Introduction
Land use change, a key global change phenomenon, greatly affects
ecosystem structure and functioning, particularly, alterations in
vegetation types and soil properties induced by land-use profoundly
alter soil carbon (C), nitrogen (N) dynamics and microbial activities
(Fu et al., 2015; Mbuthia et al., 2015; Rilling et al., 2019; Song et
al., 2012; Yue et al., 2020). Although the impacts of land use change on
soil C and N pools, and microbial community have been well demonstrated
(Cheng et al., 2013; Deng et al., 2016; Moscatelli et al., 2018), the
effects of land use change on
N-hydrolyzing enzyme activity and
stoichiometry remain controversial
(Allison et al., 2008; He et al., 2020; Zhang et al., 2019b). For
instance, some studies reported conversion of cropland to woodland
enhanced N-hydrolyzing enzyme activities and C: N enzyme ratio (Cenini
et al., 2016; Feng et al., 2018). Other studies showed that land use
change had negative or no effect on N-hydrolyzing enzyme activities or
soil enzyme stoichiometry (DeForest and Moorhead, 2020; Fichtner et al.,
2014). These inconsistent results can be attributed to different land
use management practice and microbial nutrient demand (Sinsabaugh et
al., 2016; Wang et al., 2012). Thus, more knowledge on soil
N-hydrolyzing enzyme activities in response to different types of land
use change is crucial for predictions of N cycling under future global
change scenarios.
Soil N-hydrolyzing enzyme including β-1,4-N-acetylglucosaminidase (NAG)
and leucine aminopeptidase (LAP) can serve as indicators of energy N
demand (Schimel et al., 2017), which catalyze terminal reactions to
depolymerize organic substrates for assimilation of N (Sinsabaugh et
al., 2009), and are strongly impacted by changes of soil properties
under land use change (Acosta-Martinez et al., 2007; He et al., 2020;
Sinsabaugh et al., 2008). For example, conversion of cropland to
woodland usually tends to increase soil N-hydrolyzing enzyme activities
due to increase in soil organic N content as soil organic matter is the
substrate of enzymatic reaction (Raiesi and Beheshti, 2014), whereas
cultivation decreases the soil N-hydrolyzing enzyme activities due to N
fertilizations (Zheng et al., 2020a). Meanwhile, some studies reported
that temperature was a key factor explaining the variation in soil
N-hydrolyzing enzyme activities
(Zhou et al., 2020) with a positive (Waring et al., 2014) and negative
(Peng and Wang, 2016; Xu et al., 2017) effects on it. While positive
relationship between soil N-hydrolyzing enzyme activities and soil
moisture was also be found in some studies (He et al., 2020; Xu et al.,
2017). Soil N-hydrolyzing enzyme activity was also affected by soil pH
and soil moisture (Fu et al., 2017; Pinsonneault et al., 2016). The
changes in soil pH were accompanied by changes in vegetation type which
induced positive relationship between soil pH and N-hydrolyzing enzyme
activities (He et al., 2020). Despite the importance of the above
factors in regulating soil N-hydrolyzing enzyme activities, soil
N-hydrolyzing enzyme activities driven by multiple controls under use
change are not well examined.
Soil extracellular enzyme
stoichiometry (EES) reflects the energy and nutrient controls on
microbial metabolism, indicating how shifts in substance and energy
allocation alter the according nutrient demand (Cui et al., 2018; Guo et
al., 2019; Sinsabaugh et al., 2012; Yang et al., 2020). It has been
suggested that ecosystem C: N: phosphorus (P) enzyme stoichiometry
greatly varies with land-use types (Hartman and Richardson, 2013; Li et
al., 2012). Variations in soil enzyme stoichiometry are consistent with
the patterns of microbial nutrient availability under land use change
(Sinsabaugh et al., 2008, Waring et al., 2014, Xu et al., 2017). It has
been reported that afforestation usually decreases soil enzyme C: N
ratio as higher soil C: N ratio induced N limitation for microbial (Feng
et al., 2018). But a recent study showed that differences in enzyme
stoichiometry between vegetation types were weakly related to the
microbial nutrient status (He et al., 2020). Additionally, previous
studies have found soil enzyme C: P ratio was inversely related to mean
annual temperature (MAT) and precipitation (MAP), while the soil enzyme
C: N ratio was positively related to MAP at a global scale (Sinsabaugh
et al., 2008). While Cui et al. (2018) reported that plant traits were
more important than soil physical and chemical properties in determining
EES. Thus, evaluating enzyme stoichiometry patterns and their drivers
could improve our understanding of N, P limitations in response to land
use change.
The Danjiangkou Reservoir area is a water source for the Middle Route of
China’s South-to-North Water Transfer Project. In order to protect water
quality of the reservoir and restore riparian ecosystem function, large
areas of afforestation have been conducted to reduce soil erosion, water
pollution (Zhu et al., 2010). Our previous studies reported that
afforestation greatly impacted soil C and N pools, C:N ratios and
microbial activities (Cheng et al., 2013; Deng et al., 2016). But study
on N-hydrolyzing enzyme activities can be beneficial for understanding
the interactions of soil -plant -microorganism for N cycling in this
region. Thus, we hypothesized that: (1) afforestation would increase
soil N-hydrolyzing enzyme activities primarily due to increased litter
inputs (Zhou et al., 2020; Feng et al., 2018), while cultivation (i.e.
cropland) would decrease soil N-hydrolyzing enzyme activities possibly
due the addition of N fertilizer (Carlos et al., 2021; Zheng et al.,
2020b); (2) microbial nutrient acquisition in afforested soils would be
limited by N or/and P rather than by C likely due to the nutrients (N
or/and P) competition between plants and microbes (Zheng et al., 2020a).
To test these hypotheses, we investigated seasonal patterns of NAG and
LAP enzyme activities at the top (0-10cm) and deep (10-30cm) under land
use change (i.e., woodland, shrubland, cropland and adjacent
uncultivated fields). We also examined soil properties including soil
moisture, temperature, pH and soil organic C and N (SOC and SON), labile
N (LN) and recalcitrant N (RN).