Journal of Tropical Forest Science 11 (3):599-609 (1999)
N-MINERALISATION AND INORGANIC-N UPTAKE IN A BROAD-LEAFED FOREST OF CENTRAL HIMALAYA
Samina Usman, Y. S. Rawat & S. P. Singh
Department of Botany, Kumaon University Nainital - 263 002 India
Received September 1996_____________
USMAN, S., RAWAT, Y. S. & SINGH, S. P. 1999. N-mineralisation and inorganic-N uptake in a broad-leafed forest of Central Himalaya. Net N-mineralisation and nitrification rates were measured by in situ incubation of soil in a broad-leafed forest in central Himalaya. The rate of nitrification among different seasons was in the order: rainy > winter > summer, while that of ammonification was in the order: rainy
> summer > winter. Rainy season conditions were more favourable for N mineral- isation. The size of the available nitrogen pool ranged from 10.0 u.g g1 (winter) to 35.8 ug g-1 (March). The trend for N-mineralisation rate was opposite to that of the size of the available N. The value of inorganic-N uptake was maximum in the month of July and ranged between 0 and 25 ug g-1 across different months. In the present studied forest nitrate-N was the dominant form of inorganic-N taken up by plants.
The rate of N-mineralisation was positively correlated with the root decomposition.
Key words: N-mineralisation - nitrate-N - ammonium-N - nitrification - ammonification
USMAN, S., RAWAT, Y. S. & SINGH, S. P. 1999. Pemineralan-N dan pengambilan N-tak organik di hutan daun lebar di Himalaya Tengah. Kadar bersih pemineralan-N dan penitritan disukat dengan pengeraman tanah in situ di hutan daun lebar di Himalaya Tengah. Kadar penitritan antara musim yang berbeza adalah dalam turutan:
hujan>musimsejuk>musim panas, manakala pengamoniaan adalah dalam turutan:
hujan > musim panas > musim sejuk. Keadaan musim hujan lebih sesuai bagi pemineralan-N. Ukuran kumpulan nitrogen tersedia berjulat daripada 10.0 ug g-1
(musim sejuk) hingga 35.8 ug g-1 (Mac). Kadar aliran pemineralan-N didapati bertentangan dengan ukuran N tersedia. Nilai pengambilan N-tak organik adalah maksimum pada bulan Julai dan berjulat antara 0 dan 25 ug g-1 merentasi bulan-bulan yang berbeza. Di dalam hutan yang dikaji baru-baru ini N-nitrat adalah unggul dalam bentuk N-tak organik yang diambil oleh tumbuh-tumbuhan. Kadar pemineralan-N mempunyai korelasi positif dengan pereputan akar.
Introduction
Release of nutrients by biological mineralisation is crucial in maintaining the cycling of essential nutrients immobilised in dead plant material and is vital for continued productivity of terrestrial ecosystems. Increased understanding of fac- tors influencing mineralisation may lead to management practices which enhance microbial nutrient mineralisation without addition of fertilisers (Elliott & Coleman
1988). Nitrogen mineralisation is the microbial conversion of soil organic nitrogen to ammonium-N which in turn may be oxidised to nitrate-N (Vitousek et al. 1982). Ammonium-N and nitrate-N represent the available inorganic forms of nitrogen in the soil. The total amount of N liberated from the organic matter is gross mineralisation and the quantity remaining after subtraction of in situ microbial immobilisation (uptake by microbes) is net mineralisation. The highest rate of N-mineralisation measured using in situ soil incubation in temperate forest typically occurs in spring or early summer with a secondary peak late in the growing season (Ellenberg 1977, Melillo 1977, Nadelhoffer et al. 1983). The present study highlights the N-mineralisation and available NO3- and NH4+ pool in the soil of a broad-leafed forest ecosystem.
The central Himalayan forests face the problem of landslides, landslips, erosion and year to year occurrence of fire. This leads to massive removal of soil nutrients from the steep hill slopes, making the area poor in nutrients. The loss of nutrients is responsible for the poor regeneration of several forest species. Therefore, the present study was conducted to find out the changes in inorganic-N, variation in N- mineralisation and inorganic-N uptake in the present study forest. This will help in the regeneration and management of the forests in the region.
