Nitrogen deposition accelerates soil carbon sequestration in tropical forests

Terrestrial ecosystem carbon (C) sequestration plays an important role in ameliorating global climate change. While tropical forests exert a disproportionately large influence on global C cycling, there remains an open question on changes in below-ground soil C stocks with global increases in nitrogen (N) deposition, because N supply often does not constrain the growth of tropical forests. We quantified soil C sequestration through more than a decade of continuous N addition experiment in an N-rich primary tropical forest. Results showed that long-term N additions increased soil C stocks by 7 to 21%, mainly arising from decreased C output fluxes and physical protection mechanisms without changes in the chemical composition of organic matter. A meta-analysis further verified that soil C sequestration induced by excess N inputs is a general phenomenon in tropical forests. Notably, soil N sequestration can keep pace with soil C, based on consistent C/N ratios under N additions. These findings provide empirical evidence that below-ground C sequestration can be stimulated in mature tropical forests under excess N deposition, which has important implications for predicting future terrestrial sinks for both elevated anthropogenic CO₂ and N deposition. We further developed a conceptual model hypothesis depicting how soil C sequestration happens under chronic N deposition in N-limited and N-rich ecosystems, suggesting a direction to incorporate N deposition and N cycling into terrestrial C cycle models to improve the predictability on C sink strength as enhanced N deposition spreads from temperate into tropical systems.

With the globalization of anthropologically elevated nitrogen (N) deposition (1–3), ecosystem C sequestration can be stimulated in many places, because N limitation of net primary productivity (NPP) is widespread (4, 5). However, N is relatively abundant in many tropical forests, and experiments demonstrate that N supply does not limit NPP in such N-rich forests (1, 4). Moreover, previous studies on forest C sequestration overwhelmingly emphasized plant productivity rather than soil stocks as sink for C (6–11). Soil is the largest pool of terrestrial organic C in the biosphere, and more than half of soil C is stored in forest ecosystems (12). Accordingly, C sequestration as soil organic matter could be quantitatively more important than vegetation for forest C budgets (8).Our current understanding of soil C sequestration in response to N deposition in forests has several limitations. First, unlike biomass C sequestration, responses of forest soil C sequestration to N deposition remain inconclusive. Many studies on soil C dynamics indicate that N deposition can increase soil C sequestration by reducing the decomposition of plant litter and soil organic matter (13–15), inhibiting soil respiration (16), or changing microbial enzymatic activity (14, 17). Conversely, other studies reported that long-term N application did not affect soil C sequestration (18), whereas Van Miegroet and Jandl (19) suggested that N addition can deplete soil C pool through microbial respiration linked to transformation of excess N. The contradictory evidence points to the necessity of further examination of how soil C sinks respond to increased N deposition (3, 20). Second, most studies have been conducted in mid-to-high latitudes in the Northern Hemisphere (16, 18, 20, 21), where most forest ecosystems are N-limited, and an increase in N supply can enhance NPP and aboveground litter production (4, 9, 11, 21, 22). Until now, however, we have lacked data about changes in soil C stocks with increased N supply in tropical forests, where ecosystems are more often N-rich (1, 4). This lack of information has led to a debatable assertion of ecosystem C neutrality with N deposition in many tropical forests (21, 23–25). In fact, we do not know what N will do to C storage in N-rich ecosystems because there is a large class of such systems for which there is little information.Here, we experimentally tested the influence of elevated N deposition on soil C sequestration in an N-rich tropical forest, using more than a decade of N addition to experimental plots established in a lowland primary forest at the Dinghushan Biosphere Reserve (DHSBR) in southern China, which has received high rates of ambient N deposition (e.g., >30 kg N⋅ha⁻¹⋅y⁻¹ in precipitation) for several decades (26). We first quantified the contribution of long-term N addition to below-ground soil C stocks. To further clarify the mechanisms on which soil C storage was changed, we evaluated soil density fractions, because these fractions are closely related to soil C stability and its potential for long-term preservation. The protection of organic matter in soils generally increases with the increasing density of SOM fractions, or with increasing association with mineral particles in the heavy fraction of soils (27). To characterize the biochemical composition of organic matter residing in mineral soil fractions, we employed solid-state ¹³C-NMR spectroscopy to determine the relative abundance of alkyl C, O-alkyl C, aromatic C, and carboxyl C. Moreover, to identify whether there is a general pattern of N-induced soil C sequestration in the tropics, we further combined this field evidence with a meta-analysis of N addition experiments to quantify the responses of soil C storage in tropical forests to N additions.