Elevated CO2 reduces rate of decomposition of rice and wheat residues in soil

Agriculture, Ecosystems and Environment 139 (2010) 557–564

Contents lists available at ScienceDirect

Agriculture, Ecosystems and Environment journal homepage: www.elsevier.com/locate/agee

Elevated CO2 reduces rate of decomposition of rice and wheat residues in soil Thulasi Viswanath, Deo Pal, T.J. Purakayastha ∗ Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110012, India

a r t i c l e

i n f o

Article history: Received 4 February 2010 Received in revised form 24 September 2010 Accepted 27 September 2010 Available online 27 October 2010 Keywords: Elevated CO2 Ambient CO2 Residue decomposition Wheat residue Rice residue Phytotron

a b s t r a c t The production and quality of belowground roots and plant are likely to be affected by the increase in atmospheric CO2 level with subsequent changes in their decomposition rates in soil. However, the quality of residues has received very little attention, particularly in rice and wheat residues. The present experiment was laid out to study the decomposition of residues of rice (R) and wheat (W) grown in a Typic Haplustept soil under ambient (A) and elevated (E) CO2 conditions maintained in a phytotron. The decomposition of RA and WA was carried out in ambient atmospheric CO2 conditions, while that of RE and WE was done in elevated CO2 condition. Ambient CO2 grown rice and wheat residues were found to decompose at a faster rate compared to the corresponding elevated CO2 grown residues. The amount of residues left over after 150 days of decomposition was comparatively higher in the elevated CO2 grown residues indicating their slow rate of decomposition. However, ambient and elevated CO2 grown wheat residues did not differ significantly with respect to the amount remaining at later stages of decomposition. The RA and RE decomposed to 81% and 77% of their initial amount after 150 days of decomposition, while WA and WE decomposed to 73% and 71% of their initial amounts, while the C loss from RA, RE, WA and WE were 83%, 79%, 76% and 73%, respectively. Ambient atmospheric CO2 grown residues exhibiting narrow C:N ratios decomposed to a faster rate than the elevated CO2 grown residues. Overall, total organic carbon (TOC) content was significantly higher in WA treated soil than in WE treated soil. Net N mineralization (Nmin ), microbial biomass carbon (MBC) and Nmin :MBC were greater in soil amended with ambient CO2 grown residues than in elevated CO2 grown residues. Rice residues as compared to wheat residues decomposed at a faster rate thereby releasing higher amount of N in soil. In near future the residues produced under higher concentration of atmospheric CO2 need to be handled carefully as these are decomposed with difficulty due to wide C:N ratios. This has direct implications on N cycling in soil and therefore N fertilization needs to be modified when crop residues are incorporated in soil for optimum crop production. Though lower decomposability of elevated CO2 grown residues might cause more C sequestration in soil, N limitation might adversely affect the plant C sequestration in future. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Atmospheric concentrations of carbon dioxide (CO2 ) have been steadily rising from pre industrial values of approximately 280 ␮mol mol−1 to a current global mean of approximately 380 ␮mol mol−1 (IPCC, 2007). Concentrations are projected to increase to approximately 540–958 ␮mol mol−1 by the year 2100 (IPCC, 2001). Numerous effects of elevated atmospheric CO2 concentrations on plants have been documented (Kant Pratap et al., 2007), including changes in plant elemental composition (Taub

∗ Corresponding author at: Division of Soil Science and Agricultural Chemistry, Indian Agricultural Research Institute, New Delhi 110012, India. Tel.: +91 11 25841494; fax: +91 11 25841529. E-mail addresses: [email protected], [email protected] (T.J. Purakayastha). 0167-8809/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.agee.2010.09.016

et al., 2008). Litter decomposition plays a great role in ecosystem processes and carbon cycling and is therefore an important determinant of soil fertility and plant productivity. The ongoing increase in atmospheric CO2 concentration can potentially alter litter decomposition rates by changing: (i) the litter quality of individual species, (ii) allocation patterns of individual species, (iii) the species composition of ecosystems (which could alter ecosystem-level litter quality and allocation), (iv) patterns of soil moisture, and (v) the composition and size of microbial communities (Melillo et al., 1982; Taylor et al., 1989; Kemp et al., 1994; Dukes and Field, 2000; van Groenigen et al., 2005). Synthesis of existing data showed that an average 14% reduction of N concentrations in plant tissue generated under elevated CO2 regimes (Cotrufo et al., 2005). Cotrufo and Ineson (2000) reported that elevated CO2 significantly affected the chemical composition of beech (Fagus sylvatica) twigs, which had 38% lower N and 12% lower lignin concentrations than twigs grown under ambient CO2 . The decrease

