Stable soil organic carbon (SOC) is a globally important carbon (C) pool with the potential to significantly alter atmospheric greenhouse gas concentrations, and it is believed to originate primarily from microbial necromass. However, we are not always capable of accurately predicting the magnitude and occasionally the direction of C fluxes into and out of the soil, in part due to a lack of understanding of how C moves from plants through the microbial community. C moves from the atmosphere into the soil primarily through plant uptake and incorporation followed by senescence and decomposition, or by exudation directly from roots, which especially stimulates production of readily stabilized microbial biomass in mineral soils. Root exudation rates depend on the plant species, but variations among plant species in the timing and rate of exudation have not yet been fully characterized. The primary question this study addresses is: How different are concentrations of root exudates for plant species with varying phenology? We hypothesized that plants varying in seasonal leaf expansion/senescence times also vary in seasonal concentrations of root exudation because of the difference in the timing of leaf presence and C fixation. To test this hypothesis, we measured root exudation between two species with varying leaf expansion and senescence times from April-December 2015 by measuring soil pore water saccharides as a proxy for root exudation. We also measured nitrate, ammonium, and phosphate concentrations in soil pore water and 0.5 M K2SO4 extracts to determine their distribution within the soil matrix. We observed microbial responses to root exudation by measuring soil microbial respiration, biomass, and ecoenzyme (enzymes existing/functioning outside of cells) activities. Two woody plants with different light acquisition strategies were chosen within the same temperate deciduous forest stand and soil type (coarse-loamy, mixed, nonacid, mesic Aeric Haplaquepts): Honeysuckle (Lonicera maackii), whose leaves expand early and senesces late, and red maple (Acer rubrum), which has its leaves for a much shorter time but obtains more light and shades L. maackii when its canopy is open due to its height. These were compared with plant-free control plots that were trenched in order to reduce root inputs.
Saccharide concentrations varied less in soil pore water than expected, and did not track leaf expansion and senescence as predicted, with the exception of May 18, when saccharide concentrations significantly increased in L. maackii soils just before leaf expansion was complete, at which time the L. maackii would become maximally shaded by the A. rubrum canopy (A. rubrum leaf expansion was complete one week later). Soil microbial biomass and activity also varied little between the two plant species, but all ecoenzyme activities varied significantly by date due to a decrease in December, apparently associated with seasonal temperature decline. Furthermore, although leaf litter was removed, the legacy of L. maackii’s high quality litter was apparent, with higher nutrient (nitrate and phosphate) concentrations from March to May (when leaf expansion was complete). Overall, however, nutrient dynamics were not clearly related to exudation or phenological events.
Furthermore, the distribution of saccharides within the soil matrix is still uncharacterized. A comparison of the concentrations we observed in soil pore water and salt extracts indicates that most dissolved saccharides are physically separated from the mobile pool, spatially inaccessible within the soil matrix and isolated from organisms. To our knowledge, no other studies have measured both soil pore water and extractable saccharides, but separate measurements suggest that spatially inaccessible saccharide concentrations likely range from 10-500 times greater than pore water, or “mobile”, saccharides . This suggests that our estimate of 10 times more saccharides in the spatially inaccessible than the mobile pool may be on the low side.
Overall, we conclude that concentrations of soil saccharides did not track phenological events as expected. However, while we did not observe an overall difference in soil saccharides between species, our results still suggest that the relationships between exudation and phenological events, such as leaf expansion and senescence, are likely species dependent. For example, there was an apparent pulse of soil saccharides only around L. maackii just before leaf expansion was complete. Furthermore, we conclude that there is a spatially inaccessible pool of soil saccharides that is likely much larger ( ¿ 10 times) than the mobile pool.