Hot-spots of denitrification in soil depending on crop residue and liquid manure incorporation – models and experiments

Soil denitrification is known to be affected by incorporation of crop residues due to supply of reductants and oxygen (O₂) consumption by decomposition. The formation of anoxic microsites where denitrification can occur thus depends on quality, particle size and spatial distribution of incorporated residues in interaction with physical and chemical soil properties. Current biogeochemical models typically assume homogeneity of soil layers and thus don’t consider the size and distribution of crop residue particles. Until now, dinitrogen (N₂) and nitrous oxide (N₂O) fluxes related to residue incorporation patterns have been rarely studied and are poorly represented by current models. Here we present synergetic concepts and results of two projects addressing the spatial modelling of anoxic hot-spots („Modeling the impact of liquid organic fertilization and associated application techniques on N₂O and N₂ emissions from agricultural soils”, MOFANE) and measures to mitigate denitrification in soil („Measures to reduce direct and indirect climate-impacting emissions caused by denitrification in agriculturally used soils”, MinDen).

To test the incorporation of harvest residues and catch crops on denitrification, a full-factorial laboratory incubation will be carried out in MinDen, assessing incorporation methods and pre-crushing of catch crops, and how these interact with soil type and water content.

 

Based on a static model to take into account spatial hot-spots induced by liquid manure (Baral et al., 2016)  a dynamic model was developed in MOFANE and tested using lab experiments (Grosz et al., 2022). This model can also be applied to evaluate the aforementioned crop residue effects.

We present a conceptual model of denitrification in dependence of size and distribution of crop residues, soil type, soil moisture and soil structure. It predicts that pre-shredding and homogenous incorporation favours denitrification at low gas diffusivity given by high moisture and/or high clay content or bulk density, since small organic hot-spots suffice to create anoxia. For high gas diffusivity (e.g. due to sandy texture and/or dry conditions) it predicts that denitrification is favoured if incorporated crop residues are large, since anoxic microsites can only develop if the size of organic hot-spots is large enough so that the O₂ sink strength exceeds O₂ diffusion from the atmosphere.

Scenarios of the conceptual model will be tested using the dynamic hot-spot model and results will be presented.

 

Baral, K.R., Arthur, E., Olesen, J.E., Petersen, S.O., 2016. Predicting nitrous oxide emissions from manure properties and soil moisture: An incubation experiment. Soil Biology & Biochemistry 97, 112-120.

Grosz, B., Kemmann, B., Burkart, S., Petersen, S.O., Well, R., 2022. Understanding the Impact of Liquid Organic Fertilisation and Associated Application Techniques on N2, N2O and CO2 Fluxes from Agricultural Soils. Agriculture 12, 692.