Do long-term high nitrogen inputs change the composition of soil dissolved organic matter in a primary tropical forest?

Abstract

Dissolved organic matter (DOM) plays a key role in forest carbon biogeochemistry by linking soil organic carbon (SOC) sequestration and water fluxes, which is further shaped by elevated atmospheric nitrogen (N) deposition. Although enhanced SOC sequestration was evidenced in tropical forests due to rising N deposition, it remains unclear how long-term N inputs affect soil DOM composition, which regulates SOC sequestration capability due to its mobility and biological instability. Here, the quantity, optical properties, and molecular-level characteristics of soil DOM based on a simulative N deposition experiment with four N addition levels (0, 5, 10, and 15 g m−2 yr−1) were studied in a primary tropical forest in south China. Results showed that 18-year N additions significantly altered soil DOM composition, with an increasing trend in soil dissolved organic carbon content. Medium- (10 g m−2 yr−1) and high-N addition (15 g m−2 yr−1) markedly elevated DOM average molecular weight by 12% and aromaticity, with specific ultraviolet absorbance at 254 nm increasing by 17%, modified aromatic index by 35%, and condensed aromatics by 67%. Medium- and high-N addition also increased recalcitrant DOM components but decreased other DOM components, with increasing percentages of lignin-like, tannin-like, and carboxylic-rich alicyclic molecule-like compounds, and decreasing percentage of more bioavailable contributions with H/C ratio > 1.5. Importantly, significant correlations of the SOC content of the heavy fraction with optical properties and with recalcitrant DOM components were observed. These findings suggest that long-term N additions may alter soil DOM composition in a way to benefit soil OC storage in the primary tropical forests. It merits focusing on the mechanisms to association of soil DOM dynamics with SOC sequestration.

Temporal Variations in Fire Impacts on Characteristics and Composition of Soil-Derived Dissolved Organic Matter at Qipan Mountain, China

“…For DOM extraction, 5 g of soil was mixed with 200 mL of Milli-Q water (40:1 water-to-soil ratio) and shaken in a shaker for 2 h. , The sample was then centrifuged and passed through a 0.45 μm poly(ether sulfone) resin membrane (Millipore Express PLUS, United States) to obtain the DOM. The pH, conductivity, dissolved organic carbon (DOC), total nitrogen (DTN), ammonium nitrogen (NH 4 + –N), nitrate nitrogen (NO 3 – –N), dissolved organic nitrogen (DON), absorbance spectra, and fluorescent excitation–emission matrices of DOM extracts were analyzed using previously established methods, with experimental details provided in Text S1.…”

Towards carbon-neutral sustainable development of China

“…Thus a deeper emission cut under the carbon neutrality target would bring additional benefits to the land carbon uptake through reduction of ozone-associated vegetation damage. Meanwhile, a total of 18 year N additions in a primary tropical forest significantly increase the soil dissolved organic matter (DOM) content under medium-N (10 g m −2 yr −1 ) and high-N (15 g m −2 yr −1 ) treatments, with the DOM average molecular weight increasing by 12% [12]. Such long-term N addition increases recalcitrant DOM components but decreases other DOM components.…”