1. Introduction
Various technologies have been employed in using organic materials as a source of bioenergy. In addition to bioenergy being produced, each conversion process produces a certain type of byproduct. The type of conversion process and feedstock converted largely determine the value and characteristics of these byproducts [
1]. Pyrolysis and transesterification are common technological processes used to produce biogas and biodiesel, respectively plus generation of C-rich byproducts. These associated byproducts include biochar (BC), produced during thermal breakdown (pyrolysis) of C-based feedstocks in absence of oxygen [
2] and glycerol (GL), generated during the manufacture of biodiesel via transesterification of vegetable oils. Research into finding beneficial means of their utilization is ongoing, including their application to soil [
3,
4].
Unlike GL, BC has recently attracted a global interest due to its potential agronomic and environmental benefits. Research on BC has been rapidly expanding and literature on this subject is accumulating. It has widely been evaluated as a possible means to improve soil fertility, increase crop productivity and reduce greenhouse gas emissions in a variety of soils [
3,
5,
6]. The increases in crop productivity following biochar application may occur directly through supply of essential nutrient, or indirectly through improving soil properties and functions [
2,
7]. Effects of biochar application on soil microbial biomass and enzyme activity have also been investigated, but only to a limited extent [
8,
9]. Most of the work examining the agricultural and environmental impacts of biochar application, especially the ones showing positive benefits of biochar application, has been conducted in tropical regions [
10,
11]. However, crop and soil responses may be different when biochar is applied to soils in arid and semi-arid regions. Currently, biochar application to agricultural soils is rare in Canada, and conducting more field studies with calcareous prairie soils typical of the northern Great Plains is required.
Glycerol, also known as glycerin, comprises a significant portion of biodiesel production in which every ton of biodiesel generates 100 kg of glycerol. The global production of biodiesel is projected to reach over 140 billion L by 2016 with an average annual growth of 42%, which will lead to approximately 14 billion L of crude glycerol being generated [
12]. This will lead to a surplus of glycerol and will also have an impact on the glycerol market. There is a wide range of applications for pure glycerol in pharmaceutical, food and cosmetic industries, but the refining of crude glycerol to a high purity is costly and may not be profitable for small and medium size biodiesel production plants; especially when the market for glycerol is already saturated [
13,
14]. Glycerol has also been used as a feed ingredient in animal diets to reduce diet costs [
13,
14]. Research is ongoing to explore alternative methods of crude glycerol utilization to improve the economic feasibility of the biodiesel industry. Some recent potential applications of crude glycerol have included combustion and thermochemical conversion [
15] and biological conversion or biological production of methane from crude glycerol using anaerobic sludge [
16,
17]. Despite the existing uses of crude glycerol, more applications of this byproduct need to be developed to help sustain biodiesel production. One example of a potential use of glycerol is its direct application to soil as amendment. This potential has received little attention, probably because glycerol lacks essential plant nutrient content, such as nitrogen (N) and phosphorus (P). However, one potential benefit of its agricultural use is that it could be used as a C source amendment to improve soil quality through enhancing soil organic matter content and biological activity, especially in degraded soils that contain low organic matter due to the lack of organic inputs. Under growth chamber conditions, addition of glycerol to soil led to microbial immobilization of soil N, especially when applied at a high rate (10,000 kg·ha
−1), resulting in reduction of crop yield and N uptake; however, it did significantly increase soil C content [
18]. Glycerol addition also showed a positive impact on enzyme activity and soil microbial biomass content in a controlled environment study [
19].
Glycerol is a decomposable substrate under soil conditions, especially when supplemented with N as previously reported [
20]. This indicates that C in glycerol is less resistant to microbial breakdown and expected to go through a rapid turnover in the soil compared to BC, affecting N availability for crop uptake. In contrast to glycerol, the C in BC was found to be relatively recalcitrant to decomposition in soil, as shown by low rates of mineralized C [
1]. The contrasting availability of C in both byproducts are anticipated to be reflected in their effects on crop and soil variables. Therefore, the objective of the current study was to compare the effect of two C-rich bioenergy byproducts, biochar (derived from oat hull) and glycerol (derived from canola biodiesel production), applied once to a semi-arid prairie soil on crop yield, nutrient uptake, dehydrogenase activity, soil microbial biomass C and N and selected soil chemical properties over a three-year period. The extent to which the byproduct can affect the measured crop and soil variables is expected to be determined by the C availability in each byproduct.
4. Discussion
Application of biochar alone at a rate of 2.8 T·ha
−1 did not benefit crop yield and nutrient uptake in the immediate or subsequent two growing seasons following application. This is an indication that the biochar used in the current study did not itself supply nutrient for plant uptake. Similarly, Van Zwieten
et al. [
34] generally found little crop response to biochar addition in absence of N to acidic and alkaline soils, under controlled environment conditions. Gaskin
et al. [
35] also reported limited effects of peanut hull and pine chip biochar on yield and nutrient concentrations in plants, relating this to lack of N availability from biochar. Based on the application rate used here, the biochar is calculated to add about 50 kg total N·ha
−1 in addition to about 70 kg total P·ha
−1. However, it appears little, if any, of this nutrient in the char became available for plant use, as shown in the similar N and P uptake between biochar alone amended soil and in the control soil.
