Multilevel Nitrogen Additions Alter Chemical Composition and Turnover of the Labile Fraction Soil Organic Matter via Effects on Vegetation and Microorganisms

Abstract

Global nitrogen (N) deposition greatly impacts soil carbon sequestration. A 2-yr multiple N addition (0, 10, 20, 40, 80, and 160 kg N·ha −1 ·yr −1 ) experiment was conducted in alpine grassland to illustrate the mechanisms underlying the observed soil organic matter (SOM) dynamics on the Qinghai-Tibet Plateau (QTP). Labile fraction SOM (LF-SOM) fingerprints were characterized by pyrolysis-gas chromatography/tandem-mass spectrometry, and microbial functional genes (GeoChip 4.6) were analyzed in conjunction with LF-SOM fingerprints to decipher the responses of LF-SOM transformation to N additions. The significant correlations between LF-SOM and microbial biomass, between organic compounds in LF-SOM and compound degradation-related genes, and between LF-SOM and net ecosystem exchange implied LF-SOM were the main fraction utilized by microorganisms and the most sensitive fraction to N additions. The LF-SOM increased at the lowest N addition levels (10 and 20 kg N·ha −1 ·yr −1 ) and decreased at higher N addition levels (40 to 160 kg N·ha −1 ·yr −1 ), but the decrease of LF-SOM was weakened at 160 kg N·ha −1 ·yr −1 addition. The nonlinear response of LF-SOM to N additions was due to the mass balance between plant inputs and microbial degradation. Plant-derived compounds in LF-SOM were more sensitive to N addition than microbial-derived and aromatic compounds. It is predicted that when the N deposition rate increased by 10 kg N·ha −1 ·yr −1 on the QTP, carbon sequestration in the labile fraction may increase by nearly 170% compared with that under the current N deposition rate. These findings provide insight into future N deposition impacts on LF-SOM preservation on the QTP. Key Points:• The LF-SOM quantity increased at the lowest N additions (N10 and N20) and decreased from N40 to N160, but the decrease was weakened at the highest N addition (N160) • Plant-derived compounds in LF-SOM were more sensitive to N addition than microbial-derived and aromatic compounds • The organic compounds in LF-SOM were significantly correlated with compound degradation-related genesSupporting Information:• Supporting Information S1 , et al. (2020). Multilevel nitrogen additions alter chemical composition and turnover of the labile fraction soil organic matter via effects on vegetation and microorganisms.

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“…Growth of PA in variant 3, at first sight, contradicts the thesis that mineral forms of N inhibit protease activity (Piotrowska-Długosz, 2020; Tian et al, 2020). Obviously, straw + N 30 P 45 K 45 reduced N-restriction on microbial biomass formation and P-restriction on metabolic processes in microorganisms (Chen et al, 2017), directly or indirectly activating the growth and reproduction of bacteria and fungi by changing their ratio (Li et al, 2019;Chen et al, 2020). This may, at least in part, be the reason for the activation of enzyme synthesis and secretion (Piotrowska-Długosz, 2019), in particular, the growth of PA. Changes in hydrolase activity were accompanied by a predictable increase, probably in the content of C and N nutrients and, consequently, a documented increase in C-LH, DA (Bowles et al, 2014;Singh et al, 2018).…”

“…This is consistent with the increase in variant 2 Introduction of straw + N 30 P 45 K 45 in variant 3 obviously led to a decrease in the C:N ratio in the soil and, consequently, to an increase in the rate of consumption of high-carbon organic compounds by microorganisms compared to the control. The intake of N levelled the restriction of the activity of C-metabolic enzymes, and caused an increase in their activity (Chen et al, 2020), in particular CA. Growth of PA in variant 3, at first sight, contradicts the thesis that mineral forms of N inhibit protease activity (Piotrowska-Długosz, 2020; Tian et al, 2020).…”

“…In particular, the above-mentioned POXC, CWEOC, HWEOC, DOC can affect the values of CA, PA, DA, and serve as output pools of OC for the formation of a more "mature" C-LH. It is possible that DOC (80°C), dissolved organic nitrogen DON (25, 80°C), and hot water-extracted organic nitrogen HWEON Zhao et al, 2016) of IN form in fertilisers (Chen et al, 2020) and in the soil, easily hydrogenated nitrogen, MBC, N microbial biomass MBN, etc. Notably, the health, quality, productivity, and fertility of the soil are characterised by complex indices, which include enzymatic activities within one or more processes, the content of soluble forms of OC, soil respiration, pH, etc.…”

Regularities in the Development of Soil Biological Activity and Winter Wheat Productivity under Ecologised Fertiliser Systems

“…In particular, the above-mentioned POXC, CWEOC, HWEOC, DOC can affect the values of CA, PA, DA, and serve as output pools of OC for the formation of a more "mature" C-LH. It is possible that DOC (80°C), dissolved organic nitrogen DON (25, 80°C), and hot water-extracted organic nitrogen HWEON Zhao et al, 2016) of IN form in fertilisers (Chen et al, 2020) and in the soil, easily hydrogenated nitrogen, MBC, N microbial biomass MBN, etc. Notably, the health, quality, productivity, and fertility of the soil are characterised by complex indices, which include enzymatic activities within one or more processes, the content of soluble forms of OC, soil respiration, pH, etc.…”

Nitrogen Addition Affects Ecosystem Carbon Exchange by Regulating Plant Community Assembly and Altering Soil Properties in an Alpine Meadow on the Qinghai–Tibetan Plateau

“…For example, N addition stimulated NEE mainly by increasing the cover of dominant species in alpine meadows ( Shen H. et al, 2022 ). Nitrogen-induced shifts in plant functional groups are considered to be an essential factor affecting carbon exchange in ecosystems ( Niu et al, 2010 ; Fang et al, 2011 ; Chen et al, 2020 ), generally favoring a few nitrophilic plant species while suppressing the growth of many other species ( Duprè et al, 2010 ; Jiang et al, 2012 ). In addition, studies showed that an increase in soil respiration was positively related to soil nutrient availability under N addition ( Leff et al, 2015 ; Peng et al, 2015 ; Riggs et al, 2015 ).…”