Short-Term Impact of Multi-Species Cover Cropping on Agrochemical Properties in Cambisols Versus Histosols in Northern Lithuania

Cover cropping is an essential conservation management practice to mitigate overwinter nitrogen leaching and soil degradation in hemiboreal agroecosystems. However, short-term nitrogen dynamics and agrochemical responses across contrasting soil types during the winter-fallow period remain under-quantified. This study evaluated the six-month overwintering impact of a four-species cover crop mixture (Helianthus annuus, Raphanus sativus, Fagopyrum esculentum, and Vicia sativa) on topsoil nitrogen pools and related agrochemical properties in Northern Lithuania. The research was conducted on a commercial agricultural scale (73 ha), utilizing a paired design across four spatially replicated mineral soil fields (medium-heavy loam Cambisols, n = 4) and an exploratory, unreplicated observational zone in an organic soil (Histosol, n = 1). Due to regulatory restrictions prohibiting bare fallow, the observed agrochemical shifts represent the combined influence of cover crop biological activity and natural seasonal hemiboreal weathering, rather than isolated causal mechanisms. Despite an extended autumn vegetation period facilitating substantial aboveground fresh biomass accumulation (mean 549 ± 187 g m−2), the mineral Cambisols exhibited pronounced chemical inertia. Paired-sample analyses revealed no statistically significant changes in total nitrogen, total organic matter, pH, mobile phosphorus, or water-extractable organic carbon over the overwintering period (p > 0.05). Given the high risk of winter leaching, the stability of the total nitrogen pool (−7.08%, p = 0.333) demonstrates the efficacy of the combined cover crop and weathering system in retaining residual nitrogen. A non-significant trend toward mobile potassium accumulation (+9.20%, p = 0.063) was also observed, which is presented strictly as an unverified exploratory hypothesis regarding biological nutrient pumping. In contrast, exploratory observations in the Histosol zone indicated rapid turnover, characterized by numerical surges in total nitrogen (+42.7%) and water-extractable organic carbon (+50.3%). These findings reflect observed changes during the overwintering cover-cropping period, demonstrating that multi-species mixtures assist in maintaining baseline soil chemical equilibria in well-buffered mineral soils, while highlighting potential, unverified organic matter turnover trends in organic Histosols.