Land-use change reduces soil nitrogen retention of both particulate and mineral-associated organic matter in a temperate grassland

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 (NH4NO3)-N-15-N-15 isotope in the field in a temperate grassland in Inner Mongolia. Our results showed that the fine mineral-associated organic matter (MOM < 20 mu m) 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 mu m) represented the fewer N pool with the largest mass. MOM < 20 mu m and fPOM retained > 90% of the N-15 tracer in soil. Deepened snow did not affect N-15 retention in SOM fractions, while tillage decreased N-15 retention in MOM < 20 mu m, 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 mu m. Structural equation modeling analysis revealed that tillage-induced decrease in N-15 retention of MOM < 20 mu m was regulated by both decreased microbial N-15 retention and reduced clay and silt contents. The decrease in N-15 retention of fPOM was probably due to the decreased microbial N-15 retention along with the increased plant N-15 uptake. This research reveals divergent pathways of N-15 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.