4.2.1. Macronutrients
In general, most of the analyzed macronutrients were altered by soil amendments in the sandy loam, whereas, in the clay loam soil, only two were altered. For both studied soils, total soil C content increased as BC load increased. This result agreed with the tendency reported by Demisie et al. [
42] when wood or bamboo BC was applied in a clay loam soil, although they did not evaluate any manure amendment. In our research, clay loam soil had an increase in total C content when dairy manure was applied alone. These results agreed with a meta-analysis which showed that clay-texture soils had greater soil C pool increase rates compared to sandy textures soils when manure amendment was applied [
34]. This increase was expected since the BCs we studied had at least 23% fixed C.
Soil oxidizable C was not affected by BC application in sandy loam but increased when manure amendment was applied. These results agreed with other studies where no changes in oxidizable C content were reported in sandy loam soil when similar BC derived from wood or manure was tested in
T. incarnatum and
L. multiflorum. However, no manure treatments were tested in those studies [
36]. It is worth noting that, in case of the sandy loam, regardless of BC application, the incorporation of dairy manure amendment by itself increased oxidizable C soil levels by 86% in our study compared with the no-manure treatment. The fact that we found alterations in oxidizable C only for the sandy soil may be expected because it initially contained approximately only 24% of the oxidizable C found initially in the clay loam soil.
Total N was not affected by any factor or combination in the sandy loam soil, while, in the clay loam soil, a slight increase was observed due to the application of manure. This result agreed with Anger and N’Dayegamiye [
43], who also reported a total N increase when cattle manure was applied in silt loam soil. NO
3-N was not affected by any factor or combination in the clay loam soil. However, in the sandy loam soil, the application of any of the BCs alone tended to reduce the original NO
3-N soil content, indicating its capacity for adsorbing some nutrients on its surface [
44] and making them available for an extended period [
45]. Also, the presence of
C. dactylon, regardless of any other factor, showed a high reduction (94.1%) in NO
3-N soil content due to its own uptake for growing and because of the conversion to unavailable forms. On the other hand, the incorporation of manure BC along with dairy manure tended to increase the NO
3-N level when compared with control pots with no manure amendment. This could be attributed to the ability of BC to increase soil N retention [
46] and maybe manure nutrient utilization efficiency [
29]. In our study, BC alone did not increase N soil level, contradicting the reported general tendency that BC treatments increase soil N content [
47]. Other authors also reported the same tendency when saturated or unsaturated wood BC was applied in an experiment carried out with the same grass species and soil [
23] as ours.
P was not affected by any factor or combination in the clay loam soil. Although sandy loam P was reduced by the presence of
C. dactylon (24%) by itself, the application of manure BC increased P soil content (average 261.5%), regardless of manure amendment application, whereas wood BC did not alter P soil content independently whether manure amendment was applied or not. That we found considerable alterations in P only for the sandy soil may be expected because it initially contained only 10.8% of the P found in the clay loam soil. These results agreed with a study which reported similar effects of manure and wood BC on soil P content [
36]. Moreover, our results did not agree with a study which reported that P content increased when wood BC was applied in an experiment where
C. dactylon was grown in a sandy loam [
23]. If P binds to BC particles, the P increase detected could be positive in the long term for soils, water health and plant growth. On the other hand, because BC adsorption is linked to P concentration ([P]) (at high [P], the P sorption rate slows due to competition for binding sites) [
48], excess P in soils might increase nutrient runoff with negative impacts on non-targeted downstream ecosystems like ground and surface water sources [
7]. Regarding N and P alterations in sandy soils due to BC and/or organic amendment application, neither of them alone increased these nutrients’ availability when applied in a peanut–wheat rotation, but there was synergistic effect when they were combined [
31].
K was altered by BC application in both studied soils. In general, K increased by 174 to 425% in sandy loam soil when BC was applied, showing the highest increase percentage with the incorporation of manure BC. This result agreed with Nystrom [
24], who reported a soil K increase when
C. dactylon was amended with BC in a sandy loam field trial, and with El-Shony et al. [
31], who reported the same effect in a peanut–wheat rotation in a sandy soil. Major et al. [
30] reported the same effect in a maize–soybean rotation in an oxisol. BC application tended to maintain clay loam K soil level, compensating
C. dactylon uptake as well as possible leaching and/or immobilization. The considerable alterations in K only for the sandy soil may be expected because it first contained around 12.5% of the K found in the clay loam soil. Both soils already had the critical K level (165 ppm) to avoid
C. dactylon yield losses [
24].
