Biochar and Nitrogen Fertilizer Alters Soil Nitrogen Dynamics and Greenhouse Gas Fluxes from Two Temperate Soils

Biochar (BC) application to agricultural soils could potentially sequester recalcitrant C, increase N retention, increase water holding capacity, and decrease greenhouse gas (GHG) emissions. Biochar addition to soils can alter soil N cycling and in some cases decrease extractable mineral N (NO 3 − and NH 4 + ) and N 2 O emissions. These benefits are not uniformly observed across varying soil types, N fertilization, and BC properties. To determine the effects of BC addition on N retention and GHG flux, we added two sizes (>250 and <250 µm) of oak‐derived BC (10% w/w) to two soils (aridic Argiustoll and aquic Haplustoll) with and without N fertilizer and measured extractable NO 3 − and NH 4 + and GHG efflux (N 2 O, CO 2 , and CH 4 ) in a 123‐d laboratory incubation. Biochar had no effect on NO 3 − , NH 4 + , or N 2 O in the unfertilized treatments of either soil. Biochar decreased cumulative extractable NO 3 − in N fertilized treatments by 8% but had mixed effects on NH 4 + . Greenhouse gas efflux differed substantially between the two soils, but generally with N fertilizer BC addition decreased N 2 O 3 to 60%, increased CO 2 10 to 21%, and increased CH 4 emissions 5 to 72%. Soil pH and total treatment N (soil + fertilizer + BC) predicted soil N 2 O flux well across these two different soils. Expressed as CO 2 equivalents, BC significantly reduced GHG emissions only in the N‐fertilized silt loam by decreasing N 2 O flux. In unfertilized soils, CO 2 was the dominant GHG component, and the direction of the flux was mediated by positive or negative BC effects on soil CO 2 flux. On the basis of our data, the use of BC appears to be an effective management strategy to reduce N leaching and GHG emissions, particularly in neutral to acidic soils with high N content.