Carbon sequestration and credit potential of gamhar (Gmelina arborea Roxb.) based agroforestry system for zero carbon emission of India

The agroforestry system is the best option to achieve the net zero carbon emissions target for India. Keeping this view, carbon sequestration and credit potential of gamhar based agroforestry system has been assessed. The experiment was carried out in randomized block design in seven different treatments with five replications. Gamhar tree biomass accumulation was higher in gamhar based agroforestry system compared to sole gamhar. Among different tree components, stem contributed a maximum to total gamhar tree biomass followed by roots, leaves and branches. The average contributions of stems, roots, leaves and branches in total tree biomass in two annual cycles (2016–17 and 2017–18) varied between 50 and 60, 19.8 and 20, 19.2 and 20, and 10.7 and 12.7 percent, respectively. In case of crops, above ground, below ground and total biomass was significantly higher in sole intercrops than gamhar based agroforestry system. Total (Tree + interrops + Soil) carbon stock, carbon sequestration, carbon credit and carbon price were significantly affected by treatments, and was maximum in Sole Greengram-Mustard. Net carbon emission was also recorded lowest in Sole Greengram-Mustard for which the values were 811.55% and 725.24% and 760.69% lower than Sole Gamhar in 2016–17, 2017–18 and in pooled data, respectively.


Estimation of total (Tree + intercrops + Soil) carbon stock, carbon sequestration, carbon credit and carbon price
Total (Tree + intercrops + Soil) carbon stock, carbon sequestration, carbon credit and carbon price of sole crops and gamhar based agroforestry system were significantly affected by treatments (

Estimation of total carbon emission, carbon sequestration and net emission
The annual total (Tree + interrops + Soil) carbon emission, carbon sequestration and net emission in tonnes per hectare (t ha -1 ) has been presented in the Fig. 1.Total (Tree + interrops + Soil) carbon sequestration and net emission of sole intercrops and gamhar based agroforesrty system were significantly affected by treatments.

Discussion
Total (tree + interrops + soil) carbon stock, carbon sequestration, carbon credit and carbon price of sole intercrops and gamhar based agroforestry system were significantly affected by the treatments.Carbon stock, carbon sequestration, carbon credit as well as carbon price were maximum in Sole Greengram-Mustard.All parameters were higher in sole intercrops than gamhar based agroforestry system because gamhar plants were very small (five months old) at the time of planting and the total dry biomass production was lower in trees than in intercrops.The height, diameter, basal area and volume of Gamhar after completing 12 years was lower under sole plantation compared to agri-silvicultural system 16 .Similar results have also been confirmed by 17 observed that the cash flow analysis of the carbon trading neutral products reported substantial initial investments during the first 3 years of the project, while benefits are obtained after completing 4 years.Kumar et al. 18 reported that the total tree biomass (3.707 t ha −1 ), carbon stock (1.597t ha −1 ), carbon sequestration (5.862 t ha −1 ), carbon credit (5.86) and carbon price ($ 103.76) respectively were estimated at the age of two years of poplar tree-based agroforestry system.Tamang et al. 19 studied the carbon sequestration potential of gamhar (Gmelina arborea) being
The above ground estimated carbon stock of Gmelina arborea approximately 13 Mg ha −1 or 47 Mg CO 2 e ha −1 in 9 months, making it a valuable and promising species for CO 2 sequestration under the context of climate change 21 .Similar results and reasons have been also confirmed by 22,23 .
But several researchers have found that in long term basis agroforestry systems contains very high potential for enhancing total biomass, carbon sequestration, carbon credit, carbon trading, support local peoples, and strengthen the climate resilience in agriculture compared to sole cropping system.Because agroforestry have more carbon sequestration potential and lower carbon emission through trees, intercrops and soil; similar results have also been confirmed by Orwa et al. 24 reported significantly lower net carbon emission ha -1 in agroforestry system (− 40.998 t ha −1 ) as compared to open farming (− 37.263 t ha −1 ) despite higher emission (1.052 t ha −1 as compared to 0.998 t ha −1 in open farming) due to more carbon sequestered by trees in the agroforestry system (42.049t ha −1 as compared to 38.261 t ha −1 in open farming).Azeez et al. 25 reported that in mustard field, the CO 2 emission values ranging from 1.083 to 1.683 t C ha −1 were not significantly affected by the crop cultivation www.nature.com/scientificreports/treatment.According to 26 the average yearly GHG emissions ranged from 0.93 to 1.60 t CO 2 e ha −1 yr −1 , which may be considered low when compared to other systems, probably due to adoption of agroforestry systems with reduced fuel inputs, land practices, machinery use and CO 2 emissions.The common management practices in agroforestry systems, such as zero-tillage farming and optimal fertilizer/manure regimes can increase carbon sequestration while reducing carbon and other GHG emissions 27 .Hung et al. 28 also reported that on different types of agroforestry systems, the total greenhouse gas emissions were 7.98, 4.25, 4.04 and 2.80 t CO 2 e ha −1 .Similar results and reasons have been also confirmed by [29][30][31][32] .

