Plant quality controls on carbon in soil organic matter fractions across grassland ecosystems

Abstract:

Litter quality plays a critical role in soil organic matter (SOM) stabilization through interactions with microbes and the soil matrix, contributing to the fate of soil organic carbon (SOC). While it is recognized that multiple characteristics contribute to plant litter quality, measurements are often constrained to litter carbon (C) to nitrogen (N) ratios to simplify calculations. To add insight for future models, we are assessing soil chemistry and plant chemical and physical traits for their contributions to C stored in mineral-associated organic matter (MAOM) and particulate organic matter (POM) soil fractions. We used data available through NutNet and the TRY Plant Trait Database to populate a database with soil chemistry for world grassland sites, as well as plant chemical and physical traits. We are measuring C in both the MOAM and POM fractions for each of the selected sites. For preliminary analysis, we used ANOVA and linear regression models to determine predictors of total SOC. We found that plant and soil chemistry predict SOC differently, and these were not correlated. Leaf potassium predicted SOC, but calcium and magnesium concentrations were more important in soil than in leaves. This highlights the distinct functions of the nutrients; potassium increases plant productivity, which in turn is positively associated with SOC; calcium and magnesium are crucial for cation bridging and influence soil pH. Legumes, known to produce high quality litter and provide soil with N it requires to store C, predicted higher SOC than other plant types. The C3 photosynthetic pathway was a strong predictor of SOC, suggesting that this pathway may impact litter quantity, quality, or both. Other plant physical traits were not correlated with SOC, but we expect physical traits relating to roots to predict POM formation due to recalcitrance of root tissues, while nitrogen fixing ability will predict MOAM formation due to stimulation of microbial growth. Our early results indicate that soil and plant chemistry may provide distinct mechanistic clues to SOC storage and that plant type and photosynthetic pathway are indicators of SOC. By assessing the various contributions of plant traits to the different soil fractions, we will be able to develop a more complete index for integrating litter quality into SOM models.