Abstract

It is unclear whether increases in photosynthesis as a consequence of rising CO2 will result in long-term carbon stabilization in soil fractions with slow turnover times. In addition, there are potential feedbacks between plant responses to CO2 and mineral nutrient availability. We measured soil carbon and nitrogen dynamics in an intact C3-C4 grassland that has been exposed to a gradient in CO2 from 200-µmol mol-1 to 550-µmol mol-1 since 1997. There were no significant changes in bulk soil carbon or nitrogen between 1997 and 2000. Both free POM and macroaggregate POM increased from 1997 to 2000 under the superambient CO2 treatment and decreased under subambient CO2. There was higher soil CO2 flux from the superambient chamber than the subambient chamber from 1998 through 2000. The magnitude of the difference between the two chambers increased annually through the four years of the experiment. In mid-summer of 1999 and 2000 there was a curvilinear relationship between CO2 and net N-mineralization, with much higher N-mineralization under the subambient treatment than in the elevated treatment. The C/N ratio of POM was positively correlated with CO2, supporting the hypothesis that there are feedbacks between carbon uptake and nutrient availability. This indicates that the nitrogen cycle in these grasslands is more responsive to CO2 changes that have occurred in the past than those that are forecast for the future

Objectives

• Evaluate the sensitivity of both bulk soil organic matter and a soil C pool with an intermediate turnover time (20-150 yr.) to historical and future [CO2]a
• Determine whether soil CO2 efflux is sensitive to changing [CO2]a
• Determine the sensitivity of Net N Mineralization to alterations in [CO2]a

Methods

• Particulate organic matter fractionation entails both a density (1.85 g ml-1) and a physical separation. We followed the method of Six et al. (1999) and determined free, macroaggregate, and microaggregate POM for soils collected in September 1997, April 1999, and September 2000.
• We measured soil CO2 flux monthly during the growing season using two permanent rings in each subsection of the chambers using a LiCOR-6200. These data were collected three times prior to the initiation of the experiment and throughout 1997-2000.
• Net N-mineralization was measured using an in situ paired core technique. These assays were conducted in the spring, summer, and fall of 1999 and 2000.

Prairie CO2 Gradient

• Two tunnel shaped chambers (1-m wide X 60-m long) provide a continuous gradient from superambient to subambient CO2 concentrations.
• Photosynthesis progressively depletes CO2 in the air as it passes through the chamber.
• Gradients are maintained during the growing season by altering flow rate through the chambers.
• Air is cooled and dehumidified at 5-m intervals to maintain air temperature and vapor pressure deficit.
• The vegetation is dominated by the C4 perennial bunchgrass Bothriocloa ischaemum and the C3 forbs Solanum dimidiatum and Ratibida columnaris.

Results

Objective 1: Evaluate the sensitivity of both bulk soil organic matter and a soil C pool with an intermediate turnover time (20-150 yr.) to historical and future [CO2]a

Objective 2: Determine whether soil CO2 flux is sensitive to changing [CO2]a

• No pre-treatment differences in CO2 flux between subambient and superambient chambers
• Mid-growing season divergence, with flux from the superambient chamber higher than the flux from the subambient chamber
• Magnitude of difference between the two chambers has increased over the course of the experiment

Objective 3: Determine the sensitivity of Net N-mineralization to alterations in [CO2]a

• Divergence in N Mineralization rates during mid-summer
• Exponential decrease in N Mineralization rates with increasing [CO2]
• Correlation between POM C/N and CO2

Conclusions

• Soil C storage in organic matter pools with decadal-scale residence times are sensitive to [CO2] in this grassland.
• Increasing CO2 apparently leads to greater carbon allocation belowground coupled with higher microbial activity
• The negative exponential relationship between net N-mineralization and CO2 during mid-summer indicates that the nitrogen cycle in these grasslands is more responsive to CO2 changes that have occurred in the past than those that are forecast for the future.
• The relationship between POM C/N ratios and [CO2] may indicate a link between the increase in POM storage and decrease in N availability with rising CO2.