Variations in water availability and N cycling across different seasons in cover crop systems

Context: Cover crops (CCs) exert significant influences on both soil water content (SWC) and nitrogen (N) cycling, with their effects often varying across different stages of crop succession. Many research endeavors overlook this seasonal variability by focusing solely on single sampling time points. Objective: This study seeks to explore how the introduction of diverse CCs influences the seasonal fluctuations of soil nitrate nitrogen (NO3-N) and biological (soil N functional genes – NFGs) components of the N cycle, SWC, and cash crop yield over a three-year maize-soybean succession in northeastern Italy. Methods: Three CC management systems were compared: a fixed treatment with triticale; a 3-year succession of rye, crimson clover, and mustard; and a weedy fallow as control. Soil N cycling was assessed using real-time PCR and Ion Chromatography, SWC through Sentek's Diviner2000. Results: CCs had no impact on cash crop yields and did not display water competition with subsequent cash crops compared to the weedy fallow. At CCs termination, grasses decreased soil NO3-N content (as catch crops) while enhancing potential microbial N-fixing activity (nifH), whereas clover led to the highest residual NO3-N and potential N nitrification (AOA). Agronomic operations likely mitigated differences in NFG abundances following CC residue incorporation. During the cash crop season, clover, mustard, and weeds (including wild legumes) were estimated to release higher amounts of N according to the CC-NCALC model, compared to rye and triticale which exhibited immobilization. Nonetheless, consistent N nitrification and denitrification potentials were observed across all treatments except for weedy fallow, with higher NFG abundance when soybean was cultivated instead of maize, underscoring the influence of cash crop species on N transformation dynamics. Conclusions: CC and cash crop species, seasonality of crops sequence, and management operations represent pivotal factors shaping the soil N cycling dynamics intricately governed by N-cycling microbial communities and the temporal variability of the SWC. Upon CC termination, grass CCs decrease soil NO3-N content, whereas clover CC sustain high NO3-N content, enhancing microbial nitrification. Agronomic operations disrupt differences in N processes subsequent to the incorporation of different CCs. However, all CC residues enhance microbe-mediated nitrification and denitrification by cash crop harvest time, potentially more pronounced in the presence of soybean cash crop compared to maize. Significance: The substantial seasonal variability observed emphasizes the necessity of carefully timing sample collection within a crop succession (e.g., at CC termination) for effectively utilizing NFGs and chemical indicators to assess the impacts of CCs on N dynamics.