Digging Deeper: The Importance of Litter Quality and Rooting Depth on Soil Organic Matter Stabilization in Agroecosystems

Abstract:

Soil organic matter (SOM) is a critical component of healthy soils. It improves soil structure, increases aeration, water infiltration, water holding capacity, and nutrient cycling. Increasing the SOM pools of soils has been repeatedly identified as a carbon (C) sequestration strategy to mitigating climate change. However, our understanding of the mechanisms that lead to SOM stabilization is limited, particularly in agricultural ecosystems where significant potential for C sequestration exists. Roots and shoots fundamentally differ in their ability to contribute to SOM formation both in litter quality and in the soil depths they impact. Roots are primary contributors to SOM, particularly in the deep soil, due to fine root turnover and exudation and proximity to the soil matrix. In agriculture, roots are often the primary organic matter input to the system after aboveground materials are harvested. Litter quality, such as differences in C to N ratio and % lignin, is a critical factor in the early stages of decomposition, but its relevance to SOM formation has been questioned in recent years. The recently proposed Microbial Efficiency - Matrix Stabilization hypothesis states that more labile substrates are rapidly incorporated into microbial biomass based on the stoichiometric needs of the microbes, and therefore less C is lost from the system as CO₂. Thus, more labile compounds are primary contributors to SOM. In order to address these differences in roots versus shoots, we have designed an experiment to answer the following questions: 1) how does initial quality of the root litter versus shoot litter lead to differences in the efficiency with which they are decomposed and incorporated into SOM; and 2) how does root contribution to SOM formation change as rooting depth increases? We combined ¹³C and ¹⁵N stable isotope enrichment of the litter with an innovative in situ decomposition method in order to accurately quantify root and shoot contribution to SOM up to 90cm soil depth. The isotopic enrichment allows us to trace the movement of decomposition products (root-derived C and N) to physical soil organic matter fractions (such as aggregates, light fraction and mineral fractions), informing us on the flow and likely stability of C and N over time. We will present data on the first harvest of a two-year experiment.