Elucidating relationships of mineral-associated and particulate organic matter stocks, distribution, and stoichiometry across contrasting grassland ecosystems

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Katherine Rocci, Soil and Crop Sciences, Colorado State University, The NutNet Consorteum, Nutrient Network (www.nutnet.org) and M. Francesca Cotrufo, Department of Soil and Crop Sciences, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, CO

Background/Question/Methods
Fertilization studies are crucial for understanding limiting resources to ecosystems, but often find conflicting responses of soil carbon (C), a key pool for C-climate feedbacks. This lack of consensus could be attributed to environmental variation, nutrient type, or methods of measuring soil C. To characterize this variation, we measured amounts and stoichiometry of two distinct soil organic matter (SOM) pools in response to nitrogen (N), phosphorus (P), and potassium (K) addition in grassland soils that vary in climate and soil texture. We measured mineral-associated organic matter (MAOM), which is thought to form from efficient microbial transformations of plant inputs and subsequent protection of microbial and soluble plant products on minerals or in fine (<53μm) aggregates, and particulate organic matter (POM), which is thought to form from partially decomposed plant and fungal structural compounds and persists through inherent biochemical recalcitrance. MAOM and POM may inform variation in total SOM-C through (1) variation in their amount relative to total SOM (the fraction of MAOM in SOM; fMAOM) or by (2) changes in their stoichiometry (C:N). If nutrient addition increases microbial efficiency, we might expect greater fMAOM, whereas abiotic desorption or lack of microbial stimulation may favor stoichiometric changes in MAOM and POM, respectively. Additionally, fraction responses to nutrient addition will likely vary edaphically, as environments that tend be more limited in a given nutrient (high MAT and MAP for P) or allow for more MAOM formation (clay soils) may have unique responses.
Results/Conclusions
Preliminary results indicate that SOM-C marginally significantly increased under N addition, compared to the control, but otherwise did not respond to nutrient addition. SOM-C was significantly related to fMAOM and POM C:N, suggesting these predictors are indeed helpful in explaining SOM-C response to nutrient addition, whereas MAOM C:N may provide unique information. MAOM C:N was marginally significantly lower under N addition compared to the control whereas fMAOM and POM C:N did not vary with nutrient treatment, suggesting unclear linkages between fractions and overall SOM-C. Contrary to our expectations, nutrient addition effects were not stronger where that nutrient was limited, but rather were dependent on water availability, plant productivity, and other nutrients. These initial results indicate assessing interactions between environmental variables, nutrient identity, and soil fractions, may help elucidate plant feedbacks and SOM-C responses to nutrient addition, and provide unique information not obtained by assessing solely SOM-C.