Materials and methods Study site
The study site (29° 24' N, 79° 28' E; 1950 m above sea-level) is approximately 2 km from Nainital, north India. The mean monthly temperature varies from 18.0 °C in January to 30.5 °C in June. Climatically, the year is divisible into three seasons, viz. summer (April-June), rainy (June-September) and winter (November-February).
October and March constitute transition months between winter and summer seasons respectively. The annual rainfall is 1963 mm, of which 90% occurs in the rainy season. Quercus leucotrichophora (banj oak) forest covers an extensive area at lower elevation (1500-2100m). The canopy is closed (more than 80%) when undisturbed; shrub growth is conspicuous and the less developed herb layer does not include grasses.
Geology
The central axis of the Himalaya consists of crystalline rock gneisses and metamorphosed sediments, ranging in age from the Precambrian to as late as Miocene. Geologically, the young age of the Himalaya, the pressure of residual stress and highly compressed and tectonised conditions of the rocks make the Himalayan mountains susceptible to weathering, erosion and damage by seismic activity.
The rocks present in this area are commonly black carbonaceous and pyritous locally oxidised to ash-grey colour with characteristic oxidation rings on parting planes. Light green and grey banded slates intercalated within layer of silt stone are other typical elements of lithology (Valdiya 1980).
Journal of Tropical Forest Science 11(3) :599-609 (1999) 601
The soil is brown earth type with sandy clay texture and pH of 6.2-6.6. Bulk density ranges from 0.73 to l.04g cm-3. Water holding capacity and moisture range from 46 to 61% and 32 to 50% respectively. Total N, organic-C, available P and Rare 0.21-0.30%, 2.5-3.6%, 0.11-0.20% and 0.5 to 0.18% respectively.
Sample collection
Ten samples were randomly collected from different aspects (facing north, south, east and west) up to the upper 10 cm soil layer at monthly intervals for an annual cycle (March 1990 to March 1991). Each sample was collected at a distance of 1 m from the base of the tree. The soil was mixed thoroughly and recognisable roots were hand picked and sieved in moist condition through 2-mm mesh screen.
Each soil sample was divided into three parts, of which one part was used for NO3- + NH4+ which are mineral-N (at 0 time) and the second part was used for assessing the N-mineralisation rate. The remaining part of the soil was air dried and used for analysis of particle size distribution, pH, water holding capacity, organic carbon, total nitrogen, available phosphorus and potassium.
Soil analysis
Soil analysis was carried out on dry soil condition. Particle size analysis was done using sieves and a hydrometer (Indian Standards 1965). Soil pH was measured with a glass electrode (1:5 soil: water ratio). Gravimetric water content and water holding capacity were determined following Piper (1944). Organic carbon was determined by Walkey and Black's titration method (Jackson 1958) and total P was measured following Jackson (1958). The total N was measured by the micro Kjeldahl's method (Jackson 1958). The results were analysed by correlation analysis (linear & power regression) and analysis of variance.
N-mineralisation
Nitrogen mineralisation was measured by the buried bag technique (Eno 1960).
A portion of the fresh soil sample (150 g) was incubated at 10 cm soil depth using a sealed large polythene bag. Coarse roots and large fragments of organic debris were removed from the soil through sieving in order to avoid any marked immobilisation during incubation (Ross et al. 1985, Schimel & Parten 1986).
Nitrate-N and ammonium-N were determined at time-zero and after 30 days of field incubation. Nitrate-N was measured by ion-chromatography autoanalyser (Dionex Model No LDQP-914086) and NH/-N by the distillation method (Jackson 1958).
The increase in the concentration of NH4+-N plus NO3--N during the course of field incubation is defined as net mineralisation (Melillo et al. 1982, Pastor et al. 1984).
The increase in NO3--N during incubation is referred to as net nitrification.
These rates of N-mineralisation and inorganic-N uptake were calculated using the equation developed by Nadelhoffer et al. (1984), i.e.
