Effect of biochar and liming on soil nitrous oxide emissions from a temperate maize cropping system

Biochar, a carbon-rich, porous pyrolysis product of organic residues may positively affect plant yield and can, owing to its inherent stability, promote soil carbon sequestration when amended to agricultural soils. Another possible effect of biochar is the reduction in emissions of nitrous oxide (N2O). A number of laboratory incubations have shown significantly reduced N2O emissions from soil when mixed with biochar. Emission measurements under field conditions however are more scarce and show weaker or no reductions, or even increases in N2O emissions. One of the hypothesised mechanisms for reduced N2O emissions from soil is owing to the increase in soil pH following the application of alkaline biochar. To test the effect of biochar on N2O emissions in a temperate maize cropping system, we set up a field trial with a 20 tha biochar treatment, a limestone treatment adjusted to the same pH as the biochar treatment (pH 6.5), and a control treatment without any addition (pH 6.1). An automated static chamber system measured N2O emissions for each replicate plot (n= 3) every 3.6 h over the course of 8 months. The field was conventionally fertilised at a rate of 160 kgNha in three applications of 40, 80 and 40 kgNha as ammonium nitrate. Cumulative N2O emissions were 52 % smaller in the biochar compared to the control treatment. However, the effect of the treatments overall was not statistically significant (p = 0.27) because of the large variability in the data set. Limed soils emitted similar mean cumulative amounts of N2O as the control. There is no evidence that reduced N2O emissions with biochar relative to the control is solely caused by a higher soil pH.


Introduction
Agriculture faces major challenges regarding world food security because of climate change, continued population growth and resource-depleting practises (IAASTD, 2009).Accounting for roughly 12 % of anthropogenic greenhouse gas (GHG) emissions per year, agriculture is a sector with a considerable mitigation potential and, at the same time, is highly vulnerable to the consequences of a changing climate (IPCC, 2014).With its 300-fold warming potential compared to CO 2 , nitrous oxide (N 2 O) from soil is a downside of the large productivity increase in agriculture, mostly due to synthetic nitrogen fertiliser application.Reducing agricultural N 2 O emissions would reduce the GHG induced radiative forcing (IPCC, 2014), improve the stability of the strato-spheric ozone layer (Ravishankara et al., 2009) and reduce agriculture's energy intensity when achieved with a lower nitrogen fertiliser use (IAASTD, 2009).
Biochar is produced by thermal decomposition of organic material in a low-oxygen environment, called pyrolysis.This stable charcoal-like material has the potential to contribute to the mitigation of climate change by increasing soil carbon (C) (Lehmann, 2007;Woolf et al., 2010;Lal et al., 2011).In addition, biochar can increase crop yields (Jeffery et al., 2011;Biederman and Harpole, 2013;Crane-Droesch et al., 2013) and reduce water stress, which helps to adapt to climate change (Mulcahy et al., 2013).Its application to soils that have a small cation exchange capacity and low organic carbon content is associated with higher crop yields (Crane-Droesch et al., 2013), with an overall mean response of 10 % (Jeffery et al., 2011).For fertile, temperate soils improvement of soil quality is not key to biochar application.Rather, biochar effects on soil-borne GHG emissions, N 2 O in particular, have become a strong argument for its amendment.
Biochars are often alkaline and therefore increase soil pH after application (Joseph et al., 2010).Denitrifying bacterial communities have the potential to increase their N 2 Oreducing activity with increasing pH, which may reduce N 2 O emissions from soils (Cavigelli and Robertson, 2001;Simek and Cooper, 2002;Čuhel et al., 2010).Low pH possibly impedes the synthesis of a functional N 2 O reductase enzyme (Bakken et al., 2012).Some authors suggest that the elevated soil pH is responsible for reduced N 2 O emissions following biochar application through increased activity of N 2 O reducing bacteria (Van Zwieten et al., 2010;Zheng et al., 2012).In contrast, Yanai et al. (2007) argue that the suppression of N 2 O emissions by biochar is not through increased N 2 O reduction activity because biochar ash also increases soil pH but does not reduce N 2 O emissions.Cayuela et al. (2013) showed that biochar's acid buffer capacity was a more important factor in denitrification than the pH shift in soil.There are indications that biochar enhances nosZ expression, the gene responsible for the transcription of the N 2 O reductase in denitrifying microorganisms (Harter et al., 2014;Van Zwieten et al., 2014).This could be a mechanistic link to the observed reduction in N 2 O emissions through biochar increasing soil pH and microbial activity.In contrast, under conditions favouring nitrification and not being as sensitive to pH as total denitrification, biochar addition increased N 2 O emissions in the lab (Sánchez-García et al., 2014) and possibly in the field (Verhoeven and Six, 2014).
In this study, we test (i) whether N 2 O emissions are reduced following the application of biochar to soil of a temperate maize cropping system and (ii) whether this possible reduction in N 2 O emissions is similar when soil pH is increased by other means.The latter was tested by a treatment where limestone was added to increase soil pH to the same level as that from the addition of 20 t ha −1 biochar.N 2 O emissions and maize yield were quantified during one growing season in the field.

