Data for: Nitrous oxide emissions from groundnut and millets farms in semi-arid peninsular India
Data files
Dec 16, 2022 version files 368.65 KB
Abstract
Nitrous oxide (N2O) emissions response curves for crops grown outside temperate regions have been rare and have thus far arrived at conflicting conclusions. Most studies reporting N2O emissions from tropical cropping systems have examined only one or two nitrogen fertilizer application rate(s) which precludes the possibility of discovering nonlinear changes in emission factors (EF, % of added N converted to N2O-N) with increasing fertilizer-N rates. To examine the relationship between N rates and N2O fluxes in a tropical region, we compared farming practices with three or four N rates for their yield-scaled impacts from three crops in peninsular India. We measured N2O fluxes during nine seasons between 2012 and 2015, with N application rates ranging between 0 and 70, 0 and 90, and 0 and 480 kg-N ha-1 for foxtail-millet (Setaria italica L., locally called korra), groundnut (Arachis hypogaea L., also called peanut) and finger-millet (Eleusine coracana L., locally called ragi), respectively. In two cases, the highest N application rate greatly exceeded crop-N needs. Potential climate smart farming agricultural practices (with low/optimized N rates) led to a 50-150% reduction in N2O emissions intensity (per unit yield) along with a reduction of 0.2-0.75 tCO2e ha-1 season-1 as compared to high N conventional applications. We found a non-linear increase in N2O flux in response to increasing applied N for both N-fixing and non N-fixing crops and the extent of super-linearity for non N-fixing crops was much higher than what has been reported earlier. If a linear fit is imposed on our datasets, the emission factors (EFs) for finger-millet and groundnut were ~3.5% and ~1.8%, respectively. Our data shows that for low-N tropical cropping systems, even when they have low soil carbon content, increase in N use to levels just above crop needs to enhance productivity might lead to relatively small increase in N2O emissions as compared to the impact of equivalent changes in fertilizer-N use in systems fertilized far beyond crop N needs.
Methods
The five study farms were in the Indian states of Karnataka and Andhra Pradesh. Emissions during cultivation of Groundnut (peanut), Foxtail- and Finger- millet were studied at two, one and two farms, respectively. The measurement of GHG emissions, yield and other agro-economic indicators was performed for a total of nine seasons at three regional laboratories established by a coalition of partners interested in promoting climate smart farming in agro-ecological regions 8.2 and 3.0 of the semi-arid peninsula of India.
Soil characteristics and weather conditions
Each of the five experimental sites was a farmer owned and managed small-holder plot and was located in peninsular India between 12.77-14.66 N (Latitude), 77.20-77.75 E (Longitude) and 350-790 m (elevation above sea level). The experimental sites had sandy-loam and loamy-sand texture (680-750 g kg-1 Sand, 120-170 g kg-1 Silt and 130-170 g kg-1 Clay) and soil organic matter concentration varying between 3.2 and 14.3 g kg-1 (i.e., between 1.9 and 8.3 g kg-1 soil C). Except in the case of foxtail millet (which was a newly cultivated site), the groundnut and finger millet plots were under continuous groundnut or finger-millet systems, respectively, for over a decade before establishment of our experiments. The soil characteristics of each site are given in S1 Table.
The climate of all study locations was semi-arid with measured seasonal rainfall varying from 56-480 mm during the experimental period. The lowest and the highest temperatures observed at our sites varied from 10-21 and 33-40 °C, respectively (see S1 Table for details of each site). All experimental sites were between 0.1 and 0.42 ha in size and the experimental treatments were implemented by the farmer under supervision of a trained field and laboratory research team. There were three replicates for each treatment and each subplot received one treatment with stratified randomized block design.
Nitrous oxide emissions were measured for both finger-millet and groundnut during four cropping seasons each, along with some fallow periods flanking these growing seasons between July 2012 and December 2015. Groundnut was sown between July 10-September 4 and harvested between November 3-December 25. Finger-millet was sown between August 3-August 25 and harvested between November 25-January 1. Due to severe drought and other complications, N2O emissions data from the foxtail-millet farm could be collected only for one season between October 12, 2014 and January 19, 2015. The data from two groundnut growing seasons (dry kharif and irrigated rabi in 2012) was published earlier (Kritee et al, 2015) and is presented here with new estimates of mineralized organic nitrogen which impacted the calculation of EFs. During the fallow periods, there were no inputs of water or fertilizer to the experimental sites, except to prepare for the upcoming cropping season.
Treatments
We compared N2O emissions from three or four broad categories of treatments: Very-high-N (VHN, conventional practices with N rates varying from 91 to 276 kg N ha-1), High-N (HN, conventional practices identified via our local farmer surveys with total N rate varying from 53 to 248 kg N ha-1; see S3 Table for farmer survey results), Low-N (LN, farm-specific potential climate-smart farming practices including completely organic practices for groundnut farms, total N varying from 17-78 kg N ha-1) and a zero N (control). We explored changes in N2O emissions with changing N fertilizer inputs under scenarios where water input was either below or above water requirements for groundnut (>280 mm) and finger-millet (>450 mm). The dry sites for groundnut had water input between 100-200 mm in the rainfed season (locally called kharif) whereas the wet site had a water input of 370 mm (irrigated winter season locally called rabi). The dry and wet rainfed sites for finger-millet had water inputs between 100-350 mm and ~480 mm, respectively.
The Low-N treatment (Table 1 and S3-S4 Tables) represented farm-specific “alternate” practices that were investigated for their potential to deliver similar (or higher) yields and economic benefits to farmers as well as lower climate impacts. The potential climate-smart farming practices investigated for foxtail-millet and groundnut farms in agro-ecological region (AER) 3.0 involved completely organic (with no synthetic) inputs.
