Chapter- V Carbon budget in agroecosystems Introduction Fossil fuel consumption and agricultural practices are the two main sources of greenhouse gas emissions. Any change in agricultural practices, viewed in agricultural extensification or intensification perspectives, also has impacts on carbon source-sink attributes of non-agricultural landscape elements, such as forests and grazing lands, and rates of use of fossil fuels. During the past few years, contribution of agricultural practices like conversion of forests to ranches and shifting cultivation to carbon budget has been estimated by a few workers (Duxbury et ai., 1993; Houghton, 1991; Fujisaka et ai., 1998). Within intensive agricultural zone, attention has largely been paid to evaluate the contribution of wet paddy cultivation to global emission of methane (Sinha, 1997; Sass et ai., 1990; Parashar et ai., 1994). Potential of agroforestry and tree plantations in sequesteration of atmospheric carbon has been assessed by a few workers (Schroeder, 1994; Montagnini and Porras, 1994). A comparative assessment of different crops and cropping patterns on carbon dynamics in intensive agricultural zone has been lacking. This chapter deals with carbon stocks in soil, weeds and crop components in different crops and management practices in village Rohad representing a typical village of Haryana-Punjab intensive agricultural zone. Methods As also described in preceding chapter, spatio-temporal variability in agroecosystems of village Rohad was classified based on two features: irrigation and crop type. Based on the attributes of irrigation three agroecosystems types could be differentiated: rainfed or unirrigated agroecosystems, agroecosystems receiving irrigation from canal water and agroecosystems receiving irrigation from ground water drawn through tube wells (hereafter referred to as unirrigated, canal irrigated and tubewell irrigated agroecosystems/land use respectively. Further differentiation in each
63 of these three types of agroecosystems/land use was based on the crop grown. As described earlier, two harvests are taken in a year but many a times land is fallowed in one or both growing seasons of the year. Wheat, paddy and sorghum were grown in all the three agroecosystem types differing in respect of irrigation, pearl millet only in unirrigated and canal irrigated areas, berseem in unirrigated and tubewell irrigated land, and mustard and pigeon pea only in unirrigated land. Paddy/fallow-wheat rotation was most prevalent rotation. Eight households (land holding size ranging from 5 ha to 7 ha, the most dominant land holding class in the village), were selected for carbon budgeting. At the time of weeding, density of different weed species was.counted in 10 quadrats each of 1m x 1m size in each crop type and each household. Twenty individuals of each weed species were sampled for biomass estimation. Weeds were classified into palatable and unpalatable weeds. Twenty individuals of each weed species were sampled, separated into belowground and aboveground parts, dried at 80±5 0 C in a hot air oven and weighed. Similarly, crop density was enumerated in twenty quadrats for each crop type in each household. At the time of harvest, twenty individuals of a crop in a given agroecosystem type were randomly sampled, separated into belowground, human food component, fodder component and other crop-bypro ducts. Crops were harvested at a level of 6-12 cm above soil surface. The left-over aboveground biomass burnt resembled the biomass burnt by the farmers. This component was estimated. Review of literature showed minor variations in carbon concentration in annual crops and herbaceous species (Fisher et a!., 1994; Fujisaka et ai., 1998; Nepstad et al., 1994; Clement et ai., 1995; Sen unpublished). Based on the reported values on carbon concentration, it was assumed that all plant biomass pools contained 48% of carbon. Soil was sampled from 0-15 cm and 15-30 cm horizons from 5 random locations in three fields for each crop in each agroecosystem type just before sowing and just after harvesting. Three composite samples were obtained by mixing the
64 samples from a field for each crop in each agroecosystem type. Replication was thus done considering a field as a sampling unit. Samples were air dried and passed through a 210 Jll11 sieve. Soil organic carbon was estimated by the Walkley-Black wet digestion method. Bulk density was estimated following the method given in Okalebo et al. (1993). The following steps were taken to measure the soil bulk density -a thin-sheet metal tube of 5 cm diameter of known weight (W) and volume (V) 5 cm 3 was inserted into the soil surface. Then the soil were cut beneath the tube bottom after excavating the soil from around the tube and simultaneously the excess soil from the tube ends were removed by using knife. This was dried at 105 e for two days and weighed (W2). The soil bulk density was calculated by using the following fonnular i.e. W2-Wl glcm 3 N. Bulk density varied from 1.22 to 1.26 g per cm 3 (least significant difference (P = 0.05) was 