Abstract
Proteins constitute the single largest soil organic nitrogen (SON) reservoir and its decomposition drives terrestrial N availability. Protein cleavage by extracellular enzymes is the rate limiting step in the soil organic N cycle and can be controlled by extracellular enzyme production or protein availability/stabilization in soil. Both controls can be affected by geology and land use, as well as be vulnerable to changes in soil temperature and moisture/O2. To explore major controls of soil gross protein depolymerization we sampled six soils from two soil parent materials (calcareous and silicate), where each soil type included three land uses (cropland, pasture and forest). Soil samples were subjected to three temperature treatments (5, 15, 25 °C at 60% water-holding capacity (WHC) and aerobic conditions) or three soil moisture/O2 treatments (30 and 60% WHC at 21% O2, 90% WHC at 1% O2, at 20 °C) in short-term experiments. Samples were incubated for one day in the temperature experiment and for one week in the moisture/O2 experiment. Gross protein depolymerization rates were measured by a novel 15N isotope pool dilution approach. The low temperature sensitivity of gross protein depolymerization, the lack of relationship with protease activity and strong effects of soil texture and pH demonstrate that this process is constrained by organo-mineral associations and not by soil enzyme content. This also became apparent from the inverse effects in calcareous and silicate soils caused by water saturation/O2 limitation. We highlight that the specific soil mineralogy influenced the response of gross depolymerization rates to water saturation/O2 limitation, causing (I) increasing gross depolymerization rates due to release of adsorbed proteins by reductive dissolution of Fe- and Mn-oxyhydroxides in calcareous soils and (II) decreasing gross depolymerization rates due to mobilization of coagulating and toxic Al3+ compounds in silicate soils.
Highlights
Between 50 and 90% of soil N occurs in the form of high molecular weight (HMW) peptidic N, comprisingproteins and peptide side chains in peptidoglycan (Martens and Loeffelmann, 2003), the rest deriving from amino sugar polymers, aromatic N compounds and to date uncharacterized organic N compounds (Schulten and Schnitzer, 1997)
phospholipid fatty acids (PLFAs) contents were highest in calcareous soils, but did not vary with land use, and there was no significant parent material or land use effect on fungi:bacteria ratios (Table S1)
The presented study aimed at deciphering major controls of gross protein depolymerization and microbial amino acid uptake rates in soils considering temperature, soil moisture/O2 concentration, parent material and land use
Summary
Between 50 and 90% of soil N occurs in the form of high molecular weight (HMW) peptidic N, comprising (glyco)proteins and peptide side chains in peptidoglycan (Martens and Loeffelmann, 2003), the rest deriving from amino sugar polymers (e.g. chitin and glycan strands in peptidoglycan), aromatic N compounds (e.g. nucleic acids) and to date uncharacterized organic N compounds (Schulten and Schnitzer, 1997) These HMW-organic N compounds represent the largest soil N reservoir but are not directly available for microorganisms and plants due to molecular size constraints on cellular uptake mechanisms (Schimel and Bennett, 2004). Despite its importance and first insights into the process of gross protein depolymerization in soils, we still lack a thorough mechanistic understanding of major controls on this process
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