In the case of Na, both soils showed alteration due to BC and/or manure application. Thus, a Na increase was observed in sandy loam soils when either manure BC or manure amendment was incorporated alone. In the clay loam soil, Na increased only when both amendments were incorporated simultaneously. This may be expected since sandy loam control pot Na levels showed only 25% of the Na found in clay loam; therefore, a higher input of this cation would be necessary to alter its content in clay loam soils. Finally, Ca soil level was not affected by BC or manure amendment application in either of the soils. This last result did not agree with Major et al. [
30], who reported a Ca level increase when wood BC was applied in a maize–soybean rotation.
4.2.3. Metals
Two out of the three analyzed metals (i.e., Cu, Fe) were altered in the sandy loam, whereas none was affected in the clay loam soil. Cu and Fe decreased when dairy manure was applied alone and when dairy manure or BC was applied alone, respectively. In the case of Fe, it also decreased when
C. dactylon was included. Cu and Fe content was more than two and four times greater, respectively, in the clay loam than in the sandy loam soil. In our short-term study, no limiting or toxic effect of soil metal content on the studied forage grass species was detected. Moreover, the application of manure amendment reduced Cu and Fe content. This effect was also reported by Kusiemska et al. [
32] for Cu when cattle manure was applied as an amendment in a 2-year pot experiment in a sandy soil. This might be because the organic component in manure by-products has a high affinity for metal cations because of the presence of ligands or functional groups that can chelate metals [
51]. Also, Zhu et al. [
52] reported a higher Cu adsorption rate in manure-treated soils attributed to an increase in organic-matter-induced cation exchange capacity. However, long-term applications of manure may enrich metal levels in soil, thereby exceeding crop requirements and leading to eventual phytotoxicity [
53]. In the case of Fe alteration, it might be also because cations in soils can be taken up by the BC in an anion/cation exchange and held there as an adsorption site for anions [
48,
51]. Regarding Cu and Fe, soil-incorporated BC can stabilize it and reduce their bioavailability through enhanced sorption and chemical precipitation [
15]. As the water-soluble bioactive fraction of heavy metals in soil decreases, potential uptake and bioaccumulation of heavy metals by soil organisms (including plant roots) are minimized [
54].
4.2.4. pH and Conductivity
Conductivity and pH were affected by BC and/or manure amendment application in both studied soils. In the case of sandy loam soil, pH and conductivity increased by 9% and 94%, respectively, when manure BC was incorporated alone, although an increase (34%) and even a slight decrease (1%) were also observed when manure amendment was applied alone for conductivity and pH, respectively. pH increased by 3% in the clay loam only when manure BC was applied. Even though the conductivity was not affected in this type of soil when the studied soil amendments were applied by themselves or combined, incorporation of BC alone contributed to avoiding this parameter reduction caused by the inclusion of C. dactylon.
In sandy loam soil, both BC and manure amendment altered (increased) conductivity. In general, conductivity increases as clay content increases [
55]. This agrees with our study, in which 10 times more conductivity was observed in clay loam soil control pots than in the sandy loam soil ones. The high conductivity value of the clay loam soil mainly explains the lack of this parameter alteration when BC and/or manure amendment were applied. The neutral (clay loam soil) or the slight decreasing (sandy loam soil) effect of dairy manure amendment on pH did not agree with Eghball et al. [
56], who reported a pH increase when manure or compost was incorporated into the top 10 cm in a clay loam soil based on
Zea mays N and P removal; however, there is an inconsistent relationship between manure and the soil pH [
57]. Hays et al. [
33] also reported a neutral effect on pH when dairy manure was applied on sandy and clay loam soil in a field trial.
Although BC can increase soil cation exchange capacity (CEC) and pH [
58], it might be possible that the alkaline nature of the tested BC (
Table 1) was the cause of the slight pH increase in both studied soils. However, because of the similarity between the controls (sandy loam soil, pH 7.73; clay loam soil, pH 7.52) and the highest values (sandy loam + manure BC, pH 8.40; clay loam soil + manure BC, pH 7.75), it may be that these differences were too slight to have an impact in soil and plant parameters. This result partially agreed with Hays et al. [
33], who also reported a slight pH increase in sandy loam and clay loam soil fields, respectively, when BC was applied in a field trial where
C. dactylon was growing.