J. S. OWEN, Department of Forest Resources Protection, Kangwon National University, Chuncheon, Korea. Email: email@example.com
We used a laboratory incubation approach to measure rates of net N mineralization and nitrification in forest soils from Fu-shan Experimental Forest WS1 in northern Taiwan. Net mineralization rates in the O horizon ranged from 4.0 to 13.8 mg N kg−1 day−1, and net nitrification rates ranged from 2.2 to 11.6 mg N kg−1 day−1. For mineral (10–20 cm depth) soil, net mineralization ranged from 0.06 to 2.8 mg N kg−1 day−1 and net nitrification rates ranged from 0.02 to 2.8 mg N kg−1 day−1. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. We compared the rates of these processes in three soil horizons (to a soil depth of 30 cm) on a single sampling date and found a large decrease in both net N mineralization and nitrification with depth. We estimated that the soil total N pool was 6,909 kg N ha−1. The present study demonstrates the importance of the stock of mineral soil N in WS1, mostly organic N, which can be transformed to inorganic N and potentially exported to surface and ground water from this watershed. Additional studies quantifying the rates of soil N cycling, particularly multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in nitrogen cycling.
Nitrogen mineralization and nitrification are among the most important internal ecosystem processes affecting the availability of inorganic N to plants and microbes, export to surface and groundwater, and losses through gaseous pathways. Factors affecting the rates of these processes, patterns in seasonality and temporal changes, and the relationships among ecosystem characteristics are reasonably well understood (e.g. Hill and Shackleton 1989; Nadelhoffer et al. 1984). However, most of our data and understanding of these complex relationships are from sites located over a limited range of latitude and geographic coverage. In particular, relatively few studies have quantified rates of N mineralization and nitrification and the controlling factors or related ecosystem variables in forest soils in the East Asia–Pacific region.
Awareness of the need for efforts to improve our understanding of temporal and spatial patterns in rates of ecosystem N cycling, in addition to causal factors and the consequences of these forest processes in regions outside of the temperate zone, is growing. For example, Matson et al. (1999) highlighted the need for more research on the consequences of increased N deposition to ecosystems other than temperate zone systems. One of the implications of the 15N addition studies summarized by Nadelhoffer et al. (1999) was the need for experimental N addition experiments in regions where these studies have not occurred. Tokuchi et al. (1999) used a laboratory incubation approach to study soil N cycling along a topographic sequence in Japan. Inagaki et al. (2004) quantified N mineralization and nitrification rates for forest soils in Japan to compare patterns in N cycling among forests of differing type and age.
Chen and Mulder (2007) discussed the results of their field study of N pools and fluxes in two subtropical forests in China and measured smaller soil N pools, but higher rates of net N mineralization and net nitrification compared with many temperate forest systems. More recently, Ito et al. (2008) combined laboratory measurements of net nitrification rates with a modeling approach based on an artificial neural network to accurately predict nitrification potential in the Yahagi watershed in central Japan. Among the soil characteristics that are known to affect rates of N mineralization and nitrification in soils, soil moisture, temperature and the C : N ratio are particularly important (e.g. Nadelhoffer et al. 1984; Robertson 1982). However, there are still questions regarding fundamental patterns in concentrations and fluxes of N in forested ecosystems within the East Asia–Pacific region.
The objective of the present study was to use a laboratory incubation approach to compare rates of net N mineralization and nitrification at two locations using organic and mineral soil horizons from a forest ecosystem in Taiwan. We previously studied N transformations in these forest soils in Taiwan using a field technique (Owen et al. 2003) and we hope that the present study will encourage future multi-site comparisons of watershed N cycling processes within the East Asia–Pacific region.
