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N2O emissions during Brassica oleracea cultivation: Interaction of biochar with mineral and organic fertilization
European Journal of Agronomy 115 (2020) 126021
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N2O emissions during Brassica oleracea cultivation: Interaction of biochar with mineral and organic fertilization
T
M. Sánchez-García*, M.A. Sánchez-Monedero, M.L. Cayuela* Department of Soil and Water Conservation and Organic Waste Management, CEBAS-CSIC, Campus Universitario de Espinardo, 30100, Murcia, Spain
ARTICLE INFO
ABSTRACT
Keywords: Charcoal Nitrous oxide Whole plant chamber H2S Intensive vegetable systems Sustainable intensification
The impact of different forms of nitrogen input, biochar amendments and their combination on the yield-scaled N2O emissions were investigated during the cultivation of a representative commercial crop. A field randomized block design with inorganic/organic fertilization and biochar amendment was established during a crop cycle of dripirrigated broccoli. N2O emissions were measured with a static chamber in crop rows and in the bare soil control. N2O emissions were triggered by N fertigation and heavy rainfall events and increased as the plants grew. Organic fertilization resulted in higher N2O emissions than mineral fertilization and these treatments also resulted in the highest peak emissions after fertigation events. Biochar had a significant mitigation effect in hot moments registered immediately after fertilization in organic fertilization treatments. However, biochar caused a slight but not significant reduction in cumulative N2O emissions in all treatments. Peak emissions after heavy rainfall were similar in all the treatments and were not affected by the biochar amendment. Biochar usage decreased the soil bulk density in the inorganic fertilization treatments and facilitated N uptake by the plants. Biochar addition resulted in a significant reduction in yield-scaled emissions, which was more pronounced in the inorganic fertilizer treatments.
1. Introduction The widespread use of nitrogen (N) fertilizers has increased crop production and yields needed to meet the growing food demand worldwide. However, not all the N applied is used by the crops, resulting in N-losses via multiple pathways such as nitrate (NO3−) leaching and N2O emissions, a potent greenhouse gas that is highly relevant due to its detrimental effects on climate change (IPCC, 2013). In fact, the input of N fertilizers is considered the main source of N2O emissions from agricultural soils, representing the main source of anthropogenic N2O emissions (Smith et al., 2007). In addition to the use of N fertilization, other factors have been identified that also influence N2O emissions, such as climate, irrigation management, soil and crop type (Aguilera et al., 2013; Cayuela et al., 2017). Thus, the present challenge is to optimize current management practices to achieve a reduction of the N2O emissions derived from agriculture while maintaining or even improving crop yields. Vegetable cropping systems are characterized by intensive agricultural management with high N inputs, frequent irrigation and crop rotations with multiple harvests in a single year. This intensive crop management makes these systems susceptible to N losses in the form of N2O, which are especially triggered by the high rates of N fertilization (Cayuela et al., 2017; De Rosa et al., 2016). ⁎
A potential strategy for soil N2O mitigation is its amendment with biochar, the C-rich product derived from biomass pyrolysis. A recent meta-analysis of field studies showed that biochar decreased yieldscaled N2O emissions across several cropping systems including maize, wheat, rice, vegetables and pasture (Verhoeven et al., 2017). However, the effect of biochar amendment on soil N2O emissions has been found to be positive or negative depending on the specific characteristics of the biochar, the properties of the amended soil, the N input and N form as well as weather conditions and the soil water regime (Borchard et al., 2019; Cayuela et al., 2013, 2014). Despite the great uncertainty about the effects that biochar amendments will have on greenhouse gas (GHG) mitigation, biochars derived from lignocellulosic materials produced at pyrolysis temperatures above 550 °C have been found to have the greatest potential for N2O mitigation (Borchard et al., 2019; Weldon et al., 2019). Additionally, biochar has been found to promote plant productivity through several mechanisms related to improvement in the soil’s physical properties, water and nutrient retention, increased cation exchange capacity, changes in nutrient conditions and/or liming in acidic soils (Biederman and Harpole, 2013). Moreover, the fertilizer type used in combination with biochar is a crucial factor that affects N availability for plants, where inconsistent results have been found including an increase, decrease and no effect of biochar addition on the soil’s inorganic N (Nguyen et al., 2017).
Corresponding authors. E-mail addresses: [email protected] (M. Sánchez-García), [email protected] (M.L. Cayuela).
https://doi.org/10.1016/j.eja.2020.126021 Received 26 July 2019; Received in revised form 5 February 2020; Accepted 10 February 2020 1161-0301/ © 2020 Elsevier B.V. All rights reserved.
European Journal of Agronomy 115 (2020) 126021
M. Sánchez-García, et al.
The combined reduction of N2O emissions and increase in crop yield associated to the use of biochar could result in a positive impact on yield-scaled emissions (Verhoeven et al., 2017). The concept of “yieldscaled emissions” (i.e. N2O emissions in relation to N uptake of the above-ground crop) takes into account the impact of agricultural management not only on GHG emissions but also on crop productivity, allowing a comparison of the efficiency among different cropping systems to minimise GHG emissions without compromising productivity (Johnson et al., 2012; Van Groenigen et al., 2010). Recently, there has been an increasing interest in the estimation of yield-scaled emissions, which should be minimized to meet environmental sustainability and agronomic production (Van Groenigen et al., 2010). The purpose of this study was to evaluate the effect of two different N fertilization sources and biochar and their interaction on N2O emissions during the cultivation of broccoli (Brassica oleracea, var. Italica. This cultivar is a representative commercial crop in the Mediterranean area, and particularly in Spain, where more than 200,000 T are produced annually SpanishGovernment, 2017). Despite the importance of this crop, there is little information about the N2O emissions related to its cultivation and, to our knowledge, no studies have taken place in arid and semi-arid Mediterranean systems. To address this objective, a randomized field experiment was established where inorganic and organic fertilization and their combination with biochar amendments were tested during broccoli cultivation. Biochar derived from olive tree pruning and created at 600 °C was selected, as it was considered to have the optimum properties needed to reduce soil N2O emissions. Our hypotheses were that (i) organic Nfertilization would result in lower absolute N2O emissions but higher yield-scaled N2O emissions compared to the same rate of inorganic Nfertilizer; (ii) biochar amendment would reduce yield-scaled N2O emissions in both fertilization treatments due to reduced N2O emissions and improved crop yields.
Table 1 General properties of the soil at the experimental plot (soil layer 0–25 cm). Soil property Sand (%) Clay (%) pHw (1:2.5, 20 °C) EC (1:5, 25 °C; μS cm−1) TOC (%) C/N CaCO3 (%) Active lime (%) Available P (meq 100 g−1) K+ (meq 100 g−1) Ca2+ (meq 100 g−1) Mg2+ (meq 100 g−1) Na+ (meq 100 g−1) NO3− -N (mg kg-1) NH4+ -N (mg kg−1) DN (mg kg−1) DOC (mg kg−1)
70.5 18.4 8.4 580 1.4 9.2 38.6 18.6 89 1.4 13 3.8 0.81 38.11 3.88 54.6 192.7
EC: Electrical Conductivity; TOC: Total Organic Carbon; DOC: Dissolved Organic Carbon (water extract 1:10, dw:v); DN: Dissolved Nitrogen (water extract 1:10, dw:v).
for gas measurements, described in the 2.2 section). The PVC anchor used as the base of the gas chamber was 15 cm in height and inserted at a depth of 5 cm into the soil. The 10 cm of the PVC collar left aboveground served as a physical barrier to retain the applied treatments. Considering that the crops were drip irrigated regularly, and that in this system roots do not develop outside the wet bulb (Mmolawa and Or, 2000), any potential cross contamination was discarded. The biochar was produced from olive tree pruning by slow pyrolysis at 600 °C (atmospheric pressure, 2 h of residence time) and was characterized by a high degree of aromatic condensation (H/C and O/C atomic ratios of 0.23 and 0.07, respectively), a surface pore area of 93.1 m2 g−1 and high pH (11.1). The nutrient supplied by the biochar was negligible. A full characterization of this biochar is available in Sánchez-García et al. (2019). Biochar in its original particulate form (particle size < 6 mm) was applied in the corresponding collars at 2 kg m−2 and, afterwards, the topsoil layer (0−25 cm) was ploughed with a hand-shovel in all treatments before planting and fertilization. Fertilization in a water solution was applied inside the collar area through irrigation (fertigation) in three equal, split doses: 1 day after planting, 24 days after planting and 62 days after planting (dates 10/ 02/2018, 10/25/2018 and 12/02/2018, respectively). All fertilized treatments (I – Inorganic, IB – Inorganic + Biochar amendment, O – Organic and OB – Organic + Biochar amendment) received 5.93 g plant−1 of total N (equivalent to 120 kg N per hectare) and 15.66 g plant−1 of K2SO4 according to the local recommendations for broccoli plant cultivation (SpanishGovernment, 2010). N was applied in the form of Ca(NO3)2 in the I and IB treatments. A commercial organic fertilizer (PROBELTE inc.), with a N concentration of 12.2 %, derived from amino acids resulting from the chemical hydrolysis of collagen and keratin, was applied to the O and OB treatments. K was applied in the form of K2SO4 in all fertilised treatments (I, IB, O and OB). Drip irrigation was installed during the experiment, and the total water consumption was 15.8 L m−2. Two drips with a flow of 2 L h-1 were installed in each PVC collar in order to moisten the whole surface inside the chamber and to maintain water-filled pore space (WFPS) within the range of 24–45%. Irrigation was applied 3 days per week during the entire cropping period and suspended during precipitation events. An additional drip irrigation line was installed outside the chamber to allow for the measurement of gravimetric soil water content without disturbing the soil area inside the PVC collars. The total amount of precipitation during the crop cycle was 163.8 L m−2. Soil was maintained free of weeds manually.
