DOI: 10.5061/dryad.cfxpnvx5r
title: Data for: Nitrous oxide emissions from groundnut and millets farms in semi-arid peninsular India

description: 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.
description: 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.
 <strong>Soil characteristics and weather
 conditions</strong> 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<sup>-1</sup> Sand, 120-170 g kg<sup>-1</sup>
 Silt and 130-170 g kg<sup>-1</sup> 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,
 N<sub>2</sub>O 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
 <em>kharif</em> and irrigated <em>rabi</em> 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.
 <strong>Treatments</strong> We
 compared N<sub>2</sub>O 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<sup>-1</sup>), High-N
 (HN, conventional practices identified via our local farmer surveys with
 total N rate varying from 53 to 248 kg N ha<sup>-1</sup>; 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<sup>-1</sup>) and a zero N (control). We explored changes
 in N<sub>2</sub>O emissions with changing N fertilizer inputs
 under scenarios where water input was either below or above water
 requirements for groundnut (&gt;280 mm) and finger-millet (&gt;450
 mm).  The dry sites for groundnut had water input between 100-200 mm in
 the rainfed season (locally called <em>kharif</em>) whereas
 the wet site had a water input of 370 mm (irrigated winter season locally
 called <em>rabi</em>).  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<sup>-1</sup>, 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
 N<sub>2</sub>O 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<sup>-1</sup>, 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
 <em>kharif</em>), sowing was done manually at a seed rate 146
 ± 27 kg ha<sup>-1</sup> 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<sup>-1</sup> for foxtail millet (local variety called
 <em>Jadda Korra</em>) at a 30 cm row spacing, 8-12 cm plant
 spacing and to a depth of 3-6 cm and 24.7 kg ha<sup>-1</sup>
 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).
 <strong>N<sub>2</sub>O flux monitoring
 </strong> 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
 N<sub>2</sub>O emissions rates based on methodology developed
 in our labs. Because most N<sub>2</sub>O emissions occur
 within 1-4 days following N addition and/or irrigation/rainfall,
 N<sub>2</sub>O 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 N<sub>2</sub>O flux. The GC was calibrated daily with
 four standards: 0.197, 0.393, 0.795 and 1.615 ppmv
 N<sub>2</sub>O (Bhuruka Gases, Bengaluru; NIST certified at 2%
 RSD).  For chamber deployment period of half hour, four sampling points
 under linear regression, minimum detectable N<sub>2</sub>O
 flux was 33.8 ppb which translates into ~20 µg m<sup>-2</sup>
 h<sup>-1</sup> for our chambers with a volume of ~36 L,
 ambient temperatures in the range of 35-45°C and baseframe footprint of
 0.09m<sup>2</sup>. 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 N<sub>2</sub>O flux during a cropping season was
 calculated by linear interpolation as explained earlier.
 <strong>Crop yield, yield scaled GHG flux and N
 mineralization rates</strong>  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
 N<sub>2</sub>O flux into CO<sub>2</sub>e after
 multiplying by 298 (Global warming potential of
 N<sub>2</sub>O) 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.
description: 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.