Nitrous oxide emission in response to N application in irrigated sugarcane

Emissão de óxido nitroso em resposta à aplicação de N em cana-de-açúcar irrigada R E S U M O Objetivou-se neste estudo comparar as emissões de óxido nitroso (N2O) decorrentes da aplicação de doses de nitrogênio (N) e potássio (K) em cana-de-açúcar fertirrigada, comparando-as às emissões provenientes da adubação convencional. O estudo foi conduzido na área experimental da Embrapa Meio Norte, Teresina, PI, de agosto de 2014 a janeiro de 2015. O delineamento utilizado foi o de blocos ao acaso, analisado em esquema fatorial (2 x 2) +1, com quatro repetições. Os tratamentos consistiram na combinação de duas doses de N e K2O (60-120 e 120-180 kg ha -1, respectivamente) e duas formas de aplicação (via solo e fertirrigação) e uma testemunha. O uso da fertirrigação reduziu as emissões de N2O em cana-de-açúcar em comparação ao cultivo fertilizado de forma convencional. O aumento da dose de N de 60 para 120 kg ha-1 aplicado via fertirrigação não afetou as emissões de N2O, enquanto que a aplicação de 60 e 120 kg ha -1 via solo proporcionou aumento de 40,6 e 50,2% nas emissões de N2O, respectivamente. A aplicação de 60 e 120 kg ha-1 de N via solo obtiveram maior fator de emissão de N2O, sendo superior em 1,39 e 2,08% ao registrado no cultivo fertirrigado com 60 e 120 kg ha-1 de N, respectivamente.


Introduction
Nitrous oxide (N 2 O) is an important greenhouse gas (GHG).Despite its low concentration in the atmosphere, 324 ppb (IPCC, 2013), it stands out for permanence time (approximately 114 years) and high global warming potential (GWP).The GWP of the N 2 O is 298 times greater than that of CO 2 .In Brazil, it is estimated that 93% of the N 2 O released into the atmosphere every year comes from agricultural activity (MCTI, 2013).
The amount of N 2 O emitted by the use of nitrogen fertilizers, according to the estimate of IPCC (2006), is 1% of the N applied (variation from 0.03 to 3%).However, in practice, different amounts of N 2 O are emitted, depending on fertilizer, adopted management, type of soil, and environmental conditions.Crutzen et al. (2008) have questioned the methodology adopted and the values proposed by the IPCC, claiming that they underestimate N 2 O emissions.These authors proposed that the amount of N 2 O for nitrogen fertilizers in agricultural systems should be 3 to 5% of the N applied.
Sugarcane is a crop of great importance in global agriculture, due to the demand for renewable fuels such as ethanol, which is less polluting and less costly than fossil fuels (Oliveira et al., 2016).Because of that, it is important to conduct research aiming to reduce the environmental impacts caused by the use of nitrogen fertilizers throughout crop cycle, seeking management practices which aim to reduce N 2 O emissions caused by the use of these fertilizers.
Several studies have evaluated the effect of N doses on N 2 O emissions in sugarcane (Barbosa, 2014;Carmo et al., 2013;Signor et al., 2013).However, there are no studies in Brazil on N 2 O emission associated with fertigated sugarcane.Therefore, the present study was carried out to compare N 2 O emission resulting from the application of N doses in sugarcane under subsurface drip fertigation with emissions from conventional fertilization (in the soil).