2. Materials and methods 2.1. Field experiment description The experiment was conducted at the experimental field of the University of Murcia located in the Campus of Espinardo (Murcia, Spain), coordinates 38˚01′15″N, 1˚09′56″W, 90 m above sea level, from October to December 2018. This area has a semi-arid Mediterranean climate, with mild winters and dry summers with rain mainly occurring in autumn and spring. The typical mean annual temperature is 18.0 °C and the cumulative annual rainfall is 275 mm (average from 2003 to 2018). Temperatures and precipitation were recorded during the growing season by an automated weather station. The topsoil at the experimental site (soil depth 0–25 cm) was a calcaric fluvisol (WRB-FAO classification, 2014) characterized by a sandy loam texture (NCRSS-USDA classification). The experimental land had been organically managed with horticultural crop rotations. Prior to our study, the soil was laid fallow for 4 months. The general properties of the soil are shown in Table 1. The experimental plot slope was negligible. The randomized block design was established in an experimental area measuring 42 m2 and comprised 7 different treatments with five replicates per treatment, which resulted in 35 plots with an area of 0.071 m2 with one plant per plot (in treatments with plant). The crop density consisted of two plants m−2 (shown in Fig. 1). The treatments were the following: i) – Bare soil control (C, without plants or irrigation); ii) - Control plants (P, with no fertilizer and no biochar amendment); iii) – No fertilized plants in biochar-amended soil (B); iv) – Plants treated with inorganic fertilizer (I); v) – Plants treated with inorganic fertilizer in biochar-amended soil (IB); vi) – Plants treated with organic fertilizer (O); and vii) – Plants treated with organic fertilizer in biochar-amended soil (OB). Broccoli plants (Brassica oleracea var. italica) were 10 cm seedlings of the cultivar ‘Belstar’ purchased from a local nursery. Broccoli plants were planted inside PVC collars measuring 300 mm in diameter (used 2
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Fig. 1. Experimental plot layout. PVC-collars are represented by circles in C treatment (with no plant) and by squares in the rest of treatments (with a broccoli plant inside the chamber). The green line represents the watering line (drip-irrigation, numbered from 1 to 5).
2.2. Gas measurements
Yield scaled N2O-N emissions were calculated according to Van Groenigen et al. (2010), by using the equation:
Broccoli plants were located inside a especially-designed PVC collar (300 mm in diameter and 150 mm in length) to allow gas measurements using a whole-plant static chamber technique. Chambers were constructed following the procedure specified by Parkin and Venterea (2010). These were 200–500 mm in height, which were adapted to crop growth. Crop volume at harvest and weekly size measurements throughout the growing period were assessed, and these corresponded to the broccoli growth curve reported by Rincón et al. (1999). The headspace volume of the chamber was corrected by the volume occupied by the plant as the crop was growing. Gas measurements were performed between 9:00 – 11:00 am, as this time period is considered the best sampling time for the estimation of daily mean N2O flux from soils (Alves et al., 2012). Gas fluxes were usually measured once a week and every other day after the application of fertilizer and precipitation events (decreasing frequency as gas emissions were stabilized). A total of 17 gas measurements were recorded during crop cultivation and two additional measurements were performed after harvest. N2O concentrations were measured immediately and 1 h after closing the chambers by using a photo-acoustic gas monitor 1412i (Lumasense Technologies A/S, Ballerup, Denmark) that was directly attached to the chambers by two Teflon tubes and needles through septa. This instrument automatically compensates for temperature and pressure fluctuations in its analysis cell and was set to compensate for water and gas cross-interferences according to the manufacturer's instruction (Iqbal et al., 2013). The accuracy of N2O measurements was checked by analysing N2O/CO2 mixtures made with certified standards. Additionally, H2S fluxes were measured using a portable NDIR analyser (Dräger X-am 166 7000, Dräger Safety, Lübeck, Germany) also directly from the static chambers at the same sampling intervals. N2O and H2S were calculated assuming a linear increase during the accumulation period. Linearity was regularly checked and corroborated at different points throughout the experiment by recording additional measurements during the accumulation period. Results from linearity tests are available in the Supplementary Material (Fig. SM-1). The seasonal (cumulative) gas emissions were calculated assuming linear changes in fluxes between two adjacent measurements. Bare soil control collars were used to estimate the N2O released between crop rows and to extrapolate our results to the field scale, where half of the surface area was considered bare soil according to our experimental framework of 0.5 × 1 m2.
Yield
scaled N2 O
N emissions =
N2 O N emissions , plant N uptake
(1)
where N2O-N emissions are expressed in g and plant N uptake in Kg. Additionally, N2O-N intensity was calculated as the quantity of emitted N (in the form of N2O) per ton of product as follows:
N2 O
N intensity =
N2 O
N emissions , product
where N2O-N emissions are expressed in kg ha to fresh inflorescence yield in T ha−1.
(2) −1
and the product refers
2.3. Soil and plant sampling and analyses Soil samples were collected before planting and at the end of the crop cycle (after harvesting) from each PVC collar. Plant residues were removed from the soil, including roots. Bulk density (BD) was determined in all soil core samples (5 cm diameter and 5 cm depth). After air-drying, soil samples were sieved below 2 mm and the contents of total nitrogen (N), total organic carbon (TOC) were determined in ballmilled samples by automatic elemental analysis (LECO CHNS-932, USA). NO3−-N, dissolved nitrogen (DN) and dissolved organic carbon (DOC) were determined with a water extract (1:10 dw/v) and NH4+ -N in 2 M KCl extract (1:10 dw/v) after 2 h shaking, centrifugation (15 min at 2509 x g) and filtration (0.45 μm). NO3− was determined by ion chromatography (HPLC, model 861, Metrohm AG, Herisau, Switzerland). NH4+ was determined by a colorimetric method based on Berthelot’s reaction (Sommer et al., 1992). DOC and DN content in the extracts were measured using a TOC–TN analyzer (TOC-VCSN Shimadzu). Additionally, the initial soil sample was used for agronomical characterization (Table 1). Broccoli plants were harvested when commercial stage 3 was reached 2.5 months after planting (Vallejo et al., 2003). Afterwards, the plants were washed with deionized water and oven dried at 60 °C until constant weight was reached to determine dry matter. Total N was determined in ball-milled samples by automatic elemental analysis (LECO CHNS-932, USA). N uptake was calculated from the determined N content in the harvested biomass. 3
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Table 2 Cumulative N2O-N emissions, plant N uptake (above ground biomass), yield scaled N2O-N emissions and N2O-N intensity. Treatment
Cumulative N2O-N emissions (mg N m−2)
Plant N uptake (g N plant−1)
Yield scaled N2O-N emissions (g N2O-N kg−1 aboveground N)
N2O-N intensity (kg N2O-N T−1 product)
C P B I IB O OB
88 ± 5 (a) 392 ± 21 (b) 353 ± 32 (b) 373 ± 13 (b) 369 ± 19 (b) 536 ± 64 (c) 496 ± 38 (c)
– 4.29 ± 0.46 4.16 ± 0.36 3.96 ± 0.07 4.57 ± 0.53 4.44 ± 0.78 4.20 ± 0.17
– 46.1 ± 5.1 42.5 ± 1.5 47.1 ± 1.9 40.6 ± 3.2 61.0 ± 5.1 59.1 ± 4.8
– 1.67 ± 0.09 1.79 ± 0.27 1.77 ± 0.35 1.29 ± 0.04 2.05 ± 0.22 2.52 ± 0.13
ANOVA Fertilizer Biochar F*B
0.000 0.071 0.492
0.899 0.642 0.101
0.000 0.001 0.503
(a) (a) (a) (a) (b) (b)
(ab) (b) (b) (a) (b) (c)
0.000 0.677 0.000
C: Bare soil control; P: Control plants (with no fertilizer and no biochar amendment); B: No fertilized plants in biochar amended soil; I: Plants treated with inorganic fertilizer; IB: Plants treated with inorganic fertilizer in biochar amended soil; O: Plants treated with organic fertilizer; OB: Plants treated with organic fertilizer in biochar amended soil. Average values per treatment ± standard deviations (n = 5). F*B indicates the interaction between fertilization and biochar. Significant P-values from two-way ANOVA analyses are indicated in bold (P < 0.05). Letters in brackets indicate significant differences between treatments from one-way ANOVA analyses (Tukey test, P < 0.05). The absence of letters indicates that the observed differences were not significant. Cumulative emissions are referred to crop rows (except in C treatment).
Fig. 2. N2O (A) and meteorological data (B) registered during the experiment. The vertical dashed line indicates harvesting (79 days after plantation). Black arrows indicate fertilization events and the blue arrow indicates a heavy precipitation event (99.3 mm in 6 days). Letters in brackets in the legend indicate significant differences between treatments (Repeated measures, Tukey test, P < 0.05).
2.4. Statistical analyses Significant differences between treatments were assessed through a two-way ANOVA with biochar and N fertilization as factors. A one-way ANOVA was also performed to group treatments according to Tukey’s post-hoc test. In addition, N2O-N fluxes were analyzed by repeated measures ANOVA with treatment as the independent variable and time as the within-subject factor (Kravchenko and Robertson, 2015). The ANOVAs were performed with IBM SPSS Statistics v.21, Sommers, USA. For all analyses, significance was defined as p < 0.05.
O treatment the 7th day after planting, with 78 ± 9 g H2S-S ha−1 day -1 as compared to 21 ± 20 g H2S-S ha-1 day-1 in OB treatment. Afterwards H2S emissions were only detected in the O treatment, with 37 ± 28 and 7 ± 15 g H2S-S ha−1 day -1 on days 10 and 14, respectively. 3.2. Plant N uptake and yield-scaled emissions The type of fertilization treatment affected yield-scaled emissions differently during the experiment (Table 2). Organic fertilization resulted in higher yield-scaled N2O emissions, which were mostly associated to the increase in N2O emissions rather that the impact on N plant uptake, which was neither affected by fertilizer addition, the N source nor the addition of biochar. However, treatments that received inorganic fertilization had an increased N uptake with the biochar amendment. As a consequence, biochar reduced yield-scaled emissions in the inorganic fertilizer treatments. Yield-scaled emissions were similar with and without biochar when no fertilizer was added (P vs B treatments). Similarly, fertilization management was the principal factor affecting N2O intensity whereas biochar addition did not result in a significant impact (Table 2). Organic fertilization resulted in the highest N2O intensity as their cumulative N2O emissions were also the highest. Despite the lack of significant differences in yields between treatments, there were some trends that could explain some of the significant differences observed in N2O intensity as a consequence of the interaction between the fertilizers and the biochar. Thus, biochar addition in the inorganic fertilizer treatments resulted in a significant reduction in the N2O intensity, referring to both the fresh product weight (Table 2) as well as the dry product yield basis (Supplementary material, Table SM1). This result is consistent with the observed reduction in yield-scaled emissions when combining inorganic fertilization with biochar. On the contrary, the OB treatment showed an increased N2O intensity as compared to the O treatment, although this difference was not significant when N2O intensity was calculated in a dry product weight basis (Table SM-1).
