An in situ lysimeter experiment on soil moisture influence on inorganic nitrogen discharge from forest soil

Journal

Journal of Hydrology 195 (1997) 78-98

An in situ lysimeter experiment on soil moisture influence on inorganic nitrogen discharge from forest soil Nobuhito Ohtea**, Naoko Tokuchi’, Ma&am

Suzukib

‘Division of EnvironmentalScience and Technology, Graduate School of Agriculture, Kyoto University, Kyoto 6&Wl, Japan bDepartnaentof Forestry, Faculty of Agriculture. University of Tokyo, Tokyo 113, Japan

Received30 Januaty 19%; revised 18 July 1996 accept4 7 August 19%

The influences of the soil moisture condition on ammonification, nitrification and denitrification were examined by in situ experiments on forest floors using soil column lysimeters. Distinct differences were detected in the discharge concentrations of NH; and NOT from lysimeters with different soil water content. Three types of lysimeters (diameter 19.5 cm, depth 30.0 cm) were prepared by containing undisturbed soil samples. Inorganic nitrogen discharged from lysimeters under natural rainfall was measured approximately once a week from 5 July to 29 November 199 1. Soil moisture conditions of the three lysimeters were controlled using an average pressure head at the bottomof each column at -268.6 hPa, 0 hPa. and 19.8 hPa. These three moisture. conditions corresponded approximately to the conditions in the upper or mid-upper hillslopes with relatively dry soil, lower hillslopes with relatively wet soil, and riparian zones around spring points and streams with saturated soil, respectively. For these soil moisture conditions, three different types of inorganic nitrogen discharges were observed; (1) ammonium and nitrate nitrogen; (2) nitrate nitrogen; (3) neither ammonium nor nitrate nitrogen. The result clearly showed the restraint of nitrification by soil moisture deficit and the reduction of NO; by denitrification under saturated conditions. This suggests that the different stages of the process in nitrogen dynamics could occur owing to only the influence of soil moisture variations even in the shallow soil layer. The nature of inorganic nitrogen discharge from each lysimeter helped to explain the spatial heterogeneity of the nitrogen dynamicscausedby distributed soil moisture conditions within a watershed system.

1. lntroduetion

Nitrogen cycling plays an important role in the hydrobiochemical phenomena in forest ecosystems. Many forest ecologists have extensively investigated to evaluate the * coln?s~llg

author.

0022-1694/97/$17.00 0 1997- Elsevicr Scknce B.V. All rights reserved PII SOO22-1694(96)03240-4

IV. Ohte et al./Journal of Hydrology 195 (1997) 78-98

79

influences of forest disturbance on soil nutrient condition as a factor that limits forest growth. Forest treatment impact on nitrogen cycling has been previously evaluated using the nitrogen budget method in watershed-scale experiments with hydrological measurements (Likens et al., 1970; Vitousek and Melillo, 1979; Iwatsubo et al., 1982; Feller and Kimmins, 1984; Reynolds et al., 1992). However, in terms of the discussion on nitrogen dynamics on a watershed scale, one of the major problems is that the soil environment within a watershed generally includes great heterogeneity controlled by spatial variances in hydrological conditions which are due to the topography and vegetation (Allan et al., 1993). Groffman and Tiedje (1989) have investigated the relationship between denitrification and the landscape-scale factor using soil core sampling and incubation. Davidson and Swank (1990) focused on the dissolved N20 concentration of soil water to evaluate the variances of denitrification owing to topography. The information from these investigations and other studies indicates that the conditional heterogeneity within a watershed must be taken into account to explain spatial patterns of nitrogen components within a system. Therefore, to understand and interpret the results of nitrogen budget studies at a watershed scale, it is necessary to obtain information from smaller-scale experiments involving hillslopes or soil-layer-scale observations, taking spatial heterogeneity into account. On the other hand, soil biochemists have frequently utilized the incubation technique to measure rates and to investigate the controlling factors such as 02 and CO2 gas concentrations, organic carbon, pH, temperature, and soil moisture content on nitrogen mineralization and denitrification in farm or forest soils (Christensen et al., 1990; Tietima and Wessel, 1992; Davidson et al., 1992; Hart et al., 1994). Discussions on the controlling factors have been reviewed by Binldey and Hart (1989) for nitrogen mineralization, and by Knowles (1982) and Beauchamp and Trevors (1989) for denitrification. Although a laboratory experiment has the advantage of being able to clearly determine controlling factors, an experimental environment does not always reflect the actual conditions in a watershed. The information required for a watershed-scale study of nitrogen dynamics must be obtained by an experimental technique in which the spatial heterogeneity of the soil environment in a watershed is reflected. In this paper, we present an in situ lysimeter experiment in a temperate forest watershed. This experimental approach can not only measure both inorganic nitrogen and water budget under natural rainfall conditions, owing to the in situ procedure, but is also effective in simulating the different soil environments that are hypothesized in various individual hydrologic processes in the watershed, owing to the lysimeter environment. That is, the in situ lysimeter experiment has both controllability of the environment, as in a laboratory experiment, and the ability to reflect the actual conditions and heterogeneity in a watershed. Even though the limitation of a biochemical environment is caused by disturbance with the installation of a lysimeter, a lysimeter technique is still effective for simulating the conditions created by forest disturbance such as clear cutting. The purpose of the study reported in this paper is to evaluate the influence of soil moisture variance on the nitrogen dynamics within relatively shallow soil layers in a forest watershed, because spatial heterogeneity of soil biochemical environment is frequently generated by variances in the hydrologic pathway within a small low-order watershed. To

N. Ohte et aL/Journal of Hydrology 195 (1997) 78-98

80

142'E 44"~~ Sea

ofJ'~Oapan [ I ~

40°N • Station

ExpedmenUd Sb 130OE Fig. 1. Locationof the experimentalsite.

reflect the real hydrological aspects, three lysimeters with different moisture conditions were prepared. The discharge of water and inorganic nitrogen was measured under natural rainfall conditions.