0.04). Thus the sites did not differ significantly in respect of bulk density. Soil carbon per unit area for 0-15 cm and 15-30 cm horizones was computed using soil carbon (%) and bulk density data. Dead roots carried over from the previous crops and carbon in fauna were not included in the sampling and this could be a source of underestimation of soil carbon pool. Sampling was done over a period of two years. The means of two year data are presented. Results Carbon Pools Among the three crops grown in all the three types of agroecosystems (unirrigated, canal irrigated and tubewell irrigated), carbon was stored in much larger quantities in wheat (3770-5478 kglha carbon in aboveground parts and 135-225 kglha in belowground organs) compared to paddy (2282-3081 kglha in aboveground and 85-113 kglha in belowground) and sorghum (2370-2984 kglha in aboveground and 173-220 kglha in belowground organs). Belowground carbon pool was significantly higher in unirrigated wheat crop as compared to the irrigated ones while the reverse was true
65 for the aboveground pool. Such a difference in belowground carbon pool was not observed in sorghum and paddy. Total carbon pool in standing crop (crop+weed) varied from 3995-5622 kglha in wheat, 2367-3194 kglha in paddy and 2542-3200 kglha in sorghum. Soil organic carbon pool increased (the difference between soil organic carbon pool at the time of sowing and that at the time of harvesting) in both soil horizons in wheat except for a decline in 15-30 cm horizon in tubewell irrigated crop. In case of paddy and sorghum soil carbon pool declined in both horizons in all the three agroecosystem types. There was an increase of 2420-3612 kglha carbon in 0-30 cm soil profile in wheat and decline of 3258-5323 kg/ha in paddy and 2096-3810 kglha in sorghum. The net carbon sequestration capacity depended on carbon sequestered in crop and weed biomass and increase or decrease in soil carbon pool. There was accumulation of 7607-8649 kglha of carbon in wheat while a loss of 631-2956 kg/ha of carbon from paddy. In case of sorghum, there was a net loss of 125 and 709 kglha of carbon from unirrigated and canal irrigated crop systems and net increase of 1104 kglha of carbon in tubewell irrigated crop (Table 5.1). Carbon pools in plant biomass and net change in soil carbon pool in berseem and pearl millet, the crops grown in two types of agroecosystems, are given in Table 5.2. Carbon accumulation in aboveground as well as belowground plant parts in berseem was substantially larger (4044-4576 kglha in aboveground and 235-260 kglha in belowground organs) than in pearl millet (2205-2657 kg/ha of carbon in aboveground and 156-178 kglha in belowground organs). However, while in berseem there was an increase in soil organic carbon as a result of crop growth (1525 and 953 kglha in unirrigated and tubewell irrigated areas, respectively), soil carbon was depleted in case of the pearl millet (2858 kglha in unirrigated and 1144 kglha in canal irrigated system). When plant and soil carbon are considered together, about 5800 kglha of carbon accumulated in case of berseem and 1692 kg/ha in canal irrigated pearl millet but there was a depletion of about 500 kglha of carbon from unirrigated pearl millet crop.
r 66 In case of mustard and pigeon pea, the two crops grown only in unirrigated conditions, about 3300 kg/ha of carbon was accumulated in plant component of pigeon pea crop (including weeds) and 2607 kg/ha in mustard. There was an increase of 1524 kg/ha of carbon in surface soil of mustard but a decline of 1905 kg/ha in pigeon pea. In both the crops, soil carbon in deeper soils (15-30 cm) declined due to cropping. When plant and soil component are considered together, mustard cultivation led to accumulation of about 3200 kg/ha of carbon and pigeon pea to 437 kg/ha (Table 5.3). Comparison of all crop systems shows that while there was a positive carbon balance or sequesteration of carbon in all winter crops in all types of agroecosystems, there was a negative carbon balance or release of carbon from the system in case of Kharif crops, except for sorghum tubewell irrigated and pearl millet grown in canal irrigated areas (Table 5.1 and 5.2). Wheat showed the highest rate of carbon accumulation in agroecosystem followed by berseem and mustard. Irrigation showed a significant positive effect on carbon accumulation in the system in all the cases, except for insignificant difference between unirrigated and tubewell irrigated berseem crops and negative effect observed from comparison of unirrigated and canal irrigated sorghum crops. Carbon flows Carbon accumulated in crop and weed component flows as four components: (a) human food, consumed partly in the village and partly transported out of the village (b) fodder, derived from crop by products and weeds, partly fed to village livestock, partly exported/sold and partly burnt for cooking (c) biomass carbon quickly released as a result of burning within farm fields (d) biomass carbon recycled within farm fields and largely consisting of below ground plant parts (Tables 5.4-5.5).