558

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

in N concentration resulted in significant increase in the C:N and lignin:N ratios of the beech wood grown under elevated CO2 . However, the elevated CO2 treatment did not reduce the decomposition rates of twigs, neither were the dynamics of N and lignin in the decomposing beech wood affected by the CO2 treatment despite initial changes in N and lignin concentrations between the ambient and elevated CO2 beech wood. It is further assumed that these reductions would lead to decreased rates of nutrient cycling and dampen the potential CO2 stimulation of C sequestration in native ecosystems (Rastetter et al., 1992). Frederiksen et al. (2001) reported that lower decomposition rate and fewer bacterial grazers in the straw of wheat grown at elevated CO2 indicate reduced microbial activity and turnover. However, O’Neill and Norby (1991) (tree seedlings) and Kemp et al. (1994) (tall grass prairie species) found only minor changes in the quality of litter produced by plants grown on native soils in open top chambers under elevated CO2 and no change in decomposition rates when exposed in situ in native plant communities. It is still commonly assumed that rising atmospheric CO2 will reduce leaf litter quality by increasing C:N and lignin:N ratios ˜ (O’Neill and Norby, 1996; Penuelas et al., 2001; Frederiksen et al., 2001; Lee et al., 2003; Nowak et al., 2004; Torbert et al., 2004) and lower decomposition rates in terrestrial ecosystems would lead to reductions in nutrient availability (Melillo, 1983; Agren et al., 1991; McGuire et al., 1995; Torbert et al., 2000). Very little is known about the potential effects of rising atmospheric CO2 on the in situ decomposition of native plant litter that has been produced and exposed under natural conditions. Most of the studies as stated above are concentrated on species of forest trees (Cotrufo and Ineson, 2000; Johnson et al., 2000), grass and some field crops like wheat (Frederiksen et al., 2001), soybean and grain sorghum (Torbert et al., 2000) and there is a paucity of information on potential effects of rising atmospheric CO2 on the in situ decomposition of residues of important field crops like rice and wheat. Rice–wheat cropping system is the major cropping system occupying an area of 10.5 Mha in the Indo-Gangetic Plain (IGP) of India that produces huge amounts residues posing serious disposal problem. One of the viable options for sustainable crop productions is to incorporate the residues in situ. As India has been predicted as one of the countries to be affected badly due to elevated CO2 concentration and its associated consequences, it is therefore logical to study the decomposition pattern of wheat and rice residues grown under elevated levels of CO2 . This information is extremely vital for efficient nutrient management for sustainable crop production and possible carbon sequestration in changing scenario of climate change. The hypothesis set for the present study was that the C:N of residues of wheat and rice grown under elevated and ambient CO2 concentration would vary which might influence the decomposition pattern and N mineralization. The objectives of the present study were to observe the changes in the C:N ratio of ambient and elevated CO2 grown rice and wheat residues and impact of these residues on total organic carbon (TOC), microbial biomass carbon (MBC), N (ammoniacal: NH4 + -N + nitrate: NO3 -N) mineralization in soil.

2. Materials and methods 2.1. Collection of rice and wheat residues The residues of elevated atmospheric CO2 (600 ± 50 ␮mol mol−1 ) grown rice (RE) and wheat (WE) were collected from free air carbon dioxide enrichment (FACE) experiment in the farm of Indian Agricultural Research Institute, New Delhi. The octagonal FACE ring is made up of PVC pipes of 200 mm

diameter and wall thickness of 4.5 mm. Each arm of the octagon was fitted with a centrifugal air blower at one end for blowing air into the pipe and was closed at the other end. Four rows of holes (4 mm diameter) were drilled on each arm for the outlet of CO2 enriched air. The CO2 was injected at the input of each air blower for pre-mixing with air within the pipe before its injection in the fields. All the eight arms of the octagon had independent supply of CO2 controlled by 8 on/off valves. A common computer controlled proportional integral differential (PID) valve controlled the quantity of CO2 released into the arms. The fumigation of CO2 gas into the field from the plenum was made at the crop canopy height. The residues of ambient atmospheric CO2 (372 ␮mol mol−1 ) grown rice (RA) and wheat (WA) crops were also collected from the land adjacent to the FACE experiment. The rice crop was grown during wet season (July–October). Twenty-five days old rice seedlings (three plants hill−1 ) were transplanted in the fields of both FACE and ambient experiment. The field was submerged with water for one week before transplanting of rice followed by preparation of land by hoeing. Farmyard manure (FYM) was applied at the dose of 5 t ha−1 during the time of field preparation. The N:P:K applied was at the dose 100:40:40 kg ha−1 in the form of urea, single super phosphate and muriate of potash, respectively. The N was applied in three splits, with application of 50% of the total N as basal, 25% at mid-tillering stage and the remaining 25% at panicle initiation stage. Fields were flooded throughout the season with the exception of 10 days before harvest. Manual weeding was done to remove the weeds. After harvest of rice, wheat was grown in the same plots during winter season (November–February) under both FACE and ambient condition. After thorough ploughing and land preparation FYM and fertilizers were applied at the dose 5 t ha−1 (FYM) and 100:40:40 kg ha−1 N:P:K, respectively. Seeds of wheat were sown by broadcasting and uniformity was maintained by thinning out the excess seedlings after one week of emergence. Nitrogen was supplied in three splits as that of rice; 50% basal, 25% at crown root initiation stage and the rest 25% at anthesis. After maturity both the wheat and the rice crops were harvested. Only the aboveground straw portion was used for the study. The straws were analyzed for total C (Snyder and Trofymow, 1984), total N (Buresh et al., 1982) and C:N was computed (Table 1). 2.2. Decomposition study Decomposition study on residues of wheat and rice obtained from both FACE and ambient field was carried out in National PhyTable 1 Changes in amounts of ambient and elevated atmospheric CO2 grown rice and wheat residues during decomposition in soil. Days

Residue remaining (g) Rice

0 15 30 45 60 75 90 105 120 135 150

Wheat

Ambient

Elevated

Ambient

Elevated

5.625 3.871a 3.287b 2.795d 2.402f 2.074g 1.795i 1.562j 1.365k 1.198l 1.054m

5.625 4.00a 3.513b 3.056c 2.658e 2.337g 2.06h 1.819j 1.606k 1.420l 1.272l

5.625 4.098b 3.685d 3.307f 2.924h 2.621j 2.333k 2.085l 1.872n 1.684m 1.515q

5.625 4.251a 3.702c 3.384e 2.985g 2.651i 2.399j 2.125k 1.894l 1.682m 1.657o

Values (days × CO2 ) in the same column (days) or row (ambient and elevated) in residue remaining measurements for a particular crop followed by different lower case letters are significantly different at P = 0.05 according to Duncan’s Multiple Range Test.