2. M e t h o d s

2.1. Study site

The experiment was conducted at a site in the Kiryu Experimental Watershed (5.99 ha) located in the southern part of Shiga Prefecture. This area is included in the Lake Biwa watershed in the central part of Japan (Fig. 1). The Kiryu watershed is situated at 35°N, 136°E, with elevations ranging from 190 to 255 m above sea level. Hillside and planting work have been periodically carried out over the last 100 years around the Kiryu watershed to prevent soil erosion. At present, therefore, the watershed has attained a good cover of semi-mature vegetation (a mixed stand of Japanese red pine (Pinus den~iflora Sieb. et Zucc.) and Japanese cypress (Chamaecyparis obtusa Sieb. et Zucc)). The experimental plot was covered by only Japanese cypress uniformly planted around 1957. The entire area of the watershed consists of weathered granitic rock with abundant amounts of albite. The experimental plot was located in one of the small headwater catchments in the Kiryu watershed. The average air temperature of the Kiryu watershed is 12.6°(2, and the average precipitation, evapotranspiration and runoff during the decade from 1972 to 1981 were 1671.8 mm, 740.2 mm, and 936.0 mm, respectively (Fukushima, 1988).

N. Ohte et al./Journal of Hydrology 195 (1997) 78-98

81

Table 1 Concentration of exchangeablecations in soil and pH at the experimental site Depth (cm)

Ca

Mg

Na

K

pH (1:5, soil:H20)

0-5 5-10 10-20 20-30 30-50

447A 55A 169.7 247.2 506.3

1.06 1.44 1.44 1.62 1.65

1.33 4.57 1.67 1.77 2.29

13.40 7.57 6.41 5.02 7.54

6.07 6.13 6.19 6.26 6.29

Cation concemrationsare in milligrams in 100 g of dry soil.

2.2. Soil characteristics The soil profile of the experimental site does not have an apparent structure. Organic matter is not mixed in to a notable degree. The soil type is typical brown forest soil and its profile on the experimental plot includes an A0 layer o f 7 cm, an A layer o f 7 cm, and a B layer of more than 40 cm in depth. The A0 horizon has a Moder humus form without remarkable H horizon. Neither the A nor B horizon has any apparent aggregates. The exchangeable cation contents and pH o f the soft are presented in Table 1. The Ca 2+ content is relatively high, varying from 55.4 to 506.3 m g per 100 g o f dry soil.

2.3. Experimental system and preparation o f soU lysimeters The experimental system is illustrated in Fig. 2. The s y s ~ m consisted of three soil lysimeters with drainage collectors and one automatic soil suction control unit. The lysimeters were prepared by in situ undisturbed soil sampling with PVC cylinders (inner diameter 19.5 cm, length 40.0 cm) that were installed in parts o f the lysimeters.

Fig. 2. Experimental system. I, Stainless net; 2, collecting bottle (polyethylene); 3., porous cell (ceramic); 4, collecting bottle for sucked water, 5, air pump; 6, power switch; 7, mercury manometer. Sucking pressure was maintained by the mercury manometer and the switch connected to it by the electric wire.

N. Ohte et al./Journal of Hydrology 195 (1997) 78-98

82

The basic structure of the type of lysimeter used was presented by Tokuchi et al. (1993). This type of lysimeter is a modification of the type used by Nilsson and Bergkvist (1983) and Rasmussen et al. (1986). The sampling technique was that proposed by Ohte et al. (1989). A PVC cylinder equipped with a sharp-edged ring was inserted into the soil on the forest floor without disturbing the soil structure. During the process of insertion, root systems were carefully cut off from the soil layer around the sampled column. The height of the soil samples in all lysimeters was approximately 30 cm. After 30 cm of soil was collected, a basement part was installed with stainless wire mesh at the bottom of the soil column. Three soil samples were taken from within a 3 m x 3 m area at the middle part of the hillslope. Therefore, there were no remarkable differences in the soil material and vegetation cover above the three lysimeters. To maintain the original temperature conditions of the sites from which the samples were taken, the cylinders were set back into their former places and the level of the soil surface and surface condition of the forest floor were adjusted to their original conditions before Sampling the soil around the lysimeters. After installation, the surface condition of the lysimeter was kept as natural as possible, and litter fall was not removed. 2.4. Soil moisture conditions in the three lysimeters The vertical profile of the hydraulic pressure head under equilibrium conditions, after completion of gravitational infiltration without evaporation from the surface, is expressed

by ~,(z) = -z+~,(zo)

(1)

where ~(z) indicates the hydraulic pressure head at height z from reference height z0, which is defined as the bottom of the soil column in the lysimeters. To set the lysimeters with three different soil moisture contents, the hydraulic pressure head of each lysimeter was controlled as follows: 1. Control condition (referred to here as the 'Control' lysimeter). No soil moisture control was performed. The ~(z0) was set at 0 hPa. Only gravitational drainage water was collected in a plastic bottle. 2. Dried condition (referred to here as the 'Dry' lysimeter). The ~b(z0)was kept at around -268.6 I d a by an automatically controlled vacuum system, which consisted of ceramic porous cells, an air pump, and a pressure-maintaining switch activated by a mercury manometer (see Fig. 2). When soil moisture content increased and hydraulic pressure head exceeded-286.8 Ida, excess soil water was sucked off through the porous cells to the collector until the pressure head returned to -268.6 hPa. If the water supply at the bottom resisted the suction force, the surplus portion was drained into a different collecting bottle. 3. Saturated condition (referred to here as the 'Saturated' lysimeter). The saturated water level condition was made by hanging the drainage tube up at a height of 20 cm from the bottom of the lysimeter (see Fig. 2). The ~b(z0) was set to be 19.8 hPa. Drainage occurred when the water level exceeded the highest point of the tube. After completing gravitational redistribution, the hydraulic equilibrium of the vertical

