Mitigation of Greenhouse Gas Emissions with Biochar Application in Compacted and Uncompacted Soil

: Biochar may offer a substantial potential as a climate change mitigation and soil improve-ment agent; however, little is known about its effects in fertile soils subjected to standard agricultural practices. The aim of this short-term (60 days) lab experiment, under controlled temperature and soil moisture regimes, was to investigate the interaction between soil compaction and fertiliser and biochar addition in relatively fertile Luvisol. Three different biochar types and two soil compaction levels were investigated to describe their interactive effect on soil greenhouse gas emission (GHG). A very strong effect of soil compaction on N 2 O emission (+280%) and an interaction with biochar were found. The cumulative N 2 O emissions from the compacted soil were higher (from +70 to +371%, depending on the biochar type) than the uncompacted soil. Soil compaction resulted in a faster onset and a faster decrease of N 2 O production. Biochar did not affect the temporal dynamics of N 2 O evolution from either soil. The addition of digestate/crop biomass biochar has resulted in a signiﬁcant increase in CO 2 evolution both in compacted and uncompacted soils, compared to softwood from spruce (mixture of branches and wood chips) and wood pallets from softwood (spruce without bark) biochar. In the compacted soil, NH 4+ availability was positively related to N 2 O efﬂux, and CO 2 emission was positively correlated to both NH 4+ and SOC content. An increase in GHGs as a result of an increase in NH 4+ availability was seen both in compacted and uncompacted soils, while the rates of N 2 O emission were modiﬁed by biochar type. Our results show a strong interaction between biochar and soil conditions and a strong effect of biochar type on GHG emissions from agricultural soils.


Introduction
Agricultural soils are one of the most important anthropogenic sources of GHG emissions to the atmosphere [1]. According to the IPCC [2], agriculture generates 11% of global GHG emissions due to soil and nutrient management and livestock farming. Modern agriculture is characterised by its reliance on mechanised agronomic operations and intensive soil management practices. Heavy vehicular traffic accompanying these operations increases the risk of soil compaction in arable soils [3][4][5], with a consequent change in GHG emissions [6]. At the same time, soil compaction is among the most significant drivers of soil had 12.1 g kg −1 of SOC on average, its pH (KCl) was 6.0, and the bulk density (BD) was 1.49 g cm −3 . The soil was homogenised, air-dried at 22 • C for 7 days, and finally sieved through a 10 mm sieve to remove larger debris and coarse materials to prepare the soil substrate for the experiment.
This study used three different biochars, pyrolyzed from different feedstocks by varying methodologies (Table 1). A mesocosm experiment was set up in a complete factorial design with 5 replicates per treatment for GHGs measurements and another set of 6 replicates per treatment for soil properties measurements. All treatments featured the addition of the equivalent of 70 kg N ha −1 to mimic typical arable farm soil management. Mesocosms were established by filling 1000 cm 3 polypropylene buckets (surface area: 70.9 cm 2 , height: 14.3 cm) with 0.7 kg of dry soil. They were pre-incubated for 7 days until the initial flush of CO 2 flux decreased to the background level. After that, four soil treatments were established, one with N addition only (NPK 15:15:15) and three with the addition of N and a specific type of biochar (B1, B2, and B3) at the rate corresponding to 30 t ha −1 . Each set of treatments (set for GHGs measurements and set for soil properties measurements) was established twice to test compacted and uncompacted soil. In loamy soils such as those used here, optimal bulk density (BD) values range from 1.1 to 1.3 t m −3 . The critical BD value indicating soil compaction in loamy soils is 1.45 t m −3 [40]. At this BD, the physical condition deteriorates to such an extent that the growth of plant roots is limited, resulting in a reduction in crop yield. Correspondingly, the first series of mesocosms was set up to represent compacted soil as sampled in the field, with an average BD of 1.49 g cm −3. Adequate mass of soil was weighed into each mesocosm and then manually compacted to the required volume. The second series of mesocosms featured uncompacted soil at 1.02 g cm −3 , simulating uncompacted conditions after the tillage of the soil. Soil water content of 18% by weight was established to represent the mean water content in field conditions at the agricultural field in Kostelec nad Ohří during the vegetation period. Soil water content was adjusted gravimetrically after each air sampling event throughout the experiment.

