## 4 Discussion
### Amendments and Soil Properties
#### 4.1.2 Biogeochemistry in the Absence of Plants  
Adding organic amendments modified soil TOC, TN, and the corresponding C:N ratio. Composition of these amendments depends on many factors. As noted above, biochar composition is heavily influenced by pyrolysis temperature and feedstock choice [46]. Our biochar originated from cottonwood chipped material and contained very high TOC (65.1%) and very low TN (only 0.42%). Consequently, BIOC had over double the soil TOC as UNAM. Knowing the chemical composition of biochar is therefore critical for making application recommendations. The high C:N ratio of the biochar used in this study (157) suggests that inherently low TN in native soil combined with low TN in biochar added will likely result in microbial N immobilization due to the initial stimulation of microbial growth that often accompanies the introduction of biochar-delivered DOC [45, 52]. This scenario can induce plant N deficiencies. This was further supported by the fact that the application of biochar to native soil in this study (7.5% _w/w_) resulted in a C:N ratio that was twice as high as the UNAM soils. Compost TOC and TN contents depend on the length of time the compost ages and whether it originates from animal or plant organic materials [78]. In general, compost applications result in short-term increases in TN, which depends on the frequency of application and the quantity applied [79, 80], along with the organic residue material from which it is derived [81]. The aged livestock manure used in this study contained three times as much TN and eight times less TOC than biochar. The compost addition therefore resulted in the greatest soil TN of all treatments, but similar TOC to INF andUNAM. High TN and low TOC in compost resulted in a very low C:N ratio (7) and hence stimulated rapid mineralization and formation of INN. However, by the end of the experiment, the C:N ratio of COMP was statistically similar to that of both UNAM and INF, suggesting that much of the readily decomposable content in COMP amendment had been mineralized. Alternatively, an application rate greater than 7.5% might have been required to substantively change the C:N ratio relative to UNAM. Soil incubation in the absence of amendments or plants (UNAM Begin and UNAM End Lab Inc) resulted in significant declines to INN and DON over time. Local native soils are known to have high nitrification potential; hence the majority of INN represents soil nitrate (unpublished data). Inorganic N was comparable between COMP and INF, but it is unclear how the low soil C:N ratio of the native soil predetermines soil N mineralization given UNAM had a similar C:N ratio to COMP and INF. Native soil enriched with organic amendments high in N (COMP and INF) had comparable INN and DON concentrations at the beginning of the experiment, however it was clear that COMP was more effective than INF in promoting soil labile N and N mineralization through decomposition (ca. 20-30% annually), as demonstrated by the significantly greater concentrations of INN and DOC at the end of the experiment. In contrast, the trend observed in INF with respect to reductions in labile N over time, though not significant, point to the lack of a continual source of DON and INN without OM or an alternative slow-release N source. The high soil C:N ratio in BIOC can partially explain the lowest INN and labile DON concentrations in the absence of plants at the beginning of the experiment. Biochar is also known to sequester labile forms of N, particularly NO\({}_{3}\)-N, primarily through sorption to base functional groups on the biochar surface [82; 83] generated during pyrolysis. For this reason, we do not recommend that gardeners use most biochar as a primary N source. Instead, a supplemental source of N may be necessary to offset potential deficiency among crops. Nonetheless, significant increases in INN and DON suggests that, even though concentrations were the lowest of all treatments, biochar underwent decomposition and N mineralization.