Association of Earthworm-Denitrifier Interactions with Increased Emission of Nitrous Oxide from Soil Mesocosms Amended with Crop Residue

ABSTRACTEarthworm activity is known to increase emissions of nitrous oxide (N2O) from arable soils. Earthworm gut, casts, and burrows have exhibited higher denitrification activities than the bulk soil, implicating priming of denitrifying organisms as a possible mechanism for this effect. Furthermore, the earthworm feeding strategy may drive N2O emissions, as it determines access to fresh organic matter for denitrification. Here, we determined whether interactions between earthworm feeding strategy and the soil denitrifier community can predict N2O emissions from the soil. We set up a 90-day mesocosm experiment in which15N-labeled maize (Zea maysL.) was either mixed in or applied on top of the soil in the presence or absence of the epigeic earthwormLumbricus rubellusand/or the endogeic earthwormAporrectodea caliginosa. We measured N2O fluxes and tested the bulk soil for denitrification enzyme activity and the abundance of 16S rRNA and denitrifier genesnirSandnosZthrough real-time quantitative PCR. Compared to the control,L. rubellusincreased denitrification enzyme activity and N2O emissions on days 21 and 90 (day 21,P= 0.034 andP= 0.002, respectively; day 90,P= 0.001 andP= 0.007, respectively), as well as cumulative N2O emissions (76%;P= 0.014).A. caliginosaactivity led to a transient increase of N2O emissions on days 8 to 18 of the experiment. Abundance ofnosZwas significantly increased (100%) on day 90 in the treatment mixture containingL. rubellusalone. We conclude thatL. rubellusincreased cumulative N2O emissions by affecting denitrifier community activity via incorporation of fresh residue into the soil and supplying a steady, labile carbon source.

Soil N2O emissions in a Mediterranean shrubland disturbed by experimental fires

In the present work, post-burning soil N2O fluxes and related microbial processes were investigated in a Mediterranean shrubland subjected to experimental fires. Nine plots were selected, of which three were used as controls, three were burned with low-intensity fire and three with higher intensity fire. N2O fluxes, soil humidity and temperature were measured starting 2 days before burning and for 1 year after fire. Potential net nitrification, denitrification enzyme activity, mineral N and organic C were measured from soil samples collected periodically after burning. Cumulative data indicate a doubling of N2O production in burned plots over 1 year. Burned plots showed an increase of frequency of hot spots of N2O production. A slight detrimental effect of fire on the analysed biological activities was detected only immediately after burning. After 3 months, both potential net nitrification and denitrification enzyme activity had mostly recovered and potential net nitrification further increased over control levels in the following months. Fire seemed to induce a change in the main source of N2O, which in control plots was represented by heterotrophic activity (50–75%), whereas in burned plots it was mostly of autotrophic origin, most probably due to the significant increase of soil NH4+ after burning.

Grassed buffer strips as nitrate diffuse pollution remediation tools: management impact on the denitrification enzyme activity

The management of grassed buffer strips proved to be an efficient remediation technique in controlling nitrogen losses to surface water. In south Belgium, agri-environmental policies have encouraged farmers to seed buffer strips along rivers, in zones where the soil was previously devoted to agricultural production. We wanted to assess how important denitrification is in a buffer strip in comparison with a cropped field. The study investigated the denitrifiying enzyme activity (DEA) of two contiguous buffer strips with different management stories. The eastern part of the buffer strip was seeded in 1999. The western part of the buffer strip is a piece of crop field abandoned by the farmer 20 years ago and not managed for the last 10 years. This experimental study demonstrates that the denitrification enzyme activity in a riparian buffer strip is significantly higher than in the adjacent cropped field (3.67 and 2.12 mgNkg–1d−1 respectively). The DEA was significantly different between the two buffer strips under comparison, assessing that the management of the buffer strips has a dominant effect on DEA. The old unmown buffer strip is potentially more efficient in the nitrate removal process than the 6-year-old seeded buffer strip.