Vertical Redistribution of Soil Organic Carbon Pools After Twenty Years of Nitrogen Addition in Two Temperate Coniferous Forests

Cooling has stimulated soil carbon storage in forest ecosystems

The interactive effect of soil cooling and nitrogen (N) addition can accurately simulate climatic and anthropogenic effects on terrestrial and other land-based ecosystems, but direct empirical measurements on the effects of cooling and N addition on soil carbon (C) and N are lacking. Hence, transplanting soils into colder regions was used to evaluate the effects of cooling and N addition on soil C and N. We used PVCs of 30 cm in height and 8 cm in diameter to extract soil samples. Soil C and N were significantly (P < 0.05) increased by transplanting soils into colder regions. In contrast, cooling has insignificantly (P > 0.05) increased the soil dissolved organic C (DOC) and dissolved organic (DON), but the effect was negatively significant on soil pH compared to the C/N ratio. Similarly, N addition significantly increased the measured soil N stock. However, the effect was negatively significant on soil pH (P < 0.05) compared to the C/N ratio (P > 0.05). Nevertheless, the interaction of cooling and N addition did not affect the soil C and N storage. A similar effect was observed on the soil DOC and DON. The results presented here illustrate that transplanting soils into colder regions and N deposition has perfectly simulated the effects of climate-forcing factors on soil C and N storage in terrestrial and other land-based ecosystems. Accordingly, this study suggests that low temperatures have stimulated the accumulation of the measured soil organic and dissolved properties, but the effect is less consequential when low temperature interacts with N addition in high-elevation areas where ecosystem structures and functions are limited by temperature and may serve as a baseline for future research on land feedbacks to the climate system.

Experimental evidence shows minor contribution of nitrogen deposition to global forest carbon sequestration

Human activities have drastically increased nitrogen (N) deposition onto forests globally. This may have alleviated N limitation and thus stimulated productivity and carbon (C) sequestration in aboveground woody biomass (AGWB), a stable C pool with long turnover times. This 'carbon bonus' of human N use partly offsets the climate impact of human-induced N₂ O emissions, but its magnitude and spatial variation are uncertain. Here we used a meta-regression approach to identify sources of heterogeneity in tree biomass C-N response (additional C stored per unit of N) based on data from fertilization experiments in global forests. We identified important drivers of spatial variation in forest biomass C-N response related to climate (potential evapotranspiration), soil fertility (N content) and tree characteristics (stand age), and used these relationships to quantify global spatial variation in N-induced forest biomass C sequestration. Results show that N deposition enhances biomass C sequestration in only one-third of global forests, mainly in the boreal region, while N reduces C sequestration in 5% of forests, mainly in the tropics. In the remaining 59% of global forests, N addition has no impact on biomass C sequestration. Average C-N responses were 11 (4-21) kg C per kg N for boreal forests, 4 (0-8) kg C per kg N for temperate forests and 0 (-4 to 5) kg C per kg N for tropical forests. Our global estimate of the N-induced forest biomass C sink of 41 (-53 to 159) Tg C yr^-1 is substantially lower than previous estimates, mainly due to the absence of any response in most tropical forests (accounting for 58% of the global forest area). Overall, the N-induced C sink in AGWB only offsets ~5% of the climate impact of N₂ O emissions (in terms of 100-year global warming potential), and contributes ~1% to the gross forest C sink.

Altered plant carbon partitioning enhanced forest ecosystem carbon storage after 25 years of nitrogen additions

Decades of atmospheric nitrogen (N) deposition in the northeastern USA have enhanced this globally important forest carbon (C) sink by relieving N limitation. While many N fertilization experiments found increased forest C storage, the mechanisms driving this response at the ecosystem scale remain uncertain. Following the optimal allocation theory, augmented N availability may reduce belowground C investment by trees to roots and soil symbionts. To test this prediction and its implications on soil biogeochemistry, we constructed C and N budgets for a long-term, whole-watershed N fertilization study at the Fernow Experimental Forest, WV, USA. Nitrogen fertilization increased C storage by shifting C partitioning away from belowground components and towards aboveground woody biomass production. Fertilization also reduced the C cost of N acquisition, allowing for greater C sequestration in vegetation. Despite equal fine litter inputs, the C and N stocks and C : N ratio of the upper mineral soil were greater in the fertilized watershed, likely due to reduced decomposition of plant litter. By combining aboveground and belowground data at the watershed scale, this study demonstrates how plant C allocation responses to N additions may result in greater C storage in both vegetation and soil.