Except in the case of finger-millet, the High-N treatment represents the conventional “business-as-usual” crop management practices as currently implemented by farmers with average to large land-holdings in this region. The conventional practices were identified via regional farmer surveys conducted during the study. The recommended inorganic N use for groundnut, finger- and foxtail- millet is 20-30, 50, and 30 kg N ha-1, respectively. Farmer surveys conducted during this study or by the Indian government indicated that farmers were using much higher fertilizer N application rates than the crop-specific recommendations by the state/district governments and/or academic institutions. Please see S3 Table for comparison of survey results with “High N” treatments.
The Very-High-N treatments for finger-millet and groundnut included addition of nitrogen fertilizers much higher than the respective crop’s nitrogen needs. These treatments were included specifically to test the extent of super-linear response in N2O emissions when N inputs are very high.
Overall, the N fertilization rates for groundnut, finger-millet and foxtail millet varied from 0 to 77, 0 to 470 and 0 to 49 kg N ha-1, respectively The rate and timing of all organic and inorganic fertilizer applications are provided in S2 Table and total N rate (including contribution from mineralized organic N) for each treatment is presented in Table 1.
In general, the soils in the two agro-ecological regions are not amenable to cultivation without ploughing. For groundnut and foxtail-millet, tillage was done once in each season about 25 days before sowing. For finger-millet, tillage was done 2-4 times between March and July soon after rainfall depending on soil hardness and manure (if any) was incorporated during the last 1-2 tillage events. Bullock cart ploughing tills soil to a depth of 12 cm and local tractors (used only when the soil is very hard) plough to the depth of up to 18 cm. There was no tillage done to control weeds and there was no use of herbicides and pesticides.
During the rainfed south-west monsoon season (from July to December; locally called kharif), sowing was done manually at a seed rate 146 ± 27 kg ha-1 for groundnut (Kadiri 6 variety) at a 30 cm row spacing, 10 cm plant spacing and to a depth of 5 cm, 12 kg ha-1 for foxtail millet (local variety called Jadda Korra) at a 30 cm row spacing, 8-12 cm plant spacing and to a depth of 3-6 cm and 24.7 kg ha-1 for finger-millet (MR1 variety) at a 25 row spacing to a depth of 3-6 cm. Both millets are sown with a seed drill attached to a bullock and the plots are thinned/weeded 12-20 and 20-25 days after sowing of finger- and foxtail-millet, respectively. The seed rates used in a given crop and season were the same for all treatments. All of the aboveground biomass (as well as belowground biomass for groundnut) was harvested manually 110-130 days after sowing (see exact dates in S1 Table).
N2O flux monitoring
Manual closed chambers were used to collect air samples from each of the three replicate treatment plots and the air samples were analyzed by electron capture detector (ECD) in a gas chromatograph (Thermo Fisher Trace GC 600) to quantify N2O emissions rates based on methodology developed in our labs. Because most N2O emissions occur within 1-4 days following N addition and/or irrigation/rainfall, N2O flux measurements are more reliable when the sampling frequency is high and the sampling schedule captures spatio-temporal variability in emissions. We performed sampling on 34-60% of the total days in each season (S1 Table), with continuous sampling for 3-5 days after all “events” e.g. sowing, fertilizer application, irrigation/rainfall and weeding with an average of three measurements every week. Stackable manual chambers (size: 30*30*40 cm) were deployed on base-frames. A second 30*30*40 cm chamber was stacked on top of the first chamber if (and only when) the plant height exceeded 40 cm (Fig 2). On each sampling day, four air samples (60 ml each) were collected at 10 minute intervals for 30 minutes between 10 AM and 12 noon to calculate the hourly N2O flux. The GC was calibrated daily with four standards: 0.197, 0.393, 0.795 and 1.615 ppmv N2O (Bhuruka Gases, Bengaluru; NIST certified at 2% RSD). For chamber deployment period of half hour, four sampling points under linear regression, minimum detectable N2O flux was 33.8 ppb which translates into ~20 µg m-2 h-1 for our chambers with a volume of ~36 L, ambient temperatures in the range of 35-45°C and baseframe footprint of 0.09m2. Following the recommendation of Parkin and Venterea (2010), we used the actual measured value even if it falls below the minimum detection limit (MDL). The details of the design of chambers and base-frames, methods employed to achieve uniform mixing of headspace air, chamber volume and temperature corrections, sample storage, data analysis, treatment of negative emissions, calculation of seasonal fluxes and curve fitting have been described previously. Cumulative emissions were calculated separately for each replicate plot before calculating the average emissions for each treatment. For a given farm, when available, the results from different years were averaged for treatments with similar N input rates (Table 1 and S2 Table) to perform multiple regression. The cumulative N2O flux during a cropping season was calculated by linear interpolation as explained earlier.
Crop yield, yield scaled GHG flux and N mineralization rates
Yields were measured from each treatment at maturity at the end of a season after separating groundnut pods or millet grains from the plant/straw and sun drying to a constant weight. It is customary in this region to use crop residue to feed cattle and not return it to the plots. Yield scaled GHG flux (i.e., GHGI) for each treatment was calculated by converting average N2O flux into CO2e after multiplying by 298 (Global warming potential of N2O) and dividing by average yield for that treatment. The errors (SE) associated with GHGI were calculated by standard error propagation method. Mineralization rates for the applied organic nitrogen were estimated using methods described earlier. For more details, please look at the submitted pre-print.
Usage notes
No specific programs or softwares are needed to open the data files. The preprint provided with this submission will be available as a technical white paper on the website of Environmenetal Defense Fund.