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

totron Facility, Indian agricultural Research Institute, New Delhi. Phytotron chambers maintained at ambient (370 ␮mol mol−1 ) and elevated (650 ␮mol mol−1 ) CO2 concentrations were used for investigating the kinetics of residue decomposition. Temperature and relative humidity of both the chambers were programmed at 25 ◦ C and more than 80%, respectively. Natural day light hours and dark hours were simulated as such with the help of incandescent lamps. Soil used in the decomposition study was collected from ambient field of IARI Farm. The soil belongs to Typic Haplustept. It has pH 7.5, total organic C 0.6%, KCl extractable NH4 + -N 18.0 mg kg−1 and NO3 -N 30.0 mg kg−1 , NaHCO3 extractable P 22.0 mg kg−1 , CH3 COONH4 extractable K 40.0 mg kg−1 and MBC 156 mg kg−1 . In decomposition study litterbag technique as used by others (Schortemeyer et al., 2000; Frederiksen et al., 2001; Knops et al., 2007) was followed. The nylon cloths of 60 mesh and plastic threads were used to prepare litterbags. The nylon cloth was cut into square pieces of 10 cm × 10 cm size. 5.625 g of well-chopped (0.5 cm) dried residue was placed on the cloth piece, made into bags by folding from all sides and tied with the help of plastic threads. The individual litterbag was inserted into the centre of the pot containing 420 g soil. The pots were kept in multistoried racks inside the phytotron chamber at higher CO2 concentration and ambient ones. Soil moisture was maintained near field capacity level (25%, w/w) throughout the experiment by maintaining the weight of the whole pot. Destructive sampling of the pots was done at 15, 30, 45, 60, 75, 90, 105, 120, 135, 150 days interval. The litter bags were pulled out carefully from each pot and subsequently the litter inside it was removed and kept for drying in the shade in the laboratory for one week and thereafter in an oven at 70 ◦ C till constant weight is achieved. Dry weights of the samples were noted down and were ground through a Wiley mill so as to facilitate the analysis of C (Snyder and Trofymow, 1984) and N (Buresh et al., 1982). Soil in each pot was mixed well and two subsamples were taken. One half of the soil was taken for determination of MBC (Horwath and Paul, 1994) and NH4 + -N and NO3 -N (Bremner and Keeney, 1965) and the other half was dried in shade and processed for the determination of TOC (Snyder and Trofymow, 1984). 2.3. Statistics The experiment was laid out in two factor (ambient/elevated residues versus days of decomposition) completely randomized design with Duncan’s Multiple Range Test (DMRT) at 5% level of significance for separation of means. The data pertaining to percent weight loss and C loss was analyzed in one way ANOVA. Statistical analysis was performed by DOS based MSTATC version C program developed by S.P. Eisensmith, and SPSS version 10. 3. Results 3.1. Decomposition of rice and wheat residues The effect of levels of CO2 showed a decrease in degradability of the elevated CO2 grown rice and wheat residues (RE, WE) compared to those grown under ambient atmospheric CO2 concentration (RA, WA) (Fig. 1A). Lower amounts of both rice and wheat residues remained in successive days over previous days (Table 1). The rate of decrease in residue was highest during initial 15 days period, while it decreased gradually for the rest of the period. The residue reaming in elevated CO2 grown rice residues (RE) were higher over ambient CO2 grown rice residues (RA) on 30, 45, 90 and 150 days of decomposition, whereas the amount remaining in WE was significantly higher over WA throughout the decomposition period (Table 1). This is supported by the data that RA and

559

Fig. 1. (A) Changes in the amount of ambient and elevated atmospheric CO2 grown rice and wheat residues and (B) their percent weight loss during decomposition in soil, the histograms for individual crop with different small letters are significantly different according to Duncan’s Multiple Range Test (DMRT) at P = 0.05, error bar in each data point represents the standard error of mean.

RE decomposed to 81% and 77% of their initial amount after 150 days of decomposition, while WA and WE decomposed to 73% and 70% of their initial amount (Fig. 1B). Hence elevated CO2 grown rice and wheat residues were found to decompose at a slower rate compared to the corresponding ambient CO2 grown residues. Nevertheless, the rate constants obtained by fitting the curves were 0.0704 and 0.0641 for RA and RE, respectively (data not shown). Thus elevation of atmospheric CO2 had reduced the degradation rate of rice residues by 8.9%. In case of wheat residues, increased CO2 concentration reduced the rate constant by 4.4%. Overall during decomposition, the C contents in RE was significantly higher than in RA, while C contents in WA and WE were at par (Table 2). Elevated atmospheric CO2 grown rice residues (RE) showed higher C contents over RA up to 105 days (except 45, 120, 135 and 150 days), whereas C contents in WA were higher over WE only at 15 and 30 days of decomposition period. On the contrary, N contents in ambient atmospheric CO2 grown rice and wheat residues (RA and WA) were significantly higher over that in elevated atmospheric CO2 grown residues (RE and WE) (Table 2). RA had significantly higher N content than RE throughout the decomposition period, while WA had significantly higher N content than WE on 15, 45, 105, 120 and 150 days of decomposition. The C:N ratio of elevated atmospheric CO2 grown rice (72.28) and wheat residues (90.42) were greater than that in ambient atmospheric CO2 grown rice (67.12) and wheat (86.27) residues. The average C:N ratio of RA and RE during decomposition were 29.1 and 32.1, while that in WA and WE were 35.1 and 36.1, respectively (Fig. 2). The C:N ratio decreased rapidly for the initial period of decomposition but thereafter it became almost static for the rest

