose

183               17.41b      15.63b    15.58a
190               17.74b      17.05ab   10.64b
197               19.34b      17.26ab   9.62c
204               22.16a      18.84a    10.53b


Table 2. Paddy soil properties at rice harvest as affected by age of
hairy vetch biomass incorporated

Within rows, means followed by the same letter are not significantly
different (at P=0.05, Tukey's honestly significant difference test)

Parameters                           Age of incorporated vetch (days):

                                      183      190      197      204

pH                                   7.44a    6.99a    7.25a    7.29a
Total organic C (g [kg.sup.-1])      9.22b    9.27b    9.24b    9.67a
Total N (g [kg.sup.-1])              0.70c    0.72b    0.72b    0.75a
C : N ratio                          13.17a   12.88a   12.83a   12.89a
DOC (mg [kg.sup.-1])                  369a    342ab    317bc     298c
DON (mg [kg.sup.-1])                  83a      53b      53b      49b
Available [P.sub.2][O.sub.5] (mg      122c     137c     177b     202a
  [kg.sup.-1])
Bulk density (g [cm.sup.-3])         1.38a    1.30b    1.30b    1.24c

Table 3. Some parameters of rice crops and C[H.sub.4], C[O.sub.2] and
[N.sub.2]O fluxes as affected by age of vetch biomass incorporated
for rice cultivation

Within rows, means followed by the same letter are not significantly
different (at P = 0.05, Tukey's honestly significant difference test)

Parameters                          Age of incorporated vetch (days):

                                     183      190      197      204

Greenhouse gas emission rate
  C[H.sub.4] (mg [m.sup.-2]         541.6b   535.0b   584.1a   491.6c
    [day.sup.-1])
  C[O.sub.2] (g [m.sup.-2]          2.42b    2.44b    2.49b    2.75a
    [day.sup.-1])
  [N.sub.2]0 (mg [m.sup.-2]         0.10a    0.08a    0.07a    0.07a
    [day.sup.-1])
Seasonal flux (kg [ha.sup.-1])
  C[H.sub.4]                         650b     642b     701a     590c
  C[O.sub.2]                        2908c    2924c    2983b    3294a
  [N.sub.2]O                        0.12a    0.10a    0.09a    0.09a
Rice yield component
  Grain yield (Mg [ha.sup.-1])      6.15b    6.65ab   6.85a    6.60ab
  Straw yield (Mg [ha.sup.-1])      6.28b    7.22a    7.40a    6.92ab
  No. of tillers                     16a      15a      15a      16a
  1000-grain weight (g)             23.7b    23.5b    24.4a    23.8b

Table 4. Changes in soil carbon dynamics as influenced by fertilisers
and variably aged vetch biomass incorporation for rice cultivation

Within columns, means followed by the same letter are not
significantly different (at P=0.05, Tukey's honestly significant
difference test). NPP, Net primary production; NECB, net ecosystem
carbon balance

Age of        Organic C input (kg C [ha.sup.-1])
vetch
(days)                                          Vetch
         Aboveground   Root    Litter   Urea   biomass

183         4786c      1087a    239b    20a     1270a
190         5340a      1087a    267a    20a     1269a
197         5486a      1094a    274a    20a     1268a
204         4986b      1085a    249b    20a     1288a

Age of    Organic C output (kg C [ha.sup.-1])        NECB
vetch                                           (kg [ha.sup.-1]
(days)   Harvest   C[O.sub.2]-C   C[H.sub.4]C
         removal     emission      emission

183       4786c        793b          488b            1335b
190       5340a        797b          482b            1364a
197       5486a        814b          526a            1316c
204       4986b        898a          443c            1301d

Table 5. Comparison of global warming potential (GWP) of emitted
greenhouse gases, net ecosystem carbon balance (NECB), and net GWP
value in rice soil

Within columns, means followed by the same letter are not significantly
different (at P<0.05, Tukey's honestly significant difference test)

Age of                GWP (kg C[O.sub.2]-eq.
vetch                      [ha.sup.-1])              Net GWP
(days)   C[H.sub.4]                             (kg C[O.sub.2]-eq.
                      [N.sub.2]O   C[O.sub.2]      [ha.sup.-1])
                                     (NECB)

183       12 200b        36a         4895b           17 131b
190       12 050b        30b         5001a           17 081b
197       13 150a        27b         4825c           18 002a
204       11 075c        27b         4770d           15 872c
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Author:Haque, Md. Mozammel; Biswas, Jatish Chandra; Waghmode, Tatoba R.; Kim, Pil Joo
Publication:Soil Research
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Date:May 1, 2016
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