Previous integrated or organic farming affects productivity and ecosystem N balance rather than fertilizer ¹⁵N allocation to plants and soil, leaching, or gaseous emissions (NH₃, N₂O, and N₂)

Abstract. Legumes in crop rotations are considered an ecological intensification management practice to reduce nitrogen (N) losses to the environment. However, studies on N allocation and loss on adjacent sites with the same pedoclimatic conditions but different management histories, i.e. organic farming (OF) with frequent legume cultivation and occasional organic fertilizer input, compared to integrated farming (IF) with synthetic and organic fertilizers, have remained scarce. Here, we quantified field N losses (ammonia, nitrous oxide, dinitrogen, and nitrate leaching), total N balances, and ¹⁵N labelled cattle slurry allocation to soil and plants of two adjacent sites over a two-years cropping sequence. While IF had resulted in significantly higher pH and soil organic carbon and N content, the emissions of ammonia, nitrous oxide and dinitrogen after cattle slurry application as well as nitrate leaching were not significantly different across the two farming techniques. Ammonia losses were low for all cultivation periods, indicating that drag hose application and manure incorporation successfully mitigates ammonia emissions. High ¹⁵N fertilizer recovery in plants and soil, along with a low share of unrecovered ¹⁵N agreed well with the low directly measured N losses. On average, ¹⁵N recovery was lower for OF (85 % versus 93 % in IF), likely due to unaccounted N₂ emissions which could only be measured within two weeks after fertilizer application, but the high spatial variability of ¹⁵N recovery may have turned this difference insignificant. Significantly higher harvest biomass N for IF demonstrated that management history affected productivity through increased soil organic matter mineralization. Due to the higher productivity, the cumulative N balance across all cultivation period was neutral within the limits of the measurement uncertainty for IF (-8 ± 15), indicating an optimized N management. For OF, the N balance across single cultivation period ranged from -19 to 41 kg N ha⁻¹, thus, the observations of a single cultivation period were inconclusive. The cumulative positive N balance (48 ± 14) across all cultivation periods for OF suggests that more frequent organic fertilizer additions could increase soil N (and carbon) stocks, and finally improve yield. However, the positive N balance, coupled with lower ¹⁵N recovery for OF, also points to a higher likelihood of unaccounted N losses, which would, in turn, slow down the accumulation of soil N and C over time.