ANH/-N ANO3-N A .mm
= NH4+-Na(t+l)-NH4+-Ni(t)
= NO3--Na(t+l)-NO3+-Ni(t)
= ANH4+-N + ANO3+-N where
NH4+-Ni(t)
ANH/-N
mean NH4+-N content of initial, non-incubated soil samples at the start of interval t
NH4+-Na(t+l) = mean NH4+-N content accumulated in incubated soil samples at the end of interval t
net ammonification in incubations
mean NO3- -N content of initial, non-incubated soil samples at the start of interval t
mean NO3- -N content accumulated in incubated soil samples at the end of interval t
nitrification in incubations
net N mineralisation in incubations NH4+-Ni(t)
NO3--Na(t+l) = ANO4+-N
All units are ug g-1 in 0-10 cm soil.
N-uptake
If it is assumed that net-N mineralisation and nitrification rates in incubated soils are good estimates of actual rates in the surrounding field soil, estimates of the amount of N taken up by vegetation from the 0-10 cm soil during each incubation interval can be calculated using the following equations:
NH4+-Ni(t+l) NO3--Ni(t+l) NH/-N4 upt NO3--Nupt
Nupt
mean NH4~-N content of non-incubated soil at the end of interval t
mean NO3--N content of non incubated soil at the end of interval t
NH4--N uptake between t and (t+1) NO3--N uptake between t and (t+1) total N uptake between t and (t+1)
Root decomposition
Roots were excavated and collected from Quercus leucotrichophora (having cbh 100-160 cm and fine roots up to 1mm) forest in the month of December 1990.
Roots were separated from other organic matter by hand (all sample handling was done with gloves). The litter bag technique was followed for quantifying the rate of decomposition following Upadhyay and Singh (1989). The nylon litter bags of 10 X 10 cm size containing 5 g dried root samples were placed on the forest floor to a depth of 10 cm at random locations for 18-month period. Five bags were collected from each forest site per month during the 18 months. The litter did not disappear entirely during the course of the study period.
Journal of Tropical Forest Science 11(3):599-609 (1999) 603
Results
In the present study site Quercus leucotrichophora was the dominant tree species with IVI 268 (Usman 1993) accounting for 94% of the forest which has a tree density of 540 trees ha-1. Seedlings and sapling were absent. The total density of shrubs was 10.6 shrubs 100m-2. The maximum density was exhibited by Daplne cannabina (4.8 shrubs l00m-2) followed by Myrsine africana (4.8 shrub 100m-2). The total herb density was 162 individuals m-2 in October, 49.6 individual m-2 in December and 51 individual m-2 in the month of May.
Available-N pool
Available nitrogen-N, ammonium-N and inorganic-N (nitrate-N + ammonium - N) values were maximum in March and minimum in November. Figure 1 shows that the soil ammonium-N pool was typically large (8-20 ug g-1) and more variable than the nitrate-N pool (2-15 ug g-1). The available ammonium-N pool in soil was greater in all months compared to nitrate-N. The seasonal values of nitrate-N ranged from 2 to 15 ug g-1. Thus, the ammonium-N showed greater seasonal fluctuation than the nitrate-N. The total available inorganic-N pool was 27-35 ug g-1 during summer, 12-25 ugg-1 during the rainy season to 10-31 ug g-1 during winter season. The contributions of ammonium-N to the total mineral nitrogen pool were 59-67% in summer, 62-73% in rainy season and 57-77% in winter.
The contribution of ammonium-N to the total mineral nitrogen pool across the months varied from 57 to 83%. The proportion of nitrate-N and mineral-N in soil was distributed in the order: summer > rainy > winter, while ammonium N showed the following pattern summer > winter > rainy (Figure 1).
40-i
35- 30- 25- 20- 15- 10- 5 - 0
A M J J A S O N D J F M
Months
- Nitrate-N • Ammonium-N Inorganic-N
Figure 1. Variation in mineral nitrogen in the form of inorganic-N, ammonium-N and nitrate-N in broad-leafed forest of central Himalaya
Net nitrification, ammonification and N-mineralisation
Both ammonification and nitrification peaked in July and the lowest values were recorded in May. The seasonal nitrification values ranged 1-12 ug g-1 in summer, 2-24 ug g-1 in the rainy season and 2-9 ug g-1 in winter, while those of ammonification were maximum during July and August. The rate of nitrification among different seasons was in the order: rainy > winter > summer, while that of ammonification was in the order: rainy > summer > winter (Figure 2). The percentage contribution of nitrification to total mineralisation across the months varied from 27 to 82%, while that of ammonification from 6 to 73%. Similar changes in nitrification were observed when monthly data of nitrification were regressed against mineralisation, ammonification against mineralisation and mineralisation against nitrification. The linear regression analysis shows that nitrification was more strongly correlated with mineralisation (p<0.01) than ammonification (p<0.01).
