6) Christopher S. Potter, James T. Randerson, Christopher B. Field, Pamela A. Matson, ... 14) Nyle C. Brady (1998): Nitrogen and sulfur economy of soils, in The ...
Establishing a Global Nitrogen/Carbon Cycle Model: Nitrogen Storage in Terrestrial Vegetation under Present Climate Bin-le LIN*, Ryosuke SHIBASAKI*, Naohiro GOTO** Akiyoshi SAKODA* and Motoyuki SUZUKI* * Institute of Industrial Science, University of Tokyo ** Department of Ecological Engineering, Toyohashi University of Technology
Abstract Evaluating the potential impacts of human land-use changes on terrestrial ecosystems is very essential for the sustainable use of earth. As the first step in establishing a global nitrogen/carbon cycle model to address this aim, distribution of nitrogen (N) storage in terrestrial vegetation was estimated. Since plant tissues have their own certain ratios of C/N, 20 in leaves and 150 in trunks were used in this study, respectively. Total N storage in terrestrial vegetation under present climate was estimated as 16 Pg, which was in good agreement with those previously reported in the literature. Furthermore, we found that the global distribution map of vegetation N matches spatially very well with the World Vegetation Map, based from the information of satellite remote sensing data of 1985 to 1987. The N storage in desert area was calculated as the least (0 kg/m2), while in tropical forest and forest areas were the most (0.2-0.7 kg/m2). Though further improvements are still needed, a global N cycle model integrated with C model on the vegetation compartment in this study is credible, and the estimates of N storage in terrestrial vegetation under present climate is reasonable.
1. Introduction Carbon (C) and nitrogen (N) are two basic elements in enabling life. Description of transformation and movement of C and N in the global environment are defined as global C and N biogeochemical cycles. Disturbances in these global biogeochemical cycles, in particular the cycle of N, initiated by a variety of human activities, lead to global, regional and local environmental problems, such as photochemical smog, stratospheric ozone depletion, soil acidification, and nitrate pollution of ground and surface water (1). Recent decades have seen many model studies of the global C cycle, whereas only a few global N cycle models exist (2). Research has hitherto mainly focused more on the separate global C and N cycles than on interactions between both cycles. Beside this unfortunate point, there are still few studies on evaluating the potential impacts of human land-use changes on natural ecosystem from the standpoints of these two material balance cycles, especially from N cycle (2-7). In order to satisfy the above two demands, we therefore try to develop a global biogeochemical N cycle model, integrated with our existent C cycle (8-9) and mechanistically based on the N transformation processes and its fluxes between terrestrial biosphere and atmosphere. 1.1 Global nitrogen cycle Nitrogen (N) is both an essential nutrient for plants and animals and a major pollutant in terrestrial ecosystems. Before N can be used in life, the inert dinitrogen gas (N2) has to be ‘broken’, resulting in various reactive N compounds (e.g. NH4+, NO3-). Although the atmosphere consists of about 78% N2, many terrestrial ecosystems, especially those in middle and high latitudes, are N-limited (10-13). This is because those ecosystems are severely deficient in reactive N compounds. Therefore N2 molecule in atmosphere must firstly be converted to reactive forms by the combination of N with C, hydrogen (H), and/or oxygen (O), called as N fixation process, so that it can be used by terrestrial biosphere until it is converted back to N2 by denitrification. Since N has five valence electrons and can take on oxidation states between +5 and –3, many varied N
compounds that play important roles in a wide range of contemporary environmental issues (1), occur during its biogeochemical cycle. It is important to remember that even though N biogeochemical cycle involves some abiotic processes such as deposition of N compounds, ammonia volatilization, and nitrate leaching, most of the processes are biological ones that play key roles in the cycle. These important processes are indicated schematically in Fig. 1. In the absence of human disturbances, overall abiotic and biological processes interact each other that
6
1. Nitrogen fixation is any process in which N2 reacts to form any N compound. Biological N fixation is the enzyme-catalyzed reduction of N2 to NH3, NH4+, Deposition N2 (or N2O) Deposition or organic N compound. 2. Ammonia assimilation is the process by which NH3 or NH4+ is taken up by 1 organism to become part of its biomass in the form of organic compounds. 3 + + - 3. Nitrification is the oxidation of NH3 or NH4 to NO2 or NO3 by an organism. NH3 /NH4 NO2 /NO3 4. Assimilatory nitrate reduction is the reduction of NO -, followed by uptake of 3 4 the N by the organism as biomass. 5. Ammoniafication is the breaking down of organic N compounds into NH3 or Organic N NH4+. Leaching Volatilization 6. Denitrification is the reduction of NO3- to any gaseous N species, generally N2 or N2O.