67 Discussion Terrestrial vegetation plays a vital role in global carbon cycle. Not only tremendous amounts of carbon is stored in the vegetation component, but large amounts are also actively exchanged between vegetation, soil and atmosphere (Waring and Schlesinger, 1985). Carbon pools in annual crop based agricultural systems are indeed much smaller than those in grasslands, agroforestry systems, tree plantations and natural forests (Houghton, 1991; Schroeder, 1994) but a rapid turnover and impacts of agronomic practices on soil carbon dynamics could have significant effect on global carbon budgets. Land use-land cover change occurs at a local scale, field by field, but it has a global dimension because of the sheer extent at which it is taking place (Schroeder, 1994). The amount of carbon accumulation in crop biomass depends upon the photosynthetic potential of the crop together with factors limiting crop productivity. Carbon accumulation in weed component would depend upon weeding intensity and reproductive strategies of weed species. The soil organic carbon balance would depend, upon the release of complex organic compounds as a result of decomposition, quality and quantity of organic inputs to the system, soil factors determining organic carbon retention capacity and loss of carbon largely through soil respiration and burning but also to some extent through leaching (Zinke et ai., 1984; Post and Mann, 1990; Woomer and Swift, 1994; Gupta and Kaur, 1998). In intensive agricultural zone of arid/semiarid climate, moderation of soil moisture and nutrient (particularly nitrogen) regime as result of irrigation and fertilizer application alters the rate of accumulation of carbon as crop biomass as well as soil organic carbon. Fast development of a dense crop canopy may significantly reduce soil temperature which in turn may bring down soil carbon oxidation rates. The relationship between temperature and soil moisture may vary depending upon the chemical characteristics of the residue (Singh and Shekhar, 1989a,b; Clement et al., 1995; Gupta and Kaur, 1998). The impact of soil moisture and nitrogen amelioration on crop and soil component could be variable. The
68 rate of increase in soil microbial activity in response to increase in temperature has been found to be higher than the increase in plant growth (Lloyd and Taylor, 1994). Small increase in temperatures can result in significant net losses of unstabilized soil organic matter (Ineson et al., 1998). The data presented here show that loss of carbon from soil exceeded the gain through crop and weed component in case of paddy grown in all the three types of agricultural systems (unirrigated, canal irrigated and tubewell irrigated), pearl millet in unirrigated system, sorghum in unirrigated and canal irrigated systems. In all these cases, decrease in soil temperature due to crop canopy thus was too low to protect soil carbon from oxidation. In case of pigeon pea, there was a substantial loss of soil carbon but carbon accumulated in crop and weed components compensated for this loss resulting in a positive carbon balance. Legumes, because of their high quality residue, could substantially improve soil carbon and nutrient status provided the crop residues are recycled. In the present case, all above ground parts excluding grains are burnt for cooking. All winter crops faced to lower soil temperature and moisture conditions showed a positive carbon balance for soil. The increasing emphasis to wheat-paddy rotation thus implies increased risks of soil carbon depletion as well as reduced carbon sequestration capacity of the landscape. Wet paddy cultivation has received