Contrasting impacts of long-term nitrogen and phosphorus additions on the quantity and quality of soil dissolved organic matter

Abstract

Elevated atmospheric nitrogen (N) and phosphorus (P) depositions are progressively modifying the dynamics of soil dissolved organic matter (DOM) in terrestrial ecosystems. However, the long-term effects on DOM quantity and quality remain poorly understood, especially regarding indirect regulation by plant inputs and microbial decomposition. We conducted a 12-year nutrient addition experiment with N and P in an alpine grassland on the Tibetan Plateau to investigate changes in soil organic matter (SOM), DOM quantity, and quality. SOM was derived from soil organic carbon using an elemental analyzer, while the DOM quantity was determined from dissolved organic carbon using a total organic carbon analyzer. DOM quality was assessed using UV-Visible and 3D-EEM fluorescence spectroscopy. Using linear mixed-effects models, we evaluated the effects of N and P additions on SOM, DOM quantity, and qua-lity. We found that P addition reduced SOM by 16.8%, an effect likely mediated by decreases in root biomass and soil moisture. These changes reduced organic matter inputs and likely destabilized soil aggregates, driving the observed SOM decline. Conversely, N addition altered DOM quality by shifting the balance from plant-derived inputs to microbial signatures. Specifically, increased soil electrical conductivity and inhibited cellobiohydrolase activity reduced the availability of plant-derived cellobiose, thereby favoring the accumulation of microbial-derived signatures. This shift toward a microbially-processed DOM pool was supported by increases of 7.4% and 3.1% in the biological and fluorescence indices, respectively. These contrasting pathways highlight that accurate prediction of carbon persistence requires distinguishing between nutrient-specific controls on DOM quantity versus quality.

Graphical abstract

Keywords

alpine grassland / dissolved organic matter / enzyme activity / nitrogen deposition / phosphorus deposition / plant inputs

Highlight

	● Long-term P addition reduces topsoil SOM by 16.8%.
	● P addition enhances DOM stability via 50.1% rise in humification index (HIX).
	● N addition shifts DOM quality: biological index +7.4% and fluorescence index +3.1%.
	● N addition via conductivity promotion/enzyme suppression; P via roots/moisture.
	● Contrasting N/P pathways reshape DOM quantity and quality.

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