560

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

Table 2 Changes in C, N contents in ambient and elevated CO2 grown rice and wheat residues during decomposition in soil. Days

C content (%)

N content (%)

Rice

Wheat

Rice

Wheat

Ambient

Elevated

Mean

Ambient

Elevated

Mean

Ambient

Elevated

Mean

Ambient

Elevated

Mean

0 15 30 45 60 75 90 105 120 135 150

41.28 39.34cde 38.51efgh 38.86def 37.58h 37.57h 37.58h 37.61gh 37.64fgh 37.5efgh 37.63gh

42.2 41.93a 41.1ab 39.94bcd 40.12bc 40.05bc 39.21cde 39cdef 38.96cdefg 38.56efgh 38.61defgh

40.6A 39.8AB 39.3BC 38.8BC 38.8BC 38.4C 38.3C 38.3C 38.5BC 38.1C

44 42.58a 42.00 a 41.89ab 41.68bc 41.34bcd 40.87cde 40.6def 39.89g 39.54fg 39.51fg

44.21 41.64bc 41.44bcd 41.28bcd 41.31bcd 41.18bcd 41.03cde 40.18efg 39.66fg 40.01g 40.00g

42.1A 42.0A 41.6AB 41.5AB 41.3AB 41.0BC 40.4CD 39.8D 39.8D 39.7D

0.62 0.95n 1.08l 1.19k 1.27i 1.36h 1.43g 1.48ef 1.52cd 1.56b 1.61a

0.59 0.90o 1.03m 1.10l 1.22j 1.29i 1.36h 1.42g 1.46f 1.50de 1.54bc

0.92J 1.06I 1.14H 1.25G 1.32F 1.39E 1.45D 1.49C 1.53B 1.58A

0.51 0.80l 0.98jk 1.13i 1.19h 1.2gh 1.27ef 1.32d 1.36c 1.39bc 1.45a

0.49 0.76m 0.95k 1.01j 1.16hi 1.18h 1.23f 1.27e 1.31d 1.37bc 1.41b

0.78I 0.97H 1.07G 1.17F 1.19F 1.25E 1.29D 1.34C 1.38B 1.43A

Mean

38.1B

39.8A

41.1A

40.8A

1.34A

1.28B

1.21A

1.16B

Values (days × CO2 ) in the same column (days) or row (ambient and elevated) in a particular measurement for a specific crop residues followed by different lower case letters and the values in the row (CO2 ) or column (Days) pertaining to “Mean” for a particular measurement followed by different upper case letters are significantly different at P = 0.05 according to Duncan’s Multiple Range Test.

ratio in elevated CO2 grown rice residues was significantly higher than that in ambient CO2 grown rice residues up to 105 days while elevated grown wheat residues showed significantly higher C:N over ambient grown wheat up to 45 days of decomposition. The decrease in C content and increase in N content in rice and wheat residues during decomposition in soil led to the concomitant decrease in C:N ratio of the said residues. It is further supported by the data that about 83%, 79%, 76% and 73% of the C lost from RA, RE, WA and WE over 150 days of decomposition (Fig. 3). The C loss from rice residues was more than the wheat residues. 3.2. TOC, MBC, NH4 + -N + NO3 -N

Fig. 2. Changes in C:N ratio in ambient and elevated atmospheric CO2 grown rice (RA, RE) and wheat residues (WA, WE) during decomposition, the histograms for individual crop with different small letters are significantly different according to Duncan’s Multiple Range Test (DMRT) at P = 0.05, error bar in each data point represents the standard error of mean.

of the period (Table 3). The C:N ratio in rice residues decreased up to 120 days of decomposition and thereafter it almost remained static up to 150 days. The difference in C:N ratio between elevated and ambient grown residues was higher in rice than in wheat. The C:N

Total organic carbon (TOC) in RA treated soil was higher than in RE treated soil, but TOC in WE and WA amended soil did not differ significantly (Fig. 4). The interaction effect of levels of CO2 and days of decomposition showed no significant difference between the values in soils amended with rice (RE, RA) and wheat (WA, WE) in any of the day (data not shown). The microbial biomass C (MBC) in soil amended with ambient CO2 grown rice and wheat residues (RA, WA) were higher than that in elevated CO2 grown rice and wheat residues (RE, WE) (Table 4). The MBC in RA and RE amended soils were 317.1 and 293.4 mg kg−1 , while that in WA and WE were 275 and 257.3 mg kg−1 , respectively.

Table 3 Changes in C:N ratio in ambient and elevated CO2 grown rice and wheat residues during decomposition in soil. Days

C:N Rice

0 15 30 45 60 75 90 105 120 135 150

Wheat

Ambient

Elevated

Ambient

Elevated

67.12 41.39b 35.74c 32.55de 29.58fg 27.63hi 26.26ijk 25.46jk 24.75jk 24.62kl 23.35l

72.28 46.92a 39.82b 36.45c 32.87d 31.01ef 28.87gh 27.54hi 26.65ij 25.62jk 25.12jkl

86.27 53.03b 43.82c 37.32e 35.06f 34.50f 32.32hi 30.80jk 29.28lm 28.39mn 27.37n

90.42 55.16a 43.63c 40.90d 35.62f 35.05f 33.50gh 31.74ij 30.32kl 29.13lm 28.43mn

Values (days × CO2 ) in the same column (days) or row (ambient and elevated) in C:N ratio in residues of a particular crop followed by different lower case letters are significantly different at P = 0.05 according to Duncan’s Multiple Range Test.