Conclusion
This study concludes that the total (tree + intercrops + soil) above ground, below ground, total biomass production, carbon stock, carbon sequestration, carbon credit and carbon trading were found higher in the treatment Sole Greengram-Mustard as compared to all treatments.Whereas carbon emission was lower in tree components of all the system and kept on declining in the successive years.Gmahar is a fast-growing tree species, therefore agroforestry based on this tree have high potential of carbon sequestration and lower carbon emission in long term basis.It can be a tool to increase the tree coverage, reducing 1 billion tonnes of carbon emissions by 2030,

Site description and experimental setup
The research experiment was conducted during monsoon and winter season of 2016-17 and 2017-18 at the field experimental site close to Faculty of Forestry, Birsa Agricultural University, Kanke, Ranchi, Jharkhand, India.It is a national government institution.The research experimental site is located between 23°26′54.6′′N to 23°26′55.0′′N Latitude and 85°18′53.0′′E to 85°18′53.7′′E longitudes and at an altitude of 625 m above the mean sea level (MSL).It is the eastern section part of the Deccan plateau region and comes under the agro-climatic zone (Zone VII) of the India known as Eastern Plateau and Hill Region.The experimental site is shown in Fig. 2. The entire experiment field site was laid out as per plan of All India coordinate research project on agroforestry under ICAR, govt. of India.The planted gamhar tree (Gmelina arborea Roxb.) and four different intercrops are Arhar (Cajanus cajan), Cowpea (Vigna unguiculata), Greengram (Vigna radiata), Mustard (Brassica juncea) under gamhar based agroforestry system and in sole (open) conditions.The field experiment design adopted was randomized block design (RBD) with seven treatments and five replications.They were: T 1 : Gamhar + Arhar, T 2 : Gamhar + Cowpea-Mustard, T 3 : Gamhar + Greengram-Mustard, T 4 : Sole Gamhar, T 5 : Sole Arhar, T 6 : Sole Cowpea-Mustard, T 7 : Sole Greengram-Mustard.The experimental field plot size was 24 × 7.5 m 2 and nursery raised five months old quality seedlings of uniform size gamhar tree were transplanted in the field and collection of plant material, complies with relevant institutional, national, and international guidelines and legislation on June, 2016; in pits of 45 cm × 45 cm × 45 cm size at spacing of 8 m × 2.5 m (500 plants ha -1 ) and intercrops being cultivated during monsoon and winter season of 2016-2017 and 2017-2018.

