Forest in south China: An important sink for reactive nitrogen and a regional hotspot for N2O?

Nitrous oxide (N2O), an important greenhouse gas (GHG), has seen increasing concentrations in the atmosphere since the industrial revolution. N2O is produced in soils as a bi-product of nitrogen turnover processes. The production of N2O is stimulated primarily due to global intensification of nitrogen turnover, both in response to extensive fertilizer use and increased deposition of atmogenic nitrogen (from fossil fuel burning). Emission of N2O from agriculture has received much attention, particularly due to increased fertilization. Non-agricultural soils (forest, unmanaged lands) have received far less attention. Eastern Asia, China in particular, has become a new hot-spot for nitrogen, both due to increased fertilizer use (agriculture) and not in the least to unintended enrichment with nitrogen of forest and other non-agricultural lands (via atmospheric transport). Recent research, a.o. conducted at NMBU, has shown that N2O production in soils depends strongly on soil acidity (pH). The more acid the soil is (the lower the pH), the larger is the N2O production. This is primarily due to the production of N2O-reductase (the enzyme necessary to process N2O to non-dangerous N2), which is difficult at high soil acidity (low pH). In this project, a meta-analysis based on a large global data set, showed that IPCCs «tier 1» approach, which assumes that the emission of N2O equals 1% of the added nitrogen fertilizer, underestimates N2O emission from acid agricultural land (low pH). This means that agriculture on marginal soil (with low pH), as is common in the tropics and sub-tropics, tends to release much of its nitrogen fertilizer as N2O. This knowledge is important both for improving our estimates of the global emission of N2O and locally to adapt fertilizer regimes to soil characteristics. Commonly, forest and non-managed lands have a significantly higher acidity (lower pH) than agricultural soils, so that they may have elevated N2O production and emission despite the relatively small nitrogen input. This is confirmed by our data for subtropical forest on acid soils in South China (Chongqing, Hunan, Jiangxi). N2O emission is particularly large in soils with a highly variable water content. Results indicate that in wet years the emission of N2O represents nearly 10% of the nitrogen input derived from the atmosphere (after long-range transport). In dry summers (2013 og 2014) the emission of N2O is significantly smaller. Thus, increased precipitation intensity stimulates the emission of N2O and therefore its concentration in the atmosphere. High soil pH (as in North China and in Zhejiang) and little nitrogen deposition from the atmosphere (Guizhou) causes significantly smaller emission rates of N2O. Soils, which have a permanently high water content (e.g. groundwater discharge zones near streams) have a more stable, but smaller N2O production. Lab experiments show that in these soils, with little available oxygen, denitrifiers have developed permanant gene expression for N2O-reductase, so that denitrification produces primarily N2 and limited amounts of N2O. Our data support the hypothesis that the activity of N2O reductase is limited in acid soils (low pH) and in soils with highly variable water content (e.g. on hill slopes). As expected, the emission of N2O is generally small in the cold season with low microbial activity. Studies of the relationship between 15N, a stable isotope of nitrogen, and 14N in soil water nitrate (often termed natural abundance) show a significant enrichment of 15N in the ground water discharge zone at all sites of South and North China. This indicates strong denitrification (the dominant nitrogen sink in the system). The 15N/14N ratio of nitrate in the groundwater discharge zone varies little from year to year, indicating that denitrification is efficient even in dry years. 15N-labeling experiments in the field show that 15N labeled nitrate-nitrogen leaches from the soil?s root zone rather quickly, even though some (ca. 10%) denitrifies to N2O. Ammonium-nitrogen on the other hand does not leach; little of the 15N label leaches as nitrate-nitrogen following nitrification. Most of the 15N labeled ammonium immobilizes in the large soil-nitrogen reservoir, where it is liberated gradually through mineralization. Because nitrogen immobilization is directly linked to carbon, these results are also of great importance for the estimation of carbon sequestration in sub-tropical soils This project has resulted in important knowledge about both N2O emissions and denitrification in nitrogen-saturated sub-tropical forest. Results should be included in both regional and global N2O accounts. Quantitative insight in denitrification as a nitrogen sink (removing an important nutrient for primary production) at the landscape level improves our estimates of the carbon sequestration potential of the landscape.

A systematic assessment in the 4.2 ha. forested TieShanPing catchment (Chongqing, PR China) lends support to our earlier hypothesis that nitrogen-saturated, sub-tropical forests in south China are regional hotspots for the emission of N2O, a potent GHG. TieShanPing receives about 40 kg ha-1 yr-1 of atmogenic reactive nitrogen, predominantly as ammonium derived from agriculture. Denitrification is the major sink removing about 80% of the input of reactive nitrogen. In wet years as much as one quarter of t his may be emitted as N2O. Recent research suggests that soil acidity, which is particularly high in many of the forest soils in south China, may be an important factor stimulating high N2O emission. If our findings in the TieShanPing catchment are repres entative for forest soils in southern China in general, these systems make an importantcontribution to China's national N2O budget. In addition, it would suggest that agriculture, through its contribution to atmogenic ammonia, which is subsequently deposi ted in forests, contributes far more to the national N2O budget than estimated from direct N2O emissions in croplands alone. The proposed research project has a two-step approach, where we 1. assess if south China is a regional hot spot for the emission o f N2O due to long-term high deposition of reactive nitrogen. This will be done through synoptic measurements of atmogenic deposition and runoff of nitrogen as well as measurements of N2O emission fluxes from five selected small, nitrogen-saturated catchme nts characterized by hot-humid summers. Results will be compared with nitrogen depositions and N2O emissions from two catchments in northern China with drier summers and more neutral soils. 2. characterize the underlying microbial processes by state-of-t he-art molecular methods to understand how N-removal and N2O emissions are regulated by soil physical and chemical factors in forest soils in south China.