Soil moisture variations alter the chemical and biological stability of mineral-associated organic matter

Abstract:

Clay minerals are responsible for the long-term sequestration of soil nutrients, but they are also dynamic zones of exchange. Organic compounds can rapidly enter and exit mineral associations (i.e., via sorption-desorption), but the conditions and processes that favor this turnover are not well understood. As any change or behavior in MAOM depends on the presence of water, soil moisture is likely a major control on the formation and destabilization of organo-mineral associations. There is a need to evaluate how wet-dry cycles alter the formation of MAOM and moreover, if the new MAOM is more vulnerable to turnover in the rhizosphere, especially in the face of projected shifts to hydrologic cycles. The primary objectives in our experiment were to assess 1) how fluctuating moisture influences the formation of new MAOM, 2) if the effect of moisture mediates the chemical stability and priming response of new and old MAOM, and 3) if wet-dry cycles lead to a decoupling of C and N mineralization from MAOM.

We created MAOM-sand mixtures to which we added ¹³C and ¹⁵N-labelled glycine while also applying two distinct moisture treatments. Soils were either maintained at a constant moisture level (60% of water-holding capacity) or were treated with three-day wetting-drying cycles, in succession, over a span of three weeks. Following the moisture-manipulation, we then assessed the priming response and chemical stability of the MAOM. To one subset of soils, we measured microbial biomass via chloroform fumigation extraction and conducted a sequential extraction of DOM using either KCl and sodium pyrophosphate. To another subset of soils we added either water, glucose, oxalic acid, or glucose + oxalic acid to simulate the rhizosphere effect. After substrate additions, we measured CO₂ respiration over a five day period and subsampled the gas for ¹³C-CO2 analysis. Stable isotope analyses will be used to determine the relative partitioning of newly-formed and old MAOM C and N into various soluble and gaseous phases. Preliminary results indicate that compared to soils under constant moisture, wetting-drying caused a 36% and 20% increase in dissolved C and N, respectively. This work will inform our growing understanding of the biochemical and physical mechanisms that influence MAOM formation and turnover.