3. Results 3.1. N2O and other gaseous emissions N2O pulses were recorded after fertilization and rainfall events (Fig. 2A). When the emissions were associated to fertigation events, the type of fertilizer was the most influencing factor on N2O release. Organic fertilization (in treatments O and OB) significantly increased N2O emissions as compared to the rest of crop treatments, including mineral fertilization (in treatments I and IB), which did not show any significant differences to the control. Biochar addition significantly reduced N2O emissions in organic fertilizer treatments during “hot moments” of N2O pulses following fertilization (Fig. 2A), but it did not significantly affect the total cumulative N2O emissions over the crop cycle (Table 2). The rest of treatments with plants had similar total cumulative emissions. The heavy rainfall event of nearly 100 mm of precipitation that occurred between 44 and 50 days after crop plantation (Fig. 2B) also caused N2O flux peaks in all treatments. The C treatment had a low N2O flux peak, with 28 and 18 g N2O-N ha−1 day−1 registered emissions in days 52 and 56, respectively, whereas the rest of treatments had higher and similar values, with averaged emissions across treatments of 94 ± 4 and 99 ± 6 g N2O-N ha−1 day−1 in days 52 and 56, respectively. H2S emissions were observed within the first flux peak in treatments O and OB, associated to the first split fertilizer application (Supplementary material, Fig. SM-2). H2S fluxes were recorded on days 7, 10 and 14 with a large variability. The highest peaks were recorded in 4
European Journal of Agronomy 115 (2020) 126021
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Table 3 Soil properties at the end of the experiment (soil layer 0–25 cm). Treatment
BD (g cm−3)
C P B I IB O OB
1.09 ± 0.04 1.11 ± 0.05 1.00 ± 0.06 1.09 ± 0.04 0.89 ± 0.04 1.09 ± 0.05 0.96 ± 0.01
ANOVA Fertilizer Biochar F*B
0.023 0.000 0.053
(d) (d) (bc) (d) (a) (cd) (ab)
pHw (1:2.5; 25 °C)
TOC (%)
8.28 ± 0.15 8.28 ± 0.11 8.22 ± 0.11 7.94 ± 0.04 7.92 ± 0.03 7.94 ± 0.07 7.90 ± 0.05
2.19 ± 0.09 2.22 ± 0.11 3.51 ± 0.31 2.72 ± 0.11 4.01 ± 0.73 2.52 ± 0.44 3.20 ± 0.58
0.000 0.142 0.895
(b) (b) (b) (a) (a) (a) (a)
DOC (mg kg−1) (a) (a) (cd) (abc) (d) (ab) (bcd)
0.025 0.000 0.222
144.4 ± 11.5 164.5 ± 13.7 153.7 ± 24.6 136.0 ± 18.1 128.9 ± 23.0 220.0 ± 19.2 230.3 ± 34.5 0.000 0.754 0.568
(a) (a) (a) (a) (a) (b) (b)
NO3− -N (mg kg−1)
NH4+ -N (mg kg−1)
DN (mg kg−1)
5.3 ± 1.2 (a) 2.6 ± 0.4 (a) 2.5 ± 0.4 (a) 95.1 ± 54.3 (b) 103.6 ± 22.4 (b) 81.2 ± 8.9 (b) 78.2 ± 9.4 (b)
3.58 ± 0.58 (a) 3.59 ± 0.48 (a) 3.33 ± 0.60 (a) 3.45 ± 0.59 (a) 3.19 ± 0.47 (a) 28.68 ± 10.66 (c) 13.30 ± 2.86 (b)
20.7 ± 2.7 (a) 19.9 ± 3.7 (a) 18.3 ± 3.6 (a) 146.8 ± 54.8 (bc) 178.8 ± 54.7 (c) 115.6 ± 8.9 (b) 107.4 ± 5.7 (b)
0.000 0.847 0.865
0.000 0.004 0.001
0.000 0.542 0.336
BD: Bulk Density; TOC: Total Organic Carbon; DOC: Dissolved Organic Carbon; DN: Dissolved Nitrogen; C: Bare soil control; P: Control plants (with no fertilizer and no biochar amendment); B: No fertilized plants in biochar-amended soil; I: Plants treated with inorganic fertilizer; IB: Plants treated with inorganic fertilizer in biochar-amended soil; O: Plants treated with organic fertilizer; OB: Plants treated with organic fertilizer in biochar-amended soil. Average values per treatment ± standard deviations (n = 5). F*B indicates the interaction between fertilization and biochar. Significant P-values from two-way ANOVA analyses are indicated in bold (P < 0.05). Letters in brackets indicate significant differences between treatments from one-way ANOVA analyses (Tukey test, P < 0.05).
3.3. Relevant soil properties at the end of the experiment
crop rotation (including lettuce, winter cover crop, bell pepper and Swiss chard) under Mediterranean climatic conditions. They included plants inside the gas measurement chambers, covering the wet bulb similarly as in our study, and the emissions ranged from 70 to 100 g N2O-N ha−1 day−1, which were higher than in our study but so was their fertilization rate (271 Kg N ha−1). The soil of the studied area was characterized by a good natural fertility subjected to an intensive crop management with a high overdose of N additions, conditions that resembled our experiment. On the other hand, De Rosa et al. (2016) and Scheer et al. (2014) reported substantial post-harvest emissions in intensive broccoli cultivation, although this was not corroborated in our experiment (Fig. 2A). This was probably related to the extremely dry post-harvest conditions in our study as drip irrigation was cancelled and precipitation was almost negligible (Fig. 2B) and to the fact that in our study broccoli residues were removed from the field. Longer measurement periods, including many years, are advisable for recording possible gas flux fluctuations subjected to changing weather conditions.
Soil bulk density was significantly affected by the different treatments (Table 3). The lowest BD values corresponded to treatments receiving a biochar amendment, ranging from 0.89 to 1.11 g cm−3 in the following order: IB < OB < B < O < C < I < P (Table 3). Biochar did not affect soil pH, whereas fertilization caused a significant decrease. Nevertheless, all treatments had alkaline soil pH values at the end of the experiment. Biochar addition increased soil TOC (Table 3). The lowest TOC values corresponded to the control treatments C and P. Nevertheless, the increases in TOC caused by biochar addition did not result in increased DOC. On the other hand, organic fertilization resulted in higher DOC values than in the rest of the treatments. Total N content was similar in all treatments at the end of the crop cycle and not very different from the total N content of the soil at the beginning of the experiment, with an average value of 0.15 %. The remaining soluble N forms recorded at the end of the experiment indicate that there was no N limitation during the crop growing cycle. All fertilized treatments (I, IB, O and OB) had similar soil NO3− concentrations at the end of the crop cycle, ranging from 78 ± 9–104 ± 22 mg NO3−-N kg-1 soil (Table 3), which were significantly higher (p < 0.05) than the non-fertilized treatments (with or without plants). Soil NH4+ concentration increased in organic fertilizer treatments, whereas it did not change in the rest of treatments (Table 3). This difference was significant in both organic fertilizer treatments (O and OB) as compared to the rest. However, combined biochar amendment with organic fertilization decreased the soil NH4+ as compared to the O treatment.
4.1. Fertilization and biochar impact on N2O emissions Contrary to our first hypothesis, the organic fertilizer treatments had the highest N2O emissions whereas the mineral fertilization did not show any significant differences with the planted control treatment. This is a non-expected result as N2O emissions are well known to be stimulated by mineral N fertilization (Borchard et al., 2019; Van Groenigen et al., 2010), and even more so considering that there was a N surplus (recorded as soil NO3− at the end of the cultivation). C limited conditions would explain the absence of enhanced N2O emissions in the inorganic fertilizer treatments and their increase in the treatments with organic fertilizer, as the latter was a source of available C. Complex interactions involving microbial-controlled processes, nutrient dynamics (i.e. C and N) and environmental conditions (i.e. temperature, soil physical conditions such as pH, aeration and water content) affect N2O emissions from agricultural soils. Additionally, within the same soil matrix, there are several microsites with varying mechanisms responsible of N2O production (Stevens et al., 1997). Soil WFPS varied between 24–45 % throughout the experiment, which indicates a predominance of aerobic conditions favourable to nitrification processes (Bateman and Baggs, 2005). Moreover, NH4+ soil concentrations in the treatments receiving organic fertilization were above the initial stage whereas the rest of the treatments had similar low concentrations. The recorded NH4+ concentrations in organic fertilizer treatments derived from the mineralization of the amino acids contained in the organic fertilizer, which would be available for
4. Discussion The N2O-N emissions recorded in our experiment, taking into account 1 m of bare soil between rows, ranged between 28 and 40 g N2O-N ha−1 day−1. This range of emissions is consistent with previous literature, despite the limited information available for N2O emissions from broccoli cultivation. Riches et al. (2016) reported N2O-N emissions between 1.2–6.5 g N ha−1 day−1 in a broccoli trial in temperate Australia with fertilizer rates from 39 to 252 kg N ha−1. Scheer et al. (2014) and De Rosa et al. (2016) measured soil N2O-N emissions between crop rows of broccoli in sub-tropical Australia reporting lower emissions (0.8–3.0 g N2O-N ha−1 day−1), which was attributed to C limitation. We have not found data from broccoli cultivation in weather conditions similar to ours. Suddick and Six (2013) studied N2O-N emissions in a vegetable 5
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nitrification. Thus, the increased N2O emissions from organic fertilizer treatments would seem to be associated with the nitrification process. However, anaerobic hot spots may develop under high microbial activity (Kuzyakov and Blagodatskaya, 2015) as in our treatments under organic fertilization. The enhanced microbial activity after organic fertilization was reflected by the associated respiration peaks right after fertilization (data not shown). Additionally, the registered H2S fluxes after the first fertilization event in organic fertilizer treatments also reflect the presence of anaerobic spots. In addition, the organic fertilizer treatments had the highest levels of DOC, which could have enhanced heterotrophic microbial activity, resulting in increased N2O emissions. Therefore, both nitrifier-denitrification and denitrification could have played a role. Nevertheless, the relative importance of these different mechanisms could not be assessed in our field conditions. The biochar effect on N2O emissions was only relevant when N2O hot moments were associated to organic fertilization, where biochar amendment reduced the fluxes. However, this effect did not result in a significant impact on cumulative emissions. Previous literature reported lower N2O emissions associated to denitrification (Cayuela et al., 2013; Weldon et al., 2019) and enhanced N2O emissions associated to nitrification pathways after biochar addition (Sánchez-García et al., 2014; Wells and Baggs, 2014). In our study, perhaps several mechanisms (e.g. nitrification, nitrifier-denitrification, heterotrophic denitrification) simultaneously contributed to N2O formation and emissions in those hot moments, and biochar application led to an overall moderate decrease in the N2O fluxes. Soil reached water saturation conditions favourable for denitrification between days 44 and 50 after plantation. This event resulted in similar increased fluxes in all the treatments except in the bare soil control, where there was a lower increase as compared to the rest of treatments. This event may have produced the leaching of soil nutrients to deep soil layers. Therefore, it is also possible that there were some contribution to the N2O released from soil layers below the wet bulb during water-saturated conditions. Our results are consistent with previous findings that reported the importance of soil moisture conditions for N2O emissions under Mediterranean ecosystems (Cayuela et al., 2017), which may be even more important than the fertilization strategy (Guardia et al., 2017).
