Hydrothermal conditions determine soil potential net N mineralization rates in arid and semi-arid grasslands

Soil net nitrogen (N) mineralization is a key biogeochemical process
influencing plant available N and net primary productivity (NPP) in
terrestrial ecosystems. However, the spatial variations and controlling
factors of soil net N mineralization (RPNM) in arid and semi-arid
grasslands are less studied and unclear. In this study, we investigated
the soil RPNM by performing a laboratory incubation experiment. Soil
samples were collected from 30 sites in three east-west transects on the
Inner Mongolia Plateau (MP), Loess Plateau (LP), and Tibetan Plateau (TP)
along a 3,200 km arid and semi-arid grassland gradient, with each transect
containing three different grassland types (meadow steppe, typical steppe,
and desert steppe, respectively). Results showed that the average RPNM
values ranged from -0.37 to 1.29 mg N kg–1 d–1, with a significantly lower
RPNM found in the desert steppe (0.08 ± 0.01 mg N kg–1 d–1) compared with
those in the meadow steppe (0.30 ± 0.03 mg N kg–1 d–1) and in the typical
steppe (0.33 ± 0.03 mg N kg–1 d–1) in the MP and LP transects (p <
0.05). This difference could be explained by variations in climatic and
soil factors, such as hydrothermal index (HT), the soil pH, soil organic
matter (SOM) and precipitation. However, no significant differences in
RPNM were found among different grassland types in the TP transect,
possibly due to the similarly low microbial activity, as indicated by the
MBC values. Across all three grassland transects, HT, SOM, and microbial
variables were the major factors controlling RPNM, which together
explained 20.7% of the variation in RPNM. Further SEM analysis indicated
HT was an integral predictor of RPNM, directly or indirectly via SOM,
under different conditions of precipitation and temperature. Our findings
provide field evidence and parameters for biogeochemical cycling to better
predict future N transformation processes under changing precipitation and
temperature regimes across a wide range of arid and semi-arid grassland
ecosystems.