Fig. 3. Percent carbon loss in ambient and elevated atmospheric CO2 grown rice and wheat residues during decomposition, the histograms for individual crop with different small letters are significantly different according to Duncan’s Multiple Range Test (DMRT) at P = 0.05, error bar represents the standard error of mean.

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

561

Table 4 Changes in microbial biomass C (MBC) contents in soil during decomposition of ambient and elevated atmospheric CO2 grown rice and wheat residues. Days

MBC content (mg kg−1 ) Rice

Wheat

Ambient

Elevated

Mean

Ambient

Elevated

Mean

0 15 30 45 60 75 90 105 120 135 150

153.6 423.2a 419.1a 395.0b 367.5c 334.6d 301.7e 271.6f 244.3gh 219.1ij 195.3kl

153.6 378.1bc 378.7bc 362.6c 339.5d 307.3e 279.1f 252.7g 234.5hi 210.7jk 191.2l

400.6A 398.9A 378.8B 353.5C 320.9D 290.4E 262.2F 239.4G 214.9H 193.3I

156.2 358.9a 355.8a 341.6b 323.4c 294.0d 266.0e 238.0f 212.8gh 190.4ij 168.7l

156.2 358.9bc 355.8bc 341.6c 323.4d 294.0e 266.0f 238.0g 212.8hi 190.4jk 168.7l

343.9A 341.7AB 329.0B 312.1C 283.8D 257.3E 231.0F 207.9G 189.1H 165.6I

Mean

317.1A

293.4B

275.0A

257.3B

Values (days × CO2 ) in the same column (ambient or elevated) or row (ambient and elevated) for a particular crop followed by different lower case letters and the values in the row (CO2 ) or column (Days) pertaining to “Mean” for a particular measurement followed by different upper case letters are significantly different at P = 0.05 according to Duncan’s Multiple Range Test.

The MBC did not change up to 30 and 45 days in soil amended with RA and RE residues, respectively, after which it decreased significantly for the rest of the period. The similar trend was also observed for wheat residues. When compared between the residues on different days, MBC in soil was found to be higher during decomposition of ambient residues than that of elevated residues in both the crops. The total N mineralization as measured in terms of NH4 + N + NO3 − -N was significantly higher in soil amended with ambient atmospheric CO2 grown residues than with elevated CO2 grown residues (Fig. 5). Nitrogen mineralization was 13.4% higher in soil treated with RA than RE, while it was only 9% higher in soil treated with WA than WE. Overall, the N mineralization was 1.5 times higher in soil treated with rice residue than in wheat residue. Nitrogen mineralization increased gradually during the decomposition of rice and wheat residues in soil (Table 5). The NH4 + -N + NO3 − N contents in RA treated soil were significantly higher over RE treated soil throughout the decomposition period (except 15, 90 days), while the that in WA treated soil were significantly higher over WE on 15, 45, 60, 120, 135 and 150 days. The increased microbial population under ambient grown residues was also responsible for increased N mineralization. Thus N mineralization per unit MBC (Nmin :MBC) was wider in soil

Fig. 5. Changes in total N mineralization (NH4 + -N + NO3 − N) in soil during decomposition of ambient and elevated atmospheric CO2 grown rice and wheat residues, the histograms for individual crop with different small letters are significantly different according to Duncan’s Multiple Range Test (DMRT) at P = 0.05, error bar in each data point represents the standard error of mean.

Table 5 Total N (NH4 + -N + NO3 -N) mineralization in soil during decomposition of ambient and elevated atmospheric CO2 grown rice and wheat residues. Days

NH4 + -N + NO3 — -N content in soil (mg kg−1 ) Rice

0 15 30 45 60 75 90 105 120 135 150 Fig. 4. Changes in total organic carbon (TOC) in soil during decomposition of ambient and elevated atmospheric CO2 grown rice and wheat residues, the histograms for individual crop with different small letters are significantly different according to Duncan’s Multiple Range Test (DMRT) at P = 0.05, error bar in each data point represents the standard error of mean.

Wheat

Ambient

Elevated

Ambient

Elevated

47.8 30.8jk 29.5kl 31.8j 36.2i 41.1g 46.3h 51.6e 57c 61.8b 66.1a

47.8 29.7kl 27.6m 28.0lm 31.7j 35.7i 39.9h 45.0g 49.4f 53.2d 57.9c

47.8 25.8g 22.0hi 21.3hi 22.9h 28.0g 28.0f 35.0d 35.5c 39.3b 42.9a

47.8 21.7hi 20.5ij 19.1j 20.5ij 25.5g 29.2ef 30.2de 32.0d 34.2c 38.2b

Values (days × CO2 ) in the same column (ambient or elevated) or row (ambient and elevated) in total N mineralization measurements for a particular crop followed by different lower case letters are significantly different at P = 0.05 according to Duncan’s Multiple Range Test.

