Long-Term N Addition Effects the Stability of Soil Carbon Pool
The total carbon pool of the earth is 1800Pg C where 1480Pg C exists as organic matter and dead plant parts (litters) and 45% consisted by forest. A little change in soil organic C may cause significant change in atmospheric CO2 concentration.
So the dynamics of Soil carbon pool of forest ecosystem has become one of a key factor in assessment of carbon budget of the global terrestrial ecosystem on the background of global changes and dealing with the coming global climate crises.
Since the industrial revolution, the use of fossil fuel and the increase in intensive agricultural production has induced in rapid increase in active N release into atmosphere.
Correspondingly, the atmospheric N deposition rapidly increase also and in some region, N saturation has appeared. Recent years, along with the advancement in the research work, it is realized that N cycling is a key factor in terrestrial ecosystem C budget and global change.
The effects of variation in precipitation and N deposition on the C capture ability of soil has become a hot spot in the investigation on global C budget.
Prof. ZHANG Junhui from Institute of Applied Ecology, Chinese Academy of Sciences, led his group and Dr. CHEN Zhijie take the typical Changbai Mountain forest plots that had treated with successive six years N addition as materials investigated the variation of the stability of soil organic C pool by field observation and laboratory incubation.
The results show that in contrast to the zero control and low dose (25kg N ha-1 yr-1) addition, the long-term high dose N (25 kg N ha-1 yr-1) addition resulted in the increase in soil C pool stability.
The rate of soil C release was 30% lower than control. The mechanism is the increase in the protection of macro-aggregates on micro-aggregates and the enhancement in the resistance of soil organic matter to decomposition (use δ13C as an indicator).
Physicochemical protection of soil carbon provided by soil aggregates is critical to carbon (C) sequestration in terrestrial ecosystems. However, the stability of soil organic matter (SOM) in terrestrial ecosystems in response to atmospheric nitrogen (N) deposition is unclear. In this study, N was added to a forest soil dominated by deciduous trees on Changbai Mountain, China, at three different rates (0, 25 and 50 kg N ha−1 year−1) from 2007 to 2012. Its effect on C content and stabilization was evaluated by soil fractionation and stable isotope (δ13C) analyses. The results showed that large macroaggregates (2–8 mm) decreased and small macroaggregates (0.25–2 mm) increased with increasing rates of N addition, whereas soil C content remained unchanged. Irrespective of the N treatments, the C content of soil organic matter (SOM) fractions differed significantly between large and small macroaggregates, which suggests that the size of aggregate classes regulates C content in the SOM fractions. A slight increase in the C content of microaggregates within macroaggregates (Mm) and that of silt and clay fractions was recorded with the addition of N at 50 kg N ha−1 year−1. This increase also occurred in the silt and clay fraction within microaggregates (Intra-SC). Unprotected C (comprising the free light fraction (Free-LF) and coarse particulate organic matter (CPOM)) accounted for 18.9% only of the total C and decreased in response to the addition of N. Theδ13C signature and C/N ratios obtained for SOM fractions showed that newly formed C was transferred from POM to Intra-SC. Overall, our results suggested that long-term addition of N might promote stabilization of C by increasing small macro- and micro-aggregation within macroaggregates in temperate forest soil.
Purposeemission was decreased about 30 %. Similar patterns in GHG emission/uptake rates expressed by per soil organic matter basis were observed in response to N addition treatments, indicating that N addition might decrease the decomposability of SOM in mixed Korean pine forest. The global warming potential (GWP) which was mainly contributed by CO2 Anthropogenic-induced greenhouse gas (GHG) emission rates derived from the soil are influenced by long-term nitrogen (N) deposition and N fertilization. However, our understanding of the interplay between increased N load and GHG emissions among soil aggregates is incomplete.
Materials and methods
Here, we conducted an incubation experiment to explore the effects of soil aggregate size and N addition on GHG emissions. The soil aggregate samples (0–10 cm) were collected from two 6-year N addition experiment sites with different vegetation types (mixed Korean pine forest vs. broad-leaved forest) in Northeast China. Carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4) production were quantified from the soil samples in the laboratory using gas chromatography with 24-h intervals during the incubation (at 20 °C for 168 h with 80 % field water capacity).
Results and discussion
The results showed that the GHG emission/uptake rates were significantly higher in the micro-aggregates than in the macro-aggregates due to the higher concentration of soil bio-chemical properties (DOC, MBC, NO3−, NH4+, SOC and TN) in smaller aggregates. For the N addition treatments, the emission/uptake rates of GHG decreased after N addition across aggregate sizes especially in mixed Korean pine forest where CO2 .
These findings suggest that soil aggregate size is an important factor controlling GHG emissions through mediating the content of substrate resources in temperate forest ecosystems. The inhibitory effect of N addition on the GHG emission/uptake rates depends on the forest type.
Publication Name:CHEN Zhijie, Heikki SETALA, GENG Shicong, HAN Shijie, WANG Shuqi, DAI Guanhua, ZHANG Junhui.