Carbon Availability and Nitrogen Mineralization Control Denitrification Rates and Product Stoichiometry during Initial Maize Litter Decomposition

Reinhard Well,Klaus Dittert,Birgit Pfeiffer,Johanna Pausch,Pauline Sophie Rummel

doi:10.3390/app11115309

Reinhard Well, Klaus Dittert + Show 3 more

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https://doi.org/10.3390/app11115309

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Abstract

Returning crop residues to agricultural fields can accelerate nutrient turnover and increase N2O and NO emissions. Increased microbial respiration may lead to formation of local hotspots with anoxic or microoxic conditions promoting denitrification. To investigate the effect of litter quality on CO2, NO, N2O, and N2 emissions, we conducted a laboratory incubation study in a controlled atmosphere (He/O2, or pure He) with different maize litter types (Zea mays L., young leaves and roots, straw). We applied the N2O isotopocule mapping approach to distinguish between N2O emitting processes and partitioned the CO2 efflux into litter- and soil organic matter (SOM)-derived CO2 based on the natural 13C isotope abundances. Maize litter increased total and SOM derived CO2 emissions leading to a positive priming effect. Although C turnover was high, NO and N2O fluxes were low under oxic conditions as high O2 diffusivity limited denitrification. In the first week, nitrification contributed to NO emissions, which increased with increasing net N mineralization. Isotopocule mapping indicated that bacterial processes dominated N2O formation in litter-amended soil in the beginning of the incubation experiment with a subsequent shift towards fungal denitrification. With onset of anoxic incubation conditions after 47 days, N fluxes strongly increased, and heterotrophic bacterial denitrification became the main source of N2O. The N2O/(N2O+N2) ratio decreased with increasing litter C:N ratio and Corg:NO3− ratio in soil, confirming that the ratio of available C:N is a major control of denitrification product stoichiometry.

Highlights

Returning of crop residues is a common agricultural management strategy to prevent nutrient losses and to increase soil fertility
Water-soluble C:N was highest in maize straw and lowest in maize roots. 13C-CPMAS NMR spectroscopy of maize litter revealed that maize straw and maize leaves were closer in their chemical composition than maize roots (Table 1, spectra in Supplementary Figure S1)
We found a significant negative correlation between the ratio of cumulative N2O/(N2O+N2) emissions during anoxic incubation and the ratio of water-soluble C:N of maize litter types

Summary

Introduction

Returning of crop residues is a common agricultural management strategy to prevent nutrient losses and to increase soil fertility. Plant litter provides nutrients for decomposing and denitrifying microorganisms. For litter with C:N < 25:1, mineralization increases soil NO3− content leading to increased denitrification [5,6], while for C:N > 25:1, N is immobilized by soil microorganisms to decompose litter C compounds [7] and restricts N2O emissions [8]. When litter quality was analyzed in more detail, degradable fractions explained a large share of the variability of N2O emissions [9,10], while the lignin content was not relevant [11,12]. Recent studies confirm that the quality of C compounds (especially water-soluble C) from litter is a main driver of denitrification after litter addition [13,14]

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