562

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

Fig. 6. Ratio of total N mineralization (Nmin ) with microbial biomass carbon (MBC) during decomposition of ambient and elevated atmospheric CO2 grown rice and wheat residues, the histograms for individual crop with different small letters are significantly different according to Duncan’s Multiple Range Test (DMRT) at P = 0.05, error bar represents the standard error of mean.

amended with ambient CO2 grown residues than elevated CO2 grown residues (Fig. 6). 4. Discussion 4.1. Decomposition of residues and changes in C:N ratio During 150 days of decomposition, about 81% and 77% of biomass of ambient atmospheric CO2 grown rice (RA) and wheat residues (WA) lost, while the values were 77% and 71% biomass of elevated atmospheric CO2 grown rice (RE) and wheat (WE) residues. The magnitude of decomposition of residues of our study did not match with the data published for a variety of other plant species reported elsewhere. About 15–18% of original mass of Populas spp. lost during eight months of field incubation (Cotrufo et al., 2005). Hirschel et al. (1997) reported that 14% and 21% of the initial litter mass of ambient and elevated CO2 grown Carex curvula decomposed over a 61 day exposure period. Frederiksen et al. (2001) reported less than 50% decomposition of wheat straw after five months. In contrary to the above findings, Torbert et al. (2000) reported that increased levels of easily decomposable cellular components compensated for higher C:N ratios in cotton residues resulting in similar decomposition rates among residues from different CO2 treatments. Finzi and Schlesinger (2002) showed that the greater forest floor mass and nutrient content in the plots under elevated CO2 had no consistent or long-term effect on litter decomposition. Similarly, de Graaff et al. (2004) reported that the rate of decomposition of Lolium perenne and Trifolium repens plant materials was unaffected by elevated atmospheric CO2 and rate of N fertilization. Residue decomposition when measured in terms of C loss, it was observed that about 82.9% and 79.3% C was lost from RA and RE residues, respectively, while 75.9% and 73.3% C was lost from WA and WE residues in the same order. It is worth mentioning that the C loss was 2.4% and 7.2% lower in elevated atmospheric CO2 grown rice and wheat residues, respectively. The decomposition of residues and the C loss was greater in rice residues than in wheat residues. In this context Knops et al. (2007) reported a 2.5% lower rate of C loss under elevated CO2 in first and second year of decomposition of Bromus inermis. Substrate quality has been recognized as one of the most important factors regulating decomposition processes (Swift et al., 1979), and quality parameters such as N concentration, C:N and lignin:N ratios have been correlated with decomposition rates (Melillo et al., ˜ 1982; Taylor et al., 1989; Penuelas et al., 2001; Norby et al., 2001;

Cotrufo et al., 2005). The effect of increasing CO2 levels on plant material quality has been the subject of many studies and a general reduction of N concentration, with a concomitant increase in C:N ratio is likely to occur (Coleman et al., 1993). A similar reduction in N concentration and increment in C concentration resulting in concomitant increase in C:N ratio in elevated CO2 grown rice (72.28) and wheat (90.42) residues was also observed in the current investigation. The corresponding values of C:N in ambient atmospheric CO2 grown rice and wheat residues were 72.28 and 82.67. During the decomposition the C:N of rice and wheat residues gradually narrowed down over 150 days. Overall, wheat residues as compared to rice residues maintained higher C:N up to 45 days of decomposition. Therefore, the difference between the C:N ratio in RA and RE was short lived. Contrarily, difference between C:N ratio in WA and WE was significant even up to 105 days of decomposition. This was mainly due to slower decomposition of wheat residues being wider in C:N than rice residues. Further, narrow C:N in ambient grown residues than in elevated grown residues during decomposition indicate possibility of an early mineralization. The effect of elevated CO2 concentration, manifested in terms of N concentration is less in the case of wheat when compared to rice residues. A decrease in N concentration by 4.1% was observed in wheat residues as a result of elevation of atmospheric CO2 , while the corresponding decrease was by 5.1% in rice residues. Nevertheless, the changes in N concentration and C:N ratio have got specific effects on the rate of decomposition of the residues. The decrease in per cent N composition might have resulted in the decreased decomposability of the elevated CO2 grown residues. Usually when high C:N materials are incorporated into soil the initial immobilization follows by mineralization as the residues approaches a break-even point of C:N value. Since ambient grown residues had lower C:N values during decomposition, the breakeven point of C:N indicating the transition from immobilization to mineralization to reach soon. Hence the lower C:N of RA and WA in comparison to RE and WE during decomposition has enormous consequences in nitrogen cycling in soil. But a meta-analysis by Norby et al. (2001) found a pattern of lower tissue N in leaf litter produced under elevated CO2 , but did not find consistent changes in decomposition rates. 4.2. Changes in total organic carbon (TOC), microbial biomass carbon (MBC) during decomposition In the present study, elevated CO2 -grown residues were found to be associated with lower TOC contents in soil during decomposition in rice residues as compared to the ambient CO2 grown residues. However, difference in TOC during decomposition of elevated CO2 grown and ambient CO2 grown residues were less in wheat compared to rice. This trend is in conformity with the observed change in degradability of the residues. A decrease in the rate of mineralization has been observed during the decomposition of elevated CO2 -grown rice and wheat residues. Contrarily, although soil C mineralization rates of residue amended soils were similar for ambient compared to FACE, increased storage of C in soil could still occur under elevated atmospheric CO2 conditions because of increased biomass production under CO2 enriched condition (Torbert et al., 2000). As the decomposition of elevated CO2 grown residues is slow, it has tremendous implications on C sequestration in soil. The mean residence time of C in rice and wheat residues applied in soil would increase and less emission of major greenhouse gases like, CO2 responsible for global warming. The higher C sequestration in soil amended with residues grown under elevated CO2 could be negated to some extent due to lesser availability of soil N that might limit plant C sequestration in future (Hu et al., 2001). However, this effect could be reversed if N is supplemented through fertilizer in