events, which resulted in a slight and non-significant reduction of yieldscaled emissions. The increased N2O intensity in the organic fertilizer treatments combined with biochar addition was rather contradictory. Nevertheless, the effect of biochar addition on N2O intensity calculated in a dry product basis was similar to the observed results in yield-scaled emissions. Soil mineral N content was significantly affected by biochar amendment, resulting in lower NH4+ concentration at the end of the crop cycle in the OB treatment. This could be due to a biochar effect enhancing nitrification, as soil pH was already alkaline and biochar addition did not affect soil pH and did not shift the equilibrium from NH4+ towards NH3 volatilization (Liu et al., 2018). Previous findings have reported that biochar stimulates NH4+ oxidation (Sánchez-García et al., 2014; Wells and Baggs, 2014). However, this did not have an impact on soil NO3− concentration or in an increased plant N uptake in organic fertilizer treatments. A recent meta-analysis has shown that the effect of biochar increasing nitrification is less common in neutral to alkaline soils than in acidic soils (Liu et al., 2018). Nevertheless, the low soil NH4+ concentration compared to NO3− soil content could have masked the biochar impact on nitrification by the variability of the soil NO3− concentration. Plant N uptake may also affect these results and we were not able to elucidate if biochar promoted the reduction of N2O to N2 in our field conditions. Despite this significant impact of biochar amendment in soil mineral N content in the treatments receiving organic fertilization, plant N uptake was not affected and further research must be conducted to find a satisfactory explanation for this result. Regarding the inorganic fertilizer treatments, biochar amendment reduced yield-scaled emissions in the IB treatment compared to I, confirming our second hypothesis. Our second hypothesis stated that biochar amendment would reduce yield-scaled N2O emissions as a consequence of reduced N2O emissions and improved crop yields. Nevertheless, this was due to the increased plant N uptake rather than a reduction in cumulative N2O emissions, as the latter were similar in I and IB treatments. In a similar manner, N2O intensity was improved with biochar addition. Biochar, in combination with inorganic fertilization, did not affect N2O emissions associated to fertilization events or when high moisture conditions were favourable towards denitrification process. This observation is in contrast to previous research, which reported lower N2O emissions related to denitrification processes after biochar amendment (Cayuela et al., 2013). The improved plant N uptake may have been due to the better physical properties of the soils in IB as compared to the I treatment. Biochar amendment improved soil bulk density in all the treatments, where the major decrease in BD was recorded in the treatments receiving inorganic fertilization. The minor decrease in BD of the OB treatment as compared to the O treatment was probably a consequence of organic fertilization also improving the soil’s physical properties. The decreased BD in biochar-amended soils would enhance root development and, therefore, promote plant N uptake (Olmo et al., 2016). However, crop yield was similar across all treatments (data available in the Supplementary material, Table SM-1), so that it is very likely that the initial conditions of the soil were optimal for crop growth, as there were not nutrient limitations (Li et al., 2015). The similar plant N uptake between fertilized and non-fertilized treatments reinforces our assumption of the absence of N limitation in the soil. We believe that the effect of biochar amendment on yield-scaled emissions would be more evident in soils with nutrient limitations or a deficient physical structure. Furthermore, the beneficial effect of biochar on plant N uptake could alleviate the mineral N input in intensive vegetable production without a negative impact on N2O emissions or crop yields. However, this hypothesis should be validated by testing the crop performance with lower inorganic N inputs in biochar amended soils.
4.2. Agronomic evaluation of N2O emissions Mitigation strategies should focus not only on reducing GHG emissions but also on maintaining or improving crop yields, and thus, not compromising productivity and global food availability. The assessment of yieldscaled emissions is an indicator of the N2O emitted per plant N uptake, as N is a main driver of N2O emissions as well as the main plant nutrient and crop growth limiting factor after water availability (Van Groenigen et al., 2010). Our starting hypothesis was that biochar amendment would result in reduced yield-scaled emissions as a consequence of better crop performance with lower N2O emissions. High temperature-created biochars have been found to consistently reduce N2O emissions under denitrification conditions (Weldon et al., 2019). In our study, N2O emissions were similar in P and B treatments as well as plant N uptake. Consistently, N2O intensity was also similar in both treatments. Thus, soil amendment with a highly recalcitrant wood-derived biochar did not result in reduced yieldscaled emissions in non-fertilized treatments. Moreover, there was neither an effect of biochar amendment on mineral N content at the end of the crop cycle and the increases in N2O release associated to hot moments were similar. The increased TOC in biochar-amended treatments did not reflect an impact in N turnover nor in the soil-available C content due to the high recalcitrance of the added C, which would improve soil C sequestration. Thus, biochar alone did not reflect an impact in N dynamics in the conditions recorded during our experiment. According to our first hypothesis, the highest yield-scaled emissions as well as N2O intensity corresponded to the organic fertilizer treatments, where biochar amendment reduced N2O pulses associated to fertilization
5. Conclusions Organic fertilization resulted in the highest yield-scaled emissions and the co-application of biochar did not significantly affect plant N uptake nor yield-scaled emissions. 6
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On the contrary, in inorganic fertilizer treatments, biochar amendment did not affect N2O emissions but improved plant N uptake, resulting in a 13 % reduction in yield-scaled emissions. Additionally, biochar led to a significant increase of the soil storage of organic C and a decrease of soil bulk density. The amendment with biochar resulted in a positive impact in both fertilization strategies, which was more significant in inorganic fertilizer treatments.
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CRediT authorship contribution statement M. Sánchez-García: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Visualization, Writing - original draft. M.A. Sánchez-Monedero: Conceptualization, Validation, Investigation, Resources, Writing - original draft, Supervision, Project administration, Funding acquisition. M.L. Cayuela: Conceptualization, Validation, Investigation, Resources, Writing - original draft, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was performed under the framework of the Projects # CTM2015-67200-R from the Spanish Ministry of Economy and Competitiveness co-funded by EU FEDER funds, and # 19281/PI/14 of Fundación Séneca (Agencia Regional de Ciencia y Tecnología de la Región de Murcia). The authors are very grateful to Dr Claudio Mondini from CREA (Italy) for revising the manuscript and to Mr Mario Fon for editing the final version of the manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.eja.2020.126021. References Aguilera, E., Lassaletta, L., Sanz-Cobena, A., Garnier, J., Vallejo, A., 2013. The potential of organic fertilizers and water management to reduce N2O emissions in Mediterranean climate cropping systems. A review. Agric. Ecosyst. Environ. 164, 32–52. https://doi.org/10.1016/j.agee.2012.09.006. Alves, B.J., Smith, K.A., Flores, R.A., Cardoso, A.S., Oliveira, W.R., Jantalia, C.P., Urquiaga, S., Boddey, R.M., 2012. Selection of the most suitable sampling time for static chambers for the estimation of daily mean N2O flux from soils. Soil Biol. Biochem. 46, 129–135. https://doi.org/10.1016/j.soilbio.2011.11.022. Bateman, E.J., Baggs, E., 2005. Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space. Biol. Fertil. Soils 41, 379–388. https://doi.org/10.1007/s00374-005-0858-3. Biederman, L.A., Harpole, W.S., 2013. Biochar and its effects on plant productivity and nutrient cycling: a meta‐analysis. GCB Bioenergy 5, 202–214. https://doi.org/10. 1111/gcbb.12037. Borchard, N., Schirrmann, M., Cayuela, M.L., Kammann, C., Wrage-Mönnig, N., Estavillo, J.M., Fuertes-Mendizábal, T., Sigua, G., Spokas, K., Ippolito, J.A., Novak, J., 2019. Biochar, soil and land-use interactions that reduce nitrate leaching and N2O emissions: a meta-analysis. Sci. Total Environ. 651 (Part 2), 2354–2364. https://doi.org/ 10.1016/j.scitotenv.2018.10.060. Cayuela, M.L., Sánchez-Monedero, M.A., Roig, A., Hanley, K., Enders, A., Lehmann, J., 2013. Biochar and denitrification in soils: when, how much and why does biochar reduce N2O emissions? Sci. Rep. 3, 1732. https://doi.org/10.1038/srep01732. Cayuela, M.L., van Zwieten, L., Singh, B.P., Jeffery, S., Roig, A., Sánchez Monedero, M.A., 2014. Biochar’s role in mitigating soil nitrous oxide emissions: a review and metaanalysis. Agric. Ecosyst. Environ. 191, 5–16. https://doi.org/10.1016/j.agee.2013. 10.009. Cayuela, M.L., Aguilera, E., Sanz-Cobena, A., Adams, D.C., Abalos, D., Barton, L., Ryals, R., Silver, W.L., Alfaro, M.A., Pappa, V.A., Smith, P., Garnier, J., Billen, G., Bowman, L., Bondeau, A., Lassaletta, L., 2017. Direct nitrous oxide emissions in Mediterranean
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M. Sánchez-García, et al. oleracea L. var. italica) health-promoting compounds with inflorescence development. J. Agric. Food Chem. 51, 3776–3782. https://doi.org/10.1021/jf0212338. Van Groenigen, J.W., Velthof, G.L., Oenema, O., Van Groenigen, K.J., Van Kessel, C., 2010. Towards an agronomic assessment of N2O emissions: a case study for arable crops. Eur. J. Soil Sci. 61 (6), 903–913. https://doi.org/10.1111/j.1365-2389.2009. 01217.x. Verhoeven, E., Pereira, E., Decock, C., Suddick, E., Angst, T., Six, J., 2017. Toward a better assessment of biochar-nitrous oxide mitigation potential at the field scale. J.