N. Ohte et al./Journal of Hydrology 195 (1997) 78-98

83

•

0

10'

coeuel

30

i

i

0 0.2 0.4 0.6 Water content (era 3 em-3)

Fig. 3. Vertical distribution of soil water content under hydraulic equilibrium after gravitationaldrainage. profiles of volumetric water contents should be as shown in Fig. 3. These values were estimated using the water retention characteristics (the relationship between moisture content and hydraulic pressure head, Fig. 4) determined by Ohte and Suzuki (1990). Using the water retention function in Fig. 4 by Kosugi (1994), the retained water amounts of the three 30 em soil columns are estimated as 74.4 m m for the Control lysimeter, 25.9 mm for the Dry lysimeter, and 131.9 m m for the Saturated lysimeter, respectively. 2.5. Sampling and chemical analysis

Four types of water samples were taken from the lysimeters: the gravitational drainage water from the Control, Dry and Saturated lysimeters and the sucked water from the Dry lysimeter. Although the purpose of sucking the soil solution in the Dry lysimeter was to keep the soil moisture condition dry, the sucking also removed solutes with water. Therefore, the sucked water from the Dry lysimeter was sampled and analysed for its quantity and chemical composition to measure the mass of total outgoing solutes and solution. Open rainfall water and throughfall water were also sampled at the experimental site using bottles with funnels of 20 cm diameter. Open rainfall rate was measured by a tipping -120

~-80-

!

O

Meumed Pitted

0 0 0.2 0.4 0.6 Water c~mtem(era3 cm-3)

Fig. 4. The waterretentioncharacteristicof the soil at the experimentalsite.The measureddata are fittedusin8 the function by Kosugi (1994).

84

N. Ohte et alJJournal of Hydrology 195 (1997) 78-98

bucket type rain gauge. The interval amount of open rainfall and throughfall were measured by a storage type rain gauge. Routine sampling was performed on a weekly basis (therefore, the samples of rainwater and throughfall water may have contained water from multiple events in some of the samplings). The water quantity, pH, electric conductivity (EC), and water temperature of these samples were measured in situ at each routine sampling. The water samples were then brought back to the laboratory in 100 ml polyethylene bottles for chemical analysis. The analysed chemical species were major cations (Na +, K +, NH~, Ca 2+, and Mg 2+) and anions (NO], CI- and SO2"). The concentrations were measured by suppressed ion chromatography (Shimazu, Kyoto, HIC-6A). / For the discussion in this paper, to focus on the nitrogen dynamics, we present the analytical results of the concentrations of NH~, NO~, and the other anions ((21- and SO2-). A discussion on the interaction between other cations and inorganic nitrogen will be presented in another paper. 2.6. Experimental period

Field observations and sampling were initiated on 5 July 1991, and continued to the end of November 1991. The starting time of this experiment was defined as the time when the first sample was collected based on the following considerations. The soil moisture profiles in Fig. 3 show the difference among the three lysimeters in a steady-state condition. The actual profiles were not always the same as those in Fig. 3 under natural rainfall conditions: in the first few weeks after installing the three lysimeters they were also in an unsteady-state condition, reaching the conditions expressed in Fig. 3. However, the conditional difference in terms of soil moisture among the three lysimeters was established just after setting up the experimental devices, and the soil moisture contents of these should have been Saturated > Control > Dry. Even during this phase, the moisture difference has been influencing the nitrogen dynamics in the three lysimeters. Thus it is practically impossible to set the Fig. 3 condition with the same status of nitrogen dynamics as the initial condition. Therefore, in this experiment the initial time was defined as above, and the samples collected during the early unsteady-state condition are also treated in the following discussion.

3. R ~ 3.1. Soil moisture conditions and water budgets of the lysimeters

The throughfall and drained water rate and their cumulative values for each lysimeter during the experimental period are shown in Fig. 5. The vertical axis is for the averaged rate during the intervals between two observations. Table 2 shows the water budget of the three lysimeters during the period. The estimated amount of retained water of each lysimeter in hydraulic equilibrium is also listed. Two types of collected water-gravitationally drained and sucked watermare listed for the Dry lysimeter in Table 2. The actual amount of throughfall that collected during the observation period was larger

N Ohte et alJJournal of Hydrology 195 (1997) 78-98

~T~---R o , ~ ~o11I..........1I--~m'-:" ...................... ~

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,

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150

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Date (days from July 5, 1991) []

lnte~al avcmg¢ rate ~

Cumulative rate

Fig. 5. Throughfall and drained water rate from each lysimeter.

than the totals of output water from the three lysimeters and slightly less than that observed in open rainfall. It is generally accepted that the throughfall rate varies considerably with location even within a small area. Therefore, the observed amount of throughfall in this experiment could be treated only as an approximate standard. However, the fluctuations of the rates of drained water of the three lysimeters corresponded consistently to the fluctuation of the throughfall rate shown in Fig. 5. Considering the homogeneity of canopy materials around the experimental plot (as mentioned above, the plot was covered by cypress stands that have been uniformly planted), it can be stated that the chemistry of throughfall has no remarkable variance within the experimental plot. Moreover, Table 2 shows that the sums of the output amount and the estimated storage of three lysimeters are also consistent. This suggests that the differences of the outgoing water amount among

86

N. Ohte et alJJournal of Hydrology 195 (1997) 78-98

Table 2 The waterbudgetof the Control,Dry and Saturatedlysimeters Control Rainfall(mm) Open rainfall Throughfall EstimatedsUagein lysimeter (a) (ram) Drained water(b) (mm) Suckedwater(c) (mm) a + b + c (nun)

Dry

Saturated

786 778 74.4

25.9

131.9

554 0 628.4

214 407 646.9

513 0 644.9

three lysimeters corresponded to the differences of the retained water amount in the soil lysimeters, and the throughfall amount that reached each lysimeter was estimated to be approximately the same. In the case of the Saturated lysimeter, a delay of the first discharge was observed at the beginning of the experimental period (Fig. 5). This indicates that the throughfall input at the beginning was used to fill up the bottom space of the lysimeter (see Fig. 2) and to saturate the soil pores within a depth of less than 20 cm from the bottom. 3.2. Discharge of inorganic nitrogen

