Crop plant root effects on the soil environment for denitrification in agricultural soils

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Easily degradable organic compounds in soil such as residues deposited by crop plants and soil nitrate are among the key drivers of microbial denitrification in agricultural soils. The activities and composition of denitrifying microbial communities are controlled by numerous environmental factors. This project focuses on the denitrification controls and effects exerted by crop plants. Plants take up nutrients and particularly nitrate rapidly and consume water by transpiration and photosynthesis. This leads to alteration of nitrate availability and soil moisture which may lower denitrification. At the same time plants may accelerate denitrification by translocating assimilates to their roots. Here these carbon compounds lead to root respiration consuming soil oxygen and to rhizodeposition providing easily degradable carbon compounds to soil microbes. Thus, these plant activities not only lead to changes in abiotic conditions for denitrification but they also influence microbial communities in soil and especially rhizosphere communities. We propose a series of experiments under controlled conditions for assessing denitrification controls through plants and one them will extend to field conditions. 15N and 13C stable isotope labelling will be employed to trace denitrification gas fluxes (N2O, NO and N2) as well as rhizodeposition effects on microbial communities. The objective is to assess the influence of crop plants and their roots as well as crop residue effects on soil denitrification rates to provide quantitative data on plant effects for parametrization and validation of denitrification modelling in agricultural systems. Building on methods and results of the first project phase there will be three major working fields: i) effects the composition, i.e. quality of plant shoot and root litter on soil organic carbon availability for denitrification and denitrification product stoichiometry. Here our activities are closely linked to project P3 of the DASIM research unit where we jointly address soil organic matter quality descriptors; ii) quantification of the dynamics of rhizodeposition and plant N uptake and their effects on denitrification and gaseous product rates. This makes up Associated Experiment 13 of the DASIM Research Unit and stable isotope labelling will be used not only for gas flux analysis but also for functional gene analysis and assessment of the diversity of microbial N cycling communities in cooperation with P3; iii) effects of growing plants and N uptake on denitrification and the rates of different gaseous denitrification products in an agricultural field situation. The study will also employ 15N stable isotope labelling using an improved field methodology developed by P6 and it will include a number of qualitative assessments mentioned before to provide data for parametrization and validation of the LandscapeDNDC model which is a key focus of project P7.