Abstract
Fundamental questions of how plant species within secondary forests and plantations in northeast China partition limited nitrogen (N) resource remain unclear. Here we conducted a 15N tracer greenhouse study to determine glycine, ammonium, and nitrate uptake by the seedlings of two coniferous species, Pinus koraiensis (Pinus) and Larix keampferi (Larix), and two broadleaf species, Quercus mongolica (Quercus) and Juglans mandshurica (Juglans), that are common in natural secondary forests in northeast China. Glycine contributed 43% to total N uptake of Pinus, but only 20, 11, and 21% to N uptake by Larix, Quercus, and Juglans, respectively (whole plant), whereas nitrate uptake was 24, 74, 88, and 68% of total uptake for these four species, respectively. Retention of glycine carbon versus nitrogen in Pinus roots indicated that 36% of glycine uptake was retained intact. Nitrate was preferentially used by Larix, Quercus, and Juglans, with nitrate uptake constituting 68∼88% of total N use by these three species. These results demonstrated that these dominant tree species in secondary forests in northeast China partitioned limited N resource by varying uptake of glycine, ammonium and nitrate, with all species, except Pinus, using nitrate that are most abundant within these soils. Such N use pattern may also provide potential underlying mechanisms for the higher retention of deposited nitrate than ammonium into aboveground biomass in these secondary forests.
Highlights
In Northeast China, natural secondary forests (NSFs) account for as much as 70% of the regional forests (Hao et al, 2000)
The calculated whole plant δ15N values ranged from −3.0 for Larix to −1.1 for Quercus (Table 1), with Quercus having the highest δ15N values of leaf, stem, coarse, and fine roots (Table 1). δ15N of soil N pools, on the other hand, ranged from 5.0 to 7.2 for dissolved organic N (DON), 3.1 to 15.7 for ammonium, and −5.2 to −2.1 for nitrate (Figure 1A)
DON was the dominant N form within these soils (Table 2), with concentration ranging from 4.4 ± 1.4 to 7.0 ± 0.7 μg g−1, followed by nitrate (1.5 ± 0.2 to 3.5 ± 1.3 μg g−1) and ammonium (1.2 ± 0.1 to 2.5 ± 0.4 μg g−1)
Summary
In Northeast China, natural secondary forests (NSFs) account for as much as 70% of the regional forests (Hao et al, 2000). They are formed through natural regeneration of primary forests following afforestation (Zhu and Liu, 2007). Tree Species Preference for N-Forms mosaic plantation/secondary forest landscapes. These two forest types are common in China and are regionally important in providing ecosystem services of regulating regional climate. Lower soil nutrient concentrations, especially available nitrogen, in these secondary forests and larch plantations compared to those well-developed and less-disturbed forests may constrain tree growth (Yang et al, 2012). Fundamental questions of how plant species within these forests partition and compete for limited soil N resource remain unclear
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