Time-sequential variations in the concentrations of inorganic nitrogen in throughfall and discharge water are presented in Fig. 6. Three noteworthy results have to be pointed out in the figure. First, the concentration of NH~ was high during the initial 25 days of the experimental period in all three lysimeters, and then began to decline gradually. The concentration of NO~ then began to increase during the period from 20 to 50 days after the beginning in the Dry and Control lysimeter, and it peaked at about 80 days. Second, there was a peak of NO~ concentration in throughfall at around 40-50 days, and the peaks of discharge water from the Control and Dry lysimeters were also detected at about 30 days after the throughfall peak. However, the concentrations of discharge water from these two lysimeters remained at higher levels than that observed before peaking. Third, there was no NOg in the discharge water from the Saturated lysimeter. Fig. 7 shows the cumulative load of inorganic nitrogen in discharge water from each lysimeter. This figure indicates that the input of NH~ and NO~ by througlffall was fairly small throughout the experimental period, although there was a peak in NO~ concentration in the throughfall in the 30-60 day period. The greater part of the NH~ leachate was observed before 30 days for three lysimeter. The order of the accumulated NH~ discharge was Dry > Control > Saturated. This order was found even during the early unsteady period. On the other hand, the NO~ discharge started at about 60 days, and the order for NO~ discharge was Control > Dry > Saturated. 3.3. Examination of discharge variance of inorganic nitrogen by Cl- discharge

The accumulated ion discharge of C1- for the three lysimeters is shown in Fig. 8. CI- is

N. Ohte et al./Journai of Hydrology 195 (1997) 78-98

0.2~

NO3-

NH4÷

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Date (days from July 5th, 1991)

~

~

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Fig. 6. Variations in the concentrations of NH~ and NO~ of drained and throughfali water.

generally free from adsorption by soil particles in the weak acidic soil layer such as in this experiment. Also, C1- has neither a source nor a sink factor in the soil layer. Therefore, the difference in discharge CI- between the Control and Saturated lysimeters was mainly the result of variation of the initially stored portion and partially the result of throughfall input. The difference between the Dry lysimeter and the other two lysimeters was obviously caused by less drainage water from the Dry lysimeter. In comparison between C1- and inorganic nitrogen, it can be seen that the accumulated changes of inorganic nitrogen in Fig. 7 had a totally different pattern in the three lysimeters. The differences of NH~ and NO~ discharge between the Control and Saturated lysimeters were significant. Also for the Dry lysimeter, the NH~ leachate was larger than those for the other two lysimeters, although the drainage water volume was much smaller than for the other two lysimeters. These observations suggested that the inorganic nitrogen discharge from the three lysimeters was strongly influenced by nitrogen reactions in each lysimeter, and the difference among the lysimeters was significant enough to allow examination of the influence of the conditional difference among the three lysimeters. 4. Discussion

4.1. Influences of soil moisture condition on inorganic nitrogen discharge To understand the experimental results represented in Fig. 6 and Fig. 7, first, a compartment model of the nitrogen reaction in soils is kept in mind. Fig. 9 shows a schematic

88

N. Ohte et al./.lournal of Hydrology 195 (1997) 78-98

2 NI'I4+'N, 13t-- ~

0

. 0

5 3o

~O~--N

30

~ 60

90

120

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i :

. . .

-

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. ~

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8/4

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90 120 150 10/3 11/2 12/2

Date (days from July 5th, 1991) ---o--- Control

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--o--- Saturated

Fig. 7. Variationsin accumulatedNH] and NO~loadof drainedand throughfallwater. representation of this model, presenting the generally accepted major processes that affect the pool sizes of the two differently formed nitrogen pools in forest soils. In this experiment, these pool sizes in lysimeter soils should have been affected by cutting the root systems or by soil water sucking in the Dry lysimeter. Therefore, timesequential change and the variance of inorganic nitrogen discharge have to be explained as the responses of pool size changes caused by these experimental disturbances. One of the most critical effects is the cutoff of the inorganic nitrogen uptake by plant roots. Additionally, the relationship between the pool size and discharge rate should be carefully examined, because NI-I] and NO] have different mobilities in soil water. It is generally accepted for forest soils in the temperate region that NO] as an anion has a greater mobility in soil water compared with NH~ as a cation, owing to the difference of adsorptivity of soil particles. In fact, Fig. 6 shows that the maximum drained concentrations of NH~ of the Control, Dry and Saturated lysimeters are 0.13 mequiv 1-1, 0.12meqnivl -! and 0.11 mequivl -l, respectively; on the other hand, those of NO] drainage are 0.82 mequiv 1-1, 0.31 mequiv 1-! and 0.0 mequiv 1-1, respectively. Therefore, it is reasonable to say that the discharge rate of NO] should directly correspond to the pool

N. Ohte et aLIJournal of Hydrology 195 (1997) 78-98

89

CI" ,-, 30 ! 2o

...........

0 7/5

i ................ - ..........

30 8/4

60 9/3

i. . . . . . . .

