We collected five soil cores to 0-1 m in depth at five locations spaced 25 m apart along a 100-m transect in the middle of the macro plot. Soil cores were partitioned into 5 depth layers and soil samples (n = 25) were collected for each layer to determine soil bulk density, and soil C and N concentrations, which were used to calculate soil C and N stocks. Soil samples were sieved to remove particles >2 mm in diameter, and ground to allow passage through a 60-mesh screen (250-µm pore size). Total C and N concentrations in the soils were determined using the same methods as for our litter/grass samples. Generally, soil C and N stocks were calculated by
multiplying soil C and N concentrations with soil bulk density estimates, the length of the soil layer, and a unit conversion factor (Chapter 2). We applied a correction for soil bulk density differences between mature reference forests and pastures (i.e., soil compaction; referred to as corrected soil C and N estimates, Chapter 2). Most mature forests that served as reference forests for the pastures were sampled using another gouge auger (tool B: volume = 23.7 cm3, width between vertical cutting edges = 3.45 cm) than the gouge auger used to sample pasture soils (tool C: volume = 13 cm3, width between vertical cutting edges = 2.4 cm). Two reference forests were sampled with the same gouge auger as was used to sample pasture soils (tool C). To adjust for this potential tool bias, we also calculated adjusted soil C and N stocks. We
standardized (i.e., adjusted) all reference forest soil bulk densities by estimating soil bulk densities based on forest soil C concentrations using a regression equation of forest data sampled with tool C. Additional information on soil sampling and adjustments of soil C and N stock estimates were described in Chapter 2.
We calculated ecosystem C and N stocks by summing aboveground C or N stocks and 0-1 m soil C or N stocks. In this dissertation we specifically state if estimates were adjusted and in all other cases statements refer to our original estimates. Adjusted ecosystem C and N stocks are the sum of aboveground C or N stocks and soil C or N stock estimates adjusted for potential tool bias. We assumed
69 that the differences in C and N stocks between pastures and mature forests (∆ C and N stocks) were due to the conversions from forest-to-pasture. Our original ∆ ecosystem C and N stocks were calculated by subtracting ecosystem C and N stock estimates of reference mature forests from estimates of pastures. Adjusted ∆ ecosystem C and N stocks were defined as ecosystem C and N stock estimates adjusted for potential tool bias from pastures minus adjusted estimates from reference mature forests.
3.2.5 Statistical analyses
We determined Pearson coefficients of correlation (r) between response variables (aboveground and ecosystem C and N stocks, absolute and relative aboveground and ecosystem C and N stock difference between pastures and forests [∆ aboveground and ecosystem C and N stocks], %C and N and C:N ratio for grass/litter) and explanatory variables (pasture age, elevation, temperature, and precipitation) for our 31 pastures. We compared six regression lines that described the change of response variables with pasture age for the different life zones for each response variable. Regression line comparisons tested whether the: (1) slopes of the regression lines were similar to each other (homogeneity of slopes); (2) response variable correlated with pasture age (slope
≠ 0); and (3) chronosequences (and thus life zones) were different from each other,
while accounting for the effect of pasture age (unequal intercepts). If the slopes of the regression lines differed among life zones (test 1), we did not conduct test 2 and 3 because of the age by life zone interaction. If we detected an age effect (test 2), then we conducted test 3, which was similar to an analysis of covariance with pasture age as a covariate. If we failed to detect an age effect (test 2), then test 3 resembled an analysis of variance. If we detected an effect of life zone in test 2 or 3, we conducted pair wise multiple comparisons with Tukey-Kramer adjustments to test for differences among life zones. We only corrected for age in the multi comparison test for ∆
ecosystem C stocks. We natural log-transformed aboveground and ecosystem C stocks to correct for unequal variance and we backtransformed those results; hence, we
70 reported differences between median life zone estimates (Ramsey and Shafer 2002). All statistical tests were conducted in PROC CORR and PROC MIXED using SAS1 software v 9.1 for Windows (SAS Institute 2002-2003).
Although the ranges of pasture age for the different chronosequences were not identical, they all overlapped substantially (Table 2.1), and therefore, we assumed that our comparison of regression lines was an appropriate procedure. Insufficient sites were available for years immediately after deforestation, therefore only pastures ≥8 years were used to test for age effects in the comparison of regression lines. We could not determine the exact age of five pastures due to constraints in time and resources (Table 2.1). Therefore, the following nominal ages were used for those five pastures in the analyses: >75 years = 75 years, >47 years = 50 years, >69 = 70 years, >35 years = 40 years. We report results on the sensitivity of our regression analyses by using the ages: >75 years (site a) = 100 years, >75 years (site b) = 150 years >47 years = 50 years, >69 = 80 years, >35 years = 50 years.
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3.3 Results