Material and Methods
The experiment was carried out in an experimental area of Embrapa Mid-North, cultivated with sugarcane, variety RB 92579, in the plant-cane crop, in Teresina, PI, Brazil (5° 05' S; 42° 29' W; 72 m).According to Köppen's classification, the local climate is Aw.Average annual temperature is 28.2 °C and average annual rainfall is 1,343.4mm, and the rains in this region are susceptible to large oscillations (Bastos & Andrade Júnior, 2014).The soil of the area is a dystrophic Red Yellow Argisol, with sandy loam texture (Melo et al., 2014).
Prior to sugarcane planting, the area had been cultivated with jatropha for seven years.After jatropha was removed, this experiment began, soil samples were collected for chemical analysis (Silva, 2009) (Table 1), and 2 Mg ha -1 of limestone (RNV = 90%) were applied.Soil tillage consisted of plowing, harrowing and opening of planting furrows.Sugarcane was manually planted in June 2014, at 0.3 m depth, with six cuttings, containing three buds each, per linear meter.Plots consisted of three 10-m-long double rows at spacing of 1.5 x 0.5 x 2.0 m.
Fertilizations with other nutrients were uniform in all plots.Phosphorus (P 2 O 5 ) was applied in the amount of 100 kg ha -1 , as follows: 30% at planting (TSP -triple superphosphate) and 70% by fertigation (monoammonium phosphate), in monthly applications.In treatments with soil fertilization, P was entirely applied at planting, as TSP.The micronutrients B, Zn, Mn, Cu and Mo were applied by fertigation, split into six applications in all treatments, monthly performed.
In fertigated treatments, N and K 2 O applications were split into 24 portions along six months of the crop cycle (Table 2), with 7 day interval between applications, and the first one was carried out 60 days after planting (DAP).In treatments with soil fertilization, N and K 2 O fertilizations were conventionally applied in two steps, 50% at 68 DAP and 50% at 144 DAT (Andrade Júnior et al., 2012).
N and K 2 O were simultaneously applied, using urea as N source and white potassium chloride as K 2 O source.Urea and potassium chloride were diluted in a 50 L tank and injected Table 1.Chemical characteristics of the soil before the experiment *30% of K 2 O dose was applied at planting for treatment of fertilization in soil and by fertigation Table 2. Splitting of fertigation and its respective proportional doses into the irrigation system by a positive displacement hydraulic pump (TBM pump).
In treatments with soil fertilization, urea and potassium chloride were homogeneously applied in furrows (0.05 m deep) opened on the sides of the planting rows, which were subsequently closed.After that, to ensure soil moistening, 2 L of water were applied per meter of furrow.This measure was adopted because we believed the wet bulb, formed from the subsurface drip system, would not reach the fertilizer furrows, which were situated beside the double row, since the irrigation line was located in the center of the double row.
Irrigation depth was uniform and applied based on reference evapotranspiration (ETo), estimated by the Penman-Monteith method and crop coefficients (Kc) for sugarcane, determined in the region (Andrade Júnior et al., 2017), on a daily basis.Regarding the application frequency, irrigation was performed on the following days of the week: Mondays, Wednesdays and Fridays.A subsurface drip irrigation system was used (85% efficiency), with 2 m between drip lines (drippers spaced by 0.60 m, flow rate of 2.3 L h -1 , 200 kPa pressure), buried at 0.25 m depth, in the center of the double rows.
N 2 O fluxes were quantified by the static closed-chamber method.The chambers consisted of one metal base partially buried in the soil (0.05 m deep), installed at the beginning of the experiment and kept fixed until the final period of evaluation, and one PVC base, which was fitted onto the other during sample collection, containing an upper hole to attach the syringe used in the collections.To quantify the fluxes on the collection day, three samples of N 2 O were collected per chamber: immediately after the chamber was closed (time zero); at 10 min (time 10) and at 30 min (time 30).The samples were collected in the morning (between 7 h 30 min and 10 h a.m.), stored in hermetically sealed flasks and sent to analysis.N 2 O concentration was determined using a gas chromatograph (Trace TM 1310 GC) with ECD 1 detector at 350 °C (nitrogen at 20 mL min -1 as make-up gas), and helium as carrier gas.
In fertigated and control treatments, the chamber base was installed in the center of the experimental plot, between the double rows, above the drip line.In treatments of conventional fertilization, the base was installed in the center of the experimental plot, in the interrows, above the fertilizer furrow.N 2 O emissions were estimated using one static chamber per replicate, totaling 20 chambers in the test.
In fertigated treatments, N 2 O emission evaluations were carried out at 83, 104, 146, 186 and 230 DAP, one day after fertigation, in a total of 5 evaluations.In treatments of conventional fertilization, the collections were made at 69, 70, 73 and 145, 146 and 147 DAP to quantify the emissions resulting from the 1 st and 2 nd top-dressing fertilizations, respectively, totaling 6 evaluations.
Variations of N 2 O concentrations in the samples as a function of the time after chamber closing (0, 10 and 30 min), associated with the data of chamber volume, chamber area and soil temperature, were used to calculate the N 2 O flux in µg m -2 h -1 (Jantalia et al., 2008) shown in Eq.1.
where: F -N 2 O flux; ∆c/∆t -change in gas concentration inside the chamber within the time interval during the collection; V -chamber volume, L; A -soil area covered by the chamber, m²; M -molecular mass of N atoms in the N 2 O molecule; and, V m -molar volume of the gas at temperature of sampling (22.4 µL µmol -1 , under normal temperature and pressure -NTP).
During the collection period, soil temperature and moisture were monitored.Soil moisture was obtained by gravimetric method using soil samples (0-0.1 m) collected beside the chamber during the collection.Soil temperature was measured using a thermometer, which was inserted at 0.10 m depth during the collection.
Daily N 2 O fluxes were entered in an electronic worksheet.After that, the emission accumulated in the period was obtained by trapezoid integration of the daily fluxes over time.Cumulative N 2 O emission was used to calculate the emission factors (EF) for each N dose applied.This calculation was carried out following the methodology proposed by IPCC (2006), Eq. 2.
where: EF x -N 2 O emission factor for each N dose applied, %; E x -total N 2 O flux for each N dose, kg ha -1 ; E o -total N 2 O emission for the control treatment, kg ha -1 ; and, N -quantity of N applied through fertilizer, kg ha -1 .
Analysis of residues of the total N 2 O emission was carried out.The differences between cumulative N 2 O emissions were statistically assessed by variance analysis.Based on the t-test, contrasts of interest between two means of cumulative emission and the N 2 O emission factor were estimated.All analyses were carried out using the statistical program SAS 14.1 (SAS Institute, 2015).