Incubation Experiment and Soil Analysis
The 60-day incubation experiment was carried out at a constant room temperature of 22 • C, and all mesocosms were left open throughout the experiment and kept in the dark to prevent potential autotrophic C fixation. Half of the mesocosms were randomly allocated to the gas flux observations, while the other half were assigned to soil sampling. For the GHG emission mesocosms, the headspace of each bucket was hermetically closed during the time of observation by a polypropylene lid equipped with a rubber septum. Direct fluxes of N 2 O and CO 2 from the soils were then measured by a variation of the closed chamber technique [41]. Air samples were taken four times during the first week, then two to three times a week for three weeks, and then once a week for four weeks. In total, there were 16 measurement episodes during the experiment. Mesocosm lids were closed for 30 min, and air samples from each mesocosm were collected using an air-tight syringe (Hamilton, Bellefonte, PA, USA) through the rubber septa. Air samples were immediately transferred to hermetically close pre-evacuated 10 mL glass vials (Labco Exetainer, Lampeter, UK). A gas chromatograph (Shimadzu GC-2010 Plus, Kyoto, Japan) was used, fitted with an electron capture detector (ECD) for N 2 O and a thermal conductivity detector (TCD) for CO 2 analysis. The chromatograph was calibrated using three certified standard gas mixtures (N 2 O, CO 2 , and N 2 ) in the expected concentration range. Daily and cumulative N 2 O and CO 2 fluxes were then calculated [42].
Samples were collected from the soil sampling mesocosms on the first day and then every 10-14 days throughout the experiment: 6 times throughout the experiment, each mesocosm was destructively sampled only once. We used a 2 cm diameter corer to take three subsample cores, these were mixed together to create a single composite sample per mesocosm. Samples were then analysed for soil mineral N (NO 3 − , NH 4 + ) content, soil pH (KCl), and soil organic carbon (SOC). Inorganic forms of N (NH 4 + and NO 3 − ) was isolated in 1% K 2 SO 4 as described by Yuen and Pollard [43] and determined using the calorimetric spectrometer method (WTW SPECTROFLEX 6100, Weilheim, Germany). The SOC was estimated by the Tyurin wet oxidation method using a mixture of 0.07 mol dm −3 of H 2 SO 4 and K 2 Cr 2 O 7 with titration using 0.01 mol dm −3 of Mohr's salt [44]. Soil pH was measured potentiometrically in 1 mol dm −3 KCl (1 g soil to 2.5 mL KCl) using a pH meter (HI 2211, HANNA Instruments, Smithfield, RI, USA).

Statistical Analyses
A mesocosm was the unit of replication in this study; all observations carried out within a mesocosm were averaged to this level. GHG emission data were examined by fitting a series of models to the timeline of gas measurements and then choosing the best-fitting model (second-order polynomial, apart from cumulative N 2 O data where exponential plateau was fitted). The cumulative totals of CO 2 and N 2 O emissions were used to compare the treatments. A two-way ANOVA was performed where biochar type was nested within soil compaction. All data were tested for ANOVA assumptions (Levene and Shapiro-Wilk test), no correction was necessary. Where an overall significant effect of biochar or compaction was detected, a post-hoc pairwise comparison with Bonferroni correction was performed. Statistical significance of effects is reported at p < 0.05. Simple and multiple linear regression models were used to assess the contribution of selected soil parameters to GHG emissions. Mean values per treatment were used for each data point for gas and soil variables (n = 24), not allowing for comparison of biochar type.

Effects of Soil Compaction and Biochar on N 2 O Emission
We found a very strong positive effect of soil compaction on N 2 O emission, as well as interaction with biochar. The cumulative N 2 O emissions were about three times higher in the compacted soil than in the uncompacted soil (p < 0.001, Figure 1). Biochar addition did not have an overall effect on N 2 O emission (p = 0.317). We saw a significant difference in the production of N 2 O as a result of biochar type only in compacted soils (p = 0.047). Looking at the pairwise comparisons, we did not find any difference between the effects of biochar type on N 2 O emissions.  Figure 2 shows the temporal dynamics of cumulative N 2 O emissions over the observed period. As well as higher totals, the compacted soil is characterised by a faster onset and faster decrease of N 2 O production. The evolution of N 2 O reached 90% of its final value on day 16 of the experiment, whereas on average, it took 36 days to reach this threshold in the uncompacted soil. Interestingly, biochar did not affect the temporal dynamics of N 2 O production from either soil compaction type.

