Abstract
Abstract. Chemical composition of root and shoot litter controls decomposition and, subsequently, C availability for biological nitrogen transformation processes in soils. While aboveground plant residues have been proven to increase N2O emissions, studies on root litter effects are scarce. This study aimed (1) to evaluate how fresh maize root litter affects N2O emissions compared to fresh maize shoot litter, (2) to assess whether N2O emissions are related to the interaction of C and N mineralization from soil and litter, and (3) to analyze changes in soil microbial community structures related to litter input and N2O emissions. To obtain root and shoot litter, maize plants (Zea mays L.) were cultivated with two N fertilizer levels in a greenhouse and harvested. A two-factorial 22 d laboratory incubation experiment was set up with soil from both N levels (N1, N2) and three litter addition treatments (control, root, root + shoot). We measured CO2 and N2O fluxes, analyzed soil mineral N and water-extractable organic C (WEOC) concentrations, and determined quality parameters of maize litter. Bacterial community structures were analyzed using 16S rRNA gene sequencing. Maize litter quality controlled NO3- and WEOC availability and decomposition-related CO2 emissions. Emissions induced by maize root litter remained low, while high bioavailability of maize shoot litter strongly increased CO2 and N2O emissions when both root and shoot litter were added. We identified a strong positive correlation between cumulative CO2 and N2O emissions, supporting our hypothesis that litter quality affects denitrification by creating plant-litter-associated anaerobic microsites. The interdependency of C and N availability was validated by analyses of regression. Moreover, there was a strong positive interaction between soil NO3- and WEOC concentration resulting in much higher N2O emissions, when both NO3- and WEOC were available. A significant correlation was observed between total CO2 and N2O emissions, the soil bacterial community composition, and the litter level, showing a clear separation of root + shoot samples of all remaining samples. Bacterial diversity decreased with higher N level and higher input of easily available C. Altogether, changes in bacterial community structure reflected degradability of maize litter with easily degradable C from maize shoot litter favoring fast-growing C-cycling and N-reducing bacteria of the phyla Actinobacteria, Chloroflexi, Firmicutes, and Proteobacteria. In conclusion, litter quality is a major driver of N2O and CO2 emissions from crop residues, especially when soil mineral N is limited.
Highlights
Chemical composition controls decomposition of both roots (Birouste et al, 2012; Redin et al, 2014; Silver and Miya, 2001) and plant litter (Jensen et al, 2005; Kögel-Knabner, 2002; Zhang et al, 2008) and, subsequently, C availability for biological nitrogen transformation processes in soils
The aims of this study were (1) to evaluate how fresh maize root litter affects N2O emissions compared to fresh maize shoot litter, (2) to assess to what extent N2O emissions are related to the interaction of C and N mineralization from soil and litter, and (3) to analyze the changes in soil microbial community structures related to litter input and N2O emissions
Maize shoot litter was characterized by higher concentrations of water-soluble C and N and a higher share of degradable compounds like hemicellulose and cellulose compared to maize roots
Summary
Chemical composition controls decomposition of both roots (Birouste et al, 2012; Redin et al, 2014; Silver and Miya, 2001) and plant litter (Jensen et al, 2005; Kögel-Knabner, 2002; Zhang et al, 2008) and, subsequently, C availability for biological nitrogen transformation processes in soils. When O2 concentrations are low, denitrifying soil microorganisms may use nitrate (NO−3 ) as an electron acceptor in the respiratory chain to break down organic compounds (Zumft, 1997). This leads to loss of plant-available N (Müller and Clough, 2014) and makes soils an important source of the greenhouse gas N2O (Ciais et al, 2013). When different types of litter were compared, quality parameters of plant residues, such as C : N ratio, lignin : N ratio, and chemical composition of structural components explained a large share of variances in N2O emissions (Baggs et al, 2000; Chen et al, 2013; Millar and Baggs, 2004). Plant litter enhances local anaerobicity by absorbing water from surrounding pores and retaining high moisture concentrations (Kravchenko et al, 2017, 2018)
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