35- 30- 25- b> 20-
™ is- le- s'
A S O N Months
Nitrification-N Ammonification-N Mineralisation
Figure 2. Variation in nitrogen mineralisation, ammonification and nitrification in broad-leafed forest of central Himalaya
Inorganic-N uptake
Inorganic-N uptake calculated for each incubated period and for the entire year showed that the nitrate-N was the dominant form of inorganic-N taken up by the plants from the soil. The value of nitrate-N uptake was maximum in the month of July and ranged between 0 and 25 ug g-1 across different months.
However, the values of ammonium-N uptake peaked in August and September and ranged between 1 and 11 ug g-1 across different months (Figure 3). Power
During the course of the study the values of nitrate-N uptake were always positive. However, the values of ammonium-N uptake during December, January and February were negative; this has no biological meaning and resulted from sampling errors associated with measurement both of ammonium-N accumula- tions in incubated soil and ammonium-N pool in incubated soil (Nadelhoffer et al.
1984).
After the 18-month interval in the banj oak forest, the roots decomposed at a somewhat faster rate (47%) compared to those of chirpine (45%) (Usmanl993).
The bimonthly decomposition rates of fine roots during the study period are shown in Figure 5.
Figure 5. Percentage weight of roots decomposed
Discussion
The oak (Quercus leucotrichophora) forest, which covers an extensive area at lower elevations (1500-2100 m), is classified as low to mid-montane hemi-sclerophyllous broad-leafed forest. The canopy is closed (more than 80%) when undisturbed, shrub growth is conspicuous and the less developed herb layer does not include grasses (Champion & Seth 1968).
The values of nitrate-N, ammonium-N and inorganic-N pools were maximum in the early summer season and minimum during the late rainy season (Figure 1).
Increases in nitrate-N and ammonium-N during the dry summer period and winter reflect the low demand of available nutrient by higher plants and upward movement of soil solution (Singh et al. 1989, Singh 1990). Although Figure 4 does not clearly show, except for the month of March, the moisture regime was changed in the incubated cores compared to the surrounding soil (as no throughfall occurred in the covered cores, their water contents tended to be slightly lower than in the uncovered cores). Suppression of uptake within all incubated cores obviously not only affected their inorganic nitrogen content but also their moisture regimes;
thus the water content of the uncovered cores was regularly higher than that of the
Journal of Tropical Forest Science 11(3):599-609 (1999) 607
surrounding soil which in turn had its impact on the nitrogen mineralisation balance (Becquer et al. 1990).
Peak nitrification and mineralisation rates were substantially lower than those or equivalent to the lowest values reported for tropical ecosystems. Vitousek and Matson (1988) reported that the rate of N-mineralisation ranged from 36 to 48 ug g-1 m1 for the Amazonian forest of Brazil and semi-deciduous forest of Panama and from 78 to 204 ug g-1 m-1 for old growth tropical forests of Costa Rica.
Old field and repeatedly disturbed experiment plots of Costa Rica also showed very high mineralisation rates of 30-99 ug g-1 m-1. Singh et al. (1991) reported that rate of nitrification varied from 0.8 to 15 ug g-1 m-1. The rate of N-mineralisation ranged from 0.33 toll ug g-1 m-1 for dry tropical forest (Raghubanshi 1992). The values of N-mineralisation in the present study fall within the range reported by Robertson and Vitousek (1981) for oak (10-39 ug g-1 m-1) and pine (10.8-16.6 ug g-1 m-1) forests of Indian dunes. Nitrification rates were always statistically signi- ficant and positive because initial N-pool in soil was typically small and because at least some ammonium was always oxidised to nitrate-N in incubated soil. Nitrifica- tion and mineralisation rates were maximum in the rainy season and minimum in the summer season. In semi-arid ecosystems, nutrient dynamics are closely linked to seasonal variations in temperature and moisture (Burke 1989). N- mineralisation is a microbial process with temperature and moisture conditions in soil as controlling factors. A power regression analysis based on data of different months indicated that the soil moisture (%) was positively correlated with the rate of ammonification, nitrification and N-mineralisation according to the following regression equations:
y1 = 0.0104x2 - 0.1868x + 1.8741 (r = 0.94; p<0.01) y2 = 0.0101lx2 + 0.0875x + 3.1349 (r = 0.63; p<0.01) y3 = 0.0206x2 - 0.0994x + 5.0091 (r = 0.73; p<0.01)
Where y1 y2, y3 are the rates of ammonification, nitrification and N-mineralisation (ug g-1 m-1) respectively and x indicates soil moisture (%).
