Root-derived inputs are major contributors to soil carbon in temperate forests

While we didn't find support in the greenhouse for mycorrhizal type differences in the rapid allocation of C belowground, previous research suggests total belowground C inputs may vary between mycorrhizal type in the forest. How? On the one hand, theory suggests distinct plant and fungal traits between AM and ECM species could lead to greater belowground C inputs by ECM compared AM trees due to 1) more extensive, C-rich external mycelial networks formed by ECM fungi compared AM fungal intracellular arbuscules and vesicles, 2) the greater extracellular oxidative enzymatic capacity of ECM fungi, which incurs a significant C cost, and 3) evidence that ECM plants have more slowly-decaying litters (Keller and Phillips, 2019; See et al. 2019) and grow in lower nutrient environments (Phillips et al. 2013), thereby increasing the need to allocate C belowground to enhance nutrient uptake. On the other hand, recent work within select temperate forests shows that plots dominated by AM species store more C in their soils, especially at depth, compared to ECM-dominated plots (Craig et al. 2018), suggesting there may be greater belowground C inputs in AM compared to ECM systems. To test these competing hypotheses, we used a dual-isotopic ingrowth core technique to quantify root-derived C inputs into the soil across six temperate ForestGEO sites in plots varying in AM and ECM tree dominance. We found that root-derived C was 54% greater in AM- versus ECM-dominated plots. This resulted in nearly twice as much root-derived C in putative slow-cycling mineral-associated pools in AM compared to ECM plots. Given that our estimates of root-derived inputs were often equal to or greater than leaf litter inputs, our results suggest that variation in root-derived soil C accumulation due to tree mycorrhizal dominance may be a key control in soil C dynamics in forests.  

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Coordination between above- and below-ground nutrient strategies drive soil carbon and nitrogen cycling

I fell in love with ecosystem ecology as a framework for thinking about how various components of nature interact with one another, and I've spent a lot of time thinking about how the 'hidden half' belowground relates to better-understood processes and traits aboveground. To that end, I used a multi-site field study to test the hypothesis that trees have coordinated aboveground and belowground nutrient use strategies. Specifically, I tested if conservation of nitrogen aboveground (via foliar resorption) relates to a tree's need to allocate C belowground to acquire soil nitrogen. In line with my work on belowground C allocation, I expected that the C cost of acquiring soil nutrients both reflects and determines the efficiency of nutrient resorption from senescing leaves. On one hand (what I call the 'tree-perspective'), high foliar resorption efficiency (NRE) may reduce C investment belowground, resulting in a negative relationship between aboveground NRE and belowground root production. Alternatively, given that NRE dictates leaf litter N concentrations, high NRE reduces litter N return to the soil and should drive greater tree C investment belowground (i.e., the 'soil perspective'). We found evidence of the 'tree-perspective' across two natural forest sites and support for the 'soil-perspective' at a young common garden site. These results suggest that above- and belowground nutrient strategy coordination can arise from both a 'tree' and 'soil' perspective, owing to differences in energetic trade-offs and the cost of N acquisition from soil.