Estimation of standing tree biomass
Above ground biomass of tree Gamhar (Gmelina arborea) trees were measured for their height from ground level to top of the trees and girth at collar diameter.The volume of stem was calculated with the help of girth and height.Standing tree volume of stem was calculated by the quarter girth formula: where, V ob , G and H represent volume of tree over bark, girth of tree and height of the tree, respectively.
The above ground stem biomass was calculated by the formula: Gamhar trees were then divided into individual components such as stem, branches and leaves.The branches were counted in the standing tree and detached ten different sizes of reference branches from the standing tree by random selection with the help of cutting scissors.These branches contained different sizes of leaves.The leaves were removed from branch.The fresh weights were determined for branches and their leaves by using a balance.The entire samples (branch and leaf) were packed in the bags and brought into laboratory for drying in (1) Total stem volume m 3 (V ob ) = (G/4) 2 × H (2) Biomass = volume × specific gravity of wood 530 kg m −3 for gamhar tree given by 33,34 .oven at 72 °C for 48 h.The oven dry weight of each sample was estimated.The dried weights of collected branches and leaves were used for estimation of standing biomass of tree.Total above ground biomass was computed by summing the biomass of stem, branch and leaves components.

Below ground biomass of tree
Below ground biomass contains the root of the gamhar tree.The below ground biomass was estimated by using a simple default value of 25 percent (for hardwood species) of the total above ground biomass as suggested by the 35 .

Total biomass of tree
The total biomass was estimated by adding biomass of all the components (above ground and below ground).

Estimation of intercrops biomass
Above ground biomass of intercrops Above ground biomass of intercrops (arhar, cowpea, greengram and mustard) were recorded for per meter square.All plants were uprooted from the ground level and divided into two components viz., above ground (stem, branches, leaves and pod or siliqua) and below ground (root).The separated components were oven dried at 70 ± 2 °C in an electric oven till constant weight.The oven dry weight of intercrop samples measured on digital pan balance.Dry intercrop yields the same as above ground biomass.

Below ground biomass of intercrops
Below ground biomass of intercrops (arhar, cowpea, greengram and mustard) were recorded per meter square from net plot at harvesting stages.Below ground biomass contains roots of the intercrops.Similarly, dry intercrop root yields the same as below ground biomass (root).

Total biomass of intercrops
(3) Below ground biomass = Above ground biomass × 0.25 (4)   Total biomass of tree = Above ground biomass (stem, branch and leaves) + Below ground biomass (root) (5)  Total biomass of intercrops = Above ground biomass stem, branches, leaves and pod or siliqua + Below ground biomass (root) www.nature.com/scientificreports/Total biomass (tree + intercrops) The total biomass was recorded on above ground biomass (tree + intercrops) and below ground (tree + intercrops) of gamhar and intercrops (arhar, cowpea, green gram and mustard) under sole cropping system and gamhar based agroforestry system.

Estimation of total carbon stock
Carbon stock in tree Carbon stock was derived from above ground and below ground biomass by assuming that nearly 50% of the biomass is made up by carbon [35][36][37] .So, the carbon stock for tree was determined by multiplying total biomass (above ground + below ground) with carbon conversion factor of 0.50

Carbon stock in intercrops
The carbon stock in herbs and shrub species was determined by multiplying total biomass (above ground + below ground biomass) with carbon conversion factor of 0.45 38,39 .

Carbon stock in soil
According to 40 the capacity of carbon storage in soil is higher than vegetation and atmosphere; and giving it a play major role in global carbon sequestration 41 .The carbon stock in soil was calculated by the formula as follows 42,43 .

Total carbon stock
The total carbon stock was estimated by adding of all the components (tree + intercrops + soil).

Estimation of total carbon sequestration
Carbon sequestration is the procedure of capturing and storing of atmospheric carbon dioxide by the plant.Total carbon sequestration was obtained by addition of carbon sequestrated by total carbon stock (total tree carbon stock + total intercrops carbon stock + total soil carbon stock).The estimated total carbon stocks were converted into carbon sequestration, multiplied by 44/12 or 3.666 44 .