Field site
The experiment was established on a cropland field near the Agroscope research station in Zurich, Switzerland (47.427 • N, 8.522 • E, 437 m a.s.l.).The climate is temperate with a mean annual air temperature of 9.4 • C and mean annual rainfall of 1054 mm (Climate data 1981-2010, Meteoswiss, 2013 from the MeteoSwiss station Zurich Affoltern 500 m from the experimental site).The field was under conventional management with maize in 2013, the year prior to the experiment.
The soil is a clay loam with a particle size distribution of 37 % sand, 27 % silt and 36 % clay.The soil is a Eutric Mollic Gleysol (Drainic) (IUSS Working Group WRB, 2014).The untreated soil has a pH of 6.3 in water (1 : 2.5 w/v), total organic carbon content of 26.2 g kg −1 , total N of 2.9 g kg −1 and bulk density of 1.3 g cm −3 .

Biochar
Several biochars were screened in advance to pick one with a high liming capacity and with properties in agreement to the guidelines for contents of polycyclic aromatic hydrocarbons (PAHs), C and N of the European Biochar Certificate (EBC, 2012).The chosen biochar was produced in a Pyreg reactor (Pyreg GmbH, Dörth, Germany) by Verora in Edlibach ZG, Switzerland in late 2013 (see chapter 30, case study 2 in Lehmann and Joseph, 2015).Pyreg reactors use slow pyrolysis in a continuous system with an average residence time of circa 25 min and a peak temperature of approximately 650 • C. The feedstock was green waste mainly from tree pruning.The biochar has the following properties: 64.9 % total C; 62.1 % C org , pH 9.8 (1 : 10 in water); liming capacity 17.2 % CaCO 3 , 148 m 2 g −1 BET surface area and ash content 20 %.Elemental ratios are 0.11 O / C and 0.33 H / C molar and 94 C / N by mass.Moisture content at the time of application was 12 %.Biochar was sieved < 3 mm shortly before it was spread on the field.

Experimental setup
Three different treatments were introduced: 20 t ha −1 biochar, control without additions and a limestone treatment to increase the soil pH to the same level as with biochar.The field was split into 3 × 3 plots with a size of 2 by 3 m (6 m 2 per plot and three replicates for each treatment).One metre buffer zones were established between plots on all sides.
The three different treatments were arranged in a randomised complete block design with the 3×3 grid accounting for spatial variability.The whole field, including the buffer zones, were planted with maize (Zea mays).Initial pH values were not different among treatment plots (see pH measurement from January 2014 in Fig. 2).

Field management
The field was ploughed in autumn 2013 after the maize harvest.In January 2014, 20 t ha −1 biochar and 2 t ha −1 limestone, respectively, were spread on the wet, ploughed field surface.Freshly applied biochar was gently mixed with the first 1-3 cm of soil by hand at the same time.In mid-February 2014, the automated GHG chamber system was installed and in March the field was harrowed by a rototiller to a depth of circa 15 cm.The chamber frames were reset into the soil again and Decagon TE5 temperature and humidity sensors (Decagon Devices Inc., Pullman, WA, USA) were placed at a depth of 8 cm in the centre of each plot.The TE5 sensor measures the volumetric water content in soil by time domain reflectometry (TDR) at 70 MHz.
In May, potassium (K) and phosphorus (P) fertiliser was applied at a rate of 41.4 kg P ha −1 and 132 kg K ha −1 .Nitrogen was applied in three portions of 40, 80 and 40 kg N ha −1 on the 26 May, 16 June and 16 July, respectively, as ammonium nitrate (LONZA-Ammonsalpeter 27.5 % N).At each event fertiliser was spread on each plot of 6 m 2 and chamber frame of 0.03 m 2 separately to ensure equal distribution.On the 5 May, two of the three lime replicates were treated with another 1 t ha −1 of limestone because the pH was not in the same range as the biochar plots.Maize (Padrino from KWS SAAT AG, Einbeck, Germany) was sown on the 8 May with 0.14 m distance within rows that were 0.6 m apart from each other.For plant protection only one herbicide application was conducted on the 19 June with 1 L ha −1 Dasul (Syngenta, Basel, Switzerland), 1 L ha −1 Mikado (Bayer CropScience, Germany) and 1 kg ha −1 Andil (Omya AG, Switzerland).Despite manual weeding and herbicides a considerable amount of weeds emerged.Plots were harvested on the 13 October.

