DeepRootLab collaborative project: Nitrogen effects on carbon deposition

In 2019, Deep Frontier initiated a joint research experiment with University of Göttingen and China Agricultural University regarding “Nitrogen effects on carbon deposition and sequestration in deep soil from deep-rooted crops”.
The joint research experiment contributes to Deep Frontier specific research objective no. 5: Study the effect of deeper and more permanent rooting on soil C storage, and develop DNA based methods for understanding root interaction with soil biology in deep soil during growth and subsequent decomposition

The experiments took place in the DeepRootsLab at Højbakkegaard. The work examined the potential of tracing N transport from surface to deeper soil layers alleviating N limitations, and subsequent compound specific analysis and microbial measurements. This could lay the foundation for future projects on drought, and P and N fertilization effects on C sequestration from deep-rooted crops.

The objectives regarding understanding C and N dynamics in deep-rooted crops were:

1.       Analysis of the potential of lucerne and kernza under N addition in promoting the stability of root-derived C, both the quantity and partitioning, based on compound specific analyses of amino acids, amino sugars and PLFA throughout the soil profile.

2.       Exploration of microbial responses to changes in N between lucerne and kernza in deep soil layers using molecular and functional approaches (e.g. changes in rhizosphere microbiome and functional genes).



The project will continue beyond the duration of Deep Frontier with expected completion by the end of 2021. Expected results are:

Carbon allocation and partitioning: The transport of C to deeper soil layers (i) quantified via 14C and 13C, where the presence of tracer in root tissue is related to rhizosphere and bulk soil - we expect high N kernza to produce greater root biomass than normal N kernza and thus have a greater deposition in deeper soil layers; and (ii) the partitioning of plant derived C in deep soil is determined via compound specific isotope analysis (CSIA) - the CSIA of PLFA’s (PLFA-SIP) will show incorporation of plant derived C in living microbial groups and the same for the CSIA of amino sugars (AS-SIP) with the AS-SIP also showing incorporation in microbial necrosis. The CSIA of amino acids fingerprint (AA-SIF) will show whether C bound in proteinaceous compounds are in plant or microbial form. We expect that high N kernza will result in a greater microbial incorporation of C-tracer compared to normal N kernza, and we expect lucerne to have a similar or greater microbial incorporation than high N kernza with lucerne to a greater extent stimulating the bacterial populations than kernza. 

Nitrogen allocation and partitioning: If N is transported to deep soil layers via plant roots we will identify this by: (i) partitioning of 15N among root tissue and soil - we expect 15N rhizosphere to be greater than bulk soil, and the 15N in rhizosphere of high N kernza to be higher than in the rhizosphere of normal N Kernza. Lucerne is expected to look like high N kernza, when looking at bulk 15N and total N content; and (ii) the specific incorporation of 15N in amino acids where the amino acid stable isotope fingerprint will allow us to identify whether 15N is predominantly found in root derived compounds or in the microbial biomass.


Some of the results are already published in:

Peixoto, L., Elsgaard, L., Rasmussen, J., Kuzyakov, Y., Banfield, C. C., Dippold, M. A., & Olesen, J. E. (2020). Decreased rhizodeposition, but increased microbial carbon stabilization with soil depth down to 3.6 m. Soil Biology and Biochemistry 150, 108008.

Peixoto, Leanne; Rasmussen, Jim; Elsgaard, Lars; and Olesen, Jørgen E. (2021). Nitrogen and phosphorus co-limits mineralization of added carbon in deep subsoil. European Journal of Soil Science (in press, doi: 10.1111/ejss.13083)