Results and Discussion
The irrigation depths applied one day before collections and rainfall accumulated along the evaluation period are presented in Figures 1A and B. Lowest soil temperature was recorded at 230 days after planting (DAP) during the measurements in treatments with fertigation (Figure 1C).In this period, the crop was already fully established with the leaves completely expanded, shading the soil and preventing the incidence of solar radiation directly on soil surface.In addition, the beginning of the rainy period in the region contributed to the reduction in soil temperature.
In treatments with conventional fertilization, the mean soil temperature did not vary between collection days (Figure 1D). (2) (1) Since temperature is a limiting factor in the nitrification process, it is estimated that the optimal maximum temperature for nitrification is 35 to 37 °C (Stark, 1996).Highest soil moisture percentage occurred at 146 DAP in fertigated treatments, when the largest water volume was applied (Figure 1C).However, in treatments with conventional fertilization, highest soil moisture percentage was observed at 145 DAP (Figure 1D), when gas samples relative to the second top dressing fertilization were collected.
Among fertigated treatments, the highest N 2 O fluxes were determined at 146 DAP (Figure 2A).The mean values of soil moisture (19.5%) and temperature (30 °C at 0.1 m depth) may have contributed to increasing the N 2 O emission measured one day after fertigation with N.
Besides soil temperature and moisture, N availability in the soil is another important factor for N 2 O emissions because it favors the biological processes of nitrification and denitrification (Smith et al., 2003).At 146 DAP, 60% of the N dose had already been applied in each treatment.

Figure 1 .
Figure 1.Irrigation depths (A), cumulative rainfall (B), soil moisture and soil temperature along the experimental period in sugarcane under subsurface drip fertigation (C) and fertilization in soil (D) Figure 2. N-N 2 O flux in sugarcane crop under subsurface drip fertigation (A) and conventional fertilization (B)