Interactions between plants and soil nutrient cycling under elevated CO2

The atmospheric concentration of the greenhouse gas CO₂is rising and may stimulate plant production and soil C input. If soil C input rates exceed soil C respiration rates under elevated CO₂, global warming may be mitigated by long-term soil C sequestration. However, whether soils will serve as CO₂ sinks is still debated, since it is uncertain how elevated CO₂ will affect the interactions between plant growth and soil nutrient cycling. In the first part of this dissertation, I explored how long-term elevated CO₂ affects soil C inputs <i>versus</i> SOM decomposition, and how these changes ultimately feedback to soil C sequestration. This research was carried out in a Free Air Carbon dioxide Experiment (FACE) in Switzerland that had been exposed to elevated CO₂ and N fertilization treatments for 10 years. The isotopic label of the applied CO₂ and N allowed for tracing new C and N dynamics in the system. In addition, I summarized available data related to plant growth and soil nutrient cycling from long-term CO₂-enrichment experiments using the statistical tool Meta analysis. By incubating litter and soil derived from Swiss FACE, I concluded that the impact of elevated CO₂ on litter quality and litter decomposition rates was minor. Therefore, elevated CO₂ is not expected to affect soil C contents through its impact on litter quality and decomposition. The Meta analysis showed that the main driver of soil C sequestration is not SOC decomposition, but soil C input through plant growth, which is strongly controlled by nutrient availability. If soil nutrient availability was high, soil C input outweighed C decomposition leading to net C sequestration. However, if soil nutrient availability was low, soil C input rates lagged behind soil C decomposition rates due to CO₂-inducednutrient immobilization, which had reduced plant growth. Thus, for soil C sequestration under elevated CO₂ample soil nutrient availability is required. In the Swiss FACE experiment however, soil C sequestration did not increase under elevated CO₂, despite high fertilization rates, concurrent increases in plant growth, and relatively low decomposition rates. This may be due to frequent harvests and shows that the potential for soil C sequestration in individual agro-ecosystems is still uncertain, due to management practices that can affect soil C input and/ or soil C decomposition. The potential for soil C sequestration in individual unfertilized/ natural ecosystems is also unclear, since unexplained processes appear to prevent N limitation in some of these FACE systems. These processes may occur in the rhizosphere, which is often overlooked, but plays a vital role in mechanistically coupling plant production and soil nutrient cycling. In the second part of this dissertation I focused on how rhizodeposition affects microbial regulation of soil N availability. Elevated CO₂ stimulated the amounts of root-derived C and N substrates entering the soil, but without specific exudation of amino acids. Enhanced rhizodeposition was accompanied by a proportional increase in root production, suggesting that rhizodeposition under elevated CO₂ only increases when root biomass production is stimulated. The increase in rhizodeposition of N under elevated CO₂ comprised a significant portion of the plant assimilated N, and was quickly immobilized by microbes upon entering the soil. This shows another pathway by which elevated CO₂may enhance nutrient limitation in low N-input systems. Alternatively, elevated CO₂ may alleviate N limitation by stimulating rhizodeposition induced decomposition, leading to the release of N retained in stable SOM pools. This dissertation shows that increased rhizodeposition of C under elevated CO₂ may be responsible for sustained plant growth in low nutrient input FACE systems. Since this mechanism did not increase plant tissue N concentrations, and does not contribute to a net gain of ecosystem N, however, it is not expected to offset nutrient limitation under elevated CO₂in the future (<i>i.e.</i> decades to centuries). A third aim of this dissertation was to increase the understanding of plant specific responses to elevated CO₂. Therefore, I compared the responses of plants with genetic similarity but contrasting C allocation patterns, so reducing the number of plant traits that can explain a plants’ response to elevated CO₂. In addition, C allocation to roots is a key plant trait for explaining differential responses in C and N cycling as it affects both rhizodeposition and nutrient uptake. I showed that agronomic selection has resulted in a morphological tradeoff, where C allocation to organs associated with C assimilation compared to organs associated with nutrient uptake is favoured in modern cultivars. As a result modern cultivars are more likely to increase shoot biomass production under elevated CO₂ than their wild relatives in fertilized ecosystems. On the other hand, greater root production and N uptake rates indicate a greater potential for sustained plant growth and soil C sequestration under elevated CO₂for the wild compared to the cultivated genotypes in low N-input systems. These data showed that sink strength is an important trait for controlling plant responses to elevated CO₂. In conclusion, elevated CO₂ can increase soil C sequestration when sufficient nutrients are available. The extent of the increase however is still unclear in agro-ecosystems, due to a set of management practices that affect soil C decomposition and soil C input. In unfertilized ecosystems, simultaneous increases in N demands of microbes and plants reduce nutrient availability. Increased C allocation to roots under elevated CO₂ will benefit nutrient acquisition and C sequestration in low N systems but this mechanism is expected to be transient. Therefore, in natural ecosystems soil C sequestration is likely to be constrained in the future (<i>i.e.</i> decades to centuries) by progressive nutrient limitation