Do carbon farming practices accumulate nitrogen in northern Europe agricultural soils?

In agricultural soils, Nitrogen (N) is an essential nutrient in crop production, and its supply comes from various pools. Understanding the N pools in soil could help farmers make better decisions about N fertilisation, increase productivity, reduce costs, and decrease environmental impacts. N is a challenging element to track in soil due to its seven oxidation states that are large-ly cycled and intermediated by soil organisms between various organic, mineral, and gaseous forms. This thesis aims to find whether C farming practices accumulate N in soil through investi-gating the size of soil N pools and how they relate to each other through a literature survey and data from a four-year carbon (C) farming experiment. The thesis also investigates whether meas-uring the fast-cycling N pools and including them in fertilisation recommendation calculations results in financial savings from N fertiliser cuts. Twenty different Finnish farms were investigated, split between six C farming practices: subsoil-ing, undersown cover crops, ley farming, soil organic amendments, planned grazing, diverse grasses lay. Soil samples were collected at two different depths (0-17 and 17-30 cm) and ana-lysed for total N and C (Total N, Total C), mineral N, Illinois Soil N Test (ISNT N), autoclave citrate extractable protein N (ACE-Protein N), water extractable organic N and C (WSON and WSOC), and microbial activity derived N (PMN) from CO2 burst after rewetting of dry soil (Resp C).The analysis recovered on average 25% of total N, while 75% remained in the non-recovered pool. Clay%, C% and C:Clay explained the variation in Total N (r2=0.90), ISNT N (r2=0.90), pro-tein N (r2=0.93) and the non-recovered N (r2=0.83), but it did not for WSON (r2=0.50), PMN (r2=0.21) and inorganic N (r2=0.20). Four-year C farming management did not result in statisti-cally significant differences in the N pools, even though PMN and WSON were 19% and 11.6% higher in treatment than in control, respectively. The variation was high for both (SD=48 and 42), explaining the non-significance. Strong correlations (Spearman rho, ρ) were observed between Total N - ISNT N, Residual N, and WSON, indicating their predictive capability for total N, while protein N showed weaker correlations. PMN correlated strongly with bacterial respiration C (ρ = 0.94) and WSON (ρ = 0.75) and had a negative strong correlation with the WSOC:WSON ratio (ρ = -0.73). In conclusion, soil's non-recovered N fraction, likely associated with the mineral surfaces and undecomposed particulate organic matter (POM), is the largest portion of total N. Clay%, C% and C:Clay explains the variation in the slow-cycling N pools but not in the fast-cycling ones. The quality of soil organic matter (SOM) is better assessed using WSON, PMN, and the WSOC:WSON ratio. Utilising PMN as a predictor of N release from SOM results in lower N ferti-liser recommendations, leading to cost savings for farmers. This research shows that four years of C farming do not increase the N stock in soil. Further research is needed to determine if C farming practices in a long-term experiment would increase the soil N stocks, significantly affect soil N pool relationships and result in financial savings for farmers.