2
5
Fig. 1 Processes involved in nitrogen biogeochemical cycle. Number 1-6 are biological processes. result in an efficient and healthful natural ecosystem. N fixation occurs spatially and temporally at the demand of natural ecosystem, providing about 90-130 Tg/y N on the continents, which is in balance with the natural denitrification (2). However, human land-use changes, such as integrated agriculture, over-fertilization, deforestation, biomass burning etc., have significantly disturbed N biogeochemical cycling that resulted in various serious environmental problems at regional and global scales. Therefore evaluating quantitatively the mutual relationship between human land-use changes and natural ecosystem from the standpoint of global N balance concept become one of the most important issues of international concern. 1.2 An Overview of existent global C model The model structure of global C model in terrestrial ecosystems was based on the simplified material balance concept as shown in Fig. 2. Carbon was firstly absorbed from atmosphere into LEAF by photosynthesis, then allocated into TRUNK, litter-fell to form DEAD BIOMASS then HUMUS, and at last emitted to atmosphere by respiration and decomposition. The details of C exchange kinetics among these four sub compartments and the atmosphere compartment were described in related papers (8-9).
ATMOSPHERE
CO2
LEAF
Photosynthesis CO2
TRUNK Respiration
CO2
Litter-fall
DEAD BIOMASS Decomposition Huminification
CO2 Decomposition
HUMUS
Fig. 2 Model structure of global carbon model
Only three types of vegetation were firstly considered: tropical forest, temperate forest, and boreal forest. By using the data sets (Table 1) of environmental factors organized in a geographical information system (GIS) with spatial resolution of 0.5° in latitude-longitude, together with the three types vegetation parameters, the global C storage in each vegetation for each mesh were calculated until reaching the steady state. Global land area was finally classified into five types of vegetation based on the calculation results of C storage in vegetation. The vegetation with the maximum C storage was assumed to be the vegetation that dominates the mesh. And if the dominant vegetation C storage was less than 0.5 kgC/m2 then the mesh was classified into no vegetation, and if less than 5.0 kgC/m2 and above 0.5 kgC/m2 then the mesh was classified into crops/glass-land. Table 1 Data sets used in the model calculation Name DLE RAD TEX TMP SW WHCAGRI VEGE SOIL
Attributes monthly day length of sunlight monthly illumination intensity soil texture distribution monthly temperature monthly soil water content agricultural land distribution 3 types vegetation parameters 12 types soil texture parameters
Units hour/day klux – °C cm – – –
Resolution 0.5º x 0.5º 0.5º x 0.5º 0.5º x 0.5º 0.5º x 0.5º 0.5º x 0.5º 0.5º x 0.5º – –
Source EPA EPA EPA EPA EPA EPA – –
Although the C storage in terrestrial vegetation can be successfully estimated under present climate by this cycle model, two lack considerations in this model are evidently seen: influence of N nutrient on photosynthesis and C fixation by soil microbiology. Therefore this two weak points will be improved, while we consider the global N model structure integrated with this C model latter.
2. Model description 2.1 Model structure The model structure and its concepts are depicted on the Fig. 3. The compartments and processes involved in each compartment are summarized in Table 2. We focus mainly on modeling the N transformation and movement during two major reservoirs: atmosphere and terrestrial biosphere.
Table 2 Definitions of reservoirs, compartments, components, and processes Reservoirs Atmosphere Terrestrial Biosphere
Compartments one well-mixed
Components Major processes nitrous oxide (NO&N2O) biomass burning Lighting
5 ecosystems: tropical forest (Tr) temperate forest (Te) boreal forest (Bo) crop/grassland (CG) desert (De)
8 state variables: C in vegetation (VC) N in vegetation (VN) C in detritus (DC) N in detritus (DN) C in humus (HC) N in humus (HN) ammonium (Amm) nitrate (Nit)
11 fluxes photosynthesis (GPP) litter-fall (FL) huminification (DH) nitrogen uptake (NUPTAKE) mineralization (NMIN) nitrification (NITRIF) denitrification (DENITR) N fixation (NFIX) NH3 volatilization (VOLA) N deposition (DEPO) NO3- leaching (LEACH)
One well-mixed reservoir
stratosphere
lighting
NH3/NH4 NOx/NO3-
CO2
Biomass burning
atmosphere
N2
N2O/NO
Biomass burning
N input N output
photosynthesis
Leaf
CO2
Vegetation
Biological fixation
deposition
respiration
N transformation C transformation
Trunk Root
CO2
Plant uptake
Litter-fall decomposition CO2
Detritus huminification
decomposition CO2
nitrification (N2O, NO) denitrification (N2, N2O, NO) volatilization (NH3)
Ammonium
nitrification
Nitrate
Mineralization Runoff/leaching
Humus
Using mesh data or user define rates Disturbance of land-use changes * over-fertilization * forest harvest * deforestation * cultivation * biomass burning * soil erosion etc
surface and ground water
Terrestrial biosphere is treated as 60156 meshes, 0.5º in latitude-longitude
Fig. 3 Model concept of global nitrogen/carbon cycle.
