Distinct fates of atmogenic NH4+ and NO3- in subtropical, N-saturated forest soils

作者：Yu, Longfei; Kang, Ronghua; Mulder, Jan; Zhu, Jing; Dorsch, Peter

来源：Biogeochemistry, 2017, 133(3): 279-294.

DOI：10.1007/s10533-017-0332-y

• N deposition
• N-15 tracer
• Gross N transformation
• Nitrification
• Denitrification
• Nitrate leaching
• N2O emission

Subtropical forests receive increasing amounts of atmogenic nitrogen (N), both as ammonium (NH4+) and nitrate (NO3-). Previous long-term studies indicate efficient turnover of atmogenic NH4+ to NO3- in weathered, acidic soils of the subtropics, leading to excessive NO3- leaching. To clarify the mechanism governing the fate of atmogenic inputs in these soils, we conducted an in situ N-15 tracing experiment in the TieShanPing (TSP) forested catchment, SW China. (NH4NO3)-N-15, NH (4) (15) NO3 and N-15-glutamic acid were applied to an upland hillslope soil and inorganic N, total soil N and nitrous oxide (N2O) were monitored for nine days. Incorporation of (NO3-)-N-15 into soil organic N was negligible and 80% of the applied label was lost from the top soil (0-15 cm) primarily by leaching within 9 days. In contrast, (NH4+)-N-15 was largely retained in soil organic N. However, instant production of (NO3-)-N-15 in the (NH4+)-N-15 treatment suggested active nitrification. In both the (NH4+)-N-15 and N-15-glutamic acid treatments, the N-15 enrichment in the NO3- pool exceeded that in the NH4+ pool one day after N-15 application, suggesting preferential nitrification of added (NH4+)-N-15 with subsequent dilution of the NH4+ pool and/or immobilization of (NH4+)-N-15 followed by heterotrophic nitrification. The cumulative recovery of N-15 in N2O after 9 days ranged from 2.5 to 6.0% in the (NO3-)-N-15 treatment, confirming the previously reported significant response of N2O emission to N deposition. Source partitioning of (N2O)-N-15 demonstrated a measurable contribution of nitrification to N2O emissions, particularly at low soil moistures. Our study emphasizes the role of a fast-cycling organic N pool (including microbial N) for retention and transformation of atmogenic NH4+ in subtropical, acid forest soils. Thus, it explains the near-quantitative leaching of deposited N (as NO3- and NH4+) common to subtropical forest soils with chronic, elevated atmogenic N inputs by (i) negligible retention of NO3- in the soil and (ii) rapid immobilization-mineralization of NH4+ followed by nitrification. Our findings point to a leaky N cycle in N-saturated Chinese subtropical forests with consequences for regional soil acidification, N pollution of fresh waters and N2O emission.

• 出版日期2017-5
• 单位广西师范大学