Materials and methods
The Fu-shan Experimental Forest is located approximately 40 km south-east of Taipei, Taiwan (24°34′N, 121°34′E). Fu-shan is a moist, evergreen broad-leaved forest with a flora of over 500 species (Mabry et al. 1998). Soil samples for the present study were collected from experimental watershed 1 (WS1; area 37 ha). Dominant tree species in the study area include Castanopsis carlesii (Helmsl.) Hayata, Litsea acuminata (Blume) Kurata and Diospyros morrisiana Hance. Common understory plants are Lasianthus microstachys Hayata, Helicia formosana Lour, Alsophila podophylla Hook and Blatus cochinchinensis Lour. Soils in WS1 are Typic and Lithic Dystrochrepts (Inceptisols) and Typic Hapludults (Ultisols) (Lin et al. 1996a). Additional information describing the soil characteristics in WS1 is provided in Owen et al. (2003). In this forest, a thin organic layer (1–2 cm) has accumulated over the surface of the mineral horizon (A horizon), which showed a thickness ranging from 5 to 20 cm. Horizonation in the lower mineral soil is generally not highly developed, as expected for Dystrochrept soils. Mean annual precipitation at the site is approximately 4,000 mm and the mean annual temperature is 18.2°C (Hsia and Hwong 1999). The watershed has a south-east aspect and the elevation of the watershed ranges from 670 m to more than 1,000 m a.s.l. at the ridge; the slope ranges from 40 to 90% (Cheng et al. 2002).
Experimental design and soil sampling
The upper and lower hillslope sampling locations in WS1 that are described in Owen et al. (2003) were used in the present study (elevations 850 and 700 m, respectively). Soil samples were collected between August 2002 and July 2003. Within each plot, three random replicate soil samples of the O horizon and at a depth of 10–20 cm were excavated. The 10–20 cm depth was selected to broadly represent the mineral soil column and to match the soil sampling depth used in field studies in the study watershed (Owen et al. 2003). During one sampling trip, an additional sample at a depth of 20–30 cm was collected to examine whether deeper mineral soils could possibly be important in the soil N budget. Three O horizon soil samples of precise area were excavated using a 20 cm × 20 cm frame to provide an estimate of bulk density. For mineral soil, bulk density was estimated by collecting replicate soil cores with a 10-cm long drop hammer soil corer. These samples were oven dried before determining the soil mass. These soil density estimates were used to calculate soil pool sizes by multiplying the nutrient concentrations in the organic and mineral horizons by the soil density for the organic layer and for the 0–10, 10–20 and 20–30 cm layers and the sample depth, respectively.
Soil samples were chilled during transport back to the laboratory and refrigerated (4°C) until processed in the laboratory. In the laboratory, the soils were sieved (mineral horizon, 2 mm; organic horizon, 4 mm) and a subsample was removed to determine the moisture content by drying at 105°C. Another subsample was used to measure initial extractable NH4-N and NO3-N concentrations. After sieving, approximately 40 g of moist soil was placed in a 120 mL plastic specimen cup, covered and incubated in the dark at 25°C for a nominal incubation period of 30 days. No additions of water to adjust soil moisture content were made during the incubation.
Soil extraction and chemical analysis
Fresh soil samples (10 g) were extracted with 100 mL of 2 mol L−1 KCl for 60 min. The soil extracts were filtered with Whatman No.1 filter paper and filter blank corrections were used (Piscataway, NJ, USA). The NH4-N concentration was measured using a manual indophenol colorimetric method (Dorich and Nelson 1983). The NO3-N concentration was measured using the manual Cd reduction method (APHA 1998). Net N mineralization was calculated as the difference between the NH4-N and NO3-N concentrations in the incubated sample and the NH4-N and NO3-N concentrations in the initial sample. Net nitrification was calculated as the difference between the NO3-N concentration in the incubated sample minus the NO3-N concentration in the initial sample. Total carbon and nitrogen concentrations were determined on oven-dried soils with a Perkin–Elmer CHN analyzer (Waltham, MA, USA). Soil pH (2:1, H2O) was measured on air-dried soils using a combination electrode.
Particle size analysis was done by the Soil Physical Characterization Laboratory at Oregon State University (Corvallis, OR, USA) using the pipette method (Gee and Bauder 1986). To compare locations and soil depths, log-transformed data were used to test for differences among the sampling units (upper and lower sites; depth) at P < 0.05 using a two-way ANOVA (GLM procedure; SAS Institute 2002).
