Biochar amendment of grassland soil may promote woody encroachment by Eastern Red Cedar (Juniperus virginiana)
Plant species can impact nitrogen availability through changes in the quality of plant derived inputs to the soil. Plant derived soil inputs (PDSIs) come in the form of plant litter and root exudates. PDSI driven feedbacks may drive exotic species invasions and woody encroachment, but PDSIs, such as biochar, often produced as an unintended byproduct of invasive species management practices (i.e. prescribed burning) may contribute to woody encroachment. In the Great Plains of North America, the simultaneous invasion of Bromus inermis and woody encroachment of Juniperus virginiana into grasslands provide an ideal context for examining PDSI-driven feedbacks on woody encroachment. Prescribed burning has been widely used to control B. inermis with the unintended production of biochar. As such, we examined the impact of soil amended with B. inermis biochar and B. inermis leaf litter on the growth of J. virginiana seedlings relative to soil amended with native grass biochar and native grass leaf litter. We hypothesized that changes in N availability resulting from B. inermis PDSI additions would positively impact J. virginiana growth. Additionally, our design also allowed us to separate the impact of grass species identity from PDSI type (biochar or litter) on soil N and J. virginiana growth.
Overall we did not find strong evidence for an exotic-woody interaction specifically, however we did find that changes in the type of PDSI (biochar vs litter) had an impact on J. virginiana success. These PDSI driven changes in J. virginiana biomass were driven by changes in soil nitrogen availability. Although many studies have highlighted species identity impacts on N cycling through litter inputs to the soil, our work shows that once litter is converted to biochar, species impacts on soil N levels disappear and any species specific litter feedbacks on plant growth are eliminated. In conclusion, our data suggests that the production of biochar through prescribed burns (regardless of grass species identity) can provide a favorable resource environment for newly deposited J. virginiana seeds and may encourage woody encroachment into grasslands.