Estimation of total carbon credit
The total carbon credit or certified emission reduction (CER) is the reduction/sequestration of one tonnes of atmospheric carbon emission.The one tonnes of sequestered carbon dioxide in the form of plant biomass is equal to one carbon credit or CER 18 .So, total carbon credits of gamhar based agroforestry system were estimated from the carbon equivalent values of retained total tree and intercrops biomass (Supplementary Material).

Carbon trading
The price of carbon credit was found very variable among different countries, so price taken from the international market.The price of one carbon credit or CER in Indian Rupees is about ₹ 1500 or $ 20 45,46 .

Carbon emission
The emission of carbon (CO 2 -e) was calculated by the software "Green House Gases Estimation Tool for Integrated Farming System Models" developed by the ICAR-Indian Institute for Farming System Research, Modipuram, Meerut, Uttar Pradesh, India All the inputs used during research period, viz.energy/fuel used for different practices of field preparation, application of water for irrigation, fertilizers, organic manures, herbicide, pesticide and any other farm machinery used for harvest of intercrops, etc. were in use into account as for the estimation of carbon emission.The net carbon emission was then estimated by subtracting the total carbon sequestered in the research field from total carbon emission from the experimental field.

Statistical analysis
The obtained data was analyzed using standard statistical procedure for Randomized Block Design (RBD) with the help of computer applying IBM-SPSS statistical analysis of variance (ANOVA) technique.Standard error of (6) Total biomass tree + intercrops = Above ground biomass tree + intercrops + Below ground biomass tree + intercrops (7) Carbon stock of tree = Total dry biomass × 0.50 (8)  Carbon stockof intercrops = Total dry biomass × 0.45 (9)   Soil carbon stock t ha −1 = Soil organic carbon % × Soil sampling depth (cm) × Bulk density g cm −3 (10)   Total carbon stock = Total tree carbon stock + Total intercrops carbon stock + Total soil carbon stock (11)  Carbon sequestration = Total carbon stock × 3.666

Figure 1 .
Figure 1.Carbon emission, carbon sequestration and net emission of sole intercrops and gamhar based agroforestry system.

Figure 2 .
Figure 2. Location of the experimental site.

Table 1 .
Stem, branch, leaf and root biomass of gamhar tree under gamhar based agroforestry system.*Sole Arhar, Cowpea-Mustard and Greengram-Mustard not included in statistical analysis.
Carbon emission was maximum in Gamhar + Cowpea-Mustard and Gamhar + Greengram-Mustard, (both being same because of same inputs used), which was 1284.21,2218.18 and 1626.67 percent higher than Sole Gamhar in 2016-17, 2017-18 and in pooled data, respectively.In case of carbon sequestration, maximum was observed in Sole Green gram-Mustard which was 817.09, 733.69 and 768.11 percent higher than in Sole Gamhar 2016-17, 2017-18 and in pooled data, respectively.Net emission was recorded 811.55, 725.24 and 760.69 percent lower than Sole Gamhar 2016-17, 2017-18 and in pooled data, respectively.The negative value of the data indicates that carbon sequestration was more than carbon emission.

Table 2 .
Above ground, below ground and total biomass of gamhar tree under gamhar based agroforestry system.*Sole Arhar, Cowpea-Mustard and Greengram-Mustard not included in statistical analysis.

Table 3 .
Carbon credit and Carbon price of gamhar tree under gamhar based agroforestry system.*Sole Arhar, Cowpea-Mustard and Greengram-Mustard not included in statistical analysis.

Table 4 .
Above ground, below ground, total biomass and carbon stock of intercropsunder gamhar based agroforestry system.*Sole Gamhar not included in statistical analysis.

Table 5 .
Carbon sequestration, carbon credit and carbon price of intercrops under gamhar based agroforestry system.*Sole Gamhar not included in statistical analysis.

Table 6 .
Carbon stock, carbon sequestration, carbon credit and carbon price of soil profile depths 0-30 cm under gamhar based agroforestry system.

Table 7 .
Total carbon stock, carbon sequestration, carbon credit and carbon price of sole intercrops and gamhar based agroforestry system.