Cover crop nitrogen uptake: summer drought constraints and mild winter recovery
摘要
Cover crops (CC) mitigate nitrate leaching through nitrogen (N) uptake and carbon (C) driven microbial immobilisation. These dynamics are heavily modulated by environmental variables, which this four-year field study on a Stagnic Luvisol (Lower Rhine, Germany) quantifies to resolve the plant-soil-microbe N-nexus in intensive arable cropping systems. Annual rotations of CC species and mixtures were evaluated between winter cereals and summer crops. CC above- and below-ground biomass and their C and N content were determined in autumn and, in 2020, also at the end of winter. Further, microbial biomass (MB) C and N at 0–30 cm and inorganic N at 0–30, 30–60 and 60–90 cm depth, were quantified in autumn and end of winter. In addition, soil prokaryotic and fungal gene abundance were determined in the first two experimental years. Regardless of mixture complexity, CC reduced soil inorganic N by 54%, fuelled by an average N uptake of 69–176 kg ha−1. Biomass recovery in winter 2020 following the 2019 drought underscores the significance of winter-active N-scavenging in mild climates like the Lower Rhine. Although C inputs (1.3 to > 3.5 t ha−1) did not consistently expand microbial biomass -with MBC and MBN holding at 602–852 and 103–224 kg ha−1, respectively- the 35–71 kg ha−1 seasonal MBN variability reveals a substantial microbial capacity for N immobilisation. These findings suggest that CC-derived C may trigger N transformations (immobilisation/mineralisation) rather than increasing MB, a dynamic requiring further field validation. Given that climate change (e.g. dry summers, mild winters) disrupts CC growth and N-uptake patterns, CC mixtures may offer functional redundancy, ensuring biomass production and N conservation across unpredictable winter conditions.