significant attention in terms of its potential to act as methane source (Sinha, 1995, 1997; Parashar et al., 1994), but the changes in carbon budget following increasing emphasis to wheatp~ddy rotation has not received much attention. Crop diversification such as replacement of paddy by legumes like pigeon pea or fodder/millet crops like sorghum and pearl millet seem to offer a greater potential of carbon sequestration or maintenance of a higher level of soil organic matter as compared to paddy. Fire is an age-old landscape management tool (Kozlowski and Ahlgren, 1974). Its favourable impacts on soil fertility and weed control are valued by traditional shifting cultivators (Ramakrishnan, 1992; Ramakrishnan et af., 1997). In the present
69 case, surface fire is not practised for these reasons. Threshing operations are carried out in the field and a lot of residue is left around these spots. Fanners find poor germination and growth around these areas because the straw being a high C/N material decomposes slowly and reduces germination rate and seedling growth. Fanners, intentionally ignite fire to burn accumulated straw on these spots but the fire spreads to the surrounding fields (plate no. 7). Carbon release through burning in the present case is indeed substantially lower as compared to carbon emissions from forest fire or shifting agriculture (Fujisaka et ai., 1998; Houghton, 1991). However, this loss is significant from the point of elimination of an important input for decomposition and soil organic matter and nutrient build up. Proper crop and soil management practices under intensive cropping could be economically profitable as well as environmentally ameliorative. Studies of Singh et al. (1983) and Brar and Singh (1986) in semi-arid agroecological zone and Tisdale and Nelson (1970) showed improvement in soil organic carbon with increase in agricultural inputs. This improvement was attributed to larger quantities of crop residues recycled under more productive agroecosystems. Wood et al. (1990) found improvement in soil carbon stock by greater cropping intensity but under no-till management in dryland agroecosystems. A negative carbon balance in the paddy crop system observed in the present case warrants appropriate changes in management practices and cropping patterns for long term sustainability of productivity from intensive agricultural lands in the arid and semiarid zone. Intermixing of trees with crops could substantially compensate for depletion of carbon or increase carbon sequestration in this zone but has not yet become popular (Schroeder, 1994). Policies that have so far concentrated on economic benefits to the fanners and achieving food self-sufficiency at national scale, need to be appropriately modified from the point of agroecosystem sustainability as well as global environmental benefits like carbon sequestration (Smith et al., 1997).
Table 5.1 Carbon pools in plant biomass and net change in soil carbon pool (kglha) of wheat, paddy and sorghum in Unirrigated : (Uni), Canal irrigated (Cai) and Tubewell irrigated (Tui) WHEAT PADDY SORGHUM Uni Cai Tui Uni Cai Tui Uni Cai Tui Grain 1307+164 1841±201 2078±167 505±30 699±40 587±30 0 0 0 Fodder Crop 2267±246 2654±277 3036±405 1492±111 2142±200 1637±106 2274±24 2754±31 2862±32 Weed 38+4 53±7 53±6 36+3 42±5 49±5 0 0 0 Other Crop 0 0 0 0 0 0 0 0 0 above ground Weed l59bo 174±29 311±47 249±39 198±22 259±27 27±5 39±3 33±5 Total 3770±444 4721±514 5478±627 2282±183 3081±267 2531±168 2370±30 2894±34 2980±37 above ground Below Crop 199±25 114±18 119±25 64±7 94±10 73±9 164±18 196±25 21O±29 ground Weed 27±2 21±2 25B 21±2 18±2 23±2 9+.65 11+.91 10+1 Total 225±27 135±20 144±27 85±9 113±11 96±11 173±18.~ 207±26 220BO below ground Total C 3995 4856 5622 2367 3194 2627 2542 3101 3200 Soil C 0-15 cm 1521±46 2465±75 2979±176-3612±83-2655±69-2327±96-1333±40-1905±45-953±41 15-30 em 2091±61 1328±68-559±20-1711±49-2655±99-931±29-1334±31-1905±43-1143±53 Total C 3612±106 3793±143 2420±195-5323±132-5310±169-3258±125-2667±72-381O±88-2096±94 soil carbon (0-30) em Total C 7607 8649 8042-2956 -2116-631 -125-709 1104 (Vegetation + Soil)
