Distribution of exogenous nitrogen fractions and their fate in moss-dominated biological soil crusts

Abstract

Nitrogen (N) labeled with 15N was evenly added into plots of moss-dominated biological soil crusts (BSCs) and bare soil on the Chinese Loess Plateau. After that, the surface BSCs and bare soil samples were continuously collected within 1-30 days. The 15N content of each N fraction in soil, microorganisms, and mosses was measured for each sample. The effects of BSCs on soil N fate and cycling was determined through analyzing the differences in the distribution of 15N fractions between the BSCs and bare soil. Our results showed that: 1) The 15N content of total N (TN), microbial biomass N (MBN), and dissolved organic N (DON) in the BSCs was 2.9, 17.5, and 9.0 times higher than that in the bare soil, respectively. The 15N content of moss plants in the BSCs was 4.73 mg kg-1. 2) The residual rate of 15N in the BSCs and bare soil was 13.0% and 3.3%, respectively, indicating that the N fixing and holding ability of BSCs was four times higher than that of bare soil. The percentage of each 15N fraction in T15N in the BSCs was in the order of MBN (54.3%)>moss plant N (22.5%)>DON (6.2%), while that in the bare soil was in the order of MBN (11.5%)>DON (2.6%). Over all, microorganisms and mosses in the BSCs had 65.3% higher capacity of N fixation as compared with the bare soil. 3) The transferred amount and storage capacity of MB15N in the BSCs were 17.2 and 20.5 times higher than that in the bare soil, respectively. Accordingly, the turnover rate of MB15N in the BSCs and bare soil was 5.8 and 7.2 times per month, respectively, with the turnover time of MB15N in the BSCs being 1.2 times longer than that in bare soil. In conclusion, BSCs fix and hold more N than bare soil and change the distribution of each N fraction, implying that BSCs play a critical role in N cycling in dryland ecosystems.