Total annual precipitation during 2002 and 2003 was 2,743 and 2,342 mm, respectively. In both years the annual precipitation was below the long-term average precipitation of 4,000 mm. The basic soil characteristics at the upper and lower sampling locations are shown in Table 1. At both locations, surface soils were typically characterized by a thin layer of organic matter accumulation, generally approximately 1 cm in thickness and often <2 cm thick; the organic matter could not be easily differentiated into organic subhorizons.
Table 1. Selected characteristics of the organic and upper mineral horizon soils at Fu-shan Experimental Forest WS1
Density (Mg cm−3)
Total C (g kg−1)
Total N (g kg−1)
C : N
Sand (g kg−1)
Silt (g kg−1)
Clay (g kg−1)
Upper (O horizon)
Upper (10–20 cm)
Upper (20–30 cm)
Lower (O horizon)
Lower (10–20 cm)
Lower (20–30 cm)
The mineral soils at the lower sampling location had a greater sand-sized fraction and a lower clay-sized fraction than the mineral soils at the upper location, and this difference became much more pronounced with soil depth. Soil moisture ranged from 0.57 to 1.84 g H2O g−1 soil in the mineral soils and from 0.85 to 1.88 g H2O g−1 soil in the O horizon (Table 2). We did find a significant difference in soil moisture between the soil depths (O horizon and 10–20 cm). Overall, we did not find evidence for any consistent differences in soil moisture between the upper and lower sampling locations (Table 2).
Table 2. Soil moisture (g H2O g−1dry soil), extractable NO3-N (mg N kg−1 dry soil) and extractable NH4-N (mg N kg−1 dry soil) at two depths in the upper and lower sampling locations of Fu-shan Experimental Forest WS1
For each sampling day, different lowercase letters indicate significant differences (P < 0.05) among the samples using Tukey’s Mean Separation Procedure (numbers in parentheses represent the standard error [n = 3]). ND, no data.
1.88 (0.03) a
0.73 (0.08) b
1.74 (0.09) a
0.80 (0.07) b
21.9 (2.45) a
1.9 (0.91) b
18.2 (3.97) a
1.0 (0.27) b
7.6 (3.59) a
1.6 (0.12) b
9.0 (1.54) a
1.6 (0.26) b
1.73 (0.21) a
0.85 (0.02) b
1.84 (0.13) a
1.03 (0.08) b
4.3 (1.47) b
2.3 (0.67) b
13.7 (4.19) a
3.4 (1.09) b
18.4 (6.89) a
3.0 (1.20) c
7.3 (2.74) b
2.1 (0.07) c
0.85 (0.11) a
0.61 (0.03) b
0.90 (0.02) a
0.64 (0.05) b
16.0 (2.4) a
4.3 (1.2) b
20.4 (6.2) a
1.0 (0.30) c
12.9 (8.18) a
26.7 (7.29) a
1.3 (0.13) b
Soil inorganic nitrogen
In the organic horizons at both sampling locations, mean extractable NO3-N concentrations ranged from 4.3 to 21.9 mg N kg−1 (Table 2). The highest extractable NO3-N concentrations were found during the summer months (July and August). Extractable NH4-N concentrations ranged from 7.3 to 26.7 mg N kg−1 and showed no clear pattern of seasonal variation (Table 2). In the mineral soil (10–20 cm depth), mean extractable NO3-N concentrations ranged from 0.5 to 4.3 mg N kg−1 and mean extractable NH4-N concentrations ranged from 1.3 to 3.0 mg N kg−1 (Table 2). In general, there were no consistent differences in extractable NH4-N and NO3-N concentrations between upper and lower hillslope locations. As expected, we generally found a significant difference in extractable NH4-N and NO3-N concentrations between soil depths (organic horizon and mineral soil; Table 2).
We measured higher net mineralization rates in organic soil than in the mineral soil. In the organic horizon, net mineralization rates varied between 4.0 and 13.8 mg N kg−1 day−1 (Fig. 1). The range for net mineralization rates in the mineral soil (10–20 cm) was 0.06–2.8 mg N kg−1 day−1. Rates of net nitrification in the organic horizon varied from 2.2 to 11.6 mg N kg−1 day−1 (Fig. 2). In the mineral soil, net nitrification rates ranged from 0.02 to 2.8 mg N kg−1 day−1. The temporal pattern for N mineralization and nitrification rates did not appear to exhibit any striking trends (Figs 1,2). In general, we did not find a significant difference in net N mineralization and net nitrification between the upper and lower sampling locations (Figs 1,2). As in our previous study using in situ incubations in WS1 (Owen et al. 2003), we did not find any sampling dates with negative net mineralization rate (net N immobilization) in the present study.
