Microbial enzyme activities are the direct agents of organic matter decomposition, and thus play a crucial role in global C (C) cycling. Global change factors like anthropogenic nutrient inputs and warming have the potential to alter the activities of these enzymes, with background site conditions likely driving responses. We hypothesized that enzyme activities in sites with high background soil nutrient and/or C availability would be less sensitive to nutrient additions than nutrient-poor sites. We also hypothesized that sites poor in nutrients and/or C would show greater sensitivity to changes in temperature. To test our hypotheses we used long- and short-term nutrient additions combined with laboratory temperature incubations to assess changes in enzyme activities for 8 common soil enzymes that acquire nitrogen (N), phosphorus (P) and C from organic matter. We collected mineral soils (0-10 cm depth) from 8 Hawaiian sites that provided maximum variation in nutrient availability and background soil C. Soils were sieved, pooled by site, and homogenized prior to a laboratory addition of a simple C (sucrose), N, and/or P in full factorial design. The 8 soils also were incubated at 7 temperatures from 4 - 40 ºC.

We found that the laboratory fertilizations altered enzyme activities, and that temperature sensitivities varied significantly among sites. Across the 8 sites, laboratory sucrose+N and sucrose+NP additions increased C-, N-, and P- acquiring enzymes activities (p < 0.05), with the strongest effect, as predicted in nutrient and C-poor soil. Phosphorus-acquiring enzymes were the most sensitive to these additions, while C-acquiring enzyme activities were less responsive. Results suggest that C-, N-, and P-acquisition enzyme activities respond positively to added nutrients across sites, regardless of background nutrient status. In particular, P-acquisition was broadly sensitive to N addition even in relatively N-rich soils, whereas C- and N-acquisition activity appeared to be generally sensitive to N and P. Overall, enzyme activities responded most strongly to the addition of sucrose+N or sucrose+NP. Temperature sensitivity varied significantly across sites and among enzymes, with greater temperature sensitivities for enzymes that acquire N and P in wetter sites than drier sites. In contrast to our hypothesis, enzyme temperature sensitivities were strongest for soils from relatively nutrient- and C-rich forests, than for drier sites poorer in soil nutrients and C. Enzyme Q10 were measured using the Arrhenius equation to determine temperature dependency of reaction rates. Enzyme Q10 values followed a log linear relationship and were most sensitive to changes from the 10 °C to 20 °C range as opposed to warmer temperatures (p < 0.05). Enzyme responses to changes in climate such as nutrient additions and warming will likely have negative implications for soil C storage as enzymes generally responded positively to warming and nutrient additions.

[WestJEM. 2009;10(4):250.]

Tropical forests contain a considerable fraction of the global soil carbon (C) stock, but the response of tropical soil C to predicted changes in primary productivity remains poorly understood. Drastic changes in soil C storage and loss are likely to occur if global change alters plant net primary production (NPP). This research assessed the effects of a decade of litter removal and addition on soil C storage in a seasonal tropical forest in Panama. Previous manipulation studies like the one used here have reported strong responses of carbon pools with addition and removal of litter in tropical forest ecosystems. Specifically, total C gains have been observed after addition while corresponding losses of C have been found with litter removal. However, past manipulation experiments fail to account for the heterogeneous nature of carbon cycling in these systems. The overarching hypothesis addressed here, is that changes in net primary production can affect the storage of soil C on various timescales. To address this, a density fractionation scheme was used to assess manipulation effects on rapidly- and slowly-cycling pools of C. Soil samples were collected from 0-5 cm depth in 15 - 45 x 45m plots with litter removal (L-), 2x litter addition (L+), and control (n=5). To account for any potential fertilization effect from added litter, soils from a long-term fertilization experiment were also used. Soil C storage in all pools was significantly different among the three litter treatments, and some seasonality resulted in every pool. L+ increased the oldest, most stable pool of C by 15% in the dry season and 28% during the wet season. Alternatively, L- decreased C concentrations in this long-term pool by 26% in the dry season and 28% in the wet season relative to control. There was no indication of any fertilization effect on the storage of C in this forest. The results here demonstrate that changes in primary productivity have the potential to alter the storage and cycling of C in lowland tropical forest soils.

