Fall-applied manure can conserve excess soil-profile inorganic-N for the subsequent cropping year

Abstract

To investigate dairy manure urea fertilizer interactions in cropped soils of the semiarid-West, we fall-applied and incorporated (0–0.3 m) soil urea-N (FertN) rates of 10 (N0), 45 (N1), and 80 mg N kg⁻¹ (N2), co-applied with either no manure or 86 Mg ha⁻¹ (dry wt.) stockpiled dairy manure. Soil net N mineralization, inorganic N (InorgN*), and water extractable organic carbon (WEOC*) to a 1.2-m depth, and silage corn yield and N uptake measurements were used to derive a simple mobile (soluble) N soil budget. The InorgN's descending-pulse, soil leaching profile contrasted with WEOC's adsorption and complexation profile, in which 95% of the manure-sourced WEOC accumulated in the 0- to 0.6-m soil layer by summer's end. At the outset, treatments influenced InorgN loading only at the 0- to 0.3-m depth, where doubling FertN from N1 to N2 increased InorgN in non-manured soils by an average 2.3-fold, while in manured soils InorgN was unchanged. Manure addition inhibited the availability of the added N2 FertN, possibly by increasing NH₄⁺ adsorption or its fixation by 2:1 type clay minerals. In response to increasing FertN, net mobile-N loss from soil profiles between late fall and summer's end: (1) increased from −26.2 to 116 kg ha⁻¹ in non-manured soils and (2) decreased from −54.7 to −338 kg ha⁻¹ in manured soils. The one-time fall manure application stimulated ongoing, variable, and nonsynchronous N-cycling, which, with recurrent cycling of NH₄-N between the soil solution and exchangeable pools, interrupted and delayed transport of excess soil InorgN through soil profiles.