Effects of Soil Compaction and Biochar on CO 2 Emission
In contrast to N 2 O, we found a very strong effect of biochar type on CO 2 production. Adding N + B1 and N + B3 did not affect cumulative CO 2 production when compared to the no biochar treatment (N) in either compacted or uncompacted soil. Mixing N + B2 into the soil, however, has resulted in a significant increase of CO 2 evolution both in compacted (p < 0.001) and uncompacted (p = 0.005) soils ( Figure 3). In compacted and uncompacted soil under N + B2 treatments, the overall cumulative increase in CO 2 was 233% and 40% higher than the N-only treatment. We also observed a significant effect of soil compaction (p < 0.001), in the uncompacted soil. All except the N + B2 treatment acted as a CO 2 sink very shortly after the start of the experiment. Interestingly, as can be seen in Figure 4, all treatments consumed CO 2 by the end of the experiment.

Relationships between Greenhouse Emissions and Soil Properties
We investigated the relationships between key soil parameters (pH, NH 4 + , NO 3 − , and SOC) and the emission of GHGs. Multiple regression models did not indicate any capacity of these four soil parameters to predict either N 2 O or CO 2 emission from uncompacted soil (Tables 2 and 3). In compacted soil, on the other hand, we found that NH 4 + availability had a positive relationship with N 2 O efflux (p < 0.05). In addition, CO 2 emission from compacted soils was positively affected by both NH 4 + and SOC (p < 0.05).  − -nitrate, pH-soil pH, SOC-soil organic carbon, bold-coefficient is statistically significant p < 0.05, *-regression through origin (assuming that intercept = 0). Simple linear relationships between CO 2 , N 2 O, and soil parameters were also constructed (Tables 4 and 5). The model fits between N 2 O and soil properties were more accurate than in the case of CO 2 . N 2 O emissions were reduced by increasing soil pH. The intensity of the relationship was influenced by the type of biochar itself but also by soil compaction (Table 4). In all biochar treatments in compacted or uncompacted soils, N 2 O emissions increased linearly as a result of increasing NH 4 + in the soil. In uncompacted soil and in N + B1, N + B2, and N + B3 treatments, N 2 O emission increased for each 1 g kg −1 NH 4 + by 6.09, 3.05, and 3.71 mg kg −1 soil, respectively. In compacted soil, the same trend was observed, however, the rates of increase were significantly lower. CO 2 emissions increased due to increasing NH 4 + content in both compacted and uncompacted soil (except N + B3 in uncompacted soil) (Table 5). Interestingly, greater SOC as a result of biochar application did not affect either N 2 O (except N + B2) or CO 2 emissions.  n.d.-non-detecated, n.s.-nonsignificant, * p < 0.05; ** p < 0.01; *** p < 0.001; Soil addition treatments: N-nitrogen fertilisation, B1-softwood from spruce (mixture of branches and wood chips) biochar, B2-digestate biochar, and B3-wood pallets from softwood (spruce without bark) biochar.