Nitrous oxide measurement
N 2 O and CO 2 emissions were measured with static chambers of a fully automated measurement system (Flechard et al., 2005;Felber et al., 2013) consisting of nine stainless steel chambers (30 × 30 × 25 cm).These chambers were placed on PVC frames inserted 3 cm deep into soil.The frame positions were moved three times during the growing season to obtain a better spatial representation of each plot.After maize had been sown, the chamber positions were between rows and no vegetation was grown within the chamber frame.Each of the nine chamber lids was automatically closed and opened sequentially (over a period of 3.6 h) allowing N 2 O and CO 2 to accumulate in the chamber headspace for 15 min.Chamber headspace air was circulated (1 L min −1 air flow) through an inlet and outlet line from each chamber through polyamide tubes (4 mm I.D.) to the analytical system and back to the chamber headspace after sample analysis.The analytical and chamber control instruments were installed in a nearby field cabin under temperature-controlled air conditioning.N 2 O concentrations were continuously measured and stored every minute using a gas filter correlation analyser (TEI Model 46c, Thermo Fisher Scientific, Waltham, MA, USA).The gas stream is exposed to infrared light from specific bands (filtered), both from N 2 O absorbing and nonabsorbing bands.From this difference a gas specific and concentration-sensitive signal is retrieved.The instruments linearity is described with ± 2 % with negligible interference of H 2 O, CO 2 or CO.CO 2 was measured with an infrared sensor from Liston Scientific Corp. (Irvine, CA, USA).The system was calibrated every 11 h with three different concentrations from certified gas standards (Carbagas, Rümlang, Switzerland).The N 2 O analyser showed a drift with room temperature variations that the air conditioning could not avoid completely.Hence a temperature correction factor was applied to the raw data from a regression of the device temperature with data during calibrations in May.The temperature correction factor used was about 9.1 ppm per • C temperature change from the 37 • C device reference temperature.The mean N 2 O analyser device temperature in June-July was 37.4 ± 2 • C (±1 SD).
N 2 O and CO 2 fluxes from soil were calculated from the continuous concentration measurement (resolution 1 per min) when chamber lids were closed.Data from the first 3 min of the total 15 min closure time were omitted from the flux calculation to remove signal noise due to gas exchange from the system during chamber switching and closing (Felber et al., 2013).The same flux estimation procedure (R-script by R. Fuss on bitbucket.org,see Fuss, 2015) was used as in Leiber-Sauheitl et al. (2014).It is a modification of the HMR package (Pedersen et al., 2010) that chooses between exponential curvature for non-linear chamber behaviour (Hutchinson-Mosier regression) and robust linear regression (Huber and Ronchetti, 1981).The exponential HMR scheme considers non-linear concentration increase in the chamber due to a possibly decreasing concentration gradient, chamber leakage and lateral gas transport.Robust linear regressions provide a more reliable flux estimate for low fluxes when there is a lot of variation due to limited measurement precision and outliers.Following the flux script's recommendation, non-linear HMR was used for 1034 fluxes, whereas for all the other 13 034 fluxes the robust linear regression was chosen.The resulting flux estimates from this procedure were then filtered for implausible large N 2 O uptake by soil (i.e. when the ambient N 2 O concentration suddenly drops with increased mixing in the boundary layer).N 2 O fluxes smaller than −10 ng N 2 O m −2 s −1 (Neftel et al., 2010) were removed as well as data associated with a likely invalid chamber functioning (i.e.frozen lids) indicated by CO 2 fluxes < −0.5 µmol m −2 s −1 (Felber et al., 2013).From the entire data set of 14 068 fluxes, 302 were rejected due to the CO 2 flux threshold and an additional 49 fluxes due to N 2 O (2.5 % of the total number of fluxes removed).Considering a cumulative sum of all fluxes in the data set, filtering reduced this number by 0.61 %.