Table 5.2: Carbon pools in plant biomass and net change in soil carbon pool (kg/ha) Barseem Uni Tui Grain 0 0 Crop 3870 ± 91.26 4315 ± 108.36 Fodder 0 0 Weed Crop 0 0 Other above ground 174 ± 27.84 260.93 ± 35.52 Weed Total above ground 4044 ± 119.1 0 4576 ± 143.88 Crop 235 ± 35.52 260 ± 46.56 Below ground 11.52 ± 1.43 16.8± 1.17 Weed Total below ground 247 ± 36.95 277 ± 47.72 0-15cm. 572 ± 25.18 381± 8.76 Soil C 953 ± 31.08 572 ± 12.07 15-30cm. Total soil C (0-30cm.) 1525 ± 56.26 953 ± 20.83 Total C ( vegetation+soil ) 5815 5806 Pearl millet Uni Cai 0 0 2117±74.28 2514 ± 86.52 0 0 0 0 88.56 ± 23.52 143 ± 16.8 2205 ± 97.8 2657 ± 103.32 148 ± 15.36 22.08 7.82 ±.62 11.76± 1.81 156 ± 15.98 178 ± 23.89-1334 ± 29.65-572 ± 17.28-1524 ± 61-572 ± 17.94-2858 ± 90.65-1144 ± 35.22-497 1691.74 Where, Uni - unirrigated Cai - Canal irrigated Tui - Tubewell irrigated
Table 5.3 : Carbon pool in plant biomass and net change in soil carbon poole kglha) Mustard Pie:eon pea Uni Uni Grain 687 ± 50.4 738 ± 51.6 1573 ± 127.68 2161 ± 152.1 Crop Fodder 53.76 ± 3.79 51 ± 2.94 Weed.. Crop Other above ground 0 0 139 ± 23.52 147 ± 22.8. Weed Total above ground 2453 ± 205.39 3097 ± 229.5 Crop Below ground 145 ± 21.36 186± 20.4 9.12 ± 0.66 11.52 ± 1.03 Weed Total below ground 154± 22 197 ± 21.43 Total C 2607 3294 0-15cm. 1524 ± 42.44-1905 ± 65.53 Soil C." -953 ± 43.80-952 ± 59.73 15-30cm. Total soil C (0-30cm.) 571 ± 86.24-2857 ± 241.125 Total C(V egetation+soil) 3178 437.48 Where, Uni - unirrigated
Table 5.4 : Carbon flows (kglha) through human food, livestock feed, burning and recycling in crops grown in Un irrigated (Uni ), Canal irrigated ( Cai ) and Tubewell irrigated (Tui ) land Wheat Rice Sorghum Uni Cai Tui Uni Cai Tui Uni Cai Tui Human food 1305±164 1841 ± 201 2078± 169 505 ± 30 699 ±40 586 ± 30 0 0 0 Livestock feed Crop by 1881 2202 2520 1238 1358 1887 2286 2375 Product. Weed 38±4 53 ± 7 53 ±6 36± 3 42± 5 45 ± 5 0 0 0 Total 1919 2255 2573 1274 1820 1407 1887 2285 2375 Bumt Crop 272 318 364 179 257 196 273 330 343 Weed 159 ± 30 174 ± 29 311 ± 47 249 ± 39 198 ± 22 259 ± 27 27±5 39± 3 33 ± 5 Total 431 492 675 428 455 455 299 369 376 Recycled/left in site Crop 312 246 270 138 201 154 277 334 353 Weed 27 ±2 21 ± 2 15 ± 3 21 ± 2 18 ± 1 23 ± 2 9±.65 11 ±.91 10 ± I Total 338 268 296 160 220 178 287 345 36;3
Table 5.5 : Carbon flows (kg/ha) through human food, livestock feed, burning and recycling in crops grown in Un irrigated ( Uni ), Canal irrigated (Cai ) and Tubewell irrigated (Tui ) condition. Barseem Pearl millet Mustard Pigeon pea Uni Tui Uni Cai Uni Uni Human food 0 0 0 0 687 ± 50 738 ± 52 Livestock feed Crop by product 3676 4099 1757 2087 522.27 168.49 Weed 0 0 0 0 54±4 51± 3 Total 3676 4099 1757 2087 576 219.11 Crop 0 0 254 302 972 1884 Weed 174 ± 28 261 ± 36 89±24 143 ± 17 139 ±24 147 ± 23 Total 174 261 343 445 328 406 Crop 429 476 254 292 224 294 Weed 12 ± 1 17 ± 1 8 ±.62 12 ± 2 9 ±.66 12± 1 440 493 262 304 233 305 Total
Plate No.7. Crop residues are burnt before ploughing; a bum patch is seen in the photograph: repeated burning leads to almost complete elimination of abovegroulld residues left in the field after harvesting. Plate No.8. Salt accumulation in a Canal irrigated field.