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[Display omitted] • MOM < 20 μm- and fPOM-N represent the two largest N pools among SOM fractions. • Tillage decreases soil N retention owing to reduced N retention of MOM < 20 μm and fPOM. • Lower N retention of MOM < 20 μm is due to lower microbial N recovery, clay, and silt. • Lower N retention of fPOM is due to lower microbial but higher plant N recovery. Soil organic matter (SOM) fractions vary in formation and microbial activities, thus playing different roles in exogenous nitrogen (N) retention in terrestrial ecosystems. However, it remains unclear how land-use and environmental changes affect the behavior of SOM fractions in retaining exogenous N. Here, we investigated N distribution among four SOM fractions and how soil N retention capacity responds to tillage and increased snowfall. We monitored N retention in SOM fractions by adding 15NH 4 15NO 3 isotope in the field in a temperate grassland in Inner Mongolia. Our results showed that the fine mineral-associated organic matter (MOM < 20 μm) had the largest N pool with a lower mass. The free particulate organic matter (fPOM) accounted for only 0.8% of total SOM mass, representing the second-largest N pool. The coarse mineral-associated organic matter (MOM > 20 μm) represented the fewer N pool with the largest mass. MOM < 20 μm and fPOM retained >90% of the 15N tracer in soil. Deepened snow did not affect 15N retention in SOM fractions, while tillage decreased 15N retention in MOM < 20 μm, fPOM, and occluded particulate organic matter within aggregates (oPOM). We suggested that the reduction in soil total N retention under tillage conditions was mainly due to the reduced N retention in fPOM and MOM < 20 μm. Structural equation modeling analysis revealed that tillage-induced decrease in 15N retention of MOM < 20 μm was regulated by both decreased microbial 15N retention and reduced clay and silt contents. The decrease in 15N retention of fPOM was probably due to the decreased microbial 15N retention along with the increased plant 15N uptake. This research reveals divergent pathways of 15N retention among different SOM fractions in response to land-use change and provides novel insights into the estimation of soil N retention capacity with SOM fractions taken into consideration. [ABSTRACT FROM AUTHOR]

Global warming has greatly altered winter snowfall patterns, and there is a trend towards increasing winter snow in semi‐arid regions in China. Winter snowfall is an important source of water during early spring in these water‐limited ecosystems, and it can also affect nutrient supply. However, we know little about how changes in winter snowfall will affect ecosystem productivity and plant community structure during the growing season. Here, we conducted a 5‐year winter snow manipulation experiment in a temperate grassland in Inner Mongolia. We measured ecosystem carbon flux from 2014 to 2018 and plant biomass and species composition from 2015 to 2018. We found that soil moisture increased under deepened winter snow in early growing season, particularly in deeper soil layers. Deepened snow increased the net ecosystem exchange of CO2 (NEE) and reduced intra‐ and inter‐annual variation in NEE. Deepened snow did not affect aboveground plant biomass (AGB) but significantly increased root biomass. This suggested that the enhanced NEE was allocated to the belowground, which improved water acquisition and thus contributed to greater stability in NEE in deep‐snow plots. Interestingly, the AGB of grasses in the control plots declined over time, resulting in a shift towards a forb‐dominated system. Similar declines in grass AGB were also observed at three other locations in the region over the same time frame and are attributed to 4 years of below‐average precipitation during the growing season. By contrast, grass AGB was stabilized under deepened winter snow and plant community composition remained unchanged. Hence, our study demonstrates that increased winter snowfall may stabilize arid grassland systems by reducing resource competition, promoting coexistence between plant functional groups, which ultimately mitigates the impacts of chronic drought during the growing season. [ABSTRACT FROM AUTHOR]