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N2O emissions during Brassica oleracea cultivation: Interaction of biochar with mineral and organic fertilization
T
M. Sánchez-García*, M.A. Sánchez-Monedero, M.L. Cayuela* Department of Soil and Water Conservation and Organic Waste Management, CEBAS-CSIC, Campus Universitario de Espinardo, 30100, Murcia, Spain
ARTICLE INFO
ABSTRACT
Keywords: Charcoal Nitrous oxide Whole plant chamber H2S Intensive vegetable systems Sustainable intensification
The impact of different forms of nitrogen input, biochar amendments and their combination on the yield-scaled N2O emissions were investigated during the cultivation of a representative commercial crop. A field randomized block design with inorganic/organic fertilization and biochar amendment was established during a crop cycle of dripirrigated broccoli. N2O emissions were measured with a static chamber in crop rows and in the bare soil control. N2O emissions were triggered by N fertigation and heavy rainfall events and increased as the plants grew. Organic fertilization resulted in higher N2O emissions than mineral fertilization and these treatments also resulted in the highest peak emissions after fertigation events. Biochar had a significant mitigation effect in hot moments registered immediately after fertilization in organic fertilization treatments. However, biochar caused a slight but not significant reduction in cumulative N2O emissions in all treatments. Peak emissions after heavy rainfall were similar in all the treatments and were not affected by the biochar amendment. Biochar usage decreased the soil bulk density in the inorganic fertilization treatments and facilitated N uptake by the plants. Biochar addition resulted in a significant reduction in yield-scaled emissions, which was more pronounced in the inorganic fertilizer treatments.
1. Introduction The widespread use of nitrogen (N) fertilizers has increased crop production and yields needed to meet the growing food demand worldwide. However, not all the N applied is used by the crops, resulting in N-losses via multiple pathways such as nitrate (NO3−) leaching and N2O emissions, a potent greenhouse gas that is highly relevant due to its detrimental effects on climate change (IPCC, 2013). In fact, the input of N fertilizers is considered the main source of N2O emissions from agricultural soils, representing the main source of anthropogenic N2O emissions (Smith et al., 2007). In addition to the use of N fertilization, other factors have been identified that also influence N2O emissions, such as climate, irrigation management, soil and crop type (Aguilera et al., 2013; Cayuela et al., 2017). Thus, the present challenge is to optimize current management practices to achieve a reduction of the N2O emissions derived from agriculture while maintaining or even improving crop yields. Vegetable cropping systems are characterized by intensive agricultural management with high N inputs, frequent irrigation and crop rotations with multiple harvests in a single year. This intensive crop management makes these systems susceptible to N losses in the form of N2O, which are especially triggered by the high rates of N fertilization (Cayuela et al., 2017; De Rosa et al., 2016). ⁎
A potential strategy for soil N2O mitigation is its amendment with biochar, the C-rich product derived from biomass pyrolysis. A recent meta-analysis of field studies showed that biochar decreased yieldscaled N2O emissions across several cropping systems including maize, wheat, rice, vegetables and pasture (Verhoeven et al., 2017). However, the effect of biochar amendment on soil N2O emissions has been found to be positive or negative depending on the specific characteristics of the biochar, the properties of the amended soil, the N input and N form as well as weather conditions and the soil water regime (Borchard et al., 2019; Cayuela et al., 2013, 2014). Despite the great uncertainty about the effects that biochar amendments will have on greenhouse gas (GHG) mitigation, biochars derived from lignocellulosic materials produced at pyrolysis temperatures above 550 °C have been found to have the greatest potential for N2O mitigation (Borchard et al., 2019; Weldon et al., 2019). Additionally, biochar has been found to promote plant productivity through several mechanisms related to improvement in the soil’s physical properties, water and nutrient retention, increased cation exchange capacity, changes in nutrient conditions and/or liming in acidic soils (Biederman and Harpole, 2013). Moreover, the fertilizer type used in combination with biochar is a crucial factor that affects N availability for plants, where inconsistent results have been found including an increase, decrease and no effect of biochar addition on the soil’s inorganic N (Nguyen et al., 2017).
Corresponding authors. E-mail addresses: [email protected] (M. Sánchez-García), [email protected] (M.L. Cayuela).
https://doi.org/10.1016/j.eja.2020.126021 Received 26 July 2019; Received in revised form 5 February 2020; Accepted 10 February 2020 1161-0301/ © 2020 Elsevier B.V. All rights reserved.
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The combined reduction of N2O emissions and increase in crop yield associated to the use of biochar could result in a positive impact on yield-scaled emissions (Verhoeven et al., 2017). The concept of “yieldscaled emissions” (i.e. N2O emissions in relation to N uptake of the above-ground crop) takes into account the impact of agricultural management not only on GHG emissions but also on crop productivity, allowing a comparison of the efficiency among different cropping systems to minimise GHG emissions without compromising productivity (Johnson et al., 2012; Van Groenigen et al., 2010). Recently, there has been an increasing interest in the estimation of yield-scaled emissions, which should be minimized to meet environmental sustainability and agronomic production (Van Groenigen et al., 2010). The purpose of this study was to evaluate the effect of two different N fertilization sources and biochar and their interaction on N2O emissions during the cultivation of broccoli (Brassica oleracea, var. Italica. This cultivar is a representative commercial crop in the Mediterranean area, and particularly in Spain, where more than 200,000 T are produced annually SpanishGovernment, 2017). Despite the importance of this crop, there is little information about the N2O emissions related to its cultivation and, to our knowledge, no studies have taken place in arid and semi-arid Mediterranean systems. To address this objective, a randomized field experiment was established where inorganic and organic fertilization and their combination with biochar amendments were tested during broccoli cultivation. Biochar derived from olive tree pruning and created at 600 °C was selected, as it was considered to have the optimum properties needed to reduce soil N2O emissions. Our hypotheses were that (i) organic Nfertilization would result in lower absolute N2O emissions but higher yield-scaled N2O emissions compared to the same rate of inorganic Nfertilizer; (ii) biochar amendment would reduce yield-scaled N2O emissions in both fertilization treatments due to reduced N2O emissions and improved crop yields.
Table 1 General properties of the soil at the experimental plot (soil layer 0–25 cm). Soil property Sand (%) Clay (%) pHw (1:2.5, 20 °C) EC (1:5, 25 °C; μS cm−1) TOC (%) C/N CaCO3 (%) Active lime (%) Available P (meq 100 g−1) K+ (meq 100 g−1) Ca2+ (meq 100 g−1) Mg2+ (meq 100 g−1) Na+ (meq 100 g−1) NO3− -N (mg kg-1) NH4+ -N (mg kg−1) DN (mg kg−1) DOC (mg kg−1)
70.5 18.4 8.4 580 1.4 9.2 38.6 18.6 89 1.4 13 3.8 0.81 38.11 3.88 54.6 192.7
EC: Electrical Conductivity; TOC: Total Organic Carbon; DOC: Dissolved Organic Carbon (water extract 1:10, dw:v); DN: Dissolved Nitrogen (water extract 1:10, dw:v).
for gas measurements, described in the 2.2 section). The PVC anchor used as the base of the gas chamber was 15 cm in height and inserted at a depth of 5 cm into the soil. The 10 cm of the PVC collar left aboveground served as a physical barrier to retain the applied treatments. Considering that the crops were drip irrigated regularly, and that in this system roots do not develop outside the wet bulb (Mmolawa and Or, 2000), any potential cross contamination was discarded. The biochar was produced from olive tree pruning by slow pyrolysis at 600 °C (atmospheric pressure, 2 h of residence time) and was characterized by a high degree of aromatic condensation (H/C and O/C atomic ratios of 0.23 and 0.07, respectively), a surface pore area of 93.1 m2 g−1 and high pH (11.1). The nutrient supplied by the biochar was negligible. A full characterization of this biochar is available in Sánchez-García et al. (2019). Biochar in its original particulate form (particle size < 6 mm) was applied in the corresponding collars at 2 kg m−2 and, afterwards, the topsoil layer (0−25 cm) was ploughed with a hand-shovel in all treatments before planting and fertilization. Fertilization in a water solution was applied inside the collar area through irrigation (fertigation) in three equal, split doses: 1 day after planting, 24 days after planting and 62 days after planting (dates 10/ 02/2018, 10/25/2018 and 12/02/2018, respectively). All fertilized treatments (I – Inorganic, IB – Inorganic + Biochar amendment, O – Organic and OB – Organic + Biochar amendment) received 5.93 g plant−1 of total N (equivalent to 120 kg N per hectare) and 15.66 g plant−1 of K2SO4 according to the local recommendations for broccoli plant cultivation (SpanishGovernment, 2010). N was applied in the form of Ca(NO3)2 in the I and IB treatments. A commercial organic fertilizer (PROBELTE inc.), with a N concentration of 12.2 %, derived from amino acids resulting from the chemical hydrolysis of collagen and keratin, was applied to the O and OB treatments. K was applied in the form of K2SO4 in all fertilised treatments (I, IB, O and OB). Drip irrigation was installed during the experiment, and the total water consumption was 15.8 L m−2. Two drips with a flow of 2 L h-1 were installed in each PVC collar in order to moisten the whole surface inside the chamber and to maintain water-filled pore space (WFPS) within the range of 24–45%. Irrigation was applied 3 days per week during the entire cropping period and suspended during precipitation events. An additional drip irrigation line was installed outside the chamber to allow for the measurement of gravimetric soil water content without disturbing the soil area inside the PVC collars. The total amount of precipitation during the crop cycle was 163.8 L m−2. Soil was maintained free of weeds manually.