The combined effect of soil temperature, soil moisture, rainfall and atmospheric temperature was more pronounced and accelerated the rate of nitrification, ammonification, and N-mineralisation (p<0.01) explaining for the 50,67 and 62%
variabilities.
Adams and Attiwill (1986), and Marion and Miller (1982) reported that the potentially mineralised nitrogen is highly correlated to both total soil nitrogen (positively) and carbon/nitrogen ratio (negatively). In the present studied forest the amount of total nitrogen in soil was higher and the C:N ratio lower than the corresponding values for the needle-leafed forest (Pinus roxburghii) (Usman 1993).
The rate of N-mineralisation was closely related to root decomposition. Newman (1985) reported that soluble carbon compound lost from roots can lead to larger microbial population in the rhizosphere compared with the bulk of the soil. This prompted the suggestion that carbon (C) lost from a plant root could enhance
mineralisation, viz. microbial activity involving inorganic nitrogen is closer to the root; that is why during the rainy season the rate of decomposition was maximum and rate of N-mineralisation was also maximum (Usman 1993). Regression analysis showed that the rates of N-mineralisation, ammonification and nitrification were strongly correlated to root decomposition according to the following regression equations (Usman 1993):
y1 = 1.356 + 63.585x (r = 0.73; p< 0.01) y2 = 0.166+ 26.148x (r = 0.85; p< 0.01) ys = 3.360 + 96.832x (r = 0.78; p< 0.01)
where y1 y2, y3 are the rates of nitrification, ammonification and mineralisation (ug g-1 m-1) respectively and x denotes the root decomposition.
Inorganic-N uptake calculated for each incubated period and for the entire year showed that the nitrate-N was the dominant form of mineral nitrogen taken up by plants from the soil. The nitrate-N uptake was maximum in July and ranged 0-25ug g-1 across different months. The ammonium-N uptake value peaked in August-September and ranged 1.0-11.3 ug g-1 across different months.
According to Nadelhoffer et al. (1983, 1984), a forest stand can sustain a high level of productivity when nitrate is supplied. However, productivity could be lowered if the species occupying the site is poorly adapted to either taking up or assimilating the particular form of available-N in the soil.
References
ADAMS, M. A. & ATTIWILL, P. M. 1986. Nutrient cycling and nitrogen mineralization in eucalypt forests of south-eastern Australia. Plant and Soil 92:341-362.
BECQUER, T., MERLET, D., BouDOT,J.-P., ROUILLER,J. & GRAS, F. 1990. Nitrification and nitrate uptake:
leaching balance in a declined forest ecosystem in eastern France. Plant and Soil 125:95-107.
BURKE, I. C. 1989. Control of nitrogen mineralization in a sage brush steppe landscape. Ecology 70:1115-1126.
CHAMPION, H. G. & SETH, S. K. 1968. A Revised Survey of the Forest Types of India. Government of India Publication, New Delhi. 404 pp.
ELLENBERG, H. 1977. Stickstoff als Standortsfaktor, inbesondes fur mitteleuropaische Pflanzenges- ellschaften. Oecologia Plantarum 12:1-22.
ELLIOT, E. T. & COLEMAN, D. C. 1988. Let the soil work for us. Ecological Bulletins (Copenhagen) 39:23-32.
ENO, C. F. 1960. Nitrate production in the field by incubating soil in polyethylene bags. Soil Science Society of American Proceedings 24: 277-279.
INDIAN STANDARDS, 272 . Part IV. 1965. Grain Size Analysis. Indian Standards Institute, New Delhi.