2.1.1 Atmosphere reservoir We assume earth’s atmosphere as a single mixed reservoir, model the concentration change of nitrogen oxides such as nitrous oxide (N2O) and nitric oxide (NO). Because this two compounds are very reactive gases and have the potential to do serious environmental damage in the way of acid rain, photochemical smog, greenhouse effect, and most significantly in the destruction of ozone (14).
2.1.2 Terrestrial biosphere About terrestrial biosphere reservoir, we treat it in a total of 60156 meshes with 0.5° in its latitude and longitude in order to use the data sets of environmental factors. We also use the results of five types classification in C model, and fix terrestrial biosphere into five ecosystems, i.e., tropical forest, temperate forest, boreal forest, crops/grassland, and no vegetation (desert, ice). Since C and N are two basic elements in plant tissues and soil organic compounds, we use their own certain C-to-N ratios in this study as the most common reported: 20 in leaves, 150 in trunks/roots, and 15 in soil organic compounds, respectively. N and C cycling are integrated here in both vegetation composed of leaf and trunk and roots, and soil organic compounds composed of dead biomass and soil organic matter (SOM) called as detritus and humus. Besides these compartments, soil inorganic N compartment composed of ammonium and nitrate are also considered. As mentioned above, the terrestrial N and C cycle largely consist of biological driven processes. These processes are geographically heterogeneous and depend on a variety of environmental factors (Table 1) such as solar irradiation, precipitation, temperature, soil texture, soil moisture, N nutrient and atmospheric CO2 concentrations, ect. These factors control N and C fluxes into and out of soils and vegetation, thereby influencing N and C masses in these compartments. We consider the model formulations to be a mechanism-based means of predicting ecosystem response, in contrast to applying regression equations or empirical response factors.
2.1.3 Land-use changes Land-use changes such as over-fertilization, cultivation like plowing and sweep tillage, biomass burning, forest harvest, and expansion or abandonment of agricultural land ect. are considered to be calculated by using
this model. Two methods of calculating these land-use changes are assumed to be: 1) using the mesh data of land-use changes as model inputs while their GIS data are available, otherwise 2) using user-define rates of land-use changes to couple with terrestrial ecosystem dynamics. 2.2 Model approach For the simplification procedure of modeling, we firstly consider to establish a steady state model. That is, we consider all N input and output fluxes together with their transformation fluxes among each compartment without human disturbances under following assumptions: 1. The amount of N uptake by plant is equal to the N contained in litter-falls. 2. Nitrate leaching and ammonia volatilization is zero due to the concept of N-limited in most natural ecosystems. After imposing the human land-use changes on this steady state model, the ‘disturbed’ state of terrestrial biosphere can be calculated. For example, by inputting the mesh data of global fertilization usage on the model and calculating the disturbance until it reach steady state again, we will get an quantitative illustration about the nitrate pollution of ground and surface water. Below we fully describe the model, the specific relationships defined between environmental factors and element flux rates, and the assumptions made in the model, although some of them are still under developing and need further considerations.
