Sulfur (S) is an element required by all plants. Plants acquire the majority of their sulfur as sulfate (\(\text{SO}_4^{2-}\)), which is taken up from the soil through the roots. Once inside the plant, this sulfate is metabolized and incorporated into complex organic molecules. Plants release this nutrient back into the environment through two primary pathways: the atmosphere, via volatile compounds, and the soil, through root secretion and the decay of biomass.
Sulfur’s Essential Role and Internal Forms
Sulfur is a structural component of the amino acids cysteine and methionine, which are the building blocks of proteins and necessary for synthesizing enzymes that facilitate metabolism. Assimilation begins when inorganic sulfate taken up by the roots is reduced, mainly within the chloroplasts of the leaves.
The first stable organic product formed is cysteine, which acts as the central sulfur donor for nearly all other S-containing compounds. This includes glutathione (GSH), a tripeptide that protects the plant from oxidative stress and heavy metal toxicity. Before being released, sulfur exists primarily in these organic forms, stored within the plant’s tissues.
Atmospheric Release of Volatile Compounds
Plants release sulfur into the atmosphere primarily as volatile sulfur compounds (VSCs), which helps regulate internal sulfur levels. Common VSCs include hydrogen sulfide (\(\text{H}_2\text{S}\)), carbonyl sulfide (COS), and dimethyl sulfide (DMS). This gaseous efflux occurs mainly through the stomata, the small pores on the leaf surface.
Hydrogen sulfide emission is a direct pathway to offload excess sulfur, especially when internal sulfate concentrations are high. This process is driven by the degradation of L-cysteine, catalyzed by enzymes such as L-cysteine desulfhydrase (LCD). The release of \(\text{H}_2\text{S}\) acts as a detoxification mechanism.
Other VSCs, like COS, are released as metabolic byproducts, while DMS is often associated with specific plant families. These volatile emissions contribute to atmospheric sulfur content and influence cloud formation. The rate of this release is dynamic, fluctuating rapidly in response to environmental changes.
Soil Release Through Roots and Decomposition
Sulfur is released back into the soil environment through two mechanisms: continuous root exudation and the decomposition of plant biomass. Living roots engage in rhizodeposition, actively secreting organic compounds, including sulfur-containing amino acids and peptides, into the surrounding rhizosphere. This continuous, regulated release helps shape the microbial community. The sulfur in these exudates becomes available to other organisms and the soil pool, representing a rapid-turnover pathway for cycling.
The second pathway is the decomposition of plant material after senescence or death. When tissues die, the organic sulfur locked within proteins is not immediately available. Microorganisms must first break down the complex organic matter through mineralization, converting the organic sulfur back into inorganic sulfate (\(\text{SO}_4^{2-}\)). This delayed release is a major contributor to the long-term cycling of sulfur in the ecosystem.
Environmental Controls on Sulfur Efflux
The rate and form of sulfur released are regulated by the plant’s internal nutrient status. When sulfate levels in the leaves are high, the plant increases the activity of enzymes like L-cysteine desulfhydrase to accelerate the release of \(\text{H}_2\text{S}\). Conversely, under sulfur-deficient conditions, the plant prioritizes retention and recycling, reducing efflux.
External variables heavily influence the atmospheric release of volatile compounds. Light intensity and temperature affect stomatal opening, which controls the pathway for VSC escape from the leaves. High light and warm temperatures increase stomatal conductance, leading to greater potential for VSCs to be vented into the air. Water stress, such as drought, causes stomata to close, suppressing gaseous sulfur efflux regardless of internal content.