Yield
The harvest (above-ground biomass) was separated into grain yield (kernels) and by-product (stems, leaves).Cobs were threshed and dried whereas the by-product was weighed freshly on the field, chaffed and a sub-sample was then dried to measure water content and for further plant nutrient analysis.From both grains and by-product, dry matter total N, P, K, Ca and Mg content were measured (FAL, 1996).For yield based N 2 O emissions, cumulative N 2 O emissions in kg N 2 O-N ha −1 were related to total above-ground biomass dry matter (DM) yield in t ha −1 (see Table 3) and to total aboveground N uptake in kg N ha −1 (see discussion).

Soil sampling and analysis
Soil samples for pH, ammonium (NH + 4 ) and nitrate (NO − 3 ) measurements were taken on the 31 January, 31 March, 26 May, 16 June and 4 September 2014.At each sampling, five randomly distributed soil cores per plot were taken (0-10 cm) and pooled.Soil pH was determined in moist soil samples using water at a ratio of 1 : 2.5 w/v and measured with a PH100 ExStik pH meter (Extech Instruments Corp., Nashua, NH, USA).Soil bulk density was measured on the 27 June at a depth of 3-8 cm using 100 cm 3 steel cores, 3 per plot.
For soil NO − 3 and NH + 4 concentrations, 20 g of moist soil were mixed with 100 mL 0.01 M CaCl 2 solution.The suspension was shaken for 30 min, filtered and then analysed by segmented flow injection analysis on a SKALAR SANplus analyser (Skalar Analytical B.V., Breda, Netherlands).

Statistical analysis
The obtained fluxes from the automated chamber system were aggregated to 6 h means producing a regular, smoothed data set.The system was able to measure each chamber three times for every 11 h calibration cycle during regular operations, hence on average 1.6 measurements for each chamber were included in each 6 h mean.Still missing values after this aggregation step were linearly interpolated for each chamber.Treatment averages and standard errors were calculated from the three chambers on the replicated plots.If not indicated otherwise, treatment means are shown with ±1 standard error (SE).
Statistical analyses were performed with R (version 3.0.1,The R Project, 2014).Significance level was chosen at p < 0.05 for all procedures, unless indicated otherwise.Sig-nificant treatment effects for cumulated fluxes were determined using ANOVA from rbase package (treatments: control, biochar and lime; n = 3).Bartlett test of homogeneity of variances showed conflicting ANOVA assumptions for the cumulative fluxes.This could be improved by log transformation of the flux data.

Environmental field conditions
The year started with above-average temperatures and low rainfall (Fig. 1).End of May to June was dry with high temperatures being on average for Switzerland 1.5 • C above the 1981-2010 norm (Meteoswiss, 2015).The soil's volumetric water content fell to circa 20 %, inducing high water stress on the young maize seedlings.The lack of soil moisture presumably hampered the dilution of the first application of 40 kg N ha −1 in the soil solution.Along with the 2nd N fertilisation the field was therefore irrigated with 33 mm water (shown as green bar in Fig. 1).The summer months July and August were rather cold and wet with daily mean air temperatures below 20 • C (Meteoswiss, 2015).
Soil volumetric water content tended to be higher in biochar plots (Fig. 1) with 37 out of 423 (8.7 %) half-daily means showing a statistically significant treatment effect.

Soil pH and nitrogen
Soil pH increased with limestone and biochar addition by circa 0.4 pH units (Fig. 2).During the time with major emissions in June, the pH between control and biochar/lime soils significantly (p < 0.001) increased from 6.1 to 6.5 with.The initial soil pH was on average 6.3 and not different among treatments.Following biochar application soil pH increased to up to 7.4 whereas with addition of limestone soil pH increased to up to 6.9 (averages across replicates).The pH sharply decreased after the initial peak, especially in those two liming plots, which were treated subsequently with another 1 t ha −1 in May.Soil pH of biochar and lime treatments were not significantly different at any sampling time, whereas soil pH of the control treatment was systematically below that of the amended soils.
Mean soil bulk density was not statistically different between treatments (1.31 ± 0.03 g cm −3 in the control, 1.29 ± 0.07 g cm −3 in biochar and 1.36 ± 0.04 g cm −3 in the liming treatment).
Soil mineral N was not statistically different between treatments at any sampling date (Tables 1 and 2).

