Well planned and carried out – solid conclusions
Joshua Gray
Problem Set 3
Title
Introduction
The purpose of this laboratory investigation was to compare ecosystem response to varying
amounts of nitrogen input. In this simulation the sole input of nitrogen is through atmospheric
deposition. Simulations were used to compare biomass production under four different nitrogen
regimes.
Methods
BIOME-BGC for Excel (version 0.99) was used for the numerical simulations. This model
estimates water and nutrient cycling through a forested patch using meteorological drivers.
Phenological parameters were typical of evergreen needle-leaf stands. The soil was set as a 1m deep
loamy soil (30% sand, 50% silt, 20% clay). Meteorological data for the period 1950-1993 from Missoula
MT was used. The pre-industrial nitrogen deposition parameter, NDEP, was varied to create four
separate nitrogen deposition regimes, with all other variables fixed. The four values selected were
equivalent to 1 kg N/ha/year, 10 kg N/ha/year, 30 kg N/ha/year and 50 kg N/ha/year. Daily summary
variables appropriate to comparing biomass production were added to output list. These variables
included four soil and litter stores of C: SOIL1C, SOIL2C, SOIL3C, SOIL4C, LITR1C, LITR2C, LITR3C and
LITR4C, heterotrophic respiration (HR), maintenance respiration (MR), growth respiration (GR), total soil
carbon (SOILC), fine root C (FROOTC), live and dead stem carbon (LIVESTEMC and DEADSTEMC) and live
and dead root C (LIVECROOTC and DEADCROOTC). A spinup period using the default N deposition (1 kg
N/ha/year) was conducted and the resulting restart file was used for all subsequent simulations.
Results
The ecosystem response to varying nitrogen deposition across all study years is shown in Fig.
1(a-d). It can be seen in Fig. 1c that higher levels of N deposition lead to a proportional increase in the
maximum LAI achieved by the patch in a given year. The NPP is also higher under higher N deposition
conditions (Fig. 1b). Both NPP and Max. LAI track well with annual precipitation; an expected result in a
water limited environment such as Missoula. However, the effect of varying levels of N deposition
seems to exert the strongest influence on NPP in wet years preceded by dry years (1980-1985 and 19891993). Fig. 1d shows GPP as a function of precipitation, an indicator of water use efficiency. Here too,
higher N deposition is correlated with increased C gain per unit H2o.
Figure 1: (a) Annual precipitation from 1950 to 1993 for Missoula, MT. (b) NPP for four different levels of N deposition (c)
Maximum LAI obtained in a year under varying N deposition (d) Water use efficiency: GPP as a function of precipitation with
linear regression shown for the four N deposition scenarios.
1987 was chosen to investigate the seasonal response of the forest stand to N deposition. Fig.
1a shows that 1987 was a relatively wet year which was preceded by dryer, but average, conditions. Fig.
2 illustrates the GPP for 1987 alongside the variation in soil water potential.
Figure 2: (a) 1987 variation in soil water potential (b) 1987 variation in GPP for the four different N deposition regimes.
Fig. 2a,b show more evidence of the water-limited nature of this ecosystem. GPP is strongly
correlated with the amount of water available in the soil (SOILPSI). The effect of increased N deposition
is to increase GPP, but this effect is stronger when soil water is abundant. For example, the period from
day 113 to 145 shows little variation in GPP across varying N deposition, but from day 65 through 113
the difference is much stronger.
Figure 3: (a) 1987 variation in litter 1 carbon store (b) 1987 variation in litter 4 carbon store (c) 1987 variation in soil 1 carbon
store (d) 1987 variation in soil 4 carbon store
Fig. 3(a-d) shows the variation in litter and carbon stores for the 1987 year. The amount of C in
soils increases with increasing N deposition (Fig.3 c and d). The amount of C in litter stores shows
different responses to varying nitrogen. Fig. 3b indicates that the amount of carbon in the deep litter
(LITR4C) is reduced with increasing N, but the amount of decrease diminishes above 30 kg N/ha/year.
The amount of carbon in the highest litter pool (LITR1C) varies throughout the year across N deposition
levels. At the beginning of the year there is an inverse relationship between N deposition and C in litter
store 1. After the growing season (around year day 160) this trend is reversed with higher amounts of C
associated with higher N deposition. This phenomenon could perhaps be explained by the higher
heterotrophic respiration earlier in the year associated with the higher soil nitrogen levels. Later in the
year the higher LAI’s achieved under elevated N contribute more C to this litter pool during leaf fall.
Conclusions
The overall effect of increased N deposition is to increase biomass production in foliage, stem
and roots. This effect is perhaps more pronounced in areas which don’t experience as much water stress
as our study area, Missoula, MT. It has been shown that elevated N deposition has the strongest
influence on NPP and GPP when water is abundant. From examination of the amount of carbon in the
litter pools we can observe the affect of nitrogen deposition on heterotrophic respiration. Higher
nitrogen levels in biomass are associated with increased heterotrophic respiration as the stability of
detritus decreases with decreasing C:N ratios. This effect can be observed in Fig. 3a,b where we see
lower amounts of carbon in both shallow and deep litter stores in a higher nitrogen environment. This
investigation has demonstrated that nitrogen can play an important role in ecosystem dynamics,
particularly in its influence on soil and litter stores of carbon. In this dry, primarily water-limited
environment, the influences of nitrogen are dominated by that of water. However, in times when water
becomes less limiting the influence of nitrogen becomes more important.