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

Abstract

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 15NH415NO3 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.