Jiangming Mo

Nitrogen deposition and its ecological impact in China: an overview.

Nitrogen (N) deposition is an important component in the global N cycle that has induced large impacts on the health and services of terrestrial and aquatic ecosystems worldwide. Anthropogenic reactive N (N(r)) emissions to the atmosphere have increased dramatically in China due to rapid agricultural, industrial and urban development. Therefore increasing N deposition in China and its ecological impacts are of great concern since the 1980s. This paper synthesizes the data from various published papers to assess the status of the anthropogenic N(r) emissions and N deposition as well as their impacts on different ecosystems, including empirical critical loads for different ecosystems. Research challenges and policy implications on atmospheric N pollution and deposition are also discussed. China urgently needs to establish national networks for N deposition monitoring and cross-site N addition experiments in grasslands, forests and aquatic ecosystems. Critical loads and modeling tools will be further used in N(r) regulation.

Old-growth forests can accumulate carbon in soils

Old-growth forests have traditionally been considered negligible as carbon sinks because carbon uptake has been thought to be balanced by respiration. We show that the top 20-centimeter soil layer in preserved old-growth forests in southern China accumulated atmospheric carbon at an unexpectedly high average rate of 0.61 megagrams of carbon hectare-1 year-1 from 1979 to 2003. This study suggests that the carbon cycle processes in the belowground system of these forests are changing in response to the changing environment. The result directly challenges the prevailing belief in ecosystem ecology regarding carbon budget in old-growth forests and supports the establishment of a new, nonequilibrium conceptual framework to study soil carbon dynamics

Nitrogen availability in disturbed, rehabilitated and mature forests of tropical China

Abstract To investigate the impact of human disturbance and subsequent recovery on soil nitrogen processes, nitrogen availability during different seasons and at two soil depths in disturbed, rehabilitated and mature forests in tropical China was estimated using the ion exchange resin (IER) bag method. Soil total mineral N (NH4+N+NO3−N) varied significantly depending on forest and season. Overall, total mineral N concentrations ranked as follows: rehabilitated>mature>disturbed (forest); and spring>fall>winter>summer (season). Of the total mineral N, NH4+N was the major form in all forests (about 50–95%) with its proportion varying depending on forest and season. The correlation between NH4+N and NO3−N was strongest in the mature forest, followed by the rehabilitated forest and by the disturbed forest. Harvesting understory and litter had significant effects on soil N—mineral N was higher in treatment plots (more disturbed) than in control plots (relatively less disturbed). Although mineral nitrogen is produced, fewer plants and low microbial activity lead to low uptake and low immobilization resulting in greater N leaching losses in treatment plots. The results of this study suggest that over the period of 50 year or so, successful rehabilitation of soil N availability on severely degraded lands is possible, however, as long as harvesting of understory and litter continues in the degraded forest, this rate and level of recovery is unlikely to be realized.

Cumulative effects of nitrogen additions on litter decomposition in three tropical forests in southern China

A field-scale experiment with nitrogen (N) addition treatments was performed in three forest types – a pine (Pinus massoniana Lamb.) forest, a pine-broadleaf mixed forest (mixed) and a mature monsoon evergreen broadleaf forest (mature) – in tropical China. Two kinds of leaf litter, Schima superba Chardn. & Champ. and Castanopsis chinensis Hance, were studied using the litterbag technique after more than 2 years of continuous N additions. The objective of this study was to understand the cumulative effect of N addition on litter decomposition in the tropical forests and to determine if the initial effects of N addition changes over time. Results indicated that leaf litter decomposition was significantly faster in the mature forest than in the mixed or pine forests. The mean fraction of mass remaining after 12-months of decomposition was: mature (0.22) < mixed (0.50) < pine (0.51) for the two litters. Nitrogen addition significantly depressed litter decomposition in the pine forest and the mature forest, but had no significant effect in the mixed forest. These results suggest that N deposition has significant cumulative effect on litter decomposition.

