Landscape-Scale Controls on C, N, and exchangeable Ca++, Mg++ and K+ in Soils of the Luquillo CZO Forest

Project Description: 

We sampled soils from 216 profiles representing 24 sites in the El Yunque National Forest to determine amounts C, N and neutral-salt-extractable Ca++, Mg++ and K+. Following the classic paradigm, we assessed the influence of climate (modeled precipitation, modeled temperature and/or elevation as a surrogate variable for both), forest type (tabonuco, colorado, palm), parent material (quartz diorite, volcaniclastics), and topography (catena positions ridge, slope, valley and % slope) on the distribution of these nutrients. To separate the effects of vegetation from those of climate, half of the sites were located between 500 and 700 m in the three forest types where rainfall and temperature were not significantly different. Using a combination of ANOVA (or Kruskal-Wallis) and univariate regression trees we determined that the amount of carbon in the top 80 cm of soil was influenced primarily by forest type (c > p > t) probably driven by differences in litter and/or root C:N ratios. Topographic position was significantly correlated with C amount (v > s, r), with the higher C amounts in the valleys probably driven by low O2 levels. Bedrock type was significantly correlated with C amount in c and p stands, but not in the tabonuco type. N was strongly correlated with C as expected. Exchangeable Ca was different across forest types (t > c, p) and bedrock type (qd > vc). Mg and K were differed by forest type, but not by bedrock type (t > c, p) or any other variables.
The next phases of this project are (1) to determine levels of these nutrients below the root zone (80-140 cm) and the factors controlling their distribution; and (2) establish field experiments to test the results of the regression trees which indicate that the C:N ratio of litter and/or root inputs is the most important variable influencing C distribution. The latter represents a first step in exploring the usefulness of regression trees as a way of sorting out the relative importance of each of the state factors (climate, topography, organisms, parent material and time) in the classic paradigm relating environmental variables to soil properties.
Soil C differs markedly across forest types (c> p> t, p<.0001), and across catena positions (v> s, r, p<.001), but not across bedrock types in spite of the higher clay content of soils derived from vc. C:N ratio (in any horizon, or in the whole profile) is the best predictor of soil C amount. The differences in soil C correspond to the differences in litter C:N. Tabonuco stands have the least soil C but the highest litter input rates (c= 9.1, p= 7.2, c= 7.2 Mg ha-1yr-1, Weaver and Murphy, 1990, Frizano 1999, Lugo 1992) and Sullivan et al. (1999) measured substantially faster decomposition of tabonuco litter over 100d. Those findings support the idea that soil C amount is driven by differences in decomposition rate related at least in part to C:N ratios. Similar results were obtained in the 500-700m elevation band where only vegetation differs (soil C: c= 21.5+ 3, p= 19.3+ 2, t= 14.7+ 1). Univariate regression trees identify soil C:N ratio as the most important variable explaining soil C in all combinations of candidate predictor variables. For the past several decades, determining the influence of individual state factors on soil properties has been difficult due to the fact that some of the state factors are correlated with each other, and all 5 of the environmental variables can influence one soil property. Such problems are inherent in areas like the EYNF where vegetation changes along climate gradients. We plan to test the regression-tree result indicating that the C:N ratio of litter (and perhaps roots) is a more important control on decomposition rate than temperature and rainfall with field and laboratory incubations.
Valley soils have more C than ridge or slope soils. Depth profiles of soil C show equal C in the 0-20 cm layer across the catena, but greater amounts of C in the 20-80 cm layer in the valley soils (data not shown). This suggests a minimal role for down-slope movement of litter, and that the greater C content of valley soils is driven more by slower decomposition related to the lower O2 levels measured in soil air in the valleys (Silver et al. xxxx).

2. Soil Nitrogen
Soil nitrogen and soil C are correlated within each forest type and across the study area, Attempts to find variables that might influence N amount other than forest type and the amount of organic C were unsuccessful.
3. Base cations
Neutral-salt-exchangeable Ca++ is different across forest types (t > p, c; p<.0001) and bedrock types (qd > vc, p<.0001). XRD analysis indicates that some of the soils derived from qd have substantial amounts of feldspar (up to 60%, Zhou 2011) and this probably accounts for the difference related to bedrock type. Mg and K differ only across forest types (t > p, c, p<.0001 in each case) and not in response to any other variable. It is still unclear whether the high exchangeable cation amounts in the tabonuco forest soils are due to the rapid decomposition of organic matter or to the chance correspondence of weatherable feldspar in some of the the tabonuco stands growing on quartz diorite (or perhaps both).
Summary
In the top 80 cm of soils, vegetation type and the C:N ratio of litter (and roots?) are the most important controls on the quantities of C and N and on the amounts of exchangeable base cations. Bedrock influences are minor, and in the data analyzed thus far, confined to the amount of neutral-salt-exchangeable Ca (and the Ca:Mg ratio).

Research Location: 
Source of Funding: 
NSF CZO Program
Relevant Publications: 

Gould, W.A., Gonzalez, G., and Carrero Rivera, G. 2006. Structure and composition of vegetation along an elevational gradient in Puerto Rico. Journal of Vegetation Science 17: 653-664.

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Contact Information
Person(s) Completing This Form: 
James Bedison
Investigators: 
Art Johnson -principal investigator-(215) 748-7446, Frederick Scatena (215) 898-6907, Alain Plante (215) 748-9269, Grizelle Gonzalez (787) 766-5335 x112, Hao Xing Xing, James Bedison.