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

high intensity cropping system practiced in semi-arid tropics of India. 4.3. Nitrogen mineralization (NH4 + -N + NO3 − -) in soil during decomposition The elevated atmospheric CO2 grown rice and wheat residues showed comparatively lower N mineralization than ambient CO2 grown residues. As the decomposition of elevated CO2 grown residues was slower than ambient CO2 grown residues driven by wide C:N, the N mineralization also reduced in the former than the latter treatment. de Graaff et al. (2006) reported that elevated CO2 did not change N mineralization of L. perenne and T. repens plant material in soils, because of available N in the fertilized and leguminous systems. As fertilizer was neither applied nor any legume residues incorporated in our soil, the N mineralization in ambient-CO2 grown rice and wheat residues was more than the elevated-CO2 grown residues. The difference in the magnitude of N mineralization between ambient and elevated atmospheric CO2 grown residues was higher in rice than in wheat. Overall, though initially there was immobilization of inorganic N (NH4 + -N + NO3 − -N) in soil, thereafter the mineralization picked up that caused rapid release of N. This was due to rapid increase in microbial biomass responsible for mineralization of N locked up in residues. This was further proved that enhanced microbial biomass proportionately increased N mineralization and therefore the Nmin :MBC was higher in soil amended with ambient CO2 grown residues than with elevated CO2 grown residues. The enhanced Nmin :MBC also proved that the microbes were more efficient in N mineralization in soil added with ambient grown residues. The experimental soil initially contained 27.0 mg kg−1 of NO3 − -N. Though the initial values were same, a higher amount of NO3 − -N was released into soil during the decomposition of ambient residues. This superiority of ambient residues with respect to the increase in N release can be ascribed to the higher nitrogen decomposition of ambient residues compared to elevated residues. In future, the rapid increase in CO2 could produce residues like rice and wheat whose decomposition with relation to N mineralization would be slow. This has greater implications on N cycling in soil. As the rate of decomposition is slow, therefore the dependency of crops over fertilizer will be more but environmental protection point of view it would preserve N in soil by promoting less N loss. 5. Conclusion Under changing scenario of elevated atmospheric CO2 , huge amount of rice and wheat residues will be produced in near future. The sustainable way to dispose of these residues will be to incorporate in soil. Elevated atmospheric CO2 grown rice and wheat residue quality will be decreased due to lowering of N content thereby widening of C:N. These attributes in elevated CO2 grown residues will result in lower decomposition in soil than ambient CO2 grown residue. Such decreased rate of decomposition might increase the residence time of the elevated CO2 -grown residue decomposition systems, thus supplementing sequestration of C in soil. In such situation lower N mineralization might increase the dependency of crops towards fertilizer N. However, the environmental benefit of lower N mineralization is ascribed due to checking of N loss through various mechanisms. Acknowledgements The authors express their gratefulness to Dr. S.C. Datta, Dr. S. Bhadraray, Dr. D.C. Uprety and Dr. N. Kalra for their kind help dur-

563

ing the investigation. Special thanks to Dr. Rajendra Singh for his kind help during conducting the experiment in National Phytotron Facility, Indian Agricultural Research Institute, New Delhi. The first author is obliged to the Council of Scientific and Industrial Research for awarding the fellowship to carry out this research.