90 10/3

120 150 11/2 12/2

Date (days from July 5th, 1991) ---o-- Comrol

A

D~

~

Saturated

Fig. 8. Variationsin accumulatedCl- loadof drainedand throughfnllwater. size, because the NO~ produced should mostly be released in soil water. On the other hand, because the NH~ ion is highly adsorptive, the discharge NH~ should be treated as a portion of the entire pool. Time-sequential variation of discharge rate, however, can be treated as an index representing the variation of the pool size. Moreover, for the Dry lysimeter, removal of the soil solution by sucking made the drained portion smaller. This should be considered for the estimation of the inorganic nitrogen production through the discharge load. A detailed discussion of this is presented in Section 4.2. F'wst, at the beginning of the experiment, although the NH~ discharge was observed for all lysimeters, NO~ was not discharged from any lysimeter (Fig. 6). In Fig. 7, the NO] discharge from the Control and Dry lysimeters began after about 60 days. Considering the initially contained soil water volume (see Fig. 3 and Table 2) and cumulative drainage (see Fig. 5) of each lysimeter, it can be stated that the initial soil water of the lysimeters did not involve an NO] pool, and that NO~ was not produced to a significant degree during the discharge of initial soil water. In contrast, a pool of NH~ existed to a significant degree at the initial stage. Around 12 days after starting the experiment, the peak concentrations of NH~ discharge were detected for the Control and Saturated lysimeters. This means that the NH~ pool increased quickly as a result of termination of root uptake. The decrease of NH~ discharge after this peak was caused by the rise of nitrification and consumption of NH] by it. As a result, the NO] discharge began at 20 days after the start for the Dry lysimeter and at 42 days for the Control lysimeter. In this phase, the pool of NH~ decreased and the pool of NO~ increased. The peak concentrations of NO~ discharge from the Control and Dry lysimeters have been influenced by throughfall at 40-50 days. From the average throughfall intensity (about 4 mm day-t at 40-80 days; see Fig. 5) and effective soil porosity (0.4 cm 3 cm-3; see Fig. 3), the convective transport velocity of NO~ peak can be roughly estimated at about 10 nun day-l. This estimation, which confirms that the velocity can be estimated

90

IV. Ohte et alJJournal of Hydrology 195 (1997) 78-98

denltrlficatlon

plantuptake mi crobial ~ l immobilization] minerallzation~ I

NH;

leaching

plantuptake' microbial immobilization I nitrification

T NO;

leaching

Fig. 9. Schematic diagram of the major processes affecting NH~ and NO~ pools in forest soils.

from the peak transport, was reasonable. Similar results on convective transport of NO~ in this soil were reported by Tokuchi (1993) from lysimeter experiments at the same experimental site. Even though the peak of NO~ concentration in throughfaU was significant, and it certainly influenced the peaks of NO~ discharge from the Control and Dry lysimeters, Fig. 7 shows that the NO~ load supplied by throughfall at this period was at most 2-3 kg ha-l; on the other hand, the output load at the peak concentration was estimated as more than 10-12kgha -I for the Control lysimeter and 8 - 1 0 k g h a -~ for the Dry lysimeter. (The leachate of NO~ from the Dry lysimeter at the peak period was 2-3 kg ha -1. However, 6-7 kg ha -I of NO~ was sucked out with soil water during the same period (see Fig. 12, below). The details of the differences between drained and sucked water from the Dry lysimeter are discussed in Section 4.2.) Moreover, the discharge concentration of NO~ after the peak were kept significantly larger than the load by throughfall. This means that the NO~ output from the Control and Dry lysimeters mainly consisted of a portion produced in each lysimeter. The difference in nitrogen discharge between the Control and Dry lysimeters has to be made clear. Although the NH~ concentration of drained water from the Control lysimeter decreased significantly and was reduced relative to the increase of NO~ concentration, the NH~ concentration of the Dry lysimeter continued even after the rise of NO~ concentration. This difference can be explained by the influence of the water condition on the nitrifying activity. It is generally accepted that many nitrogen reactions can be affected by the soil moisture condition, and that nitrification is more sensitive to water stress than mineralization (Sprent, 1987). For this lysimeter experiment, moisture deficit acted as a restricting factor against nitrification in the Dry lysimeter. This resulted in the NO~ discharge rate being smaller than that from the Control lysimeter; also, the NH~ pool should be larger than that in the Control lysimeter, although part of the NH~ pool was sucked out. Consequently, the NH~ discharge from the Dry lysimeter was larger than that from the Control lysimeter. For both the Control and Dry lysimeters, there was a significant delay for the NH~ discharge after the start of the NO~ discharge. This time lag suggests that the growth of

N. Ohte et alJJournal of Hydrology 195 (1997) 78-98

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the NO~ pool began while the growth of the NH~ pool was delayed. The time lag of the peaks between NH~ and NO~ was about 80 days, and the difference for it between the two soil moisture conditions was not significant. This kind of time lag is often identified in relatively long-term incubation (Johnson et al., 1980; Vitousek et al., 1982; Gosz and White, 1986; Van Miegroet et al., 1990). The lag has been explained by the progressive increase in the population of nitrifying bacteria, which was initially small (Sabey et al., 1959; Johnson et al., 1980; Vitousek and Matson, 1985; Van Miegroet et al., 1990), or by inactivation effects by allelopathic compounds (Vitousek and Matson, 1985; White, 1986). Another likely explanation, which was discussed by Jones and Richards (1977) and Hart et al. (1994), includes the influence on the consumption of the available carbon by ammonification and the immobilization of inorganic nitrogen. In our experiment, the cutting of roots has made carbon available for ammonification and immobilization of NH~ and NO~ by beterotrophic bacteria. In contrast, the nitrifier does not require a carbon supply. At the beginning of the experiment, both ammonification and nitrification might be occurring; also, the immobilization of both NO~ and NH~ might be occurring. During this phase, only NH~ discharge appeared, because the initial NH~ pool was larger than that of NO~, and the NO~ produced might immediately be immobilized by consuming available carbon. During the next phase, that is, after the available carbon supply became insufficient for ammonification and immobilization, ammonification declined and nitrification, which does not require a carbon supply, began to develop as the NH~ consuming pathway. Although the high concentration discharge of NH~ was observed from the Saturated lysimeter as well as the Control and Dry lysimeters at the beginning of the experiment, the NH~ discharge rapidly decreased also for the Control lysimeter. The NO~ discharge, however, did not appear at all. This demonstrated that the NO~ pool did not grow, although the NH~ pool should have been consumed. Two possible explanations can be proposed for this phenomenon: 1. the NO~ produced is deoxidized into nitrogen gas by anaerobic denitrifying bacteria in the lower half and saturated part of the lysimeter. 2. The saturated water condition might prevent organic nitrogen from mineralization and restrain nitrification from NI-l~ into NO~, so that the NO; pool does not grow at all. Consequently, denitrification might not occur in the Saturated lysimeter. The second possibility, however, can be rejected for the following reason. Even in the Saturated lysimeter, there was an unsaturated soil layer, of l0 cm depth, below the surface. Because this portion had a soil moisture condition similar to that of the l0 cm bottom layer in the Control lysimeter, there should have been nittificationat least in the l0 cm surface layer in the Saturated lysimeter. Thus, the lack of NO~ discharge indicated that denitrification was occurring under strongly deoxidizing condition in the Saturated lysimeter. Moreover, Fig. 10 shows the accumulated discharge of SO~- from three lysimeters. The SO~-discharge from the Saturated lysimeter was much smaller than that from the Control lysimeter. It is also smaller than even the Dry lysimeter's drainage, which had a lower volume than for the Saturated lysimeter. This suggests that there was even a possibility of deoxidization of SO~- into hydrogen sulphide (H2S) gas in the Saturated lysimeter.