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.

Long- and Short-Term Inorganic Nitrogen Runoff from a Karst Catchment in Austria

Excess nitrogen (N) deposition and gaseous N emissions from industrial, domestic, and agricultural sources have led to increased nitrate leaching, the loss of biological diversity, and has affected carbon (C) sequestration in forest ecosystems. Nitrate leaching affects the purity of karst water resources, which contribute around 50% to Austria’s drinking water supply. Here we present an evaluation of the drivers of dissolved inorganic N (DIN) concentrations and fluxes from a karst catchment in the Austrian Alps (LTER Zöbelboden) from 27 years of records. In addition, a hydrological model was used together with climatic scenario data to predict expected future runoff dynamics. The study area was exposed to increasing N deposition during the 20th century (up to 30 to 35 kg N ha−1 y−1), which are still at levels of 25.5 ± 3.6 and 19.9 ± 4.2 kg N ha−1 y−1 in the spruce and the mixed deciduous forests, respectively. Albeit N deposition was close to or exceeded critical loads for several decades, 70–83% of the inorganic N retained in the catchment from 2000 to 2018, and NO3- concentrations in the runoff stayed <10 mg L−1 unless high-flow events occurred or forest stand-replacing disturbances. We identified tree growth as the main sink for inorganic N, which might together with lower runoff, increase retention of only weakly decreasing N deposition in the future. However, since recurring forest stand-replacement is predicted in the future as a result of a combination of climatically driven disturbance agents, pulses of elevated nitrate concentrations in the catchment runoff will likely add to groundwater pollution.

Levels and variations of soil organic carbon and total nitrogen among forests in a hotspot region of high nitrogen deposition

Human activities have distinctly enhanced the deposition levels of atmospheric nitrogen (N) pollutants into terrestrial ecosystems, but whether and to what extents soil carbon (C) and N status have been influenced by elevated N inputs remain poorly understood in the 'real' world given related knowledge has largely based on N-addition experiments. Here we reported soil organic C (OC) and total N (TN) for twenty-seven forests along a gradient of N deposition (22.4-112.9 kg N/ha/yr) in the Beijing-Tianjin-Hebei (BTH) region of northern China, a global hotspot of high N pollution. Levels of soil TN in forests of the BTH region have been elevated compared with investigations in past decades, suggesting that long-term N deposition might cause soil TN increases. Combining with major geographical and environmental factors among the study forests, we found unexpectedly that soil moisture and pH values rather than N deposition levels were major regulators of the observed spatial variations of soil OC and TN contents. As soil moisture and pH values increased with mean annual precipitation and temperature, respectively, soil C and N status in forests of the BTH region might be more responsive to climate change than to N pollution. These evidence suggests that both N deposition and climate differences should be considered into managing ecosystem functions of forest resources in regions with high N pollution.

The Roles of Bacteria in Soil Organic Carbon Accumulation under Nitrogen Deposition in Stipa baicalensis Steppe

Grassland soil organic carbon (SOC) accounts for 15.5% of the SOC in reservoirs of terrestrial carbon (C) and is a major component of the global C cycle. Current and future reactive N deposited on grassland soils may alter biogeochemical processes and soil microbes. Microorganisms perform most of the decomposition on Earth and shift SOC accumulation. However, how variation in the identity and composition of the bacterial community influences SOC is far from clear. The objective of this study is to investigate the responses of SOC concentration to multiple rates of N addition as well as the roles of bacteria in SOC accumulation. We studied SOC storage and bacterial community composition under N addition treatments (0, 1.5, 3.0, 5.0, 10.0, 15.0, 20.0, and 30.0 g N·m^-2 yr^-1) in a 6-yr field experiment in a temperate grassland. We determined the soil inorganic nitrogen concentration and pH in a 0-10 cm soil layer. We used high-throughput genetic sequencing to detect bacteria. N addition led to significant increases in the concentrations of SOC. N addition reduced the soil pH but increased the NO₃-N and NH₄-N levels. The bacterial diversity was highest under low nitrogen addition. N addition increased the relative abundance of Proteobacteria, and Proteobacteria became the second dominant phylum under high N addition. Structural equation modeling further revealed that soil pH and bacterial community structure have an impact on SOC under N deposition. Nitrogen-regulated SOC is associated with Proteobacteria and Planctomycetes. These findings suggest that N deposition may alter the SOC content, highlighting the importance of understanding changes in the bacterial community for soil nutrients under N deposition.