During October we sampled mineral soil from a depth of 20–30 cm and found significantly lower rates of both N mineralization and nitrification compared with the O horizon and mineral soil at a depth of 10–20 cm (Fig. 3). Overall, a clear pattern of decreasing rates of net N mineralization and nitrification with soil depth was indicated, although we did not collect soil samples from below 20 cm on any other sampling dates.
Despite the importance of forest cover in Taiwan (approximately 60%; Lu et al. 2001; Cheng et al. 2002), few reports on the patterns in N concentrations and fluxes of N in forests in Taiwan are available. Previously we examined rates of net N mineralization and nitrification in Fu-shan Experimental Forest WS1 soils using in situ field incubations (Owen et al. 2003). Here we compare the results from our earlier field experiment with the results from the present study using laboratory incubations and we discuss some of the implications that our results have for steeply sloped, forested watersheds that are subject to the high precipitation inputs common in north-east Taiwan.
Sampling location comparison
The goal of the present study was to compare soil mineralization and nitrification rates at two sampling locations along a hillslope in Taiwan. Previous studies have identified many factors related to variation in rates of soil N mineralization and nitrification. Garten (1993) reported that higher NH4 concentrations were related to higher rates of net N mineralization and nitrification occurring in valley bottom soils in the Walker Branch Watershed (USA). Hirobe et al. (1998) found large differences in the net nitrification rate between lower and ridge-top positions along a forest hillslope in central Japan. Gilliam et al. (2005) used laboratory incubations to interpret large-scale spatial differences in the rates of net N mineralization and nitrification in two forested watersheds in West Virginia, USA. The results from Gilliam et al. (2005) supported the hypothesis that the vegetation composition of the herb layer, in particular the dominance of an ericaceous plant, Vaccinium vacillans, was related to low rates of net nitrification. In the Fu-shan watershed, there was no evidence for any large differences in either overstory or understory vegetation species composition between the upper and lower hillslope locations.
Factors affecting the rates of nitrogen mineralization and nitrification
In our study, we found much higher net N mineralization and nitrification rates in the O horizon compared with the rates in the mineral soil. However, we did not find support for any sizeable differences in net N cycling rates between upper and lower locations in the study watershed. As in previous studies, our findings also indicate some significant relationships between these variables and other soil characteristics. Based on the significant positive relationship between soil moisture and rates of net N mineralization and nitrification (r = 0.896 and r = 0.865, respectively, P < 0.0001; Table 3), soil moisture is one of the most important factors affecting rates of N cycling in soils at WS1. This same observation has been made in many studies (Binkley and Hart 1989; Robertson 1982). Results from a study of tropical soils in Costa Rica (Marrs et al. 1988) may have relevance to our understanding of forest soils in Taiwan because they measured decreased N mineralization at the highest soil moisture levels. One implication for research in our study watershed might be that the influence of typhoons or heavy precipitation events and litter inputs on forest N cycling should continue to be an important aspect of future research. The highest soil moisture values in the Marrs et al. (1988) study, which were related to decreases in net N mineralization rates, were much higher than the values recorded in the present study.
Table 3. Correlation coefficients among the soil variables
Net N mineralization
Total C (g kg−1)
Total N (g kg−1)
C : N
*P < 0.05; **P < 0.01; ***P < 0.001 (n = 42; for C and N, n = 9). Initial NO3 and NH4 are the extractable NO3-N and NH4-N concentrations at the beginning of the incubation period.
