Mixed effects of soil compaction on the nitrogen cycle under pea and wheat

Soil compaction is a disturbance which can be caused by highly mechanized agriculture and can constrain soil microbial diversity and functioning. Among other consequences, soil compaction decreases macropores and thereby oxygen diffusion, leading to more anaerobic niches. Such compaction-induced shifts, from aerobic to anaerobic conditions, can reduce nitrification and promote denitrification processes, leading to nitrogen (N) losses and N depletion for plant productivity. High soil moisture can intensify the negative outcomes of soil compaction, however the extent to which moisture enhances the effects of compaction is not well established for the microbial community and N cycle. To gain a better understanding of the impact of soil compaction on N partitioning in plant-soil systems, a greenhouse experiment was conducted to compare the effects of four different compaction levels on soil physico-chemical properties, soil microbial diversity and the abundance of specific functional groups within the N cycle, and the resulting impact on two different crops, i.e. pea and wheat. Soil compaction increased the bulk density from 15% (light compaction) to 25% (severe compaction). It also delayed germination for both crops and reduced yield by up to 60% for pea and up to 40% for wheat. Soil compaction induced crop specific shifts in microbial community structures. Under compaction, we observed an increase in the relative abundance of denitrifiers together with increased nitrate (NO3-) consumption and nitrous oxide (N2O) soil pore concentrations. Conversely, the relative abundance of nitrifiers stayed stable under compaction, but potentially decelerating nitrification, leading to an ammonium (NH4+) accumulation in the soil. Through an interdisciplinary approach this study showed that soil compaction effects were proportionate to the initial water content which could be a good indicator of compaction severity on agricultural fields. However, the impacts of soil compaction on crop and on microbial community and functions associated with the N cycle were not necessarily aligned. Those finding demonstrate the necessity to take into account not only the soil physical parameters but also different biological indicators in order to set up more precise recommendations for more sustainable farming systems.