Environmental effects on soil microbial nitrogen use efficiency are controlled by allocation of organic nitrogen to microbial growth and regulate gross N mineralization

Abstract

Microbial nitrogen use efficiency (NUE) is the efficiency by which microbes allocate organic N acquired to biomass formation relative to the N in excess of microbial demand released through N mineralization. Microbial NUE thus is critical to estimate the capacity of soil microbes to retain N in soils and thereby affects inorganic N availability to plants and ecosystem N losses. However, how soil temperature and soil moisture/O2 affect microbial NUE to date is not clear. Therefore, two independent incubation experiments were conducted with soils from three land uses (cropland, grassland and forest) on two bedrocks (silicate and limestone). Soils were exposed to 5, 15 and 25 °C overnight at 60% water holding capacity (WHC) or acclimated to 30 and 60% WHC at 21% O2 and to 90% WHC at 1% O2 over one week at 20 °C. Microbial NUE was measured as microbial growth over microbial organic N uptake (the sum of growth N demand and gross N mineralization). Microbial NUE responded positively to temperature increases with Q10 values ranging from 1.30 ± 0.11 to 2.48 ± 0.67. This was due to exponentially increasing microbial growth rates with incubation temperature while gross N mineralization rates were relatively insensitive to temperature increases (Q10 values 0.66 ± 0.30 to 1.63 ± 0.15). Under oxic conditions (21% O2), microbial NUE as well as gross N mineralization were not stimulated by the increase in soil moisture from 30 to 60% WHC. Under suboxic conditions (90% WHC and 1% O2), microbial NUE markedly declined as microbial growth rates were strongly negatively affected due to increasing microbial energy limitation. In contrast, gross N mineralization rates increased strongly as organic N uptake became in excess of microbial growth N demand. Therefore, in the moisture/O2 experiment microbial NUE was mainly regulated by the shift in O2 status (to suboxic conditions) and less affected by increasing water availability per se. These temperature and moisture/O2 effects on microbial organic N metabolism were consistent across the soils differing in bedrock and land use. Overall it has been demonstrated that microbial NUE was controlled by microbial growth, and that NUE controlled gross N mineralization as an overflow metabolism when energy (C) became limiting or N in excess in soils. This study thereby greatly contributes to the understanding of short-term environmental responses of microbial community N metabolism and the regulation of microbial organic-inorganic N transformations in soils.

Elevated temperature increases the accumulation of microbial necromass nitrogen in soil via increasing microbial turnover

“…Elevated temperature has been found to increase microbial growth and enzyme activities (Barcenas‐Moreno, Gomez‐Brandon, Rousk, & Baath, 2009). The increase of enzyme activities promotes turnover or depolymerization of soil organic C, N, and P, thereby increasing the substrate availability for microbial uptake (Zhang, Zheng, Noll, Hu, & Wanek, 2019). For example, we found that both microbial N immobilization rates ( and ) increased significantly with temperature.…”

Growth of comammox Nitrospira is inhibited by nitrification inhibitors in agricultural soils

“…The intensive use of nitrogen (N) fertilizers in agro-ecosystems is driven by a growing demand of food for a global human population of more than 11 billion by 2100 (Gerland et al 2014;Gruber and Galloway 2008). However, nitrogen use efficiency (NUE) is rarely exceeding 50%, as majority of N fertilizers is lost to the environment through nitrate leaching, nitrous oxide (N2O) emission, and ammonia volatilization (Wang et al 2018a;Zhang et al 2019). Application of nitrification inhibitors (NIs) together with N-based fertilizers is a practical tool to reduce N losses and to promote plant NUE in order to increase crop yields in agricultural systems ( Dinnes et al 2002;Subbarao et al 2006;Di and Cameron 2012;Shi et al 2016b).…”

Soil nitrogen substrates determine global N2O emission more than climate and other soil properties

“…Moreover, at low soil moisture microbial cell dehydration occurs which lowers the activity of nitrifying bacteria (Stark & Firestone 1995). Thus, the efficiency of soil processes are stimulated under higher soil moisture (Zhang et al 2019b). Finally, soil moisture can alter soil nitrification and denitrification where both processes can produce N 2 O (Bollmann & Conrad 1998).…”