N 2 O fluxes
Emissions were characterized by peak events, particularly in summer, and by background emissions in spring and autumn (Fig. 3).Main emissions occurred after the second fertilisa-  tion event with the highest application rate around early August.Afterwards, there were only emissions from one of the lime plots but almost none until the end of October from all the other plots.This also corresponds to the low amounts of available soil N, indicating that the plants had taken up most of it.All treatments revealed similar temporal N 2 O emission dynamics but the height of the peaks differed.During peak events emissions from the biochar treatment were often lower than those from the other treatments, especially compared to the control.This resulted in an increasing difference  the data set.Emission means from control and lime were very similar.With lime, N 2 O emissions were highly variable and this treatment included both the chamber with the highest and also the one with the lowest cumulative emission.Alternatively, we also calculated p values comparing only biochar and control treatments with a Welch Two-Sample t test.This resulted in a significant difference (p = 0.022).All p values have to be treated with caution because they were produced with a minimal number of replicates.Furthermore, a large treatment effect size is not reflected in the p value.N 2 O emissions per unit N applied calculated from the 160 kg N ha −1 with the mean cumulative emissions during the growing season, resulted in 1.06 % for biochar, 2.21 % for control and 2.25 % for the lime treatment.Like cumulated emissions, these values were not significantly different among treatments and have the same variance and p values.

Maize yields and plant growth
Maize yields were not significantly different between treatments, for both grains (product) and by-product (i.e.stems, leaves) (Fig. 5).Nitrogen uptake by maize did not differ among treatments (Fig. 6, Table 3).Table 3 shows cumulated N 2 O emissions for each plot and per area as well as per DM yield.Yield based emissions (Table 3) resulted in 0.128 ± 0.010 kg N 2 O-N per t-DM, for biochar, 0.319 ± 0.036 kg N 2 O-N per t-DM for control and 0.306 ± 0.148 kg N 2 O-N per t-DM for the liming treatment.Although the yield based emission with biochar is 60 % lower compared to the control, overall there is no significant treatment effect (p = 0.19).There was no difference between treatments for any of the measured nutrients in the yield (data not shown).