2.2.1 State variables The model contains eight variables: N in vegetation (VN), C in vegetation (VC), organic N in detritus (DN), organic C in detritus (DC), organic N in humus (HN), organic C in humus (HC), ammonium (Amm) and nitrate (Nit). All living plant leaves, trunk and roots are included in the vegetation pool, and all litter-falls from leaves, trunk and roots together with fixation by soil microbiology are included in detritus pool. The most common used ratios of C/N in leaves, trunk/roots, and detritus and humus are consider to be 20, 150, and 15, respectively. For predictions at all sites the model is run continuously until equilibrium conditions existed and all state variables remained virtually constant from year to year. The state variables do change from month to month according to differential inputs and losses driven by seasonal changes in climate:
dVC (1) = GPPt − TRRCt − ( FLCt + FTRC t ) dt dVN (2) = NUPTAKE t − ( FLN t + FTRN t ) dt dDC (3) = ( FLCt + FTRCt ) + FIXC t − DRC t − DHC t dt dDN (4) = ( FLN t + FTRN t ) + FIXN t − NMIN t dt dHC (5) = DHCt − HRCt dt dHN (6) = DHN t − NMIN t dt dAmm Amm (7) = NMIN + NMIN + DEPO − NUPTAKE − NITRIF − VOLA , , , t D t H Amm t t t t dt Amm ∗ Nit dNit Nit (8) = NITRIFt + DEPO Nit ,t − NUPTAKE t − DENITRt − LEACH t ) dt Amm ∗ Nit where t refers to the time step of the calculation (one month). Units for all state variables are kg/m2 of either C or N, and for all fluxes are kg/m2/month. Each of the fluxes included in Eqs. 1 through 8, and how it is controlled by external environmental factors, are described in details below. All acronyms are defined
in Table 2. The parameter values used in the model are either cited from reference papers, or determined by the calculation of this steady state model. 2.2.2 Nitrogen and carbon fluxes Nitrogen fluxes among compartments written in Eqs. 1-8 are expressed as following eleven equations, while C fluxes (8-9) are omitted from with the exception of two improvements on photosynthesis and biological N fixation. Photosynthesis (GPP) in leaf is modeled as a function of temperature (f(T)), soil water content (f(W)), CO2 concentration (f(CO2)), and N nutrient (f(N)). GPPmax is the basic C photosynthesis rate of leaf without consideration of environmental influence, and is evaluated by the Saeki equation described in the previous C model (8-9). f(T), f(W), and f(CO2) are cited from C cycle (8-9), while f(N) is still under developing.
GPP = f (T ) ∗ f (W ) ∗ f (CO 2) ∗ f ( N ) ∗ GPPmax
(9)
Biological N fixation (NFIX) by soil microbiology is probably the most important biochemical reaction for life on earth. Through this process, certain organisms convert the N2 gas of the atmosphere to N-containing organic compounds that become available to all forms of life through the N cycle. This process can be divided into symbiotic and nonsymbiotic fixation, carried out by a limited number of microorganisms, including several species of bacteria, a number of actinomycetes, and certain cyanobacteria (blue-green algae). NFIX is assumed to be the sum of fixation by symbiotic (SyF) and nonsymbiotic (NsyF), which can be modeled as the function of temperature ( η SyF −T ; η NsyF −T ) and mesh area of each ecosystem (AREAmesh). NFIX = SyF + NsyF = f clover * (η SyF −T + η NsyF −T ) ∗ AREAmesh
(10)
where fclover are parameters which stand for biological fixation ability (kgN/ha/y) of five ecosystems that are assumed to be: 15 in tropical forest, 10 in temperate forest, 8 in boreal forest, 30 in crop/grassland, and 0.5 in desert. Since SyF strongly depends on the plant growth, we assumed its temperature dependency (η SyF −T ) to be same as used in photosynthesis (8) expressed as cosine function.
η SyF −T
0 = COS T-Topt T
when T ≤ Tmin ; T ≥ Tmax
{
(10-1)
when Tmin ≤ T ≥ Tmax
And about NsyF, we model its temperature dependency by an exponential function as in Eq. 10-2, (( T −Topt ) / 10 )
η NsyF −T = Q10
(10-2)
where T is the temperature of each mesh, Tmin and Tmax are temperature range in which the N fixation is active. Topt is the optimum temperature for N fixation, which is assumed to be 20 degree. Q10 value is assumed to be 3. Nitrogen deposition (DEPO) including dry and wet depositions of ammonia gas, nitrate, and N compounds from atmosphere to soil through rain, snow, and dust. These compounds release from the soil and plants, as well as from the combustion of fossil and biomass. Nitrates also form in small quantities as a result of lighting in the atmosphere. Another source is the exhaust from automobile and truck engines, which contributes a considerable amount to the atmosphere, especially downwind from large cities. The quantity of ammonium and nitrate in precipitation (Crain) vary markedly with location, and typically about two-thirds is ammonium and one-third is nitrate. We model the wet deposition of ammonium (DEPOAmm) and nitrate (DEPONit) as a liner function of precipitation (Qrain), and dry deposition of them as a global average constant due to the lack of data. Each Crain of five ecosystems (gN/m2/month) is assumed as: 0.177 in tropical forest, 0.108 in temperate forest, 0.055 in boreal forest, 0.1 in crop/grassland, and 0.018 in desert.
