Cover crop C inputs; Multiple aerial and isotope insights from a long-term field trial.

Use of cover crops, mulches and other soil management practices have been widely promoted within the EU, as measures to draw down carbon dioxide and increase soil organic carbon in the fight against climate change. These approaches are being investigated in a number of programs and projects EU-wide, for example the EJP-SOIL program.

We used a long-term fully replicated maize based field trial with different crop and soil management practices, namely residue incorporation and/or inclusion of a cover crop, to explore carbon sequestration potential. We used natural abundance stable isotope approaches to follow the fate and residence time of mulched residues and to determine the most stable organic matter pools in these systems. We measured isotope signatures in particulate organic matter (POM), mineral associated organic matter (MOAM), stable aggregate bound organic matter (IA-POM) and soil microbial biomass (Mi-BIO) to characterize the impact of the soil fraction, on the fate and stability of the carbon pool and to test a number of emerging paradigms in soil science. We hypothesized that the residence time of the POM fraction and MOAM fractions would be lower under higher nitrogen inputs and that it would be possibly to detect these shifts using a stable isotope approach. Moreover, we used handheld and unmanned aerial vehicle UAV-captured multispectral data to investigate impacts of management on a plot scale.

Preliminary results show that even at low mulch rates (<3 t plant material ha^-1) mulching significantly increased soil carbon storage in the long term, albeit at lower rates than predicted. This increase was due to increases in SOM in the top-soil. As hypothesized mulch with lower C:N ratios contributed less to overall soil carbon storage, but whether this was due to differences in carbon accumulation in the short-term POM pool or long-term MOAM pool remains to be revealed by on-going isotope analysis. The high replication and detailed investigation of this long term field trial should allow us to tease out a number of processes in the carbon and nitrogen cycle and allow us recommend suitable management practices for increasing soil organic carbon stocks.