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Nitrogen-mediated effects of elevated CO2 on intra-aggregate soil pore structure
- Joshua S. Caplan, Daniel Gimenez, Vandana Subroy, Richard J. Heck, Stephen A. Prior, G. Brett Runion, H. Allen Torbert
- Global change biology 2017 v.23 no.4 pp. 1585-1597
- carbon dioxide, grasses, rhizosphere, biotic factors, organic matter, soil pore system, mineral soils, carbon dioxide enrichment, fertilizer application, nitrogen, pasture plants, pastures, porosity, exudation, wilting point, carbon sequestration, soil aggregates, sandy loam soils, nitrogen fertilizers, Paspalum notatum
- Soil pore structure has a strong inﬂuence on water retention, and is itself inﬂuenced by plant and microbial dynamics such as root proliferation and microbial exudation. Although increased nitrogen (N) availability and elevated atmospheric CO2 concentrations (eCO2) often have interacting effects on root and microbial dynamics, it is unclear whether these biotic effects can translate into altered soil pore structure and water retention. This study was based on a long-term experiment (7 yr at the time of sampling) in which a C4 pasture grass (Paspalum notatum) was grown on a sandy loam soil while provided factorial additions of N and CO2. Through an analysis of soil aggregate fractal properties supported by 3D microtomographic imagery, we found that N fertilization induced an increase in intra-aggregate porosity and a simultaneous shift toward greater accumulation of pore space in larger aggregates. These effects were enhanced by eCO2 and yielded an increase in water retention at pressure potentials near the wilting point of plants. However, eCO2 alone induced changes in the opposite direction, with larger aggregates containing less pore space than under control conditions, and water retention decreasing accordingly. Results on biotic factors further suggested that organic matter gains or losses induced the observed structural changes. Based on our results, we postulate that the pore structure of many mineral soils could undergo N-dependent changes as atmospheric CO2 concentrations rise, having global-scale implications for water balance, carbon storage, and related rhizosphere functions.