N. Ohte et alJJournal o f Hydrology 195 (1997) 78-98

92

S042,-, 3O [

!

20-

.............. "r ................... r ................... - . . . . . . . . . . .

] ................

10 ..................... 0" 0

30

60

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8/4

9/3

90

10/3

120

150

11/2

12/2

Date (days from July 5th, 1991) ---o--- ConUol

A

Dry

----0--- Saturated

Fig. 10. Variations of accumulatedSO42-load of drained and throughfallwater. As the above descriptions confirm, the influences of soil moisture condition on the characteristics of inorganic nitrogen discharge can be explained by the pool size variation that was caused by the difference in active time of each nitrogen reaction in soil. It was apparent that these phenomena occurred during the infiltration process even in the 30 cm surface layer.

4.2. Inorganic nitrogen in drained and sucked water from the Dry lysimeter As we mentioned above, soil water sucking in the Dry lysimeter removed solutes with water. Therefore, an influence of the dry condition should also have been found in the concentration of inorganic nitrogen of the sucked water, and we have compared the differences between drained and sucked water to discuss the more detailed soil moisture influence. Fig. 11 shows the concentrations of inorganic nitrogen of drained and sucked water. The concentration of NH~ was consistently higher in the drained water, but that of NO~ had the opposite tendency. The difference in the outgoing process between the sucked water and the drained water was that the gravitational drainage occurred only during storm events when the water flux at the bottom of the lysimeter exceeded the sucking ability. This means that the sucked water was continuously collected through relatively small pores when the soil was in an unsaturated condition, b u t the drained water rapidly infiltrated through relatively large pores at the bottom of the lysimeter. In the case of reactions in soil of which production is proportional to residence time, the concentration of soil solution in small pores could be higher than that of larger pores because the velocity of solution movement in small pores should be lower than that in large pores. Therefore, the higher concentration of NO~ in the sucked water than in the drained water was a result of the acceleration of nitrification owing to the residence time in the small pores. On the other hand, in the gravitationally drained water, which has a relatively

IV. Ohte et alJJournal of Hydrology 195 (1997) 78-98

0.2

93

1

0.15" I

0.1" o.05

0 0

30

60

90

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150

150

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30

60

90

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7/5

8/4

9/3

10/3

11/2 12/2

Date (days from July 5th, 1991) A Drained ~ Sucked Fig. 11. Concentrationof inorganic nitrogen in the drained and suckedwater from the Dry lysimeter. short residence time, the concentration of NH~ was significantly high because of the lower levels of nitrification in the large pores. Cumulative load is shown in Fig. 12. The cumulative load had the same order of concentration, i.e. Drained > Sucked for NH~, and Sucked > Drained for NO~. To evaluate inorganic nitrogen production in the soil from the output load of the lysimeter, it is necessary to examine the total load of the sucked and drained water. Even if the total load of the output from the Dry lysimeter is compared with the output (drainage) of the other two lysimeters, it can be found that the order was maintained as Dry > Control > Saturated for the output NH~, and Control > Dry > Saturated for output NO~. This examination does not reverse the interpretation above that there was a restriction of nitrification by water deficit in the Dry lysimeter. 4.3. Interpretation of the experimental results with respect to the watershed-scale phenomena

As we mentioned in the Introduction, the spatial variety of nitrogen dynamics in a catchment is generally influenced by spatial heterogeneity of the soil moisture condition. The result of this experiment shows that the soil moisture condition caused significant differences in nitrogen dynamics within a relatively short time period. Therefore, this

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2 NI'I4÷'N 1.5I0.5" i !

0 0

30

60

90

120

1.50

30 qO~--N "7

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30 8/4

60 9/3

90 10/3

120 1.50 1 1 / 2 121"2

Date (days from July 5th, 1991) A Drained~ Sucked Fig. 12. Accumulatedloadof inorganicnitrogenin the drainedand suckedwaterfromthe Dry iysimeter. result can provide some useful information for qualitative discussion on the spatial variability of nitrogen dynamics on a catchment scale, even though the quantitative extrapolation will not be available. To interpret the results of a lysimeter experiment in terms of watershed-scale phenomena, the following factors have to be considered. Because plant roots are cut, there is no water and nutrient uptake by them in the lysimeters, and as a result plant roots decompose. This is the conditional difference of the experimental samples from the actual soil layers in forest watersheds. Therefore, the decomposition rate of organic matter in the lysimeters is larger than outside of them. Moreover, the inorganic nitrogen pools should become larger. Additionally, the soil water suction in the Dry lysimeter can be compared with the plant uptake in terms of removing inorganic nitrogen in soil solution. Thus, the lysimeter's condition can be compared with that of soil layers in deforested land rather than forest land. The soil water conditions in the Dry, Control, and Saturated lysimeters qualitatively corresponded to three types of situations in actual watersheds: Dry lysimeter--relatively dry and unsaturated condition with solution uptake by plant roots at the upper or middle part of hillslope; Control lysimeter--wet and unsaturated condition at the lower part of hillslope; Saturated lysimeter--saturated condition around a stream or riparian zone. First, on the hillslope in an actual watershed, the ratio of NH~ and NO~ in the soil