JACKSON, M. L. 1958. Soil Chemical Analysis. Prentice Hall Inc., U.S.A. 498 pp.
MARION, G. M. & MILLER, P. C. 1982. Nitrogen mineralization in a Tussock Tundra soil. Arctic and Alpine Research 14(4):287-293.
MELILLO.J. M. 1977. Mineralization of Nitrogen in Northern Ecosystem. Ph.D. thesis, Yale University, New Haven, Connecticut. 136 pp.
MELILLO, J. M., ABER.J. D. & MURATORE, J. F. 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Canadian Journal of Botany 63:621-626.
Journal of Tropical Forest Science 11(3) :599-609 (1999) 609
NADELHOFFER, K. J., ABER,J. D. & MELILO.J. M. 1983. Leaf-litter production and soil organic matter dynamics along a nitrogen-availability gradient in southern Wisconsin (U.S.A.). Canadian Journal of Forest Research 13:12-21.
NADELHOFFER, K.J., ABER, J. D. & MELILO.J. M. 1984. Seasonal pattern of ammonium and nitrate in nine temperate forest ecosystems. Plant and Soil 80:321-335.
NEWMAN, E. I. 1985. The rhizosphere: carbon sources and microbial populations. Pp. 107-121 in Fitter, A. H. (Ed.) Ecological Interactions in Soil. Blackwell Scientific Publication, Oxford.
PASTOR, J., ABER,J. D., McCLAUGHERTY, C. A. & MELILO.J. M. 1984. Aboveground production and N and P cycling along a nitrogen mineralization gradient on Blackhawk Island, Wisconsin. Ecology 65:256-268.
PIPER, G. S. 1944. Soil and Plant Analysis. Adelaide, Australia. 386 pp.
RAGHUBANSHI, A. S. 1992. Effect of topography on selected soil properties and nitrogen mineralization in a dry tropical forest. Soil Biology and Biochemistry 24 (2): 145-150.
ROBERTSON, G. P. & VITOUSEK, P. M. 1981. Nitrification potentials in primary and secondary succession.
Ecology 62:376-386.
Ross, J. D., SPEIR, T. W., TALE, K. R. & ORCHARD, V.A. 1985. Effect of sieving on estimations of microbial biomass, and carbon and nitrogen mineralization in soil under pasture. Australian Journal of Soil Research 23:319-324.
SCHIMEL, D. S. & PARTEN, W.J. 1986. Microclimatic control of nitrogen mineralization and nitrification in shortgrass steppe soils. Plant and Soil 93:347-356.
SINGH, J. S., RAGHUBANSHI, A. S., SINGH, R. S. & SRIVASTAVA, S. C. 1989. Microbial biomass act and as a source of plant nutrients in dry tropical forest and savannah. Nature (London) 338:499-500.
SINGH, R. S. 1990. Effect of Grazing and Fire on Nutrient Cycling in Savannah Ecosystem. Ph.D. thesis, Kumaun University, Nainital.
SINGH, R. S., RAGHUBANSHI, A. S. & SINGH, J. S. 1991. Nitrogen mineralization in dry tropical savannah:
effect of burning and grazing. Soil Biology and Biochemistry 23:269-273.
UPADHYAY, V. P. & SINGH, J. S. 1989. Patterns of nutrient immobilization and release in decomposing forest litter in Central Himalaya, India. Journal of Ecology 77:127-146.
USMAN, S. 1993. Fine Root Dynamics and Nitrogen Mineralization in Oak and Conifer Forests of Central Himalaya. Ph.D. thesis, Kumaun University, Nainital. 203 pp.
VALDIYA, K. S. 1980. Geology of Kumaun Lesser Himalaya. Wadia Institute of Himalayan Geology, Dehradun. 291 pp.
VITOUSEK, P. M. & MATSON, P. A. 1988. Nitrogen transformation in a range of tropical forest floor soils.
Soil Biology and Biochemistry 20:361-367.
VITOUSEK, P. M., Gosz, J. R., GRIER, C. G., MELILO.J. M. & REINERS, J. A. 1982. A comparative analysis of potential nitrification and nitrogen mobility on forest ecosystems. Ecological Monographs 52:155-177.
