Input and output of dissolved organic and inorganic nitrogen in subtropical forests of South China under high air pollution

Abstract

Abstract. The nitrogen (N) emissions to the atmosphere and N deposition to forest ecosystems are increasing rapidly in Southeast Asia, but little is known about the fates and effects of elevated N deposition in forest ecosystems in this warm and humid region. Here we report the concentrations and fluxes of dissolved inorganic (DIN) and organic N (DON) in precipitation, throughfall, surface runoff and soil solution for three subtropical forests in a region of South China under high air pollution over two years (2004 and 2005), to investigate how deposited N is processed, and to examine the importance of DON in the N budget. The precipitation DIN input was 32-34 kg N ha −1 yr −1 . An additional input of 18 kg N ha −1 yr −1 as DON was measured in 2005, which to our knowledge is the highest DON flux ever measured in precipitation. A canopy uptake of DIN was indicated in two young conifer dominated forests (72-85% of DIN input reached the floor in throughfall), whereas no uptake occurred in an old-growth broadleaf forest. The DON fluxes in throughfall were similar to that in precipitation in all forests. In the younger forests, DIN was further retained in the soil, with 41-63% of precipitation DIN leached below the 20-cm soil depth. Additionally, about half of the DON input was retained in these forests. The N retention in two young aggrading forests (21-28 kg N ha −1 yr −1 ) was in accordance with the estimates of N accumulation in biomass and litter accretion. In the old-growth forest, no N retention occurred, but rather a net loss of 8-16 kg N ha −1 yr −1 from the soil was estimated. In total up to 60 kg N ha −1 yr −1 was leached from the old-growth forest, indicating that this forest was completely N saturated and could not retain addiCorrespondence to: Y. T. Fang (fangyt@scbg.ac.cn) tional anthropogenic N inputs. We found that the majority of DIN deposition as well as of DIN leaching occurred in the rainy season (March to August) and that monthly DIN concentrations and fluxes in leaching were positively related to those in throughfall in all three forests, implying that part of the N leaching was hydrologically driven. Our results suggest that long-term high N deposition has caused elevated N leaching in all three forest types although most pronounced in the old-growth forest where wood increment was negligible or even negative. N availability even exceeded the biotic N demand in the young aggrading forests, with intensive rain in the growing season further enhancing N leaching in these forests.

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

“…76 The total amount of 21.6 kg N ha −1 yr −1 , sequestrated in these three aboveground pools, is sufficient to explain the observed 21 kg N ha −1 yr −1 retained above the upper 20 cm soil through precipitation inputs. 81 Similarly, in the secondary forest, N accumulation in plant biomass and litter layer was probably higher than in the pine forest, due to higher litter production and higher foliar N concentration, 77 and might as well account for the 28 kg N ha −1 yr −1 retained in this forest. 81 It is also suggested that the balance between N supply and biological N demand generally controls leaching of NO 3 − and cations.…”

“…81 Similarly, in the secondary forest, N accumulation in plant biomass and litter layer was probably higher than in the pine forest, due to higher litter production and higher foliar N concentration, 77 and might as well account for the 28 kg N ha −1 yr −1 retained in this forest. 81 It is also suggested that the balance between N supply and biological N demand generally controls leaching of NO 3 − and cations. 82 Therefore, high plant demand for nutrients in the planted and secondary forests could be the main reason for slowing the rate of leaching loss of both NO 3 − and base cations and subsequently soil acidification.…”

Relationships between ammonia-oxidizing communities, soil methane uptake and nitrous oxide fluxes in a subtropical plantation soil with nitrogen enrichment

“…The humid subtropical forests are considered to be N-rich compared with the boreal and temperate forests, and respond differently to N enrichment [22]. Exogenous N inputs to the subtropical forests can significantly decrease soil CH 4 uptake [21,23] and increase N loss via NO 3 À À N leaching [24], as well as via gaseous N emission (N 2 O, NO, and N 2 ) [25,26]. Experimental N deposition increases [27,28], decreases [29,30] or does not change [31e34] soil AOA abundance.…”

Microbial denitrification dominates nitrate losses from forest ecosystems

“…Atmospherically deposited N represents a new N input to terrestrial ecosystems and may enhance carbon dioxide sequestration, potentially reducing some global climate impacts (3). On the other hand, long-term N deposition could result in N saturation and increased nitrate (NO 3 − ) losses to leaching and denitrification (4,5), pathways that have different consequences for the Earth system, including consequences on climate, biodiversity, and water and air quality for human health (6). Thus, in the context of global climate and other biogeochemical changes, it is critically important to understand terrestrial N balances and their responses to anthropogenic N inputs.…”