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solution which infiltrates through the topsoil may vary with the soil water content at each part of the hillslope. Toimchi (1993) reported for the temperate forest ecosystem in Japan that the NH~ portion of inorganic nitrogen in soil solution was higher on the upper part of the hillslope with relatively dry soil condition, and the NO] portion had an opposite tendency. Comparison of the inorganic nitrogen leachate between the Dry and Control lysimeters agrees with that phenomenon in the actual hillslope. Second, previous studies in stream chemistry frequently reported that the fluctuation of NO] concentration corresponded to change of the water discharge rate during storm events. This fluctuation has been explained by the influence of the interaction between groundwater runoff and surface or subsurface water runoff, i.e. the fluctuation is caused by the addition of rapid runoff with relatively high NO] concentration to groundwater runoff with low NO] concentration (Swank and Caskey, 1982; Duysings et al., 1983; Crabtree and Strudgill, 1985; Cooke and Cooper, 1988; Muraoka and Hirat& 1988). These observations suggest that it has been frequently found that there is a remarkable difference in NO] concentration between groundwater and soil water. One of the major possibifities for making the NO] concentration in the saturated zone lower is root uptake in the unsatmated soil. The result of our experiment, especially for the discharge chemistry of the Saturated lysimeter, suggests that denitrification in the saturated zone can be an additional major pathway to remove NO] from groundwater. If this mechanism was not negligible compared with root uptake, we could find the effective NO] removal in the watershed that generates groundwater discharge as a large portion of the entire discharge. However, whether root uptake or denitritication is more effective for the removal of inorganic nitrogen from groundwater or stream water still remains to be studied. A technique for quantifying both factors independently will have to be established. In accordance with previous watershed-scale experiments which focused on the influence of deforestation on inorganic nitrogen discharge, an increase of NO] discharge was reported in almost all cases (Likens et al.~ 1970; Feller and Kimmins, 1984; Martin et al., 1985). The general explanation has been that the load of inorganic nitrogen to streamwater is increased by the stopping of uptake by roots, decomposition of the dead roots and an increase of litter decomposition with an increase of sunshine. However, to clarify why the large variance in the NO~ discharge load was observed between the experiments, quautitative evaluation of the influence of denitrification is necessary in addition to the evaluation of increase in NO~ load, because, although the increase of NO~ discharge is caused by an increase of inorganic nitrogen supply from the hillslope, some portion of it should be consumed by denitrification in the saturated water zone before the outlet of the watershed. The efficiency of denitrification on a watershed scale must depend on the scale and distribution of the saturated groundwater zone and the ratio of the amount of the groundwater portion to the entire discharge water. Moreover, deforestation can bring about an increase in the runoff rate or a rise of groundwater level through a decrease of evatmuanspiration (Likens et al., 1970; Ruprecht and Schofield, 1989). The result of our lysimeter experiment also shows that the newly saturated soil layer can attain a desirable condition for denitrification within at most a few months. This suggests that a rise of groundwater level or extension of saturated zone caused by deforestation can provide higher denitrification potential to the watershed within a relatively short time period such as a few months.

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To extrapolate the experimental results from the lysimeter to an actual watershed, observation is necessary at first, to validate that the phenomena in the lysimeter are also detected in an actual watershed. For instance, if the inorganic nitrogen flux can be measured in the actual soil layers that are characterized by the three lysimeters, the inorganic nitrogen leachate from the three lysimeters can be used for comparison. The results of our experiment characterize the nitrogen reaction during the vertical infiltration in relatively shallow soil layers. As the nitrogen reaction in soil is active mainly in the shallow layer of the forest soil, this experiment allowed us to describe the source of inorganic nitrogen. However, inorganic nitrogen discharge in the watershed can be found by integration of not only soil water, but also of groundwater from elsewhere with various moisture conditions through the hydrologic processes within a watershed. In addition to the shallow soil water, the hydrologic processes influencing concentrations in groundwater and stream water, such as transport with infiltration beneath the surface layer and mixing in the groundwater zone, must be clarified. Thus, to extrapolate the knowledge from the lysimeter experiment to the actual watershed, information on the spatial distribution of the moisture profile in surface soil and the hydrologic pathway after the vertical infiltration in shallow soil layer will be essential. A detailed hydrochemical study is needed for these procedures. In the Kiryu Experimental Watershed in which the experiment has been carried out, research aiming to clarify the transport and mixing mechanism has recently been done (Tokuchi, 1993; Ohte et al., 1995). Quantitative extrapolation to a small catchment using the observed data is the next subject of this study.

~5. Sunmmry and conclusions To examine the influence of soil moisture condition on inorganic nitrogen discharge from the forest soil layer, a lysimeter experiment was carded out using undisturbed forest soil in a temperate forest. The results showed that the largest NH~ discharge, which was caused by termination of uptake by roots, was detected immediately after setting up all three lysimeters. After that, however, completely different types of inorganic nitrogen discharge were observed at the three lysimeters: (1) NH~ and NO]; (2) only NO]; (3) neither NH] nor NO]. These characteristics corresponded to the dry condition (Dry lysimeter), the wet condition (Control lysimeter), and the saturated condition (Saturated lysimeter), respectively. This difference and its time-domain change could be interpreted in terms of the pool sizes of NH~ and NO] and their variation in three different soil moisULre conditions. The NH] discharge from the Control lysimeter became almost zero after the NO] discharge rate rose. On the other hand, the NH] concentration of the Dry lysimeter drainage maintained a constant rate even after the start of the NO] discharge. This difference proved that the soil moisture deficit worked as the limiting factor of nitrification. Moreover, the discharge components from the Saturated lysimeter showed a more remarkable difference from the others. Lower discharge of both NH4+ and NO] after the high discharge of NH] at the beginning showed that denitrification removes NO] effectively from saturated soil water, although the inorganic nitrogen was produced in the unsaturated upper layer of the lysimeter. These results suggest that the different stages of the process in nitrogen dynamics could

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occur as a result of only the influence of soft moisture variations even in a shallow soil layer within a period of a few months. The nature of inorganic nitrogen discharge from each lysimeter helped to explain the spatial heterogeneity of the nitrogen dynamics caused by the distributed soil moisture condition within a watershed system.

