Short-term warm-season cover crops favor nitrogen retention in northern climates

Abstract Warm-season cover crops can reduce nitrogen (N) leaching, build soil organic matter (SOM), and provide N to subsequent cash crops, but the extent of these functions in northern climates remains unknown. We evaluated these ecosystem functional responses to cover crop quantity and quality in contrasting northern US soil types. Cover crop single species (buckwheat, sunn hemp) and bicultures (chickling vetch/sudex and cowpea/sudex) were established for two seasons at two organic, tilled vegetable production sites: a loamy sand (11 g SOM kg −1 ) and a silt loam (23 g SOM kg −1 ) at the 0–15 cm depth. Cover crop treatments were grown for two growing season lengths and compared to bare fallows. Soil samples were collected before and after cover crop termination for analysis of extractable organic and mineral N (EON, MinN), potentially mineralizable N (PMN), permanganate oxidizable C (POX-C), extractable organic C (EOC), and fluorescein diacetate hydrolysis (FDA). Cover crops decreased MinN compared to bare fallows by up to threefold at the low OM site (4.2 vs. 1 mg kg −1 soil), and 11-fold at the higher OM site (36 vs. 3 mg kg −1 soil), while PMN and EON showed the opposite trend but only in some site-years and treatments. Effects on FDA and soil C were inconsistent. Relationships among soil variables, cover crop biomass, and C:N were analyzed with Pearson and partial correlations controlling for cover crop variables. Significant ( $$\Delta$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>Δ</mml:mi> </mml:math> R &gt; 0.25) changes in correlations between MinN and PMN/EON/EOC were seen at the lower OM site, indicating cover crop biomass was the primary driver of correlations. Lack of significant correlation changes in the higher OM site indicate buffering effects from SOM. Warm-season cover crops can contribute to MinN retention in northern climates but do not appear to impact labile C.