In situ ¹⁵N labeling experiment reveals different long‐term responses to ammonium and nitrate inputs in N‐saturated subtropical forest

Abstract

Chronically elevated deposition of reactive nitrogen (N), as ammonium (NH 4 +) and nitrate (NO 3 À), in subtropical forests with monsoonal climate has caused widespread N leaching in southern China. So far, little is known about the effect of further increases in N input and changes in the relative proportion of NH 4 + and NO 3 À on turnover rate and fate of atmogenic N. Here we report a 15 N tracer experiment in Tieshanping (TSP) forest, SW China, conducted as part of a long-term N fertilization experiment, using NH 4 NO 3 and NaNO 3 , where effects of a doubling of monthly N inputs were compared. In June 2012, the regular N fertilizers were replaced by their 15 N-labeled forms, viz., 15 NH 4 NO 3 and Na 15 NO 3 , as a single-dose addition. Mass balances of N for the initial 1.5 years following label addition showed that for both treatments, 70% to 80% of the annual N input was leached as NO 3 À , both at ambient and at double N input rates. This confirms the earlier reported extreme case of N saturation at TSP. The 15 N, added as Na 15 NO 3 , showed recoveries of about 74% in soil leachates, indicating that NO 3 À input at TSP is subject to a rapid and nearly quantitative loss through direct leaching as a mobile anion. By contrast, recoveries of 15 N in soil leachates of only 33% were found if added as 15 NH 4 NO 3. Much of the 15 N was immobilized in the soil and to a lesser extent in the vegetation. Thus, immobilization of fresh N input is significantly greater if added as NH 4 + , than as NO 3 À .

Nitrification is the primary source for NO in N‐saturated subtropical forest soils: Results from in situ15N labeling

“…The importance of NO 3 − leaching is supported by the significant recovery of 15 N in the deeper AB horizon (Figure S5, supporting information) . Significant loss of labeled NO 3 − from the soil was also observed in a recent study at the same site . In addition to leaching, the atom% 15 N in NO 3 − also declined due to dilution by NO 3 − produced from nitrification of non‐labeled NH 4 + , which showed stable atom% 15 N close to natural abundance throughout the experiment (Figures B and D).…”

“…19 Significant loss of labeled NO 3 − from the soil was also observed in a recent study at the same site. 34 In addition to leaching, the atom% 15 production, 19 we show here that nitrification is the main source of NO.…”

A Sharp Decline in Nitrogen Input in a N‐Saturated Subtropical Forest Causes an Instantaneous Reduction in Nitrogen Leaching

“…It should be noted that nitrate leaching did not cease in this study, indicating that N saturation should still exist under current N deposition. In actuality, previous studies showed clear N saturation at TSP, with nearly all added N leached by soil water (Huang et al, ; Liu, Yu, et al, ). It is believed that N leaching only begins to increase significantly after the soil reaches N‐saturation (Aber & Magill ; Fang et al, ; Huang et al, ; Larssen et al, ).…”

“…Moreover, in addition to acidification, N saturation caused by increased N deposition may result in the eutrophication of soil and water, the emission of N 2 O (a potent greenhouse gas that is increasingly important in the destruction of stratospheric ozone; Fang et al, ; Li et al, ; Ravishankara et al, ), and reduced biodiversity and forest growth (Aber et al, ; Aber et al, ; Kreutzer et al, ; Lu et al, ; Pardo et al, ). Therefore, an “explosion” in N effect studies has occurred in Asia, especially in China, during the past few decades, both in forests and grasslands (Bai et al, ; Chen et al, ; Guo et al, ; Huang et al, ; Li et al, ; Liu, Yu, et al, ; Liu, Zhang, et al, ; Lu et al, ; Wei et al, ).…”

“…A rapid reduction in the soil N leaching was expected, but no significant change was expected in soil water pH due to buffering by Al mobilization and SO 4 2− sorption (Huang et al, ). Based on a 15 N labeling experiment, Liu, Yu, et al () suggested that some net sorption of NH 4 + may occur, probably due to accumulation in soil organic matter (including in microorganisms). Therefore, we also hypothesize that ending NH 4 NO 3 fertilization should result in lower NO 3 − leaching than in the reference plots receiving ambient deposition.…”