1 DEPO Amm = Q Rain ∗ C rain ∗ + DEPO Amm ,dry 3 2 (11) DEPO Nit = Q Rain ∗ C rain ∗ + DEPO Nit ,dry 3 Denitrification (DENITR) by soil microbiology is a very important process that convert nitrate or nitrite ions to gaseous forms of N such as nitric oxide gas (NO), NO2, and N2. DENITR is modeled as a function of soil water (KW), temperature (KT), and nitrate or nitrite available in soil (Nit). KT depends exponentially on the mean soil temperature with a Q10 of 2 and optimum temperature of 25 degree. KW is assumed to be 0 when soil water content is below field capacity, while above field capacity is linearly related to the water content.
DENITR = KT ∗ KW ∗ Nit
(12)
Ammonia volatilization (VOLA) is a reversible chemical reaction that is strongly related to soil pH. We model VOLA as a function of soil pH and available ammonium concentration (Amm).
VOLA = 5.8 ∗ 10 pH −10 ∗ Amm
(13)
Nitrate leaching (LEACH) is the most common way of N loss. Such leaching losses not only reduce the ecosystem productivity, but also cause several serious environmental problems (14). LEACH is assumed to be related to soil moisture (f(W)), soil texture (f(Tex)), and available nitrate concentration (Nit).
LEACH = f (W ) ∗ f (Tex ) ∗ Nit
(14)
Nitrogen uptake (NUPTAKE) by vegetation is assumed to be from the inorganic N pool (ammonium and nitrate). It is modeled as a function of available ammonium and nitrate (Amm+Nit), soil moisture (M), and temperature (T). We follow many others in utilizing Michaelis-Menten kinetics to model N uptake by plants (3). Nmax is the maximum rate of N uptake by the vegetation, which is determined during the calibration of the steady state model so that the annual N uptake is equal to the annual N flux in the litter production for the calibration sites. Ks is a parameter accounting for relative differences in the conductance of the soil to N diffusion, assume to be determined by 15.1. Kn1 is the concentration of ammonium and nitrate at which N uptake proceeds at one-half its maximum rate, cited as 10 g/m2.
NUPTAKE =
N max ∗ K s ∗ ( Amm + Nit ) 0.0693T ∗e K n1 + K s ∗ ( Amm + Nit )
K s = 0.90 ∗ ( M / FC ) 3 + 0.10
(15)
(15.1)
Nitrification (NITRIF) is assumed to be a function of soil moisture (KW) and temperature (KT), and available ammonium concentration (Amm). KT depends exponentially on the mean soil temperature with a Q10 of 2 and optimum temperature of 20 degree. KW is assumed to be linearly related to soil water content with a slope of 1.17 and a zero intercept of –0.165 when soil water content is below field capacity, while above field capacity is linearly related to the water content with a slop decrease from 1 to 0.1.
NITRIF = K T ∗ KW ∗ Amm
(16)
Nitrogen fluxes in litter-fall, huminification, and mineralization are assumed to be stoichiometrically related to C fluxes described in the C model.
3. Calculation of N storage in terrestrial vegetation In order to verify this model regarding the integration of the C model with the N model on vegetation and soil compartments, N storage in terrestrial vegetation under present climate by this model was firstly calculated. Fig. 4 shows the global distribution of N storage in terrestrial vegetation, and Table 3
summarizes the estimates of N storage in terrestrial vegetation. Table 3 Global estimates (1Pg=1015g) of nitrogen storage in terrestrial vegetation This Study Soderlund and Svensson, 1976 Makoto KIMURA, 1989 McElroy et al., 1976
16 11-14 12-15 10
0 50 100 150 200 250 300 350 0
100
200
300
400
500
600
700
0.0 0.1 0.2 0.3 0.4 0.6 0.7
Fig. 4 Global distribution of nitrogen storage in terrestrial vegetation
For the lack of data on actual N storage in terrestrial vegetation, we compared Fig. 4 to a World Vegetation Map (WVM), based from the information of satellite remote sensing data of 1985 to 1987. We found that Fig. 4 matches spatially very well with WVM, and the N storage in desert area is the least shown as 0 kgN/m2, while in tropical and temperate forest are the most shown as 0.2-0.7 kgN/m2. Furthermore, the total N storage in terrestrial vegetation was calculated as 16 Pg (1015g, peta-gram), which shows good agreement with those previously reported (Table 3). Besides these results, the parameters of Nmax (g/m2) of five types ecosystems were also evaluated during the calculation as 121 in tropical forest, 92 in temperate forest, 58 in boreal forest, 43 in crop/grassland, and 0 in desert. Consequently, we can confirm at this stage that the concept of integrating N and C model on the vegetation compartment is credible, and using the data sets of environmental factors (present climate) to calculate the N storage in terrestrial vegetation by this model is also reasonable.
