Decadal nitrogen addition enhanced soil C and N storage in mineral-associated organic matter by altering soil abiotic and microbial properties in a temperate grassland

ABSTRACT: Global nitrogen (N) deposition has impacted the structure and functioning of soil microbial communities, translating into important changes to the cycling of soil organic matter (SOM). Recent frameworks have proposed that portioning the particulate and mineral-associated organic matter (POM and MAOM) fractions can help us better understand SOM cycling. However, how N deposition affect the fractionation of SOM into MAOM and POM forms, and how soil microbes process these across soil profile all remain unclear. Here we examined the microbial phospholipid fatty acids and determined N and soil organic carbon (SOC) content in POM and MAOM at depths of 0–10, 30–40 and 70–100 cm after 10-year N addition at rates of 0, 2, 10 and 50 g m^-2 yr^-1 in a temperate steppe. We found that N addition remarkably shifted microbial communities by increasing the relative abundances of bacteria and gram-positive (GP) bacteria, and decreasing gram-negative bacterial across the three soil layers. These effects of N addition tended to increase with the N addition rate but diminished with soil depth probably as pH decreased with the N addition rate but increased with soil depth. Both N addition and soil depth may cause similar microbial community shifts, through which fungi and GP bacteria become dominant, but may through different mechanisms. More than 60% of total SOC and N are stored as MAOM in this grassland. The share of SOC and total N in the MAOM was slightly decreased by N addition in 0-10 cm but significantly increased in deeper soils. The ratios of POM-C/MAOM-C and POM-N/MAOM-N significantly decreased with soil depth regardless of N addition treatments. Moreover, N addition increased the two ratios in 0-10 cm soil, but decreased them in deeper soil layers. N addition increased the stocks of SOC (MAOM: +11 %; POM: +23 %) and total N (MAOM: +10 %; POM: +27 %) in 0–10 cm soil, but increased only in MAOM in 30–40 cm (SOC: +24 %; total N: +24 %) and 70–100 cm (SOC: +15 %; total N: +13 %) soils. Soil physicochemical features exerted stronger controls than microbial properties in the distribution of SOC and total N in the two fractions regardless of soil depth because of eight soil features explaining more of the total variation than eight microbial properties. Our findings imply that increase in N deposition may make more SOC stabilized as MAOM fraction in grassland soils.

Keywords: Nitrogen deposition, Soil microbiome, Mineral-associated organic matter, Subsoil