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WIMOVAC's Soil Carbon & Nitrogen ModuleThe potential effects of climate change on the biogeochemistry of C and N pools, have received little attention, particularly with respect to natural grasslands {Hall, 1991 #1946;Long, 1991 #1394}. There is a requirement therefore to incorporate knowledge about plant-soil feedback dynamics in well defined systems into ecosystem models that are robust enough to be used with a broad range of environmental conditions, and plant and soil types. A number of such models are currently being developed {Coughenour, 1984 #2056;Coughenour, 1984 #2057;Friend, 1993 #2058;McMurtrie, 1990 #1852;Parton, 1996 #2055} and are reviewed in {Ojima, 1996 #2054}. These models are capable of studying the complex interactions and feedback’s within a plant-soil ecosystem framework. The Century model was originally devised by Parton and co-workers {Parton, 1987 #2052; Parton, 1988 #2053; Parton, 1992 #2051; Parton, 1993 #1957} and consists of a computer model of plant-soil ecosystems which simulates the dynamics of grasslands, forest and crops. It incorporates simplified assimilation and turnover and a detailed model of soil carbon, nitrogen, phosphorus and sulphur (Figure 20, Figure 21, Figure 20) Nitrogen (N) is an important component of soil makeup because it plays a vital role in plant nutrition and function. It is the mineral element required in the greatest quantity by most vegetation, and soil N pools, as a primary source of N for plants, are an important component in determining the overall efficiency of plant photosynthesis and likely long term response to global climate change. Grassland ecosystems store most of their C in soils where turnover times of the slow and passive pools are relatively long (in the order of thousands of years) and so changes, though they may occur slowly, will be of a significant duration and magnitude{Schimel, 1994 #2059} and should be included in long term assessments of C production from vegetative systems. Carbon sub modelThe soil organic matter (SOM) sub-model in wimovac simulates the dynamics of C and N in the organic and inorganic parts of the soil system using a modified form of the Century model proposed by Parton (1993). The flow diagram (Figure 20) shows that soil C is divided up into three major components which include active, slow and passive soil C. Active SOM includes live soil microbes plus microbial products, the slow pool consists of resistant plant material which typically contains a high proportion of lignin like pigments. The passive pool contains material that is very resistant to decomposition and includes physically and chemically stabilized SOM. The flows of C are controlled by the inherent maximum decomposition rate of the different pools and an empirical factor that describes the influence of temperature and water on the decomposition process. A detailed description of the model system is as given in Parton, et al. (1988) and Parton, et al. (1992) {Parton, 1988 #2053; Parton, 1992 #2051}. The SOM model in wimovac is tightly integrated with above and below ground processes such as growth, photosynthesis, transpiration, water and nutrient uptake and senescence. There is an interactive effect between the modeled plant activities and soil processes on many levels, for example: plant growth affects soil shading and therefore temperature, plant water uptake effects decomposition rates and soil albedo, decomposition rates affect soil texture, water retention characteristics, rooting profiles and nutrient availability to the plant, plant nutritional content in the litter determines the decomposition rates of soil C and N pools. Experimentally it is very difficult to disentangle these processes and so the model provides an ideal way to explore, at least hypothetically, how these processes may inter-relate in a global climate change scenario.
Flow diagram for the Century soil organic C model. Reproduced from Parton et al., (1993). L/N is the lignin nitrogen ratio. A is the abiotic decomposition factor. T is the silt plus clay content expressed as a fraction. TS is the sand content fraction. TC is the clay content fraction. LS is the fraction of structural C that is lignin. LC=Exp(-3.LS). KL is the maximum decay rate. FM is the fraction of litter that is metabolic. Ki=3.9,4.8,7.3,6.0,14.8,18.5,0.2,0.0045 y-1 respectively.Nitrogen sub modelThe nitrogen sub-model has the same basic structure as the soil C model. The organic N flows follow the C flows and are equal to the product of the carbon flow and the N:C ratio of the state variable (square boxes in Figure 20 and Figure 21). The N:C ratios of the soil state variables are a function of the mineral N pool (NO3- and NH4+) and vary within the ranges 3-15, 12-20 and 7-10 respectively for active, slow and passive SOM. Details of this sub model are as given in Parton, et al. (1988) and Parton, et al. (1992) {Parton, 1988 #2053; Parton, 1992 #2051}.
Flow diagram of the Century N submodel. Reproduced from Parton et al., (1993). NATM is the atmospheric N addition. Nfert is the fertility addition. Pn is the average plant nitrogen content at senescence (g m-2 ). NO3 is the soil nitrate nitrogen content (g m-2 ). GN is the gross N mineralisation rate (g m-2 ). C is the carbon flow between state variables. Nsm, NA, NS, NP are the nitrogen to carbon surface microbes, active, slow and passive soil organic matter (SOM). Ni is the nitrogen to carbon ratio for active SOM. I is immobilisation and M is mineralisation. |
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