2. Materials and methods 2.1. Field experiment description The experiment was conducted at the experimental field of the University of Murcia located in the Campus of Espinardo (Murcia, Spain), coordinates 38˚01′15″N, 1˚09′56″W, 90 m above sea level, from October to December 2018. This area has a semi-arid Mediterranean climate, with mild winters and dry summers with rain mainly occurring in autumn and spring. The typical mean annual temperature is 18.0 °C and the cumulative annual rainfall is 275 mm (average from 2003 to 2018). Temperatures and precipitation were recorded during the growing season by an automated weather station. The topsoil at the experimental site (soil depth 0–25 cm) was a calcaric fluvisol (WRB-FAO classification, 2014) characterized by a sandy loam texture (NCRSS-USDA classification). The experimental land had been organically managed with horticultural crop rotations. Prior to our study, the soil was laid fallow for 4 months. The general properties of the soil are shown in Table 1. The experimental plot slope was negligible. The randomized block design was established in an experimental area measuring 42 m2 and comprised 7 different treatments with five replicates per treatment, which resulted in 35 plots with an area of 0.071 m2 with one plant per plot (in treatments with plant). The crop density consisted of two plants m−2 (shown in Fig. 1). The treatments were the following: i) – Bare soil control (C, without plants or irrigation); ii) - Control plants (P, with no fertilizer and no biochar amendment); iii) – No fertilized plants in biochar-amended soil (B); iv) – Plants treated with inorganic fertilizer (I); v) – Plants treated with inorganic fertilizer in biochar-amended soil (IB); vi) – Plants treated with organic fertilizer (O); and vii) – Plants treated with organic fertilizer in biochar-amended soil (OB). Broccoli plants (Brassica oleracea var. italica) were 10 cm seedlings of the cultivar ‘Belstar’ purchased from a local nursery. Broccoli plants were planted inside PVC collars measuring 300 mm in diameter (used 2
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Fig. 1. Experimental plot layout. PVC-collars are represented by circles in C treatment (with no plant) and by squares in the rest of treatments (with a broccoli plant inside the chamber). The green line represents the watering line (drip-irrigation, numbered from 1 to 5).
2.2. Gas measurements
Yield scaled N2O-N emissions were calculated according to Van Groenigen et al. (2010), by using the equation:
Broccoli plants were located inside a especially-designed PVC collar (300 mm in diameter and 150 mm in length) to allow gas measurements using a whole-plant static chamber technique. Chambers were constructed following the procedure specified by Parkin and Venterea (2010). These were 200–500 mm in height, which were adapted to crop growth. Crop volume at harvest and weekly size measurements throughout the growing period were assessed, and these corresponded to the broccoli growth curve reported by Rincón et al. (1999). The headspace volume of the chamber was corrected by the volume occupied by the plant as the crop was growing. Gas measurements were performed between 9:00 – 11:00 am, as this time period is considered the best sampling time for the estimation of daily mean N2O flux from soils (Alves et al., 2012). Gas fluxes were usually measured once a week and every other day after the application of fertilizer and precipitation events (decreasing frequency as gas emissions were stabilized). A total of 17 gas measurements were recorded during crop cultivation and two additional measurements were performed after harvest. N2O concentrations were measured immediately and 1 h after closing the chambers by using a photo-acoustic gas monitor 1412i (Lumasense Technologies A/S, Ballerup, Denmark) that was directly attached to the chambers by two Teflon tubes and needles through septa. This instrument automatically compensates for temperature and pressure fluctuations in its analysis cell and was set to compensate for water and gas cross-interferences according to the manufacturer's instruction (Iqbal et al., 2013). The accuracy of N2O measurements was checked by analysing N2O/CO2 mixtures made with certified standards. Additionally, H2S fluxes were measured using a portable NDIR analyser (Dräger X-am 166 7000, Dräger Safety, Lübeck, Germany) also directly from the static chambers at the same sampling intervals. N2O and H2S were calculated assuming a linear increase during the accumulation period. Linearity was regularly checked and corroborated at different points throughout the experiment by recording additional measurements during the accumulation period. Results from linearity tests are available in the Supplementary Material (Fig. SM-1). The seasonal (cumulative) gas emissions were calculated assuming linear changes in fluxes between two adjacent measurements. Bare soil control collars were used to estimate the N2O released between crop rows and to extrapolate our results to the field scale, where half of the surface area was considered bare soil according to our experimental framework of 0.5 × 1 m2.
Yield
scaled N2 O
N emissions =
N2 O N emissions , plant N uptake
(1)
where N2O-N emissions are expressed in g and plant N uptake in Kg. Additionally, N2O-N intensity was calculated as the quantity of emitted N (in the form of N2O) per ton of product as follows:
N2 O
N intensity =
N2 O
N emissions , product
where N2O-N emissions are expressed in kg ha to fresh inflorescence yield in T ha−1.
(2) −1
and the product refers
2.3. Soil and plant sampling and analyses Soil samples were collected before planting and at the end of the crop cycle (after harvesting) from each PVC collar. Plant residues were removed from the soil, including roots. Bulk density (BD) was determined in all soil core samples (5 cm diameter and 5 cm depth). After air-drying, soil samples were sieved below 2 mm and the contents of total nitrogen (N), total organic carbon (TOC) were determined in ballmilled samples by automatic elemental analysis (LECO CHNS-932, USA). NO3−-N, dissolved nitrogen (DN) and dissolved organic carbon (DOC) were determined with a water extract (1:10 dw/v) and NH4+ -N in 2 M KCl extract (1:10 dw/v) after 2 h shaking, centrifugation (15 min at 2509 x g) and filtration (0.45 μm). NO3− was determined by ion chromatography (HPLC, model 861, Metrohm AG, Herisau, Switzerland). NH4+ was determined by a colorimetric method based on Berthelot’s reaction (Sommer et al., 1992). DOC and DN content in the extracts were measured using a TOC–TN analyzer (TOC-VCSN Shimadzu). Additionally, the initial soil sample was used for agronomical characterization (Table 1). Broccoli plants were harvested when commercial stage 3 was reached 2.5 months after planting (Vallejo et al., 2003). Afterwards, the plants were washed with deionized water and oven dried at 60 °C until constant weight was reached to determine dry matter. Total N was determined in ball-milled samples by automatic elemental analysis (LECO CHNS-932, USA). N uptake was calculated from the determined N content in the harvested biomass. 3
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Table 2 Cumulative N2O-N emissions, plant N uptake (above ground biomass), yield scaled N2O-N emissions and N2O-N intensity. Treatment
Cumulative N2O-N emissions (mg N m−2)
Plant N uptake (g N plant−1)
Yield scaled N2O-N emissions (g N2O-N kg−1 aboveground N)
N2O-N intensity (kg N2O-N T−1 product)
C P B I IB O OB
88 ± 5 (a) 392 ± 21 (b) 353 ± 32 (b) 373 ± 13 (b) 369 ± 19 (b) 536 ± 64 (c) 496 ± 38 (c)
– 4.29 ± 0.46 4.16 ± 0.36 3.96 ± 0.07 4.57 ± 0.53 4.44 ± 0.78 4.20 ± 0.17
– 46.1 ± 5.1 42.5 ± 1.5 47.1 ± 1.9 40.6 ± 3.2 61.0 ± 5.1 59.1 ± 4.8
– 1.67 ± 0.09 1.79 ± 0.27 1.77 ± 0.35 1.29 ± 0.04 2.05 ± 0.22 2.52 ± 0.13
ANOVA Fertilizer Biochar F*B
0.000 0.071 0.492
0.899 0.642 0.101
0.000 0.001 0.503
(a) (a) (a) (a) (b) (b)
(ab) (b) (b) (a) (b) (c)
0.000 0.677 0.000
C: Bare soil control; P: Control plants (with no fertilizer and no biochar amendment); B: No fertilized plants in biochar amended soil; I: Plants treated with inorganic fertilizer; IB: Plants treated with inorganic fertilizer in biochar amended soil; O: Plants treated with organic fertilizer; OB: Plants treated with organic fertilizer in biochar amended soil. Average values per treatment ± standard deviations (n = 5). F*B indicates the interaction between fertilization and biochar. Significant P-values from two-way ANOVA analyses are indicated in bold (P < 0.05). Letters in brackets indicate significant differences between treatments from one-way ANOVA analyses (Tukey test, P < 0.05). The absence of letters indicates that the observed differences were not significant. Cumulative emissions are referred to crop rows (except in C treatment).
Fig. 2. N2O (A) and meteorological data (B) registered during the experiment. The vertical dashed line indicates harvesting (79 days after plantation). Black arrows indicate fertilization events and the blue arrow indicates a heavy precipitation event (99.3 mm in 6 days). Letters in brackets in the legend indicate significant differences between treatments (Repeated measures, Tukey test, P < 0.05).
2.4. Statistical analyses Significant differences between treatments were assessed through a two-way ANOVA with biochar and N fertilization as factors. A one-way ANOVA was also performed to group treatments according to Tukey’s post-hoc test. In addition, N2O-N fluxes were analyzed by repeated measures ANOVA with treatment as the independent variable and time as the within-subject factor (Kravchenko and Robertson, 2015). The ANOVAs were performed with IBM SPSS Statistics v.21, Sommers, USA. For all analyses, significance was defined as p < 0.05.
O treatment the 7th day after planting, with 78 ± 9 g H2S-S ha−1 day -1 as compared to 21 ± 20 g H2S-S ha-1 day-1 in OB treatment. Afterwards H2S emissions were only detected in the O treatment, with 37 ± 28 and 7 ± 15 g H2S-S ha−1 day -1 on days 10 and 14, respectively. 3.2. Plant N uptake and yield-scaled emissions The type of fertilization treatment affected yield-scaled emissions differently during the experiment (Table 2). Organic fertilization resulted in higher yield-scaled N2O emissions, which were mostly associated to the increase in N2O emissions rather that the impact on N plant uptake, which was neither affected by fertilizer addition, the N source nor the addition of biochar. However, treatments that received inorganic fertilization had an increased N uptake with the biochar amendment. As a consequence, biochar reduced yield-scaled emissions in the inorganic fertilizer treatments. Yield-scaled emissions were similar with and without biochar when no fertilizer was added (P vs B treatments). Similarly, fertilization management was the principal factor affecting N2O intensity whereas biochar addition did not result in a significant impact (Table 2). Organic fertilization resulted in the highest N2O intensity as their cumulative N2O emissions were also the highest. Despite the lack of significant differences in yields between treatments, there were some trends that could explain some of the significant differences observed in N2O intensity as a consequence of the interaction between the fertilizers and the biochar. Thus, biochar addition in the inorganic fertilizer treatments resulted in a significant reduction in the N2O intensity, referring to both the fresh product weight (Table 2) as well as the dry product yield basis (Supplementary material, Table SM1). This result is consistent with the observed reduction in yield-scaled emissions when combining inorganic fertilization with biochar. On the contrary, the OB treatment showed an increased N2O intensity as compared to the O treatment, although this difference was not significant when N2O intensity was calculated in a dry product weight basis (Table SM-1).
