Wide-spread limitation of soil organic nitrogen transformations by substrate availability and not by extracellular enzyme content

• Published in: Soil biology & biochemistry 2019-06, Vol.133, p.37-49
• Main Authors: Noll, Lisa, Zhang, Shasha, Zheng, Qing, Hu, Yuntao, Wanek, Wolfgang
• Format: Article
• Language: English
• Subjects: Amino acid uptake; Moisture; Organic nitrogen; Protein depolymerization; Temperature
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/j.soilbio.2019.02.016

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. •Major controls on protein depolymerization and microbial amino acid uptake were investigated.•Low sensitivity of protein depolymerization and amino acid uptake to short term changes in soil temperature or moisture/O2.•Prominent role of substrate availability underlined by relatively small effects of potential peptidase activities.•Abundance of Fe and Al oxyhydroxides influenced the response of gross depolymerization rates to anoxia.