Net N mineralization
Total C (g kg−1)
Total N (g kg−1)
Compared with our results from previous in situ buried-bag incubations, we found higher rates of net N mineralization and nitrification using laboratory incubations for WS1 soils (Owen et al. 2003). In our earlier study, we measured net N mineralization rates between 0.02 and 0.28 mg N kg−1 day−1, compared with 0.06–2.8 mg N kg−1 day−1 in the present study. In the buried-bag study, net nitrification rates had a similar range (0.02–0.26 mg N kg−1 day−1), compared with 0.2–2.8 mg N kg−1 day−1 in the present study using laboratory incubations. We expected to find higher N mineralization and nitrification rates using the laboratory incubations as a result of sample disturbance owing to mixing and sieving when using this approach (Binkley and Hart 1989). Boone (1992) also found large differences in N mineralization rates for forest soils in Massachusetts using buried bags and short-term anaerobic incubations as indicators of potential N mineralization.
We observed highly significant positive relationships among net N mineralization and nitrification, soil C concentration, soil N concentration and the soil C : N ratio (Table 3). This finding is more insightful than it may first appear because it reveals some fundamental characteristics of the N cycling processes in soils in Fu-shan WS1. Compilations of forest soil and ecosystem data from multiple sites have indicated that organic soil C : N ratios can, at least broadly, indicate sites where NO3 leaching is most likely occur (Dise et al. 1998). Aitkenhead and McDowell (2000), deriving biome-scale soil C : N ratios, found that the soil C : N ratio could be used as a predictive tool for dissolved organic carbon (DOC) export from watersheds. Soils in WS1 have low C : N ratios (range 10–14) relative to many soils (range 14–30) included in these studies or, at the very least, are at the low end of the tabulated data (Table 1). Because forest soils in WS1, even the organic layers, have sufficiently high N concentrations to account for lower soil C : N ratios compared with many temperate forest soils (e.g. Aitkenhead and McDowell 2000), we suggest that future studies on N cycling processes can also be linked with studies on DOC retention and export from forest watersheds in Taiwan.
Many factors have been shown to influence rates of mineralization and nitrification in soils. For example, higher N mineralization rates can occur where litter contains higher N concentrations because litter with higher N concentration can decompose faster (Hobbie 1992; Melillo et al. 1982). Based on a survey of studies on N cycling in tropical and temperate forests, Martinelli et al. (1999) did not find a significant difference in foliar N concentration between tropical and temperate systems. In WS1, atmospheric N inputs are approximately the same magnitude (∼1,800 kg N km−2 year−1) as inputs in regions of North America and Europe with the highest acid deposition inputs (Lin et al. 1997, 2000). These N inputs are likely to have been high enough to affect the loss of base cations from soils (Lin et al. 2001), in addition to foliar N concentrations and soil solution chemistry (McDowell et al. 1998).
Pool size of the soil nitrogen in WS1
We used our measured soil concentrations and bulk density measurements to estimate the pool sizes of soil total N, NO3-N and NH4-N in WS1. Using average results from both the upper and lower hillslope, we estimated that the soil total N pool was 6,909 ± 81 (mean ± standard deviation [SD]) kg N ha−1 (including organic and mineral soil to a depth of 30 cm). Unlike the estimate of Horng and Chang (1996), our estimate does not consider the mass of rock in the soil profile, which probably becomes important below a depth of 30 cm, particularly at the lower sampling location. Nonetheless, our result was close to the estimate from Horng and Chang (1996) of 6,949 kg N ha−1. For inorganic N, the pool sizes were much smaller: NO3-N in the organic layer (2–0 cm) accounted for approximately 1 ± 3 (mean ± SD) kg N ha−1 and for NH4-N, 0.6 ± 2 (mean ± SD) kg N ha−1. In the mineral soil (0–30 cm depth) we calculated the NO3-N pool to be 1.7 ± 4.1 (mean ± SD) and for NH4-N, 2.8 ± 9.3 (mean ± SD) kg N ha−1. For organic nitrogen, we estimated that approximately 682 ±74 (mean ± SD) kg N ha−1 was in the organic horizon and 6227 ± 311 (mean ± SD) kg N ha−1 was stored in the mineral layers. These N pools were compared with our annual rates of net N mineralization and nitrification in WS1 from in situ incubations of 30.9 and 31.8 kg N ha−1 year−1 (Owen et al. 2003). Similar to results from studies in temperate forested watersheds, microbial processes in soil, with its huge store of N (mostly organic N), may help account for the large amounts of inorganic N that can be exported in surface water during high flow periods (Kao et al. 2003; Piatek et al. 2005). Our results showed that <1% of the total soil N pool occurs as inorganic N (extractable NO3-N and NH4-N); this difference in the magnitude of the N pool size is typical for most temperate and tropical forests (e.g. Chen and Mulder 2007; Nadelhoffer et al. 1984).