Interactive Effects of Nitrogen and Phosphorus on Soil Microbial Communities in a Tropical Forest

Elevated nitrogen (N) deposition in humid tropical regions may exacerbate phosphorus (P) deficiency in forests on highly weathered soils. However, it is not clear how P availability affects soil microbes and soil carbon (C), or how P processes interact with N deposition in tropical forests. We examined the effects of N and P additions on soil microbes and soil C pools in a N-saturated old-growth tropical forest in southern China to test the hypotheses that (1) N and P addition will have opposing effects on soil microbial biomass and activity, (2) N and P addition will alter the composition of the microbial community, (3) the addition of N and P will have interactive effects on soil microbes and (4) addition-mediated changes in microbial communities would feed back on soil C pools. Phospholipid fatty acid (PLFA) analysis was used to quantify the soil microbial community following four treatments: Control, N addition (15 g N m−2 yr−1), P addition (15 g P m−2 yr−1), and N&P addition (15 g N m−2 yr−1 plus 15 g P m−2 yr−1). These were applied from 2007 to 2011. Whereas additions of P increased soil microbial biomass, additions of N reduced soil microbial biomass. These effects, however, were transient, disappearing over longer periods. Moreover, N additions significantly increased relative abundance of fungal PLFAs and P additions significantly increased relative abundance of arbuscular mycorrhizal (AM) fungi PLFAs. Nitrogen addition had a negative effect on light fraction C, but no effect on heavy fraction C and total soil C. In contrast, P addition significantly decreased both light fraction C and total soil C. However, there were no interactions between N addition and P addition on soil microbes. Our results suggest that these nutrients are not co-limiting, and that P rather than N is limiting in this tropical forest.

Correlation between leaf litter and fine root decomposition among subtropical tree species

Elucidating the processes of leaf litter and fine root decomposition has been a major research focus, while how the correlation between leaf litter and fine root decomposition is unclear. We studied the in situ decomposition and N dynamics of leaf litter and fine root of four subtropical tree species (Pinus massoniana, Castanopsis hystrix, Michelia macclurei and Mytilaria laosensis) to determine whether leaf litter and fine root decomposition is correlated across species as well as which factors influence decomposition above versus below ground. Decomposition rate of leaf litter was related to that of fine root across species. The strong correlation between leaf litter and fine root decomposition rates arose largely for several reasons. First, soil moisture had the similar influences on both leaf litter and fine root decomposition rates. Second, traits (i.e., initial Ca concentration) important to both leaf litter and fine root decomposition rates showed significant similarity among species. Third, initial P, N and aromatic C concentrations, and C/N ratio were uniquely important for fine root decomposition rate, while no unique traits for leaf litter decomposition rate. This also could account for the strong correlation between leaf litter and fine root decomposition rates. Our study suggests that among these subtropical trees, species effects on in situ decomposition rates of leaf litter and fine root are very similar. Thus, species differences in decomposition rates may be as large as they would be if faster decomposition of leaf litter was correlated with faster decomposition of fine root. N immobilization rate of leaf litter was unrelated to that of fine root across species. Our results help explain some important mechanisms by which tree species influence litter in situ decomposition.

Divergent Responses of Soil Buffering Capacity to Long-Term N Deposition in Three Typical Tropical Forests with Different Land-Use History

Elevated anthropogenic nitrogen (N) deposition has become an important driver of soil acidification at both regional and global scales. It remains unclear, however, how long-term N deposition affects soil buffering capacity in tropical forest ecosystems and in ecosystems of contrasting land-use history. Here, we expand on a long-term N deposition experiment in three tropical forests that vary in land-use history (primary, secondary, and planted forests) in Southern China, with N addition as NH4NO3 of 0, 50, 100, and 150 kg N ha(-1) yr(-1), respectively. Results showed that all three forests were acid-sensitive ecosystems with poor soil buffering capacity, while the primary forest had higher base saturation and cation exchange capacity than others. However, long-term N addition significantly accelerated soil acidification and decreased soil buffering capacity in the primary forest, but not in the degraded secondary and planted forests. We suggest that ecosystem N status, influenced by different land-use history, is primarily responsible for these divergent responses. N-rich primary forests may be more sensitive to external N inputs than others with low N status, and should be given more attention under global changes in the future, because lack of nutrient cations is irreversible.

