Shulan Cheng

Responses of CO2 efflux from an alpine meadow soil on the Qinghai Tibetan Plateau to multi-form and low-level N addition

AimsTo assess the effects of atmospheric N deposition on the C budget of an alpine meadow ecosystem on the Qinghai–Tibetan Plateau, it is necessary to explore the responses of soil-atmosphere carbon dioxide (CO2) exchange to N addition.MethodsBased on a multi-form, low-level N addition experiment, soil CO2 effluxes were monitored weekly using the static chamber and gas chromatograph technique. Soil variables and aboveground biomass were measured monthly to examine the key driving factors of soil CO2 efflux.ResultsThe results showed that low-level N input tended to decrease soil moisture, whereas medium-level N input maintained soil moisture. Three-year N additions slightly increased soil inorganic N pools, especially the soil NH4+-N pool. N applications significantly increased aboveground biomass and soil CO2 efflux; moreover, this effect was more significant from NH4+-N than from NO3−-N fertilizer. In addition, the soil CO2 efflux was mainly driven by soil temperature, followed by aboveground biomass and NH4+-N pool.ConclusionsThese results suggest that chronic atmospheric N deposition will stimulate soil CO2 efflux in the alpine meadow on the Qinghai–Tibetan Plateau by increasing available N content and promoting plant growth.

Simulated Nitrogen Deposition Reduces CH4 Uptake and Increases N2O Emission from a Subtropical Plantation Forest Soil in Southern China

To date, few studies are conducted to quantify the effects of reduced ammonium (NH4 +) and oxidized nitrate (NO3 −) on soil CH4 uptake and N2O emission in the subtropical forests. In this study, NH4Cl and NaNO3 fertilizers were applied at three rates: 0, 40 and 120 kg N ha−1 yr−1. Soil CH4 and N2O fluxes were determined twice a week using the static chamber technique and gas chromatography. Soil temperature and moisture were simultaneously measured. Soil dissolved N concentration in 0–20 cm depth was measured weekly to examine the regulation to soil CH4 and N2O fluxes. Our results showed that one year of N addition did not affect soil temperature, soil moisture, soil total dissolved N (TDN) and NH4 +-N concentrations, but high levels of applied NH4Cl and NaNO3 fertilizers significantly increased soil NO3 −-N concentration by 124% and 157%, respectively. Nitrogen addition tended to inhibit soil CH4 uptake, but significantly promoted soil N2O emission by 403% to 762%. Furthermore, NH4 +-N fertilizer application had a stronger inhibition to soil CH4 uptake and a stronger promotion to soil N2O emission than NO3 −-N application. Also, both soil CH4 and N2O fluxes were driven by soil temperature and moisture, but soil inorganic N availability was a key integrator of soil CH4 uptake and N2O emission. These results suggest that the subtropical plantation soil sensitively responses to atmospheric N deposition, and inorganic N rather than organic N is the regulator to soil CH4 uptake and N2O emission.

Foliar and soil 15N natural abundances provide field evidence on nitrogen dynamics in temperate and boreal forest ecosystems

The natural abundance of 15N (δ15N) in plants and soils is an ideal tool for assessing ecosystem N dynamics. However, many of the mechanisms driving the variability of foliar and soil δ15N values within and across ecosystems are still unclear. In this study, we analyzed the patterns of N concentrations and δ15N values in leaves, bulk soils and soil mineral N as well as soil N turnover rates across four temperate and boreal forest ecosystems along a mountain transect. The results showed that plant species and soil properties directly controlled soil δ15N patterns and climate factors (air temperature and precipitation) indirectly affected foliar δ15N patterns. Foliar N concentrations varied consistently with the concentrations of soil available N and soil NO3−-N, whereas foliar δ15N was most closely associated with the δ15N of soil NH4+, the most abundant form of N in soil solution. 15N enrichment in surface mineral soil in high elevation forests was mainly attributed to 15N-enriched organic N accumulation. Furthermore, the foliar enrichment factor (εp/s = δ15Nfoliage−δ15Nsoil) was significantly correlated with N transformation and loss rates, and was negatively correlated with the ratio of NH4+ to total inorganic N. These results suggest that foliar δ15N value and foliar N concentration together accurately reflect the N availability of forest ecosystems. Foliar εp/s can act as an integrated proxy to reflect the status of N cycling within or across forest ecosystems. Soil nitrification and species’ NH4+ to NO3− uptake ratios are key processes controlling foliar δ15N patterns in N-limited forest ecosystems. Our findings improve the mechanistic understanding of the commonly observed variability in foliar and soil δ15N within and across forest ecosystems.

13C abundance, water-soluble and microbial biomass carbon as potential indicators of soil organic carbon dynamics in subtropical forests at different successional stages and subject to different nitrogen loads

Chronic atmospheric nitrogen deposition affects the cycling of carbon (C) and nitrogen (N) in forest ecosystems, and thereby alters the stable C isotopic abundance of plant and soil. Three successional stages, disturbed, rehabilitated and mature forests were studied for their responses to different nitrogen input levels. N-addition manipulative experiments were conducted at low, medium and high N levels. To study the responses of C cycling to N addition, the C concentration and 13C natural abundances for leaf, litter and soil were measured. Labile organic carbon fractions in mineral soils were measured to quantify the dynamics of soil organic C (SOC). Results showed that three-year continuous N addition did not significantly increase foliar C and N concentration, but decreased C/N ratio and enriched 13C in N-rich forests. In addition, N addition significantly decreased microbial biomass C, and increased water soluble organic C in surface soils of N-rich forests. This study suggests that N addition enhances the water consumption per unit C assimilation of dominant plant species, restricts SOC turnover in N-poor forests at early and medium successional stages (thus favored SOC sequestration), and vice versa for N-rich mature forests.

Effects of soil erosion and deposition on soil organic carbon dynamics at a sloping field in Black Soil region, Northeast China

Abstract Soil erosion transports light density and fine particle soil material from hills down to low-lying land areas, which can lead to carbon loss and subsequent sequestration. In the present paper, the profile distribution of soil organic carbon (SOC) and soil 13C natural abundance (δ13C) were analyzed across five geomorphic positions, distributed along a typical rolling farmland in the Black Soil region of Northeast China. The contents of particulate organic carbon (POC) and mineral-associated organic carbon (MOC) at each geomorphic position were measured with physical fraction method. The results showed that soil erosion decreased 5.3–22.4% of SOC and increased 4.0–6.1% of δ13C of surface soils at the eroding sites. At the typical depositional sites, SOC content and δ13C value in the buried surface layer were 1.5 times and 1.1 times as much as those of the current plough layer, respectively. Soil erosion did not change the POC content, but MOC content decreased by 9.3–35.2%. At the eroding sites, the coefficient of determination between soil δ13C and MOC (R 2 = 0.52) was higher than that between soil δ13C and POC (R 2 = 0.37). Our study indicated that soil erosion decreased SOC content and increased δ13CSOC in surface layer mainly through transferring fine sized and 13C-depleted SOC fraction. Deep burial and re-aggregation of eroded materials at depositional sites were in favor of stabilization and sequestration of SOC.