To help foster discussion and collaboration with regard to several important possibilities for future studies related to N cycling in forests in Taiwan, we identify some research areas that would help carry out more complete studies on the N budget in Taiwan forests. A thorough literature review of studies on the N cycle in forests in Taiwan or the East Asia–Pacific region is outside the scope of the present study, but we would like to point out some future research needs related to N cycling studies in Taiwan because studies of this type are somewhat uncommon for forest ecosystems in Taiwan. For example, a number of studies on wet precipitation inputs to forests in Taiwan have been published (Lin et al. 1997, 2000), but few studies regarding dry inputs have been carried out. Although data on litter-fall chemistry and vegetation uptake in forests in Taiwan are available, additional studies are needed (Lin et al. 1999; Lin et al. 1996b). Only limited data are available for processes related to N export via groundwater or denitrification. Finally, studies linking N export and hydrology need to be carried out for forests in Taiwan (Kao et al. 2003).
Other characteristics of our study area are also meaningful with regard to the context of the current study. For example, the topography in Taiwan is characterized by finely dissected basins with steep slopes. Meybeck et al. (2001) proposed an updated global classification scheme for relief roughness and showed that parts of Taiwan are classified as highly dissected (relief roughness 80–160%) compared with other parts of the world undergoing recent orogenesis. Furthermore, annual precipitation can vary up to 5,000–6,000 mm, resulting from both periodic typhoons and heavy spring and summer rainfall events (Wu and Kuo 1999). These characteristics, in addition to the warm average annual temperatures relative to the temperatures recorded in many temperate forests, offer compelling motivation to support future studies on N and other nutrient cycling processes in forested landscapes in Taiwan and the East Asia–Pacific region.
We used laboratory incubations to measure rates of net N mineralization and nitrification in forest soils from the Fu-shan Experimental Forest WS1 in northern Taiwan. We recorded net mineralization rates in the O horizon ranging from 4.0 to 13.8 mg N kg−1 day−1, and net nitrification rates from 2.2 to 11.6 mg N kg−1 day−1. In mineral soils, net mineralization ranged between 0.06 and 2.8 mg N kg−1 day−1 and net nitrification rates varied from 0.02 to 2.8 mg N kg−1 day−1. We did not find any consistent differences in N mineralization or nitrification rates in soils from the upper and lower part of the watershed. For the single sampling date that we compared the rates of these processes down to a soil depth of 30 cm, we found a large decrease in both net N mineralization and nitrification with soil depth.
Rates of net N mineralization and nitrification were highly correlated with soil moisture content. We also found that the rates of these processes were highly correlated with the initial concentrations of extractable NO3-N and NH4-N. We estimated that the soil total N pool to a depth of 30 cm was 6,909 kg N ha−1. Developing a more complete understanding of the N budget for Fu-shan Experiment Forest WS1 or other forested watersheds in Taiwan will require more studies on several key components, including N concentrations and fluxes in soil water, groundwater, hydrological processes and denitrification. Additional studies quantifying the rates of soil N cycling, and in particular, multi-site comparisons within Taiwan and the East Asia–Pacific region, will greatly improve our understanding of regional patterns in N cycling.
This research was supported in part by grants from the National Science Council of Taiwan (NSC 90-2811-B-054-001). Fellowship support from the Taiwan Forestry Research Institute and Academia Sinica to J. S. Owen contributed to this research. This work was supported by the Korea Research Foundation and The Korean Federation of Science and Technology Societies Grant funded by the Korean Government (MOEHRD, Basic Research Promotion Fund). Competent assistance from Horng Fu-Wen, Ma Fu-Ching and staff, Division of Silviculture, Taiwan Forestry Research Institute, and all support staff at the Taiwan Forestry Research Institute is greatly appreciated.