Soil Compaction, Biochar Addition, and GHG Emission
Soil compaction alters soil structure and hydrology, chiefly by changing the physical arrangement of soil aggregates. In turn, alteration of soil physics in arable soil influences root and shoot growth and consequently crop production [7]. If soil properties change as a result of the compaction, the flux of GHGs is likely to change; the suggestion is confirmed by our results (Figures 1 and 3). Changes in GHG production and efflux are linked to changes in soil structure and physical properties [45]. Clearly a negative factor, soil compaction can be reduced mechanically or through the addition of manures and various organic additives such as biochar. Organic material particles typically are less dense than compacted mineral soil, and their application to the soil reduces the bulk density of the soil [31][32][33]46]. Organic material also supports the formation of the soil structure and increase of the porosity [34,47]. Tullberg et al. [48] reported that soil compaction affects GHG emissions, N 2 O production in compacted soil was increased by 30-50% compared to uncompacted soil. In our case, the average cumulative production of N 2 O was increased by 70-371% as a result of compaction (Figures 1 and 2). Our results also show that the application of different types of biochar affects the production of GHGs to a varying degree. The onset and the subsequent dynamics of GHG emission depend on the availability of more easily degradable organic substances in the soil-biochar complex [49,50]. The porosity of biochar itself and its ability to form soil aggregates and pores [33,34] can support aeration, which reduces N 2 O production through nitrification [51]. Biochar can reduce the emission of N 2 O from the soil into the atmosphere via adsorption of NH 3 [26] and decrease the inorganic N pool by enhancing the activity of nitrifiers [52]. On the other hand, if the soil is saturated with water, the soil pores and the pores of biochar itself (biochar is not part of soil aggregates), are filled with water, and an anaerobic environment is created. Such conditions typically lead to increased denitrification and subsequent N 2 O emissions [48]. Biochar properties affect its interaction with the soil and affect the GHG balance of the system [53]. For example, higher pyrolysis temperatures contribute to incorporating C and N into aromatic and heterocyclic rings and reducing mineralisation, and thus their availability once applied to the soil [54]. Conversely, a final product of pyrolisation conducted at a lower temperature is characterised by higher mineralisation in the soil [55].
In the case of CO 2 emissions, soil quality seems to be one of the most fundamental factors: more fertile and healthier soils seem better at C sequestration than their less productive counterparts [56]. Healthier, more productive soil is typically richer in stable SOM, which is less prone to oxidation and contributes to the chemical bonding capacity of the soil. This observation is likely is confirmed by our findings in uncompacted soil ( Figure 4). We used Luvisol, which usually denotes a highly fertile but very intensively used soil, subjected to extensive cultivation, fertilization, or liming [57]. As suggested by our results, an important factor influencing GHG emission could be the interaction between compaction and the type of biochar. Biochar surface contains functional groups which favour the adsorption of simple dissolved organic compounds and NH 4 + ions, thus providing a suitable microbial habitat [58]. The CO 2 flux showed a decreasing trend in all soils, but especially so in uncompacted soil ( Figure 4). This is usually attributed to decreasing substrate accessibility to microorganisms [59,60]. Here, biochar may stimulate microbial activity by providing a steady supply of organic compounds and nutrients. For example, biochar B2 was produced from 35% corn digestate residues, 35% cereal straw, and 30% green compost at lower temperatures compared to B1 and B3. B2 was also characterised by the highest macronutrients content, the narrowest C:N ratio, and the lowest specific surface area. The stimulating effect of B2 addition on microbe respiration and subsequent CO 2 emission was very clear in our study. Faster mineralisation of biochar was observed when it was produced at lower pyrolysis temperatures from grass biomass [55], whereas biochar produced at higher temperatures from wood materials had lower mineralisation rate [61]. Finally, we observed negative CO 2 emissions in our mesocosms. The growth of soil algae can sequester CO 2 from the atmosphere, we did not observe algal growth in our mesocosms, however this process cannot be entirely ruled out. The other likely process driving CO 2 sequestration in the soil in our mesocosms is the dissolution of CO 2 in deionised soil water used to maintain stable soil moisture [60].

Relationships between Greenhouse Emissions and Soil Properties
It is evident from our observations that finding a uniform mechanism affecting GHGs in soils at different levels of compaction and after the application of different types of biochar is not straightforward. Multiple regression models did not indicate any capacity of soil pH, NO 3 − , NH 4 + , and SOC to predict N 2 O or CO 2 emission from uncompacted soil. On the other hand, in compacted soil, NH 4 + had a positive relationship with N 2 O, while CO 2 emission was positively affected by both NH 4 + and SOC. An increase in CO 2 emissions as a result of increasing NH 4 + in the soil was confirmed by our linear model in both compacted and uncompacted soil, while the rates of N 2 O emissions efflux depend on biochar type. Observations published by Balashov [62] suggest that a very strong factor influencing GHG emissions is the filling of soil pores with water forcing a switch between anaerobic and aerobic conditions in soils. Horák et al. [63] stated that the soil pH, but also the NH 4 + content, have a major effect on increasing N 2 O emissions in particular since soil pH exerts control over the N 2 O:N 2 ratio during denitrification [64] which was partially confirmed in a few treatments (Tables 2 and 3).

Conclusions
Our results suggest that some biochar types offer the promise of mitigating GHG emissions from agricultural soils, however, the effects can be different in compacted and uncompacted soils. None of the biochar types tested in this experiment affected N 2 O emissions in either compacted or uncompacted soils. Soil compaction significantly enhanced both N 2 O and CO 2 emission from Luvisol used in this experiment. In addition, biochar produced from a combination of digestate and crop biomass strongly increased CO 2 production in both compacted and uncompacted soils. Clearly, more research into the interactive effects of biochar and soil properties on GHG emissions must be conducted before the GHG benefits of large-scale application of biochar to arable soils can be recommended. Data Availability Statement: The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable request.