References Agren, I.G., McMurtrie, R.E., Parton, W.J., 1991. State-of-the-art of models of production–decomposition linkages in conifer and grassland ecosystems. Ecol. Appl. 1, 118–138. Bremner, J.M., Keeney, D.R., 1965. Steam distillation methods for determination of ammonium, nitrate and nitrite. Anal. Chem. Acta 32, 485–495. Buresh, R.J., Austin, E.R., Craswell, E.T., 1982. Analytical methods in 15 N research. Fertil. Res. 3, 37–62. Coleman, J.S., McConnaughay, K.D.M., Bazzaz, F.A., 1993. Elevated CO2 and plant nitrogen use: is reduced tissue nitrogen concentration size dependent? Oecololgy 93, 195–200. Cotrufo, M.F., Ineson, P., 2000. Does elevated atmospheric CO2 concentrations affect wood decomposition? Plant Soil 224, 51–57. Cotrufo, M.F., Angelis, P.D., Polle, A., 2005. Leaf litter production and decomposition in a poplar short-rotation coppice exposed to free air CO2 enrichment (POPFACE). Glob. Change Biol. 11, 971–982. de Graaff, M.A., Six, J., Harris, D., 2004. Decomposition of soil and plant carbon from pasture systems after 9 years of exposure to elevated CO2 : impact on C cycling and modeling. Glob. Change Biol. 10, 1922–1935. de Graaff, M.A., Groeingen, K.J.V., Six, J., 2006. Interactions between plant growth and soil nutrient cycling under elevated CO2 : a meta analysis. Glob. Change Biol. 12, 2077–2091. Dukes, J.S., Field, C.B., 2000. Diverse mechanisms for CO2 effects on grassland litter decomposition. Glob. Change Biol. 6, 145–154. Finzi, A.C., Schlesinger, W.A., 2002. Species control variation in litter decomposition in a pine forest exposed to elevated CO2 . Glob. Change Biol. 8, 1217–1229. Frederiksen, H.B., Ronn, R., Christensen, S., 2001. Effect of elevated atmospheric CO2 and vegetation type on microbiota associated with decomposing straw. Glob. Change Biol. 7, 313–321. Horwath, W.R., Paul, E.A., 1994. Microbial Biomass. In: Bingham, J.M., Mickelson, S.H. (Eds.), Methods of Soil Analysis, Part 2, Microbiological and Biochemical Properties—SSSA, Book Series No. 5. ASA, SSSA, Madison, WI, pp. 753–773. Hirschel, G., Koner, C.H., Arnone, J.A., 1997. Will rising atmospheric CO2 affect leaf litter quality and in situ decomposition rates in native plant communities? Oecology 110, 387–392. Hu, S., Chapin III, F.S., Firestone, M.K., Field, C.B., Chiariello, N.R., 2001. Nitrogen limitation of microbial decomposition in a grassland under elevated CO2 . Nature 409, 188–191. IPCC, 2001. Climate change 2001—Synthesis Report. A Contribution of Working Group I, II and III to the Third Assessment Report of IPCC, Geneva, Switzerland. Cambridge University Press, Cambridge, United Kingdom, New York, NY, USA, pp. 398. IPCC, 2007. Climate Change 2007—Synthesis Report. A Contribution of Working Group I, II and II to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, IPCC, Geneva, Switzerland. Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA, pp. 104. Johnson, D.W., Cheng, W., Ball, J.T., 2000. Effects of CO2 and N fertilization on decomposition and N immobilization in ponderosa pine litter. Plant Soil 224, 115–122. Kant Pratap, C.B., Bhadraray, S., Purakayastha, T.J., Jain, V., Pal, M., Datta, S.C., 2007. Active carbon pools in rhizosphere of wheat (Triticum aestivum L.) grown under elevated atmospheric carbon dioxide concentration in a Typic Haplustept in sub-tropical India. Environ. Pollut. 147, 273–281. Kemp, P.R., Waldecker, D.G., Owensby, C.E., 1994. Effects of elevated CO2 and nitrogen fertilization pretreatments on decomposition of tall grass prairie leaf litter. Plant Soil 165, 115–127. Knops, J.M.H., Naeem, S., Reich, P.B., 2007. The impact of elevated CO2 , increased nitrogen availability and biodiversity on plant tissue quality and decomposition. Glob. Change Biol. 13, 1960–1971. Lee, T.D., Reich, P.B., Tjoelker, M.G., 2003. Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment. Oecology 137, 22–31. McGuire, A.D., Melillo, J.M., Joyce, L.A., 1995. The role of nitrogen in the response of forest net primary production to elevated atmospheric carbon dioxide. Ann. Rev. Ecol. Syst. 26, 473–503. Melillo, J.M., 1983. Will increases in atmospheric CO2 concentrations affect litter decay rates? In: The ecosystems center annual report. Marine Biological Laboratory, Woods Hole, Massachusetts, USA, pp. 10–11. Melillo, J.M., Aber, J.D., Murator, J.F., 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamic. Ecology 6, 621–626. Norby, R.J., Cotrufo, M.F., Ineson, P., O’Neill, E.G., 2001. Elevated CO2 , litter chemistry, and decomposition dynamics. Ecology 63, 621–626. Nowak, R.S., Ellsworth, D.S., Smith, S.D., 2004. Functional responses of plants to elevated atmospheric CO2 —do photosynthetic and productivity data from FACE experiments support early prediction? New Phytol. 162, 253–280. O’Neill, E.G., Norby, R.J., 1991. First-year decomposition dynamics of yellow-poplar leaves produced under CO2 enrichment. Bull. Ecol. Soc. Am. 72, 208.

564

T. Viswanath et al. / Agriculture, Ecosystems and Environment 139 (2010) 557–564

O’Neill, E.G., Norby, R.J., 1996. Litter quality and decomposition rates of foliar litter produced under CO2 enrichment. In: Koch, G.W., Mooney, H.A. (Eds.), Carbon Dioxide and Terrestrial Eco-systems. Academic press, San Diego, pp. 87–103. ˜ Penuelas, J., Filella, I., Tognetti, R., 2001. Leaf mineral concentrations of Erica arborea, Juniperus communis and Myrtus communis growing in the proximity of a natural CO2 spring. Glob. Change Biol. 7, 291–301. Rastetter, E.B., Mckane, R.B., Shaver, G.R., Melillo, J.M., 1992. Changes in C storage by terrestrial ecosystems: how C–N interactions restrict responses to CO2 and temperature. Water Air Soil Pollut. 64, 327–344. Schortemeyer, M., Dijkstra, P., Johnson, D.W., 2000. Effects of elevated atmospheric CO2 concentration on C and N pools and rhizosphere processes in a Florida scrub oak community. Glob. Change Biol. 6, 383–391. Snyder, J.D., Trofymow, J.A., 1984. Rapid accurate wet oxidation diffusion procedure for determining organic and inorganic carbon in plant and soil samples. Comm. Soil Sci. Plant Anal. 15, 1587–1597.

Swift, M.J., Heal, O.W., Anderson, J.M., 1979. Decomposition in Terrestrial Ecosystems. Blackwell Oxford. Taub, D., Miller, B., Allen, H., 2008. Effects of elevated CO2 on protein concentration of food crops: a meta analysis. Glob. Change Biol. 14, 565–575. Taylor, B.R., Parkinson, D., Parsons, W.J.F., 1989. Nitrogen and lignin content as predictors of litter decay rate: a microcosm test. Ecology 70, 97–104. Torbert, H.A., Prior, S.A., Rogers, H.H., Wood, C.W., 2000. Review of elevated atmospheric CO2 effects on agro-ecosystems: residue decomposition process and soil C storage. Plant Soil 224, 59–73. Torbert, H.A., Prior, S.A., Davis, M.A., 2004. Nitrogen and carbon cycling in a model longleaf pine community as affected by elevated atmospheric CO2 . Environ. Manage. 33, 132–138. van Groenigen, K.J., Gorissen, A., Harris, D., Kuikman, P.J., van Groenigen, J.W., van Kessel, C., 2005. Decomposition of 14 C-labeled roots in a pasture soil exposed to 10 years of elevated CO2 . Soil Biol. Biochem. 37, 497–506.