Abstract
Integrating leys, cover crops, and animal manures constitute promising avenues to reach annual soil organic carbon changes (ΔSOC) >0.4% in forage and grain-based crop rotations, rates required to offset the increasing C emissions from fossil fuels (“4 per mille” initiative). How these practices and rotations perform in reaching this aim was object of analysis in this paper. Five cropping systems (CS), including three three-year forage and grain-based crop rotations containing annual grass-clover leys (FR and MR) or cover crops (GR), and two contrasting controls (continuous silage maize (CM), and permanent grassland (PG)) were compared for their impact on SOC stocks over eight years (2010–2018). The CS were unfertilized (N0) or fertilized using cattle slurry (N1) at a rate of 240 kg N ha−1 yr−1 applied in the non-leguminous crops. The ΔSOC of the top 30 cm soil layer and the annual carbon inputs (Cin) from slurry applications and plant residues were estimated, their relationship established, and the slurry-induced C retention coefficient was determined. The FR and MR SOC stocks remained stable at N1, while the GR and CM SOC decreased over time by tendency even at N1. Only the PG reached ΔSOC >0.4%. Differences in ΔSOC between CS and N rates were highly associated with the system-specific increase in belowground Cin, induced by slurry applications. Slurry-induced C retention coefficients differed strongly between CS: CM (3%) followed by GR (12%), and by FR and MR (20–15%), and lastly by PG (24%). Promoting belowground carbon inputs was identified as an efficient way to reach significant increases in ΔSOC. We conclude that a ley in only one out of three years is not sufficient to significantly increase SOC stocks in arable crop rotations of the study region.
Highlights
Recent estimates suggest that obtaining annual soil organic carbon changes (∆SOC) in agricultural soils >0.4% could offset the amount of C emitted annually from burning fossil fuels (“4 per mille initiative”) [1,2]
These results indicate that using annual leys in forage and grain-based crop rotations might not be sufficient to increase SOC stocks according to the 4 per mille initiative at sandy loamy soils in northwest Europe
The effects of integrating crop-livestock system practices like annual leys or cover crops in combination with animal manure on the SOC stocks of arable crop rotations were compared under northwest European conditions
Summary
Recent estimates suggest that obtaining annual soil organic carbon changes (∆SOC) in agricultural soils >0.4% could offset the amount of C emitted annually from burning fossil fuels (“4 per mille initiative”) [1,2] This goal is confronted, with the increasing use of energy, protein, and fiber-rich feed crops [3]. Crop-livestock farming units are being separated into either intensively-managed crop or livestock production units, leading to simplification and specialization of agricultural systems and regions [5,6] With these changes, negative environmental impacts including higher greenhouse gas emissions, higher water pollution, and the depletion of soil organic matter associated with soil fertility and multiple ecosystems services have followed, as a result of nutrient imbalances and inadequate organic matter cycling [6,7,8]. To do so, integrating croplivestock system practices into crop rotations at farms (known as mixed farming systems or MFS) and regional scale (known as integrated crop-livestock systems or ICLS) are seen as avenues to implement these measures [10,11]
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