Joint application of biochar and urea showed equivalent or greater yield than other treatments, despite having only half as much urea N added. The treatment of 50 kg N·ha
−1 combined with BC benefited the crop yield similar to that in 100 kg N·ha
−1 applied alone treatment. This could be due to the ability of BC to reduce urea N losses through reduction of leaching or gaseous losses [
36].
Glycerol application reduced crop yield and nutrient availability in the first growing season (spring 2009), as shown specifically by reduced N uptake in GL + UR treatment, compared to urea applied alone treatment. This is very likely a consequence of microbial immobilization of soil N. The immobilized N in GL + UR treatment in spring 2009 appeared to become remineralized and plant available during the subsequent growing season (spring 2010), resulting in higher yield and N uptake. Similarly, Qian
et al. [
18] reported that N supply from urea fertilizer was adversely affected by glycerol application, especially at the high rates, leading to a significant reduction in plant growth and N uptake. Under growth chamber conditions, glycerol amendment was also shown to immobilize soil available N, as shown by small supply rates of NO
3−-N and NH
4+-N measured in the soil [
20]. This indicates that glycerol can contribute to N retention in microbial biomass when co-applied with conventional fertilizer. In a recent study, glycerol was also found to significantly reduce N loss through minimizing nitrate leaching, owing to microbial immobilization of N [
4]. Other bioenergy byproducts with highly available C have also found to cause a short-term immobilization of N [
1].
Dehydrogenase is an intracellular enzyme participating in the biological oxidation of organic compounds in soil [
37] and is frequently reported to be related to the organic matter availability in the soil [
28,
38]. In the few studies identifying the impact of biochar on soil enzymes, there are discrepancies and inconsistencies among the documented findings. Under controlled environment conditions, Ameloot
et al. [
39] revealed that dehydrogenase enzyme activity increased in soil amended with biochars from pyrolyzed swine manure digestate and willow wood at 350 °C, but the enzyme activity was suppressed in the same soil amended with the biochars produced from the same feedstocks, but pyrolyzed at 700 °C. The authors related this to the higher level of volatile compounds present in biochars produced at low temperature that can stimulate enzyme activity, as also reported elsewhere [
40,
41]. In the current study, biochar neither increased nor suppressed dehydrogenase enzyme activity, which is in line with a recent study that utilized biochar from wheat straw [
42]. Biochar C from the source used in the current study is apparently resistant to microbial breakdown and not accessible by soil microbes, and thereby did not stimulate enzyme activity. However, glycerol applied alone or with N promoted dehydrogenase activity in the year of application that was significantly higher than any other treatment. This may be explained by lower recalcitrance of C in glycerol and greater availability for soil microbes, resulting in stimulated enzyme activity. The same was observed when glycerol was added at different rates to the same soil used in this study, but under growth chamber conditions [
19].
The soil microbial biomass can enhance nutrient cycling and availability to plants following application of organic materials to soil, due to its key role in organic matter decomposition [
43]. It is the most labile pool of organic matter, and is frequently used as a sensitive indicator of changes in soil organic matter content [
44]. Few research studies have evaluated the effect of biochar addition on soil microbial biomass content and reported inconsistent findings. For instance, Kolb
et al. [
45] found increased microbial biomass content and activity in a range of temperate soil types amended with one type of biochar whereas Dempster
et al. [
46] reported decreased MBC but not MBN in a course textured soil treated with Eucalyptus biochar. In the current study and only in the fall 2009 sampling, the biochar applied alone did not alter MBC, but decreased MBN content compared to the control, as also did biochar plus N. However, when biochar was combined with N, the content of MBC was the lowest in comparison to other treatments. The reason for significantly lower MBC in the fall after harvest in this treatment is not clear, but is coincident with greatest crop yield and nutrient uptake in the first growing season (2009) prior to first soil sampling for microbial analysis. This may be related to the depletion of soil nutrients and surface soil moisture from high crop growth that subsequently limited microbial growth and N accumulation potential. Addition of fresh and labile organic matter can activate microbial biomass and contribute to soil C mineralization [
47]. However, this does not seem to be the case with this type of biochar, which appears to be recalcitrant to microbial decomposition. Changes in nutrient and C availability may increase or decrease microbial biomass growth and activity, depending on the soil background nutrient and C and the microbial groups responsible for decomposition [
3].
The low amount of biochar applied in this study and the conditions of low precipitation (semi-arid environment) may limit the ability to show a clear effect on selected soil chemical properties, especially if applied only once. Application rate of biochar is critical for the effects on plant and soil [
11], and as reported in most studies, the greatest positive effects of biochar were observed at the rates of 100 t·ha
−1 [
5]. However, given the difficulty in applying and retaining large quantities of surface applied biochar in the field in windy prairie conditions, the selected rate in the current study was more applicable to what could be practically applied in the field on a large scale.