References 1) Daniel A. Jaffe (1992): The Nitrogen Cycle, in Global Biogeochemical Cycles, edited by Samuel S. Butcher et al., pp. 263-282, Academic Press Limited. 2) Michel G.J. den Elzen, Arthur H.W. Beusen, and Jan Rotmans (1997): An integrated modeling approach to global carbon and nitrogen cycles: Balancing their budgets, GLOBAL BIOGEOCHEMICAL CYCLES, Vol. 11, No. 2, pp. 191-215. 3) J. W. Raich, E. B. Rastetter, J. M. Melillo, D. W. Kicklighter, P. A. Steudler, and B. J. Peterson (1991): Potential net primary productivity in south America: application of a global model, Ecological Applications, Vol. 1, No. 4, pp. 399-429.
4) A.D.McGuire, J.M.Melillo, L.A.Joyce, D.W.Kicklighter, A.L.Grace, B.Moore III, and C.J.Vorosmarty (1992): Interactions between carbon and nitrogen dynamics estimating net primary productivity for potential vegetation in North America, GLOBAL BIOGEOCHEMICAL CYCLES, Vol.6, No.2, pp. 101-124. 5) Robert J.M. Hudson, Steven A. Gherini, Robert A. Goldstein (1994): Modeling the global carbon cycle: nitrogen fertilization of the terrestrial biosphere and the "missing" CO2 sink, GLOBAL BIOGEOCHEMICAL CYCLES, Vol.8, No.3, pp. 307-333. 6) Christopher S. Potter, James T. Randerson, Christopher B. Field, Pamela A. Matson, Peter M. Vitousek, Harold A. Mooney, and Steven A. Klooster (1993): Terrestrial ecosystem production: a process model based on global satellite and surface data, GLOBAL BIOGEOCHEMICAL CYCLES, Vol. 7, No. 4, pp. 811-841. 7) Christopher S. Potter and Steven A. Klooster (1997): Global model estimates of carbon and nitrogen storage in litter and soil pools: response to changes in vegetation quality and biomass allocation, Tellus, 49B, pp. 1-17. 8) Motoyuki Suzuki, Naohiro Goto and Akiyoshi Sakoda (1993): Simplified dynamic model on carbon exchange between atmosphere and terrestrial ecosystems, Ecological Modelling, Vol. 70, pp. 161-194. 9) Naohiro Goto, Akiyoshi Sakoda and Motoyuki Suzuki (1994): Modelling of soil carbon dynamics as a part of carbon cycle in terrestrial ecosystems, Ecological Modelling, Vol. 74, pp. 183-204. 10) Mellio, J.M., and J.R. Gosz (1983): Interactions of biogeochemical cycles in forest ecosystems, in The Major Biogeochemical Cycles and their Interactions, edited by B. Bolin, and R.B. Cook, pp. 177-222, John Wiley, New York. 11) Schimel, D., I. Enting, M. Heimann, T.M.L. Wigley, D. Raynaud, D. Alves, and U. Siegenthaler (1994): The Carbon Cycle, in Radiative Forcing of Climate, Scientic Asseeement, edited by J.T. Houghton et al., Intergov. Panel on Clim. Change (IPCC), Cambridge Univ. Press, New York.
12) Vitousek, P.M., and R.W. Howarth (1991): Nitrogen limitation on land and in the sea: how can it occur?, Biogeochemistry, Vol. 2, pp. 86-115. 13) Mellio, J.M., (1995): Human influences on the global nitrogen budget and their implications for the global carbon budget, in Toward Global Planning of Sustainable Use of the Earth: Development of Global Eco-Engineering, edited by S. Murai, and M. Kimuar, pp. 117-134, Elsevier, New York. 14) Nyle C. Brady (1998): Nitrogen and sulfur economy of soils, in The nature and properties of soils, 12th edited by Nyle C. Brady and Ray R. Weil, pp. 492-522, Elsevier, New York.