Acknowledgments This research was supported by a grant from the Fund of the Japanese Ministry of Education and Culture for Science Research (#C, 02660159). Tsutomu Kodera is especially appreciated for his effort in the cotlection of samples and data organization. Professor Sumiji Kobashi has always supported the biogeochemical research activities in the Kiryu Experimental watershed, including this lysimeter experiment.

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following m'ea fertiliT, tion of a forest soil under field and laborat~y conditions. Soil Sci. Soc. Am. J., 44: 610--616. Jones, J.M. and Richasds, B.H., 1977. Effect of reforustefion on turnover of I~l-laheled nitrate and ammonium in relation to changes in soil microflora. Soil Biol. Biechem., 9: 383-392. Knowles, R., 1982. Danitrificafion. Microbiol. Rev., 46: 43-70. Kusugi, K., 1994. Three-parameter lognormal distribution model for soil water retention. Water Resour. Res., 30: 891-901. I.ik~ns, G.E., Bonnann, F.H., Johnson, N._M., Fisher, D.W. and Pierce, R.S., 1970. Effects of forest cutting and herbicide treatment on nutrient budgets in the Hubbard Brook watershed-ecosystem. Ecol. Monogr., 40: 2347. Martin, C.W., Noel, D.S. and Federer, C.A., 1985. Clearcuttin 8 and the biogeochemistry of streamwater in New England. J. For., 83: 686-689. Muraoka, IL and Hirata, T., 1988. Streamwater chemistry during rainfall events in forested basin. J. Hydrol., 102: 235-253. Nilsson, S. and Bergkvist, B., 1983. Aluminium chemistry and acidification processes in a shallow podzol on the Swedish west cousL Water Air Soil Pollut., 20:311-329. Ohte, N, and Suzuki, M., 1990. Hydraulic properties of forest soils II: Method of determining the volumetric water content-pressure head relationship by the saturated-unsaturated hydraulic conductivity test using a large-size soil sample (in Japanese with English summary). J. Jpn. For. See., 72: 468--477. Ohte, N., Suzuki, M. and Kubuta, J., 1989. Hydraulic properties of forest soils I: The vertical distribution of saturated-unsaturated hydraulic conductivity (in Japanese with English sunmmry). J. Jpn. For. SOc., 71: 137147. Ohte, N., Tokuchi, N. and Suzuki, M., 1995. Biogeochemical influences on the determination of water chemistry in a temperate forest basin: factors determining the pH value. Water Resour. Res., 31: 2823-2834. Rasmussen, L,, Jorgenson, P. and Kruse, S., 1986. Soil water samplers in ion balance studies on acidic forest soils. Bull. Environ. Contem. Toxicol., 36: 563-570. Reynolds, B., Emmett, B.A. and Woods, C., 1992. Variations in streamwater nitrate connections and nilrogen budgets over 10 years in a headwater catchment in mid-Wales. J. Hydml., 136: 155-175. Ruprecht, J.IC and Schofield, NJ., 1989. Analysis of streamflow generation following deforestation in Sonthem Western Australia. J. Hydrol., 105: 1-17. Sahey, B.R., Frederick, L.R. and Bartholomew, W.V., 1959. The formation of nitrate from ammonium nitrogen in soils. HI. Influence of teraperalnre and initial population of nitrifying organisms on the maximum rate and delay period. Soil Sci. SOc. Am. Prec., 23: 462--465. Spent, J.I., 1987. The Ecology of the Nitrogen Cycle. Cambridge University Press, New York. Swank, W.T. and Cuskey, W.H., 1982. Nitrate depletion in a secoud-order mountain stream. J. Environ. Qual., 11: 581-584. Tietima, A. and Wessel, W.W., 1992. Gross nitrogen transformations in the organic layer of acid forest ecosystem subjected to increased atmospheric nitrogen input. Soil Biol. Biochem., 24: 943-950. Tokuchi, N., 1993. Study on spatial distribution of dissolved nutrients in forest ecosystem. Ph.D. Thesis, Kyoto University. Tokuchi, N., Takeda, H. and Iwatsubo, G., 1993. Vertical changes in soil solution chemistry in soil profiles under coniferons forest. Gcoderma, 59: 1-17. Van Miegroet, H., Johnson, D.W. and Cole, D.W., 1990. Soil nitrification as affected by N fertility and changes in forest lloof C/N ratio in four forest soils. Can. J. For. Res., 20: 1012-1019. Vitousek, P.M. and Matson" P.A., 1985. Causes of delayed nitrate production in two Indiana forests. For. Sci., 31: 122-131. Vitousek, P.M. and Melillo, J.M., 1979. Nitrate losses from distributed forests: patterns and mechanisms. For. Sci., 25: 605-619. Vitonsek, P.M., Gosz, J.R., Melillo, J.M. and Reiners, W.A., 1982. A comparative analysis of potential nitrification and nitrate mobility in forest ecosystems. Ecol. Monogr., 52: 155-177. White, C.S., 1986. Volatile and water-soluble inhibitors of nitrogen mineralization and nitrification in a ponderosa pine ecosystem. Biol. Fertil. Soils, 2: 97-104. •