3. Results 3.1. N2O and other gaseous emissions N2O pulses were recorded after fertilization and rainfall events (Fig. 2A). When the emissions were associated to fertigation events, the type of fertilizer was the most influencing factor on N2O release. Organic fertilization (in treatments O and OB) significantly increased N2O emissions as compared to the rest of crop treatments, including mineral fertilization (in treatments I and IB), which did not show any significant differences to the control. Biochar addition significantly reduced N2O emissions in organic fertilizer treatments during “hot moments” of N2O pulses following fertilization (Fig. 2A), but it did not significantly affect the total cumulative N2O emissions over the crop cycle (Table 2). The rest of treatments with plants had similar total cumulative emissions. The heavy rainfall event of nearly 100 mm of precipitation that occurred between 44 and 50 days after crop plantation (Fig. 2B) also caused N2O flux peaks in all treatments. The C treatment had a low N2O flux peak, with 28 and 18 g N2O-N ha−1 day−1 registered emissions in days 52 and 56, respectively, whereas the rest of treatments had higher and similar values, with averaged emissions across treatments of 94 ± 4 and 99 ± 6 g N2O-N ha−1 day−1 in days 52 and 56, respectively. H2S emissions were observed within the first flux peak in treatments O and OB, associated to the first split fertilizer application (Supplementary material, Fig. SM-2). H2S fluxes were recorded on days 7, 10 and 14 with a large variability. The highest peaks were recorded in 4
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Table 3 Soil properties at the end of the experiment (soil layer 0–25 cm). Treatment
BD (g cm−3)
C P B I IB O OB
1.09 ± 0.04 1.11 ± 0.05 1.00 ± 0.06 1.09 ± 0.04 0.89 ± 0.04 1.09 ± 0.05 0.96 ± 0.01
ANOVA Fertilizer Biochar F*B
0.023 0.000 0.053
(d) (d) (bc) (d) (a) (cd) (ab)
pHw (1:2.5; 25 °C)
TOC (%)
8.28 ± 0.15 8.28 ± 0.11 8.22 ± 0.11 7.94 ± 0.04 7.92 ± 0.03 7.94 ± 0.07 7.90 ± 0.05
2.19 ± 0.09 2.22 ± 0.11 3.51 ± 0.31 2.72 ± 0.11 4.01 ± 0.73 2.52 ± 0.44 3.20 ± 0.58
0.000 0.142 0.895
(b) (b) (b) (a) (a) (a) (a)
DOC (mg kg−1) (a) (a) (cd) (abc) (d) (ab) (bcd)
0.025 0.000 0.222
144.4 ± 11.5 164.5 ± 13.7 153.7 ± 24.6 136.0 ± 18.1 128.9 ± 23.0 220.0 ± 19.2 230.3 ± 34.5 0.000 0.754 0.568
(a) (a) (a) (a) (a) (b) (b)
NO3− -N (mg kg−1)
NH4+ -N (mg kg−1)
DN (mg kg−1)
5.3 ± 1.2 (a) 2.6 ± 0.4 (a) 2.5 ± 0.4 (a) 95.1 ± 54.3 (b) 103.6 ± 22.4 (b) 81.2 ± 8.9 (b) 78.2 ± 9.4 (b)
3.58 ± 0.58 (a) 3.59 ± 0.48 (a) 3.33 ± 0.60 (a) 3.45 ± 0.59 (a) 3.19 ± 0.47 (a) 28.68 ± 10.66 (c) 13.30 ± 2.86 (b)
20.7 ± 2.7 (a) 19.9 ± 3.7 (a) 18.3 ± 3.6 (a) 146.8 ± 54.8 (bc) 178.8 ± 54.7 (c) 115.6 ± 8.9 (b) 107.4 ± 5.7 (b)
0.000 0.847 0.865
0.000 0.004 0.001
0.000 0.542 0.336
BD: Bulk Density; TOC: Total Organic Carbon; DOC: Dissolved Organic Carbon; DN: Dissolved Nitrogen; C: Bare soil control; P: Control plants (with no fertilizer and no biochar amendment); B: No fertilized plants in biochar-amended soil; I: Plants treated with inorganic fertilizer; IB: Plants treated with inorganic fertilizer in biochar-amended soil; O: Plants treated with organic fertilizer; OB: Plants treated with organic fertilizer in biochar-amended soil. Average values per treatment ± standard deviations (n = 5). F*B indicates the interaction between fertilization and biochar. Significant P-values from two-way ANOVA analyses are indicated in bold (P < 0.05). Letters in brackets indicate significant differences between treatments from one-way ANOVA analyses (Tukey test, P < 0.05).
3.3. Relevant soil properties at the end of the experiment
crop rotation (including lettuce, winter cover crop, bell pepper and Swiss chard) under Mediterranean climatic conditions. They included plants inside the gas measurement chambers, covering the wet bulb similarly as in our study, and the emissions ranged from 70 to 100 g N2O-N ha−1 day−1, which were higher than in our study but so was their fertilization rate (271 Kg N ha−1). The soil of the studied area was characterized by a good natural fertility subjected to an intensive crop management with a high overdose of N additions, conditions that resembled our experiment. On the other hand, De Rosa et al. (2016) and Scheer et al. (2014) reported substantial post-harvest emissions in intensive broccoli cultivation, although this was not corroborated in our experiment (Fig. 2A). This was probably related to the extremely dry post-harvest conditions in our study as drip irrigation was cancelled and precipitation was almost negligible (Fig. 2B) and to the fact that in our study broccoli residues were removed from the field. Longer measurement periods, including many years, are advisable for recording possible gas flux fluctuations subjected to changing weather conditions.
Soil bulk density was significantly affected by the different treatments (Table 3). The lowest BD values corresponded to treatments receiving a biochar amendment, ranging from 0.89 to 1.11 g cm−3 in the following order: IB < OB < B < O < C < I < P (Table 3). Biochar did not affect soil pH, whereas fertilization caused a significant decrease. Nevertheless, all treatments had alkaline soil pH values at the end of the experiment. Biochar addition increased soil TOC (Table 3). The lowest TOC values corresponded to the control treatments C and P. Nevertheless, the increases in TOC caused by biochar addition did not result in increased DOC. On the other hand, organic fertilization resulted in higher DOC values than in the rest of the treatments. Total N content was similar in all treatments at the end of the crop cycle and not very different from the total N content of the soil at the beginning of the experiment, with an average value of 0.15 %. The remaining soluble N forms recorded at the end of the experiment indicate that there was no N limitation during the crop growing cycle. All fertilized treatments (I, IB, O and OB) had similar soil NO3− concentrations at the end of the crop cycle, ranging from 78 ± 9–104 ± 22 mg NO3−-N kg-1 soil (Table 3), which were significantly higher (p < 0.05) than the non-fertilized treatments (with or without plants). Soil NH4+ concentration increased in organic fertilizer treatments, whereas it did not change in the rest of treatments (Table 3). This difference was significant in both organic fertilizer treatments (O and OB) as compared to the rest. However, combined biochar amendment with organic fertilization decreased the soil NH4+ as compared to the O treatment.
4.1. Fertilization and biochar impact on N2O emissions Contrary to our first hypothesis, the organic fertilizer treatments had the highest N2O emissions whereas the mineral fertilization did not show any significant differences with the planted control treatment. This is a non-expected result as N2O emissions are well known to be stimulated by mineral N fertilization (Borchard et al., 2019; Van Groenigen et al., 2010), and even more so considering that there was a N surplus (recorded as soil NO3− at the end of the cultivation). C limited conditions would explain the absence of enhanced N2O emissions in the inorganic fertilizer treatments and their increase in the treatments with organic fertilizer, as the latter was a source of available C. Complex interactions involving microbial-controlled processes, nutrient dynamics (i.e. C and N) and environmental conditions (i.e. temperature, soil physical conditions such as pH, aeration and water content) affect N2O emissions from agricultural soils. Additionally, within the same soil matrix, there are several microsites with varying mechanisms responsible of N2O production (Stevens et al., 1997). Soil WFPS varied between 24–45 % throughout the experiment, which indicates a predominance of aerobic conditions favourable to nitrification processes (Bateman and Baggs, 2005). Moreover, NH4+ soil concentrations in the treatments receiving organic fertilization were above the initial stage whereas the rest of the treatments had similar low concentrations. The recorded NH4+ concentrations in organic fertilizer treatments derived from the mineralization of the amino acids contained in the organic fertilizer, which would be available for
4. Discussion The N2O-N emissions recorded in our experiment, taking into account 1 m of bare soil between rows, ranged between 28 and 40 g N2O-N ha−1 day−1. This range of emissions is consistent with previous literature, despite the limited information available for N2O emissions from broccoli cultivation. Riches et al. (2016) reported N2O-N emissions between 1.2–6.5 g N ha−1 day−1 in a broccoli trial in temperate Australia with fertilizer rates from 39 to 252 kg N ha−1. Scheer et al. (2014) and De Rosa et al. (2016) measured soil N2O-N emissions between crop rows of broccoli in sub-tropical Australia reporting lower emissions (0.8–3.0 g N2O-N ha−1 day−1), which was attributed to C limitation. We have not found data from broccoli cultivation in weather conditions similar to ours. Suddick and Six (2013) studied N2O-N emissions in a vegetable 5
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nitrification. Thus, the increased N2O emissions from organic fertilizer treatments would seem to be associated with the nitrification process. However, anaerobic hot spots may develop under high microbial activity (Kuzyakov and Blagodatskaya, 2015) as in our treatments under organic fertilization. The enhanced microbial activity after organic fertilization was reflected by the associated respiration peaks right after fertilization (data not shown). Additionally, the registered H2S fluxes after the first fertilization event in organic fertilizer treatments also reflect the presence of anaerobic spots. In addition, the organic fertilizer treatments had the highest levels of DOC, which could have enhanced heterotrophic microbial activity, resulting in increased N2O emissions. Therefore, both nitrifier-denitrification and denitrification could have played a role. Nevertheless, the relative importance of these different mechanisms could not be assessed in our field conditions. The biochar effect on N2O emissions was only relevant when N2O hot moments were associated to organic fertilization, where biochar amendment reduced the fluxes. However, this effect did not result in a significant impact on cumulative emissions. Previous literature reported lower N2O emissions associated to denitrification (Cayuela et al., 2013; Weldon et al., 2019) and enhanced N2O emissions associated to nitrification pathways after biochar addition (Sánchez-García et al., 2014; Wells and Baggs, 2014). In our study, perhaps several mechanisms (e.g. nitrification, nitrifier-denitrification, heterotrophic denitrification) simultaneously contributed to N2O formation and emissions in those hot moments, and biochar application led to an overall moderate decrease in the N2O fluxes. Soil reached water saturation conditions favourable for denitrification between days 44 and 50 after plantation. This event resulted in similar increased fluxes in all the treatments except in the bare soil control, where there was a lower increase as compared to the rest of treatments. This event may have produced the leaching of soil nutrients to deep soil layers. Therefore, it is also possible that there were some contribution to the N2O released from soil layers below the wet bulb during water-saturated conditions. Our results are consistent with previous findings that reported the importance of soil moisture conditions for N2O emissions under Mediterranean ecosystems (Cayuela et al., 2017), which may be even more important than the fertilization strategy (Guardia et al., 2017).
events, which resulted in a slight and non-significant reduction of yieldscaled emissions. The increased N2O intensity in the organic fertilizer treatments combined with biochar addition was rather contradictory. Nevertheless, the effect of biochar addition on N2O intensity calculated in a dry product basis was similar to the observed results in yield-scaled emissions. Soil mineral N content was significantly affected by biochar amendment, resulting in lower NH4+ concentration at the end of the crop cycle in the OB treatment. This could be due to a biochar effect enhancing nitrification, as soil pH was already alkaline and biochar addition did not affect soil pH and did not shift the equilibrium from NH4+ towards NH3 volatilization (Liu et al., 2018). Previous findings have reported that biochar stimulates NH4+ oxidation (Sánchez-García et al., 2014; Wells and Baggs, 2014). However, this did not have an impact on soil NO3− concentration or in an increased plant N uptake in organic fertilizer treatments. A recent meta-analysis has shown that the effect of biochar increasing nitrification is less common in neutral to alkaline soils than in acidic soils (Liu et al., 2018). Nevertheless, the low soil NH4+ concentration compared to NO3− soil content could have masked the biochar impact on nitrification by the variability of the soil NO3− concentration. Plant N uptake may also affect these results and we were not able to elucidate if biochar promoted the reduction of N2O to N2 in our field conditions. Despite this significant impact of biochar amendment in soil mineral N content in the treatments receiving organic fertilization, plant N uptake was not affected and further research must be conducted to find a satisfactory explanation for this result. Regarding the inorganic fertilizer treatments, biochar amendment reduced yield-scaled emissions in the IB treatment compared to I, confirming our second hypothesis. Our second hypothesis stated that biochar amendment would reduce yield-scaled N2O emissions as a consequence of reduced N2O emissions and improved crop yields. Nevertheless, this was due to the increased plant N uptake rather than a reduction in cumulative N2O emissions, as the latter were similar in I and IB treatments. In a similar manner, N2O intensity was improved with biochar addition. Biochar, in combination with inorganic fertilization, did not affect N2O emissions associated to fertilization events or when high moisture conditions were favourable towards denitrification process. This observation is in contrast to previous research, which reported lower N2O emissions related to denitrification processes after biochar amendment (Cayuela et al., 2013). The improved plant N uptake may have been due to the better physical properties of the soils in IB as compared to the I treatment. Biochar amendment improved soil bulk density in all the treatments, where the major decrease in BD was recorded in the treatments receiving inorganic fertilization. The minor decrease in BD of the OB treatment as compared to the O treatment was probably a consequence of organic fertilization also improving the soil’s physical properties. The decreased BD in biochar-amended soils would enhance root development and, therefore, promote plant N uptake (Olmo et al., 2016). However, crop yield was similar across all treatments (data available in the Supplementary material, Table SM-1), so that it is very likely that the initial conditions of the soil were optimal for crop growth, as there were not nutrient limitations (Li et al., 2015). The similar plant N uptake between fertilized and non-fertilized treatments reinforces our assumption of the absence of N limitation in the soil. We believe that the effect of biochar amendment on yield-scaled emissions would be more evident in soils with nutrient limitations or a deficient physical structure. Furthermore, the beneficial effect of biochar on plant N uptake could alleviate the mineral N input in intensive vegetable production without a negative impact on N2O emissions or crop yields. However, this hypothesis should be validated by testing the crop performance with lower inorganic N inputs in biochar amended soils.
4.2. Agronomic evaluation of N2O emissions Mitigation strategies should focus not only on reducing GHG emissions but also on maintaining or improving crop yields, and thus, not compromising productivity and global food availability. The assessment of yieldscaled emissions is an indicator of the N2O emitted per plant N uptake, as N is a main driver of N2O emissions as well as the main plant nutrient and crop growth limiting factor after water availability (Van Groenigen et al., 2010). Our starting hypothesis was that biochar amendment would result in reduced yield-scaled emissions as a consequence of better crop performance with lower N2O emissions. High temperature-created biochars have been found to consistently reduce N2O emissions under denitrification conditions (Weldon et al., 2019). In our study, N2O emissions were similar in P and B treatments as well as plant N uptake. Consistently, N2O intensity was also similar in both treatments. Thus, soil amendment with a highly recalcitrant wood-derived biochar did not result in reduced yieldscaled emissions in non-fertilized treatments. Moreover, there was neither an effect of biochar amendment on mineral N content at the end of the crop cycle and the increases in N2O release associated to hot moments were similar. The increased TOC in biochar-amended treatments did not reflect an impact in N turnover nor in the soil-available C content due to the high recalcitrance of the added C, which would improve soil C sequestration. Thus, biochar alone did not reflect an impact in N dynamics in the conditions recorded during our experiment. According to our first hypothesis, the highest yield-scaled emissions as well as N2O intensity corresponded to the organic fertilizer treatments, where biochar amendment reduced N2O pulses associated to fertilization
5. Conclusions Organic fertilization resulted in the highest yield-scaled emissions and the co-application of biochar did not significantly affect plant N uptake nor yield-scaled emissions. 6
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On the contrary, in inorganic fertilizer treatments, biochar amendment did not affect N2O emissions but improved plant N uptake, resulting in a 13 % reduction in yield-scaled emissions. Additionally, biochar led to a significant increase of the soil storage of organic C and a decrease of soil bulk density. The amendment with biochar resulted in a positive impact in both fertilization strategies, which was more significant in inorganic fertilizer treatments.
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CRediT authorship contribution statement M. Sánchez-García: Conceptualization, Validation, Formal analysis, Investigation, Data curation, Visualization, Writing - original draft. M.A. Sánchez-Monedero: Conceptualization, Validation, Investigation, Resources, Writing - original draft, Supervision, Project administration, Funding acquisition. M.L. Cayuela: Conceptualization, Validation, Investigation, Resources, Writing - original draft, Supervision, Project administration, Funding acquisition. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This study was performed under the framework of the Projects # CTM2015-67200-R from the Spanish Ministry of Economy and Competitiveness co-funded by EU FEDER funds, and # 19281/PI/14 of Fundación Séneca (Agencia Regional de Ciencia y Tecnología de la Región de Murcia). The authors are very grateful to Dr Claudio Mondini from CREA (Italy) for revising the manuscript and to Mr Mario Fon for editing the final version of the manuscript. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.eja.2020.126021. References Aguilera, E., Lassaletta, L., Sanz-Cobena, A., Garnier, J., Vallejo, A., 2013. The potential of organic fertilizers and water management to reduce N2O emissions in Mediterranean climate cropping systems. A review. Agric. Ecosyst. Environ. 164, 32–52. https://doi.org/10.1016/j.agee.2012.09.006. Alves, B.J., Smith, K.A., Flores, R.A., Cardoso, A.S., Oliveira, W.R., Jantalia, C.P., Urquiaga, S., Boddey, R.M., 2012. Selection of the most suitable sampling time for static chambers for the estimation of daily mean N2O flux from soils. Soil Biol. Biochem. 46, 129–135. https://doi.org/10.1016/j.soilbio.2011.11.022. Bateman, E.J., Baggs, E., 2005. Contributions of nitrification and denitrification to N2O emissions from soils at different water-filled pore space. Biol. Fertil. Soils 41, 379–388. https://doi.org/10.1007/s00374-005-0858-3. Biederman, L.A., Harpole, W.S., 2013. Biochar and its effects on plant productivity and nutrient cycling: a meta‐analysis. GCB Bioenergy 5, 202–214. https://doi.org/10. 1111/gcbb.12037. Borchard, N., Schirrmann, M., Cayuela, M.L., Kammann, C., Wrage-Mönnig, N., Estavillo, J.M., Fuertes-Mendizábal, T., Sigua, G., Spokas, K., Ippolito, J.A., Novak, J., 2019. Biochar, soil and land-use interactions that reduce nitrate leaching and N2O emissions: a meta-analysis. Sci. Total Environ. 651 (Part 2), 2354–2364. https://doi.org/ 10.1016/j.scitotenv.2018.10.060. Cayuela, M.L., Sánchez-Monedero, M.A., Roig, A., Hanley, K., Enders, A., Lehmann, J., 2013. Biochar and denitrification in soils: when, how much and why does biochar reduce N2O emissions? Sci. Rep. 3, 1732. https://doi.org/10.1038/srep01732. Cayuela, M.L., van Zwieten, L., Singh, B.P., Jeffery, S., Roig, A., Sánchez Monedero, M.A., 2014. Biochar’s role in mitigating soil nitrous oxide emissions: a review and metaanalysis. Agric. Ecosyst. Environ. 191, 5–16. https://doi.org/10.1016/j.agee.2013. 10.009. Cayuela, M.L., Aguilera, E., Sanz-Cobena, A., Adams, D.C., Abalos, D., Barton, L., Ryals, R., Silver, W.L., Alfaro, M.A., Pappa, V.A., Smith, P., Garnier, J., Billen, G., Bowman, L., Bondeau, A., Lassaletta, L., 2017. Direct nitrous oxide emissions in Mediterranean
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