Impacts of willow and miscanthus bioenergy buffers on biogeochemical N removal processes along the soil–groundwater continuum

In this article, the belowground and aboveground biomass production in bioenergy buffers and biogeochemical N removal processes along the soil–groundwater continuum was assessed. In a sandy loam soil with shallow groundwater, bioenergy buffers of miscanthus and willow (5 and 10 m wide) were planted along a ditch of an agricultural field ( AF ) located in the Po valley (Italy). Mineral N forms and dissolved organic C ( DOC ) were monitored monthly over an 18‐month period in groundwater before and after the bioenergy buffers. Soil samples were measured for inorganic N, DOC , microbial biomass C ( MBC ) and N ( MBN ), and potential nitrate reductase activity ( NRA ). The results indicated that bioenergy buffers are able to efficiently remove from groundwater the incoming NO 3 ‐N (62% – 5 m and 80% – 10 m). NO 3 ‐N removal rate was higher when nitrate input from AF increased due to N fertilization. Willow performed better than miscanthus in terms of biomass production (17 Mg DM ha −1 yr −1 ), fine root biomass (5.3 Mg ha −1 ) and N removal via harvesting (73 kg N ha −1 ). The negative nonlinear relationship found between NO 3 ‐N and DOC along the soil–groundwater continuum from AF to bioenergy buffers indicates that DOC : NO 3 ‐N ratio is an important controlling factor for promoting denitrification in bioenergy buffers. Bioenergy buffers promoted soil microbial functioning as they stimulated plant–microbial linkages by increasing the easily available C sources for microorganisms (as DOC ). First, willow and miscanthus promoted high rates of biological removal of nitrate ( NRA ) along the soil profile. Second, rhizosphere processes activated the soil microbial community leading to significant increases in MBC and microbial N immobilization. Herbaceous and woody bioenergy crops have been confirmed as providing good environmental performances when cultivated as bioenergy buffers by mitigating the disservices of agricultural activities such as groundwater N pollution.