Effects of nitrogen deposition on carbon cycle in terrestrial ecosystems of China: A meta-analysis

Nitrogen (N) deposition in China has increased greatly, but the general impact of elevated N deposition on carbon (C) dynamics in Chinese terrestrial ecosystems is not well documented. In this study we used a meta-analysis method to compile 88 studies on the effects of N deposition C cycling on Chinese terrestrial ecosystems. Our results showed that N addition did not change soil C pools but increased above-ground plant C pool. A large decrease in below-ground plant C pool was observed. Our result also showed that the impacts of N addition on ecosystem C dynamics depend on ecosystem type and rate of N addition. Overall, our findings suggest that 1) decreased below-ground plant C pool may limit long-term soil C sequestration; and 2) it is better to treat N-rich and N-limited ecosystems differently in modeling effects of N deposition on ecosystem C cycle.

The 15N natural abundance of the N lost from an N‐saturated subtropical forest in southern China

The 15N-enrichment of plants and soils is believed to indicate characteristics of the open nitrogen (N) cycle in terrestrial ecosystems because N lost from an ecosystem is presumably 15N-depleted through isotopic fractionation. However, because of a lack of an appropriate analytical methodology to confirm that supposition, the δ15N value for total dissolved nitrogen (TDN, the sum of ammonium, nitrate, and dissolved organic N) in stream water from forests has been measured only rarely. This report describes the δ15N values for TDN, ammonium, and nitrate in precipitation and stream water, together with those for soil-emitted nitrous oxide (N2O; measured once) in an N-saturated subtropical forest in southern China. Concentration-weighted δ15N values of TDN were −0.7‰ in precipitation and +1.2‰ in stream water. The difference in δ15N between soil (+3.9‰) and TDN in the stream water was 2.7‰. In contrast, soil-emitted N2O was strongly 15N-depleted (−14.3‰): 18‰ lower than that of the soil. Our results demonstrate that the discharged N loss is 15N-depleted only slightly compared with soil N, and gaseous N losses can be a strong driver for raising the terrestrial ecosystem δ15N. Our findings suggest that the relation between ecosystem δ15N and the open N cycle can be interpreted better by considering the net discrimination against 15N determined by the balance between gaseous and discharge N losses. Steady state 15N budget calculations proposed by Houlton and Bai (2009) can provide important information about the gaseous N fluxes, which are difficult to measure directly. The steady state calculation for the relationships among gaseous N loss, apparent isotopic fractionation during gaseous N loss, and isotopic signature of N inputs suggests that precise measurements of unmeasured components (e.g., dry deposition, NO and N2 emission) are quite important for better estimation of gaseous N losses from the ecosystem.

Soil nitric oxide emissions from two subtropical humid forests in south China

[1] Due to the dense population, rapid industrialization, and intensified agricultural activities, some regions in Asia are hot spots of airborne nitrogen oxides and also areas with increasing nitrogen deposition. Therefore the cycling of nitrogen gases in Asia might be of increasing importance on both a regional and a global scale for atmospheric chemistry and budgets of nitrogen. Yet, to date, knowledge of soil NO emission is quite limited in Asia, particularly in forest ecosystems. In this study, soil NO emissions in two subtropical humid forests, a broadleaf forest in climax successional stage and a pine forest in primary successional stage, were measured throughout the year 2005 in Dinghushan Biosphere Reserve, south China. In the broadleaf forest, mean NO emission in wet season (14.9 ng N m � 2 s � 1 ) was lower than in dry season (23.8 ng N m � 2 s � 1 ). In the pine forest, however, mean NO emission in wet season (17.1 ng N m � 2 s � 1 ) was higher than in dry season (7.9 ng N m � 2 s � 1 ). In both forests, soil water content was the dominant factor controlling the seasonal patterns of NO emissions, and soil NO emission was significantly correlated to percent water filled pore space (%WFPS) in a quadratic manner (p < 0.001). Annual NO emissions in the broadleaf forest and the pine forest were preliminarily estimated to be 6.1–6.9 and 4.0–4.3 kg N ha � 1 yr � 1 , respectively, by using three