Ecosystem processes are fundamentally limited by nutrient availability, but the effects of nutrient limitation or co-limitation on key processes like root nutrient uptake and carbon (C) exudation into soils is relatively unknown. New paradigms of nutrient limitation suggest that colimitation by multiple nutrients is more common than single nutrient limitation, therefore the established view that ecosystem processes are limited by phosphosus (P) in lowland tropical forests on highly weathered soils ecosystems should be revisited. Rates of nutrient uptake are of particular importance because these govern downstream plant physiological processes like photosynthesis and basic metabolism. Also, nutrient uptake is directly linked to root C exudates, which are increasingly recognized as an important plant contribution to soil C pools. Here, we explored the effect of changes in nutrient availability on root nutrient uptake in a well-studied Panamanian forest, and linked nutrient uptake to C exudates. We used a long-term factorial nutrient fertilization experiment, and focused our study on a relatively abundant canopy tree species (Tegtragastris panamensis). We found a significant interacting effect of nitrogen (N) and potassium (K) on root uptake rates for ammonium, nitrate, and phosphate. Nitrogen and K added in combination produced uptake rates indistinguishable from control plots, however, N or K added individually increased the uptake rates of all nutrients compared to control and NK plots. No significant effects of N or P addition were observed on root C exudation rates. These results suggest that increased N or K availability may accelerate N and P uptake rates by roots. These results support the growing literature indicating that most ecosystem processes in tropical forests are governed by the availability of multiple nutrients, with implications for understanding plant productivity in this biome.

Understanding the mechanisms of soil carbon (C) formation and loss is essential for predicting the C storage capacity of soils under ongoing global change scenarios. Climatic variables, vegetation structure, microbial activity, soil mineralogy, and tissue C chemistry each have the potential to affect the fate of C in soils, and the interactions among these controls vary in different environments. Our mechanistic understanding of how these factors interact with each other to determine soil C storage is still rudimentary. This dissertation used a series of field and laboratory studies to assess the interacting roles of vegetation, soil mineralogy, microbial activity, C chemistry, and temperature in regulating the fate of C in soils.

In the first experiment, we sought to understand how soil mineralogy, soil nutrient and C status, and C chemistry interact to determine warming effects on the fate of newly added soil C using a 13C isotopic tracing approach. By tracking the added 13C label in soil pools at 4 days and 255 days in tropical forest soils with differing weathering and mineralogical conditions, we found that initial microbial uptake of 13C and average carbon use efficiency (CUE) by microbes were strongly correlated with longer-term C retention in mineral soils. Overall, warming had a negative effect on 13C retention in soil in the youngest, least-weathered soil only, with no warming effect on moderately to strongly weathered soils. Thus, soil C stocks in less weathered soils, and with lower microbial CUE, may be most vulnerable to C loss with a warming climate.

Our second study assessed the fate of newly added organic 13C-labeled compounds in soils of differing fertility along weathering gradients. Comparing additions of two low molecular weight compounds, 2.9x greater retention occurred for 13C-labeled glucose versus 13C-labeled glycine after two years, suggesting that glucose may be a better precursor for soil organic matter formation. Soil mineralogy and nutrient availability were not significant factors in 13C retention in soil. Soil spectra from 13C NMR revealed an increase in the proportion of alkyl C in glucose and glycine amended soil relative to control soils, and alkyl C are commonly associated with relatively stable organic C. Thus, our results indicate that microbial incorporation of labile organic compounds like glucose into biomass may be associated with greater C retention in stable soil components.

Our third study estimated the long-term effect of grass cover loss on soil organic C (SOC) and total nitrogen (TN) storage, and the spatial heterogeneity of SOC and TN in two arid grasslands. The nine years of experimental grass removal resulted in soil deflation and 30% and 35% declines in SOC and TN respectively in 100% grass removal plots (TU100). Grass removal also led to soil deposition in downwind areas of the plot (TD100). Soil organic C and TN concentrations in the deposition plot (TD100) was variable, and likely depended on the structure of the vegetation community trapping wind-blown particulates. Geostatistical analysis showed that weaker and smaller fertile islands, compared to the control, developed in TD100 plots over nine years of aeolian transport.

The outcomes of this dissertation will add to the current body of knowledge about mechanisms of soil C stabilization across environmental conditions and with warming.