N 2 O emissions
Our high-frequency automated N 2 O chamber measurements give a detailed picture of the emissions from a biochar and lime field trial.Neither soil NO − 3 nor NH + 4 concentrations can explain N 2 O emission patterns at any point in time.Although cumulative N 2 O emissions were not significantly different among the three treatments, emissions with added biochar were 52 % below the control treatment.The magni-   tude of reduction is in agreement with the meta-analysis of Cayuela et al. (2015), who showed a general reduction of N 2 O emissions by biochar of 49 ± 5 % (lab and field experiments) but it is larger than the reduction found by the same authors under field conditions (28 ± 16 %).In our maize field in the temperate zone, N 2 O emissions can thus decrease with biochar addition as much as they have been shown to be reduced under controlled lab conditions.Our results show no decrease in N 2 O emissions when limestone is used to increase the soil pH to the same level as that with biochar.This finding does not support the hypothesis that biochar's N 2 O reduction effect is similar to a geochemical adjustment of soil pH.However, it must be considered that the large variability among the three replicates hampers the power of this conclusion.A post-hoc power analysis showed a 23.4 % probability of accepting a true alternative hypothesis considering the obtained results in cumulative N 2 O emission.To have at least a power of 80 % we would have needed 10 replicates for each treatment.The high variability solely in the liming treatment might be due to additional lime application to the field in May 2014 and the high spatial-temporal variability of that soil property in general.The two replicates that received additional limestone were the ones that emitted more N 2 O than the other plot.Hence, instead of reducing emissions by increasing the pH, the additional limestone application could have provoked local arbitrary disturbance to soil chemistry leading to emission hotspots.To determine the biochar effect on N 2 O emissions, we therefore also compared only the biochar and control treatments (see results); according to the analysis of that reduced data set, the cumulative emissions in the biochar amended plots were significantly lower (by 52 %) than in the control treatment.
Using the same measurement technique, application rate and similar biochar properties Felber et al. (2013) also reported N 2 O emission reductions, but smaller as compared to the difference we saw between biochar and control.In line with our results other field studies have shown significant reductions in N 2 O emissions following biochar amendment (Taghizadeh-Toosi et al., 2011;Liu et al., 2012).A number of studies found no significant effect of biochar addition in the field (Schimmelpfennig et al., 2014;Angst et al., 2014;Scheer et al., 2011;Karhu et al., 2011;Anderson et al., 2014).Often the much higher variability in the field and the low number of replications make it difficult to reproduce reduction effects observed in laboratory studies.In particular, Angst et al. (2014) found no significant difference but there was a tendency for lower emissions with biochar addition, suggesting that the variability in the field was too high to get significant effects.However there are also studies that showed increased emissions from biochar application in the field (Verhoeven and Six, 2014;Shen et al., 2014).There is a large variability of biochar properties and effect size between these field studies.Since the driving mechanism of how biochar reduces N 2 O emissions from soil are still unknown, it can only be shown by meta-analysis that a low H : C org ratio seems to beneficial for N 2 O suppression (Cayuela et al., 2015).
Biochar has been shown to increase water-holding capacity and reduce bulk density (Peake et al., 2014).Mukherjee et al. (2014) showed that 92 % decrease in N 2 O emissions by biochar coincided with reduced bulk density by 13 % and increased soil nanopore surface area by 12 % relative to the control.In our experiment there are some situations where increased volumetric water content with biochar coincide with reduced N 2 O emissions (Fig. 1 and 2).Although there was no significant difference in bulk density, supposed increase in nanopore surface provides both suitable pores for water retention and oxygen air.Overall the improved soil aeration by biochar dominates the effect of increased water content and hence does not favour denitrification (Van Zwieten et al., 2010).Sánchez-García et al. (2014) found that biochar increases soil N 2 O emissions produced by nitrification-mediated pathways.In our study, the water content (Fig. 1) was high during periods of high emissions, suggesting that during periods of high water content denitrification dominates the N 2 O production in soil.The high emissions were thus often triggered by large precipitation events.There are many indications from lab experiments that biochar can reduce N 2 O emissions in denitrifying conditions at high water content (Yanai et al., 2007;Singh et al., 2010;Felber et al., 2013;Harter et al., 2014).Under denitrification conditions, the pH between 6 and 8 exerts control over the N 2 O : N 2 ratio (Stevens et al., 1998), especially with a pH of the soil below 7, when the reduction of N 2 O to N 2 is inhibited by acid conditions (Simek and Cooper, 2002).Various studies have suggested that an elevated soil pH is responsible for reduced N 2 O emissions following biochar application through increased activity of N 2 O reducing bacteria (Van Zwieten et al., 2010;Zheng et al., 2012).In contrast, Yanai et al. (2007) argued that the suppression of N 2 O emissions by charcoal is not due to increased N 2 O reduction activity with increased soil pH because biochar ash increased the pH to the same degree as biochar, but did not reduce N 2 O emissions.In the lab, Cayuela et al. (2013) found no N 2 O mitigation when soil pH was increased to the same level as biochar did but with CaCO 3 addition.They also showed that biochar's buffer capacity but not biochar pH was highly correlated with lower N 2 O emissions compared to pH-adjusted biochars (Cayuela et al., 2013).In our case, we used a biochar with rather high liming capacity (17.2 % CaCO 3 ) and pH (9.8).We can confirm that with this kind of biochar N 2 O emissions can effectively be reduced also in real field conditions, although the high variability in the limed treatments does not allow us to reject the hypothesis of soil pH being the major driver of N 2 O emission reductions.
More recent studies show that biochar enhances nosZ abundance in soil bacteria, which can lead to lower N 2 O emissions (Harter et al., 2014;Van Zwieten et al., 2014).Some authors relate this enhancement of N 2 O reducing bacteria to biochar's redox activity that facilitates electron shuttling for the sensitive process of N 2 O reduction (Kappler et al., 2014;Cayuela et al., 2013).This shuttling might be the connection between reduced N 2 O emissions and low H : C org ratios (Cayuela et al., 2015) in biochar that refers to condensed aromatic structures and its quinone/hydroquinone moieties being electro-active by allowing electron transfer across conjugated pi-electron systems (Klüpfel et al., 2014).Such high electro-catalytic activity has also been shown in Ndoped C nanotube arrays (Gong et al., 2009).Hence, in contrast to a promotion of microbial N 2 O reduction, there is also the possibility that biochar abiotically reduces N 2 O through its electrocatalytic abilities represented by a high aromaticity with low H : C org ratios.Indeed, this is one of the various abiotic mechanisms that reduce N 2 O emissions suggested by Van Zwieten et al. (2015).

Yield and nutrients
In our experiment, grain and by-product biomass production was not increased by biochar application to soil.There is large uncertainty around the yield effect of biochar but metaanalyses reported an average increase of 10 % (Jeffery et al., 2011;Liu et al., 2013).Crane-Droesch et al. ( 2013) described a more detailed global response of biochar on yields.They identified a substantial and specific agroecological niche for biochar in soils with low organic C content and low cation exchange capacity, typical for highly-weathered tropical or sandy soils.Given these findings, we would not expect a large increase in productivity at our site which is rich in soil C and clay.Positive yield response could however increase with time (Crane-Droesch et al., 2013) and might not show clear effects within the first year of the biochar application yet.Our data are also in agreement with Jay et al. (2015), who showed that biochar had no effect on the yield of different crops after a single rotational application (20 and 50 t ha −1 ) in a sandy loam under intensive management.
Nitrogen uptake was not changed by biochar or liming.Although there was no significant difference in P uptake between the treatments, by-product material from biochartreated plots tended to have higher P uptake than the control (+100 % increase, data not shown).Vanek and Lehmann (2014) showed significant increase in P availability through enhanced interactions between biochar and arbuscular mycorrhizas.Liu et al. (2012) reported a biochar application rate dependent decrease in emission intensity per yield, from 0.17 kg N 2 O-N t −1 in the control to 0.10 and 0.07 kg N 2 O-N t −1 with 20 and 40 t ha −1 biochar applied.For an agronomic assessment of N 2 O emissions it is most relevant to relate the cumulative emissions to the yield (Van Groenigen et al., 2010).Emissions of the control per above-ground N uptake (29.6 kg N 2 O-N (kg N) −1 ) are much higher than the reported mean of 8.4 kg N 2 O-N (kg N) −1 at fertilisation rates between 180 and 190 kg N ha −1 (Van Groenigen et al., 2010).With biochar however, this number decreases by 60 %, whereas with lime it remains at the level of the control (p = 0.21).Yield-based emissions are a good way to express biochars effects both on N 2 O emissions and yield, but the experimental uncertainties of each data set are also cumulated within this number.

Conclusions
We found a 52 % reduction in N 2 O soil emissions from biochar compared to control treatment in a maize field trial.This shows that also in temperate, intensive maize cropping systems under real field conditions, N 2 O emissions can be reduced substantially by biochar.There is no evidence that the reduction with biochar, relative to control, is solely induced by a higher soil pH.The pH hypothesis is thus not supported by our data.

Figure 1 .
Figure 1.Soil moisture means for each treatment are shown in red, blue and green solid lines with 1 SE as shaded area.Blue bars show the rainfall in mm d −1 and the orange line is daily mean air temperature.The green bar indicates the irrigation of 33 mm with the second N fertilisation.

Figure 2 .
Figure 2. Soil pH (mean with 1 SE bars) during the time of the experiment.Significant differences (p < 0.05) are indicated with stars according ANOVA test and Tukey Honest Significant Differences (TukeyHSD) are indicated by different letters, n.s.= not significant.

Figure 4 .
Figure 4. Mean cumulative N 2 O fluxes as solid lines.Shaded areas represent the standard error of the mean from the three replicates (dashed lines) per treatment.

Table 1 .
Nitrate content (mg NO − 3 -N kg −1 ) in soil during the experiment.Standard error is indicated in brackets.

Table 2 .
Ammonium content (mg NH + 4 -N kg −1 ) in soil during the experiment.Standard error is indicated in brackets.for biochar, control and lime treatments, respectively (see Table 3 for plotwise results).Relative to the control, mean cumulative N 2 O emissions were 52 % smaller in the biochar treatment.The whole treatment effect was, however, not statistically significant (p = 0.26) due to the large variability in www.soil-journal.net/1/707/2015/SOIL, 1, 707-717, 2015 ] Lime Figure 3. Mean N 2 O emissions for each treatment (coloured line) with highest and lowest replicate in grey.

Table 3 .
Cumulated N 2 O emission per